US20020140942A1 - Acousto-optic monitoring and imaging in a depth sensitive manner - Google Patents

Abstract

A system monitors or images a portion of a sample. The system includes an optical interferometer with a measurement arm, a reference arm, and an optical splitter. The arms are coupled to receive light from the optical splitter. One of the arms includes an acousto-optical modulator. The interferometer is configured to interfere light output from the two arms. The system also includes a detector that receives the interfered light and uses the received light to determine a depth-dependent quantity characterizing a portion of the interior of the sample.

Description

• This application claims the benefit of U.S. Provisional Application No. 60/269,586, filed Feb. 17, 2001.[0001]
BACKGROUND OF THE INVENTION
• 1. Field of the Invention[0002]
• This invention relates to optical monitoring and imaging.[0003]
• 2. Discussion of the Related Art[0004]
• Contemporary medical technology uses x-rays, sound waves, and visible light to produce in vivo images of biological tissues. Visible light and infrared (IR) light imaging has a better potential resolution than imaging produced by sound waves, because visible and IR light has a shorter wavelength than sound waves. In spite of this advantage of visible and IR light, in vivo imaging systems often use sound waves, because visible and IR light does not penetrate thick tissues. Consequently, many in vivo imaging systems do not have the image resolutions obtainable in imaging systems based on visible or IR light.[0005]
• The resolution of in vivo examination systems is also limited by tissue motion. For many organisms internal tissue motions are always present, and these tissue motions interfere with tissue examinations that require more observation time than the time scale associated with the internal tissue motions. The internal tissue motions can cause image scans to produce multiple or smeared images. The internal tissue motions also cause the displacement of probes placed in the tissues. These displacements cause the probes to measure smeared out tissue properties.[0006]
BRIEF SUMMARY OF THE INVENTION
• Various embodiments include systems that process optical imagining data to obtain information on the position and/or velocity of a region inside a sample being imaged. In some embodiments, the position and/or velocity data is used to compensate for tissue motions during in vivo examinations of living organisms. The compensation corrects for smearing of scan images or measured properties that would otherwise result due to relative motions between the tissue and a monitoring probe.[0007]
• One system according to principles of the invention images a portion of a sample. The system includes an optical interferometer with a measurement arm, a reference arm, and an optical splitter. The arms are coupled to receive light from the optical splitter. One of the arms includes an acousto-optical modulator. The interferometer is configured to interfere light output from the two arms. The system also includes a detector that receives the interfered light and uses the received light to determine a depth-dependent quantity characterizing a portion of the interior of the sample.[0008]
• In some such systems, the detector uses the interfered light to determine a signed displacement or velocity of a portion of the sample. Such a system monitors body tissue movements and may further include a controller that receives displacement or velocity information. The controller adjusts data collection from the tissue in a manner that is responsive to changes in the tissue's location, orientation, or velocity relative to a monitoring probe.[0009]
BRIEF DESCRIPTION OF THE FIGURES
• FIG. 1A shows a system that optically monitors or images a sample;[0010]
• FIG. 1B is a flow chart for a process that uses the system of FIG. 1A;[0011]
• FIG. 2 shows a graded index (GRIN) fiber-size lens used in some embodiments of the probe of FIG. 1A;[0012]
• FIG. 3A shows a conventional GRIN fiber lens;[0013]
• FIG. 3B shows a refractive index profile for one embodiment of the GRIN fiber-size lens of FIG. 2;[0014]
• FIGS. 4A and 4B show embodiments of the system of FIG. 1A that incorporate optical interferometers;[0015]
• FIG. 4C illustrates an acousto-optical modulator used in the interferometers of FIGS. 4A and 4B; and[0016]
• FIG. 5 shows a medical diagnostic system based on the system of FIGS. 4A and 4C.[0017]
DETAILED DESCRIPTION OF THE EMBODIMENTS
• 1. Optical Micro-Probe and Imagining System[0018]
• A co-pending patent application describes optical micro-probes and systems used in some embodiments of the invention of the present application.[0019]
• FIG. 1A shows a[0020]system10 for optically monitoring or imaging a region of asample12, e.g., for endoscopic viewing of a biological tissue. Various embodiments of thesystem10 determine the velocity and/or three-dimensional position of the region being monitored or imaged, e.g., via tomography. Such monitoring or imaging functions are useful for medical diagnostics and treatment, e.g., invasive imaging of anomalous tissue structures in vivo and monitoring of tissue motion during other medical procedures.
