System and method for performing laser doppler flowmetry

Abstract

The invention relates to a system and method for performing a laser Doppler flowmetry, LDF, measurement. The system comprises a coherent light source, a photodetector and a processor. The photodetector generates an output signal. The processor computes a series of power spectral densities, PSDs, of the photodetector output signal. An LDF signal is computed, comprising computing a moment of each of the PSDs, over a selected frequency range (Fselected). The LDF signal is output. The selected frequency range is determined based on a measure of the amount of physiological information (Qj). A frequency range is selected for which the measure fulfills a predetermined criterion.

Description

[0001]SYSTEM AND METHOD FOR PERFORMING LASER DOPPLER FLOWMETRYField of the inventionThe present invention relates to a system and method for performing a laser Dopplerflowmetry measurement of a blood perfused tissue. Background of the inventionLaser Doppler velocimetry (LDV) is a technique to measure the velocity of a fluid. When LDVis 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 bloodflow signal obtained by LDV is in fact a scalar and contains no direction information. The bloodflow signal is therefore related to speed of the blood. For this reason, the present disclosure uses the term LDF. Description of the invention A problem with known systems for LDF measurements is that their accuracy depends on various uncontrollable factors. The accuracy for example depends on the subject underinvestigation. Even LDF measurements of the same subject vary depending on e.g. thelocation on the subject’s body. The accuracy of the measurement further changes for different average blood velocity. An object of the invention is to overcome the drawbacks of known devices, or at least providean alternative system. In particular, the invention aims to provide a more accurate system forLDF measurement of a blood perfused tissue.This aim is achieved by the systems and methods according to the present disclosure.According to the invention, the system comprises a light source configured to emit coherentlight, a photodetector and one or more processors. The coherent light source, e.g. a laser, isconfigured 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. Thephotodetector is configured to generate a photodetector output signal in response to receivingsaid portion. The one or more processors are configured to compute a power spectral density (Si(f)) of the photodetector output signal for a series of time intervals (ti), thereby obtaining aseries of power spectral densities (PSDs). The one or more processors are configured tocompute an LDF signal (<v>). Computing the LDF signal (<v>) comprises computing amoment (Mi) of each of the PSDs (Si(f)). The moments (Mi) are computed over a selected frequency range (Fselected). The one or more processors are configured to output the LDF signal (<v>). Computing the PSD Si(f) of the photodetector output signal for a series of time intervals (ti)for example comprises computing a short time Fourier transform (STFT). In particular,computing the PSD may comprise computing the magnitude squared of the STFT. The inventor found that, looking at the PSD of the photodetector signal, the frequency rangein which the physiological information is found varies. Therefore, the inventor proposes tocompute a measure of the amount of physiological information in the signal and then use said measure to select an adequate frequency range for further computations. For example, an optimum frequency range is selected. In some embodiments, a first series of power spectral densities is used to determine theselected frequency range, which is then used for computing LDF signal(s) from a second,subsequent, series of power spectral densities. In other embodiments, a first series of power spectral densities is used to determine the selected frequency range, which is then used forcomputing LDF signal(s) from the same first series of power spectral densities.The system may comprise multiple photodetectors. Thus, where the present application refers to “photodetector”, alternatively multiple photodetectors are used according to the invention. In a first example, the output signals of the photodetectors are combined, e.g. by summing oraveraging, and the power spectral density Si(f) is computed from the result. In a secondexample, each photodetector signal is processed independently of the other, and the resultingLDF signals <v> are combined, e.g. by summing or averaging. In these example, themeasure of the amount of physiological information can be computed from the combined signals, or the individual photodetector signals, or both. The photodetector is for example a photodiode (generating a 1D signal). In another example,the photodetector is a photodiode array, a photodiode matrix or a camera (generating a 2Dsignal). For example, the camera may be a CCD camera. When a 2D photodetector is used, the 2D signal is preferably converted into one or more 1D photodetector output signals. For example, each pixel of a camera is processed as a separate photodetector output signal. In another example, an average of the pixel values of a group of pixels of the camera is computed and