Optical device, system and method for obtaining photoacoustic image using homogenized beam from pulsed light source

Abstract

The invention relates to an optical device (1) for shaping and homogenizing a light beam (2), comprising: a source (3) of a light beam or pulse (2); a multimode optical fiber (7) having a core having a polygonal cross-section; an optical coupling module (5) for guiding the light beam (2) from the source (3) to the optical fiber (7); and an optical orientation module (13) that directs and focuses the formed and homogenized light beam or pulse (14) from the exit side (7 '') of the optical fiber (7) to the sample (19). Also disclosed is a system using the apparatus (1), which further comprises an ultrasonic receiving transducer (21) and an acquisition and processing unit for obtaining a photoacoustic image of a sample (19) in an optical or acoustic resolution photoacoustic microscope application and / or a photoacoustic tomography application.

Description

Technical Field

[0001] The present invention belongs to the technical field of ultrasound imaging. More specifically, the present invention relates to devices, systems and methods for producing photoacoustic or optoacoustic images, which allow the generation of functional, molecular and anatomical images of substances or structures present in various tissues or samples, preferably of biological origin. Background Art

[0002] The photoacoustic effect describes the generation of ultrasound in tissue previously irradiated with short, high-intensity light pulses (understood as light beams), usually generated by a laser source. The tissue molecules hit by these pulses (duration in the nanosecond range and frequency in the visible or near-infrared band) absorb part of the light pulse energy and convert it into a series of rapid thermoelastic expansions, which generate acoustic pressure waves in the ultrasonic range.

[0003] The use of this effect in obtaining images of tissues, preferably biological tissues, is known in the prior art. In recent years, this has allowed the development of various techniques to obtain functional, molecular and anatomical images in a non-invasive manner, which is particularly advantageous in the case of studies of living tissues, since it is not based on the use of ionizing radiation.

[0004] Likewise, in order to observe specific areas of the sample illuminated with appropriate contrast, the frequency of light needs to be adjusted so as to be adjusted to the absorption band of the chromophores (Spanish: cromóforos, which are regions present in the molecules of certain substances where the energy difference between the molecular orbitals is in the visible or infrared spectral range) present therein. For this frequency, these regions, which are able to re-emit the radiation absorbed in the ultrasound, have a greater optical absorption than the surrounding substances and therefore present a higher level of contrast in the final image. The chromophores used are usually endogenous, i.e. they are naturally present in the tissue being studied. Examples of substances of this type include hemoglobin in blood vessels, melanin responsible for epidermal pigmentation, some lipids in adipose tissue, and collagen and elastin in connective tissue. The fact that these substances are a natural part of the sample to be studied allows the functional response of tissue molecules to be observed without the use of external chemical reagents, in contrast to other techniques such as conventional ultrasound, which only provides anatomical images, or more advanced functional ultrasound imaging methods (called fUS) that require external contrast agents (e.g. microbubbles) to enable functional studies of the sample, and which are mainly performed by measuring blood flow using the Doppler effect.

[0005] Despite the advantages of endogenous chromophores, exogenous chromophores can also be used in some cases, which are those introduced into the sample from the outside in order to improve its photoacoustic properties. Examples of these compounds are photosensitive contrast agents based on gold nanoparticles or covalent organic frameworks (COFs), which have the advantage of being wavelength tunable, highly biocompatible, and able to be consumed by tissues of particular interest by binding to certain organic molecules, as in the case of glucose in the study of malignant tumors.

[0006] Regarding the light beam used to excite tissues and enable them to emit ultrasound, the most common method in the prior art is to use short-duration laser pulses with sufficient energy per unit area (optical flux), because this can produce broadband ultrasound waves with a peak pressure proportional to the optical flux absorbed in the biological tissue. The optimal laser pulses for photoacoustic imaging techniques have a pulse width between a few nanoseconds and a hundred nanoseconds and a wavelength between 10-100 mJ / cm 2 Those laser pulses with fluences within the range are normalized to avoid damage to biological tissue, taking into account that the fluence is limited to values ​​below the maximum allowed exposure.

[0007] As for the source of these pulses, the usual choice is to use solid-state laser devices. These lasers are based on Q-switched technology, such as Nd:YAG or similar substances with high pulse energies of 200 to 500mJ or more at two fixed wavelengths of 1064nm and 532nm, but are limited in terms of pulse repetition rate, which varies around 10-100Hz. In addition, some tunable lasers, such as optical parametric oscillators, also based on Nd:YAG in the first stage are often used to obtain multi-wavelength excitation in the visible-near infrared band up to 2500nm, but at the expense of performing at a much lower energy per pulse. However, due to the relatively high energy of the pulses produced by solid-state lasers, they are often preferred for photoacoustic tomography, because they can cover a larger excitation area or field of view, with sufficient energy density to trigger detectable ultrasound waves.

[0008] On the other hand, photoacoustic microscopy (PAM) scanners attempt to obtain images with high spatial resolution by focusing the beam on a very small excitation spot and scanning the beam pixel by pixel over the entire field of view area, as well as at different focal depths within the volume under investigation. In particular, for optically resolved photoacoustic techniques, since the light energy is concentrated on a small spot area, much less pulse energy (on the order of microjoules or less) is required to generate photoacoustic waves that can be detected by ordinary ultrasound transducers. Therefore, low-energy sources such as pulsed laser diodes (PLDs) become viable alternatives to these imaging methods. Compared to solid-state lasers, PLDs offer much higher pulse repetition rates, above 10 kHz, thus accelerating photoacoustic imaging, and are cheaper and more compact, so they can be used to build portable and cost-competitive devices.

