Ophthalmic imaging system and method for suppressing imaging artifact in ophthalmic image

Abstract

To provide a method of suppressing a banding artifact in an ophthalmic image of a patient's eye.SOLUTION: The method comprises: partitioning the ophthalmic image into a plurality of segments that partially overlap each other, and applying an image correction algorithm, which computes a discrete cosine transform of each segment of the plurality of segments to suppress the banding artifact in the plurality of segments removing at least part of the one or more overlapping regions from each segment to remove an artifact introduced by the image correction algorithm to generate respective corrected segments; and combining the corrected segments to generate a corrected ophthalmic image that comprises less of the banding artifact than the ophthalmic image.SELECTED DRAWING: Figure 8

Description

[Technical field]

[0001] The present disclosure relates to ophthalmic imaging systems and methods for suppressing banding artifacts in ophthalmic images. In particular, but not exclusively, the present disclosure relates to suppressing banding artifacts introduced into ophthalmic images by a polygon scanning mirror in an ophthalmic imaging system. [Background technology]

[0002] Ophthalmic imaging devices, such as scanning laser ophthalmoscope (SLO) and optical coherence tomography (OCT) imaging systems, typically include a light source and a scanning system that scans light emitted by a light source over a target imaging area, such as a patient's retina, by controlling the position of an imaging beam with one or more scanning mirrors.

[0003] One type of mirror that can be used in a scanning system is a polygonal scanning mirror. A polygonal scanning mirror comprises a polygonal mirror mounted on a rotating shaft. The polygonal mirror comprises reflective facets arranged in a polygon. As the polygonal scanning mirror rotates, a light beam reflects off the facets and the light beam is scanned across a target area. A polygonal scanning mirror can rapidly scan light at an imaging target. As such, polygonal scanning mirrors are often used to linearly scan a light beam across a target area. For example, if the scan pattern is a series of parallel vertical lines, the polygonal scanning mirror will linearly scan a light beam along the vertical lines of the target area, and a scanning mirror such as a scanning galvanometer can be used to control the horizontal position of the light beam across the target area. A polygonal scanning mirror and a scanning galvanometer mirror can work in conjunction to form an XY scanning system that can be used to scan a light beam across an imaging target to generate an image of the target.

[0004] Bands are optical artifacts that can occur in images that use a polygon scanning mirror to scan a beam of light at an imaging target. Banding artifacts can appear in an image as a series of dark and / or light bands that occur periodically in the image. In the example where a polygon scanning mirror scans an imaging beam vertically across an imaging target, the bands typically present as dark and / or light bands that extend vertically across the resulting image. However, banding artifacts can take other forms, such as in the form of horizontal lines.

[0005] Banding artifacts in an image are caused by several contributing factors. For example, imperfect optical components of the scanning system can introduce bands into the image. This can include irregularities or imperfections in the polygon scanning mirror as it rotates, or wobble of the polygon scanning mirror as it rotates, which can cause variations in the angular position of the facets of the polygon mirror as it rotates. These variations can cause the imaging beam to deviate from its intended scanning pattern, thereby resulting in visible lines or bands in the resulting image. Additionally, variations in the reflectivity of each of the reflective facets or mirrors of the polygon can cause variations in the intensity of the light scanned across the target area. Bands can also be caused by uneven line spacing between the lines scanned by the polygon scanning mirror, which can be introduced by imperfections in the horizontal scanning galvanometer.

[0006] Finally, while band artifacts may appear in an image for a variety of reasons, their presence in an image acquired by an imaging system impairs the quality of the resulting image. This can make it difficult for a clinician to visualize or assess the severity of pathology in the resulting image, when the image is a medical image of tissue, such as an ophthalmic image of the retina. Thus, to improve the quality of images captured by an imaging system having a polygonal scanning mirror, it is desirable to minimize or eliminate bands from the image.

[0007] Image banding can be suppressed by using high quality optical components in the imaging system. For example, using high quality reflective facets on the polygon can reduce the variation in reflectivity of each facet. However, high quality optical components can be very expensive, which increases the overall cost of the imaging system. Additionally, damping vibrations in the imaging system can help reduce the presence of banding in images captured by the imaging system, but this can be difficult due to the inherent vibrations associated with the rotating polygon scanning mirror. Summary of the Invention

[0008] A method is described for suppressing banding artifacts in an image of an imaging target, the method including: dividing the image of the imaging target into a plurality of segments that partially overlap one another, each segment of the plurality of segments comprising one or more overlapping regions, each overlapping region being an overlapping region of each segment adjacent to each segment of the plurality of segments; applying an image correction algorithm to compute a discrete cosine transform of each of the plurality of segments to suppress banding artifacts in the plurality of segments; and removing at least a portion of the one or more overlapping regions from each of the plurality of segments to remove edge effects or artifacts introduced by the image correction algorithm to generate a respective corrected segment. Combining each of the corrected segments to generate a corrected image of the imaging target having fewer banding artifacts than the image.

[0009] Here, according to a first embodiment, there is provided a method for suppressing banding artifacts, such as periodic line artifacts, in an ophthalmic image of a patient's eye, the method comprising: dividing the ophthalmic image into a plurality of partially overlapping segments, each of the plurality of segments including one or more overlapping regions, each overlapping region being an overlapping region of the segment with each adjacent segment of the plurality of segments; applying an image correction algorithm to compute a discrete cosine transform of each of the plurality of segments to suppress the banding artifacts in the plurality of segments; removing at least a portion of one or more overlapping regions of each of the plurality of segments to remove artifacts introduced by the image correction algorithm; generating each corrected segment; and combining the corrected segments to generate the corrected image having fewer banding artifacts than the ophthalmic image. This method and any of its exemplary embodiments described below may be implemented (executed) by a computer.

