Patent Publication Number: US-2022228980-A1

Title: Common axis for optical and acoustic signals

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to pending U.S. non-provisional patent application Ser. No. 16/912,450 filed Jun. 25, 2020, which claims priority to U.S. provisional patent application No. 62/867,163 filed Jun. 26, 2019. application Ser. Nos. 16/912,450 and 62/867,163 are incorporated by reference herein. 
    
    
     BACKGROUND INFORMATION 
     Imaging devices are used in contexts such as healthcare, navigation, and security, among others. Imaging systems often measure radio waves or light waves to facilitate imaging. Imaging that measures light scattered by an object is especially challenging and advances to the devices, systems, and methods to improve optical imaging are sought to increase speed, increase resolution, improve accuracy, reduce size and/or reduce cost. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Non-limiting and non-exhaustive embodiments of the invention are described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various views unless otherwise specified. 
         FIGS. 1A-1B  illustrate an example apparatus that includes a glass element, index matching fluid, an enclosure, an ultrasound emitter, and an illumination source, in accordance with an embodiment of the disclosure. 
         FIG. 2  illustrates an example apparatus that includes an image sensor in addition to the elements disclosed with respect to  FIGS. 1A-1B , in accordance with an embodiment of the disclosure. 
         FIG. 3  illustrates an example apparatus that includes a coated glass element and a weak reference beam illumination source in addition to the elements disclosed with respect to  FIGS. 1A-2 , in accordance with an embodiment of the disclosure. 
         FIG. 4  illustrates an example apparatus that includes a polarizing beam splitting prism and a polarizer in addition to the elements disclosed with respect to  FIGS. 1A-3 , in accordance with an embodiment of the disclosure. 
         FIG. 5  illustrates an example flow chart of a process of capturing images of light scattered from a target within a diffuse medium, in accordance with an embodiment of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of an apparatus, system, and method for directing optical and acoustic signals are described herein. In the following description, numerous specific details are set forth to provide a thorough understanding of the embodiments. One skilled in the relevant art will recognize, however, that the techniques described herein can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring certain aspects. 
     This disclosure will generally describe imaging a diffuse medium in the context of human tissue, however, the content of this disclosure may be applied to medical imaging (human or veterinary), agricultural, navigation, security, scientific research, industrial, or other contexts that image diffuse mediums or objects. 
     Human tissue is translucent to infrared light and to at least some wavelengths of visible light, although different parts of the human body (e.g. skin, blood, bone) exhibit different absorption coefficients. Researchers have attempted to use the properties of infrared light for medical imaging purposes, but size and cost constraints have been prohibitive for wide-scale adoption. Illuminating tissue and other diffuse mediums with visible light and near-infrared light for imaging purposes is sometimes referred to as Diffuse Optical Tomography. In one Diffuse Optical Tomography technique, time-of-flight (TOF) imaging can theoretically be employed by measuring the time it takes for “ballistic” photons (those photons that are not scattered) to pass through tissue. Since the ballistic photons reach the sensor the fastest, they are the least scattered (have the shortest optical path) and thus some conclusion can be drawn to create an image of the tissue that is illuminated by infrared light. However, TOF imaging generally requires specialty hardware (e.g. picosecond pulsed lasers and single photon detectors) to facilitate ultrafast shutters on sensors that are able to image at the speed of light and the systems are overall very expensive and bulky. TOF imaging also requires an input of approximately 10-100 fold (or more) light intensity into the body than is received at the detector. Thus, efficacy and power limitations as well as safety limits on input intensity limit TOF imaging resolution, imaging depth and utility. 
     In contrast to TOF imaging, some embodiments of the disclosure may illuminate a diffuse medium (e.g. tissue) with an infrared illumination light. An infrared image of an interference of an infrared reference beam and an infrared exit signal may be captured while the infrared illumination light is illuminating the diffuse medium. The holographic infrared image may be captured by an image sensor, for example. The infrared exit signal is a portion of the infrared illumination light that exits the diffuse medium. 
     Holographic infrared images may be captured while an ultrasonic signal is directed to (or focused to) particular voxels in a diffuse medium such as tissue. The ultrasonic signal focused to the voxel will wavelength-shift a portion of the infrared illumination light that is propagating through the particular voxel to have an infrared wavelength that is slightly different than the narrow-band infrared wavelength of the infrared illumination light. An image sensor may capture an interference pattern of the wavelength-shifted light interfering with an infrared reference beam that has a same wavelength as the wavelength-shifted light. Applicant has utilized techniques that include directing an ultrasonic signal to a particular voxel and then capturing the interference of the wavelength-shifted portion of the infrared illumination light from that pixel. In other words, the amount of wavelength-shifted light for a particular voxel is measured. In that technique, the wavelength-shifted light and the infrared reference beam are the same wavelength, but the wavelength-shifted infrared light is a different wavelength than the infrared illumination light. 