• The[0021]system10 includes asource14 of IR, visible, or ultraviolet light, an optical splitter or circulator16, an optical micro-probe18, and alight detector20.Exemplary sources14 include monochromatic sources or multi-chromatic sources, e.g., a pulsed Ti-sapphire laser with a low coherence time of about 10⁻¹⁵-10⁻¹³seconds. The optical splitter or circulator16 directs a portion of the light from thesource14 to the optical micro-probe18. Theoptical micro-probe18 has adistal end22 located either above or below thesurface23 of theremote sample12. Theoptical micro-probe18 delivers the source light to a region of thesample12. Theoptical micro-probe18 also returns to the splitter or circulator16 a portion of the light scattered or emitted by the region of thesample12 illuminated by theoptical micro-probe18. The optical splitter or circulator16 redirects the returned light todetector20. Thedetector20 uses the returned light to determine a scattering or emission characteristic of the region of thesample12 that produced the light. Somedetectors20 are configured to determine the distance of the region from the optical micro-probe18 and/or the velocity of the region.
• FIG. 2 shows one[0022]embodiment18′ ofoptical micro-probe18 shown in FIG. 1A. Theoptical micro-probe18′ includes a single-modeoptical fiber24 that transports light to and from thesample12. Thedistal end22 of thefiber24 is fused to a GRIN fiber-size lens26, which has the same outer diameter as theoptical fiber24. In some embodiments, the GRIN fiber-size lens26 also has a rounded end face28 that facilitates insertion of theend22 of theoptical micro-probe18′ into samples such as biological tissues. In some embodiments, a portion of the GRIN fiber-size lens26 adjacent the end face28 has a conical taper (not shown). The taper also facilitates insertion of theoptical micro-probe18 intosample12, i.e., the taper functions like a needle point.
• The GRIN fiber-size lens[0023]26 collimates light fromfiber24 into afocused beam30. The collimatedbeam30 illuminates a region of thesample12 located forward of the lens26.Points32 in the illuminated region scatter or emit light in response to being illuminated. The backscattered or emitted light is useable for imaging or monitoring.
• The beam collimation enables resolving transverse locations of the[0024]points32 with respect to the axis of the GRIN fiber-sized lens26, becausepoints32 producing backscattered or emitted light are located within the region illuminated by thebeam30.
• In some embodiments, a mechanical driver (not shown) drives the[0025]distal end22 ofoptical micro-probe18 to execute scanning motions parallel and/or transverse to the axis of the GRIN fiber-size lens26. These scanning motions enablesystem10 to collect optical data for two-dimensional or three-dimensional images of thesample12, i.e., a planar or full 3D image.
• [0026]Illumination beam30 has a width that varies with distance from the end surface28 of the GRIN fiber-size lens26. The beam width has a minimum value at an approximatefocal point34 of the GRIN fiber-size lens26, i.e., at a distance “f” from end face28. Typically, the distance “f” has a value from about 0.2 millimeters (mm) to about 1.5 mm, and exemplary values of “f” are greater than about 0.8 mm. Thebeam30 has a divergence that is characterized by a rayleigh range “z”. Herein, the rayleigh range is half the length of the portion of thebeam30 that has a width less than about {square root}2 times the minimum width at the approximatefocal point34. An exemplary GRIN fiber-size lens26 has a rayleigh range greater than about 200 microns (μ), e.g., z≧300μ or 8 mm≧z≧300μ.
• The focal distance and rayleigh range of GRIN fiber-size lens[0027]26 depend on the radial profile of the refractive index in the GRIN lens and on the length of the GRIN lens. GRIN fiber-size lens26 is either a conventional GRIN fiber-size lens or a new GRIN fiber-size lens with a gentler refractive index profile.
• Conventional GRIN fiber lenses are described in U.S. Pat. No. 4,701,011, which is incorporated herein by reference in its entirety. FIG. 3A shows the radial refractive index profile of one such GRIN fiber lens. The refractive index is constant over a range of values of the radius that correspond to the fiber's outer cladding and varies over values of the radius that correspond to the fiber's core. Restricting the refractive index variations to the core typically produces a GRIN fiber lens with a short focal length, less than about 0.7 mm, and a short rayleigh range, e.g., less than 200μ.[0028]
• FIG. 3B shows a radial refractive index profile of a new GRIN fiber-size lens[0029]26 for which the profile's radial curvature is smaller in magnitude than in conventional GRIN fiber-size lenses. The smaller magnitude curvature causes the new GRIN fiber-size lens to have a longer focal length than the conventional GRIN fiber lens associated with the profile of FIG. 3A. The new GRIN fiber-size lenses are described in co-pending U.S. patent application Ser. No. 09/896,789, filed Jun. 29, 2001, which is incorporated herein by reference in its entirety.
• In the profile of FIG. 3B, the refractive index varies over the whole diameter of the lens. Thus, the new GRIN fiber-size lens has no outer cladding. The absence of cladding increases the radial range over which the refractive index varies permitting a larger optical mode, which results in the associated GRIN fiber-size lens having a longer rayleigh range than the GRIN fiber-size lens associated with the profile FIG. 3A.[0030]
• Refractive index profiles are characterized by a parameter “g” that measures the radial curvature of the profile in the core of a GRIN fiber lens. In particular, the parameter g is defined as:[0031]${g=-\frac{1}{n_0}\frac{{}^2P\left(r\right)}{r^2}}_{r=0}$

