Abstract:
An optical coherence tomography system is provided. The system includes an OCT imager; a two-dimensional transverse scanner coupled to the OCT imager, the two-dimensional transverse scanner receiving light from the light source and coupling reflected light from a sample into the OCT imager; a computer coupled to receive 3D OCT data from the OCT imager, the computer further processes the 3D OCT data; wherein the processing the 3D OCT data includes: correcting motion artifacts in baseline mode; generating reference data in baseline mode; performing segmentation to identify volumes of interest; extracting feature information, the feature information including reflectivity, texture, or the combination thereof.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Application 61/521,718, filed on Aug. 9, 2011, which is herein incorporated by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates generally to the field of optical coherence tomography and applications thereof. Specifically, the invention relates generally to methods and systems for collecting and processing three-dimensional (3D) data or two-dimensional (2D) images in ophthalmology. 
         [0004]    2. Description of Related Art 
         [0005]    Optical Coherence Tomography (OCT) is an optical signal and processing technique that captures three-dimensional (3D) data sets with micrometer resolution. The OCT imaging modality has been commonly used for non-invasive imaging of an object of interest, such as cornea and retina of the human eye, over the past 15 years. A cross sectional retinal image from an OCT scan allows users and clinicians to evaluate various kinds of ocular pathologies in the field of ophthalmology. However, due to limitations of scan speed in imaging devices based on time-domain technology (TD-OCT), only a very limited number of cross-sectional images can be obtained for evaluation and examination of the entire retina. 
         [0006]    In a new generation of OCT technology, Fourier-Domain or Spectral Domain Optical Coherence Tomography (FD/SD-OCT) has significantly improved over TD-OCT with, for example, better scan speeds and resolution. 3D data sets with dense raster scan or repeated cross-sectional scans can now be achieved by FD-OCT with a typical scan rate of approximately 17,000 to 40,000 A-scans per second. 
         [0007]    These technological advances in OCT enable massive amounts of data to be generated at an ever increasing rate. As a result of these developments, myriad scan patterns were designed and employed to capture various volumes of interest (VOI) of the eye to enhance diagnostic capabilities. 
         [0008]    Current trends in OCT ophthalmology make extensive use of 3D imaging and image processing techniques to obtain and process 3D data. The 3D data can be utilized for diagnosing diseases such as glaucoma, age-related macular degeneration (AMD), corneal diseases, and other medical conditions affecting the eye. Analyses of OCT data have been mostly focused on thickness measures of various segmented layers in the cornea and the retina. However, ocular diseases may affect the scattering properties of ocular tissues without changing the thickness measures. Some other physical characteristics and properties of the cellular layers of the eye can provide additional information useful for evaluations and diagnosis of different eye conditions. 
         [0009]    Involuntary motions of the subject&#39;s eye during OCT data acquisition commonly create artifacts that can impact the accuracy and reliability of the physical characteristics and properties of the VOL. These motions introduce relative displacements of the acquired data; for example, physical features could appear discontinuous in the resulting 3D data and might deviate from the true anatomy of the eye, resulting in unreliable and inaccurate processing and evaluation. 
         [0010]    It is common in the arts to perform diagnosis of ophthalmic diseases based on qualitative visual impressions and/or quantitative computer-aided diagnosis (CAD) analysis. Therefore, it is important to employ one or more processing methods to ensure the acquired 3D data and the extracted features remain consistent and readily comparable due to differences in system to system or modality to modality variations. There is a need for systems that improve the accuracy and effectiveness of the processing and evaluation of OCT data. 
       SUMMARY 
       [0011]    In accordance with some embodiments, an optical coherence tomography (OCT) system is provided. An optical coherence tomography (OCT) system according to some embodiments includes an OCT imager; a two-dimensional transverse scanner coupled to the OCT imager, the two-dimensional transverse scanner receiving light from the light source and coupling reflected light from a sample into the OCT imager; and a computer coupled to receive 3D OCT data from the OCT imager, the computer further capable of executing instructions to process the 3D OCT data; the instructions including correcting motion artifacts in baseline mode, generating reference data in baseline mode, performing segmentation to identify volumes of interest, and extracting feature information, the feature information including reflectivity, texture, or the combination thereof. 
