A system and method for determining a measure of blood volume of a blood perfused tissue

Abstract

The present invention relates to a system and method for determining a measure of blood volume of a blood perfused tissue, the system comprising: a light source configured to emit coherent light to the blood perfused tissue; and a photodetector configured to receive a portion of the coherent light scattered by the blood perfused tissue, and for generating an output signal based on the received portion of the coherent light, wherein the output signal comprises speckle noise, wherein the system is arranged to frequency shift the speckle noise in the output signal of the photodetector by a predetermined frequency shift, and the system is configured to determine the measure of blood volume of the blood perfused tissue based on the output signal with frequency shifted speckle noise.

Description

[0001] A SYSTEM AND METHOD FOR DETERMINING A MEASURE OF BLOOD VOLUME OF A BLOOD PERFUSED TISSUE

[0002] Field of the invention

[0003] The present invention relates to a system and method for determining a measure of blood volume of a blood perfused tissue. In particular, the invention relates to the determination of a measure of blood volume by emitting coherent light to the blood perfused tissue and detecting a portion of the coherent light scattered by the blood perfused tissue using a photodetector. For example, the measure of blood volume comprises a photoplethysmogram (PPG).

[0004] The present invention further relates to a computer program for performing said method, and for a non-transitory computer readable medium storing said computer program.

[0005] Background of the invention

[0006] Known systems for photoplethysmogram (PPG) measurement use a non-coherent light source, typically an LED. Co-pending European patent application EP 23168018.2, filed 14 April 2023, describes a system that includes a non-coherent light source (an LED) for performing a PPG measurement and a coherent light source (a laser) for performing laser Doppler velocimetry (LDV). When LDV is used for measuring blood flow in a body, it is commonly referred to as laser Doppler flowmetry (LDF). The term 'velocimetry' may suggest that velocity is measured but the blood flow signal obtained by LDV is in fact a scalar and contains no direction information. The blood flow signal is therefore related to speed of the blood. For this reason, the present disclosure uses the term LDF.

[0007] Description of the invention

[0008] A drawback of the known system is that the photodetector for measuring the non-coherent light for PPG also picks up some of the coherent light from the laser, which degrades the quality of the PPG signal.

[0009] An object of the invention is to overcome this drawback, or at least provide an alternative system. In particular, the invention aims to improve a PPG signal obtained from a photodetector proximate a laser.

[0010] This aim is achieved by the system for determining a measure of blood volume (e.g. PPG) of a blood perfused tissue, according to claim 1. The system comprises a light source and a photodetector. The light source (e.g. a laser diode) is configured to emit coherent light to the blood perfused tissue. The photodetector is configured to receive a portion of the coherent light scattered by the blood perfused tissue. The photodetector generates an output signal based on the received portion of the coherent light. The output signal comprises speckle noise. The system is arranged to frequency shift the speckle noise in the output signal of the photodetector by a predetermined frequency shift. The system is configured to determine the measure of blood volume of the blood perfused tissue based on the output signal with frequency shifted speckle noise.

[0011] The inventors realized that the coherent light induces a granular interference pattern on the photodetector when the coherent light is scattered. This interference pattern, also referred to as "laser speckle", is randomly fluctuating. The photodetector picks up the interference pattern, which appears as a noise contribution in the photodetector output signal. This noise contribution is referred to as speckle noise. The photodetector picks up the interference pattern even when the photodetector is positioned relatively far from the coherent light source. For example, the inventors observed the noise contribution even when the detector and laser (power class 1) were 30 mm apart. Thus, when determining a measure of blood volume (e.g. PPG) using a photodetector that picks up coherent light, the measurement is subject to speckle noise. In fact, the inventors found that the information relating to blood volume (e.g. PPG) is particularly affected by the speckle noise. Particularly, the low frequency range of the photodetector output (e.g. 0 - 35 Hz) comprises most information related to blood volume (e.g. PPG), and the inventors found that this low frequency range is most affected by the speckle noise, as the speckle noise has a power spectrum which peaks in the low frequency range (near or at 0 Hz) and then decays. In some cases, the signal-to-noise ratio (SNR) in the low frequency range is so low, that it is not even possible to extract the measure of blood volume: the low-frequency signal is drowned in the speckle noise. By frequency shifting the speckle noise, the contribution of the speckle noise to the photodetector output is reduced in the low frequency range, which improves the SNR and makes it possible to determine a measure of blood volume more accurately from the photodetector output.

[0012] Moreover, the inventors realized that reducing the speckle noise in the photodetector output enables using the coherent light itself for the measure of blood volume. In other words, a separate light source for non-coherent light (e.g. a LED) is no longer needed for performing the blood volume measurement (e.g. PPG). The invention thus enables determining a measure of blood volume (e.g. PPG) with a coherent light source.

[0013] Using a coherent light source for determining a measure of blood volume has advantages over conventional systems that employ a non-coherent light source (LED). In particular, the emission angle of the coherent light source (e.g. laser) is smaller, providing more accurate control over the area of the blood perfused tissue that is inspected. In addition, coherent light has low beam divergence, i.e. it is highly collimated, thus reducing the need for collimators or other optics as compared to non-coherent light sources. In fact, collimating non-coherent light results in energy losses, which in some cases makes the coherent light source a more energy efficient solution than a non-coherent light source. In addition, a coherent light source is less susceptible to temperature induced wavelength shift as compared to non-coherent light sources such as LEDs.

[0014] Moreover, the invention enables using the same light source, and preferably the same photodetector, for determining both a measure of blood volume (e.g. PPG) and a measure of blood speed (e.g. LDF). This reduces the number of components needed, which reduces the size of the system (which is particularly relevant for wearable devices). Furthermore, costs and manufacturing complexity and / or energy consumption are reduced.

[0015] It is stressed that the invention is not limited to the combined measurement of blood volume (e.g. PPG) and blood speed (e.g. LDF). The system of the invention also enables a blood volume (e.g. PPG) measurement with a coherent light source per se. Moreover, the invention enables a PPG measurement with a non-coherent light source (e.g. LED) in the vicinity of a coherent light source (i.e. where the photodetector for the PPG measurement picks up some light from a nearby coherent light source).

[0016] An example of a measure of blood volume is a PPG, which is a measure that changes over time and may be expressed as a function of time, e.g. x(t).

[0017] The photodetector generates an output signal that is indicative of intensity of the light it receives. Specifically, the detector is at least sensitive to light of wavelengths corresponding to the wavelength of the coherent light.

[0018] The system comprises hardware for processing the output of the photodetector. The hardware for example comprises electronic circuits (digital and / or analog) and / or integrated circuits. The system, for example, comprises one or more microprocessors, DSPs, FPGAs, ASICs or other logic circuitry. For example, the system comprises one or more processors and a memory for storing software for execution by the one or more processors.

[0019] Preferred embodiments are defined in the dependent claims and in the following paragraphs.

[0020] In an embodiment, the system comprises an actuator configured to vibrate, relative to the blood perfused tissue, at least one of the group comprising : the light source; the photodetector; and an optical element arranged in front of the light source or in front of the photodetector. The actuator is configured to vibrate the respective component relative to the blood perfused tissue at a vibration frequency for frequency shifting the speckle noise in the output signal of the photodetector. Preferably, the vibration frequency corresponds to the predetermined frequency shift.

