Ultrasound apparatus alignment

Abstract

The apparatus is configured to provide for transcutaneous ultrasound applications. The apparatus 100 comprises a body 102, a plurality of transducer apparatuses, preferably phased arrays, 110 coupled

Description

FIELD OF THE INVENTION The present invention relates to apparatuses for transcutaneous ultrasound applications and methods for configuring the same. BACKGROUND Transcutaneous ultrasound, whereby ultrasound waves are applied to a subject’s body through the skin, is a field that is experiencing rapid growth. This non-invasive technology can be used in many different applications. When used for diagnosis, reflections from tissue types and structures with the body can enable accurate imaging. When used for therapy, ultrasound energy can be focused at areas of interest to provide various therapeutic effects. However, a common problem is how to reliably and accurately steer the acoustic waves so that they interact with specific parts of the subject’s body, e.g., different regions of the brain when used for transcranial applications. Transducers may be conformally coupled to the subject’s body, meaning that the orientation of the transducers is dependent upon the local surface topology (e.g., the shape of the subject’s head). Alternatively, the transducers may be retained in some rigid structure and orientated towards a fixed position, which may not be optimal for subjects of different size and shape. Therefore, there is generally a need to ‘steer’ the acoustic waves appropriately so that they interact with the desired target region. This can be done using beam steering / phased array, wherein the transducer comprises an array of independently drivable transducer elements. The phase of these elements can be adjusted so that the acoustic wave generated by the transducer is steered in a desired direction, at least where the transducer elements of the array have sub-wavelength spatial dimensions. However, the use of phased arrays is not without its drawbacks. The use of phased arrays requires many driving channels and associated electronic components. This significantly increases the size, cost and / or complexity of the apparatus, which may be particularly limiting for wearable devices. Using an array of relatively small transducer elements may require many elements in order to obtain sufficient focal gain.. Generating high focal pressures in the deep brain in particular may require large radiating apertures, which may need an impractically large number of transducer elements in a phased array due to spatial constraint of the device (e.g., available surface area over the head). Overall, there is significant interest in developing new transcutaneous ultrasound apparatuses and methods. However, further improvements are needed to overcome at least some of the challenges discussed above, particularly with regard to steering / focusing acoustic waves within such apparatuses. SUMMARY According to an aspect of the invention, there is provided an apparatus for transcutaneous ultrasound applications, the apparatus comprising: a body; a plurality of transducer apparatuses coupled to the body and spatially separated from one another over a surface of the body, each transducer apparatus configured to generate an acoustic wave having a beam axis; an alignment apparatus configured to adjust an orientation of the beam axis of at least one of the transducer apparatuses; and an ultrasound controller configured to control a relative phase of drive signals provided to at least some of the transducer apparatuses. The alignment apparatus and the ultrasound apparatus may, respectively, be used to provided coarse and fine steering of the acoustic waves’ beam axes. The alignment apparatus (e.g., an acoustic lens or steering mechanism, as discussed further below) may be configured appropriately so that the acoustic waves generated by each of the transducer apparatuses intercept or overlap at a predetermined target region within a body of a subject, for example within the head. By performing coarse steering with the alignment apparatus, which may also be referred to as pre-steering, the steering requirements of the ultrasound controller may be reduced. For example, the pre-steering may eliminate the need for each transducer apparatus to be a sub-array or phased array. This may enable each transducer apparatus to be a single large transducer element. Large transducer elements, e.g., transducer elements with an aperture greater than the acoustic wavelength used, may allow for greater acoustic focal pressures to be produced. This may allow acoustic stimulation of the deep brain, for example. The pre-steering may also reduce the number of transducer elements and associated electronic / acoustic components that are required compared to phased arrays; a smaller number of large transducer elements may be provided. The relative phase of each of the transducer apparatuses may be adjusted appropriately so that the acoustic waves interfere at the target region, creating an acoustic stimulation region at the desired position where the acoustic power / pressure is above a certain threshold. Herein, each transducer apparatus may have a face. This may be an oscillating surface of the transducer that produces an acoustic wave when driven. The face will have a normal axis that may generally be perpendicular to the direction of propagation of the acoustic wave (at least where no beam steering is provided). It will be understood that the transducer face may not be flat / planar, but that a normal axis may still be defined perpendicular to the face. The beam axis may be defined as an axis along the direction of propagation of the acoustic wave from the face of the transducer to the target region (when aligned appropriately). When the acoustic wave is appropriately steered, the acoustic wave may follow a path along the beam axis from the transducer apparatus to the target region (when aligned appropriately). The beam axis may not be a straight line; the beam axis may be refracted due to changes in refractive index / density of a propagating media along the beam axis. For example, the acoustic wave may be refracted at a boundary layer of the apparatus and a body part of the subject, or at a boundary within the body of the subject (e.g., at the skull). The alignment apparatus may differ from phased-array beam steering in the way in which it adjusts the orientation of the beam axis. A phased array may adjust the direction of propagation of the acoustic wave, relative to the transducer face normal axis, at the face of the transducer - e.g., the acoustic waves are radiated from the face at a non-perpendicular angle. By contrast, the alignment apparatuses discussed herein my not steer the acoustic wave in this way. An acoustic lens may adjust the orientation of the beam after the acoustic wave is radiated from the face and has entered the acoustic lens. A steering mechanism may alter the orientation of the transducer apparatus, and thus the normal axis of the face, itself, without adjusting the orientation of the beam axis relative to the face of the transducer apparatus. The alignment apparatus may be configured to adjust the orientation of the beam axis of at least one of the transducer apparatuses relative to an axis normal to a local surface of the body. The body may comprise some deformable / conformable portion that is to be coupled to a body part of the user. This is usually required so that the apparatus can establish a good acoustic coupling of the transducers to the user. The face of the transducer apparatus may be arranged parallel to the body of the apparatus, which in turn conforms to the local topology of the body part on which it is placed. Alternatively, the body may be rigid and, during use, is pressed firmly into a soft body part. In either case, for existing devices, this means that orientation of the transducer apparatus may be dependent upon the body part in question. The present invention may therefore enable pre-steering of the beam axis of the transducer apparatuses relative to the local normal A coupling layer, e.g., a layer of acoustic gel or a gel-pad may be provided between the transducer and the subject to improve acoustic coupling therebetween. For example, a gel could be put onto a body part, into which the apparatus is pressed. The apparatus above comprises a plurality of transducer apparatuses. Below, features of the invention are said to relate to at least one of said transducer apparatuses. However, it will be understood that any of the features herein may be provided for any or all of the transducer apparatuses. For example, all of the transducer apparatuses may be provided with a corresponding acoustic lens or steering mechanism. However, in some embodiments, some of the transducer apparatuses may not be provided with all these features. The alignment apparatus may comprise an acoustic lens acoustically coupled to a face of at least one of the transducer apparatuses, the acoustic lens being configured to deflect the acoustic wave from an axis normal to the face of the transducer apparatus. An acoustic lens may be used to adjust an orientation of the beam axis as discussed above. Configuration of the acoustic lens may comprise fabricating a custom lens with suitable acoustic parameters such that the acoustic wave is directed towards the target region. An acoustic lens may be a reliable and durable means by which the beam axis orientation can be adjusted. The acoustic lens may be a volumetric lens, a thin-phase holographic lens or a Fermat-type lens. The acoustic lens may instead comprise a grating, such as a blazed grating, configured to redirect the acoustic wave similarly to a lens. Such lens types may be manufactured relatively easily with typical fabrication techniques and / or may provide suitable adjustment of the beam axis orientation. A distal end of the acoustic lens opposite to the transducer apparatus may comprise an acoustic coupling layer. A proximal end of the acoustic lens may be coupled to the face of the transducer apparatus. The acoustic coupling layer at the distal end may then be brought into contact with a body part of the subject (e.g., the scalp) when the apparatus is in use. The acoustic coupling layer may improve the coupling efficiency of the acoustic waves from the apparatus to the subject’s skin. For example, the distal end of the acoustic lens may be provided with a layer of a gel-like material or a soft / compressible polymer, whereas the acoustic lens may be made of a rigid or semi-rigid / flexible polymer material. For example, the acoustic coupling layer may comprise a polymer material with an elastic modulus less than that of the acoustic lens / the transducer apparatus. The acoustic coupling layer may improve the transmission of ultrasound waves from the apparatus to the skin by providing an intermediary medium, thereby providing impedance matching and thus reduced reflection at the apparatus-skin boundary. The acoustic coupling layer may have an elastic modulus between the elastic modulus of a material from which the acoustic lens is formed and a typical elastic modulus of skin. A plurality of acoustic coupling layers with sequentially decreasing acoustic impedance values may optionally be provided, to create a more gradual impedance transition to the skin. A similar acoustic coupling layer may be provided at the proximal end of the acoustic lens for efficient coupling between the acoustic lens and the transducer apparatus; said coupling layer may have an elastic modulus between that of the acoustic lens and the transducer apparatus. Additionally or alternatively to using an acoustic coupling layer, the distal end of the acoustic lens may be shaped so as to at least partially conform to the surface of the body part. For example, where the apparatus is to be used for transcranial applications, the distal end of the acoustic lens (and the acoustic coupling layer thereon, where present) may have a concave shape so as to at least partially conform to the shape of a subject’s head. The curvature of the distal end may be configured to conform to the surface of the body part. For example, a 3D scan / model of the body part could be used to inform the printing of the acoustic lens. Such shaping of the acoustic lens may be advantageous where large aperture transducers are used, since the relatively large face of the transducer may cover a significantly curved surface area. Said shaping may enable the acoustic lens to be made out of material with higher elastic modulus, which may improve the coupling between the transducer and the subject (transducers themselves typically made from materials with relatively high elastic modulus). The acoustic lens may be configured to deflect the acoustic wave in a plurality of different directions. The apparatus may be used to stimulate a plurality of different target regions, e.g. separate areas within the brain by redirecting part of the acoustic wave energy towards said target regions. The acoustic lens may comprise a plurality of lens portions, each lens portion being configured to deflect a spatial portion of the acoustic wave in a different direction. Each lens may be regarded as being a plurality of lens portions or separate lenses in an array, each lens portion / lens being configured (as discussed herein) so that the beam axis orientation is adjusted so that the acoustic wave intercepts one of the target regions. For example, there may be two target regions, and the acoustic lens may have two halves. A first half of the acoustic lens may direct a first half of the acoustic wave to a first target region, while the second half of the acoustic lens may direct a second half of the acoustic wave to a second target region. More than two lens portions may be provided and the size of these portions (and thus the acoustic energy directed to each target region) may not necessarily be equal. Additionally / alternatively, where the acoustic lens is a holographic lens (e.g., a volumetric or thin-element holographic lens), redirection of the acoustic waves may not require different spatial lens portions. The holographic acoustic lens may be configured to instead split the acoustic wave in the phase domain, so that two or more target regions may be addressed. Instead of using spatial lens portions, the required deflection for the distinct target regions is encoded over the whole surface of the acoustic lens. Use of holographic lenses may be advantageous compared to segmenting the lens spatially, since the full transducer aperture may be used for each of the target regions. E.g., each target region may be addressed by part of the acoustic wave from a large transducer aperture, rather than being addressed by only a sub-section of the transducer aperture (effectively functioning as a smaller aperture transducer). The acoustic lens may be configured to deflect the acoustic wave in a plurality of different directions based upon a frequency of the acoustic wave. As well as the ‘splitting’ of the acoustic wave (either using spatial segments or holographic lenses), the deflection of the acoustic wave may also be varied by changing the frequency of the acoustic wave. The acoustic lens may be configured such that the angle of deflection of the acoustic wave is dependent upon its frequency. At a first frequency, the acoustic lens may pre-steer the acoustic lens to a first target region, while at a second frequency the acoustic lens may pre-steer the acoustic lens to a second target region. The acoustic lens and chosen frequencies may be configured so that all the transducer apparatuses can be similarly pre-steered to a plurality of target regions. The alignment apparatus may comprise a plurality of acoustic lenses, each transducer apparatus being coupled to one of the acoustic lenses. Each acoustic lens may comprise an identifier indicating a corresponding transducer apparatus of the plurality of transducer apparatuses to which the acoustic lens is to be coupled. Practically speaking, the apparatus may comprise a plurality of transducer apparatuses, each transducer apparatus being provided with a custom acoustic lens so that all of the acoustic waves overlap at a target region. The identifier may therefore ensure that the transducer apparatuses and acoustic lens are correctly paired up (e.g., when switching being a set of acoustic lenses for a different user or a different target region). The identifier may be alphanumeric - e.g., a corresponding and unique letter / number provided on each acoustic lens and transducer apparatus. The acoustic lenses and transducer apparatus may be provided with a mechanical identifier so that only correct pairs can be physically coupled to one another. For example, each acoustic lens and a lens holder at the transducer apparatus may be complimentarily shaped uniquely and / or provided with a unique set of notches / projections thereon (like a Tock-and-key’ type arrangement). The alignment apparatus may comprise: a deformable member coupled to a face of at least one of the transducer apparatuses; and a steering mechanism configured to mechanically pivot the transducer apparatus relative to the axis normal to the local surface of the body. The use of a deformable member and a steering mechanism may provide an alignment apparatus that can be more easily configured. The orientation of the beam axis may be adjusted via a simple control signal to the steering mechanism (compared to needing to change acoustic lenses, for example). This may enable this example of the alignment apparatus to address multiple target regions relatively quickly. The deformable member may comprise a body comprising a compressible material (e.g., a deformable plastic or a rubber-like material). The deformable member may comprise a body containing a fluid (e.g., a deformable / compressible sack filed with water, aqueous liquid, oil-based liquid or an acoustic gel). Use of a deformable member may provide good contact with the skin of the subject by conforming thereto when pressure is applied, which may improve acoustic coupling by reducing air gaps at the apparatusskin boundary. The deformable member may be configured similarly to the acoustic coupling layer, so that the deformable member provides efficient coupling from the transducer apparatus to the skin. E.g., the materials of the deformable member may have an intermediary elastic modulus between that of the transducer apparatus and the skin. The steering mechanism may comprise: a piezoelectric actuator; a pneumatic actuator; and / or a control element comprising a shape memory material. Any suitable mechanism that can be used to pivot the transducer apparatus may be used. The transducer apparatus may be mounted on a ball joint, for example, so that the steering mechanism can pivot the transducer apparatus along two axes of rotational freedom. The beam axis orientation may therefore be freely pivoted anywhere within a range of motion of the ball joint / steering mechanism, so that target regions across a wide field-of-view can be acoustically stimulated. The apparatus may further comprise: a tilt sensor configured to measure a spatial orientation of the transducer apparatus; and an alignment controller configured to determine an alignment between an axis normal to the face of the transducer apparatus and the axis normal to a local surface of the body based upon the measured spatial orientation of the transducer apparatus. The alignment controller may be further configured to provide a control signal to the steering mechanism in order to change an alignment of the transducer apparatus relative to the axis normal to the local surface of the body. The tilt sensor and alignment controller may provide for automated control of the steering mechanism, and thus automated orientation adjustment of the beam axes. The steering mechanism may be controlled manually; a target region could be determined and the steering mechanism adjusted appropriately (e.g., using manual control) to reorientate the transducer apparatus. However, the tilt sensor may enable the orientation of the transducer apparatus to be measured. The orientation of the transducer apparatus may be measured relative to a reference, such as the body of the apparatus, for example. The axis normal to a local surface of the body may represent an axis normal to a body part of a subject (e.g., an axis normal to the subject’s head) where the body is at least partially conformable, meaning the orientation of the transducer apparatus may be determined relative to a local surface of a body part of the user. The target region may also be measured relative to the reference, e.g., spatial coordinates within an area partially enclosed by the body of the apparatus. Thus, any adjustment needed in the orientation may be determined so that the beam axis of the transducer apparatus intercepts the target region correctly. Each of the plurality of transducer apparatuses may be provided with a tilt sensor and the alignment controller may be configured to control each of the steering mechanisms of each transducer apparatus appropriately. The alignment controller may be configured to store the positions of a plurality of target regions, so that different control signals can be provided to realign the steering mechanism / transducer apparatus appropriately for each target region. The alignment controller may be configured to deactivate transducer apparatuses for which the determined alignment between the axis normal to the face of the transducer apparatus and the axis normal to a local surface of the body exceeds a predetermined threshold. A transducer apparatus may be deactivated if the tilt sensor measures that the transducer apparatus is tilted more than 10 degrees, more than 20 degrees or more than 30 degrees away from perpendicular to the local surface of the body (0 degrees being where the axis normal to the face of the transducer apparatus is parallel to the normal axis of the local surface). This may avoid acoustic waves being applied to the subject at large angles, which may otherwise cause shear wave generation in the skull or other bones of the subject. This may reduce uncomfortable or dangerous heating at the skin or near-surface body of the subject. The tilt of the transducer apparatus may also be determined relative to a surface of the subject (e.g., the skull or the skin of the subject), which may provide for more accurate control of the transducer apparatus compared to determining tilt relative to the body of the apparatus itself. The apparatus may be provided with prior knowledge of subject, for example the curvature of the subject’s skull based upon medical imaging data. The orientation of the transducer apparatus as measured by the tilt sensor may then be converted to an alignment between the axis normal to the face of the transducer apparatus and an axis normal to a local surface of the skull based upon the prior knowledge. Deactivation of the transducer apparatus may then be determined based upon a predetermined threshold angle relative to the skull itself. The alignment apparatus may comprise a second acoustic lens couplable to the transducer apparatus, the second acoustic lens being configured to deflect the acoustic wave in a different direction to the first acoustic lens. As discussed above, acoustic lenses of the present invention may comprise a plurality of lens portions so that different target regions can be addressed simultaneously. Alternatively, or additionally, a second lens may be provided to addresses the second target region. A plurality of lenses may be provided, each being configured (e.g., having different shape and / or acoustic properties) so that the beam axis of the acoustic waves intercepts a different target region. The transducer apparatus may comprise a lens holder, e.g., a snap-fit or a frictional fit portion, into which the acoustic lens can be removably placed and acoustically coupled to the transducer apparatus. Thus, different acoustic lenses (and different sets of acoustic lenses where there is a plurality of transducer apparatuses) can be swapped in / out for addressing different target regions. The steering mechanism may be operable to mechanically pivot the transducer apparatus to a first alignment and a second alignment, the first alignment and the second alignment corresponding to different angular orientations relative to the axis normal to the local surface of the body. Similar to above, a plurality of target regions may be addressed by configuring the steering mechanism in a plurality of different alignments. Where the steering alignment is controlled automatically / electronically (as per the alignment controller discussed above), the steering mechanism may be operated so that the transducer apparatuses are moved into different preset alignments corresponding to different target regions. The apparatus may further comprise a height adjustment mechanism configured to alter the position of at least one of the transducer apparatuses relative to the body along the axis normal to the local surface of the body. The height adjustment mechanism may be used to move the transducer apparatuses up and down relative to the surface of the body so as to bring the transducer