Apparatus for acquiring images by optical coherence tomography

Abstract

The present invention relates to an apparatus for acquiring images of a sample by optical coherence tomography. The apparatus comprises a light source, an optical detection unit and a control unit operationally coupled to each other. The optical detection unit comprises a sample interferometric arm which includes or is optically coupled to said sample. The aforementioned sample interferometric arm comprises an optical scanning assembly capable of projecting and moving an illumination beam, coming from said light source, onto a surface of said sample. Said optical scanning assembly is able to carry out one or more optical scanning cycles of said sample in response to control signals sent by said control unit. It comprises at least three optical scanning mirrors for reflecting an illumination beam of the sample.

Description

[0001] APPARATUS FOR ACQUIRING IMAGES

[0002] BY OPTICAL COHERENCE TOMOGRAPHY

[0003] DESCRIPTION

[0004] The present invention relates to the field of systems for acquiring images by optical coherence tomography. More specifically, the present invention relates to an apparatus capable of acquiring images of a sample, for example the retina of an eye, by means of optical coherence tomography.

[0005] As known, optical coherence tomography (OCT), also called phase coherence tomography, is a technology that allows the acquisition, even in real time, of high-resolution images (in the order of pm) of a sample, for example a biological tissue, such as a patient's retina.

[0006] The reconstructed information about the sample structure is obtained from the light radiation back reflected from different areas of the sample depending on their optical properties.

[0007] There are several known examples of optical coherence tomography imaging devices. These systems typically include a light source (e.g. laser light) and an interferometer optically coupled to the light source.

[0008] The interferometer divides the incoming light radiation (coming from the light source) between an optical arm (the so-called “sample arm”), where the sample being analysed is placed, and a second optical arm (the so-called “reference arm”) along which the radiation follows an optical path of a predefined length.

[0009] The interferometer outputs an interferometric optical signal (interferogram) due to the optical interference between the light radiation exiting the sample arm (essentially the light reflected by the illuminated sample) and the light radiation exiting the reference arm.

[0010] The optical interference signal depends on the phase shift of the optical path in the two arms of the interferometer. Its processing therefore allows us to reconstruct the reflectivity as a function of the position of the reflective components within the sample, or rather its structure.

[0011] The interferometric optical signal is sent to an optical detector which converts it into a corresponding (electrical) detection signal and an acquisition unit which samples the detection signal thus obtained.

[0012] A signal processing unit processes the sampling data of the detection signal and, based on this analysis, provides a reflectivity profile, i.e. a one-dimensional image (A -Scan image) of the sample at the point of incidence on the sample surface for different depth values along the axis of incidence of the light radiation. By repeating the procedure described above for a succession of points of incidence of the light beam on the sample, it is possible to acquire a series of A-Scan images of the sample along a cross-section of the sample.

[0013] By recomposing the collection of A-Scan images of the sample thus obtained, a two- dimensional image of the sample (B-Scan image) may be constructed at the above-mentioned sample scanning section.

[0014] The subsequent B-Scan images, obtained for different scanning sections, may be appropriately processed to obtain a three-dimensional image of the sample (C-Scan image).

[0015] As is known, any B-scan image of a sample is characterized by a certain speckle pattern, that is, a distribution of small light and dark areas which appears as noise in the background of the acquired B-scan image.

[0016] Speckle noise in a B-scan image of a sample is an optical interference phenomenon which depends substantially on the physical features of the illuminated portion of the sample. This disturbance considerably limits the quality of the acquired B-Scan images and may compromise correct observation of the sample.

[0017] Some prior art apparatuses are designed to obtain a B-Scan image of a sample by averaging a series of B-Scan images of a sample, preliminarily acquired for a certain scanning section.

[0018] Unfortunately, experience has shown that this solution is not very effective in eliminating or attenuating speckle noise. In the absence of sample movement, the speckle pattern of each acquired B-Scan image remains essentially identical and therefore cannot be effectively reduced by an averaging process.

[0019] Some prior art devices are designed to acquire a collection of B-scan images of the same sample section at successive optical scanning cycles. During each optical scanning cycle, the illumination beam is moved over the sample surface along a main scanning direction with small amplitude oscillations in an orthogonal direction (along the sample surface) with respect to the main scanning direction.

[0020] Thanks to the oscillatory movement of the illumination beam, each B-Scan image of the sample, thus acquired, is characterized by a different speckle pattern. By performing an averaging process of the acquired B-Scan images, it is possible to obtain a final B-Scan image of the sample section in which the speckle noise is cancelled or strongly reduced.

[0021] This latter solution, while very effective in terms of speckle noise cancellation, has some drawbacks.

[0022] Since the illumination beam is subjected to lateral oscillations with respect to the main scanning direction, during each optical scanning cycle, the subsequent B-Scan images, preliminarily acquired, refer to a cross-section of the sample with a relatively large thickness.

[0023] The final B-scan image of the sample, obtained by averaging the preliminarily acquired B-scan images, is therefore characterized by a reduced lateral resolution (in the orthogonal direction with respect to the main scanning direction).

[0024] These limitations in image quality are often unacceptable for high-performance image acquisition systems. Furthermore, they may compromise correct observation of the sample. In fact, a physical feature of the sample, having relatively small dimensions along a direction transversal to the main scanning direction, tends to disappear from a B-scan image obtained as a result of an averaging process of different B-scan images containing information relating to a relatively large area of the sample surrounding the aforementioned physical feature.

[0025] In prior art, there is still a strong need for innovative solutions which may overcome or mitigate the above-mentioned drawbacks.

[0026] The present invention intends to respond to this need by providing an apparatus for acquiring images of a sample by means of optical coherence tomography, according to the following claim 1 and the related dependent claims.

