Abstract:
Systems and methods for measuring a patient&#39;s relative ocular distention as an indicator of eye, brain, and systemic hemodynamic and physiological conditions are provided. Ocular distention is measured and displayed continuously through the closed eyelid by strain imaging techniques, for example with ultrasound or optical computed tomography.

Description:
TECHNICAL FIELD 
       [0001]    The present invention is directed to measuring a patient&#39;s relative ocular distention as an indicator of eye, brain, and systemic hemodynamic and physiological conditions. 
       RELATED APPLICATIONS 
       [0002]    This application claims the benefit and priority of U.S. Provisional Application No. 62/120,968, filed on Feb. 26, 2015, which is incorporated herein by reference. 
       BACKGROUND 
       [0003]    An acute depression in local perfusion pressure and blood flow is a direct indicator of an increased risk of ischemic damage. Direct, continuous measurement of local blood and other fluidic pressures and flows can therefore be critical in acute situations such as trauma and surgery, but is currently difficult or impossible, particularly in the case of central nervous system (CNS) pressures and flows. Direct, continuous measurement of intracranial pressure (ICP) is currently only available via invasive techniques, while intracranial blood flows are only available indirectly via measurements of velocity using Doppler ultrasound. Intraocular pressure (ICP) and blood flow can be measured noninvasively, but only with the eye open and for short durations. 
         [0004]    It would be desirable to have an apparatus to overcome the above deficiencies. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]    For a fuller understanding of the nature and advantages of the present invention, reference is be made to the following detailed description of preferred embodiments and in connection with the accompanying drawings, in which: 
           [0006]      FIG. 1  illustrates a illustrates a side view of a transpalpebral ultrasonic probe assembly according to an embodiment; 
           [0007]      FIG. 2  illustrates a bottom view of the transpalpebral ultrasonic probe assembly of  FIG. 1 ; 
           [0008]      FIG. 3  illustrates a side view of an OCE probe assembly for optical coherence elastography (OCE) according to an embodiment; 
           [0009]      FIG. 4  illustrates a bottom view of the OCE probe assembly of  FIG. 3 ; 
           [0010]      FIG. 5  is a block diagram of a system for transpalpebral incremental strain imaging according to an embodiment; 
           [0011]      FIG. 6  illustrates an exemplary graph that can be presented to the user according to an embodiment; 
           [0012]      FIG. 7  illustrates a flowchart of a method of acquiring data from a patient according to an embodiment; 
           [0013]      FIG. 8  is a schematic of a representative application of a transpalpebral transducer array to a patient&#39;s eyelid imaging the sclera according to an embodiment; and 
           [0014]      FIG. 9  is a schematic of an imaged tissue arc according to an embodiment. 
       
    
    
     SUMMARY 
       [0015]    To address the foregoing problems, and achieve other advantages, it is presented that a clinically useful substitute for direct measurements of CNS pressures and flows is via continuous, noninvasive monitoring of their volumetric effects on the eye. Continuous perioperative monitoring of ocular distention would give the clinician indications of a variety of otherwise difficult to monitor conditions, including: acute changes in ocular and cerebral perfusion pressures; possible occlusion of a feeder artery to the eye, e.g. ophthalmic or internal carotid; acute changes in eye hemodynamic conditions; changes in eye hemodynamic resistance; changes in eye hemodynamic capacitance; acute changes in intraocular pressure (IOP); occlusion of eye arterial or venous flow; occlusion of aqueous humor outflow; acute changes in intracranial pressure (ICP); changes in intracranial arterial pressure; changes in cerebrospinal fluid (CSF) pressure; exhaustion of the cerebral autoregulatory reserve; changes in blood viscosity; and clinically significant changes in patient orientation and physical support. 
         [0016]    Since pressure P=V/C for a container&#39;s volume V and capacitance C, changes in the eye&#39;s total volume directly indicates changes in the local blood pressure gradient for constant capacitance. Changes in gradient P may be from changes in one or more of arterial, venous, intraocular, or external pressures. Ocular capacitance can be treated as constant during a single cardiac cycle, but may change over a series of cycles due to changes in intraocular fluid volume and arterial muscle tone. 
