Device and method for interferometric imaging of scattering objects

Abstract

The invention relates to an interferometric imaging device (1) comprising: an illumination module (2) for generating a measurement beam (10) and a reference beam (11) that generate a line of light on an object; interferometric measurement means (8, 9) for generating an interference signal between the backscattered measurement beam and the reference beam; a two-dimensional sensor (14) with a rolling shutter; scanning means for moving the line of light on the object; a synchronization module having a first mode in which the sensor senses the interference signal with a zero time offset between the rolling shutter and the scanning means, and a second mode in which the sensor senses a non-coherent signal with a non-zero time offset between the rolling shutter and the scanning means; and a module for generating an image from the interference signal and the non-coherent signal.

Description

[Technical Field]

[0001] The present invention relates to a device for interferometric imaging of scattering objects. The present invention further relates to a method for imaging such objects. The field of the invention is that of imaging scattering media or objects such as, but not limited to, biological tissue. [Background technology]

[0002] [Prior art] Numerous imaging techniques are used in all scientific fields, particularly in medicine. For in vivo imaging of complex biological tissues, particularly the eye, it is possible to distinguish between incoherent and coherent techniques. Incoherent techniques can be based on direct reflection imaging or phase and absorption imaging. Coherent techniques are based on interferometric measurements of light backscattered by certain tissue structures. Incoherent imaging techniques are generally based on direct measurement of reflectivity.

[0003] In incoherent imaging, a full-field configuration allows the camera to measure photons backscattered by the object. This is a fast technique for recording all photons in parallel. However, its sensitivity is not very high because not only are photons backscattered by the layer of interest captured, but the camera also detects photons scattered by other layers and multiply scattered photons, resulting in high background noise.

[0004] In confocal imaging, measurements are taken one by one using a scanning system in both lateral directions to obtain a two-dimensional image. Confocal filtering is performed to remove photons originating from depths other than that of the object being measured and multiply scattered photons, thus enabling images with better contrast. However, this technique suffers from the slow acquisition speed caused by the scanning. The axial resolution of these techniques depends on the numerical aperture of the measurement system. Coherent imaging techniques are generally based on interferometry of backscattered light, and allow obtaining the field and phase of the captured signal.

[0005] An important coherent technique is optical coherence tomography (OCT). Optical coherence tomography is an optical microscopy technique specifically for imaging complex, scattering biological tissues by using low-coherence interferometry to select the imaging depth. In this interferometric technique, the interference signal between light scattered by the sample and a reference beam is detected. With low coherence, e.g., white light, the interference is localized to a narrow band that defines the axial resolution of the measurement system. The axial resolution is thus decoupled from the depth of field and can therefore be made arbitrarily small. Interferometry amplifies small signals, making OCT particularly sensitive. OCT techniques include Fourier-domain OCT (FD-OCT) and time-domain OCT.

[0006] In FD-OCT, the spectrum of the interferometric signal from a cylindrical sample (and therefore from various depths) is recorded and the axial information from the scatterers is recovered by digital processing (Fourier transform). A distinction is made between spectral domain OCT, in which the spectrum is measured using a diffraction grating and a linear detector, and scanning wavelength OCT, in which the spectrum is measured by continuous measurements with a point detector while scanning the wavelength of the source. The amplitude and phase of the sample beam are obtained directly by a Fourier transform operation.

[0007] In TD-OCT, the interference signal from a given depth is measured directly by integrating all wavelengths through a point detector. Depth information can be obtained by axially scanning a reference mirror. The amplitude and phase of the incident signal can be recovered, for example, using so-called quadrature phase modulation, which consists of acquiring four consecutive images by shifting the reference arm by π / 2 between each image. By combining the images, it is possible to filter out only the photons coming from the coherence plane and separate the interference amplitude terms from the phase difference terms.

[0008] Furthermore, it is possible to distinguish between scanning OCT, in which cross-sectional measurements are acquired scan by scan, and full-field OCT, in which the interference signal is measured in two-dimensional cross-sections and recorded by a camera.

[0009] In TD-OCT scanning, axial scanning of the reference arm and lateral scanning of the scanned position are required to image the entire volume. In FD-OCT scanning, information about the entire depth of the sample at each point in the field of view is obtained in one-dimensional measurements, typically using a spectrometer, but two-dimensional scanning is required.