apparatuses (or deformable bodies or acoustic lens thereon) into contact with a subject wearing the apparatus. This may assist in putting on the apparatus. When first putting on the apparatus, such as when wearing the apparatus on the head, the transducer apparatuses may be in an ‘up’ position where they are retracted from the head, thereby providing move space for putting on the apparatus and reducing the risk of damaging the transducer apparatuses. Once the apparatus is correctly fitted on the head, the transducer apparatuses may be moved ‘down’ toward the head, which may establish a good acoustic coupling therewith. The height adjustment mechanism may be any suitable mechanism, e.g., a series of manually repositionable notches, linear actuators, pistons, belt-driven sliding elements on shafts, etc. The height adjustment may be done manually (e.g., by setting the transducer element to a suitable or comfortable height by hand) or automatically and / or electronically. The height adjustment mechanism may comprise a pressure sensor configured to determine when the transducer apparatus has been brought into firm enough contact with the head / other body part. Where part of the apparatus is conformable (e.g., there is a deformable body for steering or a conformable acoustic coupling layer), contact between the transducer apparatus and the subject may be maintained across a range of heights. This may allow the height of the transducer apparatus to be adjusted relative to the surface of the subject’s head, for example, while still being acoustically coupled thereto. The apparatus may further comprise a reshaping lens acoustically coupled to a face of at least one of the transducer apparatuses, the reshaping lens being configured to adjust a beam profile of the acoustic wave. As well as redirecting / reorientating the beam axis of the acoustic wave as discussed above, a beam profile of the acoustic wave may be adjusted. The beam profile may refer to the spatial cross-section of the beam; different shaped beams may be preferable for certain applications. The beam profile may also refer to the amplitude / intensity of the acoustic wave across its cross-sectional area; a uniform beam intensity may be beneficial in order to avoid undesired hotspots or insufficiently stimulated regions, for example. Beam profile may refer to other variable parameters of the acoustic wave, such as phase, for example. The reshaping lens may be the same lens as the acoustic lens discussed above; a single lens may redirect and reshape the acoustic wave. However, these lenses may be separate - a reshaping lens may be retained while the acoustic lens is swapped so as to address a different target region, for example. The reshaping lens may also be used with the steering mechanism example discussed above; a reshaping lens may be provided between the transducer apparatus and the deformable member, for example. The reshaping lens may comprise a collimating lens configured to provide the acoustic wave with a uniform amplitude distribution across a predetermined angular range. A collimating lens may be advantageous in certain applications. A collimated acoustic wave may be more predictably controlled and may enable more predictable interference between the acoustic waves of the plurality of transducer apparatuses. The collimating lens may enable steering with equivalent focal gain and / or reduced acoustic side lobes. The apparatus may be optionally configured to be worn on the head of a subject. The apparatus may be used for transcranial ultrasound applications. The apparatus may be used for therapy, such as neuromodulation or localised heating. The apparatus may be used to perform any suitable ultrasound-based therapy, such as blood brain barrier opening (e.g., where microbubbles of air are injected into the bloodstream around the brain and then stimulated via ultrasound to non-invasively cause micro perforations in the blood brain barrier). At least some of the transducer apparatuses may be imaging transducers configured to detected reflected ultrasound acoustic waves, such that the apparatus may be used for imaging / diagnosis. The body may be made of a conformable fabric-like material. The body may be in the form of a hat (to be worn on the head) or in the form of a compressive wrap (to be worn around a limb, for example). The fabric-like material may be elasticated so that a suitably tight fit is provided between the apparatus and the subject. The transducer apparatuses may be coupled to the body. The body may have a plurality of holes therein through which the transducer apparatuses protrude, so that the body material does not interfere with the acoustic waves. Nonetheless, a ‘local surface’ as discussed above may still be considered to extend across these holes. The present invention may be used to counteract the variable topology of the subject’s body. For example, the human skull / head shape is complex and highly variable among subjects. Transducer apparatuses that are nominally orientated normal to the conformal hat-like body would thus be orientated in many different directions if not for the pre-steering provide by the alignment apparatus. The body may comprise: a rigid frame to which the plurality of transducer apparatuses are fixedly coupled; and a conformable layer to which the plurality of transducer apparatuses are acoustically coupled, the conformable layer comprising an acoustic coupling material. The rigid frame may be in the form of a helmet (to be worn on the head). The rigid frame / helmet may be hemispherical, for example. If the plurality of transducers were oriented normal to the rigid frame, the beam axes of the acoustic waves would intercept at the centre of the hemisphere. The pre-steering provided by the alignment apparatus may therefore enable the acoustic waves to be redirected to target regions in different regions. This may be used to stimulate different parts of the brain and / or to accommodate different head sizes / shapes and thus different positions of the same part of the brain relative to the apparatus. The conformable layer may be a sealing ring made of a conformable material (e.g., a deformable polymer) or a fluid-filled jacket (e.g., containing water or acoustic gel). The conformable layer may be provided on an inner surface of the rigid frame / apparatus. This may provide a comfortable fit for the user and / or may enable more efficient acoustic coupling from the transducer apparatuses to the skin. The conformable layer may function as the deformable member of the alignment apparatus for a plurality of, or all of, the transducer apparatuses. Each transducer apparatus may be coupled to the conformable layer and pivoted thereon individually by separate steering mechanisms, for example. The ultrasound controller may be configured to control a relative phase of the drive signals provided to at least some of the transducer apparatuses so as to adjust a position and / or size of an acoustic stimulation region. By adjusting the relative phase of the drive signals, a phase offset may be generated between the acoustic waves generated by the transducer apparatus. Depending on the phase offset, interference between the acoustic waves where they overlap can produce regions of constructive / destructive interference that can be controlled. The alignment apparatus may be used for coarse alignment, reorientating the beam axes so that the acoustic waves overlap with a target region. The phase offset may then be used for fine adjustment of an acoustic stimulation region. By adjusting the phase offset, the position and / or size of constructive interference within the target region may be altered. Thus, precise acoustic stimulation can still be provided for transducer apparatuses with larger radiating apertures and / or fewer radiating elements than phased arrays. Each transducer apparatus may comprise a face with a spatial dimension of at least 3 mm (e.g., a diameter of 3 mm where the face is circular). The face(s) may have a spatial dimension at least the wavelength, or at least twice the wavelength, of the acoustic waves generated by transducer apparatus as measured in a propagation medium. For example, for frequencies of 800 kHz, the wavelength in typical soft body tissue (e.g., brain matter) or an acoustic coupling layer would be approximately 1.92 mm (based upon a speed of sound of approximately 1540 m / s, due to body tissue generally being slightly higher density than water). Each transducer apparatus may comprise a face with a spatial dimension of at least 1.5 mm, at least 2 mm, at least 4 mm, at least 5 mm, at least 7.5 mm, at least 10 mm or at least 12 mm. Transducer apparatuses with faces (e.g., radiating elements) of this size may be considered to be large aperture transducers. Large aperture transducers may be able to provide higher focal pressures, particularly for regions of deep penetration (e.g., within the deep brain). However, large aperture transducers are generally highly directional. The alignment apparatus of the present invention may therefore be able to reorientate the acoustic waves so that they address a target region. This may enable high focal amplitudes to be achieved at the target region. The use of large aperture transducers may also reduce acoustic pressures within side lobes. The ultrasound controller may be configured to cause the transducer apparatus to provide the acoustic waves to an acoustic stimulation region with a peak acoustic pressure of less than 961 kPa. The peak pressure may be less than 1.24 MPa , less than 1.11 MPa, less than 877 kPa or less than 784 kPa. The peak pressure may be greater than 175 kPa , greater than 303 kPa , greater than 392 kPa or greater than 554 kPa. These pressures may be suitable for use in therapeutic applications, particularly for therapeutic stimulation of the brain. Pressures less than this may not provide sufficient stimulation. Pressures greater than this may result in excessive heating of body tissue. The ultrasound controller may be configured to cause the transducer apparatus to generate acoustic waves with a frequency of between 250 and 800 kHz. The acoustic waves may have a frequency of between 50 kHz and 1.5 MHz, between 100 kHz and 1 MHz, between 200 kHz and 1 MHz or between 400 kHz and 700 kHz. Such frequencies may be used for therapeutic applications. Greater frequencies may be used for diagnostic applications. For example, the transducer apparatuses may be imaging transducers (configured to also measure a reflection of the acoustic waves). Higher frequencies (e.g., 1 MHz to 10 MHz) may provide greater spatial resolution for imaging. The transducer apparatus may comprise a plurality of transducer elements. The ultrasound controller may be further configured to control a relative phase of drive signals provided to at least some of the transducer elements. As discussed above, the present invention may utilise a combination of an alignment apparatus and inter-wave phase offset to provide steering of the acoustic waves at a target region, without the need for phased-array techniques. Nonetheless, one or more of the transducer apparatuses may be a phased array comprising a plurality of transducer elements. The ultrasound controller may be configured to control the phase of the elements in each transducer apparatus in order to enable beam steering of the acoustic wave (i.e., reorientation of the beam axis relative to the face of the transducer). This may provide an additional and separate means for steering the acoustic waves. Each of the transducer elements may have a spatial dimension of at least half the wavelength of the acoustic waves used (e.g., approximately 0.94 mm for 800 kHz waves in water). However, even for smaller transducer elements (e.g., spatial dimensions less than half the wavelength), there may be some angular directivity, i.e., a reduction in amplitude at greater transmission angles. Thus, it may still be advantageous to change the bulk orientation of transducer elements to maximise the energy delivered; the present invention may still advantageous when used with phased arrays of smaller transducer elements. According to another aspect of the invention, there is provided a method of configuring an apparatus for transcutaneous ultrasound applications, the method comprising: determining a position of a target region within a body of a subject to which ultrasound is to be applied; providing an apparatus for transcutaneous ultrasound applications, the apparatus comprising: a body; a transducer apparatus coupled to the body, the transducer apparatus being configured to generate an acoustic wave having a