         [0017]    In some embodiments, the present system and method measures changes in eye volume continuously and noninvasively via physical strain of the corneoscleral envelope. This is accomplished by transpalpebral incremental strain imaging using in vivo strain imaging methods such as ultrasound speckle tracking and optical coherence elastography (OCE) via optical coherence tomography (OCT). Incremental strain is the change in local length over original length, where length for the scleral sphere is some circumferential or axial distance. In general, an ultrasonic or laser system probe is applied to the closed eyelid secured for example with a strap, surgical tape, or as a component of an adhesive patch. This probe is driven by an electronic or electro-optical system, respectively, which is also responsible for computing, transmitting, and displaying results as continuous waveforms and trends over time. 
         [0018]    In an aspect, the probe does not contact the ocular surface directly and further does not apply pressure to take measurements. This may be accomplished by the imaging system compensating electronically for intervening acoustically or optically visible layers and any oblique orientation to the imaged scleral or corneal surface. 
         [0019]    In an aspect, the invention is directed to an apparatus for noninvasive monitoring of an eye. The apparatus includes an adhesive support structure and an ultrasound device. The adhesive support structure includes an elongated flexible backing layer having a first planar surface. The adhesive support structure also includes an adhesive layer disposed on said first planar surface of said elongated flexible backing layer. The ultrasound device is disposed in an aperture defined in said adhesive support structure. The ultrasound device includes a rigid or semi-rigid backing layer. The ultrasound device also includes a driver circuitry disposed on said rigid or semi-rigid backing layer. The ultrasound device also includes an array of ultrasound transducers in electrical communication with said driver circuitry. The ultrasound device also includes a coupling gel disposed on said array of ultrasound transducers. 
         [0020]    In another aspect, the invention is directed to an apparatus for noninvasive monitoring of an eye. The apparatus includes an adhesive support structure and an optical device. The adhesive support structure includes an elongated flexible backing layer having a first planar surface. The adhesive support structure also includes an adhesive layer disposed on said first planar surface of said elongated flexible backing layer. The optical device is disposed in an aperture defined in said adhesive support structure. The optical device includes a housing. The optical device also includes a cable including a fiber optic cable and electronic cable as needed connected to a first wall of said housing. The optical device also includes a collimating lens disposed proximal to said first wall of said housing, said collimating lens forming collimated electromagnetic (EM) waves from uncollimated EM waves passing through said collimating lens, said uncollimated EM waves emitted from said fiber optic cable. The optical device also includes a fixed or electronically variable focusing lens disposed proximal to a second wall of said housing, said focusing lens configured to focus said collimated EM waves at a target in said eye. The probe cable, collimating lens, and focusing lens are disposed in said housing. 
         [0021]    In another aspect, the invention is directed to a system for noninvasive monitoring of an eye. The system includes a transpalpebral probe assembly including an adhesive support structure and an ultrasound device. The adhesive structure includes an elongated flexible backing layer having a first planar surface. The adhesive structure also includes an adhesive layer disposed on said first planar surface of said elongated flexible backing layer. The ultrasound device is disposed in an aperture defined in said adhesive support structure. The ultrasound device includes a rigid or semi-rigid backing layer. The ultrasound device also includes a driver circuitry disposed on said rigid or semi-rigid backing layer. The ultrasound device also includes an array of ultrasound transducers in electrical communication with said driver circuitry. The ultrasound device also includes a coupling gel disposed on said array of ultrasound transducers. The system also includes a microprocessor-based computer in communication with the assembly. The computer includes a probe driver complex including a driver logic to provide a control signal to said driver circuitry in said ultrasound device and analog-to-digital converters to transform an output of the ultrasound device to digital signals. The computer also includes a processor in communication with the probe driver complex, the processor outputting a signal representing a transpalpebral acoustical image of a portion of a subject&#39;s eye based at least in part on the digital signals. The system also includes a display in communication with the computer, the display displaying the image. 
         [0022]    In another aspect, the invention is directed to a system for noninvasive monitoring of an eye. The system includes an assembly including an adhesive support structure. The adhesive support structure includes an elongated flexible backing layer having a first planar surface. The adhesive support structure also includes an adhesive layer disposed on said first planar surface of said elongated flexible backing layer. The assembly also includes an optical device disposed in an aperture defined in said adhesive support structure. The optical device includes a housing and a fiber optic cable connected to a first wall of said housing. The optical device also includes a collimating lens disposed proximal to said first wall of said housing, said collimating lens forming collimated electromagnetic (EM) waves from uncollimated EM waves passing through said collimating lens, said uncollimated EM waves emitted from said fiber optic cable. The optical device also includes a focusing lens disposed proximal to a second wall of said housing, said focusing lens configured to focus said collimated EM waves at a target in said eye. The fiber optic cable, said collimating lens, and said focusing lens are disposed in said housing. The system also includes a light source having a coupling in electrical communication with the fiber optic cable. The system also includes a microprocessor-based computer in communication with the assembly and the light source, the computer including a processor to determine a corneal strain of a subject and to generate an output signal representing a transpalpebral optical image of a portion of a subject&#39;s eye, the corneal strain and the output signal based at least in part on an output signal of the assembly. The system also includes a display in communication with the computer, the display displaying the image. 