     Such implementations may use an ultrasound emitter for providing the ultrasonic signal directed through the diffuse tissue to the particular voxel, an illumination source for providing the infrared illumination light directed through the diffuse tissue to the particular voxel, and an image sensor for capturing holographic infrared images of the interference of the infrared reference beam and the infrared exit signal scattered by the particular voxel. The foregoing implicates as many as three separate emissions into and/or out of the diffuse tissue: (1) the ultrasonic signal to the particular voxel, (2) the infrared illumination light to the particular voxel, and (3) the infrared exit signal scattered from the particular voxel. Applicant discloses herein systems, apparatuses, and methods which provide for an interface of an apparatus with diffuse tissue at a single location and facilitate ingress and egress of the foregoing three separate emissions through that single interface location. These embodiments and others will be described in more detail with references to  FIGS. 1A-5 . 
     Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. 
     Throughout this specification, several terms of art are used. These terms are to take on their ordinary meaning in the art from which they come, unless specifically defined herein or the context of their use would clearly suggest otherwise. For the purposes of the disclosure, visible light has a wavelength from approximately 400 nm to 700 nm and infrared light has a wavelength from approximately 700 nm to 1 mm. Near-infrared light has a wavelength from approximately 700 nm to 1400 nm. 
       FIGS. 1A-1B  illustrate an example apparatus that includes a glass element, index matching fluid, an enclosure, an ultrasound emitter, and an illumination source, in accordance with an embodiment of the disclosure. Apparatus  100  includes the glass element  102  or  114  being immersed in index matching fluid  104  that is contained by enclosure  106 . In  FIG. 1A , the glass element is depicted as a glass slab  102 . In  FIG. 1B , the glass element is depicted as a glass prism  114 . (Other optical structures may be used in place of the glass element; depictions and discussion of the glass slab or glass prism as the glass element herein are exemplary only.) The glass element  102  has a first refractive index. The index matching fluid  104  has a second refractive index that is approximately the same as the first refractive index. 
     Proximate to the enclosure  106  is ultrasound emitter  108  and illumination source  110 . At least a portion of the ultrasound emitter  108  may be in physical contact with the enclosure  106  and/or the index matching fluid  104  contained by enclosure  106 . Specifically, while the ultrasound emitter  108  is depicted in the figures as being remote from the index matching fluid  104  for ease of illustration, at least a portion of the ultrasound emitter  108  that emits sound waves will be in fluid communication with the index matching fluid  104 . In other words, the ultrasound emitter  108  may need to be contacting the index matching fluid  104  to facilitate transfer of the acoustic energy in the ultrasound signal. The ultrasound emitter  108  (which may be a plane wave ultrasound, phased array ultrasound, or other suitable variant) emits an ultrasonic signal  109  (i.e. sound waves) into the index matching fluid  104 . The ultrasonic signal  109  propagates through the index matching fluid  104 . The glass element  102  at least partially reflects the propagating sound waves that comprise the ultrasonic signal  109  out of the enclosure  106 . The reflection of the sound waves takes place due to the difference in acoustic density between the index matching fluid  104  and the glass element  102  that is immersed in the index matching fluid  104 . Further, the glass element  102 , which may be a flat glass plate, a glass slab, or a glass prism (which may be a right angle prism or prism of some other shape), has a thickness selected to prevent deforming of the glass that might otherwise occur during interaction with sound waves such as ultrasonic signal  109 . 