[0010] The method may include performing filtering in the frequency domain after the discrete cosine transform is calculated to remove at least one of the low frequency and high frequency signals from each of the segments. Additionally, performing filtering may include removing discrete cosine transform coefficients having an absolute value equal to or exceeding a threshold. For example, the threshold may be an integer value of 1, 2, 3 or more. Removing discrete cosine transform coefficients having an absolute value equal to or greater than the threshold may include setting coefficients equal to or greater than the threshold to zero.

[0011] Additionally or alternatively, the method may include applying a window function to the computed discrete cosine transform of each segment. The window function may be applied to window frequency bins in the discrete cosine transform of each segment (which frequency bins correspond to spatial frequencies of the banding artifacts) to attenuate banding artifacts in the discrete cosine transform of each segment.

[0012] The window function may include a second component for filtering a second frequency corresponding to a harmonic of the frequency of the banding artifact. The second component may be applied to window a frequency bin in the discrete cosine transform of each segment that corresponds to the second frequency. The second frequency may be a multiple of the frequency of the banding artifact. In one embodiment, the window function may be a Hanning window filter.

[0013] Optionally, the image correction algorithm may include, for each segment of the plurality of segments, calculating an inverse discrete cosine transform of each of the segments after the discrete cosine transform has been calculated for each segment.

[0014] The method can include combining the corrected segments to generate a corrected ophthalmic image. Combining the corrected segments can include combining a peripheral region of a first corrected segment of the corrected segments with a peripheral region of an adjacent corrected segment of the corrected segment. Combining the peripheral region of the first corrected segment with a peripheral region of an adjacent corrected segment can include aligning one or more features present in both the first corrected segment and the adjacent corrected segment such that the one or more features are superimposed on one another in the corrected image such that a junction between the first corrected segment and the adjacent corrected segment is not visible.

[0015] According to a second embodiment of the present specification, a computer program is provided, which includes computer-readable instructions, which, when executed by a processor, causes the processor to perform the method according to the above embodiment or at least one of its embodiments. The computer program may be stored in a non-transitory computer-readable storage medium or transmitted by a signal.

[0016] According to a third embodiment, an ophthalmic imaging system for imaging an eye of a patient, comprising: a light source arranged to emit a light beam; a scanning system including a polygonal scanning mirror arranged to scan the light beam over an area of ​​the patient's eye; a photodetector optically coupled to the scanning system, the photodetector configured to generate a detection signal based on light reflected by the patient's eye; and data processing hardware or a processor configured to receive the detection signal from the photodetector and to generate based on the received detection signal, an ophthalmic image of the patient's eye, the ophthalmic image including banding artifacts, the data processing hardware configured to image the ophthalmic images by scanning the ophthalmic images of the patient's eye, the ophthalmic images ... an image correction algorithm that applies a discrete cosine transform of each of the plurality of segments to suppress banding artifacts in the plurality of segments, removing at least a portion of the one or more overlapping regions from each of the plurality of segments to remove artifacts introduced by the image correction algorithm to generate a corrected segment, and combining each of the corrected segments to generate the corrected ophthalmic image having less banding artifacts than the ophthalmic image.

[0017] The ophthalmic imaging system may be a scanning laser ophthalmoscope (SLO) imaging system or an optical coherence tomography (OCT) imaging system. The OCT imaging system may be, for example, a swept-source OCT, a spectral domain OCT, or a Fourier domain OCT imaging system.

[0018] In one embodiment, the data processing hardware may be further configured to perform filtering in the frequency domain to remove at least one of low frequency and high frequency signals from each of the segments after the discrete cosine transform has been calculated. Removing or filtering the low frequency and / or high frequency signals may include removing discrete cosine transform coefficients from each of the overlapping segments having an absolute value equal to or exceeding a threshold. Removing discrete cosine transform coefficients having an absolute value equal to or exceeding a threshold may include setting the discrete cosine transform coefficients to zero.

[0019] Additionally or alternatively, the data processing hardware may be further configured to apply a window function to the calculated discrete cosine transform of each segment. To attenuate banding artifacts in the discrete cosine transform of each segment, a frequency bin in the discrete cosine transform of each segment that corresponds to the spatial frequency of the banding artifact may be applied to the window. The window function may include a second component for filtering a second frequency that corresponds to a harmonic of the frequency of the banding artifact. The second component may be applied to window a frequency bin in the discrete cosine transform of each segment that corresponds to the second frequency of the harmonic. The window function may be a Hanning window.

[0020] Optionally, applying the image correction algorithm may further include calculating for each segment of the plurality of segments, i.e., calculating an inverse discrete cosine transform of each of the segments after the discrete cosine transform has been calculated for each of the partially overlapping segments.

[0021] Optionally, combining the corrected segments to generate the corrected ophthalmic image may include combining a peripheral region of a first one of the corrected segments with a peripheral region of an adjacent corrected segment of the corrected segment.