beam axis; and an alignment apparatus configured to adjust an orientation of the beam axis of the transducer apparatus; and configuring the alignment apparatus such that the apparatus is in an aligned state whereby the beam axis of the acoustic wave generated by the transducer apparatus intercepts the target region. The method may be used to manufacturer an apparatus according to any aspect herein. As discussed above, the apparatus may comprise a plurality of transducer apparatuses, each being configured to generate an acoustic wave having a beam axis. Any of the transducer apparatuses may be provided with an alignment apparatus. Configuring the alignment apparatus / apparatuses may enable one or more target regions of a user to be stimulated. Where there are a plurality of transducer apparatuses, these may be jointly configured such that the beam axes of the acoustic waves from at least some of the transducer apparatuses intercept the same target region. The apparatus may comprise a plurality a transducer apparatuses coupled to the body and spatially separated from one another over a surface of the body. The apparatus may further comprise an ultrasound controller configured to control a relative phase of drive signals provided to at least some of the transducer apparatuses. The alignment apparatus may comprise an acoustic lens acoustically coupled to a face of the transducer apparatus. Configuring the alignment apparatus may comprise fabricating the acoustic lens, the acoustic lens being configured to deflect the acoustic wave from an axis normal to the face of the transducer apparatus to the target region. As discussed above, in some examples the alignment apparatus may be lens-based. During a configuration or fabrication stage, these lenses may be configured appropriately. Customised acoustic lenses may be fabricated for a particular user, a particular target region and / or a particular application (e.g., dependent upon the ultrasound frequency used). Any suitable fabrication technique may be used. Fabricating the acoustic lens may comprise 3D printing the lens; casting the lens; and / or machining the lens. Such techniques may enable sufficiently precise lenses to be formed so that the acoustic waves overlap the target region as desired. These techniques, particularly 3D printing, may enable lenses to be fabricated quickly and cheaply, which may be advantageous where the lenses are to be used as customised, replaceable components of the alignment apparatus within the wider apparatus. Fabricating the acoustic lens may comprise determining a beam angle between the axis normal to the face of the transducer and the target region. Fabricating the acoustic lens may comprise calculating appropriate acoustic lens parameters using a thin-element approximation, an iterative Fourier transform-based method, a ray-tracing method, machine learning-based methods and / or a differentiable wave simulator. In order to fabricate the acoustic lens / lenses it may be necessary to determine parameters (e.g., focal length, numerical aperture, etc.) that will enable the alignment apparatus to achieve the desired orientation of the acoustic wave. The desired deflection angle of the acoustic lens may be determined (discussed further below) and used to inform the fabrication of the lens. The method may further comprise determining a position of a second target region within a body of a subject to which ultrasound is to be applied. The acoustic lens may comprise at least two lens portions, a first lens portion being configured to deflect a first spatial portion of the acoustic wave to the first target region and a second lens portion being configured to deflect a second spatial portion of the acoustic wave to the second target region. The acoustic lens may be configured to deflect the acoustic wave in a plurality of different corresponding to the first target region and the second target region directions. The acoustic lens may be configured to deflect the acoustic wave in a plurality of different directions corresponding to the first target region and the second target region based upon a frequency of the acoustic wave. The method may further comprise fabricating a second acoustic lens, the second acoustics lens being configured to deflect the acoustic wave from the axis normal to the face of the transducer apparatus to the second target region. As discussed above, a variety of different techniques may be used (independently or in combination) to control the redirection of the acoustic waves so that a plurality of different target regions may be addressed. For example: i) the acoustic lens may be a holographic lens configured to deflect the acoustic wave to a plurality of target regions based upon encoding of the desired deflection across the surface of the acoustic lens; ii) the acoustic lens may comprise a plurality of lens portions (or the acoustic lens may be formed from a plurality of lens), each lens portion being configured to deflect different spatial portions of the acoustic wave to the different target regions; iii) the deflection provided by the acoustic lens may have a frequencydependence, such that acoustic waves of a first frequency are directed to a first target region, acoustic waves of a second frequency are directed to a second target region, etc.; and iv) a plurality of different acoustic lens may be fabricated, each acoustic lens (or each acoustic lens set, where there are a plurality of transducers each with an acoustic lens) being configured so that the acoustic wave is directed to a different target region. The acoustic lenses may be swapped for one another, or combined in various combinations, so that different target regions are addressed as desired. The alignment apparatus may comprise: a deformable member coupled to a face of the transducer apparatus; and a steering mechanism configured to mechanically pivot the transducer apparatus relative to the axis normal to the local surface of the body. Configuring the alignment apparatus may comprise: determining an offset in a spatial orientation of the transducer apparatus between an initial state and the aligned state of the apparatus, the initial state being prior to the configuration of the alignment apparatus; and controlling the steering mechanism so that the apparatus is reconfigured to the aligned state. As discussed above, in some examples the alignment apparatus may be steering mechanism-based. The transducer apparatus may be physically pivoted using the steering mechanism from an initial state (e.g., a naturally occurring orientation where the face of the transducer apparatus is parallel to the local surface topology of the subject’s body) to the aligned state. In the aligned state, the acoustic wave may overlap with the target region. Controlling the steering mechanism may therefore comprise adjusting movable components, e.g., an actuator and a ball joint, to pivot the transducer apparatus to desired orientation. Determining the position of the target region may comprise analysing medical imaging data or identifying a biomarker signal within the body of the subject. Configuring the alignment apparatus may comprise using an acoustic propagation model of part of the body of the subject. Configuring the alignment apparatus may comprise: using a digital model of part of the body of the subject; identifying the target region within the digital model; and simulating the acoustic wave generated by the transducer apparatus to determine whether the simulated acoustic wave intercepts the target region within the digital model. For example, in order to identify target regions of the brain that are to be stimulated, MRI images of the brain may be taken and the corresponding regions identified. A 3D model may be developed of the imaged brain. Simulated transducer apparatuses may then be digitally placed on the 3D model to determine the propagation of the acoustic waves, from which suitable transducer apparatus positions and beam axes orientation determined so that the acoustic waves overlap the identified target region. The acoustic propagation may comprise a 3D model of the imaged region including acoustic propagation properties of various body tissues therein. For example, acoustic waves may refract at the boundary of the skull and the brain (due to different acoustic properties). By using an acoustic propagation model when configuring the alignment apparatus, the orientation may be determined more accurately. Rather than the acoustic wave simply being modelled as a straight line from the transducer apparatus to the target region, deflection resulting from different body tissues may be taken into account. The acoustic lenses may then be fabricated, and / or the steering mechanism pivoted, based upon these models to provide the desired beam axis orientation. The transducer apparatus may comprise a plurality of transducer elements. The apparatus may further comprise an ultrasound controller configured to control a relative phase of drive signals provided to at least some of the transducer elements. The method may further comprise determining an appropriate control signal based upon an expected response of the transducer elements to the phase offset, the configuration of the alignment apparatus in the aligned state and the position of the target region. As discussed above, the transducer apparatus may be a phased array comprising a plurality of transducer elements. These elements may be suitably controlled (e.g., by providing a phase offset via drive signals provided to the transducer elements) to enable beam steering of the phased array. This provide a further means by which the beam axis orientation can be adjusted, in addition to the use of the alignment apparatus and providing phase offsets between the transducer apparatuses. The phased array may be operated in conjunction with the alignment apparatus and / or the phase offset between each transducer apparatus to provide the desired steering. According to a further aspect of the invention, there is provided an apparatus for transcutaneous ultrasound applications, the apparatus comprising: a body; a plurality of transducer apparatuses coupled to the body and spatially separated from one another over a surface of the body, each transducer apparatus being configured to generate an acoustic wave having a beam axis; and an alignment apparatus configured to adjust an orientation of the beam axis of at least one of the transducer apparatuses relative to an axis normal to a local surface of the body for the respective transducer apparatus; wherein the alignment apparatus is configured such that the beam axis of the acoustic wave generated by each of the transducer apparatuses intercept a common target region. This aspect may comprise any of the features discussed above in relation to other aspects. The alignment apparatus may be configured using a method according to an aspect. According to a further aspect of the invention, there is provided an apparatus for transcutaneous ultrasound applications, the apparatus comprising: a body; a transducer apparatus coupled to the body, the transducer apparatus being configured to generate an acoustic wave having a beam axis; and an alignment apparatus configured to adjust an orientation of the beam axis; wherein the transducer apparatus comprises a transducer element with a face with a spatial dimension of at least 3 mm. The apparatus may comprise a plurality of transducer apparatuses coupled to the body and spatially separated from one another over a surface of the body. The alignment apparatus may be configured to redirect the acoustic waves generated by at least some of the transducer apparatuses relative to an axis normal to a local surface of the body for the respective transducer apparatus. This aspect may comprise any of the features discussed above in relation to other aspects. The alignment apparatus may be configured using a method according to an aspect. These and other aspects of the disclosure will be apparent from, and explained with reference to, the embodiments described hereinafter. BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the invention will be described, purely by way of example, with reference to the accompanying drawings, in which: Figure 1 shows diagrams illustrating the effects of pre-steering for transcranial ultrasound applications; Figure 2 shows models of a transcranial ultrasound apparatus with and without presteering; Figure 3 shows a diagram of an example of a lens-based alignment apparatus; Figure 4 shows a diagram of an example of a steering mechanism-based alignment apparatus; Figure 5 shows an illustration of