         [0012]    In some embodiments, a method of data analysis for ophthalmology includes acquiring at least one 3D OCT data set; correcting motion artifacts in baseline mode; generating reference data in baseline mode; performing segmentation to identify volumes of interest; extracting feature information, the feature information including reflectivity, texture, or a combination thereof. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    A more complete understanding of the present invention, and the attendant advantages and features thereof, will be more readily understood by reference to the following detailed description when considered in conjunction with the accompanying drawings, wherein: 
           [0014]      FIG. 1  is an example of an OCT system that can be used in some embodiments of the present invention. 
           [0015]      FIGS. 2   a  and  2   b  illustrate exemplary reflectivity and texture difference of a B-scan (x-z) image in (a) a normal case and (b) a pathology case. 
           [0016]      FIG. 3  shows a block diagram illustrating the steps of the computer-aided diagnosis (CAD) method according to some embodiments of the present invention. 
           [0017]      FIG. 4  shows an example baseline scan pattern in horizontal (x) and vertical (y) coordinates to acquire a sequence of 2D OCT images. 
           [0018]      FIGS. 5   a  and  5   b  show examples of feature spectra for (a) a normal case and (b) a pathology case. 
           [0019]      FIG. 6  shows an exemplary schematic diagram of a progression analysis with normative database displayed in various cutoff colors and confident intervals. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    Optical Coherence Tomography (OCT) technology has been commonly used in the medical industry to obtain information-rich content in three-dimensional (3D) data sets. For example, OCT technology can be used to provide imaging for catheter probes during surgery. In the dental industry, OCT has been used to guide dental procedures. In the field of ophthalmology, OCT is capable of generating precise and high resolution 3D data sets that can be used to detect and monitor different eye diseases in the cornea and the retina. Different scan configurations have been developed for different industries and for different clinical applications. These scan configurations further expand the application of OCT technology in different clinical applications and further enhance the quality and information-richness of 3D data sets obtained by OCT technologies. 
         [0021]    Some embodiments of the present invention are proposed to enhance data accuracy of OCT data. Such data can then be used to evaluate tissue structure of the eye and diagnose eye diseases. Specifically, some embodiments of the current disclosed analysis can be utilized in computer-aided diagnosis (CAD) methods and applied to the diagnoses of pathologies in the fovea (e.g. Diabetic Retinopathy), in the optic nerve head (e.g. Glaucoma), in the cornea (e.g. Keratoconus) and other regions of the eye and pathologies of interests. 
         [0022]      FIG. 1  illustrates an example of an OCT system  100  that can be utilized in processing and analyzing an OCT data set according to some embodiments of the present invention. OCT system  100  includes light source  101  supplying light to coupler  103 , which directs the light through the sampling arm to XY scan  104  and through the reference arm to optical delay  105 . XY scan  104  scans the light across eye  109  and collects the reflected light from eye  109 . Light reflected from eye  109  is captured in XY scan  104  and combined with light reflected from optical delay  105  in coupler  103  to generate an interference signal. The interference signal is coupled into detector  102 . OCT imager  100  can be a time domain OCT system, in which case depth (or A-scans) are obtained by scanning optical delay  105 , or a Fourier domain imager, in which case detector  102  is a spectrometer that captures the interference signal as a function of wavelength. In either case, the OCT A-scans are captured by computer  108 . Collections of A-scans taken along an XY pattern are utilized in computer  108  to generate 3-D OCT data sets. Computer  108  can also be utilized to process the 3-D OCT data sets into 2-D images according to some embodiments of the present invention. Computer  108  can be any device capable of processing data and may include any number of processors or microcontrollers with associated data storage such as memory or fixed storage media and supporting circuitry. 