[0010] In contrast, full-field OCT only requires scanning in the axial dimension of the object or sample, thus enabling high-speed imaging. However, this technique has relatively low sensitivity because non-interfering photons are picked up by the detector and contribute to the generation of noise (shot noise) in the detector, even after filtering. Therefore, it may be advantageous to filter these photons in other ways, for example, using a confocal filter, which is only possible in scanning OCT. In full-field OCT, the simultaneous acquisition of all image pixels precludes the use of confocal filtering. The use of a confocal filter is also important to limit the effects of crosstalk due to multiply scattered photons. Summary of the Invention

[0011] [DISCLOSURE OF THE INVENTION] The present invention aims to overcome the drawbacks of the prior art mentioned above. In particular, it is one goal of the present invention to provide a device and method for interferometric imaging of scattering objects that allows for full-field imaging with high speed and high sensitivity. It is a further object of the present invention to provide a device and method for interferometric imaging of scattering objects that allows for both interferometric imaging and incoherent dark-field imaging of scattering objects, so that different contrasts can be advantageously combined. A further goal of the present invention is to provide a device for interferometric imaging of scattering objects with a simple and compact construction.

[0012] At least one of the above goals is: - a low-coherence linear illumination module configured to generate a measurement beam and a linear cross-sectional reference beam, the measurement beam producing a line of illumination light on an object in a field of view; - interferometric means configured to generate an interference signal resulting from interference between a backscattered measurement beam and a reference beam in the line of light; - 2D sensor with rolling shutter; - scanning means configured to move a line of light over an object in the field of view; a synchronization module configured to operate in a first synchronization mode in which the time offset between the rolling shutter and the scanning means is zero and the sensor is configured to sense interference signals, and in a second synchronization mode in which the time offset between the rolling shutter and the scanning means is a determined offset other than zero and the sensor is configured to sense so-called incoherent light signals originating from non-illuminated areas of the object; and - an image processing module configured to generate a bright-field image from the coherent signal and a dark-field image from the incoherent signal; This is achieved with a device for interferometric imaging of scattering objects, comprising:

[0013] The imaging device according to the invention combines the advantages of full-field and dark-field configurations of imaging devices based on low-coherence interferometry, and combines the contrast obtained: the successive exposure of lines of pixels of the sensor by the rolling shutter of the sensor acts as a real-time spatial filter, allowing to significantly increase the detection sensitivity with respect to the full-field configuration without filtering and without the need for additional digital filtering during image processing.

[0014] The device according to the invention allows for both bright-field and dark-field interferometric imaging, although the two configurations are independent of each other: the device according to the invention can generate images in only bright field, only dark field, or both.

[0015] Thus, the device according to the present invention can be used for bright-field interferometric imaging. In the first, synchronized mode configuration, illumination is synchronized with a rolling shutter (or a variable digital aperture), so that only the pixel exposed at each time step is illuminated by light backscattered by the object and the reference arm. Low-coherence interferometry can be used to image scattering objects and obtain reflection contrast images. In particular, bright-field imaging provides images of scattering structures with very good contrast. The imaging speed in this configuration is the same as in the full-field configuration, while spatial filtering increases detection sensitivity.

[0016] The image processing module is configured to generate a bright-field image from the interference signal using known techniques such as quadrature or quintuple phase modulation, or synchronous demodulation of a sinusoidal wave.

[0017] The device according to the invention can also be used for dark-field imaging. In a second synchronous mode configuration, the illumination is synchronized with the rolling shutter so that the exposed pixel at each time step is illuminated only by multiply scattered light, and not by light backscattered by the object and reflected in the reference arm, respectively. This configuration allows for absorption and / or phase contrast images to be obtained.

[0018] The image processing module is configured to generate a dark-field image from incoherent signals, i.e., from light that is multiply scattered by the object and does not generate an interference signal.

[0019] Importantly, the device according to the invention is configured to image an object or sample in both bright-field and dark-field modes and to utilize measurements obtained in both modes. Switching from one synchronous mode to the other is achieved simply by adjusting the time offset between the sensor's rolling shutter and the scanning means, without requiring any mechanical manipulation by the operator. Thus, different types of full-field images of the same object or the same region of an object can be obtained from the same device, which can operate in different bright-field and dark-field configurations.

[0020] The device according to the invention allows easy switching from a full-field configuration, which allows for measurements of scattering of structures with high contrast intensity, to a dark-field configuration, which allows for measurements of phase contrast and / or absorption using shift detection. A sequence of different types of images can be used, in particular, to digitally correct for motion or other disturbances of the imaged object.

[0021] By projecting a line of light that scans the object to be imaged (e.g., the retina) at the same speed as the rolling shutter, with a fixed temporal offset between the shutter position and the line of light during acquisition, high-resolution images can be obtained even in the presence of aberrations. Adjusting the shift and exposure time of the rolling shutter allows for the phase gradient angle and signal-to-noise ratio to be changed. When switching from the first mode to the second mode, the sensor exposure time is ideally increased. For example, in bright-field mode, the shift can be 0 and the exposure time can be 50 μs. To switch to dark-field mode, the shift can be increased, for example, to 100 μs and the exposure time to 200 μs.