how an apparatus may be configured according to the present invention; Figure 6 shows a diagram of an example of an apparatus according to the present invention; Figure 7 shows a simulation of a holographic lens redirecting an acoustic wave towards to different target regions; Figure 8 shows a diagram of an example of an apparatus according to the present invention; and Figure 9 shows a flowchart of an example method for configuring an apparatus for transcutaneous ultrasound applications according to the present invention. It should be noted that the Figures are diagrammatic and not necessarily drawn to scale. Relative dimensions and proportions of parts of these Figures may have been shown exaggerated or reduced in size, for the sake of clarity and convenience in the drawings. The same reference signs are generally used to refer to corresponding or similar features in modified and different embodiments. DETAILED DESCRIPTION Referring to Figure 1, diagrams show general working principles of the present invention. An apparatus 100 for transcranial ultrasound applications may generally comprise a plurality of transducer apparatuses 110. Each transducer apparatus 110 is configured to output an acoustic wave 112. Where the transducer apparatuses 110 comprise one or more large transducer elements, e.g., having a transducer face with spatial dimensions larger than the wavelengths being used, the acoustic waves 112 may be highly directional. As shown in Figure 1, the acoustic waves 112 may propagate from the transducer apparatuses 110 with little or no beam divergence, such that they appear as tightly defined beams with near-constant diameter along the direction of propagation. The transducer apparatuses 110 may be provided on a conformable, wearable body, such as hat made from a fabric material (not shown). When worn on the head, such a body may generally conform to the shape of the underlying skull 10. Accordingly, the transducer apparatuses 110 may also conform to the shape of the skull 10, tilting and / or moving on the wearable body in response to the local surface topology of the skull 10 / the subject’s head. Each transducer apparatus 110 may naturally reorientate itself so that a surface of the transducer apparatus 110 (e.g., the transducer face) is parallel to the local surface of the skull 10 - i.e., an axis normal to the face of the transducer apparatus 110 may is aligned with an axis normal to the local surface of the skull 10 / the subject’s head (to which the wearable body conforms). As shown in the left-hand image a), the result of this conformity is that the transducer apparatuses 110 may naturally reorientate themselves in a many different directions; animal skulls, such as the human skull, generally have a complex surface topology. The acoustic waves 112 generated by the transducer apparatuses 110 may not overlap with one another. In general, it is desirable to direct the acoustic waves 112 to a predefined target region 114. This may be three-dimensional region within the head of the subject where ultrasound is to be applied. The target region 114 may correspond to a particular brain structure or may be a general spatial region of the brain. The target region 114 may be identified for a particular user based upon medical imaging, for example. However, due, to the natural conformity of the transducer apparatuses 110 to the skull 10, at least some of the acoustic waves 112 do not overlap with the target region 114. Each acoustic wave 112 has a beam axis 113; this may be an axis in the direction of propagation of the acoustic wave 112 and may be along the centre of the acoustic wave 112. The beam axes 113 in Figure 1 are shown as straight lines along the acoustic waves 112, but it will be understood that the beam axes 113 may not necessarily be straight lines - the beam axes 113 may be deflected by components of the apparatus 100 (discussed further below) or refracted by the skull 10 and / or other body structures within the head. Without any alignment, the beam axes 113 of at least some of the acoustic waves 112 may not intercept the target region 114, as shown in image a). Therefore, the acoustic waves 112 may not overlap in the desired spatial region. This may reduce the efficiency of the apparatus 100, since a significant portion of ultrasound energy output by the transducer apparatuses 110 is not provided to the target region 114. The lack of alignment may also result in other regions of the brain / head being stimulated unintentionally and / or in hot spots being formed undesirably. By contrast, the right-hand image b) shows an example of the present invention where the transducer apparatuses 110 have been suitably aligned (also referred to as pre-steering). The orientation of the beam axes 113 is adjusted such that they intercept the target region 114. This means that each of the acoustic waves 112 overlaps in the desired spatial region of the target region 114. This pre-steering of the transducer apparatuses’ beam axes may therefore provide a coarse steering of the acoustic waves; the pre-steering may be used to counteract the misalignment caused by the skull’s local surface topology. Once pre-steered, any suitable control scheme of the transducer apparatuses 110 may be used dependent upon the application of the apparatus 100. The power of the acoustic waves 112 may be controlled so as to provide a desired intensity at the target region 114 (i.e., a desired peak pressure). The phase of each transducer apparatus 110 may be controlled so as to provide desired interference at the target region 114; fine steering of a position of maximum acoustic intensity may be controlled by adjusting a phase offset between the acoustic waves 112. In order to correctly pre-steer the transducer apparatuses 110, not all of the beam axis orientations may need to be adjusted. In Figure 1, one of the transducer apparatuses 110a is already suitably aligned; the beam axis of this transducer apparatus intercepts the target region 114 in image a). In Figure 1, the present invention is described in terms of a conformable, wearable body; the beam axes of the acoustic waves being directed in uncontrolled / undesired orientations prior to pre-steering due to the transducer apparatuses being provided on a hat-like structure that conforms to the head / skull of subject. However, in other examples, the transducer apparatuses may instead be provided on rigid frame (such as a helmet). In such an example, the beam axes may intercept without pre-steering. If the rigid frame is hemispherical and the faces of transducer apparatuses are arranged parallel to this frame, the beam axes may intercept one another at a geometric centre of the hemisphere, for example. Nonethless, the pre-steering discussed herein may be used to adjust the beam axes orientation such that target regions at different locations than the hemisphere’s centre may be addressed. Referring to Figure 2, a model shows the results of the pre-steering principles discussed above. In the left-hand image a), beam axes of the transducer apparatuses 110 are not suitably reorientated; due to the transducer apparatuses 110 being oriented based on the local topology of the skull / head, the acoustic waves 112 point in many different directions. By comparison, in the right-hand image b), the beam axes of the transducer apparatuses 110 have been suitably pre-steered by an alignment apparatus (discussed further below), resulting in the acoustic waves 112 overlapping with one another in the target region 114. As a result of the pre-steering, an acoustic stimulation region 115 may be formed within the target region 114. The acoustic stimulation region 115 may correspond to a spatial region where the acoustic waves 112 interfere with one another so that the acoustic power density / peak pressure is within a predetermined range. The size and / or shape of the acoustic stimulation region 115 may be controlled by adjusting parameters of the acoustic waves 112, e.g., the power of each acoustic wave 112 and / or a phase offset therebetween. Referring to Figure 3, an example of an apparatus 100 for transcutaneous ultrasound applications is shown. The apparatus 100 comprises a body 102; the body may be a conformable hat-like body or a rigid helmet-like body, for example. A plurality of transducer apparatuses 110 are provided on the body 102 and are configured to generate acoustic waves 112. The body 102 may be configured to couple acoustic waves 112 from the transducer apparatuses 110 to a body part of a subject wearing the apparatus 100. The body 102 may comprise an acoustic coupling material configured to couple the acoustic waves 112 to the skin of the user. The body 102 may comprise or consist of a sack filled with water or an acoustic gel, for example. The transducer apparatuses 110 may be situated on an outer surface of said body 102, with their transducer faces directed towards an inner surface of the body 102 which would be in contact with the subject when the apparatus 100 is worn. Alternatively, as shown in Figure 3, the body 102 may comprise holes / cutouts through which the acoustic waves 112 are provided. However, in either case, the transducer apparatuses 110 may be provided parallel to the body 102; i.e., an axis normal to the face of the transducer apparatus 110 may be aligned with an axis normal to the surface of the body 102. The left-hand transducer apparatus 110 is shown with no pre-steering applied. A beam axis 113 of the acoustic wave 112 for this transducer apparatus has not been reorientated and thus is determined by the orientation of the transducer apparatus 110 (assuming no beam steering is performed) - the beam axis 113 is perpendicular to the local surface topology of the body 102 to which the transducer apparatus 110 is affixed. As discussed above, this may result in undesirable directionality of the acoustic wave 112; the beam axis orientation may be dependent upon the local surface topology of the subject’s body (where the body 102 is conformable) or may be fixed in an undesirable position (e.g., at a centre point where the body 102 is a hemispherical rigid helmet). By contrast, the right-hand transducer apparatus 110 is provided with an alignment apparatus. In this example, the alignment apparatus is in the form of an acoustic lens 120. The acoustic lens 120 is configured to deflect the acoustic wave 112; rather than the beam axis 113 being perpendicular to the face of the transducer / the local surface topology of the body 102, the beam axis 113 is at some non-zero angle. The acoustic lens 120 is provided along the direction of propagation of the acoustic wave between the transducer 110 and the skin of the subject. A proximal end of the acoustic lens 120 may be coupled to the face of the transducer apparatus, while a distal end of acoustic lens 120 may at least partially protrude through the body 102 for acoustically coupling to the skin of the user. The acoustic wave 112 can thus be directed to a target region, as discussed above. The acoustic lens 120 may be custom manufactured so that the deflection results in the beam axis 113 intercepting the target region when the apparatus is worn by a particular user. Referring to Figure 4, another example of an apparatus 100 for transcutaneous ultrasound applications is shown. Like in Figure 3, the apparatus 100 comprises a plurality of transducer apparatuses 110 configured to generate acoustic waves 112. The left-hand transducer apparatus 110 is not subject to any pre-steering; the beam axis 113 of the acoustic wave 112 is perpendicular to the local surface topology of the skin 12 of the user wearing the apparatus 100. However, the right-hand transducer apparatus 110 is provided with an alignment apparatus in the form of a deformable member 130 and a steering mechanism 132. The deformable member 130 may comprise a deformable solid material, such as a polymer or a rubber material. The deformable member may be filled with a fluid, such as water or acoustic gel. The deformable member 130 is provided between the face of the transducer apparatus 110 and the skin 12 of the user. The transducer apparatus 110 may be coupled to the surface of the deformable member 130 or may protrude partially therethrough, as shown. The deformable member 130 provides a compliant portion for coupling the acoustic waves 112 from the transducer apparatus 110 to the skin 12. The steering mechanism 132 may comprise any suitable means for physically moving the transducer apparatus 110. The steering mechanism 132 may comprise an actuator, for example. The transducer apparatus 110 is mounted a movable member 134, such as a ball