[0009] However, the use of PLDs in this field also brings some disadvantages. In particular, since semiconductor-based laser cavity emitters generate highly divergent beams, the beams obtained with this type of source have a lower quality. In addition, since the energy produced by these sources is generally low, multiple stacked emitters are used in common practice, resulting in a very wide emission area and a non-uniform intensity pattern, so that the final beam consists of many transverse modes. Therefore, the multi-peak nature of the beam makes it very difficult to focus on the point of interest, which is usually solved by using high numerical amplitude focusing lenses. Although these lenses can help focus the beam on the relevant area, they greatly reduce the working distance (WD). This means that the acoustic waves generated in the sample by the photoacoustic effect can only be generated at a very short focal length by the optical devices commonly used, thus limiting the depth of photoacoustic images obtained within the tissue. This is one of the fundamental advantages of this technology compared to more superficial (Spanish: superficiales) pure optical imaging techniques.

[0010] On the other hand, in laser diode based photoacoustic microscopy setups, the multimodal composition of the beam leads to a deterioration in the quality of the final image, since the lateral spatial resolution cannot be reduced without energy loss at the point of interest, which results in a poorer signal-to-noise ratio or, equivalently, in a lower contrast obtained in the ultrasound detector.

[0011] Some of these disadvantages are discussed in a general manner in patent application US2009109698A1. It describes a method for homogenizing or averaging a beam with an irregular lateral emission profile, which beam is typically emitted by a laser diode using a multimode optical fiber with a polygonal cross-section, and also provides various ways of focusing the averaged beam in a spatial light modulator for a projection or "display" device. Similarly, patent applications JP2003121664A, EP2228674A1 and US2013170806A1 use the same concept of transmission through optical fibers with square, rectangular or other cores to address the averaging of beam flux for other applications such as marking, photocoagulation or semiconductor lithography. However, none of these references deals with photoacoustic imaging applications.

[0012] In the field of photoacoustics, patent application CN102854142A proposes the use of a laser diode as a photoacoustic excitation source with an optical configuration of simple beam focusing, without optical fibers and without averaging of the laser diode beam. Applications US2011303015A1 and CN101813672A based on the use of a laser source other than the laser diode type with a high-quality Gaussian beam profile and a fundamental mode do propose beam averaging methods, although very different since they do not use optical fibers. In the first case, a diffuser or holographic plate is used to evenly expand the illumination area, although not in a focused manner. In the second method, beam shaping is used for large-area diffuse illumination of the object and detection by a depth-scanning piezoelectric detector array.

[0013] Finally, patent application WO2009055705A2 describes a confocal photoacoustic microscopy device (i.e. focusing the beam) with several possible configurations. In the method described, a single-mode fiber is used as a possible configuration to transmit a Gaussian fundamental mode beam generated by a solid-state laser source, such as Nd:YAG. Furthermore, spatial filtering is performed with a stop or pinhole, the purpose of which is to remove higher order modes that the main Gaussian beam may contain.

[0014] For a more comprehensive understanding of the prior art related to laser diode-based photoacoustic imaging, readers may refer to [S. Jeon, J. Kim, D. Lee, JW Bak, C. Kim, “Review on practical photoacousticmicroscopy”, Photoacoustics 15 (2019) 100141], [M. Erfanzadeh, Q. Zhu, “Photoacousticimaging with low-cost sources;A review”, Photoacoustics 14 (2019) 1-11] and [Q. Yao, Y. Ding, G. Liu, L. Zeng, “Low-cost photoacoustics imaging systems based on laserdiode and light emitting diode excitation”, Journal of Innovative Optical Health Sciences Vol. 10 No. 4 (2017) 17300038].

[0015] In view of the problems arising with respect to the standardization of the beam emitted by a laser diode in photoacoustic imaging applications, despite the working distance (WD) or depth within the tissue at which said image can be generated, the present invention proposes an improved method of generating images not present in the prior art, which solves the above-mentioned problems and allows obtaining high-quality images in photoacoustic microscopy techniques. Summary of the invention

[0016] In order to overcome the limitations of the prior art described above, the present invention aims to provide a new method for obtaining low-cost photoacoustic images, which solves the problem of inhomogeneity of the light beam exciting the studied sample, while allowing the lateral spatial resolution of the photoacoustic image to be increased by using a low-quality light beam light source (such as a laser diode or LED) without causing a reduction in the focusing distance of the light beam and therefore without reducing the maximum depth of ultrasonic sound waves generated by the light beam in the studied sample through the photoacoustic effect.

[0017] More specifically, a first object of the present invention relates to an optical device for shaping, homogenizing, directing and focusing a pulsed light beam onto a material sample, comprising:

[0018] - source of light beam or pulse;

[0019] - a multimode optical fiber having a core of polygonal cross-section, provided with a light entrance side and an exit side;

[0020] - an optical coupling module adapted to direct the light beam or pulse from the source to the entrance side of the optical fiber; and

[0021] - An optical directing module adapted to direct and focus the shaped and homogenized light beam or pulse from the exit side of the optical fiber to the sample.