[0022] Exemplary embodiments are described in detail, by way of non-limiting example, with reference to the accompanying drawings, in which: Like reference numbers appearing in different figures may indicate identical or functionally similar elements unless otherwise indicated, and in which: [Brief description of the drawings]

[0023] [Figure 1] FIG. 1 is a schematic diagram of an imaging system with a polygon scanning mirror in accordance with an example embodiment of the present disclosure. [Diagram 2] FIG. 2 is an example of an image including banding artifacts captured by the imaging system in FIG. [Diagram 3] FIG. 3 is a schematic diagram of the image of FIG. 2 segmented into partially overlapping segments. [Figure 4] FIG. 4 is a flow chart illustrating a method for suppressing banding artifacts in one of the overlapping segments of FIG. [Diagram 5] FIG. 5 is an example of a segment of the segmented image of FIG. 3 that contains banding artifacts. [Figure 6] FIG. 6 is an example of the segment of FIG. 5 after a correction algorithm has been applied to the segment to suppress banding artifacts. [Figure 7]FIG. 7 is an example of an image captured by the imaging system of FIG. 1 after banding artifacts have been suppressed in accordance with an example embodiment herein. [Figure 8] FIG. 8 is a flow chart illustrating a method for suppressing banding artifacts in an image captured by the imaging system in FIG. [Figure 9] FIG. 9 is a hardware implementation diagram of an example of an apparatus operable as data processing hardware for an imaging system, according to an embodiment herein. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] In view of the above background, the present inventors have devised an ophthalmic imaging system for capturing images of a patient's eye that utilizes a scanning system that includes a polygonal scanning mirror for scanning a beam of light across the patient's eye to generate a captured image of the patient's eye. The ophthalmic imaging system includes a control module or data processing hardware arranged to apply an image correction algorithm to an uncorrected ophthalmic image that includes banding artifacts to remove or suppress banding artifacts present in the ophthalmic image. Removing or suppressing banding artifacts in the ophthalmic image may include dividing the ophthalmic image into partially overlapping segments, applying an image correction or artifact suppression algorithm to each overlapping segment, cropping or removing the overlapping region of each of the partially overlapping segments, and recombining the cropped segments to generate a corrected ophthalmic image that is less banding artifact-free than the uncorrected image.

[0025] The use of data processing hardware configured to apply an image correction algorithm to an uncorrected ophthalmic image including banding artifacts advantageously removes or suppresses the banding artifacts from the ophthalmic image, thereby improving the image quality of the image acquired by the ophthalmic imaging system. Because banding artifacts detract from the quality of the ophthalmic image, it is advantageous to remove and / or suppress banding artifacts present in images acquired by the ophthalmic imaging system. The banding artifacts may be, for example, periodic line artifacts.

[0026] By dividing the uncorrected image into partially overlapping segments and applying an image correction algorithm to each of the overlapping segments, banding artifacts can be suppressed in each segment, and any residual edge effects or edge artifacts introduced by the image correction algorithm will typically be contained in the overlapping regions of the overlapping segments. Advantageously, the overlapping regions of each segment can be removed such that a central portion of each segment can be combined to create a corrected image, thereby removing edge effects from the corrected image and allowing the corrected image to be generated using only the central portion of each corrected segment.

[0027] FIG. 1 shows an imaging system 10 for acquiring an image of an imaging target 11. The imaging system 10 may be a medical imaging system, such as an ophthalmic imaging system, and the imaging target 11 may be a biological tissue, e.g., a patient's eye, such that the ophthalmic imaging system is positioned to acquire ophthalmic images of the patient's eye. The imaging system 10 may be, for example, a scanning laser ophthalmoscope (SLO) imaging system, or an optical coherence tomography (OCT) imaging system, such as a swept-source OCT or a spectral domain OCT imaging system. The imaging system 10 comprises a light source 14, such as a laser or a super-luminescent diode (SLD), positioned to emit a light beam Lb towards a scanning system 12 of the imaging system 10. The scanning system 12 is positioned to transmit the light beam Lb from the light source 14 to the imaging target 11 and scan the light beam Lb over a region of the imaging target 11 to be imaged.

[0028] Additionally, the scanning system 12 is configured to collect light Lc reflected or scattered by the imaging target 11 during scanning and transmit the collected light Lc to a photodetector 16 of the imaging system 10. The photodetector 16 is configured to detect the collected light Lc such that image processing hardware (not shown) of the data processing hardware 18 can generate an image of the imaging target 11 based on the detection signal Sd using well-known data processing techniques. The image generated by the data processing hardware 18 may be an uncorrected ophthalmic image 22 of the imaging target 11 when the imaging target 11 is an eye. The uncorrected image 22 generated by the data processing hardware 18 includes banding artifacts introduced by the polygonal scanning mirror, as discussed in more detail below.

[0029] The scanning system 12 may include an arrangement of optical components as shown generally in FIG. 1, as in this embodiment, where the scanning system 12 of FIG. 1 includes a first scanning element 201 and a first curved mirror 202. As in this embodiment, the scanning system 12 may further include a second curved mirror 203 (also referred to herein as a "slit mirror") and a second scanning element in the form of a polygonal scanning mirror 205. The first scanning element 201 may be a scanning galvanometer mirror arranged to scan a light beam Lb horizontally across the imaging target 11. The second scanning element is a polygonal scanning mirror 205 arranged to scan a light beam Lb vertically across the imaging target 11. The polygonal scanning mirror 205 includes reflective facets arranged in a polygon. The polygonal scanning mirror may include 16 equal-dimensional reflective facets arranged in a polygonal shape. The position of the imaging target 11 across which the light beam Lb is scanned by the first and second scanning elements 201, 205 may be controlled by a scanning system controller (not shown).

[0030] During operation of the scanning system 12, the light beam Lb emitted from the light source 14 enters the scanning system 12 and is focused by a lens (not shown) onto the polygonal scanning mirror 205. The light beam Lb received by the polygonal scanning mirror 205 is then sequentially reflected by the polygonal scanning mirror 205, the second curved mirror 203, the first scanning element 201 and the first curved mirror 202 to be incident on the imaging target 11. The imaging target 11 may take the form of a region of the retina of an eye in this embodiment, and this form of the imaging target 11 is provided as an example. Return light scattered by the illuminated region of the imaging target 11, for example the retina of an eye, travels the same optical path as the line of the light beam Lb incident on the imaging target 11, but through the scanning system 12 in the reverse order, and exits the scanning system 12 as collected light Lc, including optical aberrations or banding artifacts caused by the polygonal scanning mirror 205. The collected light Lc is received by a photodetector 16 .

