Patent Publication Number: US-2023157538-A1

Title: Optical coherence tomography system with an extended depth range

Description:
TECHNICAL FIELD 
     The present disclosure relates generally to imaging systems, and more particularly to an optical coherence tomography (OCT) system with an extended depth range. 
     BACKGROUND 
     Optical coherence tomography (OCT) is an imaging technique used to generate images of a sample, such as the interior of an eye during, e.g., a laser vitreolysis procedure. An OCT device sends light along a sample arm to the eye and a reference arm. The combination of light reflected from the sample and reference arms yields an interference pattern. In some systems, the reference arm is controlled to gather image information from different depths of the eye. 
     Known OCT devices, however, have limited imaging depths. The approaches for addressing this limitation either require trade-offs with imaging performance (e.g., speed) or use complex designs or components that can significantly increase the overall cost and complexity of the system. 
     BRIEF SUMMARY 
     In certain embodiments, an optical coherence tomography (OCT) system includes a light source, a beamsplitter, optical elements, a reference arm system, a detector, and a computer. The light source provides a light beam. The beamsplitter splits the light beam into a sample beam and a reference beam. Optical elements direct the sample beam along a sample path towards a sample, which reflects the sample beam to yield a reflected sample beam. The sample path comprises sample path ranges. The reference arm system directs the reference beam through each reference arm of a plurality of reference arms to yield a reflected reference beam. Each reference arm corresponds to a sample path range. Each reference arm is associated with a specific dispersion level with a corresponding dispersion compensation parameter set designed to address the specific dispersion level. The detector detects the reflected sample beam and the reflected reference beam, and generates a detector signal in response to detecting the reflected sample beam and the reflected reference beam. The computer performs the following for each sample path range to yield image information for the sample path ranges: select the dispersion compensation parameter set corresponding to the reference arm of the sample path range; apply the dispersion compensation parameter set to the detector signal to yield image information; and process the image information to yield image information for the sample path range. The computer generates an image of the sample from the image information for the sample path ranges. 
     Embodiments may include none, one, some, or all of the following features:
         The computer processes the image information by performing the following until an image quality satisfies an image quality criterion: perform image processing on the image information; generate a candidate image from the image information; and evaluate the image quality of the candidate image.   The detector signal comprises interference signals, where each interference signal corresponds to a reference arm of the plurality of reference arms.   The sample is within an eye.   The reference arms have reference planes including a first reference plane and a second reference plane. In an example, the axial separation between the first and second reference planes is substantially twice an OCT imaging depth minus an overlap (if any) between the planes. The first reference plane covers a positive OCT image space, and the second reference plane covers a negative OCT image space to yield an image that is substantially twice the OCT imaging depth minus the overlap between the planes. In another example, the axial separation between the first and second reference planes is substantially an OCT imaging depth minus an overlap (if any) between the planes. The first and second reference planes each cover a positive OCT image space to yield an image that is substantially twice the OCT imaging depth minus the overlap between the planes.   A first reference arm has dispersion material.   A first reference arm has dispersion material that creates a first dispersion mismatch. A second reference arm has dispersion material that creates a second dispersion mismatch different from the first dispersion mismatch.       