  
• Here, “r” is radial distance for the axis of the GRIN fiber lens, no is the value of the refractive index on the axis of the GRIN fiber lens, and P(r) is the value of the refractive index at the distance “r” from the axis of the fiber lens.[0032]
• Exemplary new GRIN fiber-size lenses have refractive index profiles whose radial curvatures are smaller in magnitude than those disclosed in Table 1 of “Analysis and Evaluation of Graded-Index Fiber-Lenses”, Journal of Lightwave Technology, Vol. LT-5, No. 9 (September 1987), pages 1156-1164, by W. L. Emkey et al, which is incorporated by reference herein in its entirety. The new GRIN fiber-size lenses[0033]26 have a “g” that is less than 1.7×10⁻⁶μm⁻², preferable less than about 0.9×10⁻⁶μm²and more preferably less than about 5.0×10⁻⁷μm⁻². For 125 μm—diameterGRIN fiber lenses18, values of “g” are selected from the range 1.7×10⁻⁶μm⁻²to 5.0×10⁻⁷μm⁻²and preferably in the range 0.9×10⁻⁶μm⁻²to 5.0×10⁻⁷μm⁻²to provide good beam collimation.
• Referring again to FIG. 2, the above-disclosed refractive index profiles produce focal lengths and rayleigh ranges for GRIN fiber-size lens[0034]26 that are consistent with the above-recited values. Some embodiments of optical micro-probe18′ use a GRIN fiber-size lens26 with a profile similar to that of FIG. 3B, because such a profile provides a longer rayleigh range. The longer rayleigh range provides a larger usable depth range for sample probing. Typically, the usable depth of theoptical micro-probe18′ is about 1 to 8 rayleigh ranges from thefocal point34.
• FIG. 1B is a flow chart for a process[0035]40 that usessystem10 of FIGS. 1A and 2. The process40 includes positioningdistal end22 of the optical micro-probe to monitor a selected portion of sample12 (step42). The positioning includes selecting an orientation of theoptical micro-probe18 with respect to thesample surface23 and selecting a lateral position and depth for thedistal end22 with respect to thesample surface23. After positioning theoptical micro-probe18,source14 transmits source light to theoptical micro-probe18 via splitter or circulator16 (step44). The transmitted source light passes through GRIN fiber-size lens26, which focuses the light into beam30 (step46). The region illuminated by thebeam30 produces the scattered or emitted light. The GRIN fiber-size lens26 collects a portion of the light that is scattered or emitted by the illuminated region of the sample (step48). Theoptical micro-probe18 returns the light collected by the GRIN fiber-size lens26 to the optical splitter or circulator16, which redirects a portion of the returned light to optical detector20 (step50). Thedetector20 determines the scattering or emission characteristics of the region of thesample12 from the light redirected thereto (step52). Since thebeam30 has an intensity that varies with the beam width, thedetector20 primarily receives light from a region of thesample12 that has a volume limited by the boundary of thebeam30. The volume includes sample points within about 1 to 8 rayleigh ranges offocal point34. The light from the sample points and known position and orientation ofoptical micro-probe18 enable using data fromdetector20 to determine lateral positions and depths of the sample points backscattering or emitting light in some embodiments ofsystem10.
• 2. Interferometric Optical Monitoring and Imaging[0036]
• Various embodiments according to principles of the invention of this application include monitoring and imagining systems that determine depth and/or velocity data for a region of a sample that backscatters or emits light delivered by an optical micro-probe. Exemplary micro-probes include both[0037]probe18′ with attached GRIN fiber-sized lens26, as shown in FIG. 2, and a single mode optical fiber without attached terminal GRIN fiber-sized lenses. FIGS. 4A and 4B show two such embodiments60,60′. To determine depths of sample regions, the systems60,60′ use “low-coherence interferometry” a method known to those of skill in the art.
• Each system[0038]60,60′ includes an interferometer with ameasurement arm62 and areference arm64. The twoarms62,64 receive light from amulti-chromatic source66, i.e., a low-temporal coherence source. Typically,source66 is spatially coherent. Themeasurement arm62 outputs light scattered by sample points in response to being illuminated by source light.
• Each system[0039]60,60′ interferometrically combines the light output bymeasurement arm62 andreference arm64. The combined light provides an output signal sensitive to optical path differences between the twoarms62,64. Because of the low-coherence nature ofsource66, depth resolution is provided by the rapid fall off of the amplitude of the interference signal with increasing path length difference. Interferometric combining of light requires that the difference in the optical path lengths traversed by the light being combined, e.g., the path difference between the twoarms62,64, be less than the light's coherence length, e.g., the coherence length of thesource66.Interference detector74 uses the interferometrically combined light to determine one or more characteristics of the region of thesample12 that produced scattered light, e.g., the intensity of the light scattered back into theoptical micro-probe18. Thus, the sensitivity to optical path differences makesdetector74 sensitive to the depth ofsample points32 producing scattered light. Thedetector74 is only sensitive to light produced bysample points32 that are located within the sample depth range for which the optical path length difference between the measurement andreference arms62,64 is less than the coherence length of thesource66.
