Improved MR performance in MR-guided ultrasound systems

Abstract

Ultrasonic transducers adjust the field distribution in an MRI device to improve signal sensitivity and uniformity in a region of interest. Methods employed include strategically placing field-altering features such as slits and / or dipoles along the outer surface of the transducer, or in some cases, within the interior of the transducer. In various embodiments, the field-altering feature is (or appears as) a passive resonator.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to and the benefit of U.S. Provisional Patent Application No. 62 / 858,410, filed on June 7, 2019, U.S. Provisional Patent Application No. 62 / 858,127, filed on June 6, 2019, U.S. Provisional Patent Application No. 62 / 858,129, filed on June 6, 2019, U.S. Provisional Patent Application No. 62 / 862,995, filed on June 18, 2019, U.S. Provisional Patent Application No. 62 / 858,409, filed on June 7, 2019, and U.S. Provisional Patent Application No. 62 / 862,996, filed on June 18, 2019, and is incorporated herein by reference in its entirety. Technical Field

[0003] The present invention generally relates to a magnetic resonance-guided (MR-guided) focused ultrasound system and method. Background Art

[0004] Focused ultrasound (i.e., sound waves with a frequency greater than about 20 kilohertz) can be used to image or treat tissues in the patient's body. For example, ultrasound can be used for applications involving tumor elimination, thereby eliminating the need for invasive surgery, targeted drug delivery, blood-brain barrier control, clot dissolution and other surgical procedures. During tumor elimination, a piezoelectric ceramic transducer is placed outside the patient and close to the tissue to be eliminated (i.e., the target). The transducer converts the electronic drive signal into mechanical vibrations, thereby emitting sound waves. The transducer can be geometrically shaped and positioned with other such transducers so that the ultrasonic energy they emit forms a focused beam together at a "focal area" corresponding to the target tissue area (or within the target tissue area). Alternatively or additionally, a single transducer can be formed by a plurality of individually driven transducer elements, the phases of which can be independently controlled. This "phased array" transducer helps to turn the focal area to different positions by adjusting the relative phase between the transducers. As used herein, the term "element" refers to a single transducer in an array or an independently drivable portion of a single transducer. Magnetic resonance imaging (MRI) can be used to visualize the patient and the target and thereby guide the ultrasound beam.

[0005] MRI devices use powerful magnets to generate a strong magnetic field that forces protons in the body to align with that field. When a radio frequency (RF) current is then pulsed through the patient, the protons are stimulated, and when the RF field is turned off, one or more receiver coils detect the energy released as the protons realign with the magnetic field. The time it takes for the protons to realign with the magnetic field, as well as the energy released, depends on the environment and the chemistry of the molecule. These differences can be used to reconstruct a 3D image of a region of interest (ROI) within the patient.

[0006] The combination of modalities, ie MRI and focused ultrasound, means that MRI images created, in particular under an MRI scanner, will not be disturbed by or interact excessively with the transducer elements creating the focused ultrasound, since the ultrasound transducer would have to be disposed within the magnetic element. Summary of the invention

[0007] Embodiments of the present invention adjust and / or Field distribution to improve signal sensitivity and uniformity at the ROI. The methods employed herein include strategically placing field-altering features such as slits and / or dipoles along the outer surface of the transducer or, in some cases, on the inner surface of the transducer. In various embodiments, the field-altering feature is (or appears as) a passive resonator.

[0008] Thus, one aspect of the present invention relates to an ultrasound transducer for use with an MRI unit having at least one transmit coil and at least one receive coil. In various embodiments, the ultrasound transducer includes a dielectric layer, and a first conductive layer conforming to at least a portion of the dielectric layer, the first conductive layer having at least one passive resonator, the at least one passive resonator being on or through the first conductive layer, wherein the passive resonator has a dielectric layer selected to achieve an optimized field of view of the MRI transmit and / or receive coil. and / or Geometry and size of magnetic field distribution.

[0009] In some embodiments, the passive resonator is at least one elongated slit that passes through the conductive layer and extends across at least a portion of the conductive layer. The slit can be, for example, a pair of slits in a cross-shaped configuration. In some embodiments, the slit is filled with a metamaterial element. The passive resonator can be composed of a material having a dielectric constant exceeding 10 and attached to the surface of the transducer. In some embodiments, the passive resonator is a dipole. The geometry, position and / or orientation of the passive resonator can be adjustable. In some embodiments, the resonator is shaped as a ring, a dipole or an 8-shaped shape. The passive resonator can be disposed on an inner surface of the transducer that forms an interface with the dielectric medium.

