Optical ultrasound imaging using camera sensors

Abstract

Disclosed are systems and processes for using a light source and an image sensor to record ultrasonic vibrations of a surface. Image data recorded by the image sensor can be processed to produce images or other information. The systems and processes described herein find use in a wide variety of applications including, but not limited to, ultrasonic imaging applications (e.g., medical and sonar) as well as detection of acoustic vibrations at any frequency, on either reflective surfaces or even inside volumes (such as reflective or scattering particles suspended in fluid, or by detecting changes in the refractive index of a medium), in nondestructive testing applications, and in directional hydrophones and microphones.

Description

ATTY DOCKET NO. MEDIALAB-085AWOOPTICAL ULTRASOUND IMAGING USING CAMERA SENSORSBACKGROUND

[0001] Ultrasound is a commonly used medical imaging technology, due to its unique combination of imaging depth, high resolution, low cost, and safety compared to other modalities such as magnetic resonance imaging (MRI) and X-ray imaging. Recent advancements in ultrasound imaging have included improved imaging algorithms, signaling techniques, the development of portable system-on-chip (SOC) devices, and increasing development and integration of machine learning (ML) and artificial intelligence (Al) tools.

[0002] Current state-of-the-art 4D ultrasound probes have up to 56k elements, with most below 10k elements, requiring sophisticated and expensive fabrication techniques. Significantly, achieving ideal imaging performance requires that elements of a transducer array within an ultrasound probe obey spatial Nyquist sampling constraints. Thus, to avoid spatial aliasing or "grating lobe" artifacts, the spacing of elements within the transducer must be A / 2, where A is the acoustic wavelength at a given frequency (e.g. approximately 75 pm spacing at 10 MHz).

[0003] Because of this, it can be challenging to fabricate transducers having an element spacing required for practical ultrasound transducers. This is particularly true when fabricating transducers from piezoelectric ceramics, which can be brittle. Thus, rather than attempting to fabricate such small elements, practical ultrasound transducers commonly sacrifice field of view of the transducer (to hide grating lobe artifacts).

[0004] These fundamental physical constraints — number of elements, and element spacing — continue to pose a significant challenge to the ultrasound and sonar industries. While approaches based upon micro-electromechanical systems (MEMS-based approaches) have been proposed to overcome these constraints, to date, such MEMS-based approaches fail to deliver performance advantages sufficient to overcome cost and other factors. Another approach, acoustic sensing using optical fibers, has not yet scaled to sufficiently large numbers of elements to compete with piezoelectric or MEMS based arrays.ATTY DOCKET NO. MEDIALAB-085AWOSUMMARY

[0005] The concepts, systems, devices and techniques described herein are directed towards use of a light source and a camera (or more generally an image sensor or imaging device) to record ultrasonic vibrations of a surface of an object or volume. The surface may be coupled to an acoustic medium. Each pixel of the image sensor captures the vibration at a point on the surface, meaning the surface vibration can be concurrently or simultaneously measured at a large number of points (e.g., millions of points, depending upon the particular type of image sensor or camera). Importantly, an image sensor frame rate (or camera frame rate) can be much lower than the vibration frequency (for example, a camera frame rate of sixty frames per second (60 FPS) versus a vibration frequency of about 5 MHz).

[0006] After optically recording the acoustic vibrations of a surface, the information can be used to form ultrasound or sonar images in much the same manner as existing, well-known piezoelectric systems. The image reconstruction, known as beamforming, can result in a variety of images including, but not limited to 1 D, 2D, or 3D images. Other modalities well-known in medical ultrasound, including Doppler detection of blood flow and shear wave elastography, are also possible.

[0007] The concepts, systems, devices and techniques described herein find use in a wide variety of applications including, but not limited to, ultrasonic imaging applications (e.g., medical and sonar) as well as detection of acoustic vibrations at any frequency, on either reflective surfaces or even inside volumes (such as reflective or scattering particles suspended in fluid, or by detecting changes in the refractive index of a medium), in nondestructive testing applications, and in directional hydrophones and microphones.

[0008] In one aspect, described is a method for using light source and a camera or image sensor to record ultrasonic vibrations of a surface. Each pixel on the camera captures the vibration at a point on the surface, meaning the surface vibration can be measured at millions of points simultaneously or substantially simultaneously.

[0009] For ultrasound imaging applications, a method includes transmitting aATTY DOCKET NO. MEDIALAB-085AWO frequency modulated continuous wave (FMCW) acoustic signal into an object or volume sought to be imaged. The reflected / scattered acoustic waves arrive at the surface of the volume, causing the surface to vibrate. An optical interferometer converts the vibrations into an optical signal. A light source of the interferometer is modulated in a manner that causes the optical signal to be down-converted to a frequency which is low enough to allow recording by a camera or image sensor. The frequency to which the signal is down-converted can be made arbitrarily low, even to the point where the vibration can be seen with the naked eye. The lower frequency optical signal (i.e., the down-converted optical signal), is then recorded by a camera or image sensor. After recording one or more frames of the acoustic signal, an ultrasound image may be formed from the image data in the one or more frames using well-known beamforming techniques.

[0010] Another ultrasound imaging method includes transmitting a pulsed acoustic signal into an object or volume sought to be imaged. The reflected / scattered acoustic waves arrive at the surface of the volume, causing the surface to vibrate. An optical interferometer converts the vibration into an optical signal. The light source of the interferometer is modulated in a manner that causes the optical signal to be down-converted to a frequency which is low enough to allow recording by a camera or image sensor. The down-conversion may be accomplished by effectively taking the cross-correlation of the signal with a reference signal by optical means, or by shifting the center frequency of the signal to a lower frequency. The lower frequency optical signal is then recorded by a camera or image sensor. After recording one or more frames of the acoustic signal, an ultrasound image may be formed from the image data in the one or more frames using well-known beamforming techniques.

[0011] With this particular arrangement, an optical receiver array can effectively have millions of elements (each camera sensor pixel becomes an element), much more than current technologies which are limited to approximately ten thousand (10k) elements. The effective spacing of the elements in the optical receiver array can be changed easily by using zoom lenses on the camera, allowing the receiver array to be any arbitrary size. TheATTY DOCKET NO. MEDIALAB-085AWO element spacing can similarly be arbitrarily large or small, down to <10 urn, which is unachievable with existing technologies. As a result, an ultrasound probe provided in accordance with the concepts described herein can have larger aperture size (leading to improved image resolution), closer element spacing (leading to a wider field of view), and higher frequency performance (also leading to higher resolution) compared with conventional ultrasound probes.

[0012] In one embodiment, an ultrasound receiver (i.e. optics and image sensor) can be re-used in multiple systems with different specifications, by appropriately changing one or more lenses (e.g., one or more zoom lenses). Thus, an ultrasound receiver provided in accordance with the concepts described herein is flexible and requires less re-development compared to existing technologies.

[0013] Furthermore, the imaging concepts, systems, devices and techniques described herein can be used to perform non-contact ultrasound imaging on the human body, by reading the reflection directly from the skin surface. The imaging technique described herein can use already-existing camera image sensors. In embodiments sensors having a frame rate of about fifty thousand frames per second or higher (e.g., >50k FPS) may be used.

[0014] Ultrasound devices provided in accordance with the concepts described herein are expected to be lower cost compared to existing systems of comparable capability. This is due to no longer requiring microfabrication of ceramics and advanced packaging technologies.

[0015] In accordance with a further aspect of the concepts, systems, devices and techniques described herein, the inventor has discovered that the use of frequency modulated continuous wave (FMCW) acoustic excitation followed by demodulation of an optical signal to a lower frequency, where the optical signal is then received using a detector or image sensor with 1 or more pixels can be used to generate an ultrasound image.

[0016] In accordance with a still further aspect of the concepts, systems,ATTY DOCKET NO. MEDIALAB-085AWO devices and techniques described herein, the inventor has discovered that the use of pulsed acoustic excitation followed by demodulation or cross-correlation of an optical signal to a lower frequency, where the optical signal is then received using a detector or image sensor with 1 or more pixels can be used to generate an ultrasound image.

[0017] In another aspect, the inventor has discovered the use in a device such as a medical probe or sonar system, where the interferometry is performed on a surface that is part of the device.

[0018] In another aspect, the inventor has discovered the use in an application where a surface or volume is sensed that is not part of the device, such as human skin, the surface of an object, a surface or particles inside an object or volume, or particles floating in a liquid volume.