[0022] In this way, after passing through the optical coupling module and the optical fiber, the already shaped and homogenized beam from the source is directed to the focus for photoacoustic excitation by the optical direction module, and any known collimation and focusing optics can be used. These optics are responsible for optically combining the plane on the fiber exit side with the focal plane or focusing plane of the beam, thereby reproducing the shape of the fiber core, or the shape of the hole located right at the end of the fiber. The scaling factor or magnification of the optical configuration adopted will ultimately determine the enlargement or reduction of the size of the laser excitation focus cross section of the photoacoustic device.

[0023] Additionally, in a preferred embodiment of the invention, the light source is a pulsed laser diode, a light emitting diode (LED), or a stacked array thereof. The invention allows the use of said sources to produce high-resolution photoacoustic images, taking advantage of their advantages, such as brightness or low cost, and addressing their limitations by shaping and homogenizing the beam using the apparatus of the invention.

[0024] In another preferred embodiment of the invention, the device comprises a surface having an opening arranged on the optical fiber exit side, said opening being substantially centered with respect to the core of said optical fiber. This allows spatial filtering of the light beam, which leads to a reduction in the size of the focus on the sample and, therefore, to a significant improvement in the lateral resolution. In addition, the aperture of the opening, or in other words, the opening size, may be mechanically adjustable, allowing the selection of the lateral resolution required for each use.

[0025] In another preferred embodiment of the present invention, the multimode optical fiber with a polygonal cross-section core has a high numerical aperture, which is particularly advantageous when using a light source that provides a very divergent light beam, because the high numerical aperture enables the optical fiber to accept light beams from a wide range of incident angles.

[0026] In another preferred embodiment of the present invention, the optical coupling module and / or the optical orientation module comprises two movable plano-convex aspheric lenses, one for collimation and the other for focusing. In addition, the orientation module may also comprise one or more motorized scanning mirrors (Spanish: espejos de barrido motorizados) that allow precise focusing on a desired point of the sample, as well as scanning the sample in order to obtain its complete image.

[0027] In another preferred embodiment of the invention, the optical device is part of a system for producing a photoacoustic image of a sample of a material, the system further comprising an ultrasound receiving transducer adapted to convert the sound waves into electrical signals and a signal acquisition and processing unit. In addition, the ultrasound transducer is preferably annular and coaxially surrounds the optical axis of the light beam; the signal acquisition and processing unit preferably comprises a preamplifier, an analog-to-digital converter and a device with processing capabilities such as a computer that can be connected to a screen or display for real-time display of the photoacoustic image.

[0028] The proposed system leads to some technical advantages regarding three main aspects of the performance of the photoacoustic microscopy setup, which will be discussed below.

[0029] First, by concentrating the energy of the laser beam in the core of the optical fiber in advance, an increase in the laser flux at the focus is obtained, thereby generating an acoustic pressure amplitude proportional to the flux. Here, the flux of the laser beam homogenized by the optical fiber has an almost constant cross-sectional distribution in the irradiated area of ​​the focal plane. Therefore, for an acoustic resolution photoacoustic microscopy (AR-PAM) device, uniform laser excitation can be provided over the irradiated area of ​​a defined field of view, where the lateral spatial resolution is determined by the focus of the ultrasonic transducer.

[0030] Secondly, an improvement in the lateral spatial resolution of the two-dimensional photoacoustic images obtained in an optical resolution photoacoustic microscopy (OR-PAM) device is determined by the size of the cross-sectional area of ​​the laser focus. The laser flux (φ) and the focal area (A0) can be adapted to the specific requirements of the photoacoustic microscopy system by selecting a fiber with a larger or smaller polygonal core area (A1) and coupling more or less energy from the laser source beam to the fiber entrance. Therefore, the average flux at the focus is φ = E0 / A0, where E0 is the energy of the beam pulse that finally reaches the focal point of the area given by Determine, where A1 is the cross-sectional area of ​​the core and m d is determined by the magnification factor given by the focusing optics of the optical orientation module. For example, for m d =1, the focal area will correspond to the cross-sectional area of ​​the fiber core.

[0031] Finally, the lateral spatial resolution is improved, or in other words, the area of ​​the focal spot A0 is reduced, although the focal depth or related working distance of the directional optics determines the maximum available displacement range of the laser focus to acquire two-dimensional images at different depths within the tissue, thereby allowing three-dimensional or volumetric photoacoustic scanning and imaging.

[0032] Due to the technical advantages set forth, it is of particular interest to use the devices and systems of the present invention to generate photoacoustic images in optical resolution photoacoustic microscopy applications, in acoustic resolution photoacoustic microscopy applications and / or in photoacoustic tomography applications.

[0033] In addition to the described apparatus and system that allows for convenient adaptation of some aspects of the beam of a low-quality multimode pulsed light source for excitation of ultrasound waves applied in a photoacoustic imaging system, there is also disclosed a method or process for producing a photoacoustic image of a sample of a material, comprising performing the following steps by operating a system as described above:

[0034] a) arranging a light source at a distance from the sample equal to or less than the length of the optical fiber, the light source being suitable for emitting a light beam or pulse having a duration in the nanosecond range and a frequency in the visible or near infrared band;

[0035] b) directing the light beam or pulse from the source to the entrance side of a high numerical aperture multimode optical fiber having a polygonal cross-section core by adjusting the positions of the elements constituting the optical coupling module;

[0036] c) focusing the homogenized light beam or pulse on a point of the sample by adjusting the positions of the elements constituting the optical directing module;

[0037] d) arranging an ultrasonic receiving transducer on the surface of the sample;

[0038] e) electronically receiving and processing the signal captured by the transducer by means of a signal acquisition and processing unit to obtain a photoacoustic image.