     In certain embodiments, a method for generating an image with an optical coherence tomography (OCT) system includes providing a light beam. The light beam is split into a sample beam and a reference beam. The sample beam is directed along a sample path (with multiple sample path ranges) towards a sample, which reflects the sample beam to yield a reflected sample beam. The reference beam is directed through each reference arm of multiple reference arms of a reference arm system to yield a reflected reference beam. Each reference arm corresponds to a sample path range and is associated with a specific dispersion level with a corresponding dispersion compensation parameter set. The dispersion compensation parameter set designed to address the specific dispersion level. The reflected sample beam and the reflected reference beam are detected by a detector, which generates a detector signal in response to detecting the reflected sample beam and the reflected reference beam. The following is performed by a computer for each sample path range to yield image information for the sample path ranges: selecting the dispersion compensation parameter set corresponding to the reference arm of the sample path range; applying the dispersion compensation parameter set to the detector signal to yield image information; and processing the image information to yield image information for the sample path range. An image of the sample is generated from the image information for the sample path ranges. 
     Embodiments may include none, one, some, or all of the following features:
         The processing the image information to yield image information for the sample path range corresponding to the reference arm includes performing the following until an image quality satisfies an image quality criterion: performing image processing on the image information; generating a candidate image from the image information; and evaluating the image quality of the candidate image.   The detector signal comprises interference signals, where each interference signal corresponds to a reference arm of the plurality of reference arms.   The sample is within an eye.   The reference arms have reference planes including a first reference plane and a second reference plane. In an example, the axial separation between the first and second reference planes is substantially twice an OCT imaging depth minus an overlap (if any) between the planes. The first reference plane covers a positive OCT image space, and the second reference plane covers a negative OCT image space to yield an image that is substantially twice the OCT imaging depth minus the overlap between the planes. In another example, the axial separation between the first and second reference planes is substantially an OCT imaging depth minus an overlap (if any) between the planes. The first and second reference planes each cover a positive OCT image space to yield an image that is substantially twice the OCT imaging depth minus the overlap between the planes.   A first reference arm has dispersion material.   A first reference arm has dispersion material that creates a first dispersion mismatch. A second reference arm has dispersion material that creates a second dispersion mismatch different from the first dispersion mismatch.       

     In certain embodiments, an optical coherence tomography (OCT) system includes a light source, a beamsplitter, optical elements, a reference arm system, a detector, and a computer. The light source provides a light beam. The beamsplitter splits the light beam into a sample beam and a reference beam. Optical elements direct the sample beam along a sample path towards a sample, which reflects the sample beam to yield a reflected sample beam. The sample path comprises sample path ranges. The reference arm system directs the reference beam through each reference arm of a plurality of reference arms to yield a reflected reference beam. Each reference arm corresponds to a sample path range. Each reference arm is associated with a specific dispersion level with a corresponding dispersion compensation parameter set designed to address the specific dispersion level. The detector detects the reflected sample beam and the reflected reference beam, and generates a detector signal in response to detecting the reflected sample beam and the reflected reference beam. The computer performs the following for each sample path range to yield image information for the sample path ranges: select the dispersion compensation parameter set corresponding to the reference arm of the sample path range; apply the dispersion compensation parameter set to the detector signal to yield image information; and process the image information to yield image information for the sample path range. The computer processes the image information by performing the following until an image quality satisfies an image quality criterion: perform image processing on the image information; generate a candidate image from the image information; and evaluate the image quality of the candidate image. The computer generates an image of the sample from the image information for the sample path ranges. In some embodiments, the reference arms have reference planes including a first reference plane and a second reference plane. In an example, the axial separation between the first and second reference planes is substantially twice an OCT imaging depth minus an overlap (if any) between the planes. The first reference plane covers a positive OCT image space, and the second reference plane covers a negative OCT image space to yield an image that is substantially twice the OCT imaging depth minus the overlap between the planes. In another example, the axial separation between the first and second reference planes is substantially an OCT imaging depth minus an overlap (if any) between the planes. The first and second reference planes each cover a positive OCT image space to yield an image that is substantially twice the OCT imaging depth minus the overlap between the planes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    illustrates an example of an optical coherence tomography (OCT) system that can generate images of the interior of an eye, according to certain embodiments; 
         FIGS.  2 A and  2 B  are diagrams of examples of the relative placement of OCT reference planes of reference arms for extended depth coverage for imaging retinal and vitreous regions; and 
         FIG.  3    illustrates an example of a method for imaging a sample and creating extended depth image, according to certain embodiments. 
     