         [0023]    In additional to the OCT system  100 , an apparatus for computer-aided diagnosis of ocular pathologies according to the present invention may include a camera  106 , which can be a fundus camera. Light from camera  106  can be coupled into the sample arm of OCT imager  100  by a coupler  107 . Coupler  107  prevents light from camera  106  from entering coupler  103  while directing reflected light from eye  109  that originates from camera  106  back into camera  106 . Data from resources of other imaging modalities  110  can also be imported to computer  108  by local or internet networking. Utilizing the combination of images, 3D OCT data, and resources imported from other imaging modalities, it is expected that more reliable CAD of ocular pathologies can be achieved. 
         [0024]      FIG. 3  shows a block diagram illustrating the steps of the computer-aided diagnosis (CAD) method according to some embodiments of the present invention. According to  FIG. 3 , the first step is to perform OCT data acquisition  305 . Depending on the determination of mode in mode selection  307 , the data acquired in data acquisition  305  is utilized in a baseline or a follow-up mode. In the baseline mode, motion correction  310  for OCT data is performed. Reference generation  315  is then performed to form a baseline reference. In follow-up mode, real-time tracking step  320  can perform a tracking and data registration  325  can be performed. Under either the baseline mode or the follow-up mode, the next step is to segment volumes of interest (VOI) in step  330 . Features of interest within the VOI can be extracted in step  335 . Feature normalization is then performed in step  340 . In some embodiments, a normative database (NDB) can then be constructed for feature comparison and detection in step  345 . In addition, in the follow-up scan mode as determined in step  347 , features from different points in time can be further analyzed and evaluated using progression analysis in step  350 . Details of the embodiments in  FIG. 3  are further described in the following descriptions. 
       Data Acquisition Step  305   
       [0025]    The first step in CAD method  300  illustrated in  FIG. 3  is the acquisition of OCT data in data acquisition  305 . In some embodiments, the OCT data can be acquired utilizing a scan pattern  410  as shown in  FIG. 4 . Other scan patterns commonly known in the arts can also be used to allow sufficient 3D information to be utilized, depending on the clinical application. The scan pattern should be capable of covering substantially the eye features of interest, for examples, the fovea or the optic nerve head. The 2D OCT image  420  in the x-z coordinates shown in  FIG. 4  is acquired through the horizontal scan (x-scan) as indicated by the bold horizontal line  412  in scan pattern  410 . Similarly, the 2D OCT image  430  in the z-y coordinates shown in  FIG. 4  is acquired through the vertical scan (y-scan) as indicated by the bold vertical line  414 . 
         [0026]    According to some embodiments, both the horizontal and vertical scans illustrated in scan pattern  410  are applied, rather than applying either horizontal or vertical scans alone, to provide sufficient and reliable information for the 3D data alignment. Motion artifacts along the z-direction are more readily detected and corrected using information from both the horizontal and vertical scans. 
       Motion Correction Step  310  in Baseline Mode 
       [0027]    Artifacts resulting from eye motion during a 3D volume scanning usually degrade data quality and directly impact the accuracy and reliability of subsequent quantitative measurements. Motion correction can be performed in motion correction step  310  and a ground truth or a pseudo-motionless data set, 3D data or 2D image, can be obtained to avoid the impact of motion artifacts to the measurements and data evaluations to be subsequently performed. 