[0019] In another aspect, the inventor has discovered the use of a down conversion technique without an acoustic excitation being transmitted in which optics down-convert and make visible acoustic vibrations on a target surface.

[0020] In another aspect, the inventor has discovered the specific detection hardware using air pocket Fizeau interferometers, with integrated piezoelectric transducers and plastic backing substrates.

[0021] In another aspect, the concepts described herein are directed toward a new modality for ultrasound transducers - optical ultrasound imaging using camera / image sensors to record the acoustic signals. In this technique, the nanoscale motion of a vibrating surface is made visible using optical interferometry. In one embodiment, a laser beam is projected on a vibrating surface, then the reflected or scattered light interferes with a reference beam reflected from a non-vibrating surface. The resulting interference pattern varies according to the changing displacement of the vibrating surface, and is captured by an image sensor (e.g., camera, or any image sensor having an array of pixels). Typical image sensors can have up to millions of pixels, meaning the pixel array effectively forms a full-matrix ultrasound receiver array with up to millions of elements, orders of magnitude more than the current state-of-the-art.ATTY DOCKET NO. MEDIALAB-085AWOBRIEF DESCRIPTION OF THE DRAWINGS

[0022] The manner and process of making and using the disclosed embodiments may be appreciated by reference to the figures of the accompanying drawings. It should be appreciated that the components and structures illustrated in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principals of the concepts described herein. Like reference numerals designate corresponding parts throughout the different views. Furthermore, embodiments are illustrated by way of example and not limitation in the figures, in which:

[0023] Fig. 1 is a block diagram of an optical ultrasound imaging system using camera sensors;

[0024] Fig. 2 is a block diagram of an acousto-optical receiver suitable for use in an optical ultrasound imaging system such as the system of Fig. 1 ;

[0025] Fig. 2A, is a block diagram of an top view of an example embodiment of a support structure used to space a surface of an acoustic material from a surface of a beamsplitter;

[0026] Fig. 2B, is a block diagram of an top view of an example embodiment of a support structure used to space a surface of an acoustic material from a surface of a beamsplitter;

[0027] Fig. 3 is a flow diagram of an example process for using a camera to record ultrasonic vibrations of an acoustically coupled surface using frequency modulated continuous wave (FMCW) acoustic signals; and

[0028] Fig. 4 is a flow diagram of an example process for using a camera to record ultrasonic vibrations of an acoustically coupled surface using pulsed acoustic signals;

[0029] Fig. 5 is a block diagram of an apparatus for recording ultrasonic vibrations of an acoustically coupled surface using a camera and Fizeau interferometer with a diverging light source;ATTY DOCKET NO. MEDIALAB-085AWO

[0030] Fig. 6 is a block diagram of an apparatus for recording ultrasonic vibrations of an acoustically coupled surface using non-polarizing Michelson interferometer and a camera; and

[0031] Fig. 7 is a block diagram of an apparatus for recording ultrasonic vibrations of an acoustically coupled surface using polarizing Michelson interferometer and a camera..DETAILED DESCRI PTION

[0032] Before describing details of the concepts, systems, devices and techniques described herein for optical ultrasound imaging using one or more imaging sensors, some introductory concepts are explained. To promote clarity in the description of the concepts sought to be protected, the below description sometimes focuses on ultrasonic imaging applications (e.g., medical and sonar). However, after reading the disclosure provided herein, those of ordinary skill in the art will recognize that the concepts, systems, devices and techniques described herein, can be used to detect acoustic vibrations at any frequency, on either reflective or scattering surfaces or even inside volumes (such as particles or objects suspended in fluid, or by detecting changes in the refractive index of a medium).

[0033] The concepts, systems, devices and techniques for optical ultrasound imaging described herein utilize an interferometer and a camera or image sensor. The interferometer measures the displacement of an acoustic surface caused by vibration. This is accomplished, in part, by directing a light signal from a light source (e.g. a laser signal from a laser source) toward an acoustic surface and detecting those portions of the light signal reflected or re-directed from the surface and interfering such reflected and / or re-directed portions with an interferometer and a reference beam. The interference pattern can then be recorded on a camera sensor. The recorded interface pattern may be processed to form an image. Using the concepts, systems, devices and techniques described herein, the displacement of the acoustic surface can be measured to the sub-nanometer scale.

[0034] It should be appreciated that to promote clarity in the description of theATTY DOCKET NO. MEDIALAB-085AWO concepts described herein, reference is sometimes made herein to example hardware implementations using specific types of devices. It should be appreciated that functionally equivalent devices may be used. For example, it should be appreciated that although reference may be made herein to specific types of interferometers, any interferometer design, including but not limited to Fizeau, Fabry-Perot, Michaelson, Fourier-transform, Mach-Zehnder, Sagnac, Twyman — Green, point diffraction, and phase-shifting point diffraction interferometers or any type of remote sensing interferometer can be used to implement the concepts described herein.

[0035] Similarly, any type of camera or image sensor can be used.

[0036] Referring now to Fig. 1 , an optical ultrasound imaging system includes an acoustic transmitter 2 and an acousto-optical receiver 3 both of which are coupled to a processor 4. Acoustic transmitter is provided to suit the needs of a particular application (e.g. underwater acoustics, medical ultrasound, sonar air acoustics). In some embodiments, the acoustic transmitter may include a piezoelectric transducer, a magnetostrictive transducer, an electrodynamic speaker, and / or a photoacoustic excitation laser configured to provide any type of continuous wave (CW) or pulsed acoustic signal (also sometimes referred to herein as “acoustic waves” or more simply “waves”).

[0037] Processor 4 provides control signals 6 to acoustic transmitter 2. Such control signals cause transmitter to provide a particular type of acoustic signal (e.g. CW or pulsed) having particular characteristics. For example, in response to control signals provided thereto, acoustic transmitter 2 may provide a CW signal having a particular frequency or range of frequencies, a particular power level or range of power levels, a waveform shape. In response to control signals provided thereto, acoustic transmitter 2 may provide an acoustic signal having a pulse shape with particular pulse widths or range of pulse widths, a particular pulse repetition frequency or range of pulse repetition frequencies and a particular power level or range of power levels.

[0038] In response to control signals 6, acoustic transmitter 2 emits (transmits) an acoustic signal 8 into an acoustic medium 10 (i.e., any medium in which acoustic signals may propagate, such as human tissue, water, or air). In embodiments,ATTY DOCKET NO. MEDIALAB-085AWO acoustic transmitter 2 may be controlled by processor 4 (via control signals 6) to transmit a frequency modulated continuous wave (FMCW) acoustic signal. In embodiments, acoustic transmitter 2 may be controlled by processor 4 (via control signals 6) to transmit a pulsed acoustic signal.

[0039] Acoustic signal 8 propagates through acoustic medium 10 and intercepts an object 12. Portions of the transmitted acoustic signal 8 are reflected or otherwise re-directed from object 12. Such reflected or otherwise re-directed acoustic signals 14 are received or otherwise intercepted by the opto-acoustic receiver 3.

[0040] Receiver 16 includes a surface which vibrates in response to the received acoustic signals . An optical interferometer converts the vibrations into an optical signal. A light source of the interferometer is modulated in a manner that causes the optical signal to be down-converted to a frequency below the vibration frequency. Such down-converted frequency optical signals may then be recorded by a camera or image sensor. After recording one or more frames of the acoustic signal, an ultrasound image may be beamformed via beamforming techniques performed in processor 4. It should be appreciated that although a single processor 4 is shown in Fig. 1 , in practical embodiments, the processing functions provided by processor 4 may be implemented in one or more separate processors. After reading the description provided herein, those of ordinary skill in the art will appreciate how to select the number of processors to use in a particular application.

[0041] Referring now to Fig. 2, an acoustic signal 14 is reflected or otherwise redirected from the object 12 located within the acoustic medium 10. A first surface 18a of acoustic material 18 is coupled with acoustic medium 10. The acoustic signal 14 is incident on first surface 18a and propagates through acoustic material 18 to a second surface 18b, thus causing the surface 18b to vibrate.