[0027] Further features and advantages of the invention will be better understood by referring to the description given below and to the accompanying figures, provided for purely illustrative and non-limiting purposes, in which:

[0028] Figure 1 schematically illustrates an embodiment of an apparatus for acquiring images by optical coherence tomography, according to the invention;

[0029] Figures 2-3 schematically illustrate some characteristic parts of the apparatus, according to the invention;

[0030] Figures 4-5 schematically illustrate a portion of a sample during the acquisition of a B-Scan image by the apparatus, according to the invention;

[0031] Figure 6 schematically illustrates a procedure for acquiring a B-Scan image of a sample carried out by the apparatus according to the invention.

[0032] The present invention relates to an apparatus 1 for acquiring images of a sample 100 by optical coherence tomography (OCT).

[0033] In principle, the sample 100 to be inspected may be of any type, for example a biological tissue or any opaque or semi-transparent object.

[0034] Preferably, apparatus l is a fundus inspection apparatus arranged to provide images of the retina of an eye E of the patient applying OCT techniques.

[0035] The apparatus 1 may be of various types depending on the OCT technique used. For example, it may be set up to acquire images of a sample using optical coherence tomography of the SD- OCT (Spectral Domain OCT) or SS-OCT (Swept Source OCT) type.

[0036] The apparatus 1 comprises a light source 2, preferably capable of emitting coherent electromagnetic radiation. For the applications of interest, such a light source may be, for example, a laser (Swept source) or a SLD Super -Luminescent Diode) preferably configured to emit light with wavelengths in the infrared range.

[0037] In some applications (SS-OCT), the light source 2 is configured to emit a light beam according to a sequence of emission cycles, during each of which the emitted radiation is of a very narrow band with a wavelength that is very quickly and progressively varied in a predefined wavelength range. For example, for the applications of interest, the light source may be configured to emit radiation with a wavelength which varies cyclically in a wavelength range centred around the value 1060 nm, with amplitude of the order of 100 nm and a cycle repetition frequency in the order of tens or hundreds of KHz.

[0038] In other types of applications, such as SD-OCT, the light source 2 is configured to emit broadband light radiation.

[0039] The apparatus 1 comprises an optical detection unit 3 optically coupled to the light source 2, for example by means of an optical fibre or other known optical coupling means.

[0040] The optical detection unit 3 is configured to receive a light radiation Ls coming from the light source 2 and to provide an output of an interferometric optical signal II obtained by optical interference between a measurement beam L- and a reference beam LR provided by a pair of interferometric arms.

[0041] The optical detection unit 3 therefore constitutes an interferometric structure which may be made according to a known configuration, for example a Michelson or Mach-Zender configuration.

[0042] The optical detection unit 3 comprises a sample interferometric arm 4 and a reference interferometric arm 6 optically coupled to the light source 2.

[0043] The sample interferometric arm 4 includes or is optically coupled to the sample 100 to be examined and is configured to receive as input a first illumination beam Li and provide as output the measurement beam LM.

[0044] The first illumination beam Li includes a first portion of the light radiation Ls coming from the light source 2 while the measurement beam LM includes light radiation backscattered and diffused by the sample 100 after the latter has been illuminated with the first illumination beam LR

[0045] The sample interferometric arm 4 preferably includes a same optical path 4a along which the first illumination beam Li is directed towards the sample 100 and the measurement beam L is directed in the opposite direction.

[0046] The reference interferometric arm 6 includes an optical reference T (e.g. a reflective element included in the reference interferometric arm) and is configured to receive as input a second illumination beam L2 and provide as output the reference beam LR.

[0047] 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 the optical reference T illuminated with the second illumination beam L2.

[0048] The reference interferometric arm 6 preferably includes a single optical path 6a along which the second illumination beam L2 is directed towards the optical reference T and the reference beam LR is directed in the opposite direction.

[0049] Preferably, the optical path 6a has an optical length adjustable in relation to the length of the optical path 4a in the sample interferometric arm 4.

[0050] Preferably, the optical detection unit 3 comprises an optical fibre array 7 adapted to optically couple the sample interferometric arm 4 and the reference interferometric arm 6 to each other and to the light source 2.

[0051] The optical fibre array 7 is configured to receive a light radiation Ls coming from the light source 2, convey a first portion of such light radiation towards the sample interferometric arm 4 so as to form the first illumination beam Li and convey a second portion of such light radiation towards the reference interferometric arm 6 so as to form the second illumination beam L2.

[0052] The optical fibre array 7 is configured to convey the measurement beam LM, coming from the sample interferometric arm 4, to an output port by combining it with the reference beam LR coming from the reference interferometric arm 6.

[0053] The optical fibre array 7 thus provides at the output an interferometric optical signal II (also called “measurement interferogram”) generated by the optical interference between the measurement beam LM (radiation backscattered and diffused by the illuminated sample 100) and the reference beam LR (radiation scattered by the illuminated optical reference).

[0054] The interferometric optical signal II is indicative of a reflectivity profile along the axial depth of the sample portion illuminated with the illumination beam Li.

[0055] Preferably, the apparatus 1 comprises a conversion unit 8 optically coupled (e.g. by optical fibre) to the optical detection unit 3 and configured to convert the interferometric signal II into a corresponding electrical detection signal Ie.

[0056] The conversion unit 8 may comprise a spectrometer, in the case where the apparatus is an SD- OCT system, or a group of photodiodes in the case where the apparatus is an SS-OCT system. The conversion unit 8 may be physically integrated with the optical detection unit 3. The apparatus 1 comprises a control unit 5 operatively coupled (in known ways) to the light source 2, to the optical detection unit 3 and to the conversion unit 8.

[0057] The control unit 5 is advantageously configured to perform control functions over the operation of the optical detection unit 3. Of course, the control unit may be configured to also control the operation of further components of the apparatus, for example the conversion unit 8 and the light source 2.

[0058] Preferably, the control unit 5 comprises 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 appropriate command signals for the components to be controlled, receive appropriate detection signals sent by other devices (sensors, local driving units, and so on) and execute appropriate control algorithms, optionally in cooperation with other modules of the control unit.

[0059] The control unit 5 is also configured to perform data and signal processing functions to process the electrical detection signal Ieprovided as output from the conversion unit 8 and obtain one or more images of the sample based on that signal.