[0010] In various embodiments, the dielectric layer and the first conductive layer are substantially hemispherical. In other embodiments, the conductive layer includes a plurality of discrete conductive elements tiled in a mosaic manner. The transducer may also include at least one additional conductive region that is spaced apart from the first conductive region but also conforms to the dielectric layer. The first conductive layer may have at least two passive resonators that are on or through the first conductive layer. The dielectric constant of at least one passive resonator may exceed 10 and may be attached to the surface of the transducer, and at least one passive resonator may be one or more elongated slits that pass through the conductive layer and extend across at least a portion of the conductive layer. At least one passive resonator may be disposed in an outer region of the transducer, such as a bottom annular surface configured to surround the patient's head. For example, the passive resonator may be a foil or a mesh. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The foregoing and following detailed description will be more readily understood in conjunction with the accompanying drawings, in which:

[0012] Figure 1A An exemplary focused ultrasound system according to various embodiments is schematically illustrated.

[0013] Figure 1B An MRI system according to various embodiments is shown.

[0014] Figure 1C The main operating components of an MRI apparatus are shown schematically in more detail.

[0015] Figure 2A and 2C They are three-dimensional views of a two-piece curved transducer design and a mosaic transducer design according to embodiments of the present invention, respectively.

[0016] Figure 2B and 2D They are shown respectively by Figure 2A and 2C Sagittal craniofacial slices showing inhomogeneities caused by the transducer geometry.

[0017] Figure 2E is a cross-sectional view of a multilayer transducer structure.

[0018] Figure 3A is a perspective view of a curved transducer design including two slits.

[0019] Figure 3B and 3C The results calculated by simulation are shown by using two vertical cross-shaped slits with uniform width of 1 mm and lengths L of 200 mm and 172 mm respectively. field.

[0020] Figure 3DGraphically depict the effect of different slit lengths on distributed.

[0021] Figure 4B is a perspective view of a mosaic transducer design including two dipoles.

[0022] Figure 4A and 4C It is shown that Figure 4B The transducer shown produces a Comparative simulation diagram of the field.

[0023] Figure 5A is a perspective view of a transducer having a dipole disposed on a lower surface.

[0024] Figure 5B is a perspective view of a transducer having a thin-film resonator disposed on a lower surface. DETAILED DESCRIPTION

[0025] First reference Figure 1A , Figure 1A An exemplary ultrasound system 100 is shown for generating and delivering a focused acoustic energy beam to a target area to perturb tissue, thereby causing an increase in tissue permeability in the tissue. In various embodiments, the system 100 includes a phased array 102 of transducer elements 104, a beamformer 106 for driving the phased array 102, a controller 108 in communication with the beamformer 106, and a frequency generator 110 that provides an input electronic signal to the beamformer 106.

[0026] The array 102 can have a curved (e.g., spherical or parabolic) shape suitable for placement on the surface of the patient's body, or can include one or more planar sections or other shaped sections. The size of the array 102 can vary between millimeters and tens of centimeters. The transducer elements 104 of the array 102 can be piezoelectric ceramic elements and can be mounted in silicone or any other material suitable for mechanical coupling between the damping elements 104. Piezoelectric composites can also be used, or any material that can generally convert electrical energy into acoustic energy. To ensure that maximum power is transmitted to the transducer elements 104, the elements 104 can be configured for electrical resonance of 50Ω to match the input connector impedance.

[0027] The transducer array 102 is coupled to a beamformer 106 that drives the individual transducer elements 104 so that they collectively produce a focused ultrasound beam or field. For n transducer elements, the beamformer 106 may include n driver circuits, each driver circuit including or consisting of an amplifier 118 and a phase delay circuit 120; each driver circuit drives one transducer element 104. The beamformer 106 receives a radio frequency (RF) input signal, typically in the range of 0.1 MHz to 10 MHz, from a frequency generator 110, such as a model DS345 available from Stanford Research Systems. The input signal may be split into n channels for the n amplifiers 118 and delay circuits 120 of the beamformer 106. In some embodiments, the frequency generator 110 is integrated with the beamformer 106. The frequency generator 110 and the beamformer 106 are configured to drive the individual transducer elements 104 of the transducer array 102 at the same frequency, but at different phases and / or different amplitudes.