[0042] A space 19 is present between top surface 18b of acoustic material 18, and the bottom surface 15a of beamsplitter 15. Space 19 may, for example, be provided as an air pocket 19 (i.e. air fills space 19). In other embodiments a clean gas such as nitrogen may fill space 19. Alternatively, space 19 may be a vacuum (or substantially a vacuum) or a vacuum-like environment. Additionally, space 19 may be wholly or partially filled with a solid or liquid material. In some embodiments, the solid or liquidATTY DOCKET NO. MEDIALAB-085AWO material within the space 19 may have an acoustic impedance of at least less than about 8 MRayls. A support structure 16 may be disposed between the surface 18b of the acoustic material and a first surface 15a of beamsplitter 15 to form space 19. The support structure 16 may include one or more structures (e.g., one or more spacing elements and / or one or more structural elements). One or more sealing elements may be disposed around the perimeter of space 19, or within space 19 thereby separating one or more sections of space 19, to prevent the entry or exchange of gases, liquids, dust, or other contaminants or materials into or out of space 19 or sections of space 19.

[0043] A light source 9 (e.g., a laser) emits a beam of light 11 (e.g. a laser signal) which passes through beamsplitter 15 having a partially reflective bottom surface 15a. Light source 9 receives control signals from processor 6 to emit a modulated optical signal, which may be either a frequency modulated continuous wave (FMCW) signal or a pulse signal depending upon the type of signal provided by transmitter 2 (Fig. 1). That is, processor 4 sends signals to each of the transmitter 2 and the receiver 16 which coordinates the transmit signal and the optical signal. It should be noted that the characteristics of the received signal 14 correspond to the characteristics of the transmitted signal 8 (Fig. 1). For example, if the transmitted acoustic signal is an FMCW signal then the received signal 14 will also be in the form of an FMCW signal.

[0044] A portion of the beam 11 is reflected as a reference beam 11 a from surface 15a, while another portion of a beam 13 propagates through surface 15a of beamsplitter 15.

[0045] Beam 13 may travel through bottom surface 15a of beamsplitter 15 and continue into the space 19. Beam 13 or at least a portion thereof (indicated by reference numeral 13a in FIG. 2) is reflected or otherwise redirected from the top surface 18b of the acoustic material 18, producing a measurement beam 13a.

[0046] Beam splitter 15 combines the reference beam 11a and measurement beam 13a to form an interference pattern on image sensor 22 (it should be appreciated that reference beam 11 a and 13a are overlayed on the return trip to the image sensor; they are illustrated separately in Fig. 1 solely for clarity). The interference pattern contains information about the thickness of the space 19 between surfaces 18b andATTY DOCKET NO. MEDIALAB-085AWO15a, and the changes in that thickness due to vibration caused by acoustic signal 14.

[0047] The modulation of the beam 11 (e.g., optical signal 11 ) emitted by light source 9 enables the vibrational component of the optical interference pattern caused by acoustic signal 14 to be changed (e.g., down-converted) to a frequency which allows capture by the image sensor 22 (e.g. camera) preferably with no loss of information (i.e. the down-converted frequency is below the Nyquist frequency). As one example and solely for purposes of clarity, the interference pattern caused by an acoustic transmit signal having a frequency of about 5 MHz can be optically down-modulated (or down-converted) to a frequency in the range of about 8 Hz to about 12 Hz or to a frequency in the range of about 9 Hz to about 11 Hz, or to a frequency of about 10 Hz thereby allowing an image sensor 22 (e.g., a camera) with a frame rate of about 60 Hz to record the down-converted signal with no loss of information (i.e. the down- converted frequency is below the Nyquist frequency). Those of ordinary skill in the art will appreciate that the concepts, systems, devices and techniques described herein may operate over a wide variety of frequencies and after reading the description provided herein, those one of ordinary skill in the art will appreciate how to select acoustic, optical and down-converted frequencies to suit the needs of a particular application.

[0048] The image sensor 22 records the interference pattern as image data. The image data 23 is sent to a beamforming processor 24, which may beamform using image data in one or more image frames to produce a sonar or ultrasound image, or to recover other acoustically measurable information such as distance and / or angle of arrival.

[0049] In the example embodiment of Fig. 2, the interferometer is substantially of the common-path Fizeau type, where the reference beam 11a and measurement beam 13a travel along substantially the same path (except in space 19), and any perturbations along that path, such as thermal fluctuations or vibrations, are substantially canceled since both beams are perturbed in substantially the same way. This embodiment generally has the advantage of improved signal quality and resistance to noise, compared to versions with split measurement and reference arms (e.g., as in a Michelson interferometer).ATTY DOCKET NO. MEDIALAB-085AWO

[0050] As noted above, in some embodiments, light source 9 may be provided as or comprise a laser. In some embodiments, light source 9 may be provided as or comprise a less-coherent light source comprising a Light Emitting Diode (LED) element. In this embodiment, the space 19 may have a height H selected to accommodate the reduced coherence length of the light source and while allowing for the production of an interference pattern.

[0051] It should be noted that interferometry may be performed with light sources having a coherence length shorter than the thickness of the space 19. This may be accomplished by various means such as by inducing one or more partial reflections of the light beams to create a reference beam with an added path length equal to or similar to the added path length of the measurement beam. In an embodiment, the beam 11 or alternatively the reference beams 11 a, 13a, may be passed through a Fabry-Perot cavity with a cavity gap that is at least a multiple of the optical thickness of space 19.

[0052] In embodiments, the acousto-optical receiver 3 may comprise one or more spaces or air pockets 19, and / or one or more support structures 16 having a size (i.e. dimensions) and shape selected to cover a particular area (e.g. to create a larger optoacoustic receiver) while maintaining the structural integrity of the one or more spaces or air pockets 19. The support structures 16 may be provided as pillars or other structures within the space 19. Support structure 16 may also be provided as a structure orwalls which define a perimeter of the space 19. A particular size and shape of a space or air pocket 19, and the design of the support structures, depends upon a variety of factors including, but not limited to, the acoustic frequency range, the coherence length of the light source, the materials used in the construction of the device, the materials filling the space, the external pressure handling requirements in the case of sonar. The size and / or shape of space 19 may depend in whole or in part upon any one or any combination of at least these factors.

[0053] It should be appreciated that in embodiments in which space 19 is provided as an air pocket, such embodiment have advantages compared with other approaches since the near-zero acoustic impedance of the air (e.g., approximately 413 Rayls) creates a near-zero pressure boundary condition for acoustic surface 18b, resulting inATTY DOCKET NO. MEDIALAB-085AWO a substantially doubled vibration amplitude, which increases the sensitivity of the interferometer. An additional advantage is that such air pocket embodiments have an acoustic bandwidth which is wider and more uniform compared to the bandwidth of conventional piezo-electric transducers (e.g., ideally an infinite bandwidth characteristic, limited in practice by the mechanical design details of the air pocket, such as the distance between the supports 16). As a result, the same opto-acoustic receiver may be used to receive at multiple different frequencies, such as between 100 kHz to 10 MHz, compared to a typical piezo-electric transducer that operates over the range 0.5-1 .5 MHz, for example.

[0054] In some embodiments, the supports 16 may be made of any material and keep the space or air pocket 19 open with a particular gap thickness. In embodiments, the supports 16 may be provided from a piezoelectrically active material. In this case, the supports 16 may serve a dual purpose: (1) the support structures 16 are disposed such that a surface 15a of the beam splitter 15 is a particular distance from a surface of the acoustic material 18; and (2) the support structures 16 may transmit the acoustic excitation waves into the imaging volume (e.g. medium 10) for capture. In embodiments in which the support structures 16 are provided from or comprise a piezoelectrically active material, it may be desirable to use acoustically attenuating materials in the beamsplitter 15, which may prevent acoustic waves from reverberating in the beamsplitter and potentially causing excessive vibration of the reference optical surface 15a.

[0055] In embodiments, beamsplitter 15 material may comprise or consist of glass, or other optically transparent materials including, but not limited to, acrylic, PETG, polycarbonate, polyolefins, or optically transparent urethanes / polyurethanes. The glass material may have superior optical properties and mechanical rigidity with reduced acoustic attenuation. The polycarbonate may be less rigid (compared with glass) but which has an acoustic attenuation characteristic which is higher than that of glass.

