Abstract:
An exemplary apparatus and/or an exemplary method can be provided using which, it is possible (e.g., with at least one first arrangement) to measure an amplitude and/or a phase of at least one electromagnetic radiation provided from a particular portion of a sample. Further, it is possible (e.g., using at least one second arrangement) to scan a location of the particular portion along a path from a first point of the sample to a second point of the sample. In addition, it is possible to control the scan (e.g., with the second arrangement) such that the scan may comprise at least one first segment having a positive velocity and at least one segment having a negative velocity. A first distance of the first segment and/or the second segment can be smaller than a second distance between the first and second points.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
       [0001]    The present invention claims priority from U.S. Patent Application Ser. No. 60/952,964 filed on Jul. 31, 2007, the entire disclosure of which incorporated herein by reference. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
       [0002]    The invention was made with the U.S. Government support, at least in part, by National Institute of Health, Grant number R21CA125560. Thus, the U.S. government may have certain rights in the invention. 
     
    
     FIELD OF THE PRESENT INVENTION 
       [0003]    The present invention relates to systems and methods for providing beam patterns, and more particularly to such systems and methods which can provide such beam scan patterns for high speed Doppler optical frequency domain imaging. 
       BACKGROUND INFORMATION 
       [0004]    Doppler optical coherence tomography (D-OCT) techniques enable high resolution (˜10 μm) imaging of microvasculature. (See Z. P. Chen et al., “Optical Doppler tomographic imaging of fluid flow velocity in highly scattering media,” Optics Letters 22, 64-66 (1997); and J. A. Izatt et al., “In vivo bidirectional color Doppler flow imaging of picoliter blood volumes using optical coherence tomograghy,” Optics Letters 22, 1439-1441 (1997)). Because of a possible significance of vascular morphology and function across a broad range of diseases, the potential of D-OCT techniques and uses thereof can extend from applications in diagnostic imaging to basic studies of vascular biology and vascular responses to therapy. The successful adoption of these techniques for such applications can depend in part on how well D-OCT systems are able to comprehensively map vascular networks in three-dimensions. It may be important for this effort to provide the capability of D-OCT systems to image rapidly and with high flow sensitivity. 
         [0005]    Current D-OCT systems can predominately utilize a phase-resolved technique, as described in Y. Zhao et al., “Phase-resolved optical coherence tomography and optical Doppler tomography for imaging blood flow in human skin with fast scanning speed and high velocity sensitivity,” Optics Letters 25, 114 (2000), that can be based on the detection of phase shifts between sequential A-lines (e.g., depth scans) that may result from motion within the sample, e.g., blood flow. The phase shift, e.g., v ∥ , that results from a given scatter translating in the direction of the imaging beam with a velocity, v ∥ , can be provided as 
         [0000]    
       
         
           
             
               
