Abstract:
The present disclosure provides an OCT imaging system having a variety of advantages. In particular, the OCT system of the present disclosure may provide a more intuitive interface, more efficient usage of controls, and a greater ability to view OCT imaging data.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of U.S. Provisional Application Ser. No. 61/824,688 filed May 17, 2013, entitled “Enhanced Frequency-Domain Optical Coherence Tomography Systems,” which application is incorporated herein by reference in its entirety. 
     
    
     TECHNICAL FIELD  
       [0002]    The present disclosure relates to optical imaging systems, in particular optical imaging systems utilizing frequency-domain interferometry. 
       BACKGROUND  
       [0003]    Frequency-domain (or “swept-source”) optical coherence tomography (OCT) systems are powerful tools that provide non-invasive, high-resolution images of biological samples at higher acquisition speeds and lower signal-to-noise ratios than time-domain OCT systems.  FIG. 1  illustrates an exemplary frequency-domain OCT system  100  at a high level. As shown, the exemplary OCT system includes a wavelength-swept laser source  95  (also referred to herein as a frequency swept source) that provides a laser output spectrum composed of single or multiple longitudinal modes to an input of a coupler  72 . The coupler  72  divides the signal fed thereto into the reference arm  80  that terminates in the reference mirror  82  and the sample arm  84  that terminates in the sample  86 . The optical signals reflect from the reference mirror  82  and the sample  86  to provide, via the coupler  72 , a spectrum of signals that are detected by a photo-detector  88 . 
         [0004]    Despite the many advantages of frequency-domain OCT, conventional implementations can be difficult to set up and optimize. Additionally, conventional implementations can have differences in measured properties and dimensions from system-to-system. It is with respect to this, that the present disclosure is provided. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1  illustrates a block diagram of a conventional frequency-domain OCT system. 
           [0006]      FIG. 2  illustrates a block diagram of a frequency-domain OCT system arranged according to examples of the present disclosure. 
           [0007]      FIGS. 3A-3C  illustrate the impact of precession on an PCT image. 
           [0008]      FIG. 4  illustrates a block diagram of a system for adjusting the angular orientation of an image according to some examples of the present disclosure. 
           [0009]      FIGS. 5A-5C  illustrate examples of aligning the angular orientation of images arranged according to examples of the present disclosure. 
       
    
    
     DESCRIPTION OF EMBODIMENTS  
       [0010]    In general, the present disclosure provides a variety of apparatuses and methods related to frequency-domain OCT systems.  FIG. 2  shows a high level diagram of a frequency-domain OCT system  200 , which may be implemented according to various embodiments of the present disclosure. The system  200  includes a wavelength-swept light source  95  that provides a light having an output spectrum composed of single or multiple longitudinal modes. The source  95  provides the light to an input of a coupler  72 . The coupler  72  divides the signal fed thereto into a reference arm  80  and a sample arm  84 . The reference arm  80  terminates in the reference mirror  82 , also referred to as a reference plane. The sample arm terminates in a sample  86 . Optical images reflected from the sample  86  and the reference mirror  82  are received by a photodetector  88  and processed by a signal processor  210 . 
         [0011]    Additionally, the system  200  includes a controller  220 . In general the signal processor  210  may be configured to implement various image processing operations on the images acquired by the system  200  while the controller  220  may be configured to control various aspects of the system  200 . This will be described in greater detail below with reference to the example embodiments. It is important to note, that the controller  220  may be operably connected to various components within the system  200 . However, these connections are not shown in  FIG. 2  for clarity of presentation. 
         [0012]    The signal processor  210  may be realized as software, hardware, or some combination thereof. The processor may also include a main memory unit for storing programs and/or data relating to the methods described herein. The memory may include random access memory (RAM), read only memory (ROM), and/or FLASH memory residing on commonly available hardware such as one or more ASICs, FPGAs, electrically erasable programmable read-only memories (EEPROM), programmable read-only memories (PROM), programmable logic devices (PLD), or read-only memory devices (ROM). In some embodiments, the programs may be provided using external RAM and/or ROM such as optical disks, magnetic disks, or other storage devices. 
         [0013]    For embodiments in which the functions of the processor are provided by software, the program may be written in any one of a number of high-level languages such as FORTRAN, PASCAL, JAVA, C, C++, C#, LISP, PERL, BASIC or any suitable programming language. Additionally, the software can be implemented in an assembly language and/or machine language directed to the microprocessor resident on a target device. 