[0031] The first curved mirror 202 and the second curved mirror 203 may be spheroidal mirrors and ellipsoidal mirrors, respectively, having conjugate first and second foci, as in this embodiment. The first scanning element 201 is located at the first focus of the first curved mirror 202, and the imaging target 11 is located at the second focus of the first curved mirror 202. If the imaging target 11 is a part of the retina of the eye 210, the pupil of the eye is located at the second focus of the first curved mirror 202, so that the light beam Lb is scanned across a certain area of ​​the retina of the eye during scanning. The second polygonal scanning mirror 205 is located at the first focus of the second curved mirror 203, and the first scanning element 201 is located at the second focus of the second curved mirror 203. However, the second curved mirror 203 (in this embodiment an ellipsoidal mirror) may be any reflective component having a non-spherical reflective surface, such as a conic section shape such as a paraboloid or hyperboloid, or more generally a shape described by one or more polynomial functions of two variables.

[0032] The collected light Lc received by the photodetector 16 includes periodic light artifacts or banding artifacts introduced into the collected light Lc by the polygon scanning mirror 205. The banding artifacts may be introduced by the polygon scanning mirror 205 as a result of imperfections and / or defects in the polygon scanning mirror 205 or due to non-uniform scan line spacing. For example, the banding artifacts may vary sinusoidally with a period of 16 pixels in an embodiment in which the polygon scanning mirror includes 16 reflective facets. The banding artifacts may be caused by variations in the reflectivity of each of the reflective facets of the polygon scanning mirror 205, thereby causing periodic variations in the intensity of the light scanned across the imaging target 11.

[0033] The data processing hardware 18 receives the detection signal Sd from the photodetector 16 and generates an image of the imaging target 11. The image generated by the data processing hardware 18 is an uncorrected image 22 that includes optical aberrations or banding artifacts introduced into the light Lc collected by the polygon scanning mirror 205 that detract from the quality of the image. The data processing hardware 18 includes an image correction algorithm 20 executable by the data processing hardware 18 to remove or reduce the amount of aberrations or banding artifacts in the generated uncorrected image 22. Once the image correction algorithm 20 is applied to the uncorrected image 22, the data processing hardware 18 is configured to output a corrected image 24 that has fewer banding artifacts than the uncorrected image 22. The level of banding artifacts present in the corrected image 24 is undetectable by the human eye.

[0034] 2, an example uncorrected image 22 captured by imaging system 10 and generated by data processing hardware 18 is shown. Uncorrected image 22 includes vertical bands, which are an example of band artifacts 25. Band artifacts 25 may be periodic optical aberrations or periodic line artifacts, for example, in the form of a set of regularly spaced linear features (bands) in uncorrected image 22. An example uncorrected image 22 is an ophthalmic image of a retina captured by imaging system 10 where imaging target 11 is an eye. Band artifacts 25 in the form of vertical bands shown in example uncorrected image 22 occur periodically, spaced at 16 pixel intervals in the horizontal direction. The 16 pixel spacing between adjacent bands may be caused, at least in part, by variations in reflectivity of each of the 16 reflective facets of polygonal scanning mirror 205. In another example embodiment where polygonal scanning mirror 205 has a different number of reflective facets, the spacing between adjacent bands may vary accordingly. The banding artifacts 25 shown in the uncorrected image 22 detract from the overall image quality and therefore it is desirable to remove the vertical bands from the uncorrected image 22 without compromising the image quality of the imaging target 11.

[0035] The data processing hardware 18 comprises an image correction algorithm 20 which, when executed by the data processing hardware 18, is configured to remove or suppress banding artifacts 25 present in the uncorrected image 22. The data processing hardware 18 is configured to segment the uncorrected image 22 into 2D partially overlapping segments. FIG. 3 shows an example of an uncorrected image 22 segmented into nine partially overlapping segments 30. Although the uncorrected image 22 is segmented into nine partially overlapping segments 30 in FIG. 3 for clarity, one skilled in the art will appreciate that in further embodiments, the image may be segmented into smaller overlapping segments. For example, the image may be segmented into a 10×10 grid, a 100×100 grid, or larger, depending on the severity of the banding artifacts 25 and / or the size of the uncorrected image 22.

[0036] The partially overlapping segments 30 in FIG. 3 are rectangular segments of equal size that partially overlap adjacent segments 30. The level of overlap between adjacent segments 30 can be varied by the data processing hardware 18. Cases with particularly severe banding artifacts 25 may have increased overlap of segments 30 because application of the correction algorithm 20 may introduce severe edge effects or edge artifacts that intrude further toward the center of each segment 30, and therefore more of each segment 30 must be removed to remove the edge effects or artifacts before recombining the corrected segments 30 to generate the corrected image 24. In the example shown in FIG. 3, the segments 30 overlap both horizontally and vertically. In another embodiment, the segments 30 may overlap only horizontally or vertically, for example.

[0037] Once the data processing hardware 18 has segmented the uncorrected image 22 into overlapping segments 30, an image correction algorithm 20 is executed by the data processing hardware 18 such that the image correction algorithm 20 is applied to each of the overlapping segments 30 to individually correct the banding artifacts 25 present in each of the segments 30 by performing the method outlined in FIG. 4 on each segment 30 of the uncorrected image 22, as described in further detail below.