[0022] Compared to prior art systems, the device according to the invention thus allows for increased detection sensitivity while simultaneously increasing imaging speed. A compromise between sensitivity and acquisition speed can be achieved by maintaining a degree of confocality in one transverse direction while using a line of light to parallelize pixel detection in the other transverse direction. The device according to the invention advantageously not only takes advantage of multi-modality with dark field measurements but also allows for easy implementation.

[0023] In the device according to the invention, illumination and detection are decoupled, i.e., independent of each other. Due to the presence of a two-dimensional sensor with a rolling shutter, the line of detection on the sensor is not fixed in the same position; it is not necessary to "unscan" the line under detection, i.e., to reverse the scanning system to nullify the set angle. By simply delaying time, it is easy to look outside the illuminated area of ​​the object and obtain phase and absorption contrast. This allows for rapid switching from one synchronous mode to the other.

[0024] The presence of a two-dimensional optical sensor (and the absence of a one-dimensional sensor) makes the device of the present invention robust with respect to nonlinearities of the scanning means, which may result from mechanical limitations of the scanning means, and optical aberrations introduced by the scanning means, which may cause artifacts and reduced spatial resolution, respectively. As a result of decoupling illumination and detection, nonlinearities of the light beam scanning means may result in a time delay between illumination and detection, resulting in a loss of signal. In the absence of a signal, it is possible to directly detect the nonlinearities of the scanning system, which is not possible with confocal or line imaging. In fact, in confocal and line systems, illumination and detection are coupled, and the nonlinearities of the scanning system result in artifacts that distort the structure of the imaged object. The time delay to switch from one mode to the other is typically a few microseconds. When imaging the eye, high speed imaging is particularly important to avoid artifacts caused by eye movement. High sensitivity allows detection of less scattering structures such as individual cells and their organelles. The two-dimensional sensor can be, for example, a CMOS camera (Complementary Metal Oxide Semiconductor).

[0025] According to one embodiment, the synchronization module may be configured to alternately operate in a first synchronization mode and a second synchronization mode for each position of the line of light within the field of view. Alternatively, the synchronization module may be configured to operate in a first synchronization mode and a second synchronization mode for the entire field of view. Advantageously, the image processing module may be configured to generate a phase contrast image from the at least two dark field images.

[0026] In fact, it is possible to obtain an image with enhanced phase contrast by subtracting two images (which are also phase contrast images) obtained in dark field using a time offset between the shutter and the positive or negative scanning means, respectively. Alternatively or additionally, the image processing module may be configured to generate an absorption contrast image from at least two dark field images.

[0027] In fact, it is possible to obtain an absorption contrast image by summing two images obtained in dark field using a time offset between the shutter and the positive or negative scanning means, respectively.

[0028] According to an advantageous embodiment, the device according to the invention may further comprise an adaptive optics module configured to correct static and / or dynamic imperfections introduced into the wavefront of the illumination passing through the object.

[0029] The adaptive optics module effectively corrects static and / or dynamic imperfections imposed on the wavefront by an object or sample, such as the human eye. Adaptive optics is particularly important when the object to be imaged has thickness, large variations in optical indices, and / or a non-planar surface. In the case of dynamic aberrations, such as those of the in vivo human eye, correction must be performed quickly. This correction maintains the high resolution of the device even in the presence of imperfections and disturbances. Adaptive optics also ensures that the illumination maintains its line shape and does not broaden, so that the direct signal of a broadened line does not obscure the phase signal and thereby limit the contrast obtainable with the device. Adaptive optics also ensures optimal overlap between the size of the light line and the rolling shutter. The adaptive optics module thus ensures both the correct size of the light line on the imaged object and the image quality of the structure of interest. Advantageously, the illumination module may comprise a two-dimensional light source combined with a line generator lens.

[0030] Alternatively, the illumination module may comprise a bulk light source combined with a line-shaped spatial filter or with a digital micromirror device (DMD). The light source may be an infrared source. Infrared wavelengths are particularly well suited to imaging biological tissue as they are only poorly absorbed by such tissue.

[0031] According to one embodiment, the light source may be a superluminescent diode (SLD). SLDs have the advantage of being relatively powerful (a few milliwatts) and highly directional. Furthermore, the short temporal coherence length of SLDs helps to reduce speckle, i.e., granular intensity distribution. According to another example, the light source may be a light emitting diode (LED) or a scanning laser. The line generator lens may be a cylindrical lens or a Powell lens. The Powell lens ensures an even intensity distribution along the line of generated light.

