Spatial frequency domain imaging system and method

Abstract

A system and method for contact-based spatial frequency domain imaging (SFDI). A medium (M) to be imaged is contacted by a sheet (10) including an array of illumination elements (11) generating illumination light patterns (Li) and an array of sensor elements (12) measuring a spatially resolved image of a reflected resultant light pattern (Lr) after interaction of the illumination light patterns (Li) with the medium (M). The illumination elements (11) generate illumination light patterns (Li) that are spatially modulated in light intensity along the surface of the illuminated medium (M) according to different spatial frequencies (Fn). Based on the measured resultant light patterns (Lr) caused by the different illumination light patterns (Li), a set of spatially modulated images (In) is determined. An image (Im) of the medium (M) can be generated based on a combination of the spatially modulated images (In).

Description

[Technical field]

[0001] The present disclosure relates to systems and methods for spatial frequency domain imaging (SFDI). [Background technology]

[0002] By way of background, the principles of SFDI are described in a review article by mGioux et al. [J. of Biomedical Optics, 24(7), 071613 (2019); DOI:10.1117 / 1.JBO.24.7.071613], the contents of which are incorporated herein by reference in their entirety. As described in the article, spatial frequency domain imaging involves projecting a two-dimensional (2-D) light pattern into a medium and analyzing the effect of multiple scattering and absorption on the amplitude of the reflected or transmitted pattern as a function of the spatial frequency of the pattern. For example, a sinusoidal pattern is commonly used to facilitate the analysis of a single spatial frequency per projected pattern. Multiple scattering and absorption in the medium cause a decrease in the amplitude of the projected sinusoidal wave. A reflected image of this diffused and absorbed sinusoidal pattern is captured by a digital camera and further processed. The amplitude modulation of each pixel of the image is calculated by a process called demodulation. Through various calibration approaches, the frequency response or system modulation transfer function of the optical system can be measured to arrive at the spatial modulation transfer function of the tissue, also known as diffuse reflection. The whole process is repeated at multiple spatial frequencies (at least two). Finally, the optical properties of every pixel of the image are extracted using a light propagation model. With spectroscopic measurement devices, this process can be further extended to multiple wavelengths via serial or parallel (multiplexed) multi-wavelength acquisition schemes. Due to its unique ability to provide rapid and wide-range quantitative images of biological samples, SFDI has rapidly evolved in the past few years and has attracted increasing interest in the field of biomedical optics. As shown, for example, in Figure 4 of the aforementioned article, a typical SFDI setup includes a light source to generate light, a spatial modulator to apply a spatial pattern to the light, a projection system to project the patterned light onto the tissue surface, an imaging system to image the resulting pattern by reflection / scattering, and a digital camera to record the resulting pattern.

[0003] There is a need to further improve known systems and methods of SFDI, for example to simplify the systems or enable broader application. Summary of the Invention [Means for solving the problem]

[0004] Aspects of the present disclosure relate to systems and methods for spatial frequency domain imaging of a media. As described herein, the media is contacted with a sheeting. The sheeting includes an array of illumination elements that generate an illumination light pattern that illuminates the media. The sheeting further includes an array of sensor elements that measure spatially resolved images of a reflected resultant light pattern after the illumination light pattern interacts with the media. The illumination elements are controlled to generate a set of different illumination light patterns. Each of the different illumination light patterns is spatially modulated in light intensity along a surface of the illuminated media, each according to a different spatial frequency. Sensor signals from the sensor elements are measured to determine a set of spatially modulated images based on the measured resultant light patterns caused by the different illumination light patterns, each having a different spatial frequency. Based on a combination of the spatially modulated images, one or more images of the media can be generated.

[0005] It will be appreciated that the presently disclosed system, in which the sheet directly contacts and images the medium, can provide a variety of new applications for SFDI compared to known projection-based systems. For example, a contact-based device can be applied locally, e.g., as a patch or wearable device, and easily positioned and maintained at a precise region of interest to enable continuous monitoring of tissue or other media. Furthermore, the use of a flexible sheet allows for the region of interest to be substantially curved or change shape, such as a body part that may be distorted or out of view using a projection-based system. Furthermore, a contact-based system can be significantly simplified, e.g., does not require projection optics or spatial modulators, since the illumination element is directly adjacent to the medium. Additionally, a contact-based device can use wavelengths of light that are largely absorbed in the air between the imaging system and the medium in a projection-based system.