[0039] Finally, additionally, step ce of the method can be repeated at least once, changing the position of the focus of the focusing lens by a second adjustment of the position of the elements constituting the optical direction module, each time generating pixels corresponding to different points of the sample in a plane perpendicular to the beam direction, and wherein the subsequent step of assembling the pixels is performed according to the irradiation position of the sample corresponding to each of the pixels, thereby obtaining a two-dimensional image of the sample. If the same method for obtaining a two-dimensional image is repeated sequentially, so that in each repetition the position of the focusing lens relative to the ultrasound transducer is changed along the beam direction, each time a photoacoustic image of a different plane of the sample will be generated in the longitudinal direction of the beam direction, so that after the subsequent step of assembling the images is performed according to the irradiation position of the sample corresponding to each of them, a three-dimensional image of the sample can be obtained.

[0040] The present invention describes a method for producing high quality photoacoustic microscopy images by using an array of pulsed laser diodes or stacks thereof whose multimode beams have been homogenized and shaped to exhibit the following properties:

[0041] - Flat and uniform distribution of beam energy and flux, providing high image contrast;

[0042] - High lateral spatial resolution down to a few microns;

[0043] -cm range, enabling access to a wide range of axial excitation depths in the sample and providing more space to place the ultrasound detector.

[0044] In the present invention, these beam characteristics are achieved by performing two main steps. First, the light beam is coupled to a multimode optical fiber with a polygonal cross-section (e.g., square, rectangular, octagonal, etc.) using conventional optical coupling means known in the art, such as: two movable plano-convex aspheric lenses, one for collimation and the other for focusing, and a fiber entrance support, or even by direct coupling. In this way, the flux and intensity pattern emitted by the PLD source is homogenized due to the mixing of the spatial modes of the original light beam achieved within the fiber through multiple total internal reflections on its core walls. In addition, better beam flux homogenization results are obtained if the fiber length is equal to or greater than 1 m. This results in a beam cross-sectional distribution with a uniform and flat or flat-topped intensity distribution at the exit side of the fiber, wherein the cross-section of the beam has the same shape as the core, and wherein the asymmetric divergence of the fast (vertical) and slow (horizontal) modes is symmetric through a common numerical amplitude of the fiber. Likewise, the lateral resolution is improved due to the aperture or pinhole provided at the exit side of the fiber.

[0045] Other objects of the invention relate to specific embodiments described in the claims of the present application.

[0046] In the interpretation of the present invention, the term "substantially" should be understood as "the same as" or included within a variation range of 10%. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] The above and other features and advantages will be more fully understood from the detailed description of the present invention, and from the preferred exemplary embodiments with reference to the accompanying drawings, in which:

[0048] Figure 1 Schematically corresponding to a photoacoustic microscope optical resolution scanner, it is shown as a possible embodiment of the invention, which is based on a set of PLDs used as laser sources coupled to the beam homogenization device of the invention.

[0049] Figure 2a A longitudinal view of a multimode optical fiber having a core with a polygonal cross section is shown, which is the optical fiber used in the method of the present invention.

[0050] Figure 2b The cross-sectional view of two multimode optical fibers with polygonal core cross-sections (one square and one octagonal) used in the method of the present invention is shown.

[0051] Figure 2c The exit side of a multimode optical fiber with a core of polygonal cross section used in the method of the invention is shown, and more particularly the position of the aperture opening or pinhole is shown.

[0052] Figure 3aAn experimental CCD camera image with 4.5 micron pixel size and the beam cross-sectional flux distribution at the emitting surface of a 3x3 PLD array with a total area of ​​200x280 square microns is shown.

[0053] Figure 3b Shown are experimental CCD camera images with 4.5 micron pixel size and the beam cross-sectional flux distribution at the emission region of a PLD focused at the entrance of a multimode optical fiber with a square fused silica core having a 150 micron edge.

[0054] Figure 3c An experimental CCD camera image with a 4.5 micron pixel size is shown, as well as the flux distribution of the homogenized beam cross section at the fiber exit side after passing through a 2 meter long fiber.

[0055] Figure 4a An experimental CCD camera image with 4.5 micron pixel size is shown, as well as the beam cross-sectional flux distribution at the focus of the optical directing module of the beam, which is homogenized with an almost flat distribution, for a square focus with 75 micron edges.

[0056] Figure 4b and the beam cross-sectional flux distribution, which is the beam cross-sectional flux distribution at the focus of the optical steering module of the light beam for a circular focus with a diameter of 10 microns after spatial filtering using an aperture or pinhole located on the optical fiber exit side.

[0057] Reference numerals

[0058]

[0059] DETAILED DESCRIPTION

[0060] The following is based on this paper Figure 1 - Figure 4 describes the invention in detail. This description is provided for the purpose of illustrating and not limiting the claimed invention.