[0021] The vibration changes the path length from the light source to the photodetector, which causes a relative phase shift of the light at the photodetector. Therefore, the interference pattern impinging on the photodetector changes at the vibration frequency, which causes a shift of the speckle noise in the photodetector. In a currently preferred embodiment, the actuator is configured to vibrate both the light source and photodetector, e.g. by vibrating a substrate that holds both the light source and photodetector. In this embodiment, the distance from the photodetector and light source to the tissue is constantly changing due to the vibration, and thus the path length of the light travelling from the light source, via the tissue, to the photodetector, is constantly changing as well.

[0022] The vibration may comprise a translational and / or a rotational movement. Preferably, the actuator is configured to vibrate at least in a direction towards and from the blood perfused tissue. For example, said direction is substantially perpendicular to the detector area of the photodetector. Additionally or alternatively, said direction is substantially parallel to the direction the coherent light is emitted. By vibrating in a direction towards and from the blood perfused tissue, a relatively small deflection of the actuator can induce a relatively large phase shift, as compared to e.g. a vibration in the transverse direction.

[0023] In an alternative embodiment, the actuator is configured to vibrate the tissue, instead of vibrating the respective component (light source and / or photodetector and / or optical element). For example, the actuator is configured to be attached to the skin of a user, to cause vibration of the tissue near the photodetector and / or near the laser. Vibrating the tissue results in a vibration of the respective component relative to the tissue.

[0024] In a further preferred embodiment, the actuator is driven by a triangular drive signal. The inventors have found that, surprisingly, a triangular drive signal effectively induces the desired frequency shift while inducing relatively little distortion in the higher frequency range, as compared to e.g. using a pure sinusoidal wave. This is particularly advantageous when the high frequency portion of the photodetector signal is used for LDF measurement, as will be described in more detail below.

[0025] In another embodiment, the actuator is configured to rotate an optical element, arranged in a light path from the light source to the detector, at a rotational frequency for frequency shifting the speckle noise in the output signal of the photodetector. Preferably, the rotational frequency corresponds to the predetermined frequency shift. Instead of rotating the optical element, the light source and / or photodetector may be rotated.

[0026] In an embodiment, the system is arranged to phase modulate the coherent light at a modulation frequency for frequency shifting the speckle noise in the output signal of the photodetector. This causes the speckle pattern to change at the modulation frequency, and thus shifts the speckle noise in the detector output. Preferably, the modulation frequency corresponds to the predetermined frequency shift. Phase modulation of the coherent light can be seen as an alternative to the mechanical vibration described above. Both can be said to achieve a relative phase shift of the coherent light as measured at the detector. For example, the system comprises a phase modulator (such as a Pockels cell or a liquid crystal) and / or a controller to phase modulate the light source (such as a phase-modulating laser).

[0027] In an embodiment, the system is arranged to control the light source for frequency shifting the speckle noise in the output signal of the photodetector. This is a further alternative to the mechanical vibration and phase modulation approaches described above. Preferably, the direction of emission of the light source is modulated at a modulation frequency, preferably a modulation frequency corresponding to the predetermined frequency. In another example, the light source comprises two lasers configured to illuminate the same region of the blood perfused tissue and the intensity of the two lasers is modulated synchronously, such that the total intensity of the illumination is substantially constant while the contribution of each laser to the total intensity fluctuates at the modulation frequency. For example, the two lasers are alternatingly switched on and off, at a rate corresponding to the modulation frequency. In another example, the first laser is controlled such that its intensity fluctuates according to sin2(t), and the second laser is controlled such that its intensity fluctuates according to cos2(t). In yet another example, the intensity of the two lasers is modulated according to a triangle wave, with a phase difference of half a wavelength between the two lasers.

[0028] In an embodiment, the system is configured to obtain a low-frequency portion of the output signal with frequency shifted speckle noise and to determine the measure of blood volume (e.g. PPG) based on the low-frequency portion. The low-frequency portion is obtained by attenuating frequencies above a low-pass cut-off frequency. The predetermined frequency shift is greater than the low-pass cut-off frequency. In other words, the peak of the speckle noise (at or near 0 Hz) is shifted to fall outside of the frequency range used for determining the measure of blood volume.

[0029] For example, the system includes a low-pass filter, or a band-pass filter with an upper bound corresponding to the low-pass cut-off frequency. Preferably the predetermined frequency shift is at least 2-5 times greater than the low-pass cut-off frequency.

[0030] Preferably, the low-pass cut-off frequency is in the range of 25-50 Hz, preferably in the range of 30-40 Hz, and the predetermined frequency shift is at least 150 Hz, preferably at least 250 Hz, e.g. at least 500 Hz.

[0031] In an example with a band-pass filter for obtaining the low-frequency portion, a lower frequency bound is in the range of 0.5-5 Hz and an upper frequency bound is in the range 25-50 Hz, preferably in the range of 30-40 Hz.

[0032] In an embodiment, the system is further configured to determine a measure of the speed of blood in the blood perfused tissue (e.g. LDF) based on the output signal with frequency shifted speckle noise. In other words, the system is configured to determine both a measure of blood volume (e.g. PPG) and a measure of blood speed (e.g. LDF). The invention thus allows the use of a single light source to measure both blood volume (e.g. PPG) and blood speed (e.g. LDF). This reduces the dimensions of the system, which is particularly relevant for wearable devices. Further, using a single light source reduces costs, manufacturing complexity and / or energy consumption. In addition, using a single light source ensures that the blood volume and blood speed measurements both relate to the same area of the tissue, at the same penetration depth (same wavelength). Interrogating the same part of the tissue is particularly important when using the correlation between volume and speed measurements to determine a physiological parameter, such as systolic and / or diastolic blood pressure, as for example described in co-pending European patent application EP 23168018.2, filed 14 April 2023, which is hereby incorporated by reference in its entirety.

[0033] The blood volume measure (e.g. PPG) and the blood speed measure (e.g. LDF) are determined based on the same photodetector output signal, in which the speckle noise has been frequency shifted as described above. Alternatively, the system is arranged to perform the frequency shift intermittently, resulting in frequency-shifted periods and non-frequency shifted periods. In this alternative embodiment, the system is configured to determine a measure of blood speed (e.g. LDF) based on the output signal obtained during non- frequency-shifted periods, and to determine the measure of blood volume (e.g. PPG) from the output signal as obtained during the frequency-shifted period.

[0034] In a further embodiment, the system is configured to obtain a high-frequency portion of: the output signal with the frequency shifted noise; or the output signal obtained during non-frequency shifted period.

[0035] The system determines the measure of blood speed (e.g. LDF) based on the high-frequency portion. The high-frequency is obtained by attenuating frequencies below a high-pass cut-off frequency.

[0036] For example, the system includes a high-pass filter, or a band-pass filter with a lower bound corresponding to the high-pass cut-off frequency.

[0037] Preferably, the predetermined frequency shift is at least 2-5 times smaller than the high-pass cut-off frequency. For example, the predetermined frequency shift is in the range of 150-250 Hz and the high-pass cut-off frequency is at least 1 kHz.