[0032] According to another aspect of the invention, a method for imaging a scattering object is proposed, which is realized by a device for interferometric imaging, and in particular by an interferometric imaging device according to the invention, comprising a low-coherence linear illumination module configured to generate a measurement beam and a linear cross-section reference beam, an interferometric measurement means, a two-dimensional sensor with a rolling shutter, a scanning means, a synchronization module, and an image processing module. The method comprises the following steps: - illuminating the object with a measurement beam while generating illumination in the form of a line of light in the field of view; - generating an interference signal resulting from interference between the backscattered measurement beam and a reference beam in the line of light; - scanning a line of light over an object in the field of view by a scanning means;

[0033] - synchronizing the scanning means and the rolling shutter by means of a synchronization module with a time offset of zero in a first synchronization mode in which the sensor is configured to sense interference signals, and with a determined non-zero time offset in a second synchronization mode in which the sensor is configured to sense so-called incoherent light signals originating from non-illuminated areas of the object; - detecting, by a sensor, an interfering signal and / or a non-coherent signal; - generating a bright-field image from the interferometric signal and a dark-field image from the incoherent signal by an image processing module. Includes. Advantageously, the synchronization and detection steps can be alternated to generate a bright field image and subsequently a dark field image.

[0034] It is easy to switch from the first to the second synchronous mode by adjusting the time offset between the sensor's rolling shutter and the scanning means, thus obtaining bright-field and dark-field images of the same object or the same part of the object, combining the advantages of both modes to obtain more information from the imaged object. According to an advantageous embodiment, the method according to the invention may further comprise the step of generating a phase contrast image from the at least two dark field images. Alternatively or additionally, the method according to the present invention may further comprise the step of generating an absorption contrast image from the at least two dark field images. The method according to the invention may further comprise the step of correcting static and / or dynamic imperfections of the illumination wavefront passing through the object. To this end, an adaptive optics module can be implemented as described herein above.

[0035] The device and method according to the invention can be used in numerous applications requiring the imaging of scattering structures, objects or samples. The device and method according to the invention can be implemented in all fields where optical coherence tomography (OCT) is used and useful, namely in medical diagnostics of surgical samples, in vivo medical diagnostics in ophthalmology (e.g., of the cornea or retina), and imaging of biological samples. This last application is particularly important in pharmacology, especially for the imaging of cellular organelles.

[0036] The device and method according to the invention can be used in particular in full-field OCT in the time or spectral domain (FF-OCT) or in Fourier OCT with wavelength scanning (FD-OCT). The devices and methods according to the present invention can also be used to image a variety of materials.

[0037] More specifically, for retinal imaging, the device and method according to the present invention allow for the observation of most structures of interest in the retina, whether they are strongly backscattering, such as cones and rods, or weakly scattering in bright-field mode, such as ganglion cells and capillaries, or even weakly scattering in dark-field mode. The high speeds achievable and the large imaged fields of view fulfill the medical need to scan the widest possible area while limiting imaging time and reducing motion artifacts for the patient. The high speeds also allow for the imaging of blood flow in the capillaries present in the retina. [Description of drawings and embodiments] Other advantages and features will become apparent upon examination of the detailed description of the entirely non-limiting embodiments and from the accompanying drawings. [Brief explanation of the drawings]

[0038] [Figure 1] 1A and 1B are schematic diagrams illustrating non-limiting example embodiments of interferometric imaging devices according to the present invention; [Figure 2]1 shows a schematic diagram of an example of an illumination module of a device according to the invention; [Figure 3] 2 shows a schematic diagram of a first synchronization mode performed by a device according to the invention; [Figure 4] 3A and 3B illustrate a schematic representation of a second synchronization mode performed by a device according to the invention; DETAILED DESCRIPTION OF THE INVENTION

[0039] It is clearly understood that the embodiments described below are in no way limiting. In particular, all of the described variations and embodiments can be combined with each other if there are no technical obstacles to the combination. In the drawings, the same reference numbers may be used for features that are common to several drawings. FIG. 1 shows a schematic representation of a non-limiting embodiment of an interferometric imaging device according to the present invention.

[0040] A device 1 can be used to perform steps of an interferometric imaging method according to the invention, as shown in Figure 1. Example embodiments of devices and methods according to the invention are described below. The device 1 comprises a linear illumination module 2, an interferometric measurement means, a scanning means 12 and a two-dimensional image sensor 14.

[0041] The linear illumination module 2 comprises a low-coherence light source. The module 2 generates a line-shaped light beam with a linear cross-section at the surface of the object 20 or sample to be imaged. An example of such an illumination module is shown in FIG.

[0042] 2, the illumination module 2 includes a fiber-coupled superluminescent diode (SLD) 3, a collimator 4, and a line generator lens 5. The line generator lens 5 is preferably a Powell lens. The lens 5 may also be a cylindrical lens.