[0028] The amplification or attenuation factor α1-α applied by the beamformer 106 n and phase shift a1-a n The invention relates to a method for transmitting and focusing ultrasonic energy on a target area and causing wave distortion in tissues between the transducer element 104 and the target area. The controller 108 is used to calculate the amplification factor and phase shift, and the controller 108 can provide the calculation function by software, hardware, firmware, hard wiring or any combination thereof. For example, the controller 108 can utilize a general or special digital data processor programmed with software in a conventional manner, and there is no need for excessive experimentation to determine the phase shift and amplification factor required to obtain the desired focus of the target area or any other desired spatial field pattern. In some embodiments, the calculation is based on detailed information about the characteristics (e.g., structure, thickness, density, etc.) of the intervening tissues between the transducer element 104 and the target and their effects on the propagation of acoustic energy. This information can be obtained from the imager 122. The imager 122 can be an MRI device as discussed below.

[0029] Figure 1BAn exemplary MRI device 122 is illustrated. The device 122 may include a cylindrical electromagnet 134 (or permanent magnet) that generates a static magnetic field within a bore 136 of the electromagnet 134. In MRI, nuclear spins within body tissue interact with an external magnetic field to precess around the magnetic field at a specific resonant frequency, also known as the Larmor frequency, ω0=γB, where B in this case is the external magnetic field and γ is the gyromagnetic ratio, which is a property of the nuclear spins. During a medical procedure, a patient is placed on a movable support table 138 and within the bore 136. A region of interest (ROI) 140 of the patient (e.g., the patient's head) may be positioned within an imaging region 142, wherein the electromagnet 134 generates a static magnetic field B0 with a spatial uniformity of a few parts per million. A set of cylindrical magnetic field gradient coils 144 may also be disposed within the bore 136 and around the patient. The gradient coils 144 generate magnetic field gradients of predetermined amplitudes at predetermined times and in three mutually orthogonal directions. By means of field gradients, different spatial locations can be associated with different precession frequencies, thereby giving the spatial resolution of the MR image. MRI data can only be obtained by exciting the spin system in the tissue to break away from the magnet equilibrium and measuring the excited spin system signal. To excite the spins, the RF transmitter coil 146 surrounding the imaging region 142 is energized at a frequency ω rf The shaped RF pulses are transmitted to the imaging region 142 to cause the patient's tissue to emit a magnetic resonance (MR) response signal. rf is centered and has a finite effective width Δω rf Frequency response curve. Only in the effective width Δω rf The precession frequency is close to ω rf The spins of the MRI device 122 will be excited by the RF pulse. The raw MR response signals are sensed by the RF coil 146 and transmitted to the MR controller 148, which then calculates an MR image that can be displayed to the user. Alternatively, separate MR transmitter and receiver coils can be used. Images acquired using the MRI device 122 can provide radiologists and physicians with visual contrast between different tissues and detailed internal views of the patient's anatomical structures that cannot be seen using traditional X-ray technology.

[0030] The MRI controller 148 can control the pulse sequence, i.e., the relative timing and strength of the magnetic field gradients and the RF excitation pulses and response detection cycles. The MR response signals are amplified, conditioned, and digitized into raw data using conventional image processing systems, and further converted into image data arrays by methods known to those of ordinary skill in the art. Based on the image data, target areas (e.g., tumors) can be identified.

[0031] exist Figure 1CThe main operating components of an MRI device are shown in more detail in FIG. The strength of the main magnet 134 can be, for example, 3T, and produces a B0 field to align the spins and achieve equilibrium. The gradient coils 144 are capable of image encoding in the x, y and z directions (i.e., frequency, phase and slice encoding directions). The RF coil 146 sends and receives signals, excites aligned spins and receives RF signals from the ROI. When a patient is placed in an MRI scanner, the tissue acquires a small magnetization intensity that is aligned with the magnetic field Mo. The RF transmit (Tx) coil generates RF pulses that generate a small magnetic field B1, most of which is perpendicular to the main magnetic field B0. Only the perpendicular magnetic field component oscillates around the spin resonance frequency The net magnetization will effectively rotate out of alignment with the main magnetic field. The stronger the RF pulse (energy), the greater the magnetization tilt or flip. The RF receive (Rx) coil detects the precessing magnetization, thereby generating an induced current by electromagnetic induction. The induced current is the MR signal and represents the mix of magnetization from tissue within the Rx coil field of view (FOV). The scanner may have multiple coils for both transmit and receive functions.