[0038] Preferably, the system is configured to frequency shift the peak of the speckle noise to a region outside the frequency range used for determining the measure of blood volume and outside the frequency range used for determining the measure of blood speed. For example, the system is configured to obtain a low-frequency portion with frequencies below 30-50 Hz for determining the measure of blood volume (e.g. PPG), and a high-frequency portion with frequencies above 1 kHz for determining the measure of blood speed (e.g. LDF), wherein the speckle noise is frequency shifted by 150-500 Hz, preferably 150-250 Hz. In an embodiment, the system comprises multiple photodetectors, each arranged for receiving a respective portion of the coherent light scattered by the blood perfused tissue and for generating a respective output signal.

[0039] Although the frequency shifting drastically improves the signal-to-noise ratio (SNR) in the lower frequencies used for the measure of blood volume, the interference pattern may still introduce some noise in the output signal in the lower frequencies and / or higher frequencies. By using multiple photodetectors, the SNR can be improved for the blood volume measurement and the optional blood speed measurement.

[0040] In a further embodiment, determining the measure of blood volume (e.g. PPG) by the system comprises summing the output signals of the multiple photodetectors. For example, summing comprises averaging or weighted summation.

[0041] In an embodiment wherein the system also determines the measure of blood speed, the system is preferably configured to determine a spectral power density of the high-frequency portion of each output signal of the multiple photodetectors. The system is further configured to determine 1) a sum of the first moments of the spectral power densities or 2) the first moment of the sum of the spectral power densities. The system determines the measure of blood speed based on 1) the sum of the first moments or 2) the first moments of the sum, respectively. Notably, the spectral power density is computed prior to computing a sum. Specifically, the inventors found that summing the high frequency portions of the different photodetector outputs before computing a spectral power density decreases the SNR. Preferably, the spectral power densities, the sum of their first moments or the first moment of their sum is normalized by the system using a zero-th moment of the spectral power density of each of the high frequency portions.

[0042] Preferably, the photodetectors are photodiodes, wherein preferably each photodiode has a surface area of 0.01 - 4 mm2, preferably 0.1 - 1 mm2, more preferably 0.2 - 0.5 mm2. Preferably, the distance between each photodetector and the light source is 0.5 - 8 mm, preferably 1 - 4 mm, more preferably 1.2 - 1.8 mm.

[0043] In an embodiment, at least two photodetectors are positioned at substantially the same distance to the light source. For example, a set of at least three photodetectors is arranged around the light source, wherein the distance between each pair of photodetectors within the set is substantially the same. For example, four photodetectors are arranged in a diamondshaped or cross-shaped pattern around the light source, all at the same distance to the light source.

[0044] In an embodiment at least two photodetectors are positioned at different distances to the light source. According to the invention, it is also possible that a first set of photodetectors has the same distance to the light source, while a second set of photodetectors has a different distance to the light source. For example, at least two photodetectors are arranged at the same distance to the light source while at least one photodetector is positioned further away from the light source. For example, four photodetectors are arranged in a diamondshaped or cross-shaped pattern around the light source, with equal distance to the light source, and at least one photodetector is positioned further away from the light source.

[0045] In an embodiment, the system is configured to apply a weighting factor to each of the photodetector output signals. The weighting factor may be different for different photodetectors. For example, the weighting factor depends on the distance of the photodetector to the light source, wherein weighting factors decrease for increasing distances.

[0046] In an embodiment, the coherent light has a wavelength of 700 - 1000 nm, preferably 850 nm.

[0047] In a currently preferred embodiment, the coherent light has a wavelength of 700 - 1000 nm, the system comprises multiple photodetectors (preferably photodiodes), with each photodetector having a surface area of 0.1 - 4 mm2, preferably 0.1 - 1 mm2, more preferably 0.2 - 0.5 mm2, such as 0.36 mm2, and the distance between each photodetector and the light source is 0.5 - 8 mm, preferably 1 - 4 mm, more preferably 1.2 - 1.8 mm. The inventors found that this combination of features ensures sufficient penetration depth of the coherent light into the tissue, while maintaining a good SNR of the signal. Preferably, the coherent light source of this embodiment comprises a power class 1 laser diode.

[0048] In an embodiment the light source comprises a laser diode, such as a Vertical Cavity Surface Emitting Laser (VCSEL). Preferably, the light source comprises a VCSEL with integrated photodetector, VCSEL-IP, configured to generate a VCSEL-IP output signal. The system is preferably further configured to determine a measure of motion based on at least the VCSEL- IP output signal. For example, the system determines a measure of motion based on the VCSEL-IP output signal and a measure of blood volume based on the output signal of the photodetector (respectively the output signal of at least one of the plurality of photodetectors).

[0049] Preferably, the system is configured to determine a measure of similarity of the VCSEL-IP output signal as compared to the output signal of the photodetector (respectively the output signal of at least one of the plurality of photodetectors), and to determine the measure of motion as a function of the measure of similarity, with the measure of motion decreasing for increasing similarity.

[0050] For example, determining the measure of similarity comprises determining a cross-correlation between the VCSEL-IP output signal and at least one other photodetector output signal. The measure of motion is determined using the cross-correlation: the cross-correlation increases for decreasing motion and vice versa. In a further example, determining the measure of similarity comprises determining a summation of said cross-correlation over a predetermined time interval and / or determining a maximum value of said cross-correlation.

[0051] In an embodiment, the system is configured to output the measure of motion. For example, the measure of motion can be displayed to a user and / or sent to a remote system.

[0052] Alternatively or additionally, the system is configured to determine whether the measure of motion meets a threshold criterion, and to determine whether to reject or accept the blood volume and / or blood speed measurement based on whether said threshold criterion is met. For example, the system rejects the blood volume and / or blood speed measurement if the measure of motion exceeds a predetermined threshold.

[0053] In an embodiment, the system comprises a wearable device that comprises the light source and the photodetector. In a first example, the wearable device further comprises one or more processors. In this case, the one or more processors of the wearable device perform the determination of the measure of blood volume and / or speed. In a second example, the wearable device comprises a communication module, preferably a wireless communication module, to communicate the output signal of the photodetectors, digitized by front-end electronics of the wearable device, to a remote computing system that for determining the measure of blood volume and / or blood speed. For example, the remote computing system is a server, a computer, a tablet, or a smartphone.

[0054] In a currently preferred embodiment, the wearable device comprises an actuator configured to vibrate the light source and / or the photodetector and / or an optical element in front of the light source or photodetector, as described above.

[0055] The wearable device for example comprises a patch, a smart watch, a wrist band, a chest band, a hearable (e.g. an ear bud or hearing aid) or a ring.

[0056] The invention further relates to a method for determining a measure of blood volume of a blood perfused tissue. The method comprises controlling a light source to emit coherent light to the blood perfused tissue. A photodetector output signal is obtained from a photodetector receiving a portion of the coherent light scattered by the blood perfused tissue. The photodetector output signal comprises speckle noise. The method further comprises frequency shifting the speckle noise in the photodetector output signal by a predetermined frequency shift. The measure of blood volume is determined based on the output signal with frequency shifted speckle noise.