[0043] The SLD3 is a two-dimensional light source. The light source emits broadband light with a short temporal coherence length. For example, the light source can emit light at a wavelength of 850 nm with a spectral bandwidth of 55 nm. Alternatively, the light source output may not have a fiber, in which case a collimator is not required if the emitted light beam is already parallel.

[0044] The beam 7 from the SLD 3 passes through a collimator 4 and a Powell lens 5, which converts the parallel beam into a diverging beam with a linear cross section, which has an aperture angle in one direction but remains parallel in the orthogonal direction.

[0045] FIG. 1 shows the shape of the light beam 7 emerging from the lighting module 2 in both the (x,z) and (y,z) planes, as well as the linear cross section 25 in the (x,y) plane.

[0046] 1, an achromatic doublet 6 placed at the output of the illumination module 2 limits the illumination field in the direction of beam divergence to minimize power loss. By overlapping the focal planes of the cylindrical lens 5 and the doublet 6, a collimated line of light is formed behind the doublet 6.

[0047] To ensure uniform illumination at the edges of the line of light, and as shown in Figure 1, a slit 13 can be placed in the image focal plane of the achromatic doublet 6, which focal plane is conjugate to the plane of the object or sample being imaged.

[0048] Device 1 further comprises an interferometric measurement means. In the embodiment shown in Figure 1, the interferometric measurement means comprises a cube beam splitter 8 and a movable mirror 9. A line-shaped light beam 7 from illumination module 2 is split by cube splitter 8 into a measurement beam 10 and a reference beam 11, which form the measurement arm and reference arm of the interferometer, respectively. Measurement beam 10 is directed towards and focused onto object 20 using lens 21 and objective lens 23, so that the object is illuminated with the line of light generated by illumination module 2. Light scattered by object 20 passes back through cube splitter 8.

[0049] The length of the reference arm can be changed by a movable mirror 9. The reference beam 11 is focused onto the mirror 9 by a lens 21 and an objective lens 24, and is reflected by the mirror 9 toward the cube splitter 8. At this time, the reference beam 11 interferes with the light scattered by the object 20. In the embodiment shown in FIG. 1, the interferometric means are arranged according to the so-called Linnik configuration. Of course, other interferometer configurations are possible.

[0050] The line of light is scanned over the object 20 to be imaged in the field of view using a scanning means. To achieve this, the light beam 7 emitted by the illumination module is directed onto a galvanometer mirror 12. This allows the measurement beam 10 to be directed at different angles to the object 20, so that the entire field of view is scanned with the line of light formed by the illumination module 2. The reference beam is also scanned over the reference mirror 9 by the galvanometer mirror 12.

[0051] The scanning amplitude can be adjusted to provide a specific field of view. The scanning frequency of the galvo mirror is the same as the frequency of full-field image acquisition. For example, to acquire an image over the entire field of view, the scanning rate can be 100 Hz, and for half-field image, the scanning rate can be 200 Hz.

[0052] The illumination of object 20 must be line-shaped at the image focal plane, and galvanometer mirror 12 must be at the pupil plane so that the angle of mirror 12 can reposition the line of light only on the object. Two lenses 17, 18 arranged on either side of the galvanometer mirror 12 make it possible to switch from the image plane to the pupil plane.

[0053] The device 1 according to the invention comprises a two-dimensional sensor 14 configured to detect interference signals as well as light scattered by an object. The light signals are focused onto a detector using a lens 22. The sensor 14 comprises a rolling shutter. This shutter is an electronically controlled shutter. It is in particular possible to adjust the width of the shutter in order to define the number of rows or lines of pixels of the sensor 14 that are exposed. The exposure time per pixel row of the sensor 14 is constant. The speed of the rolling shutter is determined by the frame rate of the sensor 14. The sensor 14 is, for example, a CMOS camera equipped with a rolling shutter. The device 1 according to the invention comprises a synchronization module (not shown) for managing the synchronization mode between the scanning means and the rolling shutter of the sensor 14 .

[0054] The first and second synchronization modes are shown schematically in Figures 3 and 4, respectively. In Figures 3 and 4, the field of view of the sensor is represented by a matrix of 15 x 9 pixels 31. Lines of light on the sensor resulting from the superposition of backscattered 32 and light reflected by the reference arm are shown in grey. The rolling shutter exposing three pixel rows is shown as a bold rectangle 33.

[0055] In the first synchronous mode, the time offset between the scanning means and the rolling shutter is zero. In this case, the sensor senses the area of ​​the object illuminated by the line of light and then, for each illuminated line-shaped area on the object, detects the interference signal between the light scattered by the object and a reference beam. This is bright-field detection. The working principle of the first synchronous mode is shown in FIG.