[0060] Preferably, the control unit 5 comprises an interface module 52 configured to receive the interferometric detection electrical signal Ieto perform sampling of the latter signal using known data and signal processing algorithms.

[0061] Preferably, the control unit 5 comprises 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 sample. To this end, the data processing module 53 may be configured to execute known data and image processing algorithms.

[0062] The control module 51, the interface module 52 and the data processing module 53 may be industrially manufactured according to known type solutions. For example, they may comprise suitable microprocessor circuits, FPGAs or other types of electronic circuits mounted on board an appropriate electronic board.

[0063] In the operation of the apparatus, the various electronic modules 51, 52, 53 cooperate with each other to perform the required functions. Furthermore, such modules may be physically integrated with each other in an industrial-level implementation of the device.

[0064] In principle, the images of the sample 100 that the apparatus, according to the invention, may acquire may be of various types, for example A -scan.jB-scan, C-scan images. As will be seen later, the present invention is advantageously aimed at obtaining B-Scan images of sample 100 with high quality. It will therefore be described with particular reference to this type of functionality. In many respects, the apparatus 1 may be implemented according to known solutions of various types. Such known parts or components will not be described in further detail for obvious reasons of descriptive brevity. In the following, therefore, the apparatus will be described only with reference to the aspects of interest for the invention.

[0065] Figure 2 schematically illustrates the structure of the sample interferometric arm 4 of the detection unit 3 of the apparatus, according to the invention.

[0066] The interferometric arm 4 comprises an optical port 45 suitable for receiving the first illumination beam Li intended to illuminate the sample 100.

[0067] The optical port 45 may be implemented according to known methods. For example, it may advantageously consist of the end of a suitably prepared optical fibre.

[0068] The optical port 45 is arranged to be optically conjugated with the sample 100, when the apparatus is operationally coupled to the eye itself. In this way, the first illumination beam Li will have a focus point located in proximity to the surface R of the sample, during the inspection of the latter.

[0069] The interferometric arm 4 further includes an optical scanning assembly 40 configured to scan (i.e., project and move) the first illumination beam Li onto the surface R of the sample 100.

[0070] The scanning assembly 40 advantageously comprises a system of optical scanning devices 41, 42, 43 controllable by the control unit 5. For example, they may be electronically controlled by a suitable driving module (not shown), in turn controlled by the control unit 5.

[0071] The scanning assembly 40 is configured to perform one or more optical scanning cycles of the sample 100 in response to appropriate command signals C sent by the control unit 5.

[0072] During optical scanning, the scanning assembly 40 projects and moves the first illumination beam Li onto the surface R of the sample 100 so as to cover a desired optical field and illuminate a desired portion of the sample.

[0073] Preferably, the interferometric arm 4 comprises a collimation assembly 46 of the first illumination beam Li optically coupled to the scanning assembly 40.

[0074] The collimation assembly 46 is advantageously arranged between the optical port 41 and the scanning assembly 40.

[0075] The collimation assembly 46 is configured to receive the first illumination beam Li from the optical port 41 and transmit such light beam to the scanning assembly 40.

[0076] When transmitted from the optical port 41, the illumination beam Li has light rays that naturally tend to diverge conically.

[0077] The collimation assembly 46 has the task of making the light rays of the first illumination beam Li entering the scanning assembly 40 substantially parallel to each other. Preferably, the sample interferometric arm 4 also includes an optical assembly 47 configured to vary the axial position of the focus point of the first illumination beam Li in response to appropriate control signals sent by the control unit 5.

[0078] To this end, the optical assembly 47 may advantageously comprise a system of lenses movable along a predefined optical axis and electronically controlled by a suitable driving module (not shown), in turn controlled by the control unit 5.

[0079] The optical assembly 47 has essentially the task of varying the position of the focal point of the first illumination beam Zy, where necessary. Such function may be advantageously performed in a first focusing step of the sample 100.

[0080] In general, the optical port 45, the collimation assembly 46 and the optical assembly 47 may be implemented according to known type solutions. In the following, therefore, these components of the inspection apparatus will not be described in further detail but only with reference to the aspects of interest for the invention, for reasons of descriptive brevity.

[0081] If the light beams Li, LM travel along the same path 4a inside the sample interferometric arm 4 (as illustrated in Figure 1), the above-mentioned optical components 40, 45, 46, 47 of the sample interferometric arm are also crossed by the measurement beam LM, in the opposite direction to the first illumination beam Li. In this case, the optical port 45 may also be used to provide the measurement beam L in output from the sample interferometric arm.

[0082] As mentioned above, the optical scanning assembly 40 comprises a system of optical scanning devices provided with mirrors which may be moved in a controlled manner.

[0083] According to the invention, the optical scanning assembly 40 comprises at least a first optical scanning device 41, a second optical scanning device 42 and a third optical scanning device 43 operationally coupled to each other.

[0084] The first scanning device 41 includes a first scanning mirror 411 for reflecting the first illumination beam Li. Such scanning mirror is rotatably movable about a first rotation axis Ai to move the first illumination beam Li along the surface R of the sample 100, according to a first scanning direction X of the sample (Figures 4-5).

[0085] Preferably, the optical scanning device 41 comprises a first actuator 412 operatively coupled to the first scanning mirror 411 so as to be able to rotate the latter about the corresponding rotation axis ^y.

[0086] In general, the first optical scanning device 41 may be implemented according to known type solutions. For example, it may be a galvanometric scanning device.

[0087] The second scanning device 42 includes a second scanning mirror 421 for reflecting the first illumination beam Li. Such scanning mirror is rotatably movable about a second rotation axis A2 to move the first illumination beam Li along the surface R of the sample 100, according to a second scanning direction Y of the sample (Figures 4-5).

[0088] Preferably, the second rotation axis A2 of the second scanning mirror 421 is perpendicular to the first rotation axis A 1 of the first scanning mirror 411.