joint. The steering mechanism 132 may then pivot the transducer apparatus on said movable member 134, thereby reorientating the transducer apparatus 110 relative to the rest of the apparatus 100 / the skin 12. As the orientation of the transducer apparatus 110 is adjusted, the deformable member 130 may alter its shape in order to maintain contact with the face of the transducer apparatus 110 and the skin 12. The steering mechanism 132 / movable member 134 may be fixedly coupled to a rigid portion of the apparatus’s body (not shown), so that the transducer apparatus 110 is held in position and can be pivoted relative thereto. The body may comprise a rigid frame, for example, from which the transducer apparatuses 110 and the deformable members 130 are suspended. The deformable member may then by brought into contact with the skin 12 when the apparatus 100 is worn. The alignment apparatus may further comprise a tilt sensor 136. The tilt sensor 136 may measure the orientation of transducer apparatus 110. The tilt sensor 136 may measure the orientation of the transducer apparatus 110 relative to a rigid frame of the apparatus 100, for example (e.g., by monitoring the angular position of the movable member 134). The rigid frame may be provided with a tilt sensor also and the orientation of each component of the apparatus 100 may be measured absolutely, so that the relative orientation between components can be determined. The tilt sensor 136 may output the measured orientation of the transducer apparatus to an alignment controller (not shown). Based upon this, the alignment controller may provide a control signal to the steering mechanism so as to control the orientation. This could be done via a feedback loop, for example; the orientation of the beam axis 113 may be adjusted and monitored via the tilt sensor 136, with the steering mechanism 132 being configured to provide a predetermined orientation. The predetermined orientation may be calculated so that the beam axis 113 intercepts a predetermined target region. The tilt sensor 136 output may also be used to measure and correct for an initial reorientation of the transducer apparatus 110 as the deformable member 130 is brought into contact with the skin 12 (which may pivot the transducer apparatus 110 if not otherwise locked in place). Referring to Figure 5, an example of a transcranial ultrasound apparatus 100 is shown. Similar to Figure 4 discussed above, the apparatus 100 comprises a plurality of transducer apparatuses 110 and an alignment apparatus comprising a deformable member 130. The transducer apparatus 110 is shown inset prior to any pre-steering of the acoustic wave 112; the beam axis 113 is perpendicular to the skull 10. For brevity / clarity, some elements discussed in Figure 4 have been omitted, e.g., the steering mechanism is not shown here and the transducer apparatuses 110 / deformable members 130 are simply shown ‘on’ the skull 10 rather than any body or frame. In the right-hand side of the image, the apparatus 100 is shown comprising four transducer apparatuses 110 and deformable members 130 like those shown inset. Each of these may be pre-steered independently and / or cooperatively in order to provide a suitable alignment for each of the beam axes 113. The transducer apparatus 110 can be pivoted relative to the skull 10 so that the acoustic waves 112 overlap within a defined target region 114. Accordingly, the beam axes 113 of the acoustic waves 112 may intercept, resulting in a suitable acoustic stimulation region 115 being formed within the target region 114. The location of the acoustic stimulation region 115 may then be adjusted via the adjustment apparatus (i.e., pivoting the transducer apparatuses 110 on the deformable members 130, which may be referred to as coarse steering). The location of the acoustic stimulation region 115 may also be adjusted by controlling parameters of the acoustic waves 112. For example, adjusting a phase offset between the acoustic waves may alter a region of maximum constructive interference between the acoustic waves 112 corresponding to the acoustic stimulation region 115 (e.g., the region in which the acoustic power density / pressure is over a threshold desired stimulation power density). In this example, at least some of the transducer apparatuses 110 are provided with a further lens 125. The further lens 125 is provided between the face of the transducer apparatus 110 and the deformable member 130, acoustically coupling the two. The further lens 125 may be an acoustic lens like that discussed above in relation to Figure and may be customised for each transducer apparatus 110. The further lens 125 may thus be configured to deflect the acoustic wave from an axis normal to the face of the transducer apparatus 110. This deflection may be configured to direct the acoustic wave 112 towards the target region 114 - i.e., the alignment apparatus of the present invention may comprise an acoustic lens and a deformable member / steering mechanism (as discussed separately above) in combination. The further lens 125 may be a reshaping lens. Each or at least some of the transducer apparatuses 110 may be provided with the same reshaping lens configuration. The reshaping lens may be configured to adjust a beam profile of the acoustic wave, e.g., by adjusting its shape, amplitude profile or phase profile. The reshaping lens may be a collimating lens, for example. The collimating lens may be configured to provide an acoustic wave with a substantially uniform amplitude across a given field of view / angular range. The collimating lens may therefore assist in steering the acoustic waves; the overlap between acoustic waves 112 may be confined to the target region 114 (at least when the transducer apparatuses 110 are suitably orientated). In some examples of the present invention, one or more transducer apparatuses 110 may be provided with an acoustic lens (which may also be referred to as a steering lens) and a reshaping lens. A single lens may be configured to perform both functions. For example, the further lens 125 may be a combination steering and reshaping lens. Alternatively, multiple lenses may be acoustically coupled together. For example, a reshaping lens (e.g., a collimating lens) may be acoustically coupled to the acoustic lens 120 of the lens-based alignment apparatus shown in Figure 3. Referring to Figure 6, an example of transcranial ultrasound apparatus 100 is shown arranged on a head 1 of a user. The apparatus 100 comprises a transducer apparatus 110 and a lens-based alignment mechanism. In this example, there is provided an acoustic lens comprising a first lens portion 121a and a second lens portion 121b. The lens portions 121a, 121b are coupled to the face of the transducer apparatus 110 and arranged to each receive a spatial portion of the acoustic wave - e.g., the first lens portion 121a may be coupled to a left-hand half of the transducer faces while the second lens portion 121b is coupled to the right-hand half. The two lens portions 121a, 121b are configured to deflect the received acoustic waves in different directions, thereby generating a first acoustic wave portion 112a and a second acoustic wave portion 112b. Each of the lens portions 121a, 121b can be configured similarly to the single acoustic lens discussed above, such that the wave portions 121a, 121b are suitably pre-steered to a first target region 115a and a second target region 115b. The lens portions 121a, 121b may be portions of the same lens (e.g., by fabricating a bifocal lens) or they may be fabricated separately and then coupled together and / or to the transducer apparatus 110. There may be more than two lens portions for addressing more than two target regions. This is just one example by which a lens-based alignment system may be used to address multiple target regions. There may be provided a plurality of acoustic lenses (each like that discussed in Figure 3) configured to provide different deflections. The acoustic lenses may then be swapped for one another depending upon the target region that is to be addressed. Where there is a plurality of transducer apparatuses, there may be a first set of acoustic lenses, a second set of acoustic lens, etc., each set of acoustic lenses being configured so that all of the transducer apparatuses address a first target region, a second target region, etc., respectively. Alternatively, a first sub-set of the transducer apparatuses may be provided with acoustic lenses configured for addressing a first target region, while a second sub-set of the transducer apparatuses may be provided with acoustic lenses configured for addressing a second target region. In another example, a holographic acoustic lens may be used to ‘split’ the acoustic wave. Rather than splitting the acoustic wave spatially as in Figure 6, a holographic lens may split the acoustic wave in the phase domain. Referring to Figure 7. the left-hand image shows a holographic lens 121c (axes refer to nominal spatial dimensions). The wavy appearance of the holographic lens 121c illustrates the relative phase-shift of the lens across its surface. The right-hand image shows the acoustic wave that is output by such a holographic lens; the acoustic wave is split into two different focal areas, which (if configured correctly) correspond to two different target regions 115a, 115b. Further still, the acoustic wave may be redirected with a frequency dependency. Any of the acoustic lenses discussed herein may redirect the acoustic wave differently based upon the frequency of the acoustic wave; at a first frequency the acoustic wave may be steered to a first target region, while at a second frequency the acoustic wave may be steered to a second target region. By configuring the acoustic lens appropriately and controlling the drive signal of the transducer apparatuses, different target regions may also be selectively addressed using this approach. The apparatus 100 of Figure 6 further comprises an acoustic coupling layer 122. The acoustic coupling layer 122 is coupled to a distal end of the acoustic lens (in this example, the acoustic lens portions 121a, 121b) and is configured to acoustically couple acoustic waves from the acoustic lens to the head 1. The acoustic coupling layer 122 may comprise a water-filled layer, a layer of acoustic gel and / or materials that have an intermediary elastic modulus between the elastic modulus of the acoustic lens material and an average elastic modulus of skin / scalp. The acoustic coupling layer 122 may be deformable, so as to reduce the presence of airgaps between the apparatus 100 and head 1 when in contact therewith. An acoustic coupling layer 122 like this may be provided similarly in any of the apparatuses discussed herein. A similar acoustic coupling layer may also be provided between the transducer apparatus 100 and the acoustic lens. Although not shown, the distal ends of the acoustic lens and / or the acoustic coupling layer may be shaped so as to at least partially conform to a subject’s body. For example, the distal end of the acoustic lens may have a concave shape so as to conform to the subject’s head. The apparatus 100 further comprises a height adjustment mechanism 104. The height adjustment mechanism 104 may comprise a linear actuator or a piston, for example. The transducer apparatus 110 may be coupled to a moving part of the height adjustment mechanism 104 so that it can be moved up and down relative to a body or frame of the apparatus (not shown). The height adjustment mechanism 104 may be used to ensure a snug fit of the apparatus 100 to the head 1, establishing good physical contact between the acoustic lens / coupling layer 122 and the skin. The transducer apparatus 110 may be moved upwards to a retracted position when putting on the apparatus 100, thereby providing the user with more room inside the apparatus 100 to comfortably position their head 1. Once worn, the transducer apparatus 110 may be moved down until contact is made with the head 1. The apparatus 100 may further comprise a height controller 105 for controlling the height adjustment mechanism 104. The height controller 105 may be configured to provide manual control, e.g., via user inputs. Additionally, or alternatively, the height controller may be configured to provide automatic control, e.g., a pressure sensor and / or feedback from the height adjustment mechanism 104 may be used to determine that suitable contact has been made with the head 1. Although Figure 6 shows just one transducer apparatus 110, it will be understood that in some embodiments the apparatus may comprise a plurality of transducer