• To increase depth resolution, a less[0040]coherent source66, e.g., a pulsed Ti-sapphire laser, is used in systems60,60′. Thesource66 has a coherence length that is at least less than one centimeter and typically is less than one millimeter. In some embodiments, thesource66 has a coherence length that is as small as 100 microns or even 1 micron. Since interferometric combination only occurs if some optical path length differences between the measurement andreference arms62,64 are less than about one coherence length, this condition defines the depth resolution of thesystem10. For a sample depth resolution of 10 microns, thesource14 should produce an output beam that is only coherent for a time equal to about 10⁻⁵meters/{3×10⁸meters/second} =3×10⁻¹⁴seconds.
• The systems[0041]60 and60′ of FIGS. 4A and 4B include a Michelson interferometer and a Mach-Zehnder interferometer, respectively. Each system60,60′ has an optical splitter/combiner68 that couples to one end of the measurement andreference arms62,64. The optical splitter/combiner68 transmits mutually coherent light from low-coherence source66 to the measurement andreference arms62,64. Themeasurement arm62 includesoptical micro-probe18. In the system60′ of FIG. 4B, theprobe18 connects to themeasurement arm62 through anoptical circulator65. Theprobe18 illuminates a sample region with source light and also collects light scattered produced by the illuminated sample region. In some embodiments, the optical micro-probe is a single-mode fiber24 having a GRIN fiber-size lens26 fused to itsdistal end22. Thereference arm64 includes amoveable reflector76, e.g. a moving mirror, and an acousto-optical modulator (AOM)70. The moveable reflector allows an operator to change the optical path length of thereference arm64, i.e., to scan different sample depths by moving thereflector76. TheAOM70 acoustically frequency shifts the source light received from the splitter/combiner68 and enables velocities ofsample points34 to be measured (see below). Some embodiments includedispersion compensator72 that corrects differences in chromatic dispersion or pulse broadening between light propagating in the measurement andreference arms62,64. The construction of dispersion compensators is known to those of skill in the art.
• The[0042]interference detector74 receives frequency-shifted light from thereference arm64 and light scattered by the sample from themeasurement arm62. Thearms62,64 have optical path lengths that are equal to within about one coherence length ofsource66 so that some light from the twoarms62,64 interferometrically combines in thedetector74, i.e., light produced by scattering at some sample depth. Thedetector74 determines characteristics of regions of the sample producing light that interferometrically combines with light from thereference arm64. The movingreflector76 enables an operator to adjust the optical path length difference between the reference andmeasurement arms64,62 so that sample depths can be scanned by theinterference detector74. Through such scans, the systems60,60′ are able to generate images of thesample12 as a function of sample depth.
• FIG. 4B also shows an[0043]exemplary interference detector74. Theexemplary interference detector74 includes a 50/50 optical splitter/combiner73 that produces signals with a 180° phase difference on its two output terminals. From the 50/50 optical splitter/combiner73, the 180° out of phase optical signals go toseparate intensity detectors75. Outputs of theintensity detectors73 couple to the inputs of adifferential amplifier77 whose output signal is representative of optical interference between signals from the reference andmeasurement arms64,62.
• Referring to FIG. 4C,[0044]AOM70 includes a radio frequency (RF)source78 and anoptical medium80. TheRF source78 excites sound waves, i.e., phonons, in theoptical medium80. The sound waves are directed along direction “Y” and have the source's RF frequency. Avoltage oscillator81 drives theRF source78. In some embodiments, theoscillator81 is variable so that the phonon frequency is variable.
• Referring to FIGS.[0045]4A-4C, a photon inreference arm64 may absorb or emit a phonon while propagating through theoptical medium80. Absorption or emission of a phonon produces both a frequency-shift, i.e., ±Δ/h, and a direction-change for the photon. Thus, the acoustically-drivenmedium80 produces both directionally undeviated output light, i.e., photons that have neither absorbed nor emitted a phonon, and directionally deviated output light, i.e., photons that have absorbed or emitted a phonon. Momentum conservation fixes the directions of the deviated output light to be different from the direction of the undeviated output light. TheAOM70 is configured to deliver deviated output light of one frequency to thedetector74 and to not deliver the undeviated output light to thedetector74. One embodiment screens out the undeviated light by imaging only deviated light, which has a new propagation direction, on an optical fiber that delivers light to movingreflector72. Thus, theinterference detector74 receives light whose frequency has been shifted by absorption or emission of a phonon in theAOM70.