[0089] The scanning mirrors 41, 42 are able to move the first illumination beam Li on the surface R of the sample 100 along scanning directions perpendicular to each other by varying two optical scanning angles so as to cover a desired optical field and illuminate a desired portion of the sample. In the case of acquiring a B-scan image, the above scanning angles are varied in order to obtain a preferably linear movement of the illumination beam Li along the surface R of the sample 100.

[0090] Preferably, the second scanning device 42 comprises a second actuator 422 operatively coupled to the second scanning mirror 421 so as to be able to rotate the latter about the corresponding rotation axis A2.

[0091] In general, the second optical scanning device 42 may be implemented according to known type solutions. For example, it may be a galvanometric scanning device.

[0092] The third scanning device 43 includes a third scanning mirror 431 for reflecting the first illumination beam Li. Such scanning mirror is rotatably movable about a third rotation axis A3 to move the first illumination beam Li along the surface R of the sample 100.

[0093] Preferably, the third rotation axis A3 of the third scanning mirror 431 is parallel to one of the rotation axes Ai, A2 of the aforementioned first and second scanning mirrors 411, 421.

[0094] According to some embodiments of the invention (Figure 3), the third rotation axis A3 of the third scanning mirror 431 is parallel to the second rotation axis A2 of the second scanning mirror 421 (and is preferably perpendicular to the first rotation axis Ai of the first scanning mirror 411).

[0095] According to other embodiments of the invention (not illustrated), the third rotation axis A3 of the third scanning mirror 431 is parallel to the first rotation axis A] of the first scanning mirror 411 (and is preferably perpendicular to the second rotation axis A2 of the second scanning mirror 421).

[0096] Preferably, the third optical scanning device 43 comprises a third actuator 432 operatively coupled to the third scanning mirror 431 so as to be able to rotate the latter about the corresponding rotation axis A3.

[0097] In general, the third optical scanning device 43 may be implemented according to known type solutions. For example, it may be a resonant or galvanometric type scanning device and may have lower dynamic performance and consequently also lower costs than the other scanning mirrors in the optical scanning assembly.

[0098] The scanning devices 41, 42, 33 are operationally coupled to the control unit 5 so that they may be controlled by the latter. In the operation of the apparatus, therefore, the first, second and third scanning mirrors 411, 421, 431 are operated in a synchronized manner in response to control signals C supplied by the control unit 5, for example to a drive unit of the aforementioned optical scanning devices.

[0099] During the optical scanning of the sample 100 (i.e. during the execution of at least one optical scanning cycle), the scanning mirrors 411, 421, 431 move in a synchronized manner around the corresponding rotation axes Ai, A2, As and cooperate with each other to reflect the illumination beam Li towards the sample 100.

[0100] Thanks to their synchronized movement, the first, second and third scanning mirrors 411, 421, 431 move the first illumination beam Li along the surface R of the sample 100 according to a corresponding first motion law Mx(t), a second motion law My(t) and a third motion law© (7), respectively.

[0101] Advantageously, the motion laws, with which the illumination beam Li is moved along the surface R of the sample 100, are predefined and may be calculated in real time or selected from a memory by the control unit 5 based on the desired scanning trajectory for the first illumination beam Zy.

[0102] Figure 3 illustrates a possible embodiment of the scanning assembly 40.

[0103] According to this embodiment, the scanning assembly 40 receives as input the first illumination beam Li, after the latter has been collimated by the collimation assembly 46 after having passed through the optical port 45.

[0104] The first illumination beam Li undergoes a first reflection at the first scanning mirror 411 of the first optical scanning device 41, a second reflection at the second scanning mirror 421 of the second optical scanning device 42 and a third reflection at the third scanning mirror 431 of the third optical scanning device 43.

[0105] After the third reflection inside the scanning assembly 40, the first illumination beam Li is directed towards the optical assembly 47 and, subsequently, towards the sample 100.

[0106] The control unit 5 provides for the control of the operation of the optical scanning devices so that the three scanning mirrors 411, 421, 431 move in a synchronized manner, with appropriately calculated rotation angles. Preferably, the third scanning mirror 431 is placed at a greater distance from the collimation assembly than the first and second scanning mirrors 411, 421 along the optical path of the illumination beam Li within the optical scanning assembly.

[0107] This allows the scanning mirrors 411, 421, 431 to be arranged in such a way as to facilitate their implementation and ensure a high acquisition speed of the B-Scan images.

[0108] The scanning mirrors 411, 421 may, in fact, have reduced moments of inertia and therefore be easily operated by the respective actuators 412, 422 with very high dynamic performances (rotation speed).

[0109] The first scanning mirror 411 may have limited dimensions (and therefore a relatively small moment of inertia) since it must reflect a stationary illumination beam Li (coming from the collimation assembly 46).

[0110] The second scanning mirror 412 must necessarily have larger dimensions but may be made so as to have an elongated shape along the respective rotation axis A2 since it must reflect an illumination beam Li which moves along the scanning direction X (set by the first scanning mirror 411) substantially parallel to such rotation axis (the rotation axes Aj, A2 are perpendicular to each other). It too may therefore be characterized by a relatively small moment of inertia.

[0111] The third scanning mirror 431 necessarily has a larger surface area since it must reflect an illumination beam Li moving along the scanning directions X, Y. However, it may have much lower dynamic performance than other scanning mirrors and may therefore be easily driven by the associated actuator 432 despite its relatively large size and moment of inertia.

[0112] According to other embodiments of the invention, however, the optical scanning devices 41, 42, 43 may be arranged in a different order than that illustrated. Of course, this may require providing more powerful and expensive actuators 412, 422, 432 to operate the scanning mirrors 411, 421, 431.

[0113] In the embodiment illustrated in Figure 3, the rotation axes A2, A3 of the second and third scanning mirrors 421, 431 are parallel to each other. As mentioned above, embodiments are possible in which the rotation axes Ai, As of the first and third scanning mirrors 411, 431 are parallel to each other.

