Method and device for optical coherence tomographic retinal imaging
Patent Information
- Application Number
- FR2019009169
- Authority / Receiving Office
- FR · FR
- Patent Type
- Patents
- Current Assignee / Owner
- Filing Date
- 2019-08-12
- Publication Date
- 2025-03-21
- Estimated Expiration
- 2039-08-12
Smart Images
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Abstract
Description
Title of the invention: Method and device for optical coherence tomographic retinal imaging Technical field
[0001] The present description relates to a retinal imaging method and a device suitable for implementing said method. More specifically, the retinal imaging method is based on optical coherence tomography imaging (or “OCT” according to the abbreviation of the English expression “optical coherence tomography”). State of the art
[0002] OCT is based on the use of a low coherence interferometer. This imaging technique allows in vivo images of tissue sections, with an axial resolution of a few microns. One advantage of OCT in ophthalmology comes from its ability to reveal tissues in vivo through other scattering tissues.
[0003] Fig. 1A illustrates in a simplified manner the main elements of a device 100 for OCT-type retinal imaging known from the state of the art and described for example in the journal article by JF De Boer et al. [Ref. 1]. Such a device comprises for example an illumination and detection module 110 and a two-dimensional scanning module 120 for scanning an illumination beam emitted by the module 110 and a beam re-emitted by the retina after illumination by said illumination beam. The module 110 comprises an illumination sub-module comprising a light source 111 with low temporal coherence, for example a light source of the SLED type, configured to illuminate a point of the retina with a low coherence illumination beam.The module 110 further comprises a detection sub-module formed by an interferometer, for example a fiber interferometer, for example of the Michelson type, comprising a fiber reference arm with a reflection element 115 and an optic 114. A coupler 113 receives the beams from the fibers 112-1, 112-2, 112-3 respectively coming from the source, the reference arm and the retina for the formation of interference on a detector 116, for example a photomultiplier or an avalanche photodiode, connected to the coupler by a fiber 112-4. The device 100 further comprises a signal processing unit 130, itself connected to a screen and / or interface 140 for a user 11. .
[0004] Such an optical coherence tomographic imaging device makes it possible to obtain an axial depth image at a measurement point of the retina, this axial image, acquired in one direction, being known as an "A-Scan". Such an axial depth image is obtained by various methods such as described in [ref. 1]. For example, an axial depth image can be obtained by means of a source with a wide spectrum and recording interferograms as a function of the wavelength using a spectrometer (this is called "FD-OCT" for "Fourier domain OCT"). In another example, an axial depth image can be obtained using a spectrally swept source (or "Swept Source") and recording an interferogram as a temporal modulation during the spectral scanning of the source (this is called "Swept Source OCT").
[0005] Diagram 101, Fig. 1B, thus illustrates an A-Scan profile of the retina 12 of an eye 10, the axial image being in this example acquired along a propagation axis (δ) of the illuminating light beam merged with the axis (A) of the eye, where (A) is defined by the axis passing through the center of the cornea 14 and the center of the fovea.
[0006] Furthermore, by scanning the illumination beam in such a way as to move the impact of the beam on the surface of the retina, it is possible to acquire a plurality of axial depth profiles along a line and thus obtain a two-dimensional sectional image of the retina, this sectional image being called a “B-Scan”. Diagram 102, Fig. 1B, thus illustrates a B-Scan or sectional image of the retina, acquired in a plane comprising in this example the axis (A) of the eye and with an angular displacement 0X of the scanning beam.
[0007] As illustrated in diagram 103, FIG. 1B, it is also possible to acquire a plurality of cross-sectional tomographic images, referenced B-Scan(i) in diagram 103. The B-scans can be acquired at the same location of the retina to obtain, by averaging over the captured tomographic images, a tomographic image with better signal to noise. The B-scans can also be acquired at different locations of the retina to obtain a volume image of the retina (“3D-Scan”).
[0008] Also known, as described in the published application WO 2018197288 [Ref. 2] in the name of the applicant, is a multi-scale retinal imaging system, with a “wide-field” optical path for acquiring wide-field images of the retina and a “small-field” optical path for acquiring small-field and high lateral resolution images, i.e. typically a few microns, the small-field path integrating a wavefront correction device. In the previously cited application, the imaging may be of the OCT type.
[0009] In practice, although with the OCT imaging devices known to date, increasingly high A-Scan acquisition frequencies are achieved (typically greater than 100 kHz), the minimum acquisition time for a cross-sectional image (B-Scan) of the retina, which comprises between 500 and 1000 A-scans, is typically between 3 ms and 10 ms. The minimum acquisition time for a volume image of the retina (3D-Scan), which comprises between 200 and 500 B-scans, is typically between 0.5 s and 4 s. However, during such durations, the eye may move during image capture and deteriorate the quality of the acquired images.
[0010] Indeed, even when the patient is asked to fixate on a point, the eyes continue to move according to different types of movements. For example, we know drift, tremor and involuntary microsaccades. In particular, drift is observed as a slow and irregular displacement of the optical axes of the eye. Drift, measured as an angular variation of the optical axis of the eye can be up to 1507s (minutes of arc per second) in a healthy subject. Tremor is a movement that is difficult to observe since it is an incessant movement, of very low amplitude (20 to 40 arc seconds) but of high frequency (70 to 90 Hz) and microsaccades are tiny saccades. They can have a minimum dimension of 2-5 arc minutes and occur involuntarily.