[0038] 4 is a flow chart illustrating a method for suppressing banding artifacts 25 present in each of the overlapping segments 30 of the uncorrected image 22. The method outlined in FIG. 4 may be performed by applying the correction algorithm 20 to each of the overlapping segments 30 of the uncorrected image 22, such that the method outlined in FIG. 4 is applied to each of the segments 30 of the uncorrected image 22. Those skilled in the art will appreciate that although the method illustrated in FIG. 4 is described in relation to a single overlapping segment 30, as in the present embodiment, the method may be performed for every overlapping segment 30 present in the uncorrected image 22 to remove or suppress banding artifacts 25 present in each overlapping segment 30.

[0039] In step 401, a discrete cosine transform (DCT) of a given partially overlapping segment 30 is calculated. Applying the discrete cosine transform to the segment 30 converts the image contained in the segment 30 from the spatial domain to the frequency domain by defining the image as a series of cosine functions, each having a different frequency corresponding to a frequency of the image. Using the discrete cosine transform to convert the image to the frequency domain, as opposed to another transform such as a discrete Fourier transform, is beneficial because the discrete cosine transform introduces fewer edge effects into each segment 30 compared to the discrete Fourier transform. Furthermore, each frequency bin in the discrete cosine transform corresponds to a wider range of frequencies than the corresponding frequency bin in the discrete Fourier transform. This is beneficial because the band artifacts 25 to be suppressed by the image correction algorithm 20 are typically sinusoidal signals and / or the peaks and / or valleys of the band artifact signal fit into single frequency bins in the discrete cosine transform, thereby allowing the band artifacts 25 to be more easily attenuated or suppressed in the discrete cosine transform than is possible in the discrete Fourier transform, for example.

[0040] At step 402, high and low frequencies are removed from the discrete cosine transform of the segments 30 to create a modified discrete cosine transform of the segments 30. For example, discrete cosine transform coefficients having an absolute value equal to or greater than a threshold, e.g., 2, are removed. Removing high and / or low frequencies may include setting to zero discrete cosine transform coefficients having an absolute value equal to or greater than a threshold. Removing high and low frequencies from the discrete cosine transform of the segments 30 prevents background noise and higher frequency components of the signal from being amplified in future steps in the image correction process by the image correction algorithm 20. Additionally, removing high and low frequencies from each segment 30 in the frequency domain allows high and low signals to be removed more effectively once the discrete cosine transform has been calculated than would otherwise be possible if the frequencies had been removed in the spatial domain prior to calculating the discrete cosine transform of each segment 30. Removing high and low frequencies has the effect of flattening the image by removing background noise, thereby improving the quality of the resulting corrected image 24.

[0041] In step 403, the band artifacts 25 in the overlapping segments 30 are attenuated. Attenuating the band artifacts 25 in the segments 30 can be done using a window function such as a Hanning window. The window function is centered on a frequency bin that corresponds to the frequency of the band artifacts 25 in that segment 30. The window function can be applied to window a frequency bin in the discrete cosine transform of each segment that corresponds to the spatial frequency of the band artifacts 25. For example, a Hanning window can be centered on a frequency bin that corresponds to the frequency associated with the band artifacts 25 occurring every 16 pixels. The parameters of the window function can be changed to select the level of attenuation to apply to the target frequencies. In one example, the window function can apply an attenuation of -10db to the frequencies associated with the band artifacts 25 present in the overlapping segments 30. Using a Hanning window to attenuate the frequencies associated with the band artifacts 25 as opposed to a bandpass filter such as a brickwall filter is beneficial because the Hanning window introduces less ring artifacts into the signal than a brickwall filter.

[0042] The window function used to attenuate the vertical bands in step 403 may be a multi-stage window function. For example, a two-stage or three-stage Hanning window may be used to attenuate the vertical bands of the segment 30. The main component of the Hanning window may be centered at a frequency bin corresponding to the frequency of the band artifact 25 contained in the segment 30. The window function may include one or more second components positioned at frequency bins corresponding to harmonics of the first frequency associated with the band artifact 25 being filtered by the window function, such that harmonics of the segment 30 that contribute to the presence of the band artifact 25 of the segment 30 may be attenuated by the window function.

[0043] At step 404, an inverse discrete cosine transform (IDCT) is computed on the segment 30 to reconstruct the image contained in the segment 30 and generate a corrected segment. Banding artifacts 25 are suppressed in the reconstructed image of the corrected segment compared to the level of banding artifacts 25 present in the segment 30 in the uncorrected image 22 prior to application of the correction algorithm 20 to the uncorrected segment. Applying the inverse discrete cosine transform to the segment 30 transforms the segment 30 back from the frequency domain to the spatial domain, thereby generating a corrected segment that includes a reduced number of banding artifacts 25 than were present in the segment 30 prior to application of the correction algorithm 20.

[0044] 5 and 6, examples of a segment 30 from an uncorrected image 22 before and after application of the correction algorithm 20 are shown. FIG. 5 shows an example of an uncorrected segment, segment 30, before application of the correction algorithm 20. The segment 30 shown in FIG. 5 includes vertical bands spanning the width of the segment 30, with substantially uniform spacing between adjacent bands of the segment 30. In the example shown, the bands appear periodic, with a period of 16 pixels. FIG. 6 shows the segment 30 after the correction algorithm 20 has been applied to the segment 30. As shown in FIG. 6, the banding artifacts 25 have been sufficiently suppressed in the central portion 60 of the corrected segment 30 that they are no longer visible in the image. However, application of the correction algorithm 20 has resulted in the manifestation of edge effects 68 in peripheral regions 62 of the segment 30, where the bands are still visible as a result of the edge effects 68.