[0114] Preferably, the optical scanning assembly 40 does not include lenses interposed between the scanning mirrors 411, 421, 431. The illumination beam Li therefore remains approximately parallel, as it exits from the collimation assembly 46, in the optical path inside the optical scanning assembly 40. This solution offers maximum compactness and minimum cost for the scanning assembly 40. However, other solutions are also possible which involve the insertion of at least one group of lenses with an optical relay function along the optical path of the illumination beam Li which affects the three mirrors of the scanning assembly.

[0115] In the embodiments of the invention (Figure 3) in which the sample interferometric arm 4 is made so that the light beams Zy, LM travel along the same optical path (as illustrated in Figure 1), the measurement beam - travels along the scanning assembly 40 in the opposite direction to the first illumination beam Li. In this case, the measurement beam - , coming from the sample 100 and passing through the optical assembly 44, will undergo three successive reflections (optical de-scans) inside the scanning assembly 40 at the scanning mirrors 411, 421, 431 before heading towards the collimator 46 and, subsequently, the optical port 45.

[0116] According to one aspect of the invention, during the optical scanning of the sample 100 (i.e. during the execution of at least one optical scanning cycle), the optical path length of the illumination beam Li, measured along an axial direction Z perpendicular to the first and second scanning directions X, Y, is variable, according to a certain predefined variation function Z(t), due to the reflection of the aforementioned illumination beam by the scanning mirrors 411, 421, 431.

[0117] During the optical scanning of the sample 100, the scanning mirrors 411, 421, 431 can take, from time to time, different combinations of rotation angles in order to reflect the illumination beam Li towards the sample 100. For obvious geometric optical reasons, each combination of rotation angles of the above-mentioned scanning mirrors corresponds to a different optical path length of the illumination beam Li.

[0118] The combination of movements of the scanning mirrors 411, 421, 431 (in particular of the scanning mirrors 421, 431 having rotation axes parallel to each other in the embodiment of Figure 3) thus determines a cyclic variation Z(t) of the length of the optical path of the illumination beam Li along the sample interferometric arm 4.

[0119] This has consequences for the acquisition of A-Scan images of the sample.

[0120] In fact, the subsequent series oi A-scan images, acquired along a certain scanning plane in order to compose a B-scan image of the sample, are necessarily affected by a cyclic distortion along an axial direction Z perpendicular to the surface R of the sample. As will better seen later, this circumstance may be advantageously exploited to remove the speckle noise from the acquired B-Scan images since the speckle pattern of each B-Scan image depends substantially on the difference (OPD - “Optical Path Difference”) in length between the optical paths 4a, 6a of the illumination beams Li, L2 along the interferometric arms 4, 6 of the detection unit 3. The variations in the optical path length of the illumination beam Li , along the axial direction Z, are typically much smaller than the displacements of the same illumination beam along the sample surface. For example, it has been found that a movement of the illumination beam Li of the order of ten mm corresponds to a variation in the optical path of the order of a tenth of a mm. Although very small, these variations are much larger (an order of magnitude) than the typical pitch that characterizes the distribution of light and dark areas of a typical speckle pattern. Therefore, they may be effectively exploited to remove speckle noise from the acquired B-Scan images.

[0121] Advantageously, the variation function Z(t) of the optical path length of the illumination beam Zy, along the axial direction Z, depends on the third law of motion© (t) with which the third scanning mirror 431 moves the illumination beam Li along the sample surface.

[0122] In other words, the variation function Z(t) of the optical path length of the illumination beam Li may be defined as:

[0123] Z(t) = Z(t, 0(t)) wherein 0(t) is the motion law with which the third scanning mirror 431 moves the illumination beam Li on the sample surface.

[0124] The variation function Z(t) of the optical path length is therefore known, calculable according to the law of motion 0(t) or identifiable through a calibration process using appropriate tools. Preferably, during the optical scanning of the sample 100, the third optical scanning mirror 431 moves the illumination beam Li with a periodic motion law 0(t). Preferably, the motion law 0(t) is of the sinusoidal type, advantageously with a relatively low frequency (for example from a few Hz to a few tens of Hz). This implies that the third scanning mirror 431 may move (e.g. oscillating about the corresponding rotation axis As with relatively reduced dynamic performance.

[0125] In the cases illustrated above, the variation function Z(t) of the optical path length of the illumination beam Li also has a cyclic behaviour.

[0126] Preferably, during the optical scanning of the sample 100 (i.e. during the execution of at least one optical scanning cycle), the third scanning mirror 431 moves continuously (preferably with oscillatory motion) so as to move the illumination beam Li with a motion law© (t) along the surface of the sample.

[0127] According to embodiments of the invention (Figure 3) where the third rotation axis As of the third scanning mirror 431 is parallel to the second rotation axis As of the second scanning mirror 421, the third scanning mirror 431 moves the illumination beam Li along the second scanning direction Y. Following the reflection on the third scanning mirror 431, the illumination beam Li is substantially stationary along the first scanning direction X. However, it may also move along this scanning direction if the rotation axes A2, A3 are not perfectly parallel to each other, for example due to mechanical tolerances or construction defects.

[0128] Similarly, according to embodiments of the invention (not illustrated) where the third rotation axis A3 of the third scanning mirror 431 is parallel to the first rotation axis Ai of the first scanning mirror 411, the third scanning mirror 431 moves the illumination beam Li along the first scanning direction X. Upon reflection on the third scanning mirror 431, the illumination beam Li is substantially stationary along the second scanning direction Y. However, it may also move along this scanning direction if the rotation axes Ai, A3 are not perfectly parallel to each other.

[0129] Preferably, during the optical scanning of the sample 100 (i.e. during the execution of at least one optical scanning cycle), the second mirror 421 moves the first illumination beam Li along the second scanning direction Y, with a motion lawAT t) which advantageously includes:

[0130] - a first motion component Myi(t, 0(t)) calculated so as to compensate for the movement of the illumination beam Li, along the second scanning direction Y, caused by the reflection of the latter on the third scanning mirror 431. If applied alone, this first motion component would make the illumination beam Li stationary along the scanning direction Y;

[0131] - a second motion component My2(t) calculated so as to move the illumination beam Li according to a desired optical scanning trajectory.