• In some embodiments, the light makes two passes through the[0046]AOM70, and theAOM70 screens out light whose frequency has not been shifted by the absorption or emission of two phonons. Then, thisAOM70 produces light whose frequency is shifted with respect to theoptical source66 by twice the frequency of theRF source78.
• Referring again to FIGS.[0047]4A-4B, thedetector74 obtains information on the displacement or velocity of the region of thesample12 that backscatters source. The displacement or velocity information is encoded in the size of the Doppler shift caused by the velocity of the scattering region of thesample12. TheAOM70 enables detection of such Doppler shifts through phase-sensitive detection, which are known to those of skill in the art. In some embodiments, this detection technique enables a determination of both the sign and the magnitude of the velocities of scattering sample particles along the axis ofoptical micro-probe18. In other embodiments, this detection technique enables a determination of both the sign and the magnitude of displacements of scattering sample particles along the axis ofoptical micro-probe18.
• The[0048]AOM70 provides light outputted by thereference arm64 with a different frequency from the frequency of light outputted by themeasurement arm62. In the absence of sample motion, this frequency difference is equal to the frequency of the RF energy driving theAOM70, i.e., if thereference arm64 produces photons that absorb or emit one phonon. Sample motion at the depth for which the path difference between the measurement andreference arms62,64 vanishes changes the frequency difference between the light from the twoarms62,64, i.e., due to Doppler shifting. Thedetector74 uses the magnitude of the change in the frequency difference between the light from the twoarms62,64 to determine the speed of a sample particle producing scattering. Thedetector74 uses the phase of the change in frequency difference, i.e., positive or negative, to determine the sign of the sample motion, i.e., towards or away from theoptical micro-probe18. Standard electronic or optical techniques are known for determining both the magnitude and sign of the frequency difference between the light from the twoarms62,64.
• The systems[0049]60,60′ use theAOM70 to determine information representative of velocities of sample points at a selected sample depth. Information representative of velocities of sample points includes signed displacements and velocities of the sample points along the axis ofprobe18. The systems60,60′ are also able to select different optical path lengths for thereference arm64, i.e., by movingreflector76. By scanning such optical path lengths, systems60,60′ are able to select different sample depths for which interferometric combination of scattered light from themeasurement arm62 and light from thereference arm64 occurs. During such a scan,detector74 determines sample region velocities as a function of distance fromend22 ofoptical micro-probe18, i.e., as a function of sample depth. This type of scan of sample velocities as a function of depth enables, e.g., for mapping blood flow rates in an artery of an animal or patient.
• The[0050]AOM70 shifts light in thereference arm64 by a single frequency. This simple form of the frequency shift enables thedetector74 to determine velocities in thesample12.
• FIG. 5 shows a medical[0051]diagnostic system90 based on system60 of FIGS. 4A and 4C. Thesystem90 includes adifferential amplifier91 and anelectronic filtering chain92 that amplify and remove input noise, respectively. Thesystem90 also includes amultiplier93 that combines a signal representative of the interferometrically combined optical signals from the measurement andreference arms62,64 with a signal representative of the RF signal drivingRF source78. The output of themultiplier93 goes to afringe counter94 that determines both the magnitude and sign of the velocity of a monitored portion of the sample. To determine the sign of the velocity, i.e., towards or away from optical micro-probe18, thecounter94 compares the signal from themultiplier93 when the multiplier receives, i.e., vialine95, different signals representative of the RFsignal driving source78. The different signals are out of phase by 90°.
• The fringe counter[0052]94 couples to a feed forward circuit96 that in turn transmits information on the velocity and/or position ofsample12 to acontroller97. Thecontroller97 is connected to a second diagnostic probe98, e.g., a monitoring electrode or a scanner for thesame sample12. Thecontroller97 uses the information fed forward by circuit96 to correct data that is output by the probe98 for the effects of sample motion. In some embodiments, thecontroller97 mechanically adjusts the position of the second diagnostic probe98 to eliminate relative motion between the sample and probe98. In other embodiments, thecontroller97 corrects the data collected by the second diagnostic probe98 to compensate for the motion of thesample12, e.g., by displacing image scan data to eliminate motion induced smearing.
• Other embodiments of the invention will be apparent to those skilled in the art in light of the specification, drawings, and claims of this application.[0053]