[0011] Thus, as illustrated in FIG. 1C, during the acquisition of a B-Scan captured in 10 ms, the ocular axis (A) may be caused to undergo a rotation of nearly one minute of arc (diagrams 104, 105) relative to the nominal position (diagram 104) if a subject exhibits a drift of 1007s, which corresponds on the retina to a displacement of the illumination beam of approximately 5 qm. This results in a lack of precision in the image obtained, in particular during the acquisition of an image in high lateral resolution, for which the resolution can reach 2 pm.
[0012] Patent application US 2010 / 0053553 [Ref. 3] describes a retinal imaging method allowing, during the acquisition of a cross-sectional image (B-Scan) of the retina, tracking and correction of eye movements. More specifically, the method described in the aforementioned document comprises the measurement of surface images of the retina by means of a first rapid acquisition device, for example a laser scanning system. In contrast to cross-sectional images, the surface images are acquired in a plane perpendicular to the ocular axis. Since the acquisition of the surface images is rapid, it allows the eye movements to be measured in real time.The method then comprises selecting on the surface image thus acquired a location and direction of a desired cross-sectional image for the acquisition of a B-Scan and then capturing the B-Scan by means of an OCT device, in which the scanning module is corrected in real time for eye movements measured by the rapid acquisition device. Stabilization of the scanning module of the OCT device allows the capture of the B-Scan at the exact location selected on the surface image.
[0013] Stabilizing the scanning module of the OCT device by the correction signal from the surface image acquisition device as described in [Ref. 3] is ideal in the sense that it allows excellent precision in capturing a B-Scan. However, this stabilization method is complex to implement because it requires sending a correction signal several times during the acquisition of a B-Scan.
[0014] Patent application US2014 / 0211155 [Ref. 4] also describes a retinal imaging method implementing stabilization of the OCT measurement by means of a device for rapid acquisition of surface images of the retina. More specifically, the method comprises measuring eye movements by means of surface image acquisition. The method also comprises acquiring a plurality of cross-sectional tomographic images or B-Scans by means of an OCT device equipped with a two-dimensional scanning module. According to the described method, before each acquisition of a new B-Scan, it is determined whether the eye movements measured during the acquisition of a previous B-Scan are below a predetermined threshold value. If this is the case, a correction of the eye movements is made by sending a correction signal to the scanning module of the OCT device before the acquisition of the new B-Scan, aiming to correct the starting position.If the eye movements are greater than the predetermined threshold value, the previous B-scan is acquired again.
[0015] The method described in [Ref. 4] is much simpler and faster to implement than the method described in [Ref. 3]. However, during the acquisition of a B-Scan, we observe the effect of the drift which results at the end of acquisition by a distance between the theoretical point that we are trying to image and the real point imaged, this distance being able in practice to be of the order of 5 qm in a healthy subject and much more in subjects with poor fixation. Thus, the method described in [Ref. 4] does not allow sufficient stabilization, particularly in the case of high lateral resolution imaging.
[0016] An object of the present description is to propose a method and a device for retinal imaging which presents a simplicity and a speed of acquisition comparable to that described in [Ref. 4] but which offers a precision compatible in particular with high lateral resolution imaging. Summary of the invention
[0017] According to a first aspect, the present description relates to a retinal imaging method, comprising:
[0018] - the successive acquisition of a plurality of cross-sectional images of the retina by means of a first tomographic image acquisition module, said first module comprising a first lighting and detection sub-module and a first scanning sub-module in two directions; - acquiring retinal surface images by means of a second retinal surface image acquisition module, said second module comprising a second illumination and detection sub-module; - the determination, from surface images acquired by the second module, of an angular velocity of eye movements in at least one of said directions; - determining, when launching the acquisition of each sectional image of said plurality of sectional images of the retina, a scanning speed to be applied by said first scanning sub-module along said at least one direction, said scanning speed comprising a nominal scanning speed corrected by a correction speed depending on said angular speed of the eye movements.
[0019] By no longer systematically correcting the initial position for the acquisition of a B-Scan but the scanning speed at the start of the acquisition of the B-Scan, as a function of the angular speed of the eye movements (or movements of the retina), the applicant has shown that it was possible to obtain much better consideration of the drift of the retina, in particular in the case of high lateral resolution imaging, while keeping a simple system of implementation.
[0020] In the present description, we will speak indifferently of lateral resolution and speed of movement of the retina on the one hand, and of angular resolution and speed of the retina as it appears seen through the anterior segment of the eye on the other hand. The lateral and angular values are in fact linked by the focal length of the eye. Thus, a resolution of 2 pm is equivalent to an angular resolution of 0.007°, or 0.4 arcmin (with the average focal length of the eye of 17 mm).
[0021] A scanning sub-module within the meaning of the present description is configured for scanning a lighting beam emitted by the lighting and detection sub-module, continuously or discretely.
[0022] A distinction is made in a known manner between raster or unidirectional scanning (also called “raster scan”), always carried out in the same direction, and bidirectional scanning carried out alternately in opposite directions. In both cases, the scanning comprises a main scanning direction, conventionally called “horizontal direction” in the present application.