[0056] In a first synchronization mode, the line-shaped light directly backscattered from the object and the reference beam are detected by a row of pixels 31 on the sensor exposed by the rolling shutter. In order to synchronize the rolling shutter with the scanning means and in particular with the galvanometer mirror, the generation and transmission of an electronic trigger signal of the TTL (Transistor Transistor Logic) type to the sensor can be synchronized with the generation and transmission of a linear voltage signal of the sawtooth wave type to the galvanometer mirror, for example by means of an analog output card.

[0057] In order to filter out the multiply scattered unfocused light, the center of the line of backscattered light must coincide with the center of the row of pixels exposed by the shutter.

[0058] In the example shown in Figure 3, a line 32 of backscattered light illuminates three rows of pixels 31 on the sensor. The rolling shutter speed is t i The exposure of the first pixel row begins at time t0, which coincides for example with the occurrence of a trigger signal, the exposure of the next pixel row begins after ti, and so on until the last pixel row in the image.

[0059] Exposure time t exp is, for example, 50 μs for an acquisition frequency of 200 Hz. This corresponds to exposing each line for a time equivalent to 10 pixel rows. For an illumination line width of 10 pixel rows on the sensor 14, an exposure time of 50 μs is optimal. For a field of view of 1000 pixel rows, this corresponds to an acquisition time of 5.05 ms per image.

[0060] A complete image is obtained when all pixels corresponding to the field of view, i.e., all 1000 rows, have been exposed, i.e., in 5.05 ms. To obtain an image from the interference signal, known methods (such as FD-OCT or TD-OCT) can be used. The galvo mirror returns to its initial position and a new image is then acquired. The image obtained according to this first synchronization mode is a reflection contrast image.

[0061] In the second synchronous mode, the time offset between the scanning means and the rolling shutter is non-zero. In this case, the sensor senses incoherent light signals from unilluminated areas of the object. This is dark-field detection of light multiply scattered by the object. The working principle of the second synchronous mode is shown in FIG.

[0062] In the second synchronous mode, neither the light directly backscattered by the line-shaped object nor the reference beam is detected by the sensor, so the sensor does not detect any interference signals.

[0063] Here, the sensor's rolling shutter is offset in time by a time 3ti relative to the galvanometer mirror (and hence the line of light 32 on the sensor). In the example shown in Figure 4, the rolling shutter 33 is forward relative to the illumination. In the plane of the drawing, the exposed pixels are below the illuminated pixels.

[0064] A line 34 of backscattered light illuminates two rows 32 of pixels 31 on the sensor, and the rolling shutter speed is t per row. i The exposure of the first pixel row begins at time t0, which coincides with, for example, the occurrence of a trigger signal. The exposure of the next pixel row begins after ti, and so on until the last pixel row in the image. The rolling shutter also exposes two rows of pixels 35 that are offset by three rows relative to the illuminated row 32. Of course, it is also possible to delay the rolling shutter relative to the illumination. The image obtained by this second synchronization mode is a dark-field image, from which phase-contrast and absorption-contrast images can be generated.

[0065] In a device according to the present invention, the synchronization module is configured to operate in first and second synchronization modes while combining bright-field and dark-field imaging to enhance the imaging quality of the device.

[0066] To obtain an enhanced phase-contrast image, two images acquired in dark-field mode, which are also phase-contrast images, can be subtracted. One of the two original images is acquired with a positive time offset between the shutter and the scanning means, and the other original image is acquired with a negative time offset. These time offsets preferably have the same value, only the sign is changed.

[0067] To obtain an absorption contrast image, two images obtained in dark field can be summed: one of the two original images is acquired with a positive time offset between the shutter and the scanning means, and the other original image is acquired with a negative time offset.

[0068] Scanning and synchronization parameters, such as the electronic trigger signal, the width and speed of the rolling shutter, or the various temporal offsets implemented, can be controlled via a user interface.

[0069] Advantageously, the device according to the invention may further comprise an adaptive optics module, which may improve the resolution of the image by measuring and correcting aberrated wavefronts, for example due to the motion of the object being imaged.

[0070] The adaptive optics module includes a light source (known as an analytical light source), such as a fiber-coupled superluminescent diode, a wavefront sensor, e.g., a Shack-Hartmann type, and a deformable mirror. The wavelength or spectral band of the analytical light source is preferably different from that of the light source used for imaging; alternatively, a spectral filter may be used to separate the light sources. The light source beam is directed toward the object. The disturbed wavefront of light backscattered by the object is directed toward the deformable mirror and reflected back to the wavefront sensor, which measures the disturbances in the wavefront. The deformable mirror, acting as a wavefront corrector, is controlled in real time to correct the wavefront detected by the wavefront sensor. In this case, the adaptive optics module must be arranged in the measurement arm of the interferometer. The wavefront is corrected for both the measurement beam heading towards the object and the beam scattered from the object, and the wavefront corrector is placed in the common illumination and imaging path. The device according to the invention further comprises an image processing module.