       DETAILED DESCRIPTION 
       [0023]      FIG. 1  illustrates a side view of a transpalpebral ultrasonic probe assembly  10  according to an embodiment. The assembly  10  includes a mounting structure  100  and an ultrasound structure  125 . The mounting structure  100  includes an adhesive layer  110  disposed on flexible backing  120 . The adhesive layer  110  is integrated into the assembly  10  to adhere the assembly  10  to a patient&#39;s eyelid while the assembly  10  is in clinical use. In some embodiment, adhesive layer  110  can include a low trauma adhesive such as a hydrogel, acrylic, silicone, or other adhesive. Alternatively, a cranial strap, headset, surgical tape, or similar device can be used in place of adhesive layer  110  to secure the assembly  10  to a patient&#39;s eyelid. Flexible backing  120  secures the ultrasound structure  125  to the eyelid via adhesive layer  110 . Flexible backing  120  can include a bandage, a woven fabric, a plastic material (e.g., polyvinyl chloride, polyethylene, polyurethane), or a latex material. 
         [0024]    In some embodiments, the mounting structure  100  is similar to an ECG lead. In general, the mounting structure  100  has a form factor (e.g., dimensions) that allow the mounting structure  100  to be mounted on a patient&#39;s eye. Exemplary assembly  10  dimensions are about 8 mm wide, about 20 mm long, and about 5 mm thick. However, other dimensions are within the scope of this disclosure, including assembly  10  dimensions of about 6 mm to about 10 mm wide, about 15 mm to about 25 mm long, and about 3 mm to about 7 mm thick. The flexible backing  120  can be about 1 mm to about 2 mm thick or about 1.5 mm thick. The ultrasound structure  125  can be about 2 mm to about 6 mm wide or about 4 mm wide, about 5 mm to about 15 mm long or about 10 mm long, and about 3 mm to about 7 mm thick or about 5 mm thick. As used herein, “about” means plus or minus 10% of the relevant value. 
         [0025]    Ultrasound structure  125  is disposed in a central region of mounting assembly  100 . Ultrasound structure  125  includes coupling gel  130 , ultrasonic transducer elements  140 , driver circuitry  150 , and rigid backing  160 . The driver circuitry  150  receives control signals through cabling  170 . The control signals can be generated by an internal or external microprocessor-based controller. The driver circuitry  150  includes electronics to convert the control signals to driver signals to drive an array of ultrasound transducer elements  140 . Rigid backing  160  provides support for ultrasound structure  125  and in some embodiments is a hard polymer such as polystyrene. Rigid backing  160  can also be semi-rigid, such as latex, in some embodiments. 
         [0026]    In some embodiments, the ultrasound transducer elements  140  include small, lightweight micromachined ultrasonic transducers (MUTs) instead of or in addition to conventional piezoelectric transducers. An example of such MUTs can be found in U.S. Pat. No. 6,359,367, which is incorporated herein by reference. Ultrasound transducer elements  140  can include an array (e.g., a linear array) of ultrasound elements, for example an array of 64 elements. 
         [0027]    Coupling gel  130  includes an acoustic coupling medium to transmit acoustic energy generated by transducer elements  140  to the patient (e.g., via the patient&#39;s eyelid). Preferred gels are non-toxic and water-soluble, e.g., SCAN Ultrasound Gel from Parker Laboratories, Inc. 
         [0028]    In some embodiments, the assembly  10  can include a cover or a removable material to cover the adhesive layer  110  and/or the coupling gel  130  prior to application of the assembly  10  on a subject, for example during storage. 
         [0029]      FIG. 2  illustrates a bottom view of the transpalpebral ultrasonic probe assembly  10  described above. As can be seen, the coupling gel  130  is generally in the center of the assembly  10 . The adhesive layer  110  surrounds the coupling gel and forms the balance of the bottom surface of assembly  10 . Cabling  170  is illustrated in  FIG. 2  for context, although cabling  170  enters ultrasonic structure  125  from the top of assembly  10 . As discussed above, a cover or removable material can cover the adhesive layer  110  and/or the coupling gel  130 . 