[0132] In other words, the motion law My(t) may be expressed by the following relation:

[0133] My(t) = Myl(t, 0(t)) + My2(t) wherein Myi(t, 0(t)) is the first motion component of the first illumination beam Li along the scanning direction Y, calculated as a function of the motion law© (t) with which said illumination beam moves along the scanning direction Y following the reflection on the third scanning mirror 431, and My2(t) is the second motion component of the first illumination beam Li along the scanning direction Y, calculated so as to move said illumination beam according to a desired scanning trajectory.

[0134] Preferably, during the optical scanning of the sample 100 (i.e. during the execution of at least one optical scanning cycle), the first mirror 411 moves the first illumination beam Li along the second scanning direction X, with a motion law c t) which advantageously includes:

[0135] - a third motion component Mxi(t, 0(t)) calculated so as to compensate for the movement of the illumination beam Li, along the second scanning direction X, caused by the reflection of the latter on the third scanning mirror 431. If applied alone, this first motion component would make the illumination beam Li stationary along the scanning direction X;

[0136] - a fourth motion component Mx2(t) calculated so as to move the illumination beam Li according to a desired optical scanning trajectory.

[0137] In other words, the motion law c t) may be expressed by the following relation: wherein Mxi(t, 0(t)) is the third motion component of the first illumination beam Li along the scanning direction X, calculated as a function of the motion law© (t) with which said illumination beam moves along the scanning direction X following the reflection on the third scanning mirror 431, and Mx2(t) is the second motion component of the first illumination beam Li along the scanning direction X, calculated so as to move said illumination beam according to a desired scanning trajectory.

[0138] According to the embodiments of the invention (Figure 3) in which the third rotation axis As of the third scanning mirror 431 is parallel to the second rotation axis A2of the second scanning mirror 421, the motion law c t) of the first mirror 411 may be expressed by the following relation:

[0139] Mx(t) = Mx2(t).

[0140] In other words, the first scanning mirror 411 moves the first illumination beam Li, along the first scanning direction X, with a first motion law c t) which includes only the fourth motion component Mx2(t) calculated so as to move the first illumination beam Li according to a desired scanning trajectory.

[0141] According to the embodiments of the invention (not illustrated) in which the third rotation axis As of the third scanning mirror 431 is parallel to the first rotation axis Ai of the second scanning mirror 411, the second motion law My(t) may be expressed by the following relation:

[0142] My(t) = My2(t).

[0143] In other words, the second scanning mirror 421 moves the first illumination beam Li, along the second scanning direction Y, with a first motion law Mx(t) which includes only the above motion component My2(t) calculated so as to move the first illumination beam Li according to a desired scanning trajectory.

[0144] Preferably, the control unit 5 is configured to perform an acquisition procedure 10 of a B-scan image of the sample 100. Such acquisition procedure is advantageously designed to obtain a B- sccm image of the sample in which the speckle noise is cancelled or reduced to negligible levels. The acquisition procedure 10 includes a step 10a in which the control unit 5 commands the optical scanning assembly 40 to perform a plurality TV >1 of optical scanning cycles of the sample 100 along the same scanning plane P perpendicular to the surface R of the sample (Figures 4-5).

[0145] Preferably, the execution frequency of the subsequent optical scanning cycles is much higher than the frequency of the periodic (sinusoidal) motion law 0(t) with which the third optical scanning mirror 431 moves the illumination beam Li.

[0146] Preferably, the execution frequency of the subsequent optical scanning cycles is different from an integer multiple of the frequency of the periodic (sinusoidal) motion law 0(t).

[0147] During each optical scanning cycle, the optical scanning mirrors 411, 421, 431 project and move the illumination beam Li according to predefined motion laws. For example, in the embodiment of Figure 3, where the third rotation axis As of the third scanning mirror 431 is parallel to the second rotation axis A2 of the second scanning mirror 421, the first scanning mirror 411 may move the illumination beam Li according to a motion law of the type Mx(t)=Mx2(t) , the second scanning mirror 421 may move the illumination beam Li according to a motion law of the type My(t)=Myi(t, 0(t))+My(t) and the third scanning mirror 431 may move the illumination beam Li according to a motion law 0(t) of the sinusoidal type.

[0148] The acquisition procedure 10 includes a step 10b in which the control unit 5 acquires, for each optical scanning cycle, a first preliminary B-scan image Wrof the sample 100 at the same scanning plane P.

[0149] The control unit 5 thus acquires a collection of first preliminary B-scan images Wr(r = 1, . . ., N) of sample 100 referred to the same scanning plane P.

[0150] Each B-scan image W r is a two-dimensional image having on the abscissa the position of the illumination beam Li along a scanning direction (for example the scanning direction Y) and on the ordinate the position of the illumination beam Li along the axial direction Z of incidence of the illumination beam.

[0151] The first acquired B-scan images Wrare characterized by a same speckle pattern since they all refer to a same useful acquisition field AZ = [OPD=0, OPD=OPDmax] in ordinate (i.e., along the axial direction Z), of the order of magnitude of a few mm (Figure 5).

[0152] However, each first B-scan image Wrshows each physical feature of the sample at different positions along the axial direction Z (ordinate).

[0153] Each B-scan image Wrin fact consists of a plurality M > 1 of A-scan images Wri, WrM of sample 100. Since the length of the optical path 4a of the illumination beam Li varies according to the variation function Z(t), during the optical scanning, each A-Scan image is referred to a different depth value along the axial direction Z of incidence of the illumination beam Li.

[0154] This means that the same physical feature of the sample is positioned at different axial depth values for the subsequent A-Scan images that make up the same B-Scan image.

[0155] The B-Scan image resulting from the composition of the A-Scan images thus shows the above- mentioned physical feature in a succession of different positions along the Z-axial direction. For example, with reference to Figure 5, the ordinate position of the same physical feature Q of the sample varies continuously, according to the variation function Z(t) of the length of the optical path of the illumination beam Li.