[0023] According to one or more exemplary embodiments, the scanning sub-module comprises a resonant or galvanometric scanner configured to deflect an illumination beam continuously using a mirror rotating about an axis. The scanning sub-module may also comprise a pair of scanners making it possible to modify the point of impact of the illumination beam on the retina in two directions and generating a bidirectional scan.
[0024] According to one or more exemplary embodiments, the scanning sub-module comprises a MEMS (“Microelectromechanical Systems”) electromechanical mirror configured to directly deflect an illumination beam in two directions.
[0025] Generally speaking, any component capable of producing a sufficient number of di rections or points of impact of an illumination beam on the retina can be used as a sub-scanning module.
[0026] According to one or more exemplary embodiments, the angular velocity of the eye movements in a direction is measured from two surface images of the retina, for example two images acquired successively; the angular velocity is then determined by the angular displacement in this direction divided by the time interval separating the acquisitions of said surface images.
[0027] According to one or more exemplary embodiments, the angular velocity of the eye movements is determined only in one direction, generally the main scanning direction. The correction velocity will then be determined only in this direction and the scanning velocity applied in the other direction by the scanning sub-module will be the nominal velocity.
[0028] According to one or more exemplary embodiments, the angular velocity of the eye movements may be determined in both directions. In this case, it will be possible to determine a correction velocity in each of the directions. According to one or more exemplary embodiments, the scanning velocity applied by the scanning sub-module is, in each direction, a nominal velocity corrected by said correction velocity.
[0029] According to one or more exemplary embodiments, the correction speed is a speed of eye movements measured before the acquisition is launched, for example the last speed of eye movements measured before the acquisition is launched.
[0030] According to one or more exemplary embodiments, the correction speed is obtained on the basis of a prediction, from a history of the speeds of the eye movements measured before the launch of the acquisition.
[0031] In the present description, the nominal scanning speed is the predetermined scanning speed for the acquisition of a B-Scan, before correction. The nominal scanning speed is determined for each of the directions, for example by the total field scanned in said direction, the number of A-scans contained in a B-scan, and the duration of an A scan; it is generally comprised (values measured in the space of the eye) between -100 7s (case of a small field image with high lateral resolution), and more than 10,000 7s (wide field image).
[0032] According to one or more exemplary embodiments, the determination of an angular velocity of the eye movements is made with a frequency identical to the frequency of acquisition of the surface images. It is also possible to determine the angular velocity with a lower frequency; in practice, it will be possible to measure between 2 and 10 values of the angular velocity of the eye movements in each direction during a B-Scan.
[0033] According to one or more exemplary embodiments, the acquisition of surface images of the retina is carried out by means of a two-dimensional camera-type detector and the acquisition frequency is the acquisition frequency of the camera.
[0034] According to one or more exemplary embodiments, the acquisition of surface images of the retina is done by scanning a beam illuminating the retina and the acquisition frequency is a function of the scanning frequency and the number of lines of the surface images of the retina acquired.
[0035] According to one or more exemplary embodiments, the retinal imaging method further comprises, before the acquisition of each sectional image of said plurality of sectional images of the retina, the determination of an offset correction to be applied by said first scanning sub-module, along at least one of said directions, to a nominal position offset.
[0036] In the present description, a nominal position offset is a predetermined offset value to be applied by the first scanning sub-module before acquiring a new B-Scan, independently of any correction of retinal movements. The nominal position offset is for example calculated with the scanning parameters defined by the user.
[0037] According to one or more exemplary embodiments, the offset correction to be applied along said at least one direction is a function of said correction speed. In particular in the case of a raster scan, it will thus be possible to correct the drift of the retina during the return time to the starting point for the acquisition of a new B-Scan.
[0038] According to one or more exemplary embodiments, the retinal imaging method further comprises:
[0039] - determining, during the acquisition of each sectional image of said plurality of sectional images of the retina, a value representative of a variation in the speed of eye movements; - applying, before the acquisition of a subsequent cross-sectional image of the retina, along said at least one direction, said offset correction to the nominal position offset, if said value representative of the variation in speed during the acquisition of the previous cross-sectional image is greater than a predetermined threshold value.
[0040] According to one or more exemplary embodiments, said value representative of the variation in the speed of the eye movements corresponds to an angular acceleration of the eye movements (variation in speed during a time interval).
[0041] Generally, when the acquisition of surface images of the retina is done by means of a two-dimensional camera-type detector or a scanning system of beam, said value representative of the variation in the speed of eye movements can be determined from the variation in the speed of the image of the retina.
[0042] According to one or more exemplary embodiments, the threshold value is determined as a function of the lateral resolution sought for imaging the retina, the focal length of the eye, the acquisition time of an A-Scan and the number of A-Scans per B-Scan. Examples of threshold values are between 0.1° / s and 10° / s.
[0043] According to one or more exemplary embodiments, the offset correction to be applied in each of the two directions if the value representative of the variation in speed during the acquisition of the previous sectional image is greater than the predetermined threshold value is directly calculated from the surface images of the retina.
[0044] In other examples, the offset correction may be determined from the previously measured angular velocity(s) of retinal movements.