         [0028]    In some embodiments of the present invention, an iterative process using blood vessels pattern from other imaging modalities can be used to correct the motion within a 3D OCT data set in step  310 . In some embodiments, fundus photography, for example from fundus camera  106 , can be used to provide highly accurate spatial information of the retina that can be utilized to perform motion correction. A fundus photograph usually contains few motion artifacts. One or more offset vectors can be calculated using the spatial information obtained in motion correction  310  to adjust the 3D OCT data. Preferably, the x, y and z dimension of the data set can be adjusted. In some embodiments, higher dimensional spatial adjustment can also be achieved. In step  310 , instead of using the fundus photography, in some embodiments a 2D simulated scanning laser ophthalmoscope (SSLO) image can be generated, which is similar to the fundus image, by manipulating all or partial tissue reflectivity of the scanned OCT data. The reflectivity values and morphological properties of the blood vessels can also be utilized to assess the quality of the motion correction with respect to each offset vector generated in step  310 . A quantitative score can be defined, such as, for example, by evaluating the accuracy of landmark positions or reflectivity contrast. The optimal alignment can be achieved by finding the optimal offset vector for each of the OCT images such that the total score from the sequence of all OCT images yields the highest score among all possible offset vectors within specified search ranges. 
         [0029]    In some embodiments, the motion correction method  310  can be based on two or more consecutive 3D-OCT volume scans with orthogonal fast scan axis directions. In this method, a complete scan pattern generates at least two 3D data sets where the first data set was acquired using only the horizontal (x) scans, and the second data set using only the vertical (y) scans, as shown in  FIG.4 . 
         [0030]    In other embodiments, high-performance computing techniques, such as the hardware-related Streaming SIMD (Single Instruction Multiple Data) Extension (SSE) and Graphic Processing Units (GPU) can be used to increase the speed performance of the motion correction step  310 . 
       Reference Generation Step  315  in Baseline Mode 
       [0031]    Since saving the scanned 3D data sets usually utilizes a large memory space, it is advantageous to extract a subset of the motion-corrected data to be the baseline reference. In step  315 , the baseline reference data is important because it can be used as a baseline to facilitate data registration and progression analyses for OCT data sets acquired in follow up visits. The baseline reference can also be used for visualization applications. In some embodiments, the blood vessels can be extracted as a subset of the motion-corrected data and saved as the baseline reference for subsequent follow-up visits. The networking structure of the blood vessels significantly reduces the needed memory for storage and processing and further provides an information rich reference for the data registration in follow-up visits. 
         [0032]    Real-time Tracking Step  320  in Follow-Up Mode 
         [0033]    In order to track disease progression or its response to treatment, it is desirable to perform OCT measurements at substantially the same location over multiple office visits. In the follow-up mode of the data acquisition as in step  320 , video-based or SLO-based real-time tracking can be used to compensate patient eye motion during the data acquisition of step  320 , which; this method can also account for any changes in the patient&#39;s fixation location from one visit to another to further reduce motion artifacts. Step  320  allows OCT scans to be acquired at substantially identical locations over multiple office visits for the same subject, further improving the quality of the OCT data for subsequent processing and analyses. 
       Data Registration Step  325  in Follow-Up Mode 
       [0034]    In step  325  of the follow-up data acquisition mode, a previously saved baseline reference in OCT can be used to register data from OCT or other imaging modalities acquired in the current or follow-up visits. In common clinical practice, patients are routinely examined by different ophthalmic instruments providing both structural and functional information. For example, the OCT data providing structural information can be registered with the color fundus photography (structural information) or the visual field test data (functional information) to provide physicians more useful diagnostic information. The landmarks (e.g. blood vessels and fovea location) can be used for the data registration step  325 . Since the scan area, scan resolutions, and noise characteristics can vary from instrument to instrument, additional data processing can be performed before the data registration to enhance data quality; such data processing can be noise reduction, contrast enhancement, and any other methods commonly used in the arts for data enhancement. The data registration step  325  can be achieved by any data registration method. In some embodiments, data registration step  325  uses auto-correlation functions with the capabilities of detecting variants in multiple dimensions, including scaling, rotating, and translation. 