[0156] Therefore, each B-scan image resulting from the composition of the corresponding AT acquired A-scan images is affected by a distortion along an axial direction Z perpendicular to the surface R of the sample 100.

[0157] This distortion depends on the variation function Z(t) of the optical path length of the first illumination beam Li. As illustrated above, it is known, calculable according to the motion law© (7) or identifiable through a calibration procedure, using a specific measuring instrument. The acquisition procedure 10 includes a step 10c in which the control unit 5 obtains a second preliminary B-scan image Wp(p = 1, . .., N) of the sample 100 from each first preliminary B- scan image Wr(r = 1, . . . , TV) of the sample.

[0158] Each second B-scan image Wpof said sample is obtained by processing the corresponding first B-scan image Wrin order to realign the A-scan images Wri, WrM that make up the aforementioned first preliminary B-scan image Wralong an axial direction Z perpendicular to the surface R of the sample 100.

[0159] The realignment process of the A-scan images Wri, WrM is advantageously realized based on the variation function Z(t) of the optical path length of the first illumination beam Li and therefore on the (sinusoidal) motion law©ri> with which the third optical scanning mirror 431 moves the illumination beam Li.

[0160] The realignment process of the A-scan images Wri, WrM may be achieved using appropriate image processing algorithms which may be of known type.

[0161] In practice, each A-scan Wri, WrM is translated with a correction value along an axial direction Z perpendicular to the surface R of the sample 100, extrapolated from a correction function, advantageously corresponding to the variation function Z(t) of the optical path length. The control unit 5 thus obtains a collection of second preliminary B-scan images p (p = 1, . . . , N) of the sample 100 referred to the same scanning plane P and without distortions along an axial direction perpendicular to the surface d of the sample 100.

[0162] Each second B-scan image Wpthus shows each physical feature of the sample in the same position along the axial direction Z (ordinate). The same physical feature of the sample is therefore repeated in terms of shape and position in different B-scan images Wp.

[0163] Each second B-scan image Wpis instead characterized by a speckle pattern modified with respect to the original speckle pattern of the corresponding first B-scan image Wrfrom which it was obtained.

[0164] The process of realigning the A-scan images Wri, WrM of each first B-scan image Wrdetermines a variation of the depth values along the axial direction Z to which the aforementioned A-scan images refer.

[0165] Therefore, the speckle pattern of each B-Scan image Wpvaries from time to time, being affected by a distortion along an axial direction Z perpendicular to the surface R of the sample 100.

[0166] The speckle pattern of several B-scan images Wpis not so repeated since it depends substantially on the length difference (OPD) between the optical paths 4a, 6a of the illumination beams Zy, L2 along the interferometric arms 4, 6 of the detection unit 3.

[0167] As mentioned above, the execution frequency of the subsequent optical scanning cycles is much higher and different from an integer multiple with respect to the frequency of the periodic (sinusoidal) motion law 0(t) with which the third optical scanning mirror 431 moves the illumination beam Li. This prevents the same speckle pattern from repeating periodically in different second B-Scan Wpimages obtained after the above alignment process. It is thus possible to select an arbitrary number of first acquired B-scan images Wrhaving the guarantee that, following the realignment process of the A-scan images described above, the second B- scan images Wphave different speckle patterns from each other.

[0168] Advantageously, step 10c of acquisition procedure 10 may be performed in real time, simultaneously with the acquisition of each first B-Scan image Wrof the sample, or in postprocessing, once the acquisition of the first N B-Scan images Wrhas been completed.

[0169] The acquisition procedure 10 includes a step lOd in which the control unit 5 obtains a final B- scan image Wf of the sample 100 by processing a plurality of second B-scan images Wpof the sample obtained during the execution of subsequent optical scanning cycles.

[0170] The B-scan image Wf of sample 100 is advantageously obtained through an image averaging process applied to a plurality (not necessarily all) of second B-scan images Wpof the sample obtained in the previous step 10c of the acquisition procedure. The averaging process of the second B-scan images Wpmay be achieved using appropriate image processing algorithms which may be of known type.

[0171] The acquisition procedure 10 allows obtaining B-scan images of sample 100 with speckle noise substantially cancelled or reduced to negligible levels since the second B-scan images Wpare characterized by speckle patterns that are different from each other.

[0172] At the same time, the acquired B-Scan images are characterized by a very high resolution, especially with regard to lateral resolution. Since the illumination beam may be moved along a linear scanning trajectory during each optical scanning cycle, the acquired B-Scan images refer to a cross-section of the sample with relatively narrow thickness.

[0173] The acquisition procedure 10 also allows significantly improving the signal / noise ratio of the acquired B-Scan images thanks to the averaging process performed on the second B-Scan images.

[0174] The apparatus, according to the invention, offers numerous advantages compared to the solutions currently available in the prior art.

[0175] The apparatus, according to the invention, comprises an interferometric arm 4 provided with an innovative structure which allows the length of the optical path of the relative illumination beam IL to be varied in a controlled manner. This allows the implementation of B-Scan image acquisition procedures of a sample capable of providing very high-quality images, characterized by a substantial absence of speckle noise, high lateral resolution and a high signal / noise ratio.

[0176] The apparatus, according to the invention, is also characterized by considerable operational flexibility. For example, when necessary, it may be operated in a “traditional” mode by driving the third optical scanning device to keep the third scanning mirror 431 in a fixed position.

[0177] This solution is particularly useful when the apparatus, according to the invention, is an apparatus for inspecting the ocular fundus and it is desired to proceed with the acquisition of an angiographic or volumetric image of the eye.

[0178] An angiographic or volumetric image of the eye would be noisier if the third scanning mirror 431 moved during the acquisition of the B-Scan images. Indeed, even small errors in the compensation of unwanted lateral movements (along the X, Y scanning directions) of the illumination beam Zy, caused by the reflection of the latter on the third scanning mirror 431, would have a significant impact on the presence of noise in the acquired angiographic or volumetric image.