         [0030]      FIG. 3  illustrates a side view of an OCE probe assembly  30  for optical coherence elastography (OCE) according to an embodiment. The OCE assembly  30  includes a mounting structure  300  and an optical assembly  325 . The mounting structure  300  includes adhesive layer  310  and flexible backing  320 , which can be the same as adhesive layer  110  and flexible backing  120  described above. 
         [0031]    Optical assembly  325  includes housing  330 , collimating lens  340 , mirror  350 , and focusing lens  360 . Housing  330  is illustrated as transparent for clarity, but in some embodiments one or more of the collimating lens  340 , mirror  350 , and focusing lens  360  are obstructed from view. 
         [0032]    In operation, electromagnetic waves (EM waves) (e.g., light) pass through probe cable  370 , which can include a fiber optic cable or a fiber optic bundle, and enter housing  330 , for example through a window that is transparent to the EM waves. The EM waves then pass through collimating lens  340  where the EM waves are collimated. The collimated EM waves are redirected by mirror  350 , which is disposed at an acute or obtuse angle with respect to an axis parallel to the collimated EM waves. The mirror  350  is configured to direct or redirect the EM waves so they pass through focusing lens  360 , which focuses the EM waves on a target on the patient&#39;s eye and/or on the patient&#39;s lower eyelid. In some embodiments, the focusing lens  360  is removable and can be replaced by a lens appropriate for a given patient. In yet another embodiment, lens  360  is an electronically-controlled liquid lens with a dynamic focal point. The probe cable  370  can include an electronic cable to power and drive the electronically-controlled liquid lens. In addition or in the alternative, the clinician can select from several optical probe assemblies  30  or optical assemblies  325 , each of which contain a different lens. 
         [0033]    The assembly  30  allows a conventional OCT system to be decoupled between the bulky electro-optical/mechanical scanning assembly and the small and lightweight assembly  30 . 
         [0034]      FIG. 4  illustrates a bottom view of assembly  30  described above. As can be seen, the optical assembly  325  is generally in the center of the assembly  30 . The adhesive layer  310  surrounds the optical assembly  325  and forms the balance of the bottom surface of assembly  30 . Probe cable  370  is illustrated in  FIG. 4  for context, although probe cable  370  enters optical assembly  325  from the top of assembly  30 . As can be seen, the assembly  30  can be applied to a patient in a similar manner as an ECG lead. 
         [0035]      FIG. 5  is a block diagram of a system  50  for transpalpebral incremental strain imaging according to an embodiment. The system  50  includes transpalpebral probe assembly  510 , probe driver complex  520 , processor complex  530 , external diagnostic device  540 , and user interface  550 . 
         [0036]    The transpalpebral probe assembly  510  can be the transpalpebral ultrasonic probe assembly  10  or the OCE probe assembly  30 , as discussed above. The assembly  510  is in electrical communication with probe driver complex  520 . Probe driver complex  520  includes components such as an interface to processor complex  530 . The interface can be physical or it can be virtualized in software. The probe driver complex  520  includes the components specific to the imaging modality. In some embodiments, the probe driver complex  520  and the processor complex  530  are disposed in the same physical device, such as a computer or a server. 
         [0037]    In the case of a typical OCT implementation using a Michelson interferometer and OCE probe assembly  30 , the probe driver complex  520  includes a light source, an aiming laser diode, a beamsplitter, a reference arm assembly, a photo-diode with amplifier and local image storage, a scan head and lens system, and a fiber-optic coupling mechanism to the OCE probe assembly  30 . The light source can be a broadband super luminescent diode (SLD) and can have a central wavelength of about 800 nm to about 1400 nm at 10 mW output, including about 900 nm, about 1000 nm, about 1100 nm, about 1200 nm, and about 1300 nm, 1310 nm, or any value between any two of the foregoing wavelengths. The bandwidth can be about 50 nm to about 100 nm, including about 60 nm, about 70 nm, about 80 nm, about 90 nm, or any value between any two of the foregoing bandwidths. The reference arm assembly can include a phase modulator, a calibrated scanning mirror, a lens assembly, and associated electronic drivers. Alternatively, the reference arm can be implemented in solid state without any moving parts. An exemplary system is described in U.S. Pat. No. 8,770,755, which is incorporated herein by reference. Scan frames are forwarded to processor complex  530 . 