[0061] Figure 1 The constituent elements of a system for producing photoacoustic images corresponding to an OR-PAM type photoacoustic imaging scanner according to a preferred embodiment of the system of the invention are schematically shown. The scanner is divided into two main parts: an optical device (1) for generating, shaping and homogenizing a beam or pulse (2), and an ultrasonic detection and acquisition device. Similar to the system for producing photoacoustic images, the optical device (1) for generating, shaping and homogenizing a beam or pulse (2) included in the system is itself also the main object of the invention.

[0062] In the optical device (1), a light beam or pulse (2) is generated by a light source (3), preferably a PLD laser, by means of current pulses from a controller (4). An optical coupling module (5) couples the power of the light beam (2) to an optical fiber (7), transforming the light beam (2) into a beam (6) focused on the entrance side (7') of the optical fiber. The optical fiber (7) is preferably of multimode type and has any length. Furthermore, the optical fiber (7) is preferably provided with an outer protective coating (8), a core coating (9) and a core (10) of polygonal cross section. Figure 2a-2b Geometric details of square and octagonal optical fibers (7) are shown as two examples of proper shaping and homogenization of the PLD multimode beam (2). Figure 2a A longitudinal cross section of an optical fiber (7) is shown, wherein its main components are indicated: inner core (10), outer protective coating (8) and core coating (9). Figure 2b Details of two examples of polygonal optical fiber cross-sections 7 with square core 10 and octagonal core 10 geometries are also shown.

[0063] Finally, the optical orientation module (13) guides the formed and homogenized light beam (14) from the exit side (7") of the optical fiber to a focal point (15) having an area size determined by the optical orientation module (13) and focuses it on an image plane (18) at a working distance of a focusing lens (16) directly related to its focal length. The scanning of the image plane (18) at a certain penetration depth inside the sample is performed by means of a system of motorized mirrors (17) located in the optical orientation module (13) (e.g. Figure 1 ), thereby moving the focus (15) of the light beam to a different x and y coordinate position in the image plane (18).

[0064] In addition, an improvement in lateral resolution can be achieved by spatial filtering with a small opening (11), for example a sheet or surface (12) equipped with a stop or pinhole, which is located in the center in front of the exit side (7") of the core of the optical fiber (7), such as Figure 2c As shown. In it can be seen that the opening (11) is located in the center of the exit side (7") of the central optical fiber 7. This is of particular interest in optical resolution photoacoustic microscopy devices. Since the flux cross section at the exit of the optical fiber (7) is almost uniform, the shape of the light beam (14) passing through the aperture area of ​​the aperture (A'1) is reflected in the focal plane (18) of the directional module (13), which has a reduced focal area (15) (A'0) and is related to the magnification factor m of the optical directional module (13). d (whose value is less than or equal to 1) is proportional, following the corresponding ratio In order to obtain a better lateral resolution with a substantially constant flux given by the homogenized beam (14) of the optical fiber. With this spatial filtering method, by reducing the aperture of the diaphragm (11) to a few micrometers (depending on the laser wavelength used), it is possible to reduce the focus (15) and therefore the lateral resolution, without changing the effective focal length of the objective lens of the directional module (13), and thus maintaining the optical focal depth and the working distance of the image. In addition, if the aperture of the diaphragm or pinhole (11) included in the optical device is mechanically adjustable, this provides excellent advantages when choosing the desired lateral resolution for observing the sample.

[0065] The photoacoustic wave (20) generates an ultrasonic wave in the region of the focal point (15) by the photoacoustic effect as a result of the optical power of the incident light beam (2), and the duration is typically a few nanoseconds. This is known by the relationship P(r) = Γμ a (r)φ(r), which shows that the light flux at a given spatial point r in the sample (19) is converted into a pressure pulse with an amplitude P proportional to it, where μ a is the pressure pulse is the light absorption at that point, and Γ is the Grüneisen parameter, which depends on the thermodynamic and elastic properties of the medium.

[0066] For its part, the ultrasonic detection and acquisition device of the present invention preferably comprises any ultrasonic transducer (UST) (21) which is coaxially arranged around the beam (2) in a so-called reflection mode and in direct physical contact with the sample (19) or through an intermediate high ultrasound transmission medium (e.g. coupling gel). The UST (21) then receives the ultrasonic waves (20) propagating at the speed of sound in this medium and converts them into electrical signals. These signals are amplified and digitized by a preamplifier (PREAMP) (22) and an analog-to-digital converter (ADC) (23), respectively, for subsequent digital processing of the image on a computer (24), such as Figure 1 The computer can then be connected to a screen or display (25) so that the resulting images can be viewed, in some cases even in real time. Thus, in the OR-PAM scanner, each photoacoustic image frame is ultimately recorded as a 2D map of the acoustic pressure amplitude (or squared amplitude), where each position of the laser focus (15) in the scanned image plane (18) corresponds to a pixel of the two-dimensional photoacoustic image.

[0067] Depth scanning can also be performed along the z-axis to produce a 3D volumetric image of the 2D frame slices, for example by moving the focusing lens (16) relative to the UST (21), which remains fixed and in contact with the surface of the sample (19). The maximum scanning range within the material sample (19) is provided by the working distance of the focusing lens (16), so that the focus (15) can be focused from the sample surface to a maximum depth given by the working distance of the lens (16). For this reason, a sufficient working distance is usually required to obtain images up to a depth of several millimeters, which is one of the main advantages of photoacoustic imaging compared to much shallower pure optical imaging techniques. In addition, the use of flexible optical fibers (7) allows for an advantageous design, since the optical orientation module (13) can be independently moved and placed at any desired position on the surface of the sample (19), thus facilitating its practical application for imaging.