[0056] The beamsplitter reflective surface 15a may be provided as a metal coating such as silver or aluminum, or any other metal that may cause the surface to be reflective or at least partially reflective. In embodiments, the metal may be protectedATTY DOCKET NO. MEDIALAB-085AWO using a passivation layer such as silicon dioxide. In embodiments, the beamsplitter coating may comprise or consist of alternating dielectric layers to produce a partial reflective effect. In embodiments, the beamsplitter may include no reflective coating, instead using the dielectric interface (e.g. between the beamsplitter material and air) to produce a relatively weak reflection (typically about 4% for at least some materials).

[0057] In embodiments, acoustic material 18 may be provided as a material having an acoustic impedance similar to that of the acoustic medium 10. For example, when imaging the human body with an acoustic impedance of approximately 1.5 MRayls, the acoustic material may also be close to 1.5 MRayls. This is because an increased difference between the acoustic impedances of the acoustic medium 10 and the acoustic material 18 may produce a larger acoustic reflection coefficient at the interface between the materials. Suitable acoustic materials include, but are not limited to: RTV silicones and other rubbers which may include composite fillers to adjust their acoustic impedance, thermoplastics such as Polymethylpentene which has low acoustic attenuation, thermoset or epoxy materials, and hydrogels which may have encapsulation to prevent them drying out.

[0058] In some embodiments, first surface 18a of the acoustic material 18 may include one or more acoustic matching layers which may modify (e.g, decrease or increase) the acoustic reflection coefficient between acoustic medium 10 and acoustic material 18. The first surface 18a may include a protection layerto mechanically protect the overall device from damage due to impacts, scratches, abrasion, or other means by which damage could occur. By way of example, the protection layer may be provided as an RTV silicone. In other embodiments, second surface 18b may be metallized or otherwise processed to increase its optical reflectivity. The second surface 18b may be provided having any texture, including but not limited to smooth, rough, or structured, leading to substantially mirror-like (specular), diffuse, or diffractive or holographic reflection behavior, respectively.

[0059] It should be appreciated that there is a concomitant relationship between a reflectivity characteristic of the acoustic surface 18b and a required (or desired) level of brightness of a light signal provided by a light source 9 for proper operation of the device. For example, the higher the reflectivity of the acoustic surface 18b, theATTY DOCKET NO. MEDIALAB-085AWO less bright the light source need be. Additionally, a brighter reflected light signal reaching the image sensor 22 concomitantly enables the image sensor to operate with shorter exposure times and higher frame rates. Thus, providing a surface having a higher reflectivity characteristic may lead to an improvement in the power efficiency of the device since the light source may require less power to provide a light signal having an acceptable strength, and an improvement in aspects of the performance of the device since acoustic data can be acquired faster. It should, of course, be appreciated that the systems and devices described herein will sufficiently operate for any non-zero reflectivity. It is noted that many materials (even if they are not mirrored) will reflect about 4% of light incident thereon, which is sufficient for operation of the systems and devices described herein.

[0060] The interferometer may also include retro reflective elements (not shown in the example embodiment of Fig. 2) to enhance the intensity of the light returned to the camera sensor. In one embodiment, a "cats-eye" retroreflector may be used (i.e., a passive optical system with a lens and a mirror that reflects light back to its source). The cats-eye retroreflector has the advantage of only using a single reflection, which tends to better preserve the polarization state of the light compared to corner-cube or prismatic retroreflectors.

[0061] In embodiments, the image sensor 22 may include one or more lenses or lens systems disposed in front of the image sensor 22 to allow focusing one or both of the interferometer surfaces 15a and 18b as an image on the image sensor 22. The one or more lenses may be configured to focus on the surfaces themselves, at another distance to provide a blurred image, or at / near infinity. A lens or lens system providing a magnification function allows the imaging region to be increased or decreased (i.e., made larger or smaller) by "zooming" the camera image. Zooming the image also allows the effective size of the pixels to be changed freely.

[0062] In an example embodiment, one or more beamsplitters are used to implement epi-illumination. In another example, one or more light sources, such as LEDs or laser diodes, may be integrated within the image sensor 22 next to the image sensor pixels. Each light source of the one or more light sources may illuminate a distinct region or spot on the interferometer surface, and one or more image sensorATTY DOCKET NO. MEDIALAB-085AWO pixels may be focused to receive from that distinct region or spot. In another embodiment, the light source can be effectively placed in-line with the image sensor without using a semi-reflective beamsplitter, for example by using an optical fiber or a small mirror to insert the light source without excessively occluding the image sensor’s view of the interferometer surface, or by placing the light source to the side of the image sensor 22 in such a manner that the light reflected from the interferometer surfaces 15a, 18b reflects onto the image sensor face.

[0063] The type of interferogram may also be varied, where each type of interferometer may be sensitive to displacement or vibration of the acoustic surface. In one embodiment, the interferogram may include amplitude or intensity variations that are captured by the image sensor 22. Certain types of interferometers, including amplitude sensing interferometers, may have uneven sensitivity to the acoustic surface vibration (i.e. dead spots), such as at local maxima or minima of the intensity, as well as ambiguities about the direction of motion (i.e. towards or away from the image sensor).

[0064] In another embodiment, the interferometry may be performed using a polarized light source 9, and the image sensor 22 may measure a polarization angle or other polarization characteristics, such as degree of polarization in the interferogram. The phase angle measurement may be measured by several means, including but not limited to placing one or more linear polarizers in front of the image sensor 22, or integrating one or more linear polarizers with the image sensor pixels. By way of example, "polarization cameras" may include linear polarizers integrated into the image sensor 22. The use of polarization allows for every region of the image to maintain a constant brightness while the measurement surface 18b moves and causes the polarization angle to rotate, leading to a constant (or substantially constant) sensitivity. Additionally, the direction of motion of the surface may also be measured.

[0065] In another embodiment, the first surface 18a may be used as the acoustic measurement surface instead of second surface 18b. In this embodiment, the acoustic material 18 may be substantially optically transparent to allow the light source to illuminate surface 18a. In this embodiment, surface 18b may be used as the reference optical surface. The acoustic material 18 may be directly mounted on beamsplitter 15,ATTY DOCKET NO. MEDIALAB-085AWO using it as a substrate. The acoustic material may have a thickness on the order of the acoustic wavelength, which allows the acoustic material to act as a resonator. In such embodiments, the space 19 and related structures 16, as well as beamsplitter surface 15a, may not be required (i.e. they may not exist).

[0066] Fig. 2A, is a top view of an example embodiment of a support structure 16 used to space a surface of an acoustic material from a surface of a beamsplitter. It should be appreciated that in Fig. 2A, beamsplitter 15 has been made transparent to better illustrate the circular (or ring-shaped) support structure 16.

[0067] Fig. 2B, is a top view of another example embodiment of a support structure 16 used to space a surface of an acoustic material from a surface of a beamsplitter. It should be appreciated that in Fig. 2B, beamsplitter 15 has been made transparent to better illustrate the pedestals (or blocks) 16 which form the support structure 16.

[0068] It should thus be appreciated that support structure 16 may be provided as a single structure (e g. as shown in Fig. 2A) or may comprise multiple structures (e.g. as shown in Fig. 2B). It should also be appreciated that support structure 16 may be provided having any size or shape suitable to fit the needs of a particular application.

[0069] Additional non-limiting examples of specific materials and implementations of the system disclosed in Fig. 2 are described below in conjunction with Figs. 3-7.

[0070] FIGs. 3 and 4 are flow diagrams illustrating processing that may be implemented within a system such as the illustrative system described above in conjunction with Figs. 1 and 2.

[0071] Rectangular elements (typified by element 30 in Fig. 3), are herein denoted “processing blocks,” and represent steps or processes performed by one or more acoustic or optical elements, one or more circuits, or one or more processors such as one or more digital signal processing circuits or one or more application specific integrated circuits (ASICs). The flow diagrams do not depict the syntax of any particular programming language, but rather illustrates the functionalATTY DOCKET NO. MEDIALAB-085AWO information one of ordinary skill in the art requires to fabricate circuits or to generate computer software or take whatever actions are necessary to perform the processing (e.g. signal processing) described. It should be noted that many routine program elements, such as initialization of loops and variables and the use of temporary variables are not shown. It will be appreciated by those of ordinary skill in the art that unless otherwise indicated herein, the particular sequence of blocks described is illustrative only and can be varied without departing from the spirit of the concepts, systems, devices and techniques sought to be protected herein. Thus, unless otherwise stated the processing described below is unordered, meaning that, when possible, the functions represented by the blocks can be performed in any convenient or desirable order.