         [0038]    In the case of a typical ultrasound strain imaging implementation using transpalpebral ultrasonic probe assembly  10 , the probe driver complex  520  includes electronic analog-to-digital converters, digital signal processing filters, local RF and digital image storage, and driver logic. The drive logic can include logic output a control signal for the driver circuitry on the ultrasound device of probe assembly  10 . There is typically one analog-to-digital converter for each transducer element in ultrasonic transducer elements  140 . Frames can be forwarded to processor complex  530  for speckle tracking and analysis. 
         [0039]    Since transpalpebral strain imaging requires a small scan area at only a few millimeters depth, high frequency ultrasound on the order of 10 to 30 MHz can be used to optimize strain measurement accuracy. To provide a continuous waveform, the system further preferentially uses ultrafast B-mode imaging at a high (&gt;1000 Hz) frame rate. One or more embodiments may employ ultrafast B-mode image processing using modern graphics processing platforms for high computational throughput. The ultrasonic frequency, waveforms, and imaging are controlled by the driver logic. 
         [0040]    Processor complex  530  is responsible for the computationally intense aspects of image processing, elastography, hemodynamics, external I/O, user control, and results storage and forwarding. In addition, processor complex  530  can implement a speckle-tracking algorithm, e.g. as described in Tang, J. and Liu, J.,  Ultrasonic Measurements of Scleral Cross - Sectional Strains During Elevations of Intraocular Pressure: Method Validation and Initial Results in Posterior Porcine Sclera,  Journal of Biomechanical Engineering 2012; 134(9), 091007-1-091007-10, which is incorporated herein by reference. In addition, processor complex  530  can compute the strain (e.g., strain of stromal or corneal tissue) and combine the results of strain with inputs provided by external diagnostic device(s)  540 , such as ocular spherical diameter, to compute derivable diagnostic metrics. An exemplary algorithm for optical coherency micro-elastography is presented in Kennedy, B. et al., “Optical Coherence Micro-Elastography: Mechanical-Contrast Imaging of Tissue Microstructure,” Biomedical Optics Express, 2014 Jun. 9; 5(7):2113-24, which is incorporated herein by reference. In an additional embodiment, processor complex  530  estimates the nominal total scleral volume from the 3D local images of the sclera via the radius of curvature of the en face segment. The processor complex  530  can provide this information and results to the user through user interface  550 . These functions are generally independent of the imaging implementation and are preferentially implemented in software. 
         [0041]    User interface  550  can include a touch-screen monitor or a display that can accept menu-driven user input. Such user input can include configuration parameters for probe assembly  510 , probe driver complex  520 , and/or processor complex  530 . User input can also include data obtained from external diagnostic devices  540  or commands to synchronize external diagnostic devices  540  with processor complex  530 . User interface  550  also allows the user to display data, waveforms, trends, and other information generated and obtained by processor complex  530 . An example of the data that can be provided on user interface  550  is graph  60  illustrated in  FIG. 6 . As illustrated in the graph  60 , the ocular diameter is observed to be decreasing. As ocular diameter is related to strain, the decreasing trend may indicate an acute decrease in retinal and ciliary artery perfusion pressures due to a developing thrombus in the ophthalmic artery. 
         [0042]    Continuous intracranial pressure, intraocular pressure, ocular perfusion pressure (OPP), ocular pulse amplitude (OPA), and pulsatile ocular blood flow (POBF) estimates in absolute terms are derived from ocular distention, additional estimates of baseline total volume from the 3D images of the local sclera, and via supplemental data provided by the user and/or external diagnostic devices  540  such as brachial artery blood pressures. 
         [0043]      FIG. 7  illustrates a flowchart  70  of a method of acquiring data from a patient using system  50  described above. In step  710  ultrasound probe  10  or OCE probe  30  is affixed to the patient&#39;s eyelid by the user/clinician. The patient&#39;s eyes remain closed for the duration of the measurements. In step  720 , an acoustically imaged (using probe  10 ) or optically imaged (using probe  30 ) region of the surface or volume of the stroma of either the sclera or cornea of the patient&#39;s eye is acquired electronically. Target stromal/corneal tissue is distinguished from intervening layers of acoustically/optically visible tissue and fluids by the relative characteristics and depth of the return signal as compared to empirically-based profiles. For example, in a transpalpebral ultrasound image at 30 MHz, the changes in acoustic impendences between eyelid, conjunctiva, sclera, and the vitreous chamber readily delineate the boundaries those tissues. The speckle densities further distinguish these tissues. For the higher-resolution optical method tissues are similarly readily distinguished by coarse-grained histological features, e.g., cell volumetric density. Modified profiles are input by the user in step  750  for abnormal physiologies or pathologies. As anterior scleral circumferential elastic properties are largely independent of circumferential direction, the array  10 ,  30  can be oriented at any angle with respect to the closed eyelid seam. Corneal elastic properties are strongly directional, requiring for corneal applications a standard transducer orientation or alternatively an indication to the device of the transducer orientation with respect to the orientation of additionally provided parameters. 