    
    
     DESCRIPTION OF EXAMPLE EMBODIMENTS 
     Referring now to the description and drawings, example embodiments of the disclosed apparatuses, systems, and methods are shown in detail. The description and drawings are not intended to be exhaustive or otherwise limit the claims to the specific embodiments shown in the drawings and disclosed in the description. Although the drawings represent possible embodiments, the drawings are not necessarily to scale and certain features may be simplified, exaggerated, removed, or partially sectioned to better illustrate the embodiments. 
     Known optical coherence tomography (OCT) devices have limited imaging depths. Accordingly, OCT systems described herein includes a reference arm system with multiple reference arms that provide multiple reference signals. Each reference signal covers a different depth range in the sample, so the multiple signals provide extended depth coverage. In known devices with multiple reference arms, however, the multiple reference signals provide image data from multiple reference planes, which could yield multiple superimposed images that degrade image quality. Hence, to improve image quality, the reference arms of the systems herein have differential dispersion properties that can be used to separate the images. Each reference arm has a particular dispersion mismatch relative to the sample arm, which is compensated for with particular dispersion compensation parameters. Accordingly, different dispersion compensation parameters can be applied to the image data to generate an image for a particular reference arm and the corresponding depth range in the sample. 
     The OCT systems described herein provide several advantages for ophthalmic procedures. For example, the extended depth imaging can show the retina and floater in the same image, so the spatial coordinates of the floater and proximity of the floater to the retina can be determined. This allows the surgeon for laser procedures such as laser vitreolysis to check that the floater is not too close to the retina and to direct the beam to the floater coordinates. As another example, the design is relatively simple and does not require additional electro-optical components, so does not add to the system cost or complexity. As yet another example, the images of different regions can be acquired concurrently and separated during post-processing, so the image acquisition is relatively robust against various motion-related artifacts. 
       FIG.  1    illustrates an example of an optical coherence tomography (OCT) system  10  that can generate images of the interior of an eye, according to certain embodiments. OCT system  10  utilizes multiple reference arms and dispersion compensation to provide extended depth imaging. Multiple reference arms yield image information from different sample depths. Each reference arm has a particular dispersion mismatch relative to the sample arm, which is compensated for with corresponding dispersion compensation parameters. The dispersion compensation parameters are applied to the image data from the corresponding reference arm to yield image data for the associated sample depth. Image data from the different sample depths are joined together to yield an extended image. 
     In general, OCT dispersion mismatch results from the different optical path lengths of the reference arm and sample arm paths for different wavelengths. To address this, dispersion compensation parameters are used to apply a corrective dispersive phase to obtain clearer, higher contrast images. As an example of dispersion compensation parameters, consider a simple example of an interferometric setup with a beam splitter that splits and directs light towards reference and sample arms. At the reference arm, light travels through a free-space medium and is reflected by a mirror located at a physical distance of Z R  from the beam splitter. At the sample arm, light travels through a dispersive element with refractive index n(ω) and physical thickness of Z D  and is reflected by a sample located at a physical distance of Z S  from the beam splitter with a power reflectivity R S . The beam splitter receives and recombines the light reflected from the reference and sample arms. 
     The recombined light can be used to generate an interferometric OCT signal. The interferometric OCT signal can be approximated as ˜2Re{E S (ω)E* R (ω)}, where E S (ω) and E R (ω) represent the electric field signals from the sample and reference arms, respectively. The electric field signals E R (ω) and E S (ω) may be expressed as: 
     
       
         
           
             
               
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     The dispersive phase φ Disp (ω) represents the dispersive phase effect due to chromatic dispersion: 
       φ Disp (ω)=( k (ω)− k   0 )·2 Z   D  
 
     Using the Taylor series expansion of angular frequency dependent wavenumber, the dispersive phase can be represented as: 
       φ Disp (ω)=( c   1   ω+c   2 ω 2   +c   3 ω 3  . . . )·2 Z   D  
 