                 
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         [0000]    where n is the refractive index of the material, v ∥  is the mean wavelength of the OCT imaging system, and T is the Doppler integration window which is equal to the time separation between phase measurements. The acquisition of the exemplary Doppler images may proceed by measuring the phase difference as a function of depth (z) and transverse coordinate (x), Δφ(z,x)). While the majority of exemplary D-OCT systems may utilize a ramp beam-scan pattern that translates the beam at a constant velocity across the field of view (FOV), alternate scan patterns may be used to affect both the Doppler integration window, T, as well as the phase noise background, and improve Doppler imaging performance. 
         [0006]    There may be a need to overcome certain deficiencies associated with the conventional arrangements and methods described above. 
       OBJECTS AND SUMMARY OF EXEMPLARY EMBODIMENTS 
       [0007]    To address and/or overcome such deficiencies, exemplary embodiments of systems and methods can provide such beam scan patterns for high speed Doppler optical frequency domain imaging. Thus, as described herein below for the exemplary embodiment of the present invention, it is possible to overcome at least some of the deficiencies associated with the prior art systems and/or methods, and achieve an improved performance resulting from a specific beam scan patterns: segmented sawtooth beam scanning. 
         [0008]    For example, segmented sawtooth scanning techniques may deviate significantly from conventional ramp scanning and accordingly enables Doppler imaging techniques with a combination of high sensitivity and rapid imaging speed that fall significantly outside the capabilities of the traditional approach. Through the parallelized acquisition of Doppler measurements across the transverse FOV, segmented sawtooth scanning can allow the Doppler sensitivity to be tuned across a broad range without affecting the imaging frame rate. The exemplary details of this scan pattern, and its ability to achieve highly sensitive Doppler imaging over a large FOV at 24 frames per second (fps) are described below. For example, three-dimensional vascular maps acquired in human skin can illustrate the capability of segmented sawtooth scanning technique, and may represent a significant advance in the demonstrated utility of D-OCT to image microvasculature in human skin. 
         [0009]    Accordingly, an apparatus and/or a method can be provided using which it is possible (e.g., with at least one first arrangement) to measure an amplitude and/or a phase of at least one electromagnetic radiation provided from a particular portion of a sample. Further, it is possible (e.g., using at least one second arrangement) to scan a location of the particular portion along a path from a first point of the sample to a second point of the sample. In addition, it is possible to control the scan (e.g., with the second arrangement) such that the scan may comprise at least one first segment having a positive velocity and at least one segment having a negative velocity. A first distance of the first segment and/or the second segment can be smaller than a second distance between the first and second points 
         [0010]    The second arrangement can comprises a galvanometric reflecting arrangement and/or an acousto-optic deflecting arrangement. A time of the scan may be less than about 200 msec and/or less than about 50 msec. The first arrangement can be used to receive an interference signal which is associated with the electromagnetic radiation and at least one further radiation received from a reference. A frequency of the electromagnetic radiation can change over time. The first distance and/or the second distance may be less than a fourth or an eighth of a third distance the first point to the second point. The sample can include a biological structure. It is possible to measure a blood flow within the sample and/or determine a response by the biological sample to a treatment thereof. 
         [0011]    These and other objects, features and advantages of the present invention will become apparent upon reading the following detailed description of embodiments of the invention, when taken in conjunction with the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    Further objects, features and advantages of the invention will become apparent from the following detailed description taken in conjunction with the accompanying figures showing illustrative embodiments of the invention, in which: 
           [0013]      FIG. 1(   a ) is a block diagram of an optical frequency domain imaging (“OFDI”) system according to an exemplary embodiment of the present invention which can be used for implementing the exemplary embodiments of methods and techniques of the present invention; 
           [0014]      FIG. 1(   b ) is an illustration of light being conveyed to a sample by a high-speed galvanometric beam scanner, in accordance with one exemplary embodiment of the present invention; 
           [0015]      FIG. 2(   a ) is an exemplary graph of an extrapolation of phase shifts measured from a pair of reference signals to yield a phase shift at the zero differential depth; 
           [0016]      FIG. 2(   b ) is an exemplary graph of a phase differential vs. depth as an exemplary acousto-optic frequency shifter which provides the results shown in the graph of  FIG. 2(   a ); 
           [0017]      FIG. 2(   c ) is an exemplary graph of a measured distribution of phase shifts at zero differential depth to cluster around discrete values of +p/2, 0, and −p/2, e.g., due to a phase locking of the frequency shifter drive signal at about 25 MHz and an acquisition clock at about 100 MHz; 
           [0018]      FIG. 3(   a ) is graph of an exemplary segmented sawtooth scan pattern for N=8 segments; 
           [0019]      FIG. 3(   b ) is a graph of a single segment from the graph of  FIG. 3(   a ) provided in greater detail; 
           [0020]      FIG. 3(   c ) is graph of an exemplary segmented sawtooth scan pattern for N=32 segments; 
           [0021]      FIG. 3(   d ) is graph of an exemplary segmented sawtooth scan pattern for N=2 segments; 
           [0022]      FIG. 4(   a ) is a set of exemplary images, e.g., a reflectivity image of a phantom and a Doppler image showing a flow signal; and 
           [0023]      FIG. 4(   b ) is a set of exemplary Doppler images within a row from each exemplary scan pattern for a fixed flow velocity. 
       