[0045] As illustrated in FIG. 3, the segments 30 are dimensioned such that the edges of each segment 30 overlap. When the correction algorithm 20 is applied to the segments 30, the resulting edge effects 68 contained in the peripheral region 62 of a given segment 30 correspond to the overlapping portions of the segments 30. As such, the peripheral region 62 of each segment 30 can be removed or cut away such that the peripheral region 62 containing banding artifacts as a result of the edge effects 68 can be discarded. The remaining central portion 60 of each segment 30 can be recombined or stitched together by the data processing hardware 18 to generate the corrected image 24. The central portion 60 is an example of a corrected segment. The dimensions and amount of overlap of the segments 30 in FIG. 3 can be selected such that the peripheral region 62 of a segment 30, including the bands from the edge effects 68, is completely contained within the overlapping region of an adjacent segment 30.

[0046] 7 shows an example of a corrected image 24 produced by combining corrected segments produced by applying the correction algorithm 20 with segments 30 of the uncorrected image 22. The data processing hardware 18 is arranged to combine the corrected segments 64 to produce the corrected image 24 such that the joints between adjacent segments 30 are not visible or perceptible to the human eye. The bands in the corrected image 24 have been suppressed sufficiently (or eliminated entirely) that they are no longer visible in the corrected image 24.

[0047] 8, there is shown a flow chart illustrating a method for suppressing banding artifacts 25 in an image of an imaging target 11. The method illustrated in FIG 8 may be performed by data processing hardware 18 of imaging system 10. The method may be performed for each uncorrected image 22 to produce a corrected image 24.

[0048] In step 801, the uncorrected image 22 is divided into at least partially overlapping 2D segments. The number and dimensions (sizes) of the segments 30 can be selected based on parameters such as the severity of banding artifacts 25 present in the uncorrected image 22, the resolution of the uncorrected image 22, and the processing power of the data processing hardware 18. The segments are typically rectangular and equal in size. Furthermore, the amount of overlap between adjacent segments is usually equal. If the banding artifacts 25 present in the uncorrected image 22 are severe, the uncorrected image 22 can be divided into smaller segments, such as, for example, the uncorrected image 22 being divided into 1000×1000 overlapping segments or larger. Increasing the number of segments 30 into which the uncorrected image 22 is generally divided increases the effectiveness of the application of the image correction algorithm 20. However, increasing the number of segments 30 into which the uncorrected image is divided also increases the computation time to perform the method of suppressing banding artifacts 25 in the uncorrected image 22.

[0049] Next, in step 802, banding artifacts 25 are corrected or suppressed in each of the overlapping segments 30 by applying an image correction algorithm 20 to each of the segments 30 of the uncorrected image 22. Correcting or suppressing banding artifacts 25 present in each of the overlapping segments 30 may be performed by performing the method outlined in FIG. 4 as described above. Applying the image correction algorithm 20 to each of the overlapping segments 30 suppresses banding artifacts 25 present in each of the overlapping segments 30. Applying the image correction algorithm 20 to each segment 30 includes computing a discrete cosine transform for each of the overlapping segments 30.

[0050] In step 803, at least a portion of the overlapping region 62 is removed from each of the segments 30 to remove edge effects 68 or remove artifacts introduced in each of the segments 30 by the image correction algorithm 20 in step 802. Removing at least a portion of the overlapping region 62 may include cropping at least a portion of the peripheral region 62 from each of the segments 30 to remove artifacts or edge effects 68 introduced in each of the segments 30 by the image correction algorithm 20. Cropping the peripheral region 62 from the segments 30 beneficially removes artifacts and / or edge effects 68 introduced in each of the segments 30 by the image correction algorithm 20. By removing at least a portion of the overlapping region 62 from each of the overlapping segments 30, a corrected segment is generated. Removing at least a portion of the overlapping region 62 may be performed after the image correction algorithm 20 has been applied to the segments 30.

[0051] At step 804, each of the processed segments 30 are recombined to generate the corrected image 24. Recombining the cropped segments 30 may include mosaicking, and optionally combining the segments 30 to generate the corrected image 24. Combining adjacent segments 30 when recombining the segments 30 to generate the corrected image 24 prevents artifacts due to misalignment of the recombined segments 30 from being introduced into the corrected image 24. Combining each of the corrected segments 30 to generate the corrected image 24 may include blending and / or aligning peripheral regions of adjacent segments 30 to generate the corrected image 24 such that the join between the adjacent segments 30 is imperceptible in the corrected image 24. Aligning the adjacent segments may include identifying common features present in the adjacent segments 30 and aligning the adjacent segments 30 in the corrected image 24 by superimposing the common features of the adjacent segments 30.

[0052] 9 is a schematic diagram of a programmable signal processing hardware 600 configured to remove or suppress banding artifacts 25 in an image captured by the imaging system 10. The programmable signal processing hardware 600 may perform at least a portion of the functionality of the data processing hardware 18 of the imaging system 10, and in one embodiment herein, at least a portion of the hardware 600 is included in the data processing hardware 18. The programmable signal processing device 600 includes a communication interface 610 for receiving the detection signal Sd from the photodetector 16 and / or for outputting the corrected image 24. In one embodiment herein, the communication interface (I / F) 610 may input / output any information obtained as part of the method described herein.

[0053] The signal processing device 600 further includes a processor (e.g., a central processing unit, CPU, and / or a graphics processing unit, GPU) 620, a working memory 630 (e.g., a random access memory), and an instruction store 640 that stores computer programs 645 including computer readable instructions that, when executed by the processor 620, cause the processor 620 to perform various functions, including those of the processor 18 of FIG. 1, and / or functions of the methods described herein. In one embodiment herein, only the processor 620 is included in the data processing hardware 18, although one or more additional components of the hardware 600 are also included in the data processing hardware 18.