[0068] As for the optical method for beam shaping in the OR-PAM scanner, the beam or pulse (2) is generated by a stack of PLDs used as light sources (3), which are connected to their current pulse controller (4) or PLD driver through their positive and negative electrodes with direct polarity. The PLD controller (4) provides pulse trains (Spanish: de pulsos de corriente) that are above the PLD current intensity threshold to emit laser light, with a duty cycle given by D=t p F p Given and programmed by the controller (4) the duration t of the pulse width p and the pulse repetition rate F p Therefore, the PLD generates a pulsed light beam (2) with the same time distribution, and the power of the light pulse is proportional to the current intensity supplied to the PLD by the controller (4). In order to prevent damage to the PLD, the current intensity, duty cycle and pulse width are set below the maximum values ​​provided in the instructions of the PLD manufacturer.

[0069] To achieve higher maximum optical powers, the PLD source (3) is constructed with multiple individual laser diode emitters, typically arranged in horizontal monolithic strips (Spanish: barras monolíticas), each with a specific number of emitters, while the strips can also be stacked vertically to cover the global emission area A2. Therefore, the cross-section of the emitted light beam (2) will have a non-uniform intensity or flux pattern corresponding to any configuration of the array of individual emitters.

[0070] Figure 3a For a luminescent device having an emission area A2 = 200 × 280 μm 2 (exist Figure 3aAn image of the emission cross section of the light beam (2) taken with a CCD camera for a particular stacked array of 3×3 PLDs (framed with black squares in FIG. 1 ). For this PLD, the maximum power and the central wavelength are approximately 200 W and 905 nm, respectively. Then, for a beam width (2) of 100 ns, the energy of the beam (2) of this PLD beam is estimated to be 20 μJ, and the repetition frequency of the light pulses (2) can reach 10 kHz.

[0071] like Figure 1 As shown in the OR-PAM scanner embodiment, a light beam or pulse (2) from a PLD light source (3) is coupled to an optical fiber (7) having a core (10) with a polygonal cross-section through an optical coupling module (5), which is capable of emitting most of the power of the light beam (2) from the PLD emission area A2 to the core (10) area A1 of the optical fiber. For an area of ​​the core (10) of the optical fiber that is larger than the PLD emission area, A1>A2, direct coupling without optical devices can be achieved after properly aligning the core (10) relative to the center of the PLD area. Otherwise, due to It may be necessary to have a value represented by m c The optical coupling module (5) is provided with an amplification factor of 100 to match the cross section of the light beam (2) at least in the region of the fiber core (10). It is usually necessary to align the position of the entrance side (7') of the fiber core (10) with the position of the light beam (6) focused on the fiber side (7') by the optical module to maximize the entrance power sent to the fiber core (10), which can be achieved, for example, by accurately adjusting the position of the fiber entrance support (26) by micro-translation in two directions transverse to the optical axis.

[0072] The coupled beam (2) is guided along the optical fiber (7) until it exits through an exit side (7") of the optical fiber (7) with a divergence given by the numerical aperture of the optical fiber (7) and with an approximately uniform or homogenized flux cross section.

[0073] The numerical aperture (NA) of the optical fiber (7) provides the maximum acceptance angle for an incident light beam (2) that will be guided through the core (10) by multiple total internal reflections at the core / coating interface. The numerical aperture is calculated from the well-known ratio (Spanish: is given based on the refractive index of the optical fiber (7) element. Therefore, for a highly divergent light beam (2) from the PLD source (3) and the optical coupling module (5), a high numerical aperture fiber (7) is usually required in order to achieve efficient mode mixing along the core (10) and thus obtain better uniformity of the light beam (2) at the exit of the optical fiber (7). The length of the optical fiber (7) will also improve the uniformity of the light beam (2), typically with a nearly constant flux distribution for one or two meters of optical fiber (7).

[0074] Figure 3b and Figure 3c CCD camera images of the flux distribution of the cross section of the light beam or pulse (2) at the entrance of the optical fiber (7) and at the exit of the optical fiber (7) in the focal plane of the optical coupling module (5) are shown respectively. These experimental data are provided to evaluate the performance of the optical fiber homogenization (7) and for Figure 1 A specific embodiment of a system corresponding to a 3x3 transmitter configuration (see Figure 3a The above PLD stack includes a square core (10) with a size of 150 μm per side, a multimode optical fiber (7) with a numerical aperture of 0.39 and a length of 2 m, and an optical coupling module (5) consisting of two plano-convex aspheric lenses, the first lens (27) being used to collimate the light beam, and the second lens (28) being used to focus it on the core, and their respective nominal focal lengths being f1=20.1 mm and f2=10.5 mm. Thus, the optical coupling module (5) has a given increase in m c =f2 / f1≈0.5, so that the PLD emission area A2 is reduced by a factor of about 4 to fit the cross section of the core (10) of the optical fiber A1. Ray tracing simulations replicating the same optical configuration show good uniformity of the cross section of the flux of the light beam (2) at the exit of the optical fiber (7) both for a square core (10) of 150 μm per side and for an octagonal core (10) with an aperture (octagonal diameter within the circumscribed circle) of 150 μm.