     In the example, the dispersion compensation parameters c 1 , c 2 , etc. can be used to perform dispersive phase correction in OCT signal processing. 
     Turning to an example embodiment, an eye has a z-axis (e.g., a visual or optical axis). OCT system  20  has reference arms R 1  and R 2  used to image z-ranges Z 1  and Z 2 , respectively, relative to (e.g., on or parallel to) the z-axis. Dispersion compensation parameters D calibrate the sample and reference signals, e.g., dispersion compensation parameters D 1  calibrate sample and reference arm R 1  signals, and dispersion compensation parameters D 2  calibrate sample and reference arm R 2  signals. When dispersion compensation parameters D 1  are applied, reference arm R 1  signal yields a clearer, higher contrast image, and reference arm R 2  yields a blurrier, lower contrast image. Iterative image processing is performed on the image data to separate the images until a desired image quality is achieved. Any suitable image quality metric may be used to determine the sharpness or blurriness of an image, e.g., analysis of peak signal levels, sharpness or blur metrics, or  2 D Fourier transforms of the images. The resulting image is used for z-range Z 1 . Analogous steps are performed with dispersion compensation parameters D 2  to yield an image for z-range Z 2 . 
     In the illustrated example, OCT system  10  includes a light source  20 , a splitter  22 , reference arm system  24 , a lens  26 , an xy-scanner  28 , a lens  30 , an objective lens  32 , a detector  34 , and a computer  40 , coupled as shown. Reference arm system  24  includes a lens  50 , reference arms R 1  and R 2 , a beamsplitter  52 , mirrors  54  ( 54   a ,  54   b ), and dispersion material  56 . Computer includes logic  60 , a memory  62  (which stores one or more computer programs  64 ), and an interface (IF)  66 . 
     As an overview of operation, light source  20  provides a light beam. Beamsplitter  22  splits the light beam into a sample beam and a reference beam. Optical elements (e.g., lens  26 , xy-scanner  28 , lens  30 , objective lens  32 ) direct the sample beam towards an eye, which reflects the light to yield a reflected sample beam. Reference arm system  24  directs the reference beam along each reference arm of multiple reference arms to yield a reflected reference beam. Each reference arm corresponds to a sample path range of multiple sample path ranges of the sample. Also, each reference arm has a specific dispersion level and a corresponding dispersion compensation parameter set designed to address the specific dispersion level. 
     Continuing with the example of operation, detector  34  detects the reflected sample beam and the reflected reference beam, and generates a detector signal in response to detecting the beams. Computer  40  performs the following for each sample path range to yield image information for the sample: select the dispersion compensation parameter set of the reference arm corresponding to the sample path range; apply the dispersion compensation parameter set to the detector signal to yield image information; and process the image information to yield image information for the sample path range. Computer  40  then generates an image of the sample from the image information for the sample path ranges. 
     Turning to the components, OCT system  10  may include any suitable OCT technology, e.g., a Fourier domain type (such as a swept source, spectral domain, or line-field type) that utilizes a fast Fourier transform (FFT) of the interference signal. Light source  20  may be a variable wavelength light source that changes the wavelength of the emitted light. Light source  20  provides light for the interferometer beam. Examples of light source  20  include a super-luminescent diode or swept-source laser. For example, light source  20  may be super-luminescent diode that provides light with an 850 nm wavelength, a greater than 7 nm spectral bandwidth, and a z-resolution of approximately 50 um. 
     Beamsplitter  22  splits the light beam into a sample beam and a reference beam, and may comprise any suitable beamsplitter, such as a dichroic mirror. Optical elements (e.g., lens  26 , xy-scanner  28 , lens  30 , objective lens  32 ) direct the sample beam towards an eye. In general, an optical element can act on (e.g., transmit, reflect, refract, diffract, collimate, condition, shape, focus, modulate, and/or otherwise act on) a laser beam. Examples of optical elements include a lens, prism, mirror, diffractive optical element (DOE), holographic optical element (HOE), and spatial light modulator (SLM). 
     In the example, lens  26  collimates beams. Xy-scanner  28  scans beams transversely in xy-directions. Examples of scanners include a galvo scanner (e.g., a pair of galvanometrically-actuated scanner mirrors that can be tilted about mutually perpendicular axes), an electro-optical scanner (e.g., an electro-optical crystal scanner) that can electro-optically steer the beam, or an acousto-optical scanner (e.g., an acousto-optical crystal scanner) that can acousto-optically steer the beam. Lens  30  and objective lens  32  collimate and focus beams towards the eye. 