    
    
       [0024]    Throughout the figures, the same reference numerals and characters, unless otherwise stated, are used to denote like features, elements, components or portions of the illustrated embodiments. Moreover, while the subject invention will now be described in detail with reference to the figures, it is done so in connection with the illustrative embodiments. It is intended that changes and modifications can be made to the described embodiments without departing from the true scope and spirit of the subject invention. 
       DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0025]    An optical frequency domain imaging (“OFDI”) systems with an A-line rates of 50 kHz was used in this study. An exemplary embodiment of an OFDI system which can be used with the methods and arrangements according to the present invention is shown schematically in  FIG. 1(   a ). (See, e.g., B. J. Vakoc et al., “Phase-resolved optical frequency domain imaging,” Optics Express 13, 5483-5493 (2005); and S. H. Yun et al., “High-speed optical frequency-domain imaging,” Optics Express 11, 2953-2963 (2003)). For example, light from a swept source  100  can be split into a fiber optic reference arm  105  and a sample  110  preferably incorporating optical circulators  115 A,  115 B. 
         [0026]    A reflected light  121  from a sample  120  can be combined with a light  125  from the reference arm  105  in a free-space optical circuit utilizing a non-polarizing broadband beam splitter (BBS)  131  and polarization beam splitters (PBS)  132   a,    132   b.  Thus, both a balanced-detection and a polarization-diversity detection can be enabled. For example, a 1% tap coupler  135  in the sample arm  110  can be used to generate a continuous phase recalibration signal which may allow for phase variations due to timing jitter to be removed. The imaging beam can be scanned across the sample with a high-speed galvanometer  140 , as shown in  FIG. 4(   b ) A polarization controller (PC)  101  can be placed in the interferometer reference arm. An acousto-optic frequency shifter (FS)  102  was incorporated in the reference arm. Multimode fibers (MMF)  141  may be used to couple the optical output signals to balanced receivers (Rx)  142 . Light was conveyed to the sample by a high-speed galvanometric beam scanner  143 . 
         [0027]    The acousto-optic frequency shifter  102  can be operated at about 25 MHz, and may be used to double the imaging range through the removal of depth degeneracy. (See, e.g., S. H. Yun et al., “Removing the depth-degeneracy in optical frequency domain imaging with frequency shifting,” Optics Express 12, 4822-4828 (2004)). Interference fringes may be digitized at about 100 MS/sec. 
         [0028]    As described above, an exemplary use of a laser source that sweeps asynchronously with the acquisition board digitization clock can produce a variability in both the starting time and the starting wavelength of each acquired A-line. The variability in the starting wavelength may result in a spurious, depth-dependent phase shift. The variability in the starting time in combination with the use of an acousto-optic frequency shifter may induce a spurious, depth-independent phase shift. It may be difficult to discern the relative contributions of a starting wavelength variation and a starting time variation using a single recalibration mirror signal. Therefore, it may be difficult to determine how to scale the measured phase shift with depth to generate an appropriate correction signal. To address this, a modified phase recalibration approach can be implemented. For example, instead of a single phase recalibration signal, a pair of phase recalibration signals spaced approximately 100 microns apart can be generated from a reflection from a front surface and a back surface of a glass slide  137 . The following exemplary procedure can be used to remove spurious timing and starting wavelength induced phase shifts: 
         [0029]    (A) The phase difference between two recalibration signals can be measured, and the phase difference at zero differential depth may be determined by a linear extrapolation.  FIG. 2(   a ) illustrates an exemplary measured A-line showing exemplary phase recalibration signals  200 A,  200 B.  FIG. 2(   b ) shows an exemplary extrapolation of the measured phase difference from each signal  202 A,  202 B to that at a zero differential delay  201 . Because the phase shift due to variations in the starting wavelength can scale with depth, the phase shift at zero differential delay therefore may provide such component resulting from the frequency shifter. Further, by phase-locking the data acquisition clock (e.g., at about 100 MHz) and the frequency shifter drive signal (e.g., at about 25 MHz), the value of this exemplary phase variation may be restricted to an integer multiple of π/2 (2π*25/100). The extrapolated value may therefore be rounded to the nearest integer multiple of □/2. This extrapolated phase shift can then be subtracted from the measured phase difference at all depths, possibly removing the errors resulting from the frequency shifter.  FIG. 2(   c ) shows an exemplary distribution of measured phase shifts at zero differential delay; and 
         [0030]    (B) After completion of exemplary procedure (A) above, same or similar approach previously described to remove phase shifts origination from the variations in the starting wavelength can be applied. The signal from either surface of the glass slide can be used as the phase recalibration signal. 
         [0031]    The generalization of the exemplary embodiment of a phase recalibration approach described above may provide for a phase-sensitive Doppler imaging in the exemplary embodiment of OFDI systems incorporating a frequency shifter. 
         [0032]    An exemplary embodiment of a beam scan pattern can be used that may, in contrast to conventional ramp scan patterns, decouple a Doppler integration window and an imaging frame rate by acquiring exemplary Doppler measurements in parallel. As a result, high frame rate systems with extremely high Doppler sensitivities can be achieved. 
         [0033]    The specific exemplary beam scan pattern discussed herein may have, e.g., a segmented sawtooth waveform. For example,  FIG. 3(   a ) illustrates one of the possible exemplary configurations of this scan pattern, and can define the relevant parameters of the exemplary scan. The overall scan pattern may comprise of a series of N identical concatenated segments (e.g., N=8 as shown in  FIGS. 3(   a ) and  3 ( b )). Each segment may acquire Doppler measurements over a portion (1/N) of the full transverse field of view. Within each segment, as illustrated in  FIG. 3(   b ), the beam can be scanned for P A-lines (e.g., P=22 shown in  FIGS. 3(   a ) and  3 ( b )) at, e.g., a constant velocity to provide a first phase measurement over such exemplary limited transverse extent. The beam can then be rapidly returned and allowed to stabilize over a series of M A-lines (e.g., M=18 as shown in  FIGS. 3(   a ) and  3 ( b )), and subsequently rescanned over the same transverse extent previously sampled. Measurements of exemplary phase shift between the first and second scan may provide the flow information, with the Doppler integration window given by (M+P) A-lines. By changing the number of segments, N, in a given scan, this Doppler integration time can be reduced or increased, as illustrated in  FIGS. 3(   c ) and  3 ( d ) which shows similar scans for (N=32) and (N=2) with correspondingly shorter and longer integration times. 
         [0034]    Table 1 provided herein below can define several (e.g., seven) scan patterns with a range of segments per frame (N) and corresponding Doppler sensitivities. In these exemplary patterns, the number of A-lines per frame can be held constant at about 2048, and the resulting image width may be held constant at 704 pts/pixels. Additionally, 64 A-lines can be discarded from the beginning of each scan to allow for the flyback/settling time of the galvanometer between frames. The flyback time within the segments (M) can be scaled with the flyback distance, which may scale with the number of segments (N). Taken together, these exemplary seven scan patterns can each be acquired at about 24 fps, and may provide the Doppler sensitivity to be adjusted by a factor of about 64. The measured phase sensitivities and minimum detectable flow velocities listed in Table 1 are described herein. 
         [0000]    
       