[0075] The homogenized beam (14) exiting from the optical fiber (7) is transmitted to the final focus (15) through the optical orientation module (13) of the OR-PAM system or scanner, which holds the collimating lens (29) and the focusing lens (16) and the xy scanning mirror system (17) on the same mechanical support and is coupled to a ring-shaped ultrasonic transducer (21) coaxially surrounding the beam (14), as shown in Figure 1 With this optical configuration, the cross-sectional area A1 of the flux of the beam (14) formed at the exit of the optical fiber (7) is shown as being in its entire area at the final focus (15). The same uniform flux distribution is obtained on the optical directional module (13) and its size is proportional to the amplification factor m. d =f2 / f1 are proportional to the focal length ratio of the collimating lens (29) and the focusing lens (16), respectively.

[0076] Figure 4a The CCD camera image shows the cross-sectional shape of the laser focus (15) and the homogenized flux distribution obtained for the same 150 μm square core (10) optical fiber (7) of the above experimental example. Figure 3cThe light beam (14) with the flux cross section shown is transmitted to the focal plane (18) by means of a specific optical guidance module (13) consisting of two optical guides with a magnification or enlargement factor m. d =f2 / f1≈0.5, wherein the focal lengths of the first lens (29) and the second lens (16) are f1=20.1 mm and f2=10.5 mm, respectively. Therefore, the final focus (15) has the same square shape as the core (10), but the area is reduced by 4 times relative to the area of ​​the core (10) at the fiber outlet, and the side of the square area is reduced by 2 times, reaching about 75 μm per side. The maximum penetration range of the focus (15) in the sample is given by the working distance of the focusing lens (16), which specifically has a focal length of f2=10.5 mm, for which a working distance of WD=6.4 mm in air and a working distance of WD=8.8 mm for the lens (16) immersed in water are obtained.

[0077] Since the size of the laser focus (15) directly determines the lateral resolution of the photoacoustic image formed by the scanning laser beam (14), the PLD beam (2) shaping and homogenization method using a square-core multimode fiber (7) allows this important parameter to be controlled in the OR-PAM scanner by selecting the core (10) size and the amplification factor of the optical directional module (13). Due to its approximately constant flux distribution and steep flanks, high-contrast illumination spots are also obtained, such as Figure 4a The final average flux achieved at the square focus (15) with a side of about 75 μm is about 160 mJ / cm 2 , which corresponds to a measured power of 90 W and an energy of 9 μJ in beam (2) (measured in air). This flux level is more than three times the maximum allowed by the 905 nm PLD wavelength used in this example to avoid damage to human skin, although the excess energy of beam (2) will be used to compensate for energy losses that are typically present in practical applications.

[0078] Furthermore, as described above, an increase in lateral resolution can be achieved using the same optical directional module (13) by spatial filtering with a small opening (11) centered in front of the exit side (7") of the core (10). The focal spot size (15) will then be reduced to the size of the opening (11) multiplied by the magnification factor m d , because as mentioned before, the optical directing module (13) will focus the outgoing light beam (14) in the same shape as the opening (11) in the image focal plane (18). Figure 4b The cross-sectional distribution of the focus (15) is shown, which is produced by spatial filtering with a diaphragm (11) having a circular aperture of 20 μm for the optical orientation module (13) m d= f2 / f1≈0.5. Therefore, the diameter of the focus (15) is about 10 μm, as Figure 4b It should also be noted that the pixel size of the CCD camera, 4.5 μm, prevents a better definition of the focal point (15).

[0079] Despite Figure 4b The energy of the beam (14) reaching the focus (15) is thus reduced by spatial filtering with a smaller aperture (11), but the flux at the focus (15) remains approximately constant due to the prior homogenization of the cross section of the beam (2) at the end of the core (10). Thus, in this example, the lateral resolution is about 160 mJ / cm2, as with the larger focus (15) of a square core (10) fiber (7) without spatial filtering. 2 The same average flux level is increased to at least a focal diameter of 10 μm (15), thus producing the same sound pressure amplitude.

[0080] In addition to the described OR-PAM technique and the corresponding embodiments of its scanning device, this optical method for shaping and homogenizing the light beam (2) based on an optical fiber (7) with a polygonal core (10) can be applied to other photoacoustic imaging techniques, such as acoustic resolution photoacoustic microscopy and photoacoustic tomography, in order to obtain a region illuminated by a uniform flux from an inhomogeneous and low-quality light source (3).

[0081] Therefore, all of these imaging techniques can benefit from the higher light pulse (2) repetition rates of semiconductor-based light sources (3) compared to the more powerful and expensive solid-state lasers that are typically used as light sources (3) for these techniques.

[0082] In acoustic resolution photoacoustic microscopy and photoacoustic tomography, the lateral and axial resolutions depend only on the center frequency and bandwidth of the UST (21). However, in order to obtain a good field of view, a larger focal area with uniform illumination is usually required. This can be achieved by using an optical fiber (7) with a larger polygonal cross-section, so that a higher power beam (2) from a PLD with a larger emission area can be efficiently coupled to the optical fiber (7) to provide very good illumination uniformity and the necessary average flux level in the imaging focal plane (18), such as Figure 1 In addition, although a focusing lens (16) with a wider effective aperture and focal length is used, a larger focal area (15) can also be scanned through the imaging plane (18) using a motorized mirror (17).