[0057] The invention further relates to a computer program comprising instructions which, when executed by one or more processors, cause the one or more processors to perform the method as described in any of the embodiments of this disclosure. The invention further relates to a non-transitory computer readable medium storing said computer program. According to a second aspect, the present invention provides a system, method and corresponding computer program for determining a measure of blood volume and / or blood speed of a blood perfused tissue that comprises a coherent light source and multiple photodetectors. Specifically, the system comprises a light source configured to emit coherent light to the blood perfused tissue and a plurality of photodetectors. The photodetectors are each configured to receive a respective portion of the coherent light scattered by the blood perfused tissue, and for generating a respective output signal based on the received portion of the coherent light. The system is configured to determine the measure of blood volume and / or blood speed of the blood perfused tissue based on the output signals of the plurality of photodetectors.

[0058] According to a third aspect, the present invention provides a system, method and corresponding computer program for determining a measure of blood volume and / or blood speed, and a measure of motion based on a comparison between the output signal of a VCSEL-IP and at least one further photodetector. Specifically, the system comprises: a VCSEL-IP configured to emit coherent light to the blood perfused tissue, wherein the integrated photodetector is configured to generate a VCSEL-IP output signal; and a further photodetector configured to receive a portion of the coherent light scattered by the blood perfused tissue, and for generating a further output signal based on the received portion of the coherent light. The system is configured to determine the measure of blood volume and / or blood speed of the blood perfused tissue based on the VCSEL-IP output signal and / or the further output signal. The system is further configured to: determine a measure of similarity of the VCSEL-IP output signal as compared to the further output signal; and determine a measure of motion as a function of the measure of similarity, with the measure of motion decreasing for increasing similarity.

[0059] Brief description of the

[0060] In the following, example embodiments will be described with reference to the drawings, wherein:

[0061] Figure 1 schematically illustrates a first embodiment of the device or system according to the invention;

[0062] Figure 2 schematically illustrates a second embodiment of the device or system according to the invention;

[0063] Figure 3 schematically illustrates a third embodiment of the device or system according to the invention;

[0064] Figure 4 illustrates a first example of a front-end processing of a device or system according to the invention; Figure 5 shows photodetector signals obtained with and without vibration, wherein the upper graph shows the photodetector signal with a bandpass filter of 0.5 - 30 Hz, the second graph shows the photodetector signal with a bandpass filter of 5 - 30 Hz and the third graph shows the photodetector signal with a bandpass filter of 15-30 Hz, whereas the bottom graph shows the drive signal for the actuator,

[0065] Figures 6 and 7 illustrate a second and third example of a front-end processing of a device or system according to the invention, respectively;

[0066] Figure 8 is a schematic top view of a second embodiment of the device or system according to the invention.

[0067] Detailed description of the drawings

[0068] A first embodiment of the invention is depicted in Figure 1. The figure shows a cross section view of a body-worn device or system 10 and a cross-sectional view of a region of blood perfused tissue 30. The device 10 comprises a coherent radiation source 12 (e.g. a laser diode such as a VCSEL). The coherent radiation 122 exposes and penetrates the skin 300 and other parts of the tissue 30 at exposed tissue region 32.

[0069] Discontinuities of optical properties in the tissue 30 can scatter the radiation in other directions than that of the incident direction, wherein moving discontinuities 34, e.g. blood cells, moving in blood vessels 301 can Doppler-shift the radiation. A photodetector 16 receives scattered radiation 162 from the tissue 30.

[0070] The general amount of radiation absorbed in the tissue 30 is related to the amount of blood therein, as the absorption of the blood reduces the amount of scattered light, and can thus be used to derive a measure of blood volume (e.g. PPG). The Doppler shifting is related to a blood speed and can thus be used to derive a measure of blood speed (e.g. LDF). Both the measure of blood speed and the measure of blood volume can be derived from the output signal 164 of the photodetector 16.

[0071] For example, a low-frequency component of the output signal of the photodetector 16 is proportional to the total amount of radiation, while a high-frequency component contains contributions caused by the interference of the Doppler-shifted and non-Doppler-shifted coherent radiation.

[0072] The system 10 is further provided with an actuator 14 for vibrating the photodetector 16. In the example of figure 1, the actuator is a microelectromechanical system (MEMS) device, e.g., a piezoelectric element or electrostatic tweeter. In the illustrated example, the actuator 14 is configured to vibrate the detector 16 in direction A. Direction A is substantially perpendicular to the detection area of photodetector 16. Alternatively (not shown), the actuator 14 is configured to vibrate the light source 12. Figure 2 shows a system 11 that is a variant of system 10 of figure 1. In system 11, the coherent light source 12 and photodetector 16 are mounted on a substrate 17, and additionally an actuator 14 is mounted on the substrate 17. The actuator 14 is configured to vibrate the substrate 17 in direction A (perpendicular to the detection area of photodetector 16), which causes the light source 12 and the photodetector 16 to vibrate together. Figure 3 shows another variant of figure 1, wherein a system 13 comprises a light source 12 and a photodetector 16, that are each provided with an actuator 14 for vibrating both the light source 12 and the photodetector 16.

[0073] Figure 4 shows a block diagram of the front-end signal processing for systems according to the invention. Actuators 14 are drawn with dashed lines to indicate that one or more actuators are optional. The actuators 14 are depicted at different possible locations, in accordance with the options described in figures 1-3.

[0074] The front-end signal processing determines a measure of blood volume (e.g. PPG) from the photodetector output 164. Preferably, a pre-amp 18 is provided, as the output signal 164 from the detector 16 is usually quite weak.

[0075] The optional actuator 14 is used to vibrate the photodetector 16 and / or light source 12. In the example, the vibration frequency is 250 Hz. The photodetector output 164 (preferably amplified by pre-amp 18) is fed to a low-pass filter 184. In this example, the low-pass filter is embodied as a bandpass filter that extracts frequencies between 0.5 Hz and 30 Hz. The resulting signal 24 is indicative of blood volume changes in tissue region 32 (as shown in figure 1) of tissue 30. In particular, the resulting signal 24 represents a PPG signal x(t), wherein t represents time.

[0076] Figure 4 also illustrates an optional optical element 142, such as a lens, an optical fibre for guiding the light, a transparent plate or a waveplate. Instead of vibrating the light source 12 or photodetector 16, actuator 14 may be configured to vibrate the optical element 142. In the drawing, the optical element 142 is arranged in front of the photodetector 16. Alternatively, the optical element 142 is arranged in front of the light source 12.

[0077] In an alternative embodiment, a frequency shift of the speckle noise is achieved by phase modulating the laser 12 at a predetermined vibration frequency, e.g. 250 Hz. In a further alternative embodiment, the direction of the laser light is slightly altered (e.g. within 0.1 - 2 degrees) at a predetermined vibration frequency, e.g. 250 Hz.

[0078] The vibration (e.g. by actuators 14) or modulation (by phase modulators or laser control) frequency shifts the speckle noise in the spectrum of the photodetector's output signal. Figure 5 presents results of an experiment performed with a system similar to figure 2, wherein the photodetector 16 and light source 12 are vibrated at 250 Hz. In the experimental setup a speaker coil was used as actuator. The light source 12 emitted light at a wavelength of 850nm (infra-red). The speaker coil exhibits a deflection of 560 nm during vibration. In figure 5, the x-axis represents time in seconds. The lower graph shows the drive signal, with the y-axis indicating deflection (in arbitrary units). This lower graph clearly shows that the drive signal is switched on and off intermittently. The top three graphs show the corresponding photodetector signal, with different bandpass filters applied to select frequency ranges of 0.5-30 Hz, 5-30Hz and 15-30 Hz, respectively.