[0072] Turning now to Fig. 3, for ultrasound imaging applications, one method for implementing the concepts described herein is as follows.

[0073] Processing begins in processing block 30 in which a frequency modulated continuous wave (FMCW) acoustic signal is transmitted through an acoustic medium toward an object sought to be imaged (e.g., objects in water, or tissue structures inside human tissue).

[0074] In embodiments, this FMCW signal may be provided as a linear chirp waveform, but other waveforms are possible such as triangular, sawtooth, sinusoidal, or stepped frequency modulations. The reflected or otherwise redirected acoustic waves (e.g. acoustic signals or waves 14 in Figs. 1 and 2) arrive at a surface, causing the surface to vibrate.

[0075] The surface vibrations are detected as shown in processing block 31 and the surface vibration is converted into an optical signal as shown in processing block 32. This may be accomplished, for example, via an optical interferometer. The light source of the interferometer is modulated in a manner that causes the optical signal to be down-converted to a lower frequency. This lower frequency can be made arbitrarily low, even to the point where the vibration can be seen with the naked eye.

[0076] The lower frequency optical signal may be recorded by a camera or image sensor (e.g., image sensor 22 in Fig. 2) as shown in processing block 34. AfterATTY DOCKET NO. MEDIALAB-085AWO recording one or more frames of the optical signal, an ultrasound or sonar image may be reconstructed via a beamforming processing technique as shown in processing block 36. Any suitable beamforming technique may be used including but not limited to: Delay-and-Sum (DAS), Delay-Multiply-and-Sum (DMAS), Frequency domain or FFT beamforming techniques, Plane wave or coherent compounding methods, Minimum Variance (MVDR) and Capon beamformers, Synthetic Aperture methods that combine data across multiple acquisition cycles, nonlinear or super-resolution methods such as MUSIC or ESPRIT, and back-propagation or wave equation solver methods.

[0077] In one embodiment where a linear chirp signal is used, the frequency modulated excitation and demodulation works as follows. A frequency modulated continuous wave (FMCW) acoustic signal (e.g. signal 11 in Figs, 1 and 2 and referred to below as fChirP) is transmitted into an acoustic medium (e.g. acoustic medium 10 in Fig. 1 ). Means of transmitting the signal include, but are not limited to, an electromechanical transducer method (i.e. bulk piezo, PMUT, CMUT), an opto-acoustic or photoacoustic method (i.e. using a laser to create a thermo-acoustic excitation), or any other acoustic excitation method. Equation 1 below describes the linear chirp:Equation 1

[0078] where the variables are the initial (or starting) phase angle (4); ), the initial (or starting) frequency (0), t is time and the chirp ramp rate (df / dt The acoustic excitation signal (chi,.p), is transmitted into the acoustic medium; then it reflects (e.g., signal 14 in Figs. 1 and 2) or is otherwise redirected back to the surface or region (e.g. surface 10a of imaging volume 10 in Figs. 1 and 2) being optically interrogated, which may be the imaging volume's surface (e.g., wherein surface 10a may correspond to the skin in the case of on-body medical applications) or part of the ultrasound or sonar sensor (e.g., surface 18a in Fig. 2). The signal arriving at the optical sensing region is thus the time-delayed version, as illustrated by Equation 2 below:ATTY DOCKET NO. MEDIALAB-085AWO flix ) = fchirp t - t)Equation 2

[0079] in which t is time and At is the round-trip time delay of the acoustic signal. The surface being optically interrogated then oscillates or vibrates according to the fRXsignal.

[0080] To demodulate the optical signal to a lower frequency suitable for the camera or image sensor, the optical signal acquired by the image sensor may be modulated. By way of example, the optical signal may be modulated by modulating the light source (e.g., light source 9 in Fig. 2), by interrupting or diverting the light before it reaches the image sensor, or by modulating or gating the image sensor sensitivity. The optical signal acquired by the image sensor is described below in Equation 3:Equation 3

[0081] where A(t) represents the modulation function. Due to the modulation function A(t) being multiplied with the acoustic signal fRX(t), the resulting signal fIF(t) captured by the image sensor has a frequency domain spectrum that is the convolution of the frequency domain spectra of the input signals A(t) and fRX(.t). The frequency spectrum of fIF(t) may thus contain one or more signals at lower frequencies than either of the input signals A(t) or fRX(t).

[0082] When using a linear chirp, the light source may be modulated with the fchirp ( signal, or a modified version of it; for example, square waves can be used, and sometimes a frequency offset might be applied to fchirpto shift the received spectrum of fIF. This produces a lower frequency optical signal as described below in Equation 4:ATTY DOCKET NO. MEDIALAB-085AWO

[0083] This fIFsignal has the property of translating the echo time delay (At) into a proportional frequency (— dtAt) determined by the chirp ramp rate. By setting the chirp ramp rate to a sufficiently low value, the fIFsignal can be reduced to a frequency capturable by a camera or image sensor, or even by the human eye.

[0084] For actual ultrasound imaging, the fchirpsignal sweeps the desired acoustic bandwidth (i.e. from an initial frequency foto a final frequencywhile the camera or image sensor detects or records one or more frames (i.e. , video) of the fIFinterferogram image. Each pixel in the image may include information about the vibration at that point.

[0085] After capturing the fIFsignal, the signal may be transformed back into the equivalent pulse-echo ultrasound signal by, for example, performing matched filtering on the signals captured by each pixel. The actual ultrasound image can then be reconstructed by beamforming according to well known algorithms such as delay-and- sum (DAS), or others. Such processing may be performed in a beam forming processor 24 which may be provided as part of processor 4 (Fig. 1) or which may be a processor separate from processor 4. In either case, the output of the beamforming processor (e.g., an ultrasound image) is provided to an output interface (not explicitly shown in Fig. 2).

[0086] This technique of creating an acoustic receiver array using the camera or image sensor 22 may be used in a wide variety of applications. For example, it can be used to replace the piezo array inside a handheld ultrasound probe, allowing higher resolution imaging with an expanded field of view. It can also be used in a non-contact laser ultrasound application, where the movement of the skin surface is measured from a distance. In that case, either electromechanical or optoacoustic excitations could be used. In another example, the technique could be used to detect the movement of particles inside a volume instead of on a surface. In another example, the technique could be used for sonar, permitting the construction of very large area and high- resolution sonar arrays. In another example, the optical receiving technique could be used to image and record the vibrations of structures such as car bodies, bridges, and buildings, which might use the optical downconversion method to scan or otherwise acquire the vibrational spectrum without any transmitted acoustic excitation.ATTY DOCKET NO. MEDIALAB-085AWO

[0087] Referring now to Fig. 4, another method for implementing the concepts described herein for ultrasound imaging applications begins in processing blocks 40 and 42 in which a pulsed acoustic signal is transmitted toward an object sought to be imaged.

[0088] Although the pulsed acoustic signal is preferably encoded to improve the signal to noise ratio (SNR), this is not a requirement (i.e. encoding is optional). Suitable encoding techniques include, but are not limited to: pulsed signals, amplitude modulation, phase modulation, or frequency modulation of pulse trains, fast chirps, Golay waveforms, random or pseudorandom waveforms, or any other transmit excitation.

[0089] The reflected or otherwise redirected acoustic waves arrive at the surface of the volume, causing the surface to vibrate. As shown in processing block 44, the surface vibrations are converted into an optical signal. This may be accomplished via an optical interferometer. In one embodiment, a light source of the interferometer may be modulated with the same (or similar) signal as the acoustic excitation, which effectively performs a cross-correlation on the received acoustic signal and generates an optical signal at a lower frequency. This can alternatively be thought of as an all- optical matched filtering operation as shown in processing block 45.

[0090] Performing this cross-correlation with the light source signal having time delays of different lengths relative to the acoustic signal allows the magnitude and phase of the reflected acoustic signal to be measured for those different time delays. If this is performed across a suitable range of time delays, the acoustic echo signal may be recovered in its entirety.

[0091] In another embodiment, the acoustic excitation signal is a pulse having a center frequency and a bandwidth. For example, the pulse may be a sine wave one or more periods in length, or an exponentially decaying sine wave, a wavelet such as a Morlet, a chirp, or a chirplet.