     The sample beam penetrates the eye in the z-direction and is reflected by the interior of the eye. The reflected sample beam provides information about the internal portion in the z-direction. For example, the sample beam may indicate the location of the surfaces, e.g., the anterior and/or posterior surfaces, of a floater, the lens (natural or intraocular lens (IOL)), and/or the retina. 
     Reference arm system  24  includes any suitable number of reference arms R (R 1 , R 2 ) that have different optical paths bounded by reference mirrors  54  ( 54   a  and  54   b , respectively). Each reference arm is used to scan a different z-range of the eye. The z-ranges may overlap slightly, e.g., 5 millimeters (mm) or less such as 1 mm or less, to facilitate fusion of adjacent images to obtain continuous imaging. The arrange of the reference arms may have any suitable optical path difference between the reference arms, as described in more detail with reference to  FIGS.  2 A and  2 B . 
     Each reference arm has its own dispersion mismatch relative to the sample arm. In certain embodiments, dispersion material  56  creates dispersion in one or more arms to yield the different dispersion mismatches for different arms. Any suitable dispersion material  56  may be used, such as a dispersive prism, grating, fiber-stretcher, or dispersion glass (e.g., BK7, which is a pure optical borosilicate-crown glass material). The same or different dispersion material  56  may be used for the reference arms. Beamsplitter  52  directs the beam to the arm R for a particular z-range, and may switch between arms in, e.g., less than 5 millisecond (ms), such as approximately one ms. 
     Detector  34  detects the reflected sample and reference beams, which form interference signals. Detector  34  aggregates the photon reflections in the z-direction to yield an A-scan, i.e., the reflection intensity distribution of the sample light in the z-direction. Multiple A-scans may be performed in another direction (e.g., the x- or y-direction) to generate multiple adjacent A-scans, which may be compiled into a B-scan. A-scans may be performed at any suitable rate, e.g., once every 10 to 30 ms, such as every approximately 20 ms, to determine the z-location of a target such as a floater. Examples of detector  34  include a high-resolution spectrometer or fast interferometer diode. 
     Computer  40  sends instructions to components of system  10  and performs image processing to generate images of a sample. For example, computer  40  instructs the components to send sample and reference signals and to detect the reflected signals. Computer  40  then applies dispersion compensation parameter sets to the detected signals to generate images of different z-ranges of an eye and joins the images together to yield an image of the length of the eye. 
       FIGS.  2 A and  2 B  are diagrams  70  ( 70   a  and  70   b , respectively) of examples of the relative placement of OCT reference planes P 1  and P 2  of reference arms for extended depth coverage for imaging retinal and vitreous regions. In some applications, e.g., laser vitreolysis for removing floaters  72 , a laser beam is directed to a target, e.g., a floater  72 . The target should be sufficiently far from the retina such that the laser beam does not overexpose the retina. Extended depth imaging can show the retina and floater  72  in the same image to determine the proximity of the floater  72  to the retina. In the examples, placement of reference planes P 1  and P 2  extends the original OCT imaging depth Z IM  to cover mid-vitreous and posterior vitreous/retinal regions. 
       FIG.  2 A  illustrates an example of the relative placement of reference planes P 1  and P 2 . In the example, the axial separation between reference planes P 1  and P 2  is approximately twice the imaging depth Z IM  of the OCT system minus overlap ΔZ (if any) between the planes, or 2*Z IM −ΔZ. Reference plane P 1  covers a positive OCT image space and reference plane P 2  covers a negative OCT image space to yield an image across the mid-vitreous and posterior vitreous/retinal regions with an imaging depth of 2*Z IM −ΔZ. 
       FIG.  2 B  illustrates another example of the relative placement of reference planes P 1  and P 2 . In the example, the axial separation between reference planes P 1  and P 2  is approximately the imaging depth of the OCT system Z IM  minus overlap ΔZ (if any) between the planes, or Z IM −ΔZ. Reference planes P 1  and P 2  cover positive OCT image spaces to yield an image across the mid-vitreous and posterior vitreous/retinal regions with an imaging depth of 2*Z IM −ΔZ. 
       FIG.  3    illustrates an example of a method for imaging a sample and creating an extended depth image, according to certain embodiments. The method starts at step  110 , where a light source provides a light beam. A beamsplitter splits the light beam into a sample beam and a reference beam at step  112 . Optical elements direct the sample beam towards the sample at step  114 . The sample reflects the sample light to yield a reflected sample beam. 