[0045] Thus, according to one or more exemplary embodiments, no correction to the nominal position shift directly calculated from the surface images of the retina is applied for the acquisition of a subsequent B-Scan if there has not been a significant change in the speed of the movements of the retina. This absence of correction in the position shift, associated with a correction of the scanning speed, makes it possible to avoid any sudden variation in the spacing between the end of a B-scan and the start of the following B-scan. A correction to the nominal position shift depending on the speed of the eye movements can however be applied.
[0046] According to one or more exemplary embodiments, the nominal position offset is predetermined for the acquisition of a cross-sectional image of the retina at the same location as that of the previous cross-sectional image of the retina, with a view to acquiring an averaged cross-sectional image of the retina. The imaging method then comprises determining an averaged cross-sectional image of the retina, calculated from an average between several cross-sectional images acquired at the same location. According to one or more exemplary embodiments, said averaged cross-sectional image of the retina is displayed.
[0047] According to one or more exemplary embodiments, the nominal position offset is predetermined for the acquisition of a cross-sectional image of the retina at the same location as that of the previous cross-sectional image of the retina, with a view to acquiring a so-called “angiographic” image of the retina, or “OCT A”, making it possible to highlight the vessels of the retina (see [Ref. 1]); the method then comprises the determination of an image calculated from a modification of the content between several cross-sectional images acquired at the same location. For example, a dispersion indicator can be used on the information of each pixel of the image. The information of each pixel can for example correspond to a modulus of a complex value corresponding to the OCT measurement, or to the phase of this measurement. A dispersion indicator dispersion is for example the variance, the standard deviation, the mean absolute deviation, the moment of order N or any other indicator of dispersion of the information of each pixel.
[0048] According to one or more exemplary embodiments, the nominal position offset is predetermined for the acquisition of a cross-sectional image of the retina at a location on the retina different from that of the previous cross-sectional image, with a view to acquiring a three-dimensional image of the retina.
[0049] According to one or more exemplary embodiments, the method further comprises displaying a three-dimensional image of the retina formed from said plurality of cross-sectional images of the retina.
[0050] From said three-dimensional image of the retina, obtained by means of the method according to the first aspect, a user can, according to one or more exemplary embodiments, select a region of interest to be imaged. He can for example, and in a non-limiting manner, require the acquisition of an averaged B-Scan, the acquisition of a three-dimensional image of the retina in high lateral resolution over a reduced field, the acquisition of an angiographic image of the retina. For each of these applications, the method according to the present description can be applied, with in particular a correction of the scanning speed at the start of the acquisition of a B-Scan and, where appropriate, a correction of the offset.
[0051] According to one or more exemplary embodiments, the method further comprises, during the acquisition of each cross-sectional image of the retina, detecting a blink of the eye from said surface images of the retina. During a blink of the eye, the surface images of the retina obtained have a quasi-zero signal.
[0052] In the event of detection of a blink of the eye, the acquisition of the current cross-sectional image of the retina is stopped, then when the blink is finished and the surface images of the retina are again obtained, a position shift in each of the directions is applied by said first scanning sub-module to restart the acquisition of said cross-sectional image of the retina.
[0053] According to one or more exemplary embodiments, the method further comprises, during the acquisition of each cross-sectional image of the retina, the detection of a microsaccade from said angular velocity of the movements of the retina. In the event of detection of a microsaccade, the acquisition of the current cross-sectional image of the retina is stopped and when the microsaccade is finished, a position shift in each of the directions is applied by said first scanning sub-module to restart the acquisition of said cross-sectional image of the retina.
[0054] The applicant has shown in fact that in the case of a blink, the absence of a signal does not allow any measurement and in the case of a microsaccade, the displacement of the retina during the acquisition is too great to be able to be corrected. It is then better to start the B-scan acquisition again.
[0055] According to a second aspect, the present description relates to devices for implementing the methods described according to one or more exemplary embodiments of the method according to the first aspect.
[0056] Thus, the present description relates to a retinal imaging device, comprising:
[0057] - a first tomographic image acquisition module, comprising a first lighting and detection sub-module and a first scanning sub-module in two directions, said first module being configured for the acquisition of a plurality of cross-sectional images of the retina; - a second retinal surface image acquisition module comprising a second illumination and detection sub-module; - a control unit configured for • determine, from surface images acquired by the second module, an angular velocity of eye movements in at least one of said directions; • determining, when launching the acquisition of each sectional image of said plurality of sectional images of the retina, a scanning speed to be applied by said first scanning sub-module along said at least one direction, said scanning speed comprising a nominal scanning speed corrected by a correction speed depending on said angular speed of the eye movements.
[0058] According to one or more exemplary embodiments, each of said first and second modules comprises a wide-field optical path and a small-field optical path. Such modules are described for example in [Ref. 2].
[0059] According to one or more exemplary embodiments, said second retinal surface image acquisition module further comprises a second sub-module for scanning in two directions. This is, for example, a laser scanning ophthalmoscope (or "SLO" for "Scanning laser ophthalmoscope"). Such an SLO comprises, for example, a resonant scanner for scanning in a first direction and a galvanometric scanner for scanning in a second direction.