         [0014]    Additionally, the controller  220  may be realized as software, hardware, or some combination thereof. The processor may also include a main memory unit for storing programs and/or data relating to the methods described herein. The memory may include random access memory (RAM), read only memory (ROM), and/or FLASH memory residing on commonly available hardware such as one or more ASICs, FPGAs, electrically erasable programmable read-only memories (EEPROM), programmable read-only memories (PROM), programmable logic devices (PLD), or read-only memory devices (ROM). In some embodiments, the programs may be provided using external RAM and/or ROM such as optical disks, magnetic disks, or other storage devices. 
         [0015]    For embodiments in which the functions of the processor are provided by software, the program may be written in any one of a number of high-level languages such as FORTRAN, PASCAL, JAVA, C, C++, C#, LISP, PERL, BASIC or any suitable programming language. Additionally, the software can be implemented in an assembly language and/or machine language directed to the microprocessor resident on a target device. 
         [0016]    Other examples and aspects of the OCT system  200  are described in greater detail in U.S. Pat. No. 7,733,497 and U.S. patent application Ser. No. 13/412,787, the disclosures of which are both incorporated by reference herein in their entirety. 
         [0017]    It is noted, that although various examples described herein reference the OCT system  200 , this is merely done for convenience and clarity and is not intended to be limiting. 
         [0018]    In conventional OCT systems, maintaining a consistent angular orientation of the cross-sectional images can be difficult. In particular, because the rotation of the catheter and the acquisition of data are typically not synchronized, the angular position of the image may be different each time a new image acquisition begins. This can manifest as blurring and/or the features in the image changing locations during viewing. Furthermore, conventional OCT systems typically suffer from precession. Precession occurs where the orientation of the image drifts during acquisition, due to, for example, variations in the rotational speed of the catheter. 
         [0019]      FIGS. 3A-3C  illustrate the impact of precession on the orientation of OCT images. In particular,  FIG. 3A  depicts an acquired OCT frame  710  corresponding to correct rotational speed of the catheter. More specifically, the OCT frame  710  is captured as the catheter completes a single rotation. Accordingly, the angular positioning  711  (e.g., 90 degrees, 180 degrees, 270 degrees, 360 degrees, or the like) will be correctly represented in the frame.  FIG. 3B  depicts an acquired OCT frame  720  where the rotation of the catheter is too slow. More specifically, as can be seen, the OCT frame  720  is captured before the catheter completes the rotation. As such, the angular positioning  721  (e.g., 90 degrees, 180 degrees, 270 degrees, 360 degrees, or the like) will be incorrectly represented in the frame. In particular, the angular position  721  will be compressed.  FIG. 3C  depicts an acquired OCT frame  730  where the rotation of the catheter is too fast. More specifically, as can be seen, the OCT frame  730  is captured while the catheter completes more than one full rotation. As such, the angular positioning  731  (e.g., 90 degrees, 180 degrees, 270 degrees, 360 degrees, or the like) will be incorrectly represented in the frame. In particular, the angular position  731  will be stretched. 
         [0020]    In either case (e.g.,  FIGS. 3B-3C ) where the angular rotation of the catheter is incorrect, the corresponding OCT image shows incorrect data and the errors accumulate over multiple frames, which can cause rotation of the image orientation (i.e., precession). In addition to constant errors in the rotation velocity (e.g., as depicted in  FIGS. 3A-3C ), changes in the rotational speed of the catheter may occur during a frame, as such, different parts of the image can be stretched or compressed. This is typically referred to as non-uniform rotational distortion (NURD). 
         [0021]    As mentioned previously, encoders may be used to measure and correct for angular velocity deviations caused by the motor. However, the catheter itself may also cause changes in rotation speed, which generally cannot be detected by the encoders. As will be appreciated, variations in the rotational speed may be due to both inherent imperfections in system itself (e.g., the fiber optic rotary junction (FORJ)) as well as NURD. 
         [0022]    Embodiments of the present disclosure may be implemented to align the orientation of OCT images using measurements of the catheter&#39;s angular orientation. In particular, the angular position of the catheter can be measured and the OCT images aligned accordingly. In some examples, the catheter&#39;s angular orientation can be measured using encoders. For example, the system  200  may be implemented with encoders on the motor used to rotate the catheter. 