[0092] In such a case, the light source may be modulated with the center frequency of the pulse (i.e. with the carrier wave frequency, not a reproduction of the pulse itself), at a frequency offset from the center frequency, or with any other frequency. ThisATTY DOCKET NO. MEDIALAB-085AWO modulation causes the optical signal to transfer some information about the acoustic signal to a lower frequency. For example, if the acoustic excitation were a pulse with center frequency 5 MHz and bandwidth 2 MHz, modulating the light source at 5 MHz would produce an optical signal containing acoustic information centered (or substantially centered) at 0 Hz (or DC) and spanning, but not limited to, between -1 MHz to 1 MHz.

[0093] In some embodiments, it may be useful to modulate the light source at approximately the center frequency, but change the frequency over time. This may allow for compensation of effects such as dispersion and frequency-dependent attenuation that are common in many acoustic mediums. Such effects may cause the effective center frequency and bandwidth distribution characteristics of the received acoustic signal to vary depending on parameters such as the distance traveled by the acoustic echo signal. Thus, varying the frequency (or other characteristics) of the modulation of the light source can serve to compensate for such effects and optimize the received signal quality.

[0094] As shown in processing block 46, the lower frequency optical signal may then be recorded by a camera or image sensor. After recording one or more frames of the optical signal, as shown in processing block 48, an ultrasound image may be reconstructed from data in the one or more frames according to any well-known beamforming techniques.

[0095] Referring now to Fig. 5, shown is an apparatus for recording ultrasonic signals in acoustic medium 10 causing vibrations of a coupled surface 10a using a camera and a Fizeau interferometer (similar to that shown in Fig. 2) with a diverging light source 9. In this embodiment, light from the light source is inserted into the imaging path by epi-illumination using beamsplitter 50. The light is then substantially collimated using lens 52 and interacts with the interferometer comprising beamsplitter 15 with reference optical surface 15a. Beamsplitter 15 may be spaced from the measurement surface 10a by support structures 16. The back reflected measurement and reference beams may pass again through the lens 52, and propagate through beamsplitter 50, and pass through lens 54, which may eitherATTY DOCKET NO. MEDIALAB-085AWO collimate the beams, or alternatively cause the beams to be focused onto the camera or image sensor 56. An output from the image sensor 56 is provided to a processor 58. If the beams are focused onto the image sensor 56, they may be focused so as to form an image of the surfaces 15a and / or 10a, or focused in any other way. One or more polarizing elements such as linear polarizers may be included, which may enable the camera or image sensor 56 to detect aspects of the polarization of the light.

[0096] In embodiments, remote-sensing Michelson interferometers may be used as shown in Figs. 6 and 7. In such embodiments, the interferometer has separate measurement and reference arms. It should be appreciated that these embodiments are advantageous for receiving the acoustic signals from a remote acoustic measurement surface, whether it be reflective or diffusely scattering, and may be used to achieve non-contact or remotely sensed acoustic measurements or imaging (e.g. ultrasound imaging, sonar imaging, or nondestructive testing). For example, the human skin may be used as an acoustic measurement surface, enabling a non-contact medical ultrasound probe or scanner.

[0097] Fig. 6 illustrates a system for recording ultrasonic signals in acoustic medium 10 causing vibrations of a coupled surface 10a using a camera and a non-polarizing Michelson interferometer. In Fig. 6, the non-contact measurement is performed using a Michelson interferometer and a modulated light source 9 collimated by a lens 60 and directed into a non-polarizing beam splitter 62, which splits the light into a measurement beam 63 and a reference beam 65. At least portions of the measurement beam 63 may be directed towards the acoustic measurement surface 10a, while at least portions of the reference beam 65 may be directed towards a mirror 64. Each beam is reflected / scattered, and returns to the beam splitter 62 which combines the beams. The combined beam then passes through a focusing system (which may comprise one or more lenses 66, 68) which projects the interfering beam onto an image sensor 70. An output from the image sensor 70 is provided to a processor 72 where a beamforming process may be performed.

[0098] Fig. 7 illustrates a system for imaging with a polarizing MichelsonATTY DOCKET NO. MEDIALAB-085AWO interferometer. Imaging is performed by detecting the polarization angle of the light captured by the camera sensor. In this embodiment, a polarizing beamsplitter 62 is used to split the modulated light source into two orthogonally polarized beams 63, 65 by transmitting one polarization (e.g. the p-polarization), while reflecting the other polarization (e.g. the s-polarization). Each beam then passes through respective quarter wave plates 74, 78. The measurement beam 63 reflects / scatters from the acoustic measurement surface 10a, and the reference beam 65 reflects from the reference mirror surface 64. Each beam may return and pass through the quarter wave plates 74, 78 again before reaching the beamsplitter 62. Passing through the quarter wave plates 74, 78 twice causes the polarization angle of each beam to be rotated by approximately 90 degrees relative to its original orientation (i.e. p-polarization is changed to s-polarization, and the s-polarization is changed to p-polarization). As a result, the measurement beam 63 and the reference beam 65 now have the opposite behavior with respect to the polarizing beamsplitter 62 (i.e. the transmitted beam is now reflected, and the reflected beam is now transmitted). As a result, the beamsplitter recombines the beams and directs them towards the lenses 66, 68 (or more generally a focusing system) and image sensor 70. The beams pass through the one or more lenses (or focusing systems), which project the interfering beams onto one or more image sensors. The one or more image sensors may be designed to detect the polarization angle (in addition to brightness and any other image information), which may be accomplished through any means known to those skilled in the art. An output from image sensor 70 is provided to a processor 72.

[0099] In one embodiment, the image sensor 70 may include a grid or other arrangement of polarizing filters, such as linear polarizing filters, which may lead to specific pixels on the image sensor 70 to be sensitive to specific polarization angles. The information from several pixels may combine to form a single meta-pixel that encodes both brightness and polarization information, which may include characteristics such as polarization angle and degree of polarization. This may be repeated across the whole image to yield an image with both brightness and polarization angle information. In another embodiment, further polarizingATTY DOCKET NO. MEDIALAB-085AWO beamsplitters 62 may split the light according to polarization angle, sending it to multiple image sensors 70. The information from the multiple image sensors 70 may combine to form an image with both brightness and polarization information.

[0100] In both the non-polarizing and polarizing versions of the Michelson interferometer, each Michelson interferometer type may use any form of unpolarized or polarized light, comprising linear, elliptical, or circularly polarized light. Additionally, in both the non-polarizing and polarizing versions of the Michelson interferometer, additional components comprising polarizers or neutral density filters may allow for the adjustment of the relative brightness of the measurement and reference arms of the interferometer.

[0101] In another embodiment, either or both of the measurement surface and the reference surface may have retro reflective elements embedded or otherwise attached to it, to increase the intensity of light returned to the interferometer system. Such retro reflective elements may include corner-cube or prismatic retroreflectors, or in another embodiment, cats-eye retroreflectors since they better preserve the polarization state of reflected light.

[0102] It should be understood that any technique of modulating the optical signal ultimately recorded by the camera sensor may be used, and these techniques are all included in the definition of “modulated light source”.

[0103] In some embodiments, the light source is modulated using amplitude modulation. This may occur by modulating the brightness of the light source itself, or inserting an element into the light path that either interrupts or diverts the path of the light.

[0104] In another embodiment, the polarization of the light is modulated.

[0105] In another embodiment, the wavelength of the light is modulated.

[0106] In another embodiment, the reflectivity or transmissivity of an optical element in the optical path is modulated.

[0107] In another embodiment, the absorption of an optical element in the optical path is modulated, such as in an electro-absorption modulator.ATTY DOCKET NO. MEDIALAB-085AWO

[0108] In another embodiment, the angle of a mirror or other element in the optical path is modulated.

[0109] In another embodiment, the light source is modulated by placing a polarization-altering device in the optical path, such as a liquid crystal device or a Faraday rotator device.

[0110] In another embodiment, the light source is modulated by an acousto- optical modulator.

[0111] In another embodiment, a birefringent element in the optical path is modulated.

[0112] In another embodiment, the sensitivity of the image sensor is modulated.

[0113] In another embodiment, the modulation is achieved by time gating of the image sensor.

[0114] Many other implementations of remote sensing interferometers are possible. In one embodiment, the camera may be focused at the acoustic surface. This is a preferable approach when working with diffusely reflecting measurement surfaces, such as human skin or unpolished materials, and is more robust to bending and distortion of the measurement surface that occurs during use of the device.