[0006] Advantageously, the use of a collimating structure (preferably formed by a thin sheet) between the illumination element and the contact surface allows for more precise control of the light pattern projected onto the medium. For example, this can reduce blurring of the illumination pattern when the illumination element is multi-directional or diffuse (such as a typical LED). Furthermore, this blurring can be reduced even when the illumination element is some distance behind the contact surface or when there is some distance between the imaged portion of the medium and the contact surface of the device. As will be appreciated, precise control of the illumination pattern is particularly beneficial for current applications that may depend on precise measurement of the light pattern resulting from, for example, scattering within the medium as a function of the controlled illumination pattern.

[0007] By arranging the sensor element in front of the lighting element, for example close to or on the contact surface, the resulting light pattern can be accurately measured. For example, the light source of the lighting element passes between and from behind the sensor elements, and the resulting light is reflected from the medium to the front of the sensor element (impinging on the sensor surface). Furthermore, by arranging the sensor element in front of the aforementioned collimating structure, almost all the light measured directly from the medium can be measured. In principle, the sensor element can also be arranged behind the collimating structure, for example next to the lighting element. However, this may reduce the light intensity of the resulting light pattern, which may already be relatively weak due to absorption by the medium, and may be multidirectional due to scattering in the medium (so little returns through the collimator). Therefore, preferably, the collimator is arranged between the lighting element (behind the collimator) and the sensor element (in front of the collimator).

[0008] These and other features, aspects, and advantages of the presently disclosed devices, systems, and methods will become better understood from the following description, appended claims, and accompanying drawings. [Brief description of the drawings]

[0009] [Figure 1A]1 illustrates various levels of detail for contact-based SFDI systems and methods. [Figure 1B] 1 illustrates various levels of detail for contact-based SFDI systems and methods. [Figure 1C] 1 illustrates various levels of detail for contact-based SFDI systems and methods. [Figure 2A] Each shows a set of different illumination light patterns for the SFDI. [Figure 2B] Each shows a set of different illumination light patterns for the SFDI. [Figure 2C] Each shows a set of different illumination light patterns for the SFDI. [Figure 2D] Each shows a set of different illumination light patterns for the SFDI. [Figure 3A] 1 shows measurements using the contact SFDI system and method described herein. [Figure 3B] 1 shows measurements using the contact SFDI system and method described herein. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] The terms used to describe particular embodiments are not intended to limit the invention. As used herein, the singular forms "a," "an," and "the" are intended to include the plural forms unless the context clearly indicates otherwise. The term "and / or" includes all combinations of one or more of the associated listed items. The terms "comprises" and / or "comprising" specify the presence of stated features but do not preclude the presence or addition of one or more other features. When a particular step of a particular method is referred to as following another step, it is further understood that it may follow the aforementioned step directly, or that one or more intermediate steps may be performed prior to performing the particular step. Similarly, when a connection between structures or components is described, it is understood that this connection may be established directly or through an intermediate structure or component, unless otherwise specified.

[0011] The present invention is described more fully below with reference to the accompanying drawings, in which embodiments of the invention are shown. In the drawings, absolute and relative sizes of systems, components, layers, and regions may be exaggerated for clarity. The embodiments may be described with reference to schematic and / or cross-sectional illustrations of idealized possible embodiments and intermediate structures of the invention. In the description and drawings, like numbers refer to like elements throughout. Relative terms and their derivatives should be construed to refer to the orientations described at the time or shown in the drawings under discussion. These relative terms are for convenience of description and do not require the system to be constructed or operated in a particular orientation, unless specifically specified.

[0012] 1A-1C illustrate various levels of detail of contact-based SFDI systems and methods. FIG. 1A illustrates an SFDI system 100 including a sheet 10 for contacting a medium "M" to be imaged and a controller 20 for transmitting control signals "Sc" to the sheet 10 and / or receiving sensor signals "Ss" from the sheet 10. Although shown here as separate components, the controller 20 may be integrated partially or fully as part of the sheet 10. In some embodiments, the controller 20 includes or accesses a (non-transitory) computer-readable medium that stores instructions that, when executed (e.g., by one or more processors), cause the controller to perform operational acts as described herein.