[0054] The working memory 630 stores information used by the processor 620 during the execution of the computer program 645. The instruction store 640 may, for example, comprise a ROM (e.g., in the form of an Electrically Erasable Programmable Read Only Memory (EEPROM) or Flash memory) preloaded with computer readable instructions. Alternatively, the instruction store 640 may comprise a RAM or similar type of memory, and the computer readable instructions of the computer program 645 may be input thereto from a computer program product, such as a non-transitory computer readable storage medium 650 in the form of a CD-ROM, DVD-ROM, etc., or a computer readable signal 660 carrying the computer readable instructions. In any case, the computer program 645, when executed by the processor 620, causes the processor 620 to perform the methods described herein, including, by way of example and not limitation, the method of suppressing banding artifacts in ophthalmic images as described above.

[0055] In one embodiment herein, the data processing hardware 18 of the exemplary embodiment described above comprises a computer processor 620 and a memory 640 storing computer readable instructions that, when executed by the computer processor 620, cause the computer processor 620 to perform the methods described herein, including, by way of example and not limitation, a method for suppressing banding artifacts in ophthalmic images acquired by the imaging system 10, as described herein. It should be noted, however, that the data processing hardware 18 may instead be implemented with non-programmable hardware, such as a dedicated ASIC, FPGA or other integrated circuit performing the functions of the data processing hardware 18 described above, or a combination of such non-programmable and programmable hardware as described above with reference to FIG.

[0056] In the preceding description, exemplary aspects are described with reference to several exemplary embodiments. The specification should therefore be considered illustrative rather than restrictive. Similarly, the diagrams illustrated in the drawings that highlight the functionality and advantages of the exemplary embodiments are presented for illustrative purposes only. The architecture of the exemplary embodiments is sufficiently flexible and configurable such that it may be utilized in ways other than those shown in the accompanying figures.

[0057] Example software embodiments presented herein may be provided as software, such as a computer program, or one or more programs having instructions or sequences of instructions, and in one embodiment are contained or stored in an article of manufacture, such as a machine-accessible or machine-readable medium, instruction store, or computer-readable storage device, each of which may be non-transitory. The programs or instructions in the non-transitory machine-accessible medium, machine-readable medium, instruction store, or computer-readable storage device may be used to program a computer system or other electronic device. Machine-readable media, instruction stores, and storage devices may include, but are not limited to, floppy diskettes, optical disks, and magneto-optical disks, or other types of media / machine-readable media / instruction stores / storage devices suitable for storing or transmitting electronic instructions. The techniques described herein are not limited to any particular software configuration. They may find applicability in any computing environment. As used herein, the terms "computer-readable," "machine-accessible medium," "machine-readable media," "instruction store," and "computer-readable storage device" are intended to include any medium that can store, encode, or transmit instructions or sequences of instructions for execution by a machine, computer, or computer processor and cause the machine / computer / computer processor to perform any one of the methods described herein. Furthermore, it is common in the art to speak of software, in one form or another (e.g., program, procedure, process, application, module, unit, logic, etc.), as taking an action or causing a result. Such expressions are merely a shorthand way of stating that execution of the software by a processing system causes the processor to perform actions to produce a result.

[0058] Some embodiments may also be implemented by the preparation of application specific integrated circuits, field programmable gate arrays, or by interconnecting an appropriate network of conventional component circuits.

[0059] Some embodiments include a computer program product. A computer program product is a storage medium or media, instruction store, or storage device having instructions stored thereon or therein and can be used to control or cause a computer or computer processor to perform any of the procedures of the exemplary embodiments described herein. Storage medium / storage devices can include, by way of example and not limitation, optical disks, ROM, RAM, EPROM, EEPROM, DRAM, VRAM, flash memory, flash cards, magnetic cards, optical cards, nanosystems, molecular memory integrated circuits, RAID, remote data storage / archive / warehousing, and / or any other type of device suitable for storing instructions and / or data.

[0060] Some embodiments, stored on either a computer readable medium or media, instruction storage device, or storage device, include software for controlling both the hardware of the system and for enabling the system or microprocessor to utilize the results of the embodiments described herein to interact with a human user or other mechanism. Such software may include, but is not limited to, device drivers, operating systems, and user applications. Finally, such computer readable medium or storage device further includes software for carrying out exemplary aspects of the present invention, as described above.

[0061] Included in the programming and / or software of the system are software modules for carrying out the procedures described herein. In some embodiments herein, the modules comprise software, while in other embodiments herein, the modules comprise hardware, or a combination of hardware and software.

[0062] While various exemplary embodiments of the present invention have been described above, it should be understood that they have been presented by way of example and not limitation. Various changes in form and detail can be made therein, as will be apparent to those skilled in the art. Thus, the present invention should not be limited by any of the above-described embodiments, but should be defined only in accordance with the following claims and their equivalents.

[0063] It should also be understood that the procedures recited in the claims do not have to be performed in the order presented.

[0064] Although this specification contains details of many specific embodiments, these should not be construed as a limitation of the scope of any invention or what may be claimed, but rather as a description of features specific to the specific embodiments described herein. Certain features described herein in the context of separate embodiments may also be implemented in combination in a single embodiment. Conversely, various features described in the context of a single embodiment may also be implemented in multiple embodiments separately or in any suitable subcombination. Furthermore, although features may be described above as acting in a particular combination, and even as originally claimed as such, in some cases, one or more features from a claimed combination may be excluded from the combination, and the claimed combination may be directed to a subcombination or a variation of the subcombination.