[0032] The Tx coil generates electromagnetic Field, electromagnetic The field is designed to be perpendicular to the main (static) magnetic field B0 and to oscillate in an elliptically polarized mode at a resonant frequency γB0. The precession frequency corresponds to the frequency range of radio waves (MHz). The received electromagnetic field is also perpendicular to B0 and is called field.

[0033] A single "body coil" can be used alternately as a transmitter coil and a receiver coil, generating Field and sensing field. Above the FOV, the transmitted The field should be uniform to promote uniform excitation, and the coil should have high sensitivity over the entire ROI (which is less than or equal to the FOV area). Figure 1B 136. Introducing the transducer array 102 within the aperture 136 can disrupt both uniformity and sensitivity. Specifically, the geometry of the transducer array 102 and its dielectric properties (which primarily arise from highly conductive components and dielectric fluids such as water) can change the spatial distribution of electromagnetic energy within the aperture 136; in effect, the transducer array 102 acts as a resonant cavity that imposes a frequency-dependent mode pattern.

[0034] For brain imaging, ultrasound transducers can have Figure 2A A simple configuration of a two-piece curved hemisphere 200 is shown, which includes a top dome-shaped element 210 separated from a bottom skirt-shaped element 215 by a narrow annular gap. Figure 2CAs shown, the ultrasound transducer may include or consist of tile-shaped elements 220 that are arranged in a mosaic manner to be close to a hemisphere 225.

[0035] In some embodiments, the transducer may include more than one highly conductive layer through its thickness. Figure 2E , where the transducer 200 has four layers of membrane 230 that enclose and define a water-filled chamber 235. The membrane 230 includes an innermost polymer layer (e.g., silicone layer) 2401, which may, for example, have a thickness of 1 mm; a first conductive layer (e.g., copper layer) 2402, which may have a thickness of 2 mm; a second polymer layer 2403, which may also have a thickness of 1 mm; and a second (outermost) conductive layer 2404, which may have a thickness of 0.1 mm. Typically, the slit will extend through all of the highly conductive layers, so for Figure 2E , the slits may extend through layers 2402, 2404, but not necessarily through polymer layers 2401, 2403. In some embodiments, the slits (or notches) may be filled with passive resonators such as metamaterial components (e.g., capacitive and inductive elements) with sub-λ sizes (where λ is the RF wavelength in water), which will change the impedance of the dielectric shell and will change the electromagnetic field distribution therein.

[0036] Electromagnetic waves that interact with highly conductive materials are reflected. Reflections from the inside of the hemisphere produce poor The standing waves of the modes, such as Figure 2B A simplified description of the phenomenon uses a reflection coefficient Γ defined between two layers; for example, in the case of a water layer and a copper layer, Where Z w and Z c are the impedances of water and copper, respectively. Since Zw>>Zc, the reflection is approximately Γ=-1, which indicates total reflection of the wave with flipped sign, which produces a maximum at the dome-shaped element 210. strength and the minimum generated below it Strength, as shown in Figure 2.

[0037] In a particularly advantageous embodiment of the curved hemisphere configuration 200, field disordering is reduced by introducing one or more slits 310 through a highly conductive layer (e.g., a copper layer), such as Figure 3A This effectively creates a slot antenna that allows RF to penetrate between the inside and outside of the hemisphere 200 and rearrange the B1 field inside the transducer. The slot 310 can be filled with a low conductivity material (e.g., air or a polymer). In some embodiments, as Figure 3AAs shown, slits are introduced in the dome-shaped top element 210; in the skirt-shaped element 215; or in both elements 210 and 215.

[0038] like Figure 3A As shown, a cross-shaped slit 310 cut through the highly conductive shell allows the RF waves to penetrate the highly conductive shell between the inside and outside of the transducer under restorative conditions. The optimal geometry and length of the slit can be selected by simulation or actual measurement without undue experimentation to achieve the desired mode. By using two vertical cross-shaped slits with uniform width of 1 mm and length L of 172 mm, the final Field Figure 3C (The results for the slit length of 200 mm are shown in Figure 3B ). Figure 3D The results of different slit lengths L show distribution; therefore, it is straightforward to design and select from slit geometries to achieve the desired pattern. In particular, a relatively flat (and therefore maximally uniform) leftmost peak region in the FOV or ROI may be desired.