     A reference arm system directs the reference beam along each reference arm of a plurality of reference arms at step  116  to yield a reflected reference beam for each arm. Each reference arm corresponds to a sample path range of the sample. Each reference arm is also associated with a specific dispersion level and an associated dispersion compensation parameter set, where the dispersion compensation parameter set addresses the specific dispersion level. A detector detects the reflected sample beam and reference beam and generates a detector signal comprising interference information at step  120 . 
     A computer gathers image information for the sample path ranges from the detector signal at steps  122  to  134 . A sample path range is selected at step  122 . The computer selects the dispersion compensation parameter set of the reference arm corresponding to the sample path range at step  124 . The computer applies the dispersion compensation parameter set to the detector signal at step  126  to yield image information for the sample path range. The computer processes the image information to improve image quality at step  130 . An image is generated at step  132 , and image quality of the image is evaluated at step  134 . The image quality may be satisfactory at step  142 . If the image quality is not satisfactory, the method returns to step  130  to process the image information to improve image quality. If the image quality is satisfactory, the method proceeds to step  142 . 
     There may be a next sample path range to consider at step  142 . If there is a next range, the method returns to step  122  to select the next range. If there is no next range, the method proceeds to step  150 . The computer joins the images for the sample path ranges to generate an extended depth image of the sample at step  150 . The computer outputs the extended depth image (e.g., on a display) at step  152 . 
     A component (such as the control computer) of the systems and apparatuses disclosed herein may include an interface, logic, and/or memory, any of which may include computer hardware and/or software. An interface can receive input to the component and/or send output from the component, and is typically used to exchange information between, e.g., software, hardware, peripheral devices, users, and combinations of these. A user interface is a type of interface that a user can utilize to communicate with (e.g., send input to and/or receive output from) a computer. Examples of user interfaces include a display, Graphical User Interface (GUI), touchscreen, keyboard, mouse, gesture sensor, microphone, and speakers. 
     Logic can perform operations of the component. Logic may include one or more electronic devices that process data, e.g., execute instructions to generate output from input. Examples of such an electronic device include a computer, processor, microprocessor (e.g., a Central Processing Unit (CPU)), and computer chip. Logic may include computer software that encodes instructions capable of being executed by an electronic device to perform operations. Examples of computer software include a computer program, application, and operating system. 
     A memory can store information and may comprise tangible, computer-readable, and/or computer-executable storage medium. Examples of memory include computer memory (e.g., Random Access Memory (RAM) or Read Only Memory (ROM)), mass storage media (e.g., a hard disk), removable storage media (e.g., a Compact Disk (CD) or Digital Video or Versatile Disk (DVD)), database, network storage (e.g., a server), and/or other computer-readable media. Particular embodiments may be directed to memory encoded with computer software. 
     Although this disclosure has been described in terms of certain embodiments, modifications (such as changes, substitutions, additions, omissions, and/or other modifications) of the embodiments will be apparent to those skilled in the art. Accordingly, modifications may be made to the embodiments without departing from the scope of the invention. For example, modifications may be made to the systems and apparatuses disclosed herein. The components of the systems and apparatuses may be integrated or separated, or the operations of the systems and apparatuses may be performed by more, fewer, or other components, as apparent to those skilled in the art. As another example, modifications may be made to the methods disclosed herein. The methods may include more, fewer, or other steps, and the steps may be performed in any suitable order, as apparent to those skilled in the art. 
     To aid the Patent Office and readers in interpreting the claims, Applicants note that they do not intend any of the claims or claim elements to invoke 35 U.S.C. § 112(f), unless the words “means for” or “step for” are explicitly used in the particular claim. Use of any other term (e.g., “mechanism,” “module,” “device,” “unit,” “component,” “element,” “member,” “apparatus,” “machine,” “system,” “processor,” or “controller”) within a claim is understood by the applicants to refer to structures known to those skilled in the relevant art and is not intended to invoke 35 U.S.C. § 112(f).