used as photodetector output signal. For example, an average over all pixels of the camera is computed to obtain a single photodetector output signal for use in subsequent computations. Instead of an average, a median, interquartile mean or other central tendency may be used.According to a first aspect, the selected frequency range (Fselected) is determined based on ameasure of amount of physiological information in LDF signals computed for a number of different frequency ranges. In particular, according to the first aspect the one or more processors are configured to perform at least the following steps to determine the selected frequency range Fselected. A power spectral density (Si(f)) is computed of the photodetector output signal for a series of time intervals (ti), thereby obtaining a series of power spectral densities. For each respectivefrequency range Fj out of P different frequency ranges (Fj=1 to P), a measure of the amount ofphysiological information (Qj) is computed for a LDF signal <vFj> that is computed bycomputing moments (Mi) of the PSDs (Si(f)) over the respective frequency range (Fj). Theselected frequency range (Fselected) is then determined as the frequency range (Fj) for whichthe computed measure of amount of physiological information (Qj) fulfills a predetermined criterion. The one or more processors are further configured to output an LDF signal computed using the selected frequency range. For example, this includes an LDF signal that was computed during determination of the selected frequency, i.e. the LDF signal <VFj> with Fj = Fselected. Alternatively or additionally, the selected frequency range is used to compute LDF signal(s) from subsequent photodetector output signals. According to a second aspect, the selected frequency range (Fselected) is determined based on a measure of amount of physiological information in spectra of the photodetector output signal. In particular, determining the selected frequency range comprises computing a spectrum of the photodetector output signal for a series of time intervals (ti), thereby obtaining a series ofspectra. A measure of the amount of physiological information (Qf) in said spectra is, for anumber of different frequencies, is then computed. The selected frequency range (Fselected) isthen determined as the range of frequencies (f) for which the computed measure of amountof physiological information (Qf) fulfills a predetermined criterion. An LDF signal is computedusing the selected frequency range, and the LDF signal is output.Preferably, the spectra are PSDs of the photodetector output signal. Alternatively, the spectra are amplitude spectra of the photodetector output signal. According to both the first aspect and the second aspect, a frequency range is selected based on a measure of amount of physiological information. The selected frequency range ranges from a lower frequency bound fmin to an upper frequency bound fmax. Selecting the frequency range for example comprises selecting the upper frequency bound fmax, or selecting the lower frequency bound fmin, or selecting both the upper and lower frequency bound. In a current preferred embodiment, the lower frequency bound fminis predetermined (fixed) and selecting the frequency range comprises selecting the upper frequency bound fmax.For example, the predetermined criterion comprises a threshold criterion. For example, if themeasure Qi increases for increasing amount of physiological information (e.g. positive correlation), the threshold criterion comprises determining whether the measure exceeds a predetermined threshold. In another example, if the measure decreases for increasing amount of physiological information (e.g. negative correlation), the threshold criterion comprises determining whether the measure is below a predetermined threshold. In another example, the predetermined criterion is indicative of a maximum amount of physiological information. For example, the frequency range corresponding to the maximum value of Qiis selected (in case Qiincreases for increasing amount of physiological information) or the frequency corresponding to the minimum value of Qiis selected (in case Qidecreases for increasing amount of physiological information).Optionally, the predetermined criterion additionally includes a predetermined lower limit forthe measure Qi. For example, if Qi is below the predetermined lower limit, the signal does not have sufficient quality to perform the LDF measurement. In that case, the method may fall back to the previously selected frequency range or to a predetermined default frequency range.In some embodiments, the predetermined criterion comprises more than one criterion. Forexample, the method comprises a step of determining whether the bandwidth of the selected frequency range is below a minimum bandwidth, and, if it is determined that the bandwidth is below the minimum bandwidth, increasing the bandwidth to the minimum bandwidth, e.g. by increasing the upper frequency bound fmax and / or lowering the lower frequency bound fmin. For example, the minimum bandwidth is 500 Hz. Preferred embodiments are defined in the dependent claims and in the following paragraphs. Embodiments of the first aspect In an embodiment of the first aspect, the measure of the amount of