[0079] As can be seen in the top three graphs, the noise in the blood volume signal x(t) is significantly reduced when the vibration is activated, whereas the signal is noisy when the vibration is off.

[0080] Surprisingly, the inventors found that driving the actuator by a triangular waveform results in relatively less distortion in the higher frequency range, as compared to using a pure sinusoidal wave. Particularly, when vibrating the photodetector, harmonic peaks appear in the high frequency range. These harmonic peaks are more pertinent when using a sinusoidal wave than when using a triangular wave. Reducing the distortion in the high frequency range of the photodetector output signal is particularly important when combining the measurement of blood volume (e.g. PPG) with a measure of blood speed (e.g. LDF), since the latter is extracted from the high frequency range of the photodetector signal. Determination of the measure of blood speed (e.g. LDF) will be described next, with reference to figure 6.

[0081] Figure 6 shows a second example of front-end signal processing of a system of the invention. Figure 6 contains many of the same elements as figure 4, which carry the same reference numeral. Additionally, the system of figure 6 comprises a high-pass filter 182 and a computation module 186.

[0082] The high-pass filter 182 receives the photodetector output 164 (optionally amplified by preamp 18) and outputs a high-pass filtered signal. The cut-off frequency of the high-pass filter is in the range of 500 Hz - 2 kHz. In this example, the high-pass filter 182 is embodied as a bandpass filter to also attenuate very high frequencies (above 20 - 50 kHz), which often contain only noise. Particularly, the bandpass filter of figure 6 extracts frequencies in the range of 1 kHz - 40 kHz.

[0083] The high-pass filtered signal is then fed to computation module 186 to calculate the first moment of the spectrum, which is correlated to blood speed. The first moment is calculated as: where:

[0084] Mx(t) is the first moment of the spectral power density at time t, and f2are the limits of the band pass filter (e.g. 1 kHz and 40kHz), and Sv(f,t) is the spectral power density of the photodetector output.

[0085] The magnitude of the first moment is proportional to the average Doppler shift of the radiation received by the photodetector and proportional to the intensity of the radiation. The first moment may be normalized in order to remove or reduce the dependency of the intensity by dividing it by the average determined over the same frequency band (i.e. the zero-th moment). wherein: v(t) is the average Doppler shift at time t

[0086] M0(t) is the average power at time t (or "zero-th moment").

[0087] The signal v(t) represents an LDF signal which is output as signal 22. Thus, the system of figure 6 determines both x(t) and v(t), i.e. both a PPG signal 24 and an LDF signal 22, using a single coherent light source 12.

[0088] Figure 7 shows a third example of signal processing for a system according to the invention, wherein the speckle noise is frequency shifted intermittently. In the example of figure 7 this is achieved by driving actuator 14 (vibrating photodetector 16) intermittently. In this example, the measure of blood volume (e.g. PPG) is determined when the actuator 14 is on, whereas the measure of blood speed (e.g. LDF) is determined when the actuator is off.

[0089] A selector 181 is provided which is capable of routing the output of the preamp 18 to either of the filters 182 and 184. The selector 181 is controlled by a timing circuit 183 which also operates the actuator 14 to achieve the intermittent vibration. The timing circuit 183 controls actuator 14 to vibrate photodetector 16 at a predetermined vibration frequency, while instructing the selector 181 to forward the output of the preamp 18 to the filter 184 for the generation of the PPG signal 24. At a later point in time, the timing circuit 183 will instead switch off actuator 14 while instructing the selector 181 to forward the output of the preamp 18 to the filter 182 for generation of LDF signal 22.

[0090] Intermittently switching the vibration (or other type of modulation) on and off, is advantageous to supress distortions in the high frequency range of the photodetector spectrum which could otherwise reduce signal quality of the LDF signal 22.

[0091] The examples of figures 1-7 include a single photodetector. According to further embodiments of the invention, multiple photodetectors are used, preferably in combination with a single coherent light source. A currently preferred embodiment of the system using multiple photodetectors is depicted in figure 8. Figure 8 shows a system 13 comprising a coherent light source 12, specifically a VCSEL, emitting coherent light with a wavelength of 850 nm. Optionally, light source 12 is a VCSEL with integrated photodetector 168, also referred to as VCSEL-IP. In the present example, the integrate photodetector 168 is a photodiode.

[0092] The system 13 further comprises multiple photodetectors 16a, 16b. Four photodetectors 16a are arranged in a cross-shaped arrangement around the VCSEL 12, each - within tolerance - at the same distance di of 0.5 - 4 mm. In this example, di is 1.2 mm. A further photodetector 16b is positioned at a distance d2> di. In the example of figure 8, the photodetectors 16a, 16b are photodiodes. Preferably, photodetectors 16a have the same detection area. In the example of figure 8, photodetectors 16a each have a surface area of 0.36 mm2. In the example of figure 8, the surface area of photodetector 16b is the same as the surface area of photodetectors 16a. Alternatively, the photodetector 16b has a different surface area, such as a larger surface area.

[0093] Each of the components 12, 16a, 16b is mounted to a substrate (not shown), e.g. a circuit board. Optionally, an actuator 14 (e.g. a MEMS speaker) is provided to vibrate the light source 12 or one or more of the photodetectors 16a, 16b. For example, the optional actuator 14 vibrates the substrate to which the components are mounted.

[0094] The measure of blood volume (e.g. PPG) is determined based on the output of all photodetectors 16a, 16b. In particular, the output signals of photodetectors 16a, 16b are summed and the sum signal is used to determine the measure of blood volume. For example, a lower frequency portion (e.g. 0.5-30 Hz) is extracted from the sum signal, while other frequencies are attenuated.

[0095] In an embodiment, a weighted sum is calculated, wherein each photodetector 16a, 16b can be assigned a different weight. For example, the output of photodetector 16b at distance d2is weighted differently (e.g. weighted less) than the output of photodetectors 16a at distance di, which may be weighted by the same weighting factor.

[0096] Optionally, also a measure of blood speed (e.g. LDF) is determined based on the output of all photodetectors 16a, 16b. For example, a high frequency portion (e.g. 1-40 Hz) is extracted from each of the photodetector signals, while other frequencies are attenuated. A spectral power density is computed of each high-frequency portion. The spectral power densities are then combined to determine the measure of blood speed (e.g. LDF). In a first example, the sum of the spectral power densities is calculated and a first moment of said sum is computed. In a second example, the first moment of each spectral power density is calculated and the sum of said first moments is computed. In these calculations, the first moment can be normalized using the zero-th moment, as described above.

[0097] If the light source 12 comprises a VCSEL with integrated photodetector 168, optionally a measure of motion is determined based on the output of integrated photodetector 168. Optionally, the measure of motion is determined by comparing the output of the integrated photodetector 168 with the output of one or more of the photodetectors 16a, 16b. For example, a cross-correlation between the signal from photodiode 168 and a (weighted) average of the signals from photodiodes 16a, 16b is computed. The maximum value of the cross-correlation is determined. The maximum value represents a measure of similarity between the signal of photodiode 168 and the average signal of photodiodes 16a. The maximum value is converted to a measure of motion, using a function or lookup table.