[0083] In particular, in the case of a photoacoustic tomography scanner, image reconstruction is usually performed using a phased array ultrasound transducer (21) consisting of a plurality of ultrasound detector elements that allow capturing the photoacoustic waves (20) at different viewing angles relative to the field of view of the optical illumination. The phased array detector elements can be distributed around the beam (2) in an annular or hemispherical geometry, similar to Figure 1 A single-element annular transducer (21) in the optical orientation module (13) of the OR-PAM scanner shown in FIG.

Claims

1. An optical device (1) for shaping, homogenizing, directing and focusing a pulsed light beam (2) onto a sample (19) of a material, characterized in that The optical device comprises: - a source (3) of light beams or pulses (2); - a multimode optical fiber (7) having a core (10) of polygonal cross-section, provided with a light entry side (7') and an exit side (7"); - an optical coupling module (5) adapted to guide the light beam or pulse (2) from the source (3) to the entrance side (7') of the optical fiber (7); and - an optical directing module (13) adapted to direct and focus the formed and homogenized light beam or pulse (14) from the exit side (7") of the optical fiber (7) to the sample (19).

2. The device (1) according to the preceding claim, comprising a surface (12) with an opening (11), said opening (11) being arranged on the outlet side (7") of said optical fiber (7).

3. Device (1) according to the preceding claim, wherein said opening (11) is substantially centered relative to the core (10) of said optical fiber.

4. The device (1) according to any of the preceding claims, wherein the size of the opening (11) is mechanically adjustable.

5. The device (1) according to any of the preceding claims, wherein the light source (3) comprises a pulsed laser diode, a light emitting diode LED or a stacked array thereof.

6. The device (1) according to any of the preceding claims, wherein the multimode optical fiber (7) having a core (10) with a polygonal cross-section has a high numerical aperture.

7. The device (1) according to any of the preceding claims, wherein the optical coupling module (5) and / or the optical orientation module (13) comprises two movable plano-convex aspheric lenses, one for collimation (27, 29) and the other for focusing (28, 16).

8. The device (1) according to any of the preceding claims, wherein the optical orientation module (13) comprises one or more motorized scanning mirrors (17).

9. A system for producing a photoacoustic image of a sample (19) of a material, wherein the system comprises: - An optical device (1) according to any one of the preceding claims; - a transducer (21) for receiving sound waves (20); adapted to generate an electrical signal from the reception of the sound waves (20); and - a unit for acquiring and processing said electrical signals generated by said transducer (21), wherein said unit comprises software and / or hardware means adapted to generate photoacoustic image data from the processed electrical signals.

10. System according to the preceding claim, wherein the ultrasonic transducer (21) is annular and coaxially surrounds the optical axis of the light beam (2).

11. The system according to any one of claims 9-10, wherein the signal acquisition and processing unit comprises a preamplifier (22), an analog-to-digital converter (23) and / or a computer (24).

12. The system according to any one of claims 9-11, wherein the signal acquisition and processing unit is connected to a screen or display (25) for real-time display of the generated photoacoustic image data.

13. Use of the device (1) according to any one of claims 1 to 8 for generating photoacoustic images in optical resolution photoacoustic microscopy applications, in acoustic resolution photoacoustic microscopy applications and / or in photoacoustic tomography applications.

14. A method for producing a photoacoustic image of a sample (19) of a material, the method comprising performing the following steps by operating a system according to any one of claims 9 to 12: a) arranging a light source (3) at a distance from the sample (19) equal to or less than the length of the optical fiber (7), the light source (3) being suitable for emitting a light beam or pulse (2) having a duration in the nanosecond range and a frequency in the visible or near infrared band; b) directing the light beam or pulse (2) from the source (3) to the entrance side (7') of a multimode optical fiber (7) having a core (10) with a polygonal cross section by adjusting the positions of the elements constituting the optical coupling module (5); c) focusing the formed and homogenized light beam or pulse (14) onto a point of the sample (19) by adjusting the positions of the elements constituting the optical orientation module (13); d) arranging an ultrasonic receiving transducer (21) on the surface of the sample (19); e) electronically receiving and processing the signal captured by the transducer (21) by means of a signal acquisition and processing unit to obtain a photoacoustic image.

15. The method according to the preceding claim, wherein steps ce are repeated at least once, wherein: Each time the position of the focal point (15) of the focusing lens (16) is changed by a second adjustment of the position of the elements constituting the optical direction module (13), pixels corresponding to different points of the sample (19) are generated in a plane perpendicular to the direction of the beam (2) each time, and wherein a subsequent step of assembling the pixels is performed according to the irradiation position of the sample (19) corresponding to each of the pixels, thereby obtaining a two-dimensional photoacoustic image of the sample (19).

16. The method according to the preceding claim, wherein the method is repeated at least once, wherein: Whenever the position of the focusing lens (16) changes relative to the ultrasonic transducer (21) along the direction of the beam (2), a photoacoustic image of a different plane of the sample (19) in a longitudinal direction with respect to the direction of the beam (2) is generated each time, and a subsequent step of assembling the images is performed according to the irradiation position of the sample (19) corresponding to each of the photoacoustic images, thereby obtaining a three-dimensional image of the sample (19).

Images