[0065] In certain circumstances, multitasking and parallel processing may be advantageous. Furthermore, the separation of various components in the above-described embodiments should not be understood as requiring such separation in all embodiments, and it should be understood that the program components and systems described may generally be integrated into a single software product or packaged into multiple software products.

Claims

1. 1. A method for suppressing banding artifacts (25) in an ophthalmic image (22) of a patient's eye, the method comprising: Dividing the ophthalmic image (22) into a plurality of overlapping segments (30), each of the plurality of segments (30) including one or more overlapping regions, each of the overlapping regions being an overlapping region of the segment with an adjacent segment of each of the plurality of segments (30); applying an image correction algorithm (20) that computes a discrete cosine transform of each of said plurality of segments (30) to suppress banding artifacts (25) in said plurality of segments (30); removing at least a portion of one or more of the overlapping regions from each of the plurality of segments (30) to remove artifacts (68) introduced by the image correction algorithm (20) to generate a respective corrected segment; The method further comprises combining the corrected segments to generate a corrected image (24) having less of the banding artifacts (25) than the ophthalmic image (22).

2. 2. The method of claim 1, further comprising performing filtering in the frequency domain after the discrete cosine transform is calculated to remove at least one of low and high frequency signals from each of the segments (30).

3. The method of claim 2 , wherein the filtering comprises removing discrete cosine transform coefficients from each of the segments (30) having an absolute value greater than or equal to a threshold.

4. 4. The method of claim 1, further comprising applying a window function to the calculated discrete cosine transform of each of the segments (30), the window function being applied to window frequency bins of the discrete cosine transform of each of the segments (30), the frequency bins corresponding to spatial frequencies of the banding artifacts (25), and attenuating the banding artifacts (25) of the discrete cosine transform of each of the segments (30).

5. 5. The method of claim 4, wherein the window function includes a second component for filtering a second frequency corresponding to a harmonic of the frequency of the band artifact (25), the second component being applied to window a frequency bin in the discrete cosine transform of each of the segments (30) that corresponds to the second frequency.

6. The method according to claim 4 or 5, wherein the window function is a Hanning window.

7. The method of any one of claims 1 to 6, wherein applying the image correction algorithm (20) further comprises calculating, for each of the segments of the plurality of segments (30), an inverse discrete cosine transform of each of the segments after the discrete cosine transform has been calculated.

8. The method of any one of claims 1 to 7, wherein combining the corrected segments (30) to generate the corrected image (24) comprises combining a peripheral area (62) of a first one of the corrected segments with peripheral areas of adjacent corrected segments of the corrected segment.

9. A computer program comprising instructions which, when executed by a processor, cause the processor to perform the method according to any one of claims 1 to 8.

10. An ophthalmic imaging system (10) for imaging an eye of a patient, the ophthalmic imaging system (10) comprising: a light source (14) arranged to emit a light beam (Lb); a scanning system including a polygonal scanning mirror (205), the polygonal scanning mirror (205) being positioned to scan the light beam (Lb) over an area of ​​the patient's eye; a photodetector (16) optically coupled to the scanning system (12), the photodetector (16) configured to generate a detection signal (Sd) based on light reflected by the patient's eye; and data processing hardware (18) configured to receive a detection signal (Sd) from the photodetector (16) and generate an ophthalmic image (22) of the patient's eye based on the received detection signal (Sd), the ophthalmic image (22) including a band artifact (25), The data processing hardware (18) further comprises: Dividing the ophthalmic image (22) into a plurality of overlapping segments (30), each of the plurality of segments (30) including one or more overlapping regions, each of the overlapping regions being an overlapping region of the segment with an adjacent segment of each of the plurality of segments (30); applying an image correction algorithm (20) that computes a discrete cosine transform of each of said plurality of segments (30) to suppress banding artifacts (25) in said plurality of segments (30); removing at least a portion of one or more of the overlapping regions from each of the plurality of segments (30) to remove artifacts (68) introduced by the image correction algorithm (20) to generate a respective corrected segment; An ophthalmic imaging system (10) configured to combine the corrected segments to produce a corrected image (24) having less of the banding artifact (25) than the ophthalmic image (22).

11. 11. The ophthalmic imaging system (10) of claim 10, wherein the data processing hardware (18) is further configured to perform filtering in the frequency domain to remove at least one of low frequency and high frequency signals from each of the segments (30) after the discrete cosine transform is calculated.

12. 12. The ophthalmic imaging system (10) of claim 10 or 11, wherein the data processing hardware (18) is further configured to apply a window function to the calculated discrete cosine transform of each of the segments (30), the window function being applied to window frequency bins of the discrete cosine transform of each of the segments (30), the frequency bins corresponding to spatial frequencies of the banding artifacts (25), and attenuating the banding artifacts (25) of the discrete cosine transform of each of the segments (30).

13. The window function is a second component for filtering a second frequency corresponding to a harmonic of the frequency of banding artifacts (25), said second component being applied to window frequency bins in the discrete cosine transform of each of said segments (30) corresponding to said second frequency; Hanning window, The ophthalmic imaging system (10) of claim 12, comprising at least one of:

14. An ophthalmic imaging system (10) as described in any one of claims 10 to 13, wherein applying the image correction algorithm (20) further comprises, for each of the plurality of segments (30), calculating an inverse discrete cosine transform of each of the segments after the discrete cosine transform has been calculated for each of the partially overlapping segments (30).

15. An ophthalmic imaging system (10) according to any one of claims 10 to 14, wherein combining the corrected segments to generate the corrected image (24) comprises combining a peripheral region (62) of a first corrected segment of the corrected segments with a peripheral region (62) of an adjacent corrected segment of the corrected segment.

Images