[0039] Although the purpose is to manipulate the transducer and / or mode, but a strong dependence between the slot geometry and the field distribution can be observed when adjacent transducer layers (e.g., air, copper, water) have matching impedance values. The effect of a linear slot element can be qualitatively approximated as that of a linear slot antenna surrounded by a conductive sheet that linearly polarizes electromagnetic radiation above the cutoff frequency. The two perpendicular slots 310 will polarize the RF elliptically polarized pulses from the transmission. Or from the spin system signal The electromagnetic radiation of the body coil is applied with two perpendicular linear polarizations, thereby obtaining an elliptical excitation (EP) that maintains the phase of the electromagnetic wave from the source. The Tx / Rx signal of the combined body coil and slot antenna is for Generates two coherent sources that coherently produce a new mode. The length of the slit can be adjusted to change the elliptical polarization. For Tx, the slit applies passive "RF shimming" at the FOV or ROI. For Rx, the slit improves signal sensitivity and uniformity at the ROI. Typically, additional slits or notches can be applied along the sides of the transducer to further manipulate In some embodiments, each slit has an adjustable geometry to improve the field distribution in the region of interest (e.g., a portion of the slit may be covered with a conductive sheet). For example, the adjustment may be triggered by an initial MRI image. In one approach, the conductive sheet is slidably positioned within a guide rail so that the slit may be covered by a desired amount.

[0040] Similar to the slot antenna configuration, the passive resonator approach is also applicable to other transducer configurations such as Figures 4A-4C The inlaid transducer design shown in FIG. The inlaid configuration allows RF from the body coil to pass through the transducer 400, but the boundary between the water and air within the transducer 400 reflects the RF and can still result in uneven signal strength at the center of the transducer base, such as Figure 2D and 4A Using a simplified description of RF propagation, the reflection coefficient between water and air in the embedded transducer 400 is Γ≈1, which means that the RF still behaves as a standing wave within the transducer.

[0041] Based on the Babinat principle, a slot antenna in a curved transducer and a dipole antenna in an inlaid transducer will exhibit similar RF radiation effects. Therefore, adding one or more dipoles 410 to the surface of the inlaid transducer 400 will affect the transducer performance in a manner similar to adding a slot antenna to a curved transducer. The dipoles 410 (composed of strings or strips of conductive material such as copper) act as passive resonators, which beneficially The field uniformity region moves toward the ROI. The length, width and geometry of the dipole can be adjusted directly to manipulate Field uniformity. By including an adhesive or polymer layer below the dipole, the dipole 410 can be isolated from other conductive layers (e.g., copper or other metal or conductive material), the tile, and the underlying dielectric layer, and electrically isolated from the transducer. It should be understood that in addition to achieving field uniformity, the resonator should be substantially transparent to the ultrasonic waves emitted by the transducer 200. This can be achieved by keeping the width of the resonator small (e.g., within a range from sub-millimeter width to several millimeters width) and / or positioning the resonator away from the ultrasonic propagation medium. Reference Figure 2E , the dipoles may reside on the exposed (inner) surface of the non-conductive layer 2041. Alternatively, the dipoles may be isolated by plastic (or other dielectric material) and not in direct contact with the water.

[0042] In some embodiments, a set of passive resonators having annular geometries (e.g., circular, elliptical, or figure-8) and / or dipole (elongated) shapes and / or other geometries that affect the RF field within the FOV can be disposed at the surface of the transducer 200. In this context, a passive resonator refers to an element with a high (e.g., >10) dielectric constant. In other embodiments, such as Figure 5A As an alternative or in addition to the passive resonator at the upper surface of the transducer, a lower surface 245 of the transducer 200 (see also FIG. 24 ) surrounding the patient's head may be provided. Figure 2E ) to add a passive resonator (e.g., dipole 410). In this case, lower surface 245 is non-conductive (e.g., polymer) and the dipole can be inside the lower surface (in contact with the dielectric liquid) or outside. Another alternative is to use copper or aluminum foil or Figure 5B The mesh 250 shown in FIG. 24 is used to make the entire inside or outside of the lower surface 245 conductive.

[0043] Likewise, passive resonators can be adjustably configured so that the geometry, position, and / or orientation of the resonator can be changed to improve the field distribution in the ROI. For example, adjustments can be triggered by an initial MRI image. In one approach, this can be done manually by cutting the resonator and / or moving the resonator within a track until the desired performance is achieved. As described above, passive resonators can be combined with slot antenna patterns on a surface by filling the slots with a suitable material.