       Volume of Interest (VOI) Segmentation Step  330   
       [0035]    Multiple-layer segmentation algorithms for the retinal, choroidal, and cornea scans are available in the RTVue FD-OCT systems (Optovue, Fremont, Calif.). 3D surfaces are hierarchically detected starting from the highest gradient magnitudes, and ending with the most subtle ones. In some embodiments, all surfaces are smoothed using some linear and nonlinear filters with a priori histological information. For retina related scans, the segmentation can generate the following retinal surfaces, namely inner limiting membrane (ILM), nerve fiber layer (NFL), ganglion cell layer (GCL), inner plexiform layer (IPL), inner nuclear layer (INL), outer plexiform layer (OPL), outer nuclear layer (ONL), outer limiting membrane (OLM), inner segment layer (ISL), connecting cilia (CL), outer segment layer (OSL), Verhoeff&#39;s membrane (VM), and retinal pigment epithelium (RPE). For choroidal related scans, the segmentation can generate surfaces such as Bruch&#39;s membrane (BM), choriocapillaries (CC), Sattler&#39;s layer (SL), Haller&#39;s layer (HL), and choroid sclera interface (CSI). For cornea related scans, the segmentation can generate anterior segment surface/tear film (TF), epithelium layer (EPL), Bowman&#39;s membrane (BOM), stroma layer (STL), Descemet&#39;s membrane (DM), and endothelium layer (ENL). 
         [0036]    In step  330 , once the desired surfaces are segmented or identified, the volume of interest (VOI) can be defined by the 3D space between any two of the segmented or identified surfaces of interest discussed above. For examples, in the early diagnosis of glaucoma, it is useful to define the VOI as the 3D space between ILM and RNFL or between ILM and IPL. For intraocular lens implantation, the space between TF and ENL can be used to compute the corneal power. For subjects with aged-related macular degeneration (AMD), the retinal layers from ILM to RPE are especially interested for the case of cystoid macular edema (CME), from ILM to ONL for the case of diabetic macular edema (DME), and from ISL to RPE for the cases of drusen disruption and geographic atrophy (GA). 
         [0037]    In addition to the VOI defined in various depth (z) positions, it can be defined in various spatial (x,y) locations. For example, the standard ETDRS (Early Treatment Diabetic Retinopathy Study) sectors can be the VOI used to evaluate each feature locally centered at the retinal fovea pit in the (x,y) plane. Depending on the desired clinical application, a suitable VOI can be segmented from the reference data of the baseline visit or the registered data of the follow-up visits for further detection or analysis. 
       Feature Extraction Step  335   
       [0038]    In step  335 , once the layers are segmented as described in  330 , different properties of the ocular tissues in each of these layers are extracted and analyzed. It has been common in the art to segment and analyze 3D data for layer thickness, however, in some embodiments of the present invention, analysis of other important properties and characteristics of the retina or VOI are provided. These important and yet frequently overlooked properties can be layer reflectivity and tissue texture.  FIGS. 2   a  and  2   b  are diagrams showing different structural properties of the tissue layers without any thickness changes.  FIG. 2   a  shows the cellular structure of the tissue layers in a healthy case, the cellular structure  210  is regularly spaced and cells are well organized and have similar shapes and sizes.  FIG. 2   b  shows an example of changes in the cellular level; the shape, size and organization of the different tissue layers  220  change due to pathologies without any change in tissue thickness. These changes result in areas with reduced reflectivity and textural differences in the acquired data, in addition to shading produced by media opacities. Therefore, reflectivity and texture analysis methods can be more suitable for quantifying the spatial variations in these situations, where pathology changes the cellular structure and properties without any changes in tissue thickness. 
         [0039]    3D reflectivity and texture features are computed in the VOI from step  330 . Some important features, denoted by F(x,y,z|VOI), include the intensity level distribution measures, run length measures, co-occurrence matrix measures, wavelet analysis measures, and pattern spectrum measures and other measures commonly known in the arts. 
         [0040]    In some embodiments, a 2D feature map, which is an advanced feature derived from the above 3D features, can be generated by 
         [0000]        F -Map ( x,y )=Σ z in VOI    F ( x,y,z|VOI ),
 
         [0000]    where the 2D feature map can be a summation of the texture measures along the z-axis in VOI. 