[0115] In another example embodiment, a system for recording ultrasonic vibrations of an acoustically coupled surface uses a camera and Fizeau interferometer with a circularly polarized light source. In one implementation of the interferometer, the light source is a laser with circularly polarized light. The circularly polarized light may travel through the interferometer as discussed above The camera sensor is then capable of measuring the polarization angle of the light falling on its surface.

[0116] By using circularly polarized light, the amplitude modulation of the interferogram is converted into a corresponding polarization angle modulation. In other words, the interferogram is constant brightness across all pixels. Instead, the polarization angle rotates in a circle, having no maxima or minima. In signalATTY DOCKET NO. MEDIALAB-085AWO processing terms, the polarized light is like a complex-valued waveform, where the complex-value waveform contains its phase information and thus does not have phase unwrapping ambiguity.

[0117] Using polarized light (coherent or non-coherent), while not required, will usually result in improved performance of the device.

[0118] In the above embodiments, the measurement surface and reference surface are approximately described as moving and stationary, respectively. It should be noted that the interferometer measures the change in distance, or the change in the gap, between the measurement and reference surfaces. As such, the reference surfaces may move as much as, or even more than, the measurement surface.

[0119] It should be noted that the camera or image sensor may have any arrangement of pixels. That is, the pixels might be arranged in a line, or in a foveal array (i.e. with a greater density of pixels at the center), or in a sparse array, or in any other arrangement. Furthermore, the lenses and other elements focusing the image on the image sensor may focus the image in a non-standard fashion. For example, lenses or other elements may focus the image in a warped manner to achieve a higher effective density of pixels in certain regions, or to achieve an apodization of the pixel density. This type of approach may be used to allow a standard 2D grid image sensor to behave like a foveal array, among other arrangements.

[0120] The above description refers to imaging on surfaces that are generally on the surface of a particular volume (e.g. , a medium), such as a surface of an imaging device (as in Fig. 2), or on the surface of another body or object, such as human skin. However, it should also be appreciated that alternative embodiments are possible that allow imaging on surfaces or particles that are inside a particular volume, which may include the acoustic medium. For example, a sheet of plastic may be submerged within a body of water, and then used as a measurement surface in accordance with the methods described above. In another embodiment, a measurement surface may be encased within an optically clear (or substantially optically clear)ATTY DOCKET NO. MEDIALAB-085AWO material. In yet another embodiment, particles may include colloidal particles or larger floating particles that may reflect or scatter light, allowing them to act as a non-solid measurement surface from an optical perspective. The important point is that the measurement surface is a physical means of reflecting or scattering light back to the image sensor, that is also coupled to the acoustic medium.

[0121] In another embodiment, the acoustic medium and / or the acoustic medium surface may have one or more magnets coupled to the measurement surface. The magnetic field may couple the acoustic signal to a magnetic measurement surface, vibrating the measurement surface without direct physical contact. Other means of achieving non-mechanical couplings may include, but is not limited to, electric fields, electromagnetic waves, forces due to eddy currents, pressure waves through air, and / or thermal couplings. In short, any non-mechanical coupling method or technique may be used.

[0122] In another embodiment, the reference mirrors may be replaced with diffusely scattering materials. In another embodiment of the Michelson interferometer, the reference mirror may be omitted. The light source may be coupled to an optical fiber or other light guiding means including, but not limited any device for steering the path of light such as a light guide or light pipe (e.g., an optically a transparent piece of plastic). The guided light is then used to illuminate the camera or image sensor to provide the reference optical signal. Any means of redirecting part of the light from the light source to serve as a reference signal will work for this purpose.

[0123] In another embodiment, the light source and camera may be controlled to achieve effective undersampling of the acoustic signal. For example, the light source may be left unmodulated (i.e. left on), while the camera is set for a sufficiently short exposure time. In one embodiment, the camera exposure time is set to less than ! of the acoustic period to avoid averaging the optical signal. The camera may acquire one or more frames at a rate below the Nyquist rate for the acoustic signal. The acoustic signalATTY DOCKET NO. MEDIALAB-085AWO may appear aliased when captured by the camera or image sensor, since the signal may exceed the Nyquist frequency of the camera. However, since the excitation signal frequency range is known, it is possible to reconstruct the acoustic signal correctly despite sampling at below the Nyquist rate. Numerous implementations of undersampling are possible by modulating and configuring different aspects of the device in various ways, for example by modulating the light source with a short pulse instead of setting the above short exposure time on the camera.

[0124] Various embodiments of the concepts, systems, devices, structures and techniques sought to be protected are described. It should, however, be appreciated that alternative embodiments can be devised without departing from the scope of the concepts, systems, devices, structures and techniques described herein. It is noted that various connections and positional relationships (e.g., over, below, adjacent, etc.) are set forth between elements in the following description and in the drawings. These connections and / or positional relationships, unless specified otherwise, can be direct or indirect, and the described concepts, systems, devices, structures and techniques are not intended to be limiting in this respect. Accordingly, a coupling of entities can refer to either a direct or an indirect coupling, and a positional relationship between entities can be a direct or indirect positional relationship.

[0125] As an example of an indirect positional relationship, references in the present description to providing, placing or otherwise disposing or arranging element (or structure or layer) "A" over element (or structure or layer) "B" include situations in which one or more intermediate elements or layers (e.g., element, structure or layer "C") is between element or layer "A" and element or layer "B" as long as the relevant characteristics and functionalities of element or layer "A" and element or layer "B" are not substantially changed by the intermediate layer(s).

[0126] The following definitions and abbreviations are to be used for the interpretation of the claims and the specification. As used herein, the terms "comprises," "comprising, "includes," "including," "has," "having," "contains" or "containing," or any other variation thereof, are intended to cover a non-exclusiveATTY DOCKET NO. MEDIALAB-085AWO inclusion. For example, a method, or apparatus that comprises a list of elements is not necessarily limited to only those elements but can include other elements not expressly listed or inherent to such method, or apparatus.

[0127] Additionally, the term "exemplary" is used herein to mean "serving as an example, instance, or illustration. Any embodiment or design described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other embodiments or designs.

[0128] The term "one or more" is understood to include any integer number greater than or equal to one, i.e. one, two, three, four, etc. The terms "a plurality" and two or more are understood to include any integer number greater than or equal to two, i.e. two, three, four, five, etc.

[0129] References in the specification to "one embodiment, "an embodiment," "an example embodiment," etc., indicate that the embodiment described can include a particular feature, structure, or characteristic, but every embodiment can include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to affect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described.

[0130] For purposes of the description hereinafter, the terms "upper," "lower," "right," "left," "vertical," "horizontal, "top," "bottom," and derivatives thereof shall relate to the described structures and methods, as oriented in the drawing figures. The terms "overlying," "atop," "on top, "positioned on" or "positioned atop" mean that a first element, such as a first structure, is present on a second element, such as a second structure, where intervening elements such as an interface structure can be present between the first element and the second element.

[0131] The term "connection" can include an indirect "connection" and a direct "connection". The term "direct contact" means that a first element, such as a first structure or layer, and a second element, such as a second structure or layer, areATTY DOCKET NO. MEDIALAB-085AWO connected without any intermediary elements or layers at the interface of the two elements or layers.

[0132] Use of ordinal terms such as “first,” “second,” “third,” etc., in the claims to modify a claim element does not by itself connote any priority, precedence, or order of one claim element over another or the temporal order in which acts of a method are performed, but are used merely as labels to distinguish one claim element having a certain name from another element having a same name (but for use of the ordinal term) to distinguish the claim elements.

[0133] The terms “approximately” and “about” may be used to mean within ±20% of a target value in some embodiments, within ±10% of a target value in some embodiments, within ±5% of a target value in some embodiments, and yet within ±2% of a target value in some embodiments. The terms “approximately” and “about” may include the target value.

[0134] The term “substantially” may be used to refer to values that are within ±20% of a comparative measure in some embodiments, within ±10% in some embodiments, within ±5% in some embodiments, and yet within ±2% in some embodiments. For example, a first direction that is “substantially” perpendicular to a second direction may refer to a first direction that is within ±20% of making a 90° angle with the second direction in some embodiments, within ±10% of making a 90° angle with the second direction in some embodiments, within ±5% of making a 90° angle with the second direction in some embodiments, and yet within ±2% of making a 90° angle with the second direction in some embodiments.