[0060] According to one or more exemplary embodiments, said first tomographic image acquisition module is of the “Fourier domain OCT” or “Swept source OCT” type as known from the state of the art. Brief description of the figures
[0061] Other advantages and characteristics of the invention will appear on reading the description, illustrated by the following figures:
[0062] [fig.lA], already described, represents a diagram illustrating a device for OCT type retinal imaging known from the state of the art;
[0063] [fig.lB], already described, represents three diagrams illustrating OCT type retinal images (A-Scan and B-Scan) known from the state of the art;
[0064] [fig.lC], already described, represents two diagrams illustrating the effect of a drift-type eye movement on OCT (B-Scan) type retinal images;
[0065] [fig.2] represents a diagram illustrating an example of a retinal imaging device optical coherence tomography (OCT) according to the present description;
[0066] [fig.3A] represents a block diagram of an exemplary optical coherence tomographic (OCT) retinal imaging method according to the present description;
[0067] [fig.3B] represents a block diagram illustrating a detail of the retinal imaging method described in Fig. 3A, according to an example;
[0068] [fig.4A] represents a diagram illustrating two curves showing respectively, as a function of time and in a direction, an example of retinal drift and the corresponding correction signal in an example of a retinal imaging method according to the present description;
[0069] [fig.4B] represents, for comparison, a diagram illustrating the same two curves as in Fig. 4A but in an example of a retinal imaging method according to [Ref. 3];
[0070] [fig.4C] represents, for comparison, a diagram illustrating the same two curves as in Fig. 4A but in an example of a retinal imaging method according to [Ref. 4];
[0071] [fig.5] represents a block diagram of an example of a re imaging method An optical coherence tomographic (OCT) retina according to the present disclosure comprising acquisition of a 3D OCT image of the retina, and selection, by a user, of acquisitions of other images from a display of the 3D image. Detailed description of the invention
[0072] In the figures, the elements are not shown to scale for better visibility.
[0073] Fig. 2 represents a diagram illustrating an exemplary optical coherence tomographic (OCT) retinal imaging device 200 configured to implement exemplary retinal imaging methods according to the present disclosure.
[0074] The device 200 comprises a first module 201 for acquiring tomographic images, a second module 202 for acquiring retinal surface images and a control unit 203 for controlling elements of said first and second modules 201, 202 and for implementing imaging method steps according to the present description. A beam splitter element 206 allows the combination of the paths for surface imaging and for tomographic imaging for illumination and their separation for detection.
[0075] Generally, the control unit referred to in the present description may comprise one or more physical entities, for example one or more computers. When in the present description, reference is made to calculation or processing steps for the implementation in particular of method steps, it is understood that each calculation or processing step may be implemented by software, hardware, firmware, microcode or any appropriate combination of these technologies. When software is used, each calculation or processing step may be implemented by computer program instructions or software code. These instructions may be stored or transmitted to a storage medium readable by the control unit and / or be executed by the control unit in order to implement these calculation or processing steps.
[0076] The control unit 203 is in this example connected to a screen and / or interface 204 for the interface with a user 11.
[0077] The first tomographic image acquisition module 201 comprises, in a known manner, a first lighting and detection sub-module 210 and a first scanning sub-module 220 in two directions for scanning an illumination beam emitted by the sub-module 210 and a beam re-emitted by the retina after illumination by said illumination beam.
[0078] The illumination and detection sub-module 210 comprises a light source 211 with low temporal coherence, for example a light source of the SLED type, configured to illuminate a point of the retina with a low coherence illumination beam and an interferometer 213, for example a fiber interferometer, for example of the Michelson type, comprising a reference arm 214, configured to form interferences on a detector 216, for example a photomultiplier or an avalanche photodiode. The scanning sub-module 220 comprises for example two galvanometric scanners. The elements of the first tomographic image acquisition module 201 are known and configured for the generation of tomographic images of the “FD-OCT” or “Swept source OCT” type as described for example in [Ref. 1]. Only the main elements are shown diagrammatically in FIG. 2.
[0079] The second module 202 for acquiring a surface image of the retina comprises, in the example of FIG. 2, a second lighting and detection sub-module 230 and a second scanning sub-module 240 in two directions for scanning an illumination beam emitted by the sub-module 230 and a beam re-emitted by the retina after illumination by said illumination beam. The second module 202 is for example a state-of-the-art scanning laser ophthalmoscope (SLO), as described in RH Webb et al. [Ref. 5]. The illumination and detection sub-module 230 comprises a source 231, for example an infrared superluminescent diode (SLED) and a detector 236, for example an avalanche photodiode. The scanning sub-module 240 comprises a resonant scanner and a galvanometric scanner.
[0080] The second retinal surface image acquisition module 202 thus allows the acquisition of retinal surface images at a given acquisition frequency, determined by the scanning frequency of the scanning sub-module 240 and the number of points per line of the retinal surface images that one seeks to acquire.
[0081] Note that other retinal surface image acquisition modules can be used, such as a two-dimensional camera-type acquisition device. In this case, the acquisition frequency is given by the acquisition frequency of the camera. Generally, SLO, although more complex to implement, gives more contrasted images than those provided by a camera.
[0082] In the example of FIG. 2, an optical system allows on each of the retinal surface imaging and tomographic imaging paths to route the illumination beam to the patient's eye and to shape it jointly with the scanning module to scan the retina in the desired manner. The optical systems of each of the paths have at least one common element 205.
[0083] Note that according to one or more exemplary embodiments, each of said first and second modules may comprise a wide-field optical path and a small-field optical path (not shown in FIG. 2). Such modules are described for example in [Ref. 2].
[0084] FIG. 3A represents a block diagram of an exemplary optical coherence tomographic (OCT) retinal imaging method 300 according to the present description, implemented for example by means of a device as described in FIG. 2 and FIG. 3B represents a block diagram illustrating a detail of the retinal imaging method described in FIG. 3A.
[0085] In the method according to the present description, surface images of the retina are acquired at a given frequency by the surface image acquisition module (202, FIG. 2) (step 340, FIG. 3B). In practice, a series of surface images of the retina are acquired to generate a reference image 341 by registration and averaging, then the acquired images are compared to identify the presence of a blink, a microsaccade and to measure the speed of the movements of the retina.
[0086] From the surface images of the retina, the control unit determines (step 311) an angular velocity of the eye movements (or movements of the retina) along the two scanning directions or along at least one of the two directions, for example the main direction. The determination of the angular velocity of the movements of the retina can be done at the acquisition frequency of surface images, for example from successively acquired surface images.
[0087] The retinal imaging method 300 further comprises the successive acquisition (350, FIG. 3B) of a plurality of B-Scan(i) sectional images of the retina by means of the first tomographic image acquisition module (201, FIG. 2).
[0088] As illustrated in FIG. 3A, during the acquisition 301 of each B-Scan(i) sectional image, a scanning speed 312 is calculated by the control unit 203. The scanning speed is applied by the first scanning sub-module 220. The scanning speed to be applied before the start of acquisition 313 of the B-Scan(i) comprises a nominal scanning speed corrected by a correction speed depending on the angular speed of the eye movements before the start of the acquisition of the B-Scan(i) image. For example, the last value of the angular speed of the eye movements measured before the start of the acquisition of the B-Scan will be taken. It is also possible to determine a correction speed from a prediction obtained from a plurality of previously determined retinal movement speed values.
[0089] During the acquisition of a B-Scan, it is determined (step 322) whether there has been a blink of the eye from the surface images of the retina. For this, an absence of signal is detected.
[0090] It is also determined (step 324) whether there has been a microsaccade. The information of a microsaccade is determined from the speeds of the movements of the retina. For example, a very rapid shift of the image may be observed.
[0091] If there is a blink (323) or microsaccade (325), the acquisition of the B-Scan is suspended (326) for the duration of the blink or microsaccade. A new complete acquisition of the current sectional image is carried out after the end of the blink or microsaccade. For this, an offset is applied to the scanning beam (327) and a new measurement of the retinal movement speed is taken into account.
[0092] If there has been no microsaccade, the acquisition of the B-Scan continues until the end of the predetermined line for the acquisition is reached (314).
[0093] The OCT scan stops (315) to start a new B-Scan if necessary.
[0094] In order to start a new B-Scan, a nominal offset is applied (316). The nominal position offset is a predetermined offset value to be applied by the first scanning sub-module before acquiring a new B-scan, independently of any eye movement correction. The nominal position offset is for example calculated with the user-defined scan parameters. For example, the user defines a scan that gives B-scans of 30° length, spaced 0.2° vertically, in raster scan (always in the same direction). The offset nominal position between the end of a B-scan and the next B-scan is for example -30°, -0.2°.
[0095] In the case of a raster scan, an offset correction may be applied to the nominal offset in at least one direction, for example the main direction, the offset correction being a function of the angular velocity of the eye movements in said direction and of the time used by the scanner to return to the starting point. This makes it possible to correct the drift of the retina during the time taken to return to the starting point for the acquisition of a new B-Scan.
[0096] In the example illustrated in FIG. 3A, a variation in the speed of the movements of the retina is also determined (317) during the acquisition of the B-Scan(i). This measurement of the variation in speed is used to determine whether or not to apply a correction offset to be applied before the acquisition of a subsequent B-Scan.
[0097] Thus, it is determined (318) whether the variation is less than a predetermined threshold. If so, the acquisition of the following B-Scan (B-Scan(i+1), 302) is carried out without offset correction (or with only the correction linked to the speed as previously explained). If not, an offset correction is carried out (319) before acquiring the following B-Scan (B-Scan(i+1), 302). This offset correction is calculated on the basis of the surface images of the retina and is carried out according to at least one of the scanning directions.
[0098] Thus, in practice, when a plurality of cross-sectional images of the retina are acquired, during the acquisition of each of the cross-sectional images, a scanning speed comprising a nominal scanning speed corrected by the speed of the movements of the retina is applied by the first scanning sub-module. Furthermore, before the launch of each new cross-sectional image of the retina, an offset correction in at least one direction can be applied to the nominal position offset by said first scanning sub-module. This offset correction can be a correction calculated directly from the images of the retina if said value representative of the variation in the speed of the movements of the retina during the acquisition of the previous cross-sectional image is greater than the predetermined threshold value.
[0099] The threshold value may be determined as a function of the lateral resolution sought for retinal imaging, the focal length of the eye, the acquisition time of an A-Scan and the number of A-Scans per B-Scan. Examples of threshold values are between 0.1° / s and 10° / s. These values are calculated respectively for (min value) a resolution of 1 pm, an A-Scan acquisition frequency of 50 kHz, and a number of 1000 A-Scans per B-Scan and for (high value) a resolution of 5 pm, an A-Scan acquisition frequency of 200 kHz, and a number of 500 A-Scans per B-Scan.
[0100] For example, a lateral resolution on the retina of 2 pm with an eye of focal length 17 mm corresponds to an angular resolution of 0.007°. The acquisition time of a B-Scan composed of 1000 A-Scans acquired at a frequency of 200 kHz is 5 ms. The speed variation threshold to be applied in this case is 0.00770.005s = 1.37s.
[0101] FIG. 3B thus illustrates an example of acquisition of a first B-Scan(i) (step 351). During the acquisition, the control unit determines from the surface images 342 a speed of the movements of the retina (step 311). At the end of the acquisition, a new B-Scan is acquired (B-Scan(i+1), step 352). As illustrated in FIG. 3B, the B-Scan(i+1) is interrupted due to the detection of a blink or a microsaccade. It is therefore acquired again (353). Then a new B-Scan is acquired (B-Scan(i+2), 354), etc.
[0102] FIG. 4A represents a diagram illustrating two curves 401, 402 showing respectively, as a function of time and in a direction, an example of retinal drift (401) and the corresponding correction signal (402) applied to the scanning sub-module (220, FIG. 2) of the tomographic image acquisition module.
[0103] For the acquisition of the cross-sectional images of the retina B-Scan(i), B-Scan(i+1), B-Scan(i+2), B-Scan(i+3), there is no application of an offset correction but only the application of a scanning speed correction, observable by the changes in slopes. On the other hand, during the acquisition of the cross-sectional image B-Scan(i+4), the control unit has determined a variation in the angular speed of the movements of the retina greater than the predetermined threshold and an offset correction is applied in addition to the scanning speed correction.
[0104] These curves can be compared with those that would be obtained (see FIG. 4B) in an example of a retinal imaging method according to [Ref. 3]. In this example, an offset correction is applied with a high frequency during the acquisition of the B-Scan so that the two curves follow each other precisely.
[0105] The curves of FIG. 4A can also be compared with those that would be obtained (see FIG. 4C) in an example of a retinal imaging method according to [Ref. 4]. In this example there is no correction of the scanning speed and there is a systematic correction of the offset according to the surface images of the retina. As can be seen in FIG. 4C, the resulting correction is much less precise.
[0106] FIG. 5 depicts a block diagram of an exemplary optical coherence tomographic (OCT) retinal imaging method according to the present disclosure comprising acquisition of a 3D OCT image of the retina, and selection, by a user, of acquisitions of other images from a display of the 3D image.
[0107] More specifically, in this example, the method 500 comprises a step 501 during which a patient is asked to fix his gaze on a marker.
[0108] An acquisition of a three-dimensional OCT image of the retina is obtained (502) at by means of a method according to the present description, for example a method as described by means of FIG. 3A for which successive B-Scans are acquired at different locations of the retina.
[0109] The 3D OCT image is displayed (503) and a user is offered a new examination (504). Thus, in this exemplary embodiment, a user can for example select a new region of the retina to be imaged based on the previously acquired three-dimensional image.
[0110] FIG. 5 illustrates several examples of additional examinations 510, 520, 530, which can be chosen by a user, these examples not being limiting and being able to be implemented successively from the basis of the same 3D image of the retina.
[0111] For example, a user may choose to do an averaged B-Scan (510).
[0112] The position of the B-Scan to be performed is chosen (step 511) from the 3D image of the retina determined beforehand and displayed. A preview can be shown (512) before starting the acquisition (513). A plurality of B-Scans are acquired (514) according to the method according to the present description, for example as described by means of FIG. 3A. In particular, the nominal position offset is predetermined for the acquisition of a cross-sectional image of the retina at the same location as that of the previous cross-sectional image of the retina, and an averaged cross-sectional image of the retina is calculated from an average between several cross-sectional images acquired at the same location. The averaged B-scan is displayed (515).
[0113] According to another example, a user can choose to make a three-dimensional image of the retina called angiographic (520).
[0114] The position of the region to be imaged is chosen (step 521) from the 3D image of the retina determined beforehand and displayed. A preview may be shown (522) before starting the acquisition (523). A plurality of B-Scans are acquired (524) according to a method according to the present description, for example the method as described by means of FIG. 3A. In this example, an image is calculated from a modification of the content between several images acquired at the same location. The angiographic image is displayed (525).
[0115] In another example, a user may choose to make a small field, high lateral resolution three-dimensional image of the retina (530).
[0116] The position of the region to be imaged is chosen (step 531) from the 3D image of the retina determined beforehand and displayed. A preview may be shown (532) before starting the acquisition (533). A plurality of B-Scans are acquired (534) according to a method according to the present description, for example the method as described by means of FIG. 3A, using the high-resolution small-field channels of the retinal imaging device. The three-dimensional image of the retina is displayed (535).
[0117] Although described through a certain number of exemplary embodiments, the method and The retinal imaging device according to the present description includes various variations, modifications and improvements which will be obvious to those skilled in the art, it being understood that these various variations, modifications and improvements are part of the scope of the invention as defined by the following claims. References
[0118] Ref 1.: JF De Boer et al. “Twenty-five years of optical coherence tomography: the paradigm shift in sensitivity and speed provided by Fourier domain OCT\ Vol. 8, No. 7 I 1 Jul 2017 I BIOMEDICAL OPTICS EXPRESS 3248
[0119] Ref2.:WO 2018197288
[0120] Ref 3: US 2010 / 0053553
[0121] Ref. 4: US2014 / 0211155
[0122] Ref. 5: RH Webb et al. “Confocal scanning laser ophthalmoscope” Appl. Opt.26, 1492-1499(1987)
Claims
Claims
1. A retinal imaging method, comprising: - successively acquiring (350) a plurality of cross-sectional images (B-Scan(i)) of the retina by means of a first module (201) for acquiring tomographic images, said first module comprising a first lighting and detection sub-module (210) and a first sub-module (220) for scanning along two directions (x, y); - acquiring (340) surface images of the retina by means of a second module (202) for acquiring surface images of the retina, said second module comprising a second lighting and detection sub-module (230); - determining (311), from surface images acquired by the second module (202), an angular velocity of the movements of the retina along at least one of said directions (x, y);- determining (312), before the start of the acquisition of each sectional image of said plurality of sectional images of the retina, a scanning speed to be applied by said first scanning sub-module along said at least one direction, said scanning speed comprising a nominal scanning speed corrected by a correction speed depending on said angular speed of the movements of the retina.;
2. A retinal imaging method according to claim 1, further comprising, before acquiring each cross-sectional image of said plurality of cross-sectional images of the retina, determining an offset correction to be applied to a nominal position offset by said first scanning sub-module, in at least one of said directions,
3. A retinal imaging method according to claim 2, wherein the offset correction to be applied along said at least one direction is a function of said correction speed determined along said at least one direction.
4. A retinal imaging method according to any one of the preceding claims, wherein the retinal imaging method further comprises: - determining, during the acquisition of each sectional image (B-Scan(i)) of said plurality of sectional images of the retina, a value representative of a variation in the speed of eye movements in at least one of said directions; - applying, before the acquisition of a sectional image of the retina subsequent (B-Scan(i+1)), of an offset correction to be applied to a nominal position offset along said at least one direction, if said value representative of the variation in speed during the acquisition of the previous sectional image (B-Scan(i)) is greater than a predetermined threshold value.
5. An imaging method according to claim 4, wherein said shift correction is directly calculated from the surface images of the retina.
6. A retinal imaging method according to one of claims 4 or 5, wherein said value representative of the variation in the speed of retinal movements comprises a value of acceleration of retinal movements.
7. A retinal imaging method according to any preceding claim, further comprising determining an averaged cross-sectional image of the retina, calculated from an average between several cross-sectional images acquired at the same location.
8. A retinal imaging method according to any one of claims 1 to 6, further comprising determining an image calculated from an estimate of a change in content between several cross-sectional images acquired at the same location.
9. A retinal imaging method according to any one of claims 1 to 6, wherein said plurality of cross-sectional images of the retina are acquired at different locations of the retina, for the purpose of acquiring a three-dimensional image of the retina.
10. A retinal imaging method according to claim 9, further comprising: - displaying said three-dimensional image of the retina; - selecting, by a user, from said three-dimensional image of the retina, a new region of the retina to be imaged.
11. A retinal imaging method according to any one of the preceding claims, further comprising: - detecting, during the acquisition of each cross-sectional image (B-Scan(i)) of said plurality of cross-sectional images of the retina, a blink of the eye from said surface images of the retina, - stopping the acquisition of said cross-sectional image (B-Scan(i)) in the event of detection of a blink of the eye; and - applying by said first scanning sub-module an offset position according to each of the directions to restart the acquisition of said cross-sectional image of the retina.
12. A retinal imaging method according to any one of the preceding claims, further comprising: - detecting, during the acquisition of each sectional image (B-Scan(i)) of said plurality of sectional images of the retina, a microsaccade from said angular velocity of the movements of the retina, - stopping the acquisition of said sectional image (B-Scan(i)) in the event of detection of a microsaccade; - applying by said first scanning sub-module a position shift in each of the directions to restart the acquisition of said sectional image of the retina.
13. Retinal imaging device (200), comprising: - a first module (201) for acquiring tomographic images, comprising a first lighting and detection sub-module (210) and a first sub-module (220) for scanning in two directions, said first module being configured for acquiring a plurality of sectional images (B-Scan(i)) of the retina; - a second module (202) for acquiring a surface image of the retina comprising a second lighting and detection sub-module (230), said second module being configured for acquiring surface images of the retina; - a control unit (203) configured to: - determine, from surface images acquired by the second module (202), an angular velocity of the movements of the retina in at least one of the two directions;- determining, before the start of the acquisition of each sectional image of said plurality of sectional images of the retina, a scanning speed to be applied by said first scanning sub-module along said at least one direction, said scanning speed comprising a nominal scanning speed corrected by a correction speed depending on the angular speed of the movements of the retina.;
14. The retinal imaging device of claim 13, wherein each of said first and second modules comprises a wide-field optical path and a narrow-field optical path.
15. A retinal imaging device according to any one of claims 13 or 14, wherein said second module (202) retinal surface image acquisition further comprises a second sub-module for scanning in two directions.