apparatuses. Each transducer apparatus may be provided with an alignment apparatus (e.g., a lens-based alignment apparatus or a deformable member / steering mechanism-based alignment apparatus) and / or any of the other features discussed herein. Referring to Figure 8, an example of transcranial ultrasound apparatus 100 is shown arranged on a head 1 of a user. The apparatus 100 comprises a plurality of ultrasound transducers 110. The apparatus 100 further comprises a steering mechanism-based alignment apparatus, comprising steering mechanisms 130 (e.g., actuators) and moving mechanisms 134 (e.g., ball joints) upon which the transducer apparatuses 110 are mounted. The body of the apparatus 100 comprises a conformable layer 102 and a rigid frame 103. The conformable layer 102 may be a fluid filled sack, for example, and may provide an inner body portion that is to be worn on the surface of the head 1. The transducer apparatuses 110 are each acoustically coupled to the conformable layer 102; the face of each transducer apparatus 110 may be acoustically coupled to an outer surface of the conformable layer 102 so that acoustic waves are coupled from the transducer apparatuses 110 to the head 1 through the conformable layer 102. The steering mechanism 130 is coupled to the rigid frame 103; the transducer apparatuses 110 may then be pivoted and reorientated relative to this rigid frame 103. As the transducer apparatuses 110 are pivoted, the conformable layer 102 may deform locally. Therefore, the conformable layer 102 may function as a plurality of deformable members of the alignment apparatus, as discussed in relation to Figure 4. The conformable layer 102 may be segmented, each segment being coupled to one transducer apparatus 110. The apparatus 100 further comprises an alignment controller 131. The alignment controller 131 may be any suitable computer controller and is electronically coupled to each of the steering mechanisms 130. The alignment controller 131 is configured to control the orientation of the transducer apparatuses 110 by adjusting the steering mechanisms 130. As discussed in relation to Figure 4, tilt sensors may be provided and configured to determine the orientation (at least relative to other components of the apparatus, e.g., the rigid frame 103) of the transducer apparatuses 110. The measurements of the tilt sensors may be provided to the alignment controller 131, which may be configured to automatically adjust the steering mechanisms 130. Desired orientations for each of the transducer apparatuses 110 may be determined, the desired orientations resulting in acoustic waves overlapping at a target region within the head 1 such that the beam axes intercept one another. These desired orientations may be provided to the alignment controller 131, which may then automatically reorientate the steering mechanisms 131. The alignment controller 131 may be configured to provide acoustic stimulation to a plurality of target regions. The alignment controller 131 may automatically reorientate the transducer apparatuses 110 to a number of predetermined orientation configurations, for example, each orientation configuration causing the acoustic waves to overlap at a different target region. The alignment controller 131 may be configured such that, if any of the transducer apparatuses 110 are reorientated above a certain threshold (e.g., the angle of the transducer apparatus 110 relative to an axis normal to the rigid frame 103, as per the alignment shown in Figure 8, is greater than a predetermined value), said transducer apparatus 110 is automatically deactivated, so as not to generate acoustic shear waves due to large incident angles relative to a rigid body (e.g., the skull). The alignment controller 131 may be provided with prior knowledge of the subject, for example 3D models of the subject’s head or other body parts based on medical imaging. This prior knowledge may be used to convert the orientation of the transducer apparatuses 110 measured by the tilt sensors 136 to an orientation relative to the subject. For example, the alignment controller 131 may be provided with information regarding the curvature of the subject’s skull. The orientation of the transducer apparatuses 110 relative to the skull may thus be determined, and the transducer apparatuses 110 may be deactivated if the angle of the transducer apparatus 110 relative to an axis normal to the local surface of the skull is greater than a predetermined value. Where the tilt sensors 136 are used to measure the orientation of the transducer apparatuses 110 relative to the apparatus 100 (e.g., relative to a body of the apparatus 100), the orientation of the transducer apparatuses 110 relative to a subject may be determined using a variety of methods. For example, a neuronavigational-based approach may be used, wherein the position of the subject’s brain is tracked based upon prior imaging of the subject’s brain (e.g., via MRI) and real-time tracking of the subject’s head 1 using cameras. Alternatively, LIDAR could be used to determine the position of the apparatus 100 relative to the subject; reflectors could be positioned on a rigid frame of the apparatus 100 and the subject’s head 1 and tracked using LIDAR cameras. Using these methods or similar, the position of the apparatus 100 relative to the subject (or a body part thereof) may be determined. This information may be provided to the alignment controller 131 such that the orientations of the transducer apparatuses 110 relative to the apparatus (as measured by the tilt sensors 136, e.g., relative to the rigid frame) can be converted to orientations relative to the subject. The apparatus 100 further comprises an ultrasound controller 111. The ultrasound controller 111 is electronically coupled to each of the transducer apparatuses 110 and can provide drive signals thereto. The ultrasound controller 111 may therefore be configured to control one or more parameters of the acoustic waves generated by the transducer apparatuses 110. The ultrasound controller 111 may be configured to provide manual control - for example, a user may manually adjust the power / amplitude of the acoustic waves in order to provide comfortable stimulation. The ultrasound controller 111 may be configured to provide automatic control - parameters of the acoustic waves may be automatically set to predetermined values and / or varied in order to provide appropriate therapy / diagnostic performance. The ultrasound controller 111 and the alignment controller 131 may communicate with one another. For example, the alignment controller 131 may provide a signal to the ultrasound controller 111 indicating that the transducer apparatuses 110 have been reorientated, prompting the ultrasound controller 111 to activate the transducer apparatuses 110. The alignment controller 131 may provide orientation and / or target region information to the ultrasound controller 111, which may cause the ultrasound controller 111 to adjust one or more parameters. For example, in response to an indication that the transducer apparatuses have been reorientated to target a deep brain region, the ultrasound controller 111 may drive the transducer apparatuses 110 with a greater amplitude than for shallow brain regions. The ultrasound controller 111 may also be configured to provide drive signals to the plurality of transducer apparatuses 110 that cause a phase offset between the acoustic waves that are generated. By altering the relative phase of the acoustic waves, the regions of contrastive / destructive interference between the acoustic waves where they overlap may by adjusted. As discussed above, the alignment apparatus (e.g., acoustic lenses or steering mechanisms 130) may be configured to provide pre-steering. This may be regarded as a coarse adjustment of where the acoustic waves overlap resulting from adjusting the beam axis of each acoustic wave; each acoustic wave may therefore overlap with a target region and / or the beam axes may approximately intercept one another. However, adjusting the phase offset of the acoustic waves via the ultrasound controller 111 may be regarded as a fine adjustment. But adjusting the relative phase of each acoustic wave, the position(s) of constructive interference may be precisely controlled (e.g., with sub-wavelength accuracy). This may be used to control the size and / or position of an acoustic stimulation region; a region where the total acoustic power is within a desired range depending upon the application. This fine control may be advantageous over other beamforming / beam steering methods, such as phased array. This fine control may still enable transducer apparatuses with larger apertures and / or fewer elements to be used; the phase control may adjust the interaction of the acoustic waves at the overlapping target region, rather than steering the acoustic wave as it emitted from the transducer apparatus. In some examples, one or more of the transducer apparatuses 110 may comprise a plurality of transducer elements. The plurality of transducer elements may form a phased array and suitable drive signals may be provided to these transducer elements so as to steer the acoustic waves that are generated. In combination with the alignment apparatuses discussed herein and / or providing phase offsets between the transducer apparatuses 110, the use of phased array steering may provide a further means for adjusting the orientation of the beam axes so that they intercept at the target region. Referring to Figure 9, a flow chart is shown of a method 200 of configuring an apparatus for transcutaneous ultrasound applications. The method 200 may be used to configure any of the apparatuses discussed herein. The method comprises determining 202 a position of a target region within a body of a subject to which ultrasound is to be applied. The target region may be identified using medical imaging data or by identifying a biomarker with the body of the subject, for example. For example, MRI imaging data could be analysed to determine the position of the target region (e.g., the location of a particular brain structure). A plurality of target regions may be identified, and the apparatus may be configured into a plurality of aligned states corresponding to each target region (e.g., by providing a plurality of acoustic lenses or by reorientating a steering mechanism, as discussed above). The method further comprises providing 204 an apparatus for transcutaneous ultrasound applications. The apparatus may be any of the apparatuses discussed herein. The method further comprises configuring 206 the alignment apparatus such that the apparatus is in an aligned state whereby the beam axis of the acoustic wave generated by the transducer apparatus intercepts the target region. Where the apparatus comprises a lens-based alignment mechanism, configuring the apparatus may comprise fabricating an acoustic lens that provides a desired deflection so that the acoustic waves overlap with the target region. Where the apparatus comprises a steering mechanism-based alignment apparatus, configuring the apparatus may comprise controlling the steering mechanism so as to reorientate the beam axes of the acoustic wave to the target region. From reading the present disclosure, other variations and modifications will be apparent to the skilled person. Such variations and modifications may involve equivalent and other features which are already known in the art of coupled inductors, and which may be used instead of, or in addition to, features already described herein. Although the appended claims are directed to particular combinations of features, it should be understood that the scope of the disclosure of the present invention also includes any novel feature or any novel combination of features disclosed herein either explicitly or implicitly or any generalisation thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention. Features which are described in the context of separate embodiments may also be provided in combination in a single embodiment. Conversely, various features which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub combination. The applicant hereby gives notice that new claims may be formulated to such features and / or combinations of such features during the prosecution of the present application or of any further application derived therefrom. For the sake of completeness, it is also stated that the term "comprising" does not 5 exclude other elements or steps, the term "a" or "an" does not exclude a plurality. Reference signs in the claims shall not be construed as limiting the scope of the claims.