[0098] Preferably, the measure of motion increases (more motion) for decreasing maximum value (i.e. decreasing similarity).

[0099] List of embodiments

[0100] Various aspects of the present invention are described in the following list of embodiments.

[0101] 1. A system for determining a measure of blood volume of a blood perfused tissue, comprising : a light source configured to emit coherent light to the blood perfused tissue; and a photodetector configured to receive a portion of the coherent light scattered by the blood perfused tissue, and for generating an output signal based on the received portion of the coherent light, wherein the output signal comprises speckle noise, wherein the system is arranged to frequency shift the speckle noise in the output signal of the photodetector by a predetermined frequency shift, and the system is configured to determine the measure of blood volume of the blood perfused tissue based on the output signal with frequency shifted speckle noise.

[0102] 2. The system according to embodiment 1, comprising an actuator configured to vibrate, relative to the blood perfused tissue, at least one of the group comprising: the light source; the photodetector; an optical element arranged in front of the light source or in front of the photodetector, wherein the actuator is configured to vibrate at a vibration frequency for frequency shifting the speckle noise in the output signal of the photodetector.

[0103] 3. The system of embodiment 2, wherein the actuator is configured to vibrate at least in a direction towards and from the blood perfused tissue.

[0104] 4. The system of embodiment 2 or 3, wherein the actuator is driven by a triangular drive signal. 5. The system according to embodiment 1, comprising an actuator configured to rotate at least one of the group comprising : the light source; the photodetector; and an optical element arranged in front of the light source or in front of the photodetector, for frequency shifting the speckle noise in the output signal of the photodetector, wherein preferably the actuator is configured for rotation at a rotational frequency corresponding to the predetermined frequency shift.

[0105] 6. The system of embodiment 1, wherein the system is arranged to phase modulate the coherent light at a modulation frequency for frequency shifting the speckle noise in the output signal of the photodetector.

[0106] 7. The system of embodiment 6, wherein the system comprises a phase modulator and / or a controller to phase modulate the light source

[0107] 8. The system of embodiment 7, the phase modulator comprising a Pockels cell and / or a liquid crystal.

[0108] 9. The system of embodiment 1, wherein the system is arranged to control the light source for frequency shifting the speckle noise in the output signal of the photodetector, preferably by adjusting an emission direction at the predetermined frequency.

[0109] 10. The system of any one or more of the embodiments 1-9, wherein the system is configured to: obtain a low-frequency portion of the output signal with frequency shifted speckle noise, wherein frequencies above a low-pass cut-off frequency are attenuated, wherein the predetermined frequency shift is greater than the low-pass cut-off frequency, preferably at least 2-5 times greater than the low-pass cut-off frequency; and determine the measure of blood volume based on the low-frequency portion.

[0110] 11. The system of embodiment 10, wherein the low-pass cut-off frequency is in the range of 25-50 Hz, preferably in the range of 30-40 Hz, and the predetermined frequency shift is at least 150 Hz, preferably at least 250 Hz, e.g. at least 500 Hz.

[0111] 12. The system of any one or more of the embodiments 1-11, wherein the system is further configured to determine a measure of blood speed in the blood perfused tissue based on the output signal with frequency shifted speckle noise.

[0112] 13. The system of any one or more of the embodiments 1-11, wherein the system is arranged to perform the frequency shift intermittently, resulting in frequency-shifted periods and non-frequency shifted periods, and the system is further configured to determine a measure of blood speed based on the output signal obtained during non-frequency shifted periods.

[0113] 14. The system of embodiment 12 or 13, wherein the system is configured to: obtain a high-frequency portion of the output signal with the frequency shifted noise or the output signal obtained during non-frequency shifted periods, wherein frequencies below a high-pass cut-off frequency are attenuated; and determine the measure of blood speed based on the high-frequency portion.

[0114] 15. The system according to any one or more of the preceding embodiments, wherein the system comprises multiple photodetectors, each arranged for receiving a respective portion of the coherent light scattered by the blood perfused tissue and for generating a respective output signal.

[0115] 16. The system according to embodiment 15, wherein determining the measure of blood volume of the blood perfused tissue comprises summing the output signals of the multiple photodetectors.

[0116] 17. The system according to embodiment 14 in combination with embodiment 15 or 16, wherein the system is configured to: determine a spectral power density of the high-frequency portion of each output signal; determine a sum of the first moments of the spectral power densities or the first moment of the sum of the spectral power densities; and determine the measure of blood speed based on the sum of the first moments or the first moments of the sum, respectively.

[0117] 18. The system according to embodiment 17, wherein the system is configured to determine a zero-th moment of the spectral power density of each high-frequency portion and use the determined zero-th moments to normalize the sum of the first moments or the first moments of the sum.

[0118] 19. The system according to any one or more of the embodiments 15-18, wherein the photodetectors are photodiodes, wherein preferably each photodiode has a surface area of 0.01 - 4 mm2, preferably 0.1 - 1 mm2, more preferably 0.2 - 0.5 mm2.

[0119] 20. The system according to any one or more of embodiments 15-19, wherein the distance between each photodetector and the light source is 0.5 - 8 mm, preferably 1 - 4 mm, more preferably 1.2 - 1.8 mm.

[0120] 21. The system according to any one or more of embodiments 15-20, wherein at least two photodetectors are positioned at substantially the same distance to the light source.

[0121] 22. The system according to embodiment 21, wherein a set of at least three photodetectors is arranged around the light source, wherein preferably the distance between each pair of photodetectors in said set is substantially the same. 23. The system according to any one or more of the embodiments 15-22, wherein at least two photodetectors are positioned at different distances to the light source.

[0122] 24. The system according to embodiment 23, wherein the system is configured to apply a weighting factor depending on the distance of the photodetector to the light source, wherein weighting factors decrease for increasing distances.

[0123] 25. The system according to any one or more of the preceding embodiments, wherein the coherent light has a wavelength of 700 - 1000 nm, preferably 850 nm.

[0124] 26. The system according to any one or more of the preceding embodiments, wherein the light source comprises a laser diode, such as a Vertical Cavity Surface Emitting Laser (VCSEL).

[0125] 27. The system according to embodiment 26, wherein the light source comprises a VCSEL with integrated photodetector, VCSEL-IP, configured to generate a VCSEL-IP output signal.

[0126] 28. The system of embodiment 27, wherein the system is further configured to: determine a measure of similarity of the VCSEL-IP output signal as compared to the output signal of the photodetector; and determine a measure of motion as a function of the measure of similarity, with the measure of motion decreasing for increasing similarity.

[0127] 29. The system according to any of the preceding embodiments, comprising a wearable device that comprises the light source and the photodetector.

[0128] 30. A method for determining a measure of blood volume of a blood perfused tissue, the method comprising : controlling a light source to emit coherent light to the blood perfused tissue; obtaining a photodetector output signal from a photodetector receiving a portion of the coherent light scattered by the blood perfused tissue, wherein the photodetector output signal comprises speckle noise; frequency shifting the speckle noise in the photodetector output signal by a predetermined frequency shift, and determining the measure of blood volume based on the output signal with frequency shifted speckle noise.