         [0041]    Depending on the specific application, a  1 D feature spectrum can be represented by further extracting information from the above 3D features or a 2D feature map. For example, the number of retinal disruptions with a given shape B and dimension r in the VOI can be defined as: 
         [0000]        F -Spectrum ( r,B )=# of the shape B with size  r  in  VOI    
         [0042]    In this example, a shape-size spectrum is used to quantitatively evaluate the shape and size in the segmented VOL Large impulses in the feature spectrum at a certain size indicate the existence of major (protruding or intruding) substructures of the shape B at that given size. The bandwidth of the pattern spectrum, m bw , can be used to characterize the size span of the shape B, and can be defined as: 
         [0000]        m   bw   =r   max   −r   min , 
         [0000]    where the size parameters r max  and r min  denote the maximum size and minimum size in the feature spectrum, respectively. 
         [0043]    In some embodiments, an entropy-like feature complexity measure based on the feature spectrum, m ir , can further be used to characterize the shape and irregularity of the tissues in the VOL Mathematically, it can be expressed by: 
         [0000]        m   ir   =−Σp ( r,B ) log [ p ( r,B )], 
         [0000]        p ( r,B )= F -Spectrum ( r, B )/Volume( VOI ). 
         [0044]    In this example, p(r,B) is the probability function by treating F-Spectrum (r, B) from a probabilistic viewpoint. The maximum value of m ir  is attained whenever the feature spectrum is flat, indicating that the feature of interest is very irregular or complex by containing B (e.g. sphere) patterns of various sizes. It reaches its minimum value (0) whenever the feature spectrum contains just an impulse at, for instance, r=k; then VOI is simply a pattern B of size k and therefore can be considered to be the most regular (or the least irregular). 
         [0045]      FIGS. 5   a  and  5   b  show an example of a feature spectra of the entropy measures from the exemplary tissue layer structure as described in  FIG. 2 .  FIG. 5   a  demonstrates the entropy measure of the tissue structure of the healthy case as shown in  FIG. 2   a , while  FIG. 5   b  shows the entropy measure of the irregular cellular structure of the pathology case as shown in  FIG. 2   b.    
       Feature Normalization Step  340   
       [0046]    In common clinical practice, the diagnosis of ophthalmic disease is primarily based on visual impressions. Comparing the relative features (reflectivity and texture) quantitatively among different measurements can be more effective and reliable when the features are normalized against some predefined reference. In some embodiments of the present invention, the normalization performed in step  340  can be based on OCT data. The data reflectivity may be normalized based on the maximum reflectivity at a specific reference layer. For example, the RPE, CC, and TF can be used as the reference layers for the retina, choroid, and cornea scans, respectively. The normalized features can be expressed by: 
         [0000]        NF ( x,y,z|VOI )= F ( x,y,z|VOI )× K/μ 
 
         [0000]    where K and μ are the target average reflectivity value (reference value) and the measured average reflectivity value of the reference, respectively. 
         [0047]    Alternatively, histogram-based analysis can be used to provide an estimate of the data reflectivity at a specific reference layer. This characteristic reflectivity estimate can be used to compensate the scan-to-scan variation in data reflectivity. 
         [0048]    In some embodiments, the feature normalization step  340  can utilize other imaging modalities. There are a variety of imaging modalities commonly used by ophthalmologists to aid in the diagnoses of different disease conditions. In addition to OCT, some frequently used modalities include color fundus photography, fluorescein angiography (FA), indocyanine green (ICG) angiography, ultrasonography, scanning laser ophthalmoscopy (SLO) and its derivatives (tomography, polarimetry). Various imaging modalities provide data with different reflectivity and contrast for tissues of interest. For example, we may use feature contrast between the foveal area and the blood vessels in the fundus photography as the reference to normalize a 2D feature map for OCT data. That is, 
         [0000]        NF -Map ( x,y )= F -Map ( x,y )× C/c  
 
         [0000]    where C and c are the average contrast values in fundus photography (reference value) and in acquired OCT data, respectively. 