Claims

1. An apparatus for transcutaneous ultrasound applications, the apparatus comprising:a body;a plurality of transducer apparatuses coupled to the body and spatially separated from one another over a surface of the body, each transducer apparatus configured to generate an acoustic wave having a beam axis;an alignment apparatus configured to adjust an orientation of the beam axis of at least one of the transducer apparatuses; andan ultrasound controller configured to control a relative phase of drive signals provided to at least some of the transducer apparatuses.

2. The apparatus of claim 1, wherein the alignment apparatus comprises an acoustic lens acoustically coupled to a face of at least one of the transducer apparatuses, the acoustic lens being configured to deflect the acoustic wave from an axis normal to the face of the transducer apparatus, and optionally wherein the acoustic lens is a volumetric lens, a thin-phase holographic lens or a Fermat-type lens.

3. The apparatus of claim 2, wherein a distal end of the acoustic lens opposite to the transducer apparatus comprises an acoustic coupling layer.

4. The apparatus of any of claim 2 or claim 3, wherein the acoustic lens is configured to deflect the acoustic wave in a plurality of different directions;wherein the acoustic lens comprises a plurality of lens portions, each lens portion being configured to deflect a spatial portion of the acoustic wave in a different direction; and / orwherein the acoustic lens is configured to deflect the acoustic wave in a plurality of different directions based upon a frequency of the acoustic wave.

5. The apparatus of any of claims 2 to 4, wherein:the alignment apparatus comprises a plurality of acoustic lenses, each transducer apparatus being coupled to one of the acoustic lenses; andeach acoustic lens comprises an identifier indicating a corresponding transducer apparatus of the plurality of transducer apparatuses to which the acoustic lens is to be coupled.

6. The apparatus of claim 1, wherein the alignment apparatus comprises:a deformable member coupled to a face of at least one of the transducer apparatuses, wherein the deformable member optionally comprises a body comprising a compressible material or a body containing a fluid; anda steering mechanism configured to mechanically pivot the transducer apparatus relative to the axis normal to the local surface of the body.

7. The apparatus of claim 6, wherein the steering mechanism comprises:a piezoelectric actuator;a pneumatic actuator; and / ora control element comprising a shape memory material.

8. The apparatus of claim 6 or claim 7. further comprising:a tilt sensor configured to measure a spatial orientation of the transducer apparatus; andan alignment controller configured to determine an alignment between an axis normal to the face of the transducer apparatus and the axis normal to a local surface of the body based upon the measured spatial orientation of the transducer apparatus, and wherein the alignment controller is optionally further configured to provide a control signal to the steering mechanism in order to change an alignment of the transducer apparatus relative to the axis normal to the local surface of the body.

9. The apparatus of any of claims 6 to 8, wherein the alignment controller is configured to deactivate transducer apparatuses for which the determined alignment between the axis normal to the face of the transducer apparatus and the axis normal to a local surface of the body exceeds a predetermined threshold.

10. The apparatus of any of claims 2 to 5, wherein the alignment apparatus comprises a second acoustic lens couplable to the transducer apparatus, the second acoustic lens being configured to deflect the acoustic wave in a different direction to the first acoustic lens; orthe apparatus of any of claims 6 to 9, wherein the steering mechanism is operable to mechanically pivot the transducer apparatus to a first alignment and a second alignment, the first alignment and the second alignment corresponding to different angular orientations relative to the axis normal to the local surface of the body.

11. The apparatus of any preceding claim, further comprising a height adjustment mechanism configured to alter the position of at least one of the transducer apparatuses relative to the body along the axis normal to the local surface of the body.

12. The apparatus of any preceding claim, further comprising a reshaping lens acoustically coupled to a face of at least one of the transducer apparatuses, the reshaping lens being configured to adjust a beam profile of the acoustic wave; and optionallywherein the reshaping lens comprises a collimating lens configured to provide the acoustic wave with a uniform amplitude distribution across a predetermined angular range.

13. The apparatus of any preceding claim, wherein the apparatus is optionally configured to be worn on the head of a subject, and wherein:the body is made of a conformable fabric-like material; orthe body comprises:a rigid frame to which the plurality of transducer apparatuses are fixedly coupled; anda conformable layer to which the plurality of transducer apparatuses are acoustically coupled, the conformable layer comprising an acoustic coupling material.

14. The apparatus of any preceding claim, wherein the ultrasound controller is configured to control a relative phase of the drive signals provided to at least some of the transducer apparatuses so as to adjust a position and / or size of an acoustic stimulation region.

15. The apparatus of any preceding claim, wherein:each transducer apparatus comprises a face with a spatial dimension of at least 3 mm, and / orthe ultrasound controller is configured to:cause the transducer apparatus to provide the acoustic waves to an acoustic stimulation region with a peak acoustic pressure of less than 1 MPa; and / orcause the transducer apparatus to generate acoustic waves with a frequency of between 250 and 800 kHz.

16. The apparatus of any preceding claim, wherein:the transducer apparatus comprises a plurality of transducer elements;the ultrasound controller is further configured to control a relative phase of drive signals provided to at least some of the transducer elements.

17. A method of configuring an apparatus for transcutaneous ultrasound applications, the method comprising:determining a position of a target region within a body of a subject to which ultrasound is to be applied;providing an apparatus for transcutaneous ultrasound applications, the apparatus comprising:a body;a transducer apparatus coupled to the body, the transducer apparatus being configured to generate an acoustic wave having a beam axis; andan alignment apparatus configured to adjust an orientation of the beam axis of the transducer apparatus; andconfiguring the alignment apparatus such that the apparatus is in an aligned state whereby the beam axis of the acoustic wave generated by the transducer apparatus intercepts the target region.

18. The method of claim 17, wherein:the alignment apparatus comprises an acoustic lens acoustically coupled to a face of the transducer apparatus; andconfiguring the alignment apparatus comprises fabricating the acoustic lens, the acoustic lens being configured to deflect the acoustic wave from an axis normal to the face of the transducer apparatus to the target region, wherein fabricating the acoustic lens optionally comprises:3D printing the lens;casting the lens; and / ormachining the lens.

19. The method of claim 18, wherein fabricating the acoustic lens comprises:determining a beam angle between the axis normal to the face of the transducer and the target region; andcalculating appropriate acoustic lens parameters using a thin-element approximation, an iterative Fourier transform-based method, a ray-tracing method, machine learning-based methods and / or a differentiable wave simulator.

20. The method of claim 18 or claim 19, further comprising determining a position of a second target region within a body of a subject to which ultrasound is to be applied, and wherein:wherein the acoustic lens is configured to deflect the acoustic wave in a plurality of different corresponding to the first target region and the second target region directions;the acoustic lens comprises at least two lens portions, a first lens portion being configured to deflect a first spatial portion of the acoustic wave to the first target region and a second lens portion being configured to deflect a second spatial portion of the acoustic wave to the second target region;wherein the acoustic lens is configured to deflect the acoustic wave in a plurality of different directions corresponding to the first target region and the second target region based upon a frequency of the acoustic wave; and / orthe method further comprises fabricating a second acoustic lens, the second acoustics lens being configured to deflect the acoustic wave from the axis normal to the face of the transducer apparatus to the second target region.

21. The method of claim 19, wherein:the alignment apparatus comprises:a deformable member coupled to a face of the transducer apparatus; anda steering mechanism configured to mechanically pivot the transducer apparatus relative to the axis normal to the local surface of the body; andconfiguring the alignment apparatus comprises:determining an offset in a spatial orientation of the transducer apparatus between an initial state and the aligned state of the apparatus, the initial state being prior to the configuration of the alignment apparatus; andcontrolling the steering mechanism so that the apparatus is reconfigured to the aligned state.

22. The method of claim 21, further comprising:determining a position of a second target region within a body of a subject to which ultrasound is to be applied; andcontrolling the steering mechanism so that the apparatus is reconfigured to a second aligned state whereby the acoustic waves generated by the transducer apparatus intercept the second target region.

23. The method of any of claims 17 to 22, wherein:determining the position of the target region comprises analysing medical imaging data or identifying a biomarker signal within the body of the subject; and / or configuring the alignment apparatus comprises using an acoustic propagation model of part of the body of the subject.

24. The method of any of claims 17 to 23, wherein configuring the alignment apparatus comprises:using a digital model of part of the body of the subject;identifying the target region within the digital model; andsimulating the acoustic wave generated by the transducer apparatus to determine whether the simulated acoustic wave intercepts the target region within the digital model.

25. The method of any of claims 17 to 24, wherein:the transducer apparatus comprises a plurality of transducer elements;the apparatus further comprises an ultrasound controller configured to control a relative phase of drive signals provided to at least some of the transducer elements; and the method further comprises determining an appropriate control signal based upon an expected response of the transducer elements to the phase offset, the configuration of the alignment apparatus in the aligned state and the position of the target region.A

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