     The illumination source  110  emits infrared light  111  into the index matching fluid  104 , through the glass element  102 , through the index matching fluid  104  and out of the enclosure  106 . As with ultrasound emitter  108 , at least a portion of the illumination source  110  may be in physical contact with enclosure  106  and/or the index matching fluid  104  contained by enclosure  106 . As noted above, the index matching fluid  104  has a second refractive index that is approximately the same as the first refractive index. The second refractive index is selected to match the first refractive index to significantly reduce or even eliminate reflection or refraction that the glass element  102  would otherwise impose on the infrared light  111  due to a change in refractive index at the interface of the index matching fluid  104  and the glass element  102 . Through immersion of the glass element  102  in the index matching fluid  104 , the glass element  102  is therefore effectively rendered inoperable as to the infrared light  111  and/or is transparent to the infrared light  111  (while remaining highly reflective to sound waves such as ultrasonic signal  109 ). Thus, the infrared light  111  propagates—without a change in direction—along an optical path into the index matching fluid  104 , through the glass element  102 , through the index matching fluid  104 , and out of the enclosure  106  into diffuse medium  113 . Meanwhile, the ultrasonic signal  109  emitted by the ultrasound emitter  108  and propagating through the index matching fluid  104  is reflected by the glass element  102 , altering the path of the ultrasonic signal  109  out of the enclosure  106  along the substantially similar path to that of the infrared light  111 . 
     Upon encountering the glass element  102 , the ultrasonic signal  109  emitted by the ultrasound emitter  108  and the infrared light  111  emitted by the illumination source  110  travel a substantially similar path out of the enclosure  106  in the direction of a target  112 . The path of the ultrasonic signal  109  to the glass element  102  is represented in  FIGS. 1A-1B  by a line having a dash-dash-dash depiction. The path of the infrared light  111  to the glass element  102  is represented in  FIGS. 1A-1B  by a line having a dot-dot-dot depiction. The paths of the ultrasonic signal  109  and the infrared light  111  past the glass element  102  are represented in  FIGS. 1A-1B  by a line having a dash-dot-dash-dot depiction. While light  111  is referred to as “infrared light” throughout the disclosure, in some embodiments, light  111  may have a visible light wavelength such as 650 nm. Light  111  may be between 650 nm and 850 nm. Light  111  may be laser light having a very narrow linewidth. In non-medical contexts, other wavelengths (in addition to red light and near-infrared light) may be used as light  111 . 
     None of the dash-dash-dash depiction, the dot-dot-dot depiction, and/or the dash-dot-dash-dot depiction in  FIGS. 1A-1B  is intended to convey any meaning related to the shape of the waveform or any other characteristic of the ultrasonic and/or infrared emissions other than the path. Additionally, the single dash-dot-dash-dot depiction is intended only to convey that the paths of the two emissions past the glass element  102  and to the outside interface of the enclosure  106  in the direction of the target are substantially similar; no meaning is intended to be conveyed by the line depictions relating to any combining of, or interaction by, the two emissions. 
     In some embodiments, the glass element  102  is supported in place within the enclosure  106  and within the index matching fluid  104  by structure capable of immobilizing the glass element  102  without the structure interfering with the travel path of the ultrasonic signal  109  or infrared light  111 . Such structure may include, for example, a portion of the enclosure  106  itself. 
     Accordingly, an apparatus  100  may be configured to permit ingress of ultrasonic and infrared beams emitted by ultrasound and illumination emitters in disparate locations relative to and outside the enclosure  106 , while facilitating egress of the different beams towards the interface leading outside of the enclosure  106  in a substantially similar path of travel. In some configurations, the ultrasonic signal  109  may be reflected by the glass element  102 , and its path may thereby become co-located and/or coaxial with the optical path of the infrared light  111 . The enclosure  106  may be situated next to a diffuse medium  113  in which a target  112  is present. Thus, the ultrasonic and infrared beams  109  and  111  may converge outside the enclosure at the target  112  inside the diffuse medium  113  despite the beams having originated from the disparately-located emitters. 
     An exemplary application of the apparatus  100  may include visualization in turbid media such as diffuse, partially translucent tissue, in which individual locations within the diffuse tissue are visualized sequentially. Diffuse media other than tissue may similarly be visualized. The ultrasonic signal  109  and infrared light  111  may converge on a particular location within the diffuse medium  113 , that location being referenced as a voxel (volumetric pixel), such as target  112  within diffuse medium  113  depicted in  FIGS. 1A-1B . In the exemplary application, the voxel is illuminable through the diffuse medium (such as partially translucent tissue) by infrared light  111  originating from an illumination source (such as illumination source  110 ). The voxel is simultaneously targeted by an ultrasonic signal  109  originating from an ultrasound emitter (such as ultrasound emitter  108 ) and conveyed through the index matching fluid  104  and the diffuse medium  113  to the voxel (target  112  e.g.). Using the apparatus  100  disclosed herein, the illumination source  110  and ultrasound emitter  108  may be disparately located yet the respective infrared light  111  and ultrasonic signal  109  can enter the surface of the diffuse medium  113  at approximately the same location. The index matching fluid  104  contained by the enclosure  106  therefore serves as both a conveyance for the ultrasonic signal  109 , particularly where the enclosure  106  interfaces with the diffuse medium  113 , and a means for rendering the glass element  102  effectively transparent to the infrared light  111 . Interaction of the infrared light  111  and ultrasonic signal  109  at the target  112  may facilitate visualization and/or sensing of other characteristics of the voxel (i.e. target  112 ) of the diffuse medium  113 . 