         
               
             
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Exemplary parameters of the segmented sawtooth scans. A variation in segment 
               
               
                 number (N) may result in varying Doppler integration windows, T, that can set a Doppler 
               
               
                 sensitivity for each scan pattern. 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Frame Size 
                 2048 
               
               
                 (# A-lines) 
               
               
                 Pad (Frame Flyback) 
                 64 
               
               
                 (# A-lines) 
               
               
                 Frame Rate 
                 24 
               
               
                 (fps) 
               
               
                 Image Width 
                 704 
               
               
                 (# pts/pixels) 
               
             
          
           
               
                 Segments per Frame, N 
                 64 
                 32 
                 16 
                 8 
                 4 
                 2 
                 1 
               
               
                 Pad (Segment Flyback), M 
                 9 
                 18 
                 36 
                 72 
                 144 
                 288 
                 576 
               
               
                 (# A-lines) 
               
               
                 M = 9 * 64/N 
               
               
                 Segment Length, P 
                 11 
                 22 
                 44 
                 88 
                 176 
                 352 
                 704 
               
               
                 (# A-lines) 
               
               
                 P = ((2048 − 64)/N − M)/2 
               
               
                 Time Separation, T 
                 20 
                 40 
                 80 
                 160 
                 320 
                 640 
                 1280 
               
               
                 (# A-Lines) 
               
               
                 Measured Phase Sensitiivty 
                 0.20 
                 0.18 
                 0.17 
                 0.18 
                 0.20 
                 0.27 
                 0.21 
               