         [0023]    In some examples, the catheter&#39;s angular orientation can be inferred by using image processing techniques on the OCT images. In particular, the signal processor  210  may apply image-processing techniques to detect the angular position based on inherent image features present in the OCT images. As another example, the system  200  can be implemented with registration marks in the catheter. As such, the signal processor  210  can detect the angular position based on the registration marks. Correcting image orientation may be achieved either by synchronizing the acquisition of data with rotation of the catheter or by correcting angular distortions in post processing (e.g., using two-dimensional interpolation, or the like.) 
         [0024]      FIG. 4  depicts a block diagram of a system  800  that may be implemented to align the orientation of the OCT images as described herein. As depicted, the system  800  includes a rotary junction  810  (e.g., a FORJ, or the like), an optical engine  820  (e.g., the signal processor, or the like) and a data acquisition system  830 . It is important to note, that the optical engine  820  and the data acquisition system  830  (DAQ) can be implemented as a single unit or as separate units. Examples are not limited in this context. 
         [0025]    In general, OCT image data  801  is transmitted from the rotary junction  810  to the optical engine  820  while a measure of angular rotation  803  (e.g., the angular rotation of the catheter) is transmitted to the DAQ  830 . In some examples, the measure of angular rotation  803  corresponds to an electrical signal transmitted from a sensor on the motor used to rotate the catheter. For example, the sensor can indicate the “north” position, the “0 degree” position, or the like. The optical engine  820  receives optical OCT image data  801  from the rotary junction  810  and converts the optical OCT image data  801  to electrical signals, which are communicated to the DAQ  830 . Accordingly, the DAQ  830  can align the image data received from the optical engine  820  based on the received measure of angular rotation  803 . 
         [0026]      FIGS. 5A-5C  illustrate examples of aligning the angular rotation of an acquired frame with the angular rotation (or speed) of the catheter. In general, these figures depict acquired OCT frames  911 ,  912 , and  913  a corresponding rotational position signals  921 ,  922 , and  923 . The system  800  is configured to align the OCT data in the frames  911 - 913  based on the corresponding position signals  921 - 923 . For example,  FIG. 5A  depicts synchronization of image acquisition for standard OCT acquisition. More specifically, acquisition of the frames  911 ,  912 , and  913  is independent of the timing of the rotational position signals  921 ,  922 , and  923 . It is noted, that in this example, acquisition of data starts independent of the orientation of the catheter. More specifically, acquisition of the frame  911  is not synchronized with receipt of the rotational position signal  911 . It is noted, that this may result in a different angular image orientation each time an acquisition starts. 
         [0027]      FIG. 5B  depicts synchronization of the data acquisition with the rotational position signals. In particular, by waiting for the rotational position signals  911  to start the acquisition of the frame  911 , a consistent initial image orientation is achieved. 
         [0028]    Additionally, the system  800  may be implemented to correct for precession, such as, for example procession due to the rotary junction.  FIG. 5C  depicts an example where the acquisition of each frame of OCT data is aligned or synchronized with the corresponding rotational position signal. In particular, initiation of acquisition of the frame  911  is synchronized with the corresponding rotational position signal  921 . Likewise, initiation of acquisition of the frames  912  and  913  are synchronized with the corresponding rotational position signals  922  and  923 , respectively. As such, the orientation of the image will not drift during the acquisition. 
         [0029]    As will be appreciated, the catheter itself may also cause changes in rotational speed, which generally cannot be detected by the encoders. These changes may be corrected by using image processing to either track the angular changes as a function of time based on image correlation or by detecting registration marks purposely added to the catheter. One example of tracking angular changes includes using cross-correlation between adjacent segments of the image to measure how rapidly an image changes. Faster angular rotation results in a lower correlation and slower rotation results in greater correlation. This information may be used to calculate rotational speed. The use of registration marks may include adding features to the catheter sheath or balloon that cause a noticeable change in the image, such as a reduction of intensity. Image processing methods may be used to detect the locations of these features, which then provide an indirect measurement of the angular orientation. 
         [0030]    The terms and expressions employed herein are used as terms and expressions of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding any equivalents of the features shown and described or portions thereof. In addition, having described certain embodiments of the invention, it will be apparent to those of ordinary skill in the art that other embodiments incorporating the concepts disclosed herein may be used without departing from the spirit and scope of the invention. Accordingly, the described embodiments are to be considered in all respects as only illustrative and not restrictive.