     While the ultrasound emitter  108  and the illumination source  110  are depicted in  FIGS. 1A-1B  as being a distance away from the enclosure  106 , it may be that portions of the ultrasound emitter  108  and/or the illumination source  110  are affixed to, coupled with, or otherwise proximate to the enclosure  106 . Such a configuration would not modify the principle of operation of the apparatus  100 . All that is necessary is that the ultrasound emitter  108  and/or the illumination source  110  are located such that they are capable of directing their emissions into the enclosure  106 . Further, the enclosure  106  itself may be entirely transparent to facilitate ingress and egress of the infrared light  111 , for example, or the enclosure  106  may have windows or membranes at locations on its exterior surfaces for facilitating ingress and egress with minimal distortion or attenuation of the emissions. In an embodiment, such membranes or windows would be compatible with typical ultrasound gels and sterilization methods. In some embodiments, enclosure  106  allows for possible thermal expansion; without such accommodation, seals or adhesives used in conjunction with membranes, windows or other interfaces to the enclosure could leak and/or fail. 
     As the index matching fluid  104  may have a high cost, in some embodiments enclosure  106  contains the smallest volume of index matching fluid possible. In some embodiments, the index matching fluid is selected to closely match the index of the prism. In an exemplary embodiment, a BK-7 glass prism having an index of ˜1.52 is implemented as glass element  102 . Cargille Laboratories (www.cargille.com) is an example manufacturer of specific index matching oils which can be chosen for use as index matching fluid  104  to closely fit the index of BK-7 glass used in the exemplary embodiment as glass element  102 . In other embodiments, simple mineral oil may be a close enough match for use as index matching fluid  104 . 
     Waves traveling in a direction opposite to that discussed above may also pass through and/or be reflected by the glass element  102 . For example, emanations from the target  112  inside the diffuse medium  113  (reflections, e.g.) may pass through the enclosure  106  and may be captured by an image sensor. In  FIG. 2 , an apparatus related to those depicted in  FIGS. 1A-1B  is shown, in which an image sensor detects light scattered by the target which has traveled an optical path through the enclosure. Apparatus  200  includes image sensor  216  capable of detecting infrared light. In some embodiments, the image sensor  216  may include a complementary metal oxide semiconductor (“CMOS”) image sensor. Illumination source  110  emits infrared light  111  in the direction of target  112  inside diffuse medium  113 . As in the configuration depicted in  FIGS. 1A-1B , ultrasound emitter  108  emits ultrasonic signal  109  into index matching fluid  104  contained by enclosure  106 , the ultrasonic signal  109  being reflected by glass element  102  out of the enclosure  106  and in the direction of the target  112  inside the diffuse medium  113 . 
     Additionally, wavelength-shifted light  217  is generated by the ultrasonic signal  109  encountering the infrared light  111  at the target  112 . The wavelength-shifted light  217  is scattered from the target  112 , passes into the index matching fluid  104 , through the glass element  102 , and through the index matching fluid  104  where the wavelength-shifted light  217  is incident upon the image sensor  216 . The optical path of the wavelength-shifted light  217  within enclosure  106  is a substantially similar path to that of the ultrasonic signal  109  exiting enclosure  106 , albeit in an opposite direction. As with the apparatus depicted in  FIGS. 1A-1B , the index matching fluid  104  having the second refractive index that is approximately same as the first refractive index effectively neutralizes lensing or reflective effects that the interface between index matching fluid  104  and glass element  102  would have on wavelength-shifted light  217 . Thus, reflection or refraction of the wavelength-shifted light  217  from encountering glass element  102  is greatly reduced and wavelength-shifted light  217  can travel the optical path through the index matching fluid  104  and out of the enclosure  106  where it is incident upon the image sensor  216 . 