         [0044]    In step  730 , a zero incremental strain baseline is established. The user signals to the device (e.g., via user interface  550 ) that the current image represents the strain baseline. In the perioperative context this baseline often corresponds to stable patient conditions. If used, additional baseline data such as absolute intraocular pressure measurements, e.g. with dynamic contour tonometry, and baseline brachial artery pressure are measured and provided to the device at this time. Such additional baseline data is input in step  760 . 
         [0045]    In step  740 , the system continuously estimates, measures, and records regional incremental strain. Total incremental strain relative to the eye baseline geometry can be estimated as the product of the series of incremental strain measurements. 
         [0046]    In step  750 , the user inputs patient parameters to the system, e.g., through user interface  550 . Patient parameters can include ocular rigidity and ocular diameter. Such parameters can be used to estimate certain diagnostic metrics that are dependent on measured ocular distention. For example, the parameters can be used to estimate scleral meridional diameter, absolute IOP, and pulsatile ocular blood flow. Alternatively, the system can provide empirical estimates of these values from provided patient parameters such as age and gender. 
         [0047]    In step  760 , the external device data is input into the system. The data can be input manually by the user or automatically by a device through a physical or wireless connection with the external device. An example of external device data that may be beneficial is mean arterial pressure and heart rate which can be used to estimate additional diagnostic metrics such as intraocular pressure (IOP) and ocular perfusion pressure. 
         [0048]    In step  770 , derived metrics are estimated using as inputs the continuous (or near continuous) incremental data (step  740 ), patient parameters (step  750 ), and data provided by external devices (step  760 ). The strain may be volumetric or directional in various embodiments. 
         [0049]    In step  780 , continuous waveforms and trends data are displayed to the user. The data is displayed based on the continuous (or near continuous) incremental strain data (step  740 ) and estimated derived metrics (step  770 ). 
         [0050]      FIG. 8  is a schematic  80  of a representative application of a transpalpebral transducer array (e.g., array  10 ) to a patient&#39;s eyelid imaging the sclera. The device can be alternatively applied to imaging corneal strain via respective supra-corneal orientation of the array. The array is generally oriented superior and parallel to the eyelid seam, but need not be oriented along a principal circumferential direction, as discussed above. It is noted that the array will generally not be oriented perpendicular to the scleral (or corneal) radial axis, which will require incident beam and/or image correction using, e.g., beam steering in the case of ultrasound. As discussed above, intervening layers of tissue and fluid are discriminated from the target measured tissue using known idiosyncratic image characteristics of these layers. 
         [0051]      FIG. 9  is a schematic  90  of the imaged tissue arc, in this case scleral, which again need not be oriented along a principal direction. 
         [0052]    Some aspects of the above can include or modify some or all of clinical ultrasound systems and methods such as those described in Tang, J. and Liu, J.,  Ultrasonic Measurements of Scleral Cross - Sectional Strains During Elevations of Intraocular Pressure:  Method Validation and Initial Results in Posterior Porcine Sclera, Journal of Biomechanical Engineering 2012; 134(9), 091007-1-091007-10. Additional aspects can include or modify some or all of the systems and methods described in Tanter, M. and Fink, M.,  Ultrafast Imaging in Biomedical Ultrasound, IEEE  Transactions on Ultrasonics, Ferroelectrics, and Frequency Control 2014; 61(1):102-119. Additional aspects can include or modify some or all of the systems and methods described in Xie, T. et al.,  Fiber - optic - bundle - based optical coherence tomography, Optics Letters  2005; 30(14):1803-1805. Each of the foregoing references is incorporated herein by reference. 
         [0053]    The present invention should not be considered limited to the particular embodiments described above, but rather should be understood to cover all aspects of the invention as fairly set out in the attached claims. Various modifications, equivalent processes, as well as numerous structures, materials and unforeseen technologies to which the present invention may be applicable, will be readily apparent to those skilled in the art to which the present invention is directed upon review of the present disclosure. The claims are intended to cover such modifications and equivalents.