[0044] In some embodiments, the passive resonator may include metamaterial elements such as capacitive and inductive components and may have dimensions related to the RF wavelength λ. The resonator may be integrated within the dielectric liquid and / or at the surface and / or in a recess around the housing. In any of the above embodiments, the passive resonator may be replaced by an active resonator, as long as the active resonator is used to control the FOV. Field uniformity. Resonators can also be located on the upper region of the transducer 200, for example using a polymer top layer above the conductive layer 2404. Such resonators (whether passive or active) can affect the field uniformity within the transducer at the FOV when RF can penetrate the outside of the transducer. Field uniformity.

[0045] Some embodiments of the present invention are described above. However, it should be clearly pointed out that the present invention is not limited to these embodiments; on the contrary, additions and modifications to the contents clearly described herein are also included within the scope of the present invention.

Claims

1. An ultrasonic transducer (200), the ultrasonic transducer (200) being used together with an MRI unit, the MRI unit having at least one transmitting coil and at least one receiving coil, the ultrasonic transducer (200) comprising: Dielectric layer; as well as a first conductive layer conforming to at least a portion of said dielectric layer, The invention is characterized in that the first conductive layer has at least one passive resonator, the at least one passive resonator is on the first conductive layer or passes through the first conductive layer, and the passive resonator has a structure selected to allow radio frequency waves to pass through the first conductive layer and rearrange the B1 field inside the transducer (200) to achieve an optimized B1 field within the field of view of the MRI transmitting coil and / or receiving coil. + and / or B1 - The geometry and size of the magnetic field distribution, The invention is also characterized in that the at least one passive resonator is at least one elongated slit (310) passing through the first conductive layer and extending across at least a portion of the first conductive layer, or the at least one passive resonator is shaped as a ring, a dipole or a figure 8.

2. The ultrasonic transducer (200) according to claim 1, wherein the optimized magnetic field distribution is the maximum B1 in the region of interest. + and / or B1 - Uniformity.

3. The ultrasonic transducer (200) according to claim 1, wherein the optimized magnetic field distribution is the maximum B1 in the region of interest. + and / or B1 - Sensitivity.

4. The ultrasonic transducer (200) according to claim 1, wherein the at least one slit is a pair of slits configured in a cross shape.

5. The ultrasonic transducer (200) according to claim 1, wherein the dielectric layer and the first conductive layer are substantially hemispherical.

6. The ultrasonic transducer (200) of claim 1, further comprising at least one additional conductive region spaced apart from the first conductive region but also conforming to the dielectric layer.

7. The ultrasound transducer (200) according to claim 4, wherein the at least one slot is filled with a metamaterial element.

8. The ultrasonic transducer (200) according to claim 1, wherein at least a passive resonator is composed of a material having a dielectric constant exceeding 10 and is attached to a surface of the ultrasonic transducer (200).

9. The ultrasonic transducer (200) according to claim 1, wherein at least the passive resonator is a dipole.

10. The ultrasonic transducer (200) of claim 1, wherein at least one of a geometry, a position, or an orientation of the at least one passive resonator is adjustable.

11. The ultrasonic transducer (200) of claim 9, wherein the at least one passive resonator is disposed on an inner surface of the transducer that interfaces with a dielectric medium.

12. The ultrasonic transducer (200) according to claim 1, wherein the first conductive layer has at least two passive resonators, and the at least two passive resonators are on the first conductive layer or pass through the first conductive layer.

13. The ultrasonic transducer (200) of claim 12, wherein at least one passive resonator has a dielectric constant exceeding 10 and is attached to a surface of the ultrasonic transducer (200), and at least one passive resonator is at least one elongated slit passing through the conductive layer and extending across at least a portion of the conductive layer.

14. The ultrasonic transducer (200) according to claim 1, wherein at least one passive resonator is disposed in an outer region of the ultrasonic transducer (200).

15. The ultrasound transducer (200) of claim 14, wherein the outer region is a bottom annular surface configured to surround the patient's head.

16. The ultrasonic transducer (200) according to claim 15, wherein the at least one passive resonator is a foil.

17. The ultrasonic transducer (200) of claim 15, wherein the at least one passive resonator is a mesh.

18. The ultrasonic transducer (200) according to claim 1, wherein the conductive layer comprises a plurality of discrete conductive elements tiled in a tessellated manner.

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