     The path of the ultrasonic signal  109  to the glass element  102  is represented in  FIG. 2  by a line having a dash-dash-dash depiction that uses short dashes. The path of the wavelength-shifted light  217  incident upon the image sensor  216  is represented in  FIG. 2  by a line having a dash-dash-dash depiction that uses long dashes. The paths of the ultrasonic signal  109  and the wavelength-shifted light  217  within enclosure  106  are represented in  FIG. 2  by a line having a dash-dash-dash depiction with alternating short and long dashes. The single dash-dash-dash depiction with alternating short and long dashes is intended only to convey the paths of the two waves; no meaning is intended to be conveyed by the line depictions relating to any combining of, or interaction by, the two waves. 
     As discussed above, the glass element  102  is rendered effectively transparent to the infrared light  111  and the wavelength-shifted light  217  through the immersion of the glass element  102  in the index matching fluid  104 . However, if some reflection by light having certain characteristics (light having a particular wavelength, e.g.) is desirable, then coatings can be applied to an exterior face of the glass element.  FIG. 3  depicts an apparatus having a glass element with a coating that is configured to at least partially reflect certain light. Apparatus  300  includes the glass element  102  having coating  318  applied to an exterior face. In some embodiments, the coating may be a partial mirror or a  50 / 50  beam splitter operable to reflect a portion of light incident upon the coating. Other portions of light incident upon the coating may propagate through the coating and/or be lost in reflection. A reference beam source  320  emits reference beam  321  into the index matching fluid  104  towards the glass element  102 , where the coating  318  applied to the glass element  102  reflects at least a portion of the reference beam  321  to image sensor  216 . 
     The origin of the reference beam  321  may be the same as the origin of the infrared light  111  even though the reference beam  321  and infrared light  111  are emitted from two different illumination sources. For example, the illumination source  110  and the reference beam source  320  may derive their emitted light from a single laser (the single laser not being depicted herein). Additionally, while reference beam source  320  is depicted in  FIG. 3  as being separate from enclosure  106  for simplicity of illustration, some of all of reference beam source  320  may be in physical contact with enclosure  106 . 
     An exemplary application of the apparatus  300  may include imaging in turbid media such as diffuse, partially translucent tissue, in which individual locations within the diffuse tissue are imaged in a raster scanning process. Diffuse media other than tissue may similarly be imaged. Illumination source  110  is configured to direct infrared light  111  at the target  112  within diffuse medium  113 , the infrared light  111  being a strong illumination beam which passes through the partially translucent diffuse medium. Ultrasound emitter  108  emits ultrasonic signal  109  into the index matching fluid  104  contained by enclosure  106 . The glass element  102  reflects the ultrasonic signal  109  through the index matching fluid  104 , out of the enclosure  106  and at the target  112 . Upon encountering the ultrasonic signal  109 , the infrared light  111  is wavelength-shifted by the ultrasonic signal  109 . The wavelength-shifted light  217  is scattered by the target  112  and propagates into the index matching fluid  104 , through the glass element  102 , through the index matching fluid  104 , and incident upon the image sensor  216 . As noted above, the coating  318  applied to the glass element  102  may be operable to redirect at least a portion of the wavelength-shifted light  217  encountering the glass element such that the portion of the wavelength-shifted light  217  is directed towards the image sensor  216 . The reference beam  321  (a “weak” reference beam relative to the strong illumination beam because it does not pass through the partially translucent media), which may be the same wavelength as the wavelength-shifted light  217  scattered by the target, is also incident upon the image sensor  216 . The image sensor  216  captures one or more images of the wavelength-shifted light  217  scattered by the target  112  interfering with the reference beam  321 . 
     Repeating the process of imaging the interference pattern generated by directing the ultrasonic signal  109  and infrared light  111  at different targets  112  within the diffuse medium  113  provides a raster scan of the diffuse medium  113 . The raster scan can be used to derive a composite  3 D image corresponding to some or all of the diffuse medium  113 . 
       FIG. 4  depicts an alternate embodiment of the apparatus illustrated by  FIG. 3 , in which the illumination source  110  of the infrared light  111  is configured to direct the strong illumination beam (i.e. infrared light  111 ) through the enclosure  106  so that, like the apparatus  100  depicted in  FIGS. 1A-1B , there is a single interface with the diffuse medium  113  for emitting waves into the diffuse medium  113 . Like the apparatuses  200 / 300  depicted in  FIGS. 2 and 3 , that interface is also used for the wavelength-shifted light  217  scattered by the target  112 , and like the apparatus  300  depicted in  FIG. 3 , a weak illumination beam  321  is provided such that the image sensor  216  may capture one or more images of the wavelength-shifted light  217  interfering with the weak illumination beam  321 . Apparatus  400  includes a polarizing beam splitting prism  424 . Illumination source  110  emits infrared light  111  (the strong illumination beam) in the direction of the polarizing beam splitting prism  424 , which reflects the infrared light  111  into the enclosure  106 , into the index matching fluid  104 , passing through glass element  102 , through the index matching fluid  104 , and into diffuse medium  113  towards target  112 . The optical path of the infrared light  111  is represented by the large dot dot-dot-dot pattern in  FIG. 4 . 