[0071] The image processing module generates a bright-field interferogram from the interference signal. Various methods can be used to obtain the interferogram from the interference signal by obtaining successive images with various positions of the reference mirror, such as four-stage imaging, five-stage imaging, or synchronous detection. These methods are well known and will not be discussed in further detail. The image processing module further generates a dark-field image from the incoherent signal.

[0072] The image processing module comprises at least one computer, central unit or calculation unit, microprocessor and / or suitable software means.

[0073] An example embodiment of a method for imaging scattering objects is described below, which can be implemented, for example, by the example imaging device described with respect to FIGS. The method according to this embodiment comprises the following steps: - illuminating the object with a line of light within the field of view by a linear illumination module; - generating an interference signal resulting from interference between the measurement beam and the reference beam backscattered in the line of light; - scanning a line of light over an object in the field of view by a scanning means;

[0074] - synchronizing the scanning means and the rolling shutter in a first synchronization mode with a zero time offset by a synchronization module in order to detect the interference signal using the sensor and to acquire a bright field image using an image processing module. Includes.

[0075] The image or images acquired in bright field mode can then be optimized for image quality to find the correct temporal offset between the scanning of the line and the rolling shutter, which in practice may vary from sample to sample.

[0076] If the object being imaged is the retina, this step is important to ensure that the imaging parameters are correct, in particular that the photoreceptors can be imaged in the correct plane, which also allows for setting the optimal temporal offset for dark-field imaging.

[0077] In particular, in bright-field mode, this optimization step ensures that the center of the rolling shutter is always exactly located on the line of light and that the width of the rolling shutter is identical to the width of the line. This condition is confirmed when the signal of the resulting bright-field image is homogeneous and maximal across the entire image. The temporal offset is zero in this case.

[0078] Optimal settings in the bright-field imaging mode allow for the introduction of a controlled temporal offset for the phase-contrast imaging obtained from the dark-field imaging mode.

[0079] Thus, if the bright-field mode is configured correctly, applying the same temporal offset value early or late effectively shifts the rolling shutter forward (early) or backward (late) with the same spatial shift at each instant, symmetrically about the center of the line of light. Without this symmetry, the symmetry in the dark-field mode between the image obtained with the early rolling shutter and the image obtained with the late rolling shutter would be lost. This optimization or adjustment step can be performed during the calibration stage of the device.

[0080] Optionally, an adaptive optics module can be implemented as previously described herein to correct for static and / or dynamic imperfections in the illumination wavefront passing through the object or sample and ensure a well-defined line of light on the object and high imaging resolution.

[0081] In a subsequent step, a positive time offset can be applied to the scan by using a synchronization module to synchronize the scanning means and the rolling shutter with a positive time offset in a second synchronization mode in order to detect incoherent signals using a sensor and generate a dark-field image using an image processing module. One or more images acquired in dark field can be optimized for image quality to find the correct temporal offset.

[0082] In fact, in dark-field mode, the offset required to obtain optimal contrast depends on the structure being observed. A parameter study can be performed by acquiring a series of images of the same area with the same exposure time while varying the temporal offset between the illumination line and the rolling shutter, so that changes in contrast and sharpness of the imaged structure based on the temporal offset can be observed. From this data, the optimal temporal offset can be determined to maximize the contrast of the structure of interest. Using the same steps, one or more dark field images can be acquired with a positive temporal offset and a negative temporal offset of the same value. Between steps of acquiring an image in dark-field mode or a bright-field image, it may be useful to switch back to bright-field mode or dark-field mode, respectively.

[0083] According to one embodiment, during acquisition of, for example, an eye, the eye is fixated on a target and prone to involuntary movements, causing a relative shift between the acquired images. Therefore, the images must be recalibrated in post-processing. In that case, it may be easier to estimate the relative shift between images from bright-field images than from dark-field images. The estimated shift for each bright-field image can also be used to extrapolate the shift for the dark-field images following the shift just estimated for the bright-field images.

[0084] According to another embodiment, it may be useful to acquire images with absorption contrast and phase contrast for certain retinal structures, such as photoreceptors. In this case, image acquisition can be performed sequentially in bright field (e.g., 100 times), followed by image acquisition in dark field. Alternatively, image acquisition can be performed alternately in bright field and dark field. The advantage of this alternative is that the same structure can be imaged within a short time frame, potentially allowing for the dynamics of retinal cells to be investigated.