         [0049]    In some embodiments, the normalization is based on OCT light sources of multiple-bands. In most OCT systems  100 , light source  101  is a single band light source, typically centered around a wavelength of 830 nm. Alternatively, using a light source  101  of center wavelength 500 nm will yield different tissue reflectivity in the same VOL For example, a healthy OCT RNFL data acquired with the 500 nm light source may exhibit about twice the reflectivity value as it would have been with the 830 nm light source. Damage to the RNFL may result in decreased reflectivity, which is usually more evident in the 500 nm light source than in the 830 nm light source. Therefore, features extracted from OCT data at a given wavelength can be normalized by those extracted in another wavelength. The reflectivity ratio can be estimated to be close to 2.0 for a healthy RNFL and close to 1.0 for a damaged RNFL. 
       Normative Database (NDB) Comparison and Abnormalities Detection Step  345   
       [0050]    In step  345 , normalized features from  340  can be compared to a normative database for disease detection and evaluation. The normative database of ocular data (data with normal appearance) can be derived from a set of N number of OCT volumes from normal eyes. In some embodiments, the distribution of each normalized feature at each 2D feature map location (x,y) across these N volumes is defined by the average μ N (x,y) and the standard deviation σ N (x,y) of the N feature values (one normalized feature value per volume). Thus, the deviation d(x,y|VOI) between a normalized feature map NF-Map(x,y) and the average can be expressed by: 
         [0000]        d ( x, y|VOI )=( NF -Map ( x,y )−μ N ( x,y ))/σ N ( x,y ).
 
         [0051]    A simple method to classify ocular abnormalities according to some embodiments consists of computing the local deviations from the normal appearance at set cutoff points, for example the following four cutoffs:
   a. 99% cutoff: d(x, y|VOI)=2.32;   b. 95% cutoff: d(x, y|VOI)=1.64;   c. 5% cutoff: d(x, y|VOI)=−1.64;   d. 1% cutoff: d(x, y|VOI)=− 2 . 32 .   
 
         [0056]    For example, in the CAD of glaucoma patients using the RNFL reflectivity as a feature, the tissue location can be classified as normal (≧5% cutoff), suspect (≧1% and &lt;5% cutoff), and advanced glaucoma (&lt;1% cutoff) using such exemplary scheme. 
       Progression/Analysis Step  350   
       [0057]    When data is acquired in the follow-up visit mode, a progression analysis can be performed in step  350  to monitor the trend and progress of a particular medical condition. In step  350 , the reproducibility of each of the extracted features can be evaluated for both normal and pathology cases. Reproducibility of the features accounts for the variations of different contributing factors; common factors include systems, operators, and measurement repeatability. A common measure of the reproducibility can be represented by the standard deviation of the feature under evaluation. The smaller the standard deviation, the more reproducible the measurement and thus the more reliable the results of the progression analysis. An example of a progression analysis according to some embodiments of the present invention is shown in  FIG. 6 . In this example, a trend analysis is performed over a period of time, typically a period of 2 to 5 years in a common clinical setting. In this example, extracted feature of the acquired OCT data is plotted against time at about a 6-month interval for a period of 3 years. This trend analysis is also overlaid on a feature-adjusted normative database (eg. age-adjustment) as described in step  345 . The progression analysis in  FIG. 6  can also include vertical bars at each feature value, indicating the standard deviation of the feature under evaluation, to graphically demonstrate the confidence and repeatability of the results. 
         [0058]    It should be understood that certain embodiments or portions thereof may be implemented in hardware, firmware, or software. If implemented in software, the software may be any language that can cause a processor to be configured in a manner to perform embodiments discussed herein or equivalents thereof. The software may be in the form of executable instructions and stored on any non-transient or transient, computer-readable medium that can be loaded and executed by a general purpose or application-specific processor. 
         [0059]    While the methods and devices described herein have been particularly shown and described with references to example embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein.