Improvements in the acquisition of images of the retina through optical coherence tomography

Abstract

Method of tracking the movements of the retina based on images of the retina acquired through optical coherence tomography. The tracking method includes the following steps: - obtaining at least one pair of B-scan images of the retina, wherein each B-scan image of the retina is acquired for a corresponding optical scan of the retina during which an illumination beam is projected onto the retina and is moved along an optical scanning trajectory having the shape of a closed curve; - identifying pairs of characteristic areas having a similar speckle pattern in the B-scan images of said at least one pair of images selected; - calculating tracking data indicative of the movements of the retina based on the position of said characteristic areas in the B-scan images of the retina of said at least one pair of images selected.

Description

[0001] IMPROVEMENTS IN THE ACQUISITION OF IMAGES OF THE RETINA THROUGH OPTICAL COHERENCE TOMOGRAPHY DESCRIPTION

[0002] Field of the invention

[0003] The present invention relates to the field of image acquisition systems through optical coherence tomography. More in particular, the present invention relates to a method of tracking the movements of the retina based on images of the retina acquired through optical coherence tomography. In a further aspect, the present invention relates to an apparatus capable of acquiring images of the retina, for example C-Scan images or angiographic images of the retina, through optical coherence tomography.

[0004] Background art

[0005] As is known, optical coherence tomography (OCT), also called phase- sensitive optical coherence tomography, is a technology that allows the acquisition, also in real time, of high resolution images (of the order of magnitude of pm) of a sample, for example of a biological tissue. The reconstructed information relating to the structure of the sample is obtained from the light radiation back- scattered from the various regions of the sample according to their optical properties. Numerous examples of apparatus for inspection of the eye fundus through optical coherence tomography are known. These systems typically include a light source (for example laser light) and an interferometer optically coupled to the aforesaid light source.

[0006] The aforesaid interferometric device is arranged to provide as output an interferometric optical signal obtained by detecting an optical interference between light radiations reflected respectively by the retina of the eye and by an optical reference illuminated with suitable illumination beams coming from the light source.

[0007] Inspection apparatus of this type are often used to acquire volumetric (C-Scan) images or angiographic images of the retina.

[0008] As is widely known, the images acquired by these apparatus are often affected by motion artifacts due to unwanted movements of the retina during the image acquisition process. These motion artifacts often appear, for example, as stripes or areas of light which, in fact, prevent or greatly limit observation of the retina. To limit this problem, apparatus for inspection of the eye fundus have been developed which, in addition to the system for acquiring images through optical coherence tomography, also have a retinal imaging device (for example a fundus camera). This additional imaging device is advantageously configured to acquire images of the retina (for example a video) simultaneously to the acquisition of images through optical coherence tomography.

[0009] The images of the retina provided by the additional imaging device are processed in order to obtain tracking information indicative of the movements of the retina during the acquisition of images through optical coherence tomography.

[0010] These tracking data can, for example, include data indicative of the extent of movement of the retina, of the direction of movement of the retina and / or of the speed of movement of the retina. Inspection apparatus of this type are characterized by their considerable size and relatively high industrial costs due to the presence of an additional imaging device.

[0011] Moreover, experience has shown that these inspection apparatus are capable of providing images of the retina with a relatively low maximum frequency (around 25 Hz). In fact, processing of the images provided by the additional imaging device requires significant times, which considerably influence the overall performance of the inspection apparatus. To overcome this problem, the calculation resources installed could be suitably increased. However this solution can lead to an unacceptable increase in the total industrial costs.

[0012] From the above, it is evident how, in the state of the art, there is a great need for innovative solutions that allow the aforesaid problems to be overcome or mitigated.

[0013] The present invention intends to meet this need by providing a method of tracking the movements of the retina according to the appended claim 1 and the related dependent claims. Summary of the invention

[0014] In a general definition thereof, the method of tracking the movements of the retina, according to the invention, includes the following steps: obtaining at least one pair of B-scan images of the retina, wherein each B-scan image of the retina is acquired for a corresponding optical scan of the retina during which an illumination beam is projected onto the retina and is moved along an optical scanning trajectory having the shape of a closed curve; identifying pairs of characteristic areas having a similar speckle pattern in the B-scan images of said at least one pair of images thus obtained; calculating tracking data indicative of the movements of the retina based on the position of said characteristic areas in the B-scan images of the retina of said at least one pair of images thus obtained.

[0015] Preferably, the aforesaid tracking data include one or more of: an estimate value of the relative movement of the retina, an estimate value indicative of the direction of movement of the retina, an estimate value indicative of the speed of movement of the retina, an estimate value indicative of the change in speed of movement of the retina.

[0016] Preferably, the B-scan images of the retina are acquired for corresponding optical scans of the retina included in a group of optical scans of the retina which includes at least two optical scans of the retina performed consecutively to one another.

[0017] Preferably, the B-scan images of the retina are acquired for corresponding optical scans of the retina included in a group of optical scans of the retina which includes at least two optical scans of the retina during which said illumination beam is moved along the same optical scanning trajectory.

[0018] According to an aspect of the invention, the step of calculating said tracking data includes calculating the relative distance between a pair of characteristic areas of each B-scan image of the retina.

[0019] According to an aspect of the invention, the step of calculating said tracking data includes calculating the position of a symmetry plane between a pair of characteristic areas of each B- scan image of the retina.

[0020] In a further aspect, the present invention also relates to an apparatus for acquiring images of the retina, for example C-Scan or angiographic images of the retina, through optical coherence tomography according to the following claim 8 and the related dependent claims.

[0021] Brief description of the Figures

[0022] Further features and advantages of the invention will be more apparent by referring to the description provided below and to the accompanying drawings, provided purely by way of nonlimiting example, wherein:

[0023] Fig. 1 schematically illustrates an example of embodiment of an apparatus for inspection of the eye fundus, according to the invention;

[0024] Figs. 2-4 schematically illustrate some steps of the method of tracking the movements of the retina, according to the invention; and

[0025] Figs. 5-7 schematically illustrate operation of an apparatus for inspection of the eye fundus, according to the invention.

[0026] Detailed description of the invention

[0027] The present invention relates to a method 100 for tracking the movements of the retina through optical coherence tomography (OCT).

[0028] The tracking method 100 is conceived to be performed by an apparatus for inspection of the eye fundus through OCT.

[0029] Fig. 1 schematically illustrates an example of apparatus for inspection of the eye fundus arranged to provide images of the retina R of an eye E of a patient through OCT.

[0030] The apparatus 1 can be of different type as a function of the OCT technique used. For example, it can be arranged to acquire images of the retina through optical coherence tomography of SD- OCT (Spectral Domain OCT) or SS-OCT Swept Source OCT) type.

[0031] The apparatus 1 includes a light source 2, preferably configured to emit a coherent electromagnetic radiation. For the applications of interest, this light source can, for example, be a laser or a diode (Super-Luminescent Diode or SLD) configured to emit light with a wavelength in the infrared range.

[0032] In some applications (SS-OCT), the light source 2 is configured to emit an illumination beam according to a sequence of emission cycles, during each of which the radiation emitted has a very narrow band with a wavelength that is varied very rapidly and progressively in a range of predefined wavelengths. For example, for the applications of interest, the light source can be configured to emit a radiation with a wavelength variable cyclically in a range of wavelengths centred around the value 1060 nm, with amplitude of the order of 100 nm and repetition frequency of the cycles of the order of tens or hundreds of KHz.

[0033] In other types of application, for example of SD-OCT type, the light source 2 is configured to emit a broadband light radiation.

[0034] The apparatus 1 includes an optical detection unit 3 optically coupled to the light source 2, for example through optical fibre or other known optical coupling means.

[0035] The optical detection unit 3 is configured to receive a light radiation Ls coming from the light source 2 and to provide as output an optical interferometric signal 11 obtained through optical interference between a measurement beam LM and a reference beam LR.

[0036] The measurement beam LM is formed by light radiation reflected by the retina R illuminated with a first illumination beam L formed by light radiation coming from the light source.

[0037] The reference beam LR is formed by light radiation reflected by an optical reference illuminated with a second illumination beam L2 formed by light radiation coming from the light source.

[0038] In practice, the optical detection unit 3 constitutes an interferometric structure that can be produced according to a known configuration, for example a Michelson or Mach-Zehnder configuration.

[0039] The optical detection unit 3 includes an interferometric measurement arm 4 and an interferometric reference arm 6 optically coupled to the light source 2.

[0040] The interferometric measurement arm 4 is configured to receive as input the first illumination beam L and to provide as output the measurement beam LM.

[0041] The first illumination beam L includes a first portion of the light radiation Ls coming from the light source 2, while the measurement beam LM includes light radiation back- scattered from the retina R after the latter has been illuminated with the first illumination beam L.

[0042] The interferometric measurement arm 4 preferably includes a first optical path 40 along which the first illumination beam L is conveyed toward the eye E of the patient and the measurement beam LM is conveyed in the opposite direction.

[0043] Generally, the first optical path 40 has a variable optical length that substantially depends on the length of the eye of the patient.

[0044] The interferometric reference arm 6 is configured to receive as input the second illumination beam L2 and to provide as output the reference beam LR.

[0045] The second illumination beam L2 includes a second portion of the light radiation Ls coming from the light source 2, while the reference beam LR includes light radiation reflected by an optical reference T (for example a reflecting element included in the interferometric reference arm) after the latter has been illuminated with the second illumination beam L2.

[0046] The interferometric reference arm 6 preferably includes a second optical path 60 along which the second illumination beam L2 is conveyed toward the optical reference T and the reference beam LR is conveyed in the opposite direction.

[0047] Preferably, the second optical path 60 has an adjustable optical length. During use of the apparatus, this optical length is advantageously varied in relation to the length of the optical path 40 in the interferometric measurement arm 4.

[0048] Preferably, the optical detection unit 3 includes an optical fibre array 7 configured to optically couple the interferometric measurement arm 4 and the interferometric reference arm 6 to one another other and to the light source 2.

[0049] The optical fibre array 7 is configured to receive a light radiation Ls coming from the light source 2, convey a first portion L of this light radiation toward the interferometric measurement arm 4 so as to form the first illumination beam L and convey a second portion L2 of this light radiation toward the interferometric reference arm 6 so as to form the second illumination beam. The optical fibre array 7 is configured to convey the measurement beam LM, coming from the interferometric measurement arm 4, toward an output port combining it with the reference beam LR coming from the interferometric reference arm 6.

[0050] The optical fibre array 7 thus provides an optical interferometric signal 11 (also called “measurement interferogram”) generated by the optical interference between the measurement beam LM (radiation reflected and backscattered from the illuminated retina R) and the reference beam LR (radiation reflected from the illuminated optical reference T).

[0051] This optical interferometric signal is indicative of a reflectivity profile as a function of the axial depth of the portion of retina illuminated with the first illumination beam L.

[0052] Preferably, the inspection apparatus 1 includes a conversion unit 8 optically coupled (for example by optical fibre) to the optical detection unit 3 and configured to convert the interferometric signal 11 into a corresponding electrical detection signal . The conversion unit 8 can include a spectrometer in the case in which the inspection apparatus is an SD-OCT system, or an assembly of photodiodes in the case in which the inspection apparatus is an SS-OCT system.

[0053] The conversion unit 8 can be physically integrated with the optical detection unit in a production of the inspection apparatus on an industrial scale.

[0054] The inspection apparatus 1 includes a control unit 5 operationally coupled (according to known methods) to the conversion unit 8.

[0055] The control unit 5 is advantageously configured to perform control functions of the operation of the optical detection unit 3. Naturally, the control unit can be configured to also control the operation of further components of the inspection apparatus 1, for example of the conversion unit 8 and of the light source 2.

[0056] Preferably, the control unit 5 includes at least one control module 51 electrically connected to the components to be controlled, according to known methods. The control module 51 is configured to provide as output suitable control signals for the components to be controlled, receive as input suitable detection signals sent by other devices (sensors, local control units, and so forth) and perform suitable control algorithms, if necessary in cooperation with other modules of the control unit.

[0057] The control unit 5 is also configured to perform data and signal processing functions to process the electrical detection signal provided as output by the conversion unit 8 and obtain one or more images of the retina based on this signal.

[0058] Preferably, the control unit 5 comprises an interface module 52 configured to receive the interferometric electrical detection signal and perform sampling of this signal through known data and signal processing algorithms.

[0059] Preferably, the control unit 5 includes at least one data processing module 53 configured to process the sampling data provided by the interface module 52 and obtain one or more images of the retina. To this end, the data processing module 53 can be configured to perform known data and image processing algorithms.

[0060] The control module 51, the interface module 52 and the data processing module 53 can be produced industrially according to known solutions. For example, they can include suitable microprocessor circuits, FPGAs or electronic circuits of other type mounted on a suitable circuit board.

[0061] During operation of the apparatus, the various modules 51, 52, 53 cooperate with one another to perform the required functions. Moreover, these modules can be physically integrated with one another in production of the inspection apparatus on an industrial scale.

[0062] In principle, the images of the retina obtainable can be of different types, for example A-scan, B-scan or C-scan images. As will be seen below, the present invention is advantageously aimed at obtaining volumetric images Q of the retina, for example C-Scan images or angiographic images of the retina.

[0063] As indicated above, the inspection apparatus, in the embodiment of Fig. 1, is described here purely by way of example and without in any way wishing to limit the scope of the present invention. The tracking method 100, according to the invention, can in fact be performed by inspection apparatus of a different type, in principle by any apparatus (also of known type) that uses an OCT technology to acquire images of the retina.

[0064] The tracking method 100 includes a step of obtaining at least one pair of B-scan images Wi, W2 of the retina.

[0065] Each B-scan image of the retina is acquired for a corresponding optical scan of the retina during which a illumination beam L is projected onto the retina and moved along an optical scanning trajectory having the shape of a closed curve.

[0066] Preferably, the aforesaid optical scanning trajectory has a circular shape.

[0067] Preferably, the B-scan images Wi, W2 of the retina obtained refer to optical scans of the retina included in a group of optical scans of the retina that includes at least two optical scans of the retina performed consecutively to one another.

[0068] Preferably, the B-scan images Wi, W2 of the retina are acquired for optical scans of the retina included in a group of optical scans of the retina that includes at least two optical scans of the retina during which an illumination beam L is moved along the same optical scanning trajectory. The tracking method 100 involves obtaining the aforesaid tracking information based on analysis of the speckle pattern of the B-scan images Wi, W2 of the retina.

[0069] As widely known, a B-scan image of the retina shows a two-dimensional portion of the retina having a dimension along the surface of the retina, according to the optical scanning trajectory, and a dimension along the axial depth of the retina (with reference to the axis of the illumination beam L), in practice perpendicular to the surface of the retina.

[0070] Any B-scan image of the retina is characterized by a certain speckle pattern, i.e., by a distribution of small light and dark areas on the background of the image.

[0071] The speckle pattern of a B-scan image of the retina is a phenomenon of optical interference that depends substantially on the physical characteristics of the portion of retina illuminated by the illumination beam L. In practice, it unequivocally identifies the (two-dimensional) portion of retina illuminated during the optical scan and shown in the B-scan image acquired.

[0072] Based on the above, it is evident that B-scan images of the retina obtained by performing successive optical scans of the retina, during which an illumination beam is moved along the same trajectory, necessarily have a similar speckle pattern if the illumination beam illuminates the same portion of retina.

[0073] Conversely, B-scan images of the retina will have different speckle patterns from one another if the illumination beam illuminates portions of retina different from one another.

[0074] For reasons of clarity, it is specified that B-scan images or portions of B-scan images of the retina have “similar” speckle patterns if the speckle patterns of these B-scan images or portions of B-scan images are greatly correlated to one another, in practice if there is a high level of correspondence between position and intensity of the light and dark areas of the speckle patterns of these B-scan images or portions of B-scan images.

[0075] The similarity between the speckle patterns of B-scan images or portions of B-scan images of the retina compared with one another can be quantitatively evaluated by calculating a suitable correlation index between B-scan images or portions of B-scan images and comparing the correlation index calculated with a suitable threshold value.

[0076] The tracking method 100 exploits this principle to obtain tracking information of the movements of the retina.

[0077] Fig. 2 illustrates the portions of retina Ri, R2 illuminated during two successive optical scans of the retina (preferably consecutive to one another) during each of which an illumination beam L is projected onto the retina and is preferably moved along the same circular trajectory.

[0078] Fig. 3 shows a further view of the portions of retina Ri, R2 illustrated in Fig. 2 according to an observation axis parallel to or coincident with the axis of the illumination beam L.

[0079] As can be noted from the aforesaid figures, each of the portions of retina Ri, R2 has a dimension along the surface of the retina and a dimension in axial depth, i.e., along the axis of the illumination beam L (in practice perpendicular to the surface of the retina).

[0080] Given that the optical scanning trajectory is circular, each of the portions of retina Ri, R2 has a substantially tubular shape that extends along a corresponding longitudinal axis of symmetry Ai, A2.

[0081] If the retina were to remain immobile during the performance of successive optical scans, the portions of retina Ri, R2 illuminated would be perfectly coincident and overlapping one another. However, as discussed above, the retina is typically subject to unwanted movements, typically caused by voluntary or involuntary movements (saccades) of the retina.

[0082] In the latter case, the portions of retina Ri, R2 illuminated are different from one another and staggered with respect to one another as illustrated in Fig. 2.

[0083] However, given that the optical scanning trajectory has the shape of a closed curve (circular), the portions of retina Ri, R2 illuminated are partly overlapping one another at a pair of overlap regions R12, R21 which are illuminated during the performance of both the aforesaid optical scans.

[0084] Naturally, this occurs, for relatively limited movements of the retina between one optical scan and the next. Nonetheless, this circumstances occurs almost always in view of the fact that acquisition of a B-scan image of the retina is normally completed in a time range of a few ms. With reference to Fig. 3, it is hypothesized that the retina has moved according to a displacement vector d between one optical scan and the next. It is evident how positioning of the overlap regions R12, R21 between the portions of retina Ri, R2 illuminated depends on the modulus (distance between axes of symmetry Ai, A2) and on the direction (displacement angle a) of the displacement vector d.

[0085] Fig. 4 schematically illustrates a pair of B-scan images Wi, W2 of the retina which show the portions of illuminated retina Ri, R2 illuminated during two successive optical scans (preferably consecutive to one another).

[0086] Each B-scan image can refer to the same reference system having the optical scanning angle (according to a circular trajectory) on the abscissa and the thickness of the retina (along the axis of the illumination beam L) on the ordinate.

[0087] As discussed above, each B-scan image is characterized by a certain speckle pattern.

[0088] Given that the portions of retina Ri, R2, illuminated during performance of the successive optical scans are different from one another and overlapping at a pair of overlap regions R12, R21, the B-scan images Wi, W2 of the retina have speckle patterns generally different from one another. However, they have a similar speckle pattern at a pair of characteristic areas W12, W21 which correspond to the overlap regions R12, R21 between the aforesaid portions of retina Ri, R2.

[0089] For obvious reasons of geometry, given that the optical scanning trajectory is circular, the characteristic areas W12, W21 of each B-scan image Wi, W2 have the same symmetry plane D (identified by the plane perpendicular to the surface of the retina and directed according to the displacement vector d thereof).

[0090] Moreover, given that the B-scan images Wi, W2 of the retina refer to the same reference system, the characteristic areas W12, W21 of each B-scan image Wi, W2 of the retina are positioned at different relative distances a, b from one another given that the retina moved during performance of the successive optical scans.

[0091] From the above, it is evident that observation of the characteristic areas W12, W21 of each B- scan image Wi, W2 of the retina can provide tracking information that allows the movements of the retina between one optical scan and the next to be traced.

[0092] The tracking method 100 thus includes a step of identifying characteristic areas W 12, W21 having a similar speckle pattern in the images Wi, W2 of the retina obtained and, subsequently, the step of calculating tracking data Z indicative of the movements of the retina based on the position of the characteristic areas W12, W21 in each of the B-scan images Wi, W2 of the retina selected. Recognition and identification of the characteristic areas W12, W21 in the B-scan images Wi, W2 of the retina and calculation of the tracking data Z can advantageously be performed through suitable image processing algorithms (also of known type).

[0093] Preferably, the tracking method 100 includes calculating the relative distance a, b between the characteristic areas W12, W21 in each of the B-scan images Wi, W2 of the retina.

[0094] Preferably, the tracking method 100 includes also calculating the position a of the symmetry plane D between the characteristic areas W12, W21 of each of the B-scan images Wi, W2 of the retina. The position a of the symmetry plane D is advantageously an angular position measured with respect to a reference indicative of the starting point of the first optical scan of the retina performed.

[0095] Preferably, the tracking data Z of the movements of the retina include one or more of: an estimate value of the relative movement of the retina, an estimate of the direction of movement of the retina, an estimate value of the speed of movement of the retina, an estimate value of the change in speed of movement of the retina.

[0096] Advantageously, these estimate values can be calculated based on the data a, b, a indicative of the relative position between the characteristic areas W12, W21 in the B-scan images Wi, W2 of the retina (Figs. 3-4).

[0097] For example, an estimate value s / of the relative movement of the retina (modulus of the displacement vector d) can be calculated based on the following relation: si = | < / | ~ (b - )!2 where a, b are the relative distances between the characteristic areas W12, W21 in each of the B- scan images Wi, W2 of the retina.

[0098] As further example, an estimate value S2 indicative of the direction of relative movement of the retina (angle of the displacement vector d) can be calculated based on the following relation: S2 - arg(d) ~ a where a is the angular position of the symmetry plane D measured with respect to a reference indicative of the starting point of the first optical scan of the retina performed.

[0099] As further example, an estimate value ss of the speed of relative movement of the retina (change of the modulus of the displacement vector d in the unit of time) can be calculated based on the following relation: where a, b are the relative distances between the characteristic areas W12, W21 in each of the B- scan images Wi, W2 of the retina and T.s is the optical scanning period (i.e., the interval of time between two successive optical scans of the retina).

[0100] As further example, an estimate value S4 = Av / At of the change in speed of movement of the retina can be calculated in a relatively simple way based on the estimate values ss of the speed of movement of the retina calculated for successive pairs of B-scan images selected.

[0101] Advantageously, the tracking data Z of the movements of the retina can include further estimate values adapted to describe the kinematic behaviour of the retina during the successive optical scans. For example, the tracking data Z can comprise estimate values indicative of the speed of movement along the Cartesian axes of the reference system used, estimate values indicative of the changes in speed of movement along the Cartesian axes of the reference system used, and so forth.

[0102] For the sake of simplicity, the tracking method 100 has been described above with particular reference to the case in which the successive optical scans of the retina are performed along a circular optical scanning trajectory (Figs. 2-4). This is not meant to limit the scope of the present invention in any way. The tracking method 100 allows tracking data Z indicative of any movements of the retina to be obtained also based on the analysis of B-scan images of the retina obtained by performing optical scans along optical scanning trajectories having a closed shape of different type, for example an elliptical or oval shape. Naturally, in these cases, calculation of the estimate values (tracking data Z) indicative of the movements of the retina is performed based on different mathematical relations to those described above.

[0103] Being based on processing of B-scan images of the retina that can be naturally acquired by an apparatus for inspection of the eye fundus through optical coherence tomography, the tracking method 100 can be easily integrated with retinal image acquisition procedures through optical coherence tomography. The tracking method 100 allows information indicative of the movements of the retina to be obtained very quickly. Therefore, it is particularly suitable for use used in automatic tracking processes of the retina that can be performed in real time to regulate the optical scans of the retina so as to take account of any movements thereof.

[0104] The tracking method 100 can however also be used only to identify any movements of the retina during the acquisition of volumetric images of the retina, for example C-Scan images or angiographic images of the retina.

[0105] The tracking data Z can be used to verify whether any B-scan images of the retina acquired are affected by motion artifacts so as to be able to repeat acquisition of the B-scan images of the retina thus identified.

[0106] The tracking data Z can also be used during post-processing to construct volumetric images of the retina without motion artifacts.

[0107] In a further aspect thereof, the present invention refers to an apparatus for inspection of the eye fundus through optical coherence tomography, for example an inspection apparatus of the type illustrated in Fig. 1.

[0108] Preferably, the inspection apparatus is configured to perform a retinal image acquisition procedure 200, which includes the steps, described above, of the method of tracking 100 the movements of the retina simultaneously to acquisition of B-scan images of the retina.

[0109] The acquisition procedure 200 includes the performance of a series of optical scans of the retina. During each optical scan of the retina, an illumination beam L is projected onto the retina and is moved along the same optical scanning path T which is translated along the surface of the retina, according to a predefined translation direction Dr between one optical scan and the next. Figs. 5-7 schematically illustrate the performance of successive optical scans of the retina, during each of which an illumination beam L is projected onto the retina and is moved along the same optical scanning path T.

[0110] It is evident how the acquisition procedure 200 involves performing a raster of optical scans having the same optical scanning path T which is translated, time by time, along the translation direction DT.

[0111] The optical scanning path T, followed by the illumination beam L, during each optical scan, includes two interlaced optical scanning trajectories, in particular a first optical scanning trajectory Ti having the shape of a closed curve and a second optical scanning trajectory T2. Preferably, the first optical scanning trajectory Ti has a circular shape.

[0112] Preferably, the second optical scanning trajectory T2 has the shape of a closed curve and comprises a pair of rectilinear portions T21, T22 parallel to one another. Fig. 5 illustrates an example optical scanning path T followed by the illumination beam L during a generic optical scan of the acquisition procedure 200.

[0113] In this case, the optical scanning path T includes a first optical scanning trajectory Ti of circular shape and a second optical scanning trajectory T2 having the shape of a closed curve that includes a pair of rectilinear portions T21, T22 parallel to one another and a pair of semi-circular portions T23, T24 opposite one another and positioned so as to join the corresponding ends of the rectilinear portions T21, T22.

[0114] The optical scanning trajectories Ti, T2 provided for the illumination beam L could however have a different shape to the one illustrated. For example, the first optical scanning trajectory Ti could have an elliptical shape while the second optical scanning trajectory T2 could have a rectangular shape.

[0115] The relative position between optical scanning trajectories Ti, T2 could also differ from the one indicated. For example, although interlaced, the aforesaid optical scanning trajectories might not be partly overlapping one another as illustrated in Fig. 5.

[0116] During each optical scan, a first group of optical scans is performed, which includes a plurality (at least two) of first optical scan cycles, during each of which an illumination beam L is projected onto the retina and is moved along the first optical scanning trajectory Ti.

[0117] During each optical scan, a second group of optical scans is also performed, which includes one or more second optical scan cycles, during each of the which an illumination beam L is projected onto the retina and is moved along the second optical scanning trajectory T2.

[0118] The number of first optical scan cycles and the number of second optical scan cycles, performed during the same optical scan, are not necessarily correlated to one another.

[0119] For example, in the case of acquisition of a C-scan image of the retina, during each optical scan, the illumination beam L can be moved twice along the first optical scanning trajectory Ti and once along the second optical scanning trajectory T2. In this case, each optical scan involves performing two first optical scan cycles and only one second optical scan cycle.

[0120] For example, in the case of acquisition of an angiographic image of the retina, during each optical scan, the illumination beam L can be moved four times along the first optical scanning trajectory Ti and four times along the second optical scanning trajectory T2. In this case, each optical scan involves performing four first optical scan cycles and four second optical scan cycles.

[0121] During a generic optical scan, the order in which the first and the second optical scan cycles are performed can be any, according to requirements. The illumination beam L can therefore be moved first along the first optical scanning trajectory Ti and subsequently along the second optical scanning trajectory T2, or vice versa, or even in an interlaced manner.

[0122] According to the acquisition procedure 200, a B-scan image of the retina is acquired for each first optical scanning cycle performed during each optical scan of the retina.

[0123] Therefore, the acquisition procedure 200 involves acquiring a B-scan image of the retina each time the illumination beam L travels along the first trajectory Ti during the successive optical scans of the retina.

[0124] The collection of B-scan images of the retina thus obtained is advantageously used to calculate tracking data Z indicative of the movements of the retina, according to the tracking method 100 described above.

[0125] In practice, the acquisition procedure 200 includes: obtaining one or more pairs of B-scan images Wi, W2 of the retina acquired for corresponding successive first optical scan cycles of the retina; identifying pairs of characteristic areas W12, W21 having a similar speckle pattern in the B- scan images Wi, W2 included in each pair of images selected; calculating tracking data Z indicative of the movements of the retina based on the position of the characteristic areas W12, W21 in the B-scan images Wi, W2 of the retina of each pair of images.

[0126] Recognition and identification of the characteristic areas W12, W21 in the B-scan images Wi, W2 of the retina and calculation of the tracking data Z can be implemented as already described above.

[0127] Simultaneously to acquisition of B-scan images of the retina to be processed to calculate tracking data Z indicative of the movements of the retina, the acquisition procedure 200 involves acquiring B-scan images of the retina to be processed to obtain a final image Q of the retina, for example a C-scan image or an angiographic image of the retina.

[0128] The acquisition procedure 200 includes a step of acquiring at least one B-scan image of the retina for each second optical scanning cycle during which the illumination beam L is advantageously moved along the second optical scanning trajectory T2.

[0129] According to an embodiment of the invention, two B-scan images of the retina are acquired for each second optical scanning cycle. In particular, a B-scan image of the retina is acquired each time the illumination beam L is moved along a corresponding rectilinear portion T21, T22 of the second optical scanning trajectory T2. In this case, the illumination beam L is advantageously moved along the second optical scanning trajectory T2 substantially at constant speed so as to allow the acquisition of a B-scan image for each rectilinear portion T21, T22.

[0130] According to this solution, it is thus possible to obtain two collections of B-scan images of the retina: a collection containing B-scan images acquired by moving the illumination beam L in one direction (forward) and another collection containing B-scan images acquired by moving the illumination beam L in the opposite direction (return).

[0131] Given that the forward and return rectilinear portions T21, T22 of the trajectory T2 are not coincident, a movement of the retina implemented for a short time would damage two portions of the whole raster relatively distant from one another.

[0132] Continuing with the acquisition, these two damaged portions of trajectory of the raster will be reacquired naturally even without a command for reacquisition of the damaged areas.

[0133] Consequently the B-scan of the first collection of B-scan images, acquired in forward direction, and the B-scan images of the second collection, acquired in return direction, can advantageously be selectively combined to exclude portions of image affected by visual artifacts. It is thus possible to form a collection of B-scan images of the retina of high quality to be processed in order to obtain a final image Q of the retina.

[0134] According to an alternative embodiment of the invention, only one B-scan image of the retina is acquired for each second optical scanning cycle. In particular, a B-scan image of the retina is acquired only when the illumination beam L is moved along one of the rectilinear portions T21 or T22 of the second optical scanning trajectory T2.

[0135] In this case, the illumination beam L can advantageously be moved with variable speed along the second optical scanning trajectory T2. In particular, it can be moved with a lower speed along one of the rectilinear portions of trajectory so as to allow the acquisition of a B-scan image with higher resolution and with a higher speed along the other rectilinear portion of trajectory so as to complete the corresponding second optical scanning cycle in a relatively short time.

[0136] With this solution it is possible to obtain, in a relatively short time, a collection of B-scan images of the retina to be processed in order to obtain a final image Q of the retina.

[0137] Preferably, the acquisition procedure 200 includes the step of processing the tracking data Z of the movements of the retina, obtained by performing the tracking method 100, to identify any B-scan images of the retina acquired that are affected by motion artifacts. In this case, the acquisition procedure 200 can include reacquisition of any B-scan images of the retina affected by motion artifacts. Preferably, the acquisition procedure 200 includes the step of processing the B-scan images of the retina acquired for corresponding second optical scan cycles of the retina based on the tracking data Z indicative of the movements of the retina. These data are obtained by processing the B-scan images of the retina acquired for corresponding first optical scan cycles of the retina. This step of processing images of the retina can advantageously include a selection of the B- scan images of the retina acquired based on the tracking data Z calculated as above. This allows only B-scan images of the retina not affected by significant motion artifacts to be used to obtain a final image Q of the retina.

[0138] This step of processing the B-scan images of the retina can also be advantageously performed through suitable image processing algorithms (also of known type).

[0139] As is apparent from what is illustrated above, the method of tracking 100 the movements of the retina allows the acquisition of volumetric images of the retina, for example of C-scan or angiographic type, of very high quality based on only the B-scan images acquired through optical coherence tomography, without the need for additional imaging devices.

[0140] However, according to some embodiments of the invention, the apparatus for inspection of the eye fundus 1 is operationally coupled to or includes a retinal imaging device 9 configured to acquire images of the retina .S'.

[0141] Acquisition of images S of the retina is performed during the acquisition of the B-scan images of the retina of the acquisition procedure 200.

[0142] Preferably, the apparatus for inspection of the eye fundus 1 is configured to process the images .S' of the retina provided by the imaging device 9 so as to calculate additional tracking data ZJ of the movements of the retina.

[0143] Preferably, the apparatus for inspection of the eye fundus 1 is configured to process the B-scan images of the retina acquired for corresponding second optical scan cycles of the retina also based on the aforesaid further tracking data ZJ of the movements of the retina in order to obtain a final image of the retina Q.

[0144] Apparatus for inspection of the eye fundus, according to this embodiment of the invention, are capable of tracking the movements of the retina in a very fast and precise manner.

[0145] However, they require high calculation powers for real time processing of the B-scan images acquired.

[0146] Therefore, these machines are capable of providing high level performance against relatively high industrial costs. Advantages of the invention

[0147] The method of tracking 100 the movements of the retina, according to the invention, offers numerous advantages with respect to prior art solutions available.

[0148] In general, it allows the acquisition of images of the retina, for example of C-scan or angiographic type, of very high quality based only on the B-scan images acquired through optical coherence tomography, without the need for additional imaging devices.

[0149] The tracking data of the movements of the retina obtained can in any case be combined with additional tracking data of the movements of the retina obtained by processing the images of the retina provided by a retinal imaging device.

[0150] Being based on processing only B-scan images of the retina, with the method of tracking 100 the movements of the retina it is possible to obtain tracking data Z of the movements of the retina with very high acquisition frequencies, for example in the order of a few hundreds of Hz (tracking data / second).

[0151] The method of tracking 100 the movements of the retina, according to the invention, is relatively simple to implement on an industrial scale, without requiring additional optical, mechanical and electronic elements.

[0152] It can be performed by apparatus for inspection of the eye fundus through optical coherence tomography having a very compact structure and which are relatively simple and inexpensive to produce on an industrial scale through standardized production techniques and processes.

Claims

CLAIMS1. Method (100) of tracking the movements of the retina through optical coherence tomography characterized in that it includes the following steps:- obtaining at least one pair of B-scan images (Wi, W2) of the retina, in which each B-scan image of the retina is acquired for a corresponding optical scan of the retina during which an illumination beam (L) is projected onto the retina and is moved along an optical scanning trajectory having the shape of a closed curve;- identifying pairs of characteristic areas (W12, W21) having a similar speckle pattern in the B-scan images Wi, W2) obtained;- calculating tracking data (Z) indicative of the movements of the retina based on the position of said characteristic areas W12, W21) in the B-scan images (Wi, W2) of the retina obtained.

2. Method, according to claim 1, characterized in that said B-scan images (Wy, W2) of the retina are acquired for corresponding optical scans of the retina included in a group of optical scans of the retina which includes at least two optical scans of the retina performed consecutively one to another.

3. Method, according to one or more of the previous claims, characterized in that said B-scan images (Wi, W2) of the retina are acquired for corresponding optical scans of the retina included in a group of optical scans of the retina which includes at least two optical scans of the retina during which said illumination beam is moved along the same optical scanning trajectory.

4. Method, according to one or more of the previous claims, characterized in that the step of calculating said tracking data (Z) includes calculating the relative distance between a pair of characteristic areas (W12, W21) of each B-scan image (Wi, W2) of the retina.

5. Method, according to one or more of the previous claims, characterized in that the step of calculating said tracking data (Z) includes calculating the position of a symmetry plane (D) between a pair of characteristic areas (W12, W21) of each B-scan image (Wi, W2) of the retina.

6. Method, according to one or more of the previous claims, characterized in that said tracking data (Z) includes one or more of: an estimate value indicative of the relative movement of the retina, an estimate value indicative of the direction of movement of the retina, an estimate value indicative of the speed of movement of the retina, an estimate value indicative of the change in speed of movement of the retina.

7. Method, according to one or more of the previous claims, characterized in that said optical scanning trajectory is circular.

8. Apparatus (1) for inspection of the eye fundus through optical coherence tomography configured to perform a method of tracking (100) the movements of the retina, according to one of the previous claims.

9. Apparatus, according to claim 8, characterized in that it is configured to perform a retinal image acquisition procedure (200) which includes the following steps:- performing a series of optical scans of the retina, wherein, during each optical scan of the retina, an illumination beam (L) is projected onto the retina and is moved along an optical scanning path (T) comprising a first optical scanning trajectory (Ti ) having the shape of a closed curve and a second optical scanning trajectory (7z), wherein said optical scanning path (T) is translated along a predefined translation direction (Dr) between one optical scan and the next; wherein, during each optical scan of the retina, first optical scan cycles of the retina are performed, during each of which an illumination beam (L) is projected onto the retina and is moved along said first optical scanning trajectory (Ti ), and one or more second optical scan cycles are performed, during each of which an illumination beam (L) is projected onto the retina and is moved along said second optical scanning trajectory 2)~- acquiring a B-scan image of the retina for each first optical scan cycle;- acquiring at least one B-scan image of the retina for each second optical scan cycle;- processing B-scan images of the retina acquired for corresponding first optical scan cycles according to said method of tracking (100) the movements of the retina to calculate tracking data (Z) indicative of the movements of the retina.

10. Apparatus, according to claim 9, characterized in that said acquisition procedure (200) includes the step of processing said tracking data (Z) to identify possible B-scan images of the retina acquired during said second optical scan cycles and affected by motion artifacts.

11. Apparatus, according to one of claims 9 to 10, characterized in that said acquisition procedure (200) includes the step of obtaining a final image of the retina (Q) by processing the B-scan images of the retina acquired for corresponding second optical scan cycles of the retina based on said tracking data (Z).

12. Apparatus, according to one or more of claims 9 to 11, characterized in that said second optical scanning trajectory (T2) has the shape of a closed curve and includes at least onerectilinear portion (T21, T22), wherein at least one B-scan image of the retina is acquired for every second optical scan cycle, when said illumination beam (L) is moved along said at least one rectilinear portion (T21, T22).

13. Apparatus, according to one or more of claims 11 to 12, characterized in that said final image (Q) of the retina is a C-scan image or an angiographic image of the retina.

14. Apparatus, according to one or more of claims 9 to 13, characterized in that it is operationally coupled to or includes a retinal imaging device (9) configured to acquire images of the retina (S) during the performance of said series of optical scans of the retina.

15. Apparatus, according to claims 14, characterized in that it is configured to process the images (S) of the retina provided by said imaging device (9) to calculate additional tracking data (ZJ ) indicative of the movements of the retina, wherein said apparatus is configured to process the B-scan images of the retina acquired for corresponding second optical scan cycles of the retina also based on said additional tracking data (ZJ) in order to obtain a final image of the retina (Q).

Images