[0179] Preferably, the control unit 5 is configured to command the third optical device 43 to maintain the third scanning mirror 31 in a fixed position, during the acquisition process of an angiographic or volumetric image of the eye.

[0180] In the apparatus, according to the invention, the provision of an innovative scanning assembly 40 in the interferometric arm 4 has a marginal impact on the overall dimensions and production costs.

[0181] The third scanning mirror 431, in fact, may move around its rotation axis with relatively low frequencies. The third optical scanning device 43 may therefore ensure satisfactory performance even if it has relatively low dynamic capabilities and is consequently manufactured in a cost-effective manner.

[0182] The apparatus, according to the invention, has a very compact structure, relatively simple and cost-effective to manufacture on an industrial level using standardized production techniques and processes.

Claims

CLAIMS1. Apparatus (1) for acquiring images of a sample (100) by optical coherence tomography, wherein said apparatus comprises a light source (2), an optical detection unit (3) and a control unit (5) operationally coupled to each other, wherein said optical detection unit (3) comprises a sample interferometric arm (4) which includes or is optically coupled to said sample (100), wherein said sample interferometric arm (4) includes an optical scanning assembly (40) capable of projecting and moving an illumination beam (Zy), coming from said light source (2), onto a surface (R) of said sample (100), said optical scanning assembly being able to carry out one or more optical scanning cycles of said sample in response to control signals (C) sent by said control unit, characterized in that said optical scanning assembly (40) includes:- a first optical scanning device (41) including a first scanning mirror (411) for reflecting said illumination beam (Zy), said first scanning mirror being rotatably movable about a first axis of rotation (Aj) to move said illumination beam along the surface of said sample according to a first scanning direction (X);- a second optical scanning device (42) including a second scanning mirror (421) for reflecting said illumination beam (Zy), said second scanning mirror being rotatably movable around a second axis of rotation (A2) to move said illumination beam along the surface of said sample according to a second scanning direction (Y) perpendicular to said first scanning direction (X);- a third optical scanning device (43) including a third scanning mirror (431) for reflecting said illumination beam (Zy), said third scanning mirror being rotatably movable around a third axis of rotation (A2) to move said illumination beam along the surface of said sample; wherein, during an optical scan of said sample, said scanning mirrors (411, 421, 431) move in a synchronous way and cooperate one to another to reflect said illumination beam (Zy), wherein, during an optical scan of said sample, the optical path length of said illumination beam (Zy), along an axial direction (Z) perpendicular to said first and second scanning directions (X, Y), varies upon the reflection of said illumination beam (LI) by said scanning mirrors (411, 421, 431), wherein said optical scanning devices (41, 42, 43) operate said scanning mirrors (411, 421, 431) in response to control signals (C) provided by said control unit.

2. Apparatus, according to claim 1, characterized in that said scanning assembly (40) does not include optical lenses interposed between said scanning mirrors (411, 421, 431) along the optical path of said illumination beam (Zy).

3. Apparatus, according to one of the previous claims, characterized in that the rotation axis (As) of said third scanning mirror (431) is parallel to the rotation axis (Aj, As) of one of said first and second scanning mirrors (411, 421).

4. Apparatus, according to one of the previous claims, characterized in that, during an optical scan of said sample, said third optical scanning mirror (431) moves said illumination beam Li) along the surface of said sample with a sinusoidal motion law.

5. Apparatus, according to one of the previous claims, characterized in that, during an optical scan of said sample, said second scanning mirror (421) moves said illumination beam Li) along said second scanning direction (Y) with a motion law that includes:- a first motion component calculated so as to compensate for a movement of said illumination beam along said second scanning direction (Y) upon a reflection of said illumination beam by said third scanning mirror (431);- a second motion component calculated so as to move said illumination beam according to a desired optical scanning trajectory.

6. Apparatus, according to one of the previous claims, characterized in that, during an optical scanning of said sample, said first scanning mirror (421) moves said illumination beam (Li) along said first scanning direction (X) with a motion law that includes:- a third motion component calculated so as to compensate for a movement of said illumination beam along said first scanning direction (X) upon a reflection of said illumination beam by said third scanning mirror (431);- a fourth motion component calculated so as to move said illumination beam according to a desired optical scanning trajectory.

7. Apparatus, according to one of the previous claims, characterized in that said first interferometric arm (4) includes a collimation assembly (46) optically coupled to said scanning assembly (40), said third scanning mirror (431) being arranged at a longer distance from said collimation assembly compared to said first and second scanning mirrors (411, 421) along the optical path of said illumination beam (Li).

8. Apparatus, according to one of the previous claims, characterized in that said control unit (5) is configured to carry out an acquisition procedure (10) of a. B-scari image (Wf) of said sample (100) which includes the following steps:- commanding (10a) said optical scanning assembly (40) to carry out a plurality of optical scanning cycles of said sample along a same scanning plane ( ) of said sample;- for each optical scanning cycle, acquiring (10b) a first B-scan image (JTr) of said sample at said scanning plane (P), each acquired first B-scan image ( Tr) being composed of a plurality of A-scan images (Wri, ... , WFM) of said sample;- obtaining (10c) a second B-scan image (Wp) of said sample from each first B-scan image (PFi) of said sample, each second B-scan image (Wp) of said sample being obtained by processing said first B-scan image ( Tr) to realign the A-scan images (Wrj, Wr\i) composing said first B-scan image (Wr) along an axial direction (Z) perpendicular to the surface (P) of said sample;- obtaining said B-scan image ( f) of said sample by processing a plurality of second B- Scan images (Wp) of said sample through an image averaging process.

9. Apparatus, according to one of the previous claims, characterized in that it is an apparatus for inspection of the ocular fundus using optical coherence tomography.

10. Apparatus, according to claim 9, characterized in that said control unit (5) is configured to command said third optical device (43) to maintain said third scanning mirror (431) in a fixed position, during an acquisition process of an angiographic or volumetric image of the retina of an eye.

Images