[0135] The term “substantially equal” may be used to refer to values that are within ±20% of one another in some embodiments, within ±10% of one another in some embodiments, within ±5% of one another in some embodiments, and yet within ±2% of one another in some embodiments.

[0136] It is to be understood that the disclosed subject matter is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The disclosed subject matter is capable of other embodiments and of being practiced and carriedATTY DOCKET NO. MEDIALAB-085AWO out in various ways. Also, it is to be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting. As such, those skilled in the art will appreciate that the concepts, upon which this disclosure is based, may readily be utilized as a basis for the designing of other structures, methods, and systems for carrying out the several purposes / functions of the disclosed subject matter. Therefore, the claims should be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the disclosed subject matter.

[0137] Although the disclosed subject matter has been described and illustrated in the foregoing exemplary embodiments, it is understood that the present disclosure has been made only by way of example, and that numerous changes in the details of implementation of the disclosed subject matter may be made without departing from the spirit and scope of the disclosed subject matter.

Claims

ATTY DOCKET NO. MEDIALAB-085AWOWhat is claimed is:1 . A system comprising an acoustic receiver comprising:(a) an acoustic material having a first surface and a second surface the acoustic material configured such that an acoustic signal 14 incident on first surface, propagates through acoustic material the second surface thereby causing the second surface of the acoustic material to vibrate;(b) a beamsplitter having a first surface spaced apart from the second surface of the acoustic material to define a space between the second surface of the acoustic material and the first surface of the beam splitter;(c) a light source configured to emit a modulated beam of light toward the beamsplitter wherein at least a portion of the light from the light source passes through beamsplitter and is reflected from the second surface of the acoustic material to form a measurement beam which propagates back through the beam splitter and wherein at least a portion of the light from the light source reflects off a surface of the beam splitter to form a reference beam and wherein the beam splitter combines the reference beam and measurement beam to form an interference pattern; and(d) an image sensor disposed to record the interference pattern wherein the modulation of the beam emitted by the light source is selected to enable the vibrational component of the optical interference pattern caused by the acoustic signal to be down-converted to a frequency which allows capture by the image sensor.

2. The system of claim 1 further comprising an acoustic transmitter configured to produce at least one of: a frequency modulated continuous wave (FMCW) acoustic signal; and a pulsed acoustic signal.ATTY DOCKET NO. MEDIALAB-085AWO3. The system of claim 1 further comprising one or more support structures disposed between the second surface of the acoustic material and the first surface of the beamsplitter.

4. The system of claim 1 wherein the support structures are provided from a piezoelectrically active material and are configured to transmit one or more acoustic excitation waves into on object or volume to be imaged.

5. A method comprising a. transmitting an acoustic signal into an acoustic medium, wherein reflected / scattered acoustic waves cause vibrations of a surface; b. converting the surface vibrations into an optical signal using an optical interferometer; c. down-converting the optical signal to produce a down-converted optical signal having frequency which is low enough to allow recording by an image sensor; and d. recording image data of the down-converted optical signal using an image sensor.

6. The method of claim 5 further comprising forming an image from the image data.

7. The method of claim 5, wherein forming an image from the image data comprises forming an image using a beamforming technique.

8. The method of claim 5, wherein forming an image from the image data comprises using beamforming technique to form one or more ultrasound images.

9. The method of claim 5, wherein transmitting an acoustic signal comprises transmitting one of: a frequency modulated continuous wave (FMCW) acousticATTY DOCKET NO. MEDIALAB-085AWO signal; and a pulsed acoustic signal.

10. The method of claim 5, wherein transmitting an acoustic signal into an acoustic medium comprises transmitting an acoustic signal into a portion of a human body and wherein the surface vibrations are measured on human skin.

11. A method for ultrasound imaging an object, the method comprising:(a) transmitting an acoustic signal toward the object to be imaged wherein the FMCW acoustic signal causes a surface of the object to vibrate;(b) converting the surface vibration into an optical signal;(c) down-converting the optical signal to a frequency which is lower than a frequency of the surface vibration;(d) recording the lower frequency optical signal with an image sensor having multiple pixels;(e) reconstructing an ultrasound image from the recorded signals.

12. The method of claim 11 wherein the acoustic signal is a frequency modulated continuous wave (FMCW) acoustic signal.

13. The method of claim 12 wherein the acoustic signal is a frequency modulated continuous wave (FMCW) acoustic signal is provided as one of: a linear chirp; a triangular chirp; or a sinusoidal chirp.

14. The method of claim 11 wherein converting the surface vibration into an optical signal comprises receiving the surface vibration in an optical interferometer configured to convert the surface vibration into an optical signal.

15. The method of claim 11 wherein the down-converted frequency can be made an arbitrarily low frequency.ATTY DOCKET NO. MEDIALAB-085AWO16. The method of claim 11 wherein the image sensor is a camera and recording the lower frequency optical signal comprises recording the lower frequency optical signal with the camera.

17. The method of claim 11 wherein: recording the lower frequency optical signal with an imaging device having multiple pixels comprises recording one or more frames of image data; and reconstructing the ultrasound image comprises reconstructing the ultrasound image from the image data.

18. The method of claim 11 wherein reconstructing the ultrasound image comprises reconstructing the ultrasound image using a beamforming method.

19. The method of claim 11 wherein reconstructing the ultrasound image comprises reconstructing the ultrasound image using a delay-and-sum (DAS) beamforming method.

20. A method for ultrasound imaging a volume, the method comprising:(a) transmitting a pulsed acoustic signal into the volume to be imaged;(b) the reflected / scattered acoustic waves arrive at the surface of the volume, causing the surface to vibrate. An optical interferometer is used to convert the surface vibration into an optical signal. The light source of the interferometer is modulated with the same (or similar) pattern as the acoustic excitation, which effectively performs a cross-correlation on the received acoustic signal and generates an optical signal at a lower frequency. This can alternatively be thought of as an all-optical matched filtering operation.(c) the lower frequency optical signal is recorded by a camera.(d) after recording one or more frames of the optical signal, the ultrasound image is reconstructed according to well-known beamforming algorithms such as Delay-and-Sum (DAS).ATTY DOCKET NO. MEDIALAB-085AWO21 . The method of claim 20 further comprising encoding the acoustic signal and transmitting the encoded pulsed acoustic signal into the volume to be imaged.

22. The method of claim 21 wherein encoding the acoustic signal comprises one of:(a) encoding the acoustic signal by amplitude-modulating the acoustic signal;(b) encoding the acoustic signal by phase-modulating the acoustic signal;(c) encoding the acoustic signal by frequency modulated pulse trains;(d) encoding the acoustic signal by fast chirps; and(e) encoding the acoustic signal by Golay waveforms.

23. A process comprising a. transmitting a frequency modulated continuous wave (FMCW) acoustic signal into a solid volume, wherein reflected / scattered acoustic waves cause surface vibrations; b. converting surface vibrations into an optical signal using an optical interferometer, wherein the optical signal comprises a lower frequency than the surface vibrations; c. recording (detecting) the optical signal as an ultrasound image using a camera (image sensor); d. using an algorithm to beamform the ultrasound images.

24. The process of claim 23, wherein the camera comprises an optical receiver array.

25. The process of claim 24, wherein the optical receiver array comprises >106elements (sensor pixels).

26. The process of claim 25, wherein the ultrasound image comprises 1 or more pixels and <10 urn.ATTY DOCKET NO. MEDIALAB-085AWO27. The process of claim 23, wherein the surface vibrations are measured on human skin or particles floating in a liquid volume.

28. The process of claim 23, wherein the optical interferometer comprises an air pocket Fizeau interferometer.

29. The process of claim 23, wherein the optical interferometer comprises integrated piezoelectric transducers and plastic backing substrates.

30. A device comprising a. means for producing a frequency modulated continuous wave (FMCW) acoustic signal to be transmitted into a solid volume; b. an optical interferometer for detecting surface vibrations, wherein the optical interferometer comprises a light source for irradiating a surface of the solid volume and producing an optical signal; and c. an image sensor for recording the optical signal.

31. The device of claim 30, wherein the image sensor comprises an optical receiver array.

32. The device of claim 30, wherein the optical signal comprises a multiplicity of pixels.

33. The device of claim 30, further comprising a computer configured to process the optical signal into an image via a beamforming technique.

34. The device of claim 30, wherein the image sensor comprises frame-rate sensors have a frame rate of about fifty thousand frames per second (FPS) or greater.

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