[0129] 31. The method of embodiment 30, wherein frequency shifting the speckle noise in the photodetector output comprises: controlling an actuator to vibrate, relative to the blood perfused tissue, at least one of the group comprising : the light source; the photodetector; and an optical element arranged in front of the light source of in front of the photodetector.

[0130] 32. The method of embodiment 31, wherein the actuator is controlled to vibrate at least in a direction towards and from the blood perfused tissue.

[0131] 33. The method of embodiment 31 or 32, wherein controlling the actuator to vibrate comprises driving the actuator by a triangular drive signal.

[0132] 34. The method of embodiment 30, wherein frequency shifting the speckle noise in the photodetector output comprises: controlling an actuator to rotate at least one of the group comprising : the light source; the photodetector; and an optical element arranged in front of the light source or in front of the photodetector, for frequency shifting the speckle noise in the output signal of the photodetector, wherein preferably the actuator is controlled to cause rotation at a rotational frequency corresponding to the predetermined frequency shift.

[0133] 35. The method of embodiment 30, wherein frequency shifting the speckle noise in the photodetector output comprises: phase modulating the coherent light at a modulation frequency corresponding to the predetermined frequency shift.

[0134] 36. The method of embodiment 35, wherein the phase modulating comprises controlling the light source and / or a phase modulator.

[0135] 37. The method of embodiment 36, wherein the phase modulator comprises a Pockels cell and / or a liquid crystal.

[0136] 38. The method of embodiment 30, wherein frequency shifting the speckle noise in the photodetector output comprises controlling the light source, preferably by adjusting an emission direction at the predetermined frequency.

[0137] 39. The method of any one or more of the embodiments 30-38, wherein determining the measure of blood volume based on the frequency shifted output signal comprises: obtaining a low-frequency portion of the output signal with frequency shifted speckle noise, wherein frequencies above a low-pass cut-off frequency are attenuated, wherein the predetermined frequency shift is greater than the low-pass cut-off frequency, preferably at least 2-5 times greater than the cut-off frequency; and determining the measure of blood volume based on the low-frequency portion. 40. The method of embodiment 39, wherein the low-pass cut-off frequency is in the range of 25-50 Hz, preferably in the range of 30-40 Hz, and the predetermined frequency shift is at least 150 Hz, preferably at least 250 Hz.

[0138] 41. The method of any one or more of the embodiments 30-40, further comprising determining a measure of blood speed in the blood perfused tissue based on the photodetector output signal with frequency shifted speckle noise.

[0139] 42. The method of any one or more of the embodiments 30-41, wherein the step of frequency shifting is performed intermittently, resulting in frequency-shifted periods and nonfrequency shifted periods, the method further comprising determining a measure of blood speed based on the photodetector output signal obtained during non-frequency shifted periods.

[0140] 43. The method of embodiment 41 or 42, comprising : obtaining a high-frequency portion of the photodetector output signal with the frequency shifted speckle noise or the output signal obtained during non-frequency shifted periods, wherein frequencies below a high-pass cut-off frequency are suppressed; and determining the measure of blood speed based on the high-frequency portion.

[0141] 44. The method according to any one or more of the embodiments 30-43, wherein multiple photodetector output signals are obtained, from multiple photodetectors that each receive a respective portion of the coherent light scattered by the blood perfused tissue and for generating a respective output signal.

[0142] 45. The method according to embodiment 44, wherein determining the measure of blood volume of the blood perfused tissue comprises summing the output signals of the multiple photodetectors.

[0143] 46. The method according to embodiment 43 in combination with embodiment 44 or 45, further comprising : determining a spectral power density of each of the high-frequency portions; determine a sum of the first moments of the spectral power densities or the first moment of the sum of spectral power densities; and determine the measure of blood speed based on the sum of the first moments or the first moments of the sum, respectively.

[0144] 47. The method according to embodiment 46, further comprising determining a zero-th moment of the spectral power density of each of the high frequency portions and use the determined zero-th moments to normalize the sum of the first moments or the first moments of the sum. 48. The method of any of the embodiments 44-47, further comprising : receiving, for each photodetector output signal, a measure of the distance between the photodetector that generated the respective output signal and the light source, applying a weighted sum as a function of the measure of the distance of the respective photodetector to the light source, with decreasing weights for increasing distances.

[0145] 49. A computer program comprising instructions which, when executed by one or more processors, cause the one or more processors to perform the method according to any of the embodiments 30-48.

[0146] 50. A non-transitory computer readable medium storing the computer program of embodiment 49.

[0147] 51. A system for determining a measure of blood volume and / or blood speed of a blood perfused tissue, comprising : a light source configured to emit coherent light to the blood perfused tissue; and a plurality of photodetectors, each configured to receive a respective portion of the coherent light scattered by the blood perfused tissue, and for generating a respective output signal based on the received portion of the coherent light, wherein the system is configured to determine the measure of blood volume and / or blood speed of the blood perfused tissue based on the output signals of the plurality of photodetectors.

[0148] 52. The system according to embodiment 51, wherein determining the measure of blood volume of the blood perfused tissue comprises summing the output signals of the multiple photodetectors.

[0149] 53. The system according to embodiment 51 or 52, wherein the system is configured to: obtain a high-frequency portion of each output signal; determine a spectral power density of the high-frequency portion of each output signal; determine a sum of the first moments of the spectral power densities or the first moment of the sum of the spectral power densities; and determine the measure of blood speed based on the sum of the first moments or the first moments of the sum, respectively.

[0150] 54. The system according to embodiment 53, wherein the system is configured to determine a zero-th moment of the spectral power density of each high-frequency portion and use the determined zero-th moments to normalize the sum of the first moments or the first moments of the sum. 55. The system according to any one or more of the embodiments 51-54, wherein the photodetectors are photodiodes, wherein preferably each photodiode has a surface area of 0.01 - 4 mm2, preferably 0.1 - 1 mm2, more preferably 0.2 - 0.5 mm2.

[0151] 56. The system according to any one or more of embodiments 51-55, wherein the distance between each photodetector and the light source is 0.5 - 8 mm, preferably 1 - 4 mm, more preferably 1.2 - 1.8 mm.

[0152] 57. The system according to any one or more of embodiments 51-56, wherein at least two photodetectors are positioned at substantially the same distance to the light source.

[0153] 58. The system according to embodiment 57, wherein a set of at least three photodetectors is arranged around the light source, wherein preferably the distance between each pair of photodetectors in said set is substantially the same.

[0154] 59. The system according to any one or more of the embodiments 51-58, wherein at least two of the photodetectors are positioned at different distances to the light source.

[0155] 60. The system according to embodiment 59, wherein the system is configured to apply a weighting factor depending on the distance of the photodetector to the light source, wherein weighting factors decrease for increasing distances.

[0156] 61. The system according to any one or more of the embodiments 51-60, wherein the coherent light has a wavelength of 700 - 1000 nm, preferably 850 nm.

[0157] 62. The system according to any one or more of the embodiments 51-61, wherein the light source comprises a laser diode, such as a Vertical Cavity Surface Emitting Laser (VCSEL).

[0158] 63. The system according to embodiment 62, wherein the light source comprises a VCSEL with integrated photodetector, VCSEL-IP, configured to generate a VCSEL-IP output signal.