     Reference beam source  320  emits reference beam  321  in the direction of the polarizing beam splitting prism  424 , which reflects the reference beam  321  into polarizer  426  and then towards image sensor  216 . The optical path of the reference beam  321  is represented by the small dot dot-dot-dot pattern in  FIG. 4 . 
     The ultrasound emitter  108  emits ultrasonic signal  109  into the enclosure  106  and into the index matching fluid  104  whereupon the ultrasonic signal  109  is reflected by glass element  102  through the index matching fluid  104  out of the enclosure  106  and into diffuse medium  113  towards target  112  along with the infrared light  111 . The path of the ultrasonic signal  109  to the glass element  102  is represented in  FIG. 4  by a line having a dash-dash-dash depiction that uses short dashes. 
     At the target  112 , the infrared light  111  is wavelength-shifted by the ultrasonic signal  109 . The wavelength-shifted light is scattered by the target (i.e. wavelength-shifted light  217 ) into the index matching fluid  104 , through the glass element  102 , through the index matching fluid  104 , out of the enclosure  106 , and through the polarizing beam splitting prism  424 . The wavelength-shifted light  217  passes through the polarizer  426 , imparting to the light the same polarization as the reference beam, whereby the wavelength-shifted light  217  becomes incident upon the image sensor  216 . The path of the wavelength-shifted light  217  within enclosure  106  to the image sensor is represented in  FIG. 4  by a line having a dash-dash-dash pattern with alternating short and long dashes. 
     As discussed in relation to  FIG. 3 , the image sensor  216  captures one or more images of the wavelength-shifted light  217  scattered by the target  112  interfering with the reference beam  321 . The apparatus thus facilitates ingress of the ultrasonic signal  109  and infrared light  111  into the diffuse medium  113  and egress of the wavelength-shifted light  217  from the diffuse medium through the same interface, even though the ultrasound and infrared emitters  108  and  110  and the image sensor  216  are located in disparate locations. Beneficially, imaging of the diffuse medium  113  may therefore occur through placement of only a single apparatus on the surface of the diffuse medium  113  itself. 
       FIG. 5  illustrates an example flow chart of a process  500  of capturing images of light scattered from a target within a diffuse medium, in accordance with an embodiment of the disclosure. The order in which some or all of the process blocks appear in process  500  should not be deemed limiting. Rather, one of ordinary skill in the art having the benefit of the present disclosure will understand that some of the process blocks may be executed in a variety of orders not illustrated, or even in parallel. Some or all of process  500  may be executed by one or more processing devices. 
     In process block  505 , an enclosure (e.g. enclosure  106 ) is situated adjacent to a diffuse medium (e.g. diffuse medium  113 ), the enclosure providing an interface proximate to a target (e.g. target  112 ) within the diffuse medium. The diffuse medium may include tissue. The target may include a voxel within the tissue desired to be imaged. 
     In process block  510 , an ultrasound emitter (e.g. ultrasound emitter  108 ) emits an ultrasonic signal (e.g. ultrasonic signal  109 ) to the target through the interface. The enclosure may include structure (e.g. glass element  102 ) to at least partially reflect at least a portion of the ultrasonic signal in the direction of the interface. 
     In process block  515 , an illumination source (e.g. illumination source  110 ) in a disparate location relative to the ultrasound emitter emits a strong illumination beam (e.g. infrared light  111 ) to the target through the interface. Upon encountering the target, the strong illumination beam may be wavelength-shifted by the ultrasonic signal. The target scatters backscattered light (e.g. wavelength-shifted light  217 ) in the direction of the interface. 
     In process block  520 , an image sensor (e.g. image sensor  216 ) is activated to capture at least one image of backscattered light scattered by the target (e.g. wavelength-shifted light  217 ) back through the interface. 
     Process block  520  may include optional operation  525 , in which the ultrasonic signal and the strong illumination beam propagate along a substantially similar optical path to the interface. 
     The above description of illustrated embodiments of the invention, including what is described in the Abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize. 
     These modifications can be made to the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification. Rather, the scope of the invention is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.