[0013] As referred to herein, sheet 10 is understood to be a relatively thin structure. In some embodiments, for example as shown in FIG. 1B, the thickness "T" of sheet 10 is less than 5 cm, less than 3 cm, less than 2 cm, less than 1 cm, such as up to 0.5 cm, up to 1 mm, or less. In other or further embodiments, the thickness "T" of the sheet is less than the maximum length and / or width along the surface of the sheet, for example at least 10 times, 20 times, 50 times, 100 times or more less. For example, the sheet has a length and / or width of 1 cm or more, 2 cm or more, 5 cm or more, 10 cm or more, such as up to 50 cm or 100 cm or more.

[0014] In a preferred embodiment, as shown, for example, in Figures 1A and 1B, the sheet 10 is flexible, e.g., can be bent to an acceptable radius without loss of essential functionality (e.g., without breaking electrical connections, components, or layers of material, and / or without essentially losing electrical and optical performance). Preferably, the acceptable bend radius is less than 1 m, less than 0.5 m, less than 20 cm, less than 10 cm, less than 5 cm, e.g., up to 1 cm or less. Smaller bend radii provide a wider variety of applications, such as conforming to non-planar media, enhancing the capabilities of the device.

[0015] Typically, as shown, for example, in FIG. 1C, the sheet 10 has a contact surface 10a for contacting a medium "M" to be imaged. In some embodiments, the sheet includes an array, preferably a two-dimensional array, of illumination elements 11. For example, the illumination elements 11 are arranged inside the sheet 10. In one embodiment, the illumination elements 11 are configured to generate a controllable illumination light pattern "Li". For example, the illumination light pattern "Li" originates from the contact surface 10a. In this way, the illumination light pattern "Li" can illuminate a medium "M", for example, contacted by the sheet 10 or at least in the vicinity of the sheet 10. In another or further embodiment, the sheet includes an array, preferably a two-dimensional array, of sensor elements 12. For example, the sensor elements 12 are arranged inside the sheet 10. In one embodiment, the sensor elements 12 are configured to measure a spatially resolved image of a resulting light pattern "Lr". For example, the resulting light pattern includes light reflected directly or indirectly to the contact surface 10a after interaction of the illumination light pattern "Li" with the medium "M".

[0016] In some embodiments, the controller 20 is configured to control the lighting elements 11 to generate a set of different illumination light patterns "Li". For example, the different illumination light patterns "Li" are generated using respective control signals "Sc". In one embodiment, each of the different illumination light patterns "Li" is spatially modulated in light intensity along the touch surface 10a according to a respective different spatial frequency "Fn". In another or further embodiment, the controller 20 is configured to determine a set of spatially modulated images "In" based on the measured resultant light patterns "Lr" caused by the different illumination light patterns "Li" each having a different spatial frequency "Fn". For example, the spatially modulated images "In" are determined by processing the sensor signals "Ss" from the sensor elements 12. In some embodiments, the controller 20 is configured to generate an image "Im" of the medium "M" based on a combination of the spatially modulated images "In". For example, the image "Im" can be transmitted to, for example, a user, an integrated or separate computing device 30 and / or a display configured to display a representation of the image "Im". Alternatively or additionally, the spatially modulated image “In” and / or image “Im” may be stored for later processing by the controller 20 and / or a separate computing device 30 .

[0017] In principle, after data has been acquired by a contact imaging device as presently disclosed, the data may be processed to generate an image of the tissue, similar to known SFDI systems that rely on remote image projection. For example, the amplitude modulation for each pixel of the image may be calculated by demodulation of the spatially modulated image. Other methods of processing the data are also contemplated.

[0018] In a preferred embodiment, the sheet 10 includes a collimating structure 13 between the illumination elements 11 and the contact surface 10a. The collimating structure is used to filter the light rays so that only light rays traveling parallel or nearly parallel to a specified direction pass through. When reprojected through the collimator onto a rear sensor element, the resolution or sharpness of the image projected by the light source through the collimator onto a nearby medium and / or the resulting image can be increased, which is advantageous for the applications described herein.

[0019] In general, the main direction in which light rays are allowed to pass through the collimator may be referred to as the main transmission direction. The collimator selectively transmits light through depending on the angle of incidence relative to the main transmission direction of the incoming light. In a preferred embodiment, the main transmission direction of the collimator is aligned with the normal vector of the collimator surface, which is typically parallel to the contact surface 10a of the sheet 10. This means that only light with normal or near-normal angles of incidence passes through the collimator, while light at higher angles is blocked. For example, the collimator only transmits light with a planar angle of less than 10 degrees, less than 5 degrees, less than 2 degrees, or less than 1 degree, relative to the main transmission direction. The smaller the threshold angle, the better the light is collimated, but this may come at the cost of blocking more uncollimated light. Light at higher angles of incidence is preferably absorbed by the collimator material, for example by (micro)openings in the collimator or holes through the collimator.

[0020] In a preferred embodiment, the collimating structure 13 comprises a (flexible) collimator sheet, which is a relatively thin or sheet-like structure with (micro) apertures and configured to pass mainly light transverse to the sheet surface. In one embodiment, the collimator sheet has a thickness "Ha" of less than 0.5 cm, preferably less than 2 mm, most preferably less than 1 mm, e.g. less than 100 μm or less. Typically, the apertures through the collimator are dimensioned to define a threshold angle. For example, the apertures have a (maximum or average) aperture diameter transverse to the main transmission direction and a (minimum or average) aperture length along the main transmission direction. For example, the threshold angle is the arctangent function (tan ) of the aperture diameter "Da" divided by the aperture length or height "Ha". -1 ), i.e. tan(θmax)=Da / Ha. For example, a threshold angle θmax of less than 10 degrees may be achieved by an aspect ratio Da / Ha<0.17, i.e. tan(10) or less than about 1:5. For example, a threshold angle θmax of less than 5 degrees may be achieved by an aspect ratio Da / Ha<0.087, i.e. tan(5) or less than about 1:10. For example, a threshold angle θmax of less than 1 degree may be achieved by an aspect ratio Da / Ha<0.017, i.e. tan(1) or less than about 1:50. The lower the aspect ratio Da / Ha, the more collimated the filtered light is. For example, the aperture is a microaperture with a cross-sectional diameter of 0.1-20 μm, preferably 1-10 μm. At the same time, the microaperture may for example have a length or height of at least 10 μm, preferably at least 50 μm or 100 μm or more, e.g. 1 mm or more.

[0021] Preferably, the walls surrounding the (micro)opening through the collimator have a relatively high light absorption, at least inside the opening, for example absorbing more than 90%, preferably more than 99%, of the used light of the lighting element 11 that hits the wall. For example, the walls may be made of carbon nanotubes (CNTs), as previously disclosed in published WO 2019 / 172763 A1. Also other methods of manufacturing collimating structures can be used, such as molding, drilling, lithographic techniques. In some embodiments, absorbing particles such as carbon can be mixed into the collimator material to increase the absorption at the wall. For example, visible or infrared light may be used. By absorbing most or all the light, non-collimated light that hits the wall material of the collimator can be prevented from passing through the opening, for example by reflection. Advantageously, the use of a collimator in front of the lighting element 11 allows to maintain a well-controlled lighting pattern while using various light sources with relatively diffuse and / or wide-angle emission. In one embodiment, the lighting element 11 is formed, for example, by LEDs or OLEDs.

[0022] In a preferred embodiment, the collimating structure 13 is disposed between the lighting element 11 and the sensor element 12. Most preferably, the sensor element 12 is disposed in front of the collimating structure 13, e.g. between the collimating structure 13 and the contact surface 10a, and the lighting element 11 is disposed behind the collimating structure 13, e.g. between the collimating structure 13 and the back layer of the sheet 10. Preferably, the lighting element 11, the sensor element 12, and the collimating structure 13 are each disposed inside the sheet 10. Accordingly, these elements or structures are preferably flexible and / or bendable like the sheet, and / or sufficiently thin.

[0023] In some embodiments, the lighting elements 11 are arranged according to a first pattern and the sensor elements 12 are arranged according to a second pattern complementary to the first pattern. In other words, the sensor elements 12 are arranged (exclusively) in the positions of the gaps between the lighting elements 11, or / and the lighting elements 11 are arranged (exclusively) in the positions of the gaps between the sensor elements 12. In one embodiment, with a collimating structure 13 arranged between the lighting elements 11 and the sensor elements 12, the sensor elements 12 are arranged in the gaps between the lighting elements 11 on the opposite side of the collimator, and / or vice versa. In this way, light from the lighting elements 11 can pass through the collimating structure 13 between the sensor elements 12. Alternatively, the sensor elements 12 may be arranged behind the collimating structure 13, for example on a separate substrate or on the same substrate interspersed between the lighting elements 11. For example, the collimating structure 13 preferably covers both the illumination element 11 and the sensor element 12 to prevent light from reaching the adjacent sensor element 12 unless it passes through the collimating structure 13 (twice).

[0024] In some embodiments, the sensor elements 12 are arranged on a substrate between the lighting element 11 and the contact surface 10a that is transparent, i.e. at least transparent to the illumination light pattern "Li", for example transmitting 80% or more, 90% or more, 95% or more, up to 99% or more. Preferably, the substrate comprises electronics and / or wiring for reading the individual and / or combined sensor elements 12. For example, the electronics and / or wiring are relatively thin and / or transparent and / or arranged according to a second pattern, while still allowing sufficient light of the lighting element 11 to pass through. Instead of arranging the sensor elements 12 on a separate transparent substrate, it is also conceivable to arrange the sensor elements 12 and / or wiring / electronics directly on the light collecting structure 13 (not shown here).

[0025] In some embodiments, the contact surface 10a is formed by a front substrate 14 that is transparent to the illumination light pattern "Li" and the resultant light pattern "Lr", for example with a transmittance of 80% or more, 90% or more, 95% or more, up to 99% or more. In one embodiment, the front substrate 14 is placed in front of the sensor elements 12. In another embodiment, the thickness of the front substrate 14 is less than half a cm, preferably less than 2 mm, and most preferably less than 1 mm. The thinner the front substrate 14, the more accurately the resultant light pattern "Lr" can be measured. For example, the sensor elements 12 can be placed just behind the front substrate 14, which can protect the sensor elements 12 from damage. Instead of using a front substrate 14, the sensor elements 12 can be part of the front substrate 14, for example placed in direct contact with the medium "M" (not shown here).

[0026] In some embodiments, the lighting elements 11 are disposed on a substrate. Preferably, the substrate also includes electronics and / or wiring for controlling the individual and / or combinations of lighting elements 11. In one embodiment, for example as shown, sheet 10 includes a back layer substrate 15 for protecting the lighting elements 11 and / or backside electronics. Instead of using a separate back layer substrate 15, the back side of the substrate housing the lighting elements 11 can form the back layer of sheet 10.

[0027] In some embodiments, the array of lighting elements 11 is configured to emit respective light patterns at controllable wavelengths of light. For example, they are capable of emitting at least two different wavelengths λa, λb. In one embodiment, the array of lighting elements 11 includes a first set of lighting elements 11a configured to emit light at a first wavelength λa and a second set of lighting elements 11b configured to emit light at a second wavelength λb different from the first wavelength λa. For example, the array of lighting elements 11 includes two, three, or more different lighting devices, such as LEDs or OLEDs with different colors. By using different wavelengths, additional (spectrally resolved) information about the medium can be measured. For example, the transmission, absorption, scattering, and / or reflection properties of light interacting with the medium and / or its substructures can be measured and / or compared at two or more different wavelengths. For example, the image may include spectral information of the medium, such as biological tissue. In some embodiments, the central or average wavelength of the light emitted by one or more lighting elements 11 includes infrared wavelengths between 780 nm and 1 mm. For example, IR-A (780nm-1.4μm), IR-B (1.4-3μm) and / or IR-C (aka far infrared, 3μm-1mm). Advantageously, infrared wavelengths can be used to probe different aspects of biological tissue. Also, other wavelengths of light can be used, such as visible light and ultraviolet light.

[0028] In some embodiments, the controller 20 is configured to control a first set of illumination elements 11a to generate a first set of different illumination light patterns with a first wavelength of light λa, and to control a second set of illumination elements 11b to generate a second set of different illumination light patterns with a second wavelength of light λb. Preferably, the first set of different illumination light patterns each have the same spatial modulation as the second set of different illumination light patterns. In this way, the resulting patterns that differ only in the wavelength of illumination can be compared. Alternatively, the spatial modulation may differ between the sets of different wavelengths.

[0029] In one embodiment, the controller 20 is configured to control the first set of illumination elements 11a during a first time interval to generate a first set of different illumination light patterns and to process the first sensor signals from the sensor elements to determine a first set of spatially modulated images based on the measured resultant light patterns caused by the different illumination light patterns at the first wavelength of light λa. In another or further embodiment, the controller 20 is configured to control the second set of illumination elements 11b during a second time interval to generate a second set of different illumination light patterns and to process the second sensor signals from the sensor elements to determine a second set of spatially modulated images based on the measured resultant light patterns caused by the different illumination light patterns at the second wavelength of light λb. In another or further embodiment, the controller 20 is configured to generate one or more images "Im" of the medium based on a combination of the spatially modulated images "In" at the first and second wavelengths of light λa, λb. Preferably, each pattern at the different wavelengths is generated sequentially. That is, the second time interval does not overlap with the first time interval. This method facilitates the use of the same sensor element while still distinguishing between different wavelengths based on time intervals, and also allows for simultaneous illumination with different wavelengths, for example using spectral filters and sensor elements sensitive to each wavelength.

[0030] According to some aspects, the present teachings can be embodied as a wearable device including the SFDI system described herein. For example, the sheet 10 is worn in contact with a body part of a subject and configured to monitor the underlying biological tissue. For example, the wearable device includes an attachment means, such as a strap or an adhesive material (e.g., a patch) for attaching the sheet to the body. The wearable portion of the device includes the sheet and may also include a controller 20. Alternatively, the controller may be provided separately, for example, operating on a mobile device communicatively connected to the wearable portion via a wireless or wired connection.

[0031] According to another or further aspect, the present teachings may be embodiments of a method related to the use of a SFDI system or related to contact-based SFDI of a medium "M". An embodiment includes contacting the medium "M" with a sheet 10 including an array of illumination elements 11 generating illumination light patterns "Li" and an array of sensor elements 12 measuring spatially resolved images of reflected resultant light patterns "Lr" after interaction of the medium "M" with the illumination light patterns "Li". Another or further embodiment includes controlling the illumination elements 11 to generate a set of different illumination light patterns "Li", each different illumination light pattern "Li" being spatially modulated in light intensity along the surface of the illuminated medium "M" according to a different spatial frequency "Fn". Another or further embodiment includes processing the sensor signals "Ss" from the sensor elements 12 to determine a set of spatially modulated images "In" based on the measured resultant light patterns "Lr" caused by the different illumination light patterns "Li", each different illumination light pattern "Li" having a different spatial frequency "Fn". Other or further embodiments include generating an image "Im" of the medium "M" based on a combination of the spatially modulated images "In."

[0032] In one embodiment, the medium "M" includes biological tissue and the image includes spectral information regarding biologically relevant information such as tissue composition, oxygenation, etc. For example, by measuring a combination of different wavelengths of light, the spectral signature of each tissue can be determined. It is also possible to calculate tissue oxygenation using the ratio of measured spectral values ​​represented by oxygenated or deoxygenated hemoglobin.

[0033] 2A-2D show an example of a set of different illumination light patterns "Li". Each light and dark color indicates the light intensity at each location. As shown, each different illumination light pattern "Li" is spatially modulated in light intensity along at least one surface coordinate X, Y, R, according to a respective different spatial frequency F1, F2, F3. In some embodiments, such as shown here, the illumination light pattern includes a gradual transition in light intensity between dark and light bands. This can be achieved by controlling the illumination devices to generate a set of different light intensities, for example according to a sinusoidal or other light intensity variation. In other or further embodiments (not shown here), the transition between high and low intensities can be abrupt or even binary. For example, each light pattern is formed by turning on or off the illumination devices along a particular row, column, or other pattern.

[0034] In a preferred embodiment, each illumination light pattern comprises a set of stripes of alternating high and low intensity (gradual or abrupt transitions), the light intensity being repeated along at least one spatial coordinate with a respective spatial period according to the spatial frequency "Fn" of each of the illumination light patterns. For example, the spatial frequency is the inverse of the spatial period (Pn=1 / Fn). Preferably, the spatial period or frequency has a fixed value for each light pattern, for example the stripe pattern is repeated multiple times periodically or at fixed spatial intervals. In principle, it may be sufficient to use only two different spatial frequencies (F1, F2), but preferably more spatial frequencies are used, for example at least 3, 4, 5, up to 10 or more. Preferably, the set of different spatial frequencies (F1, F2, F3) is equally spaced in frequency or period (inverse frequency).

[0035] In some embodiments, the illumination light pattern includes horizontal, vertical, and / or diagonal stripes, as shown in Figures 2A-2C. In other or further embodiments, other spatially repeating patterns may be used, such as circular patterns with different spatial frequencies as shown in Figure 2D. Also, combinations of differently oriented light patterns or different types of light patterns may be used.

[0036] In some embodiments, the array of illumination elements 11 is controlled to generate a first set of different illumination light patterns during a first time interval and processes a first sensor signal from the sensor elements to determine a first set of spatially modulated images based on a measured resultant light pattern caused by the first set of different illumination light patterns, where the first set of different illumination light patterns comprises a pattern of stripes with alternating light intensity oriented along a first surface coordinate X. In other or further embodiments, the array of illumination elements 11 is controlled to generate a second set of different illumination light patterns during a second time interval and processes a second sensor signal from the sensor elements to determine a second set of spatially modulated images based on a measured resultant light pattern caused by the second set of different illumination light patterns. For example, the second set of different illumination light patterns comprises a pattern of stripes with alternating light intensity oriented along a second surface coordinate Y having a non-zero angle with respect to the first surface coordinate X. In some embodiments, an image "Im" of the medium "M" is generated based on a combination of the first and second spatially modulated images at different orientations.

[0037] Preferably, the different directions of the first and second surface coordinates are transverse and perpendicular to each other, as shown in Figs. 2A and 2B, for example. Other or further directions can also be used, such as 45° as shown in Fig. 2C. For example, the measurement may include rotating the pattern at various angles to reduce or enhance the anisotropic scattering behavior of the medium in one or more of the combined images. Different directions of light patterns or different types of light patterns can also be combined with different wavelengths. For example, the same varying pattern is used at two or more different wavelengths, or different variations are used at different wavelengths. For example, it is possible to envisage using a first set of patterns in a first direction with a first wavelength and a second set of patterns with a different, e.g. transverse or perpendicular, direction with a second wavelength.

[0038] 3A and 3B show measurements using the contact SFDI system and method described herein. The horizontal axis shows the position along "X" (in mm) on the surface where light is measured by each sensor element. The vertical axis shows the relative (normalized) light intensity "I". The vertical axis shows the relative (normalized) light intensity "I". The dashed and dotted lines show the transitions where each lighting element is switched OFF (left side of the line) or ON (right side of the line).

[0039] In Figure 3A, a comparison is shown with the system in contact with the mirror surface. The open dots graph shows the measurement of a system with a collimator between the illumination element and the mirror, while the closed dots graph shows the measurement of the same device without the collimator. Comparing the two graphs, it can be seen that the collimator provides a significant improvement in maintaining a clear transition between illuminated and non-illuminated parts of the mirror. On the other hand, in the absence of the collimator, this transition becomes blurred, for example due to the diffusive nature of the light source (the mirror itself hardly diffuses light).

[0040] In FIG. 3B, a comparison is shown of the same system including the collimator in contact with a mirror surface (open point) or a tissue surface (closed point). From the comparison of the two graphs, it can be seen that compared to the mirror surface, tissue has a much higher scattering behavior causing the light to be scattered to further adjacent areas that are not directly illuminated. This may include light that is directly scattered from the surface of the medium as well as light that penetrates the medium. For example, this may depend on whether the wavelength of light is more or less transmissible through the medium. In some embodiments, the resulting pattern measured from the mirror surface is used to calibrate the SFDI system, for example to determine an actual or assumed illumination light pattern "Li" that illuminates the medium "M" or to compare with the resulting light pattern "Lr" from the medium "M" being imaged.

[0041] In interpreting the appended claims, it should be understood that the word "comprising" does not exclude the presence of other elements or acts than those recited in a particular claim, that the word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements, that reference signs in the claims do not limit their scope, that a plurality of "means" may be represented by the same or different items or implemented structures or functions, and that the disclosed apparatus or portions thereof may be combined or further divided unless specifically stated otherwise. When one claim refers to another claim, this indicates a synergistic effect achieved by the combination of the respective features. However, the mere fact that certain measures are recited in mutually different claims does not indicate that a combination of these measures cannot also be used to advantage. Thus, the present embodiment may in principle include all operative combinations of claims, each of which may refer to any preceding claim, unless clearly excluded by the context.

Claims

1. A sheet (10) having a contact surface (10a) for contacting a medium (M) to be imaged, said sheet (10) comprising: an array of lighting elements (11) configured to generate a controllable illumination light pattern (Li) projected from the contact surface (10a) to illuminate the medium (M) in contact with the sheet (10); an array of sensor elements (12) configured to measure a spatially resolved image of a resulting light pattern (Lr) reflected by the contact surface (10a) after interaction of the illumination light pattern (Li) with the medium (M); a seat (10) comprising: A controller (20), controlling the lighting elements (11) to generate a set of different illumination light patterns (Li), each of which is spatially modulated along the contact surface (10a) in light intensity according to a different spatial frequency (Fn); determining a set of spatially modulated images (In) representing the resultant light patterns (Lr) caused by the different illumination light patterns (Li) based on measurements by the sensor elements (12); generating an image (Im) of the medium (M) based on a combination of the spatially modulated images (In); a controller (20) configured to: A spatial frequency domain imaging (SFDI) system (100) comprising:

2. 2. The system of claim 1, wherein the sheet (10) is flexible and can be bent to a radius of less than 20 cm without impairing its essential function.

3. 2. The system of claim 1, wherein the sheet has a total thickness (T) of less than 1 cm.

4. The system of claim 1 , wherein the sheet (10) comprises a collimating structure (13) between the lighting element (11) and the contact surface (10a).

5. The collimating structure (13) A flexible collimator sheet having a thickness (Ha) of less than 1 mm; a set of micro-apertures configured to pass only light from the lighting element (11) having an angle of incidence less than 10 degrees from the normal vector of the contact surface (10a); The system of claim 4.

6. The system of claim 5, wherein the collimating structure (13) is arranged between the lighting element (11) and the sensor element (12).

7. The lighting elements (11) are arranged according to a first pattern, the sensor elements (12) are arranged according to a second pattern that is complementary to the first pattern; The sensor elements (12) are arranged in each gap between the lighting elements (11) on the opposite side of the collimating structure (13), The illumination elements (11) are arranged in each gap between the sensor elements (12) on the opposite side of the collimating structure (13). The system of claim 6.

8. 2. The system according to claim 1, wherein the sensor element (12) is arranged on a transparent substrate between the lighting element (11) and the contact surface (10a).

9. the contact surface (10a) is formed by a front substrate (14) that is transparent to the illumination light pattern (Li) and the resultant light pattern (Lr); The front substrate (14) is disposed in front of the sensor element (12); The thickness of the front substrate (14) is less than 1 mm. The system of claim 1 .

10. The array of lighting elements (11) includes a first group of lighting elements (11a) configured to emit light of a first wavelength (λa) and a second group of lighting elements (11b) configured to emit light of a second wavelength (λb) different from the light of the first wavelength (λa); the controller (20) is configured to control the first group of illumination elements (11 a) to generate a first set of different illumination light patterns at the first wavelength (λa) of light, and to control the second group of illumination elements (11 b) to generate a second set of different illumination light patterns at the second wavelength (λb). The system of claim 1 .

11. The controller (20) controlling the first group of lighting elements (11 a) to generate the first set of different illumination light patterns during a first time interval; processing first sensor signals from the sensor elements to determine a set of first spatially modulated images based on the measured resultant light patterns caused by different illumination light patterns at the first wavelength of light (λa); controlling a second group of lighting elements (11b) to generate the second set of different illumination light patterns during a second time interval; processing second sensor signals from the sensor elements to determine a second set of spatially modulated images based on the measured resultant light patterns produced by different illumination light patterns at the second wavelength of light (λ b); generating an image (Im) of the medium based on a combination of the spatially modulated images (Im) at the first and second wavelengths of light (λa, λb); It is configured as follows: The system of claim 10.

12. each said illumination light pattern (Li) comprises a set of stripes with alternating high and low light intensities; the alternating light intensities are repeated multiple times at each spatial frequency (Fn) of the illumination light pattern along at least one spatial coordinate of the contact surface (10a); The system of claim 1 .

13. 2. The system according to claim 1, wherein the lighting elements (11) are formed by a two-dimensional array of light-emitting diodes, each having a controllable light intensity.

14. The SFDI system of claim 1 . A wearable device comprising: The sheet (10) is configured to be worn in contact with a body part of a subject and to monitor underlying biological tissue. Wearable device.

15. A method for contact SFDI on a medium (M), comprising: The method comprises: contacting the medium (M) with a sheet (10) comprising an array of illumination elements (11) that generate an illumination light pattern (Li) that is irradiated onto the medium (M) and an array of sensor elements (12) that measure a spatially resolved image of a resultant light pattern (Lr) that is reflected after interaction of the illumination light pattern (Li) with the medium (M); controlling said lighting elements (11) to generate a set of different lighting light patterns (Li); each of said different illumination light patterns (Li) being spatially modulated in light intensity along the illuminated surface of said medium (M) according to a different spatial frequency (Fn); processing the sensor signals (Ss) from the sensor elements (12) to determine a set of spatially modulated images (In) based on measured resultant light patterns (Lr) produced by the different illumination light patterns (Li), each having a different spatial frequency (Fn); generating an image (Im) of the medium (M) based on a combination of the spatially modulated images (In); method.

Images