Claims (17)

What is claimed is:

1. A system for monitoring or imaging a sample, comprising:

an optical interferometer comprising a measurement arm, a reference arm, and an optical splitter, the arms being coupled to receive light from the optical splitter, the interferometer being configured to interfere light outputted by the arms, one of the arms having an acousto-optical modulator; and

a detector configured to receive the interfered light and to use the received light to determine a depth-dependent quantity characterizing a portion of the interior of the sample.

2. The system ofclaim 1, wherein the quantity is representative of one of a signed displacement and a velocity of the portion of the interior of the sample

3. The system ofclaim 1, wherein the quantity is representative of a signed displacement of the portion of the interior of the sample.

4. The system ofclaim 2, further comprising:

an optical source coupled to transmit light to the measurement and reference arms and capable of producing light with a coherence length of less than 1 centimeter.

5. The system ofclaim 2, further comprising:

an optical source coupled to transmit light to the measurement and reference arms and capable of producing light with a coherence length of less than 1 millimeter.

6. The system ofclaim 2, where in one of the reference arm and the measurement arm has a variable optical path length.

7. A system for medical monitoring or imaging of a patient or animal, comprising:

an optical interferometer having a measurement arm, a reference arm, and an optical splitter, the arms being coupled to receive light from the optical splitter and configured to cause light outputted by the arms to interfere; and

an interference detector coupled to receive a portion of the interfering light and configured to determine information representative of a location, an orientation, or a velocity of a portion of the patient or animal from the received light; and

a controller coupled to receive the information and to adjust collected data on the animal or patient in a manner responsive to a change in a relative location, orientation, or velocity between a probe and a portion of the interior of a tissue in the animal or patient.

8. The system ofclaim 7, wherein the controller is configured to adjust collected image data to correct the image data for motion of the interior of the tissue in the animal or patient.

9. The system ofclaim 7, wherein the controller is configured to control the position of the probe.

10. The system ofclaim 7, wherein one of the reference arm and the measurement arm has a variable optical path length.

11. The system ofclaim 7, wherein one of the reference arm and the measurement arm includes an acousto-optical modulator.

12. The system ofclaim 7, wherein the detector is configured to determine one of a velocity and a signed displacement of the portion of the interior of the tissue based on the received interfering light.

13. The system ofclaim 7, wherein the measurement arm includes an optical endoscope for sending light to and receiving light from the portion of the interior of the tissue of the patient or animal.

14. The system ofclaim 13, wherein the endoscope includes an optical fiber configured to perform the sending and receiving.

15. A process for monitoring a sample, comprising:

transmitting light to measurement and reference arms of an interferometer;

acousto-optically frequency shifting light in one of the reference arm and the measurement arm;

collecting light from the measurement arm in response to the light scattering off a portion of the interior of the sample; and

interfering light from the reference arm with the collected light.

16. The process ofclaim 15, further comprising:

determining one of a velocity and a signed displacement of the portion of the interior of the sample based on a measurement of the interfering light.

17. The process ofclaim 15, wherein the transmitted light has a coherence length of less than 1 centimeter.

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