               
                 (rad) 
               
               
                 Predicted Axial Flow Sensitivity 
                 37.1 
                 16.7 
                 8.00 
                 4.13 
                 2.26 
                 1.57 
                 0.60 
               
               
                 (μm/sec) 
               
               
                   
               
             
          
         
       
     
         [0035]    The scan patterns described in Table 1 may be used to generate Doppler images of size 704×1024 pixels using a 50 kHz A-line rate system as described herein. An exemplary bulk motion can be removed by subtracting the median phase shift for points with a signal magnitude exceeding a given threshold. (See, e.g., M. C. Pierce et al., “Simultaneous intensity, birefringence, and flow measurements with high-speed fiber-based optical coherence tomography,” Optics Letters 27, 1534-1536 (2002)). Exemplary structural images may be generated as a mean of the A-line power reflectivity of each of the A-lines between which phases may be compared for the Doppler image. The phase difference at each depth from the polarization channel with the largest signal may be used; and the phase difference measured in the orthogonal channel can be discarded. 
         [0036]    Because the Doppler integration window can be increased, e.g., almost arbitrarily with the segmented sawtooth scan pattern, the phase noise background does not have to play a limiting role in determining the Doppler sensitivity. However, it may be important that this phase noise floor can be approximately independent of the Doppler integration window to ensure that enhancements in sensitivity from the latter are not cancelled by increases in the former. To verify that the noise floor remained approximately constant, images of a homogenous rubber phantom may be acquired using each of the scan patterns described in Table 1, and the phase noise may be calculated as the standard deviation of the phase difference within a 500×280 pixel (e.g., 2.8 mm×1.2 mm) region of interest (“ROI”). Table 1 lists the exemplary measured phase noise for each scan pattern. The exemplary noise can remain approximately constant as the Doppler integration time increases. The axial flow sensitivity predicted from the measured phase sensitivity by Eq. 1 can also be provided to demonstrate the scaling of flow sensitivity with scan pattern parameters. 
         [0037]    To demonstrate the scaling of the Doppler sensitivity with the scan pattern parameters, exemplary images of a flow channel phantom may be acquired for each of the patterns of Table 1 and, e.g., for three flow velocities. For example, the flow phantom may consist of a transparent sheath  400  embedded in a silicone rubber  401  with TiO2 added to provide scattering. Glycerol with the same TiO2 scatters  410  can be effected to flow through the tube using an infusion pump with variable flow settings.  FIG. 4(   a ) illustrates a portion of an exemplary structural image of the flow phantom, and  FIG. 4(   b ) illustrates a corresponding exemplary Doppler image. The flow within the tube may be clearly indicated in the Doppler image, with the magnitude being large enough to induce phase wrapping as indicated by the rings. 
         [0038]    The exemplary embodiment of the system according to the present invention can be configured to rapidly alternate between each of the seven scan patterns in succession. Exemplary Doppler images for each of the patterns are shown in  FIG. 4(   b ), with each row presenting the images from each of the seven scan patterns for a fixed flow velocity, and each row showing these results for a different flow velocity. Each exemplary image can be mapped to the same or similar grayscale of  FIG. 4(   b ). The scaling of the sensitivity within each row, and the consistency in the level of the phase noise background within each image are provided. 
         [0039]    The foregoing merely illustrates the principles of the invention. Various modifications and alterations to the described embodiments will be apparent to those skilled in the art in view of the teachings herein. Indeed, the arrangements, systems and methods according to the exemplary embodiments of the present invention can be used with imaging systems, and for example with those described in International Patent Application PCT/US2004/029148, filed Sep. 8, 2004, U.S. patent application Ser. No. 11/266,779, filed Nov. 2, 2005, and U.S. patent application Ser. No. 10/501,276, filed Jul. 9, 2004, the disclosures of which are incorporated by reference herein in their entireties. It will thus be appreciated that those skilled in the art will be able to devise numerous systems, arrangements and methods which, although not explicitly shown or described herein, embody the principles of the invention and are thus within the spirit and scope of the present invention. In addition, to the extent that the prior art knowledge has not been explicitly incorporated by reference herein above, it is explicitly being incorporated herein in its entirety. All publications referenced herein above are incorporated herein by reference in their entireties.