[0159] 64. The system according to any one or more of the embodiments 51-63, comprising a wearable device that comprises the light source and the plurality of photodetectors.

[0160] 65. A method for determining a measure of blood volume and / or blood speed of a blood perfused tissue, comprising : controlling a light source to emit coherent light to the blood perfused tissue; obtaining a plurality of photodetector output signals from a plurality of photodetectors, each configured to receive a respective portion of the coherent light scattered by the blood perfused tissue, and to generate a respective output signal based on the received portion of the coherent light; and determining the measure of blood volume and / or blood speed of the blood perfused tissue based on the output signals of the plurality of photodetectors, preferably using the system of one or more of the embodiments 49-62. 66. A computer program comprising instructions which, when executed by one or more processors, cause the one or more processors to perform the method according to embodiment 65.

[0161] 67. A non-transitory computer readable medium storing the computer program of embodiment 66.

[0162] 68. A system for determining a measure of blood volume and / or blood speed of a blood perfused tissue, comprising : a Vertical Cavity Self Emitting Laser with integrated photodetector, VCSEL-IP, configured to emit coherent light to the blood perfused tissue, wherein the integrated photodetector is configured to generate a VCSEL-IP output signal; and a further photodetector configured to receive a portion of the coherent light scattered by the blood perfused tissue, and for generating a further output signal based on the received portion of the coherent light, wherein the system is configured to determine the measure of blood volume and / or blood speed of the blood perfused tissue based on the VCSEL-IP output signal and / or the further output signal.

[0163] 69. The system of embodiment 68, further configured to determine a measure of motion based on at least the VCSEL-IP output signal.

[0164] 70. The system of embodiment 69, wherein the system is further configured to: determine a measure of similarity of the VCSEL-IP output signal as compared to the further output signal; and determine the measure of motion as a function of the measure of similarity, with the measure of motion decreasing for increasing similarity.

[0165] 71. The system according to any one or more of the embodiments 68-70, comprising a wearable device that comprises the light source and the further photodetector.

[0166] 72. A method for determining a measure of blood volume and / or blood speed of a blood perfused tissue, comprising : controlling a Vertical Cavity Self Emitting Laser with integrated photodetector, VCSEL-IP, to emit coherent light to the blood perfused tissue; obtaining from the integrated photodetector a VCSEL-IP output signal; obtaining a further output signal from a further photodetector configured to receive a portion of the coherent light scattered by the blood perfused tissue; and determining the measure of blood volume and / or blood speed of the blood perfused tissue based on the VCSEL-IP output signal and / or the further output signal. 73. The method of embodiment 72, further comprising determining a measure of motion based on at least the VCSEL-IP output signal.

[0167] 74. The method of embodiment 73, comprising : determining a measure of similarity of the VCSEL-IP output signal as compared to the further output signal; and determining the measure of motion as a function of the measure of similarity, with the measure of motion decreasing for increasing similarity.

[0168] 75. A computer program comprising instructions which, when executed by one or more processors, cause the one or more processors to perform the method according to any one or more of the embodiments 72-74.

[0169] 76. A non-transitory computer readable medium storing the computer program of embodiment 75.

Claims

CLAIMS1. A system for determining a measure of blood volume of a blood perfused tissue, comprising : a light source configured to emit coherent light to the blood perfused tissue; and a photodetector configured to receive a portion of the coherent light scattered by the blood perfused tissue, and for generating an output signal based on the received portion of the coherent light, wherein the output signal comprises speckle noise, wherein the system is arranged to frequency shift the speckle noise in the output signal of the photodetector by a predetermined frequency shift, and the system is configured to determine the measure of blood volume of the blood perfused tissue based on the output signal with frequency shifted speckle noise.

2. The system according to claim 1, comprising an actuator configured to vibrate, relative to the blood perfused tissue, at least one of the group comprising: the light source; the photodetector; an optical element arranged in front of the light source or in front of the photodetector, wherein the actuator is configured to vibrate at a vibration frequency for frequency shifting the speckle noise in the output signal of the photodetector.

3. The system of claim 2, wherein the actuator is driven by a triangular drive signal.

4. The system of claim 1, wherein the system is arranged to phase modulate the coherent light at a modulation frequency for frequency shifting the speckle noise in the output signal of the photodetector, wherein the system preferably comprises a phase modulator and / or a controller to phase modulate the light source.

5. The system of claim 1, wherein the system is arranged to control the light source for frequency shifting the speckle noise in the output signal of the photodetector, preferably by adjusting an emission direction at the predetermined frequency.

6. The system of any one or more of the claims 1-5, wherein the system is configured to: obtain a low-frequency portion of the output signal with frequency shifted speckle noise, wherein frequencies above a low-pass cut-off frequency are attenuated, wherein the predetermined frequency shift is greater than the low-pass cut-off frequency, preferably at least 2-5 times greater than the low-pass cut-off frequency; and determine the measure of blood volume based on the low-frequency portion.

7. The system of claim 6, wherein the low-pass cut-off frequency is in the range of 25-50 Hz, preferably in the range of 30-40 Hz, and the predetermined frequency shift is at least 150 Hz, preferably at least 250 Hz.

8. The system of any one or more of the claims 1-7, wherein the system is further configured to determine a measure of blood speed in the blood perfused tissue based on the output signal with frequency shifted speckle noise.

9. The system of any one or more of the claims 1-7, wherein the system is arranged to perform the frequency shift intermittently, resulting in frequency-shifted periods and nonfrequency shifted periods, and the system is further configured to determine a measure of blood speed based on the output signal obtained during non-frequency shifted periods.

10. The system of claim 8 or 9, wherein the system is configured to: obtain a high-frequency portion of the output signal with the frequency shifted noise or the output signal obtained during non-frequency shifted periods, wherein frequencies below a high-pass cut-off frequency are attenuated; and determine the measure of blood speed based on the high-frequency portion.

11. The system according to any one or more of the preceding claims, wherein the system comprises multiple photodetectors, each arranged for receiving a respective portion of the coherent light scattered by the blood perfused tissue and for generating a respective output signal.

12. The system according to claim 11, wherein determining the measure of blood volume of the blood perfused tissue comprises summing the output signals of the multiple photodetectors.

13. The system according to claim 10 in combination with claim 11 or 12, wherein the system is configured to: determine a spectral power density of the high-frequency portion of each output signal; determine a sum of the first moments of the spectral power densities or the first moment of the sum of the spectral power densities; and determine the measure of blood speed based on the sum of the first moments or the first moments of the sum, respectively.

14. The system according to any one or more of the preceding claims, comprising multiple photodetectors, each arranged for receiving a respective portion of the coherent light scattered by the blood perfused tissue and for generating a respective output signal, wherein a set of at least three photodetectors is arranged around the light source, wherein preferably the distance between each pair of photodetectors within said set is substantially the same.

15. The system according to any one or more of the preceding claims, wherein the light source comprises a Vertical Cavity Surface Emitting Laser, VCSEL, with integrated photodetector, VCSEL-IP, configured to generate a VCSEL-IP output signal, wherein the system is further configured to: determine a measure of similarity of the VCSEL-IP output signal as compared to the output signal of the photodetector; and determine a measure of motion as a function of the measure of similarity, with the measure of motion decreasing for increasing similarity.

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