physiological information (Qi) comprises a ratio between energy in a predetermined low frequency range of the respective LDF signal <vFj> and energy in a predetermined high frequency range respective LDF signal <vFj>. In an embodiment of the first aspect, the measure of the amount of physiological information (Qi) comprises Shannon entropy of the respective LDF signal <vFj>. In an embodiment of the first aspect, the measure of the amount of physiological information (Qi) comprises a difference between the maximum and minimum points of the respective LDFsignal <vFj>. In other words, the measure comprises a mean AC envelope.In an embodiment of the first aspect, computing the measure of the amount of physiologicalinformation comprises determining a heart rate from the respective LDF signal <vFj>, andobtaining a reference heart rate, wherein the measure of the amount of physiological information (Qi) comprises the difference between the heart rate determined from the respective LDF signal and the reference heart rate . Preferably, the reference heart rate is obtained from a heart rate sensor. Alternatively, the heart rate is obtained from the output signal of the photodetector(s). For example, a heart rate is extracted from an LDF signal computed from the photodetector signal(s) without frequency optimization, e.g. an LDF signal computed over a broad frequency range (broader than all of the predetermined frequency ranges). This heart rate is then used as reference heart rate. In another example, the one or more processors maintain a running average of theheart rate of the LDF signals, e.g. over a period of 10 – 30 s, and the running average is usedas a reference heart rate In this case, the measure Qi increases for decreasing physiological information: an increasing difference between the heart rate computed from the LDF signal and the heart rate obtained from the sensor is indicative of decreasing physiological information. In an embodiment of the first aspect, computing the measure of the amount of physiological information in the respective LDF signal <vFj> comprises: subdividing the LDF signal <vFj> into individual LDF pulses. In a further embodiment, computing the measure of the amount of physiological information comprises computing the number of individual LDF pulses within a predetermined time span. For example, the number of individual LDF pulses is compared to an expected number of LDF pulses. The expected number of LDF pulses is computed as a function of heart rate, e.g. as obtained from a heart rate sensor. For example, the measure of amount of physiological information is greater when the computed number of pulses matches the expected number,and smaller when the computed number of pulses does not match the expected numberpulses. For example, computing the measure of the amount of physiological information comprises computing a ratio between the measured number of LDF pulses and the expected number of LDF pulses. For example, the one or more processors compute the minimum of Nmeasured / Nexpectedand Nexpected / Nmeasured, wherein Nexpected is the expected number of pulses (e.g. based on heart rate and Nmeasuredis the number of pulses measured in the LDF 1 represents an exact match between measured and expected number of pulses. The lower the calculated value is, the greater the mismatch between the measure and expected number. In a further embodiment of the first aspect, a central tendency of the individual LDF pulses is determined to obtain an ensemble LDF pulse, and the measure of the amount of physiological information is computed from the ensemble LDF pulse. For example, the central tendency isan average, i.e. the ensemble LDF pulse is obtained by averaging the individual LDF pulses.In a further embodiment, computing the measure further comprises determining the averageamplitude of the first N harmonics of the individual LDF pulses, wherein N is an integergreater than 1. In a first example, the first N harmonics are calculated from the ensemble LDFpulse. In a second example, the average of the first N harmonics is calculated for eachindividual LDF pulse, and then the averages are averaged. Preferably, obtaining the ensemble LDF pulse comprises time aligning the individual LDF pulses. The ensemble LDF pulse more accurately represents the central tendency (e.g.average) of the individual LDF pulses when the ensemble is computed on time aligned LDFpulses. For example, when computing the ensemble as an average of the individual pulses, an incorrect time-alignment will affect the shape of the ensemble such that it no longer represents a true average shape of the individual LDF pulses. Moreover, incorrect time alignment increases the amount of noise in the signal and therefore it is more difficult todetect features in the resulting LDF signal. Incorrect time alignment also increases the noisefloor in the frequency domain and therefore fewer number of harmonics can be detected above the noise floor. Time alignment may for example be achieved using an external trigger, e.g. a trigger obtained from an ECG sensor and / or a PPG sensor. In another example, a gradient of the individual LDF pulses is calculated, and the time point where the gradient has its maximum is identified as a time point for alignment. All