[0085] According to one embodiment, the method further comprises generating a phase contrast image from the at least two dark field images by subtracting an image acquired at a positive temporal offset and an image acquired at a negative temporal offset from each other.

[0086] By way of example, Figure 6 shows a phase contrast image obtained with a method according to an embodiment previously described herein, in which blood vessels, capillaries and fibres can be identified.

[0087] When alternating between dark-field modes corresponding to positive and negative time offsets, two successively acquired images can be subtracted to obtain a so-called "split detection" image sequence with maximum phase contrast. If the initial sequence of alternating dark-field modes with positive and negative time offsets is, for example, 800 fps, the effective rate of the "split detection" image sequence obtained from two successive images in the initial sequence will be half that, or 400 fps.

[0088] When the method according to the invention is used for retinal imaging, it is important to ensure that the light source of the illumination module, and optionally the light source of the adaptive optics module, are eye-safe when switched on.

[0089] Naturally, the invention is not limited to the examples described above, and many adjustments can be made to these examples without departing from the scope of the invention.

Claims

1. A device (1) for interferometric imaging of a scattering object (20), comprising: a low-coherence linear illumination module (2) configured to generate a measurement beam (10) and a linear cross-sectional reference beam (11), the measurement beam (10) producing a line-shaped illumination of an object (20) in a field of view; - interferometric means (8, 9) configured to generate an interference signal resulting from interference between the backscattered measurement beam and the reference beam in the line of light; - a two-dimensional sensor with a rolling shutter (14); - scanning means (12) configured to move a line of light over an object (20) in the field of view; a synchronization module configured to operate in a first synchronization mode, in which the time offset between the rolling shutter and the scanning means is zero and the sensor (14) is configured to sense interference signals, and in a second synchronization mode, in which the time offset between the rolling shutter and the scanning means is a determined offset other than zero and the sensor (14) is configured to sense so-called incoherent light signals originating from non-illuminated areas of the object (20); and - an image processing module configured to generate a bright-field image from the coherent signal and a dark-field image from the incoherent signal; A device comprising:

2. 2. The device (1) according to claim 1, characterized in that the synchronization module is configured to operate alternately in a first synchronization mode and a second synchronization mode for each position of the line of light in the field of view.

3. 3. The device (1) according to claim 1 or 2, characterized in that the image processing module is configured to generate a phase contrast image from at least two dark field images.

4. The device (1) according to any one of claims 1 to 3, characterized in that the image processing module is configured to generate an absorption contrast image from at least two dark field images.

5. The device (1) according to any one of claims 1 to 4, characterized in that it further comprises a adaptive optics module configured to correct static and / or dynamic imperfections of the illumination wavefront passing through the object (20).

6. The device (1) according to any one of claims 1 to 5, characterized in that the lighting module (2) comprises an infrared light source (3).

7. The device (1) according to any one of claims 1 to 6, characterized in that the illumination module (2) comprises a two-dimensional light source (3) combined with a line generator lens (5).

8. The device (1) according to any one of claims 1 to 7, characterized in that the sensor (14) is a complementary metal oxide semiconductor sensor.

9. 1. A method for imaging a scattering object (20) realized by a device (1) for interferometric imaging comprising a low-coherence linear illumination module (2) configured to generate a measurement beam (10) and a linear cross-sectional reference beam (11), interferometric measurement means (8, 9), a two-dimensional sensor (14) with a rolling shutter, scanning means, a synchronization module and an image processing module, the method comprising the following steps: - illuminating the object with a measurement beam (10) while generating an illumination in the form of a line of light in the field of view; - generating an interference signal resulting from interference between the backscattered measurement beam and the reference beam in the line of light; - scanning a line of light over an object (20) in the field of view by a scanning means; - synchronizing the scanning means and the rolling shutter by means of a synchronization module with a zero time offset in a first synchronization mode in which the sensor (14) is configured to sense interference signals, and with a determined non-zero time offset in a second synchronization mode in which the sensor (14) is configured to sense so-called incoherent light signals originating from non-illuminated areas of the object (20); - detecting the interference and / or non-coherent signals by means of a sensor (14); - generating a bright-field image from the interferometric signal and a dark-field image from the incoherent signal by an image processing module. A method comprising:

10. 10. The method of claim 9, wherein the steps of synchronizing and detecting are alternated to generate a bright-field image and subsequently a dark-field image.

11. 11. The method of claim 9 or 10, further comprising the step of generating a phase contrast image from at least two dark field images.

12. The method according to any one of claims 9 to 11, further comprising the step of generating an absorption contrast image from at least two dark field images.

13. Method according to any one of claims 9 to 12, characterized in that it further comprises the step of correcting static and / or dynamic imperfections of the illumination wavefront passing through the object.

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