LDF pulses are then time shifted such that said time points for alignment are aligned. Embodiments of the second aspectIn an embodiment of the second aspect, determining the selected frequency range (Fselected)comprises obtaining one or more trigger signals indicative of a timing of individual LDF pulses.Using the one or more trigger signals, a spectrogram X(f,t) for each individual isdetermined from the spectra (Si(f)). In this embodiment, the step of computing the measure of the amount of physiological information (Qf) comprises, computing the measure (Qf) from the spectrograms. The one or more trigger signals are indicative of a timing of LDF pulses. For example, the oneor more triggers is / are indicative of an onset or rising edge of an LDF pulse or of a peak of theLDF pulse. The trigger signal is for example obtained from a sensor, such as an ECG (electrocardiography sensor) or PPG (photoplethysmography) sensor. As LDF, ECG and PPG signals follow the cadence of cardiac cycle, ECG and / or PPG signals can be used to obtain a trigger signal for identifying individual LDF pulses. This holds true even when some offset exists between the onset of an ECG or PPG signal and LDF pulses. In another example, the trigger signals are obtained from an analysis of the photodetector output signal itself. For example, an LDF signal is determined from the photodetector output signal (as described above), and pulse detection is performed on the LDF signal, e.g. by detecting a rising edge, apeak or a threshold crossing. The detection of the LDF pulse in the LDF signal is then used togenerate the trigger signal. In a further embodiment, an ensemble spectrogram (XE(f,t)) is determined from the spectrograms of individual LDF pulses. The ensemble spectrogram comprises a central tendency of the spectrogram of the individual LDF pulses. In this embodiment, the step ofcomputing the measure of the amount of physiological information (Qf) comprises, computingthe measure (Qf) from the ensemble spectrogram. For example, the ensemble spectrogramXE(f,t) comprises an average, median or interquartile mean of the spectrogram of theindividual LDF pulses. The ensemble spectrogram may be considered as a “template” for the spectrogram of theindividual LDF pulses. By basing the measure of the amount of physiological information onthe ensemble spectrogram rather than on individual spectrograms, the computation becomesless prone to outliers (e.g. distorted signals). In an embodiment , the measure of the amount of physiological information comprises a dispersion d(f) of the spectra, as a function of frequency (f). Preferably, the dispersion d(f) is normalized. For example, the dispersion d(f) comprises astandard deviation or variance. Preferably, the dispersion d(f) comprises a normalizedstandard deviation or normalized variance. For example, the standard deviation is normalizedover the mean for the respective frequency. Preferably, the predetermined criterion comprises a threshold criterion for the dispersion d(f). For example, the selected frequency range is determined as the frequency range over which the normalized standard deviation is above or below the threshold. The threshold is for example set as the median or mean of the dispersion d(f). In a further embodiment, the measure of the amount of physiological information comprises a dispersion d(f) of the ensemble spectrogram XE(f,t) as a function of frequency f. Alternatively, the measure of the amount of physiological information comprises a dispersion d(f) of thespectrogram X(f,t) of individual LDF pulses. In these two embodiments, the predeterminedcriterion preferably comprises a threshold criterion for the dispersion d(f).Embodiments of the first or second aspectIn an embodiment of the first or second aspect, moments are computed as a weighted moment. For example, the moments are computed as a weighted first moment, a weighted second moment, or a combination thereof. In an embodiment of the first or second aspect, a system comprises a wearable device comprising the light source and the photodetector. In a first example, the wearable device further comprises the one or more processors for performing the computations. In a second example, the wearable device comprises a communication module for sending the photodetector output signal to the one or more processors, e.g. over a wireless connection. In both the first and second aspect, the selected frequency range may be determined basedon a first series of spectra (e.g. PSDs), after which the determined frequency range is usedfor computing the LDF signal from a second, subsequent, series of spectra (e.g. PSDs).Alternatively, the selected frequency range is determined based on a first series of spectra (e.g. PSDs), after which the determined frequency range is used for computing the LDF signalfrom the same first series of spectra (e.g. PSDs).The invention further relates to a computer program. The computer program comprisesinstructions which, when executed by a computing device, execute the method according toany of the embodiments described herein. For example, the computer program is executableby a processor of a wearable device. The present disclosure further relates to a non-transitorycomputer-readable medium storing said computer program. The same technical effects as described above in relation to the system apply to the method of the invention. Moreover, any features of the system described above can similarly beapplied in the method. Preferably, the method is performed using the system of any of theembodiments of this disclosure. Brief description of the drawings In the following, example embodiments will be described with reference to the drawings, wherein:Figure 1 shows a cross section of a wearable device or system for performing a LDFmeasurement of a blood perfused tissue;Figure 2 shows a schematic drawing of the system of Figure 1;Figure 3 is a flow diagram of a general method for determining an LDF signal from aphotodetector output, according to embodiments of the invention;Figure 4 is flow diagram of a method for selecting a frequency range for determining the LDF signal, according to an embodiment of the first aspect of the invention;Figure 5 is flow diagram of exemplary sub-steps for step S404 of figure 4;Figure 6 is flow diagram of a method for selecting a frequency range for determining the LDFsignal, according to another embodiment of the first aspect of the invention;Figure 7 is flow diagram of a method for selecting a frequency range for determining the LDFsignal, according to an embodiment of the second aspect of the invention;Figure 8 shows an example of an ensemble average spectrogram of LDF pulses;Figure 9 shows a normalized standard deviation of the spectrogram of Figure 8 (plot on a logarithmic scale); andFigure 10 schematically illustrates an exemplary computing device for implementing any ofthe method of the present disclosure. Detailed description of the drawings 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 bloodperfused tissue 30. The device 10 comprises a laser 12 (e.g. a laser diode such as a VCSEL).The coherent light 122 of the laser 12 exposes and penetrates the skin 300 and other parts of tissue 30 at exposed tissue region 32. Discontinuities of optical properties in the tissue 30 can scatter the laser light 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. The Doppler shifting isrelated to a blood speed and can thus be used to derive a measure of blood speed (e.g. LDF).The device 10 comprises a photodetector 16 arranged to receive scattered light from thetissue 30 and to generate a corresponding output signal 164. The drawing illustrates thephotodetector 16 receiving scattered light 162 from the moving blood cells 34. Additionally, the photodetector 16 receives scattered light from stationary discontinuities (not illustrated). The light from moving discontinuities 34 and stationary discontinuities interferes, resulting in a measurable Doppler shift. The device 10 further comprises a processor 18 for generating an LDF signal based on the photodetector output 164. The processor 18 is not shown in the cross section of Figure 1.Reference is made to figure 2, that shows a schematic drawing of the laser 12, photodetector16 and processor 18. The lines in figure 2 indicate the functional connection between the processor 18 and the laser 12, and between the processor 18 and the photodetector 16. Theprocessor 18 is configured to control the laser 12 to emit laser light 122 onto the skin 300.The processor 18 is further configured to process the output 164 of the photodetector 16. In the illustrated examples, the processor 18 is a digital processor and the photodetector output 164 is a digital signal. The device 10 may include an ADC to convert analogue output of the photodetector 16 into the digitized photodetector output 164. Alternatively, processor 18 may comprise an analogue processing circuit for operating on an analogue photodetector signal. The processing of the photodetector output 164 to produce an LDF signal is illustrated in theflow diagrams of figures 3-6. Figure 3 describes the general method for producing an LDFsignal from photodetector output 164, whereas Figures 4-6 describes determining a suitablefrequency range for performing the method of figure 3.In step S400 (figure 3), a power spectral density Si(f) is computed for consecutive timeintervals (ti, with i = 1, 2, …, ψ) of the photodetector output 164 to obtain a series of powerspectral densities. This is done by computing a short-time Fourier transform (STFT), Ω(ti, f), of the photodetector signal 164 and computing the power spectral densities Si(f) as thesquare of the magnitude of the STFT: Si(f) | Ω(ti,f)|2.The time interval is predetermined and preferably smaller than a typical duration of an LDFpulse. In the present examples, the time intervals Tido not overlap. Alternatively, the timeintervals Ti may overlap. The time interval determines the sampling rate (or vice versa). Thesampling rate is set to at least 100 Hz. With a sampling rate of 100 Hz, a frequency response up to 50 Hz is obtained (according to the Nyquist criterion). With a heart rate of 180 bpm, up to 17 harmonics can be detected using this sampling rate. This is considered sufficient. Asampling rate of at least 100 Hz corresponds to a time interval of 10ms or shorter. Forexample, the predetermined time interval is 1 ms – 10ms. In some embodiments, highersampling rates are used, e.g. 200 Hz, 400 Hz or even higher, corresponding to predetermined time intervals of 5 ms, 2.5 ms or even shorter.Step S402 then computes a moment M(i) of each of the power spectral densities Si(f)computed in step S400. The Nth moment MN(i) of power spectral density Si(f) is computed as:^^^^^^(^) = ^ ^^ ⋅ ^^(^)^^^^(equation 1) where:^^(^) is the Nth moment of the spectral density Si(f) at time ti,^^^^and ^^^^are the lower and upper bounds of the frequency range over which the moment is to be calculated, and^^(^) is the power spectral density at time ti.The magnitude of the first moment M1(i) is proportional to the average Doppler shift of thelight received by the photodetector 16 and proportional to the intensity of the light. The firstmoment 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. ^(^) =^^(^) ^^(^) (equation 2) wherein:^(^) is the average Doppler shift at time ^i,^^(^) is the average spectral density at time ^i.Optionally, a weighted Nthmoment is computed, wherein the summation includes a frequency- dependent weighting factor w(f): ^^^^ (equation 3)The weighting factors w(f) may for example be chosen to provide less weight to thefrequencies near fmin and near fmax than to more central frequencies. For example, the weighting factor is based on a triweight function or a Gaussian function. Optionally, the weighting factor w(f) is set to zero for a small subset of frequencies within thefrequency range (fmin, fmax). This effectively excludes these frequencies from the computation.The number of frequencies excluded is small, e.g. less than 20% of the frequency range. Inother words, most of the weighting factors are set to non-zero values, preferably at least 80%of the weighting factors. When computing a weighted moment, preferably a predetermined set of weighting factors wj(f) (j = 1 to P) is used for each of the predetermined frequency ranges Fj(j = 1 to P). Likewise, a weighted and normalized Nthmoment can be computed. Preferably, a weightedand normalized 1st moment is calculated as above, wherein both M1(i) and M0(i) are computedusing a weighting factor w(f) or wj(f). Optionally, more than one moment is computed, e.g. afirst moment (optionally normalized and / or weighted) and a second moment (optionally normalized and / or weighted). As illustrated in Figure 3, step S402 receives the selected frequency range Fselectedas input. In this example, Fselectedincludes the variable fmax, while fminis fixed to 500 Hz. The inventor found that the quality of the LDF signal is highly dependent on the selected frequency range. Selecting a frequency range that results in a good quality LDF signal is an object of the present invention. Different methods for determining the frequency range Fselected will be described below with reference to figures 4 and further.The end result of step S402 is a series of average Doppler shifts <vi> = M1(i) / M0(i) for times ti(t1 to tψ). The series is denoted in figure 3 as the vector <v> (<v> = <v1>, <v2>, … <vψ >),and represents the LDF signal.Figure 4 shows a first embodiment of a method for performing a LDF measurement thatincludes steps for selecting a frequency range Fselected. Step 400 is the same as in Figure 3 and computes, from the photodetector output 164, power spectral densities Si(f) for consecutive time intervals (ti, with i = 1, 2, …, ψ). Step S401 initializes a frequency range variable Fj for aloop L1. Loop L1 loops through a predefines set of P frequency ranges (i.e. Fj with j = 1 to P).For example, the frequency ranges Fj comprise P predefined frequency ranges that span 500Hz each. In other examples, the frequency ranges Fj comprise P predefined frequency rangeswith different bandwidths, e.g. frequency ranges with the same minimum frequency fmin butdifferent maximum frequency fmax. In step S401, j is set to 1 and then the method continuesto loop L1. Within loop L1, step S402 is executed in the same manner as described with reference to figure 3. In this example, step S402 computes a normalized first moment of each of the spectral densities Si(f) computed in step S400. The moments computation is executed for the current frequency range Fj. The result of S402 is a series of average Doppler shifts <vFj,i> =M1(i) / M0(i) for times ti (t1 to tψ) for the frequency range Fj. The series is denoted in figure 4 asthe vector <vFj> (<vFj> = <vFj,1>, <vFj,2>, … <vFj,ψ>), and represents an LDF signal. In step S404, a measure of the amount of physiological information is computed for the specific frequency range Fj. This measure is denoted as Qj and may also be referred to as “figure of merit” or “FOM”. More details of the computation of Qjwill be described below. Preferably, Qjis a scalar or a vector with a predetermined number of vector elements, to allow comparison between different measures Qjcomputed for different Fj.Step S406 checks whether the loop L1 has been performed for all frequency ranges Fj.Particularly, step S406 checks whether the condition j = P is true. If the condition is false, themethod increments j by 1 in step S408, and returns to step S402. If the condition is true, allfrequency ranges Fjhave been processed, and the method moves to step S410.When arriving at step S410, the method has computed – and stored – the measure Qj for all Pfrequency ranges Fj. Step S410 then selects one of the frequency ranges Fj based on thecomputed measures Qj. In particular, steps S410 determines whether Qj fulfils apredetermined criterion. For example, step S410 determines the maximum or minimum of thecomputed Qj values. The selected frequency is denoted as Fselected. Optionally, Fselected is output.Step S410 then outputs the LDF signal <vFselected> that corresponds to the selected frequency Fselected. Preferably, in the loop L1, the vectors <vFj> are stored, such that in step S410, the <vFselected> does not have to be recalculated. Alternatively or additionally, the selected frequency Fselected thus is used for computing an LDF signal from a subsequent series of PSDs. For example, Fselectedis used in the LDF computationfor consecutive time intervals (ti, with i = ψ + 1, ψ + 2, …).In the following, different options for the computation of the measure of the amount ofphysiological information Qj are described. Qj may be based on a single metric of the amountof physiological information, but preferably Qj is based on a combination of different metricsfor the amount of physiological information. For example, a number of different metrics iscombined in a vector Qj, or a weighted average of the different metrics is computed to arriveat a single scalar Qj.A first example of computing a metric for Qj is illustrated in Figure 5. In this example, themetric comprises a ratio between low frequency energy and high frequency energy of the LDF signal <vFj> for frequency range Fj. The ratio is calculated (i.e. in step S404 of figure 4), by inputting <vFj> into processing steps S412 and S414. Step S412 applies a low pass (LP) filter to <vFj> and step S414 applies a high pass (HP) filter to <vFj>. The cutoff frequencies of the filters are preferably chosen to be the same. Cutoff frequencies can be typically above 5Hz (such that, for a heart rate of 60bpm, the first five harmonics of the signal are included). Optionally, the LDF signal <vFj> is pre-processed by applying a further high pass filter with acut-off frequency around 0.5Hz, to suppress low frequency movement artefacts. In stepsS416 and S418, the energy of the low and high pass filtered signal is computed. The energyof a signal is defined as the integral of the squared magnitude of the signal. For a discretesignal this boils down to the summation of the squared magnitudes of the signal values.Step 420 computes a ratio RFj between the two energies. The ratio gets higher for the signalwith more physiological content, since the physiological information typically resides in a lower part of the frequency spectrum. In a second example of a metric for Qj, the spectral entropy EFj of the signal <vFj> is computed. The spectral entropy is a measure of the signal’s spectral power distribution. The spectral entropy treats the signal’s normalized power distribution in the frequency domain asa probability distribution and calculates the Shannon entropy of the distribution. The higherthe spectral entropy, the more random the signal is. Therefore, this metric gets lower for signals having a larger amount of physiological information.In a third example of a metric for Qj, a difference ACFj between maximum and minimumpoints of the LDF signal <vFj> is computed. This is also referred to as “mean AC envelope”.This metric gets higher when more physiological content is present in the signal. For example,ACFj is computed as ACFj = max(<vFj>) – min(<vFj>).In a fourth example of computing a metric for Qj, a heart rate is estimated from therespective LDF signal <vFj>. For example, the estimation computes the number of peaks inthe LDF signal per unit time. In addition, a heart rate is obtained from a heart rate sensor. Then, a difference or ratio between the estimated heart rate HRestimated (determined from <vFj>) and the heart rate HRmeasured obtained from the heart rate sensor. This results in ametric ΔFj. For example = - HRmeasured or ΔFj = HRestimated / HRmeasured is e.g. bycomputing a weighted combination. For example, to compute a measure Qj that increases foran increasing amount of physiological information, the metrics described above may be combined according to: Qj = w1 RFj + w2 / EFj + w3 ACFj + w4 ΔFj,wherein wiare predetermined weight factors. In this computation, the inverse of EFjis used, as this metric decreases for increasing amount of physiological information, whereas the other metrics are multiplied by the weighting factor as they increase for increasing physiological content. Figure 6 illustrates computing Qj based on an ensemble LDF pulse. The ensemble LDF pulse iscomputed based on a central tendency of individual LDF pulses, e.g. as an average pulse.Steps S400, S401, S402 are identical to those described in relation to Figure 4. In step S422, the LDF signal <vFj> for the current frequency range Fj is split into individual LDF pulses. Step S424 computes an average pulse based on the individual pulses computed in step S422. Step S426 determines the measure Qjbased on the ensemble LDF pulse.As in figure 4, step S406 check whether the loop L2 has been performed for all frequencyranges Fj. Particularly, step S406 checks whether the condition j = P is true. If the condition isfalse, the method increments j by 1 in step S408, and returns to step S402. If the condition istrue, all frequency ranges Fjhave been processed, and the method moves to step S410. As previously described in relation to figure 4, step S410 selects a frequency range based on thecomputed measures Qj, and the LDF signal <VFselected> corresponding to the selectedfrequency range is output. Optionally, also Fselected is output. For example, the computation of Qjincludes computing the metrics described above (energyratio RFj, entropy EFj, AC envelope ACFj, heart rate difference ΔFj) for the ensemble averagecomputed in steps S424. Another example of a metric is the average amplitude of the first Nharmonics of the ensemble LDF pulse, wherein N is an integer greater than 1. Preferably thefirst three harmonics are used for the computation. In a further example, computation of Qj comprises computing an average of the first N harmonics divided by an estimate noise floor. The noise floor is estimated by computing a median of the part of the spectrum in between the detected harmonics.Note that the metrics computed in Figures 4 and 6 may be combined. Particularly, themeasure Qj may combine metrics computed from the signals <vFj> and metrics computedfrom the ensemble average derived from the signals <vFj>. Figure 7 illustrates an embodiment of a method for selecting the frequency Fselected, directlyfrom the power spectral densities Si(f). Notably, the method of figure 7 does not require aloop for calculating moments for a number of different frequency ranges. Step S400 is identical to step S400 of figures 4 and 6: power spectral densities Si(f) are computed from the photodetector signal 164, for different discrete time point ti. In step S428the consecutive PSDs Si(f) are processed to generate an ensemble average spectrogram, i.e.a spectrogram representative of an average LDF pulse. Step S428 uses a trigger signal T togenerate the ensemble average spectrogram. The trigger signals are indicative of timing ofthe LDF pulses. In this example, the trigger signal is obtained from an ECG sensor and indicates the start time of an ECG pulse. As ECG and LDF pulses both follow the cardiac cycle, the start time of an ECG pulse is indicative of the start time of an LDF pulse. In a first sub-step of S428, the PSDs Si(f) (i = 1 to ψ) are subdivided into Z pulses, based on the timinginformation from the trigger signal T. The number of pulses Z is at least an order ofmagnitude smaller than ψ. For example, 1000 PSDs Si(f) are subdivided into 5 pulsescomprising 200 PSDs Si(f) each. Each pulse thus corresponds to a different subset of the PSDsSi(f). The subset of PSDs Si(f) of a single LDF pulse describes a spectrogram X(f, t). In thisnotation, t is renumbered with respect to i, such that the spectrograms of the pulses span thesame time window. In a second sub-step of S428, the spectrograms X(f, t) of the individualLDF pulse are averaged, to obtain an ensemble average spectrogram XE(f,t). An example ofsuch an ensemble average spectrogram is illustrated in figure 8. In this figure, the horizontalaxis represents t (time), and the vertical axis represents frequency (f). The colors indicate themagnitude of XE(f,t), on a logarithmic scale. In step S430, a dispersion d(f) is computed from the ensemble average spectrogram XE(f,t).In other words, for each frequency f of the spectrogram XE(f,t), the dispersion along the timeaxis is computed. In this example, the dispersion d(f) comprises the normalized standarddeviation σ(f). Specifically, the standard deviation is normalized by the mean. Figure 9 shows a plot of the normalized standard deviation σ(f) computed from the spectrogram of figure 8,on a logarithmic scale. The horizontal axis represents frequency. The vertical axis representsthe logarithm of the normalized standard deviation. In step S432, the normalized standard deviation σ(f) is compared to a threshold λ. The frequency where the threshold λ is crossed for the first time while σ(f) has a positive slope isdetermined as the minimum frequency, fmin, and the frequency where the threshold λ iscrossed for the first time while σ(f) has a negative slope is determined as the maximum frequency, fmax. The result is a selected frequency range of Fselected= (fmin, fmax).The threshold λ may be a fixed value. Preferably λ determined as acentral tendency of the normalized standard deviation σ ,λ is selected asthe mean or median of the normalized standard deviation σ(f). The result of step S432 is a selected frequency range Fselected. Referring back to figure 7,Fselected may optionally be output by step S432. The next step in the process is step S402 that,computes the LDF signal by calculating moments of the power spectral densities the selected frequency Fselected. The end result of the method is the LDF signal <vFselected>. of figure 7, the selected frequency range Fselected is computed from a series of PSDs Si(f). Step S402 is then applied to the same series of PSDs to determine an LDF signal. Alternatively or additionally, step S402 is applied to a subsequent series of PSDs. In the example of figure 7, steps S400 computes PSDs. Alternatively, step S400 of figure 7 computes an amplitude spectrum, e.g. as a magnitude of the STFT: Ai(f) = |Ω(ti,f)|.The examples above describe the use of an output signal 164 of a single photodetector.Alternatively, the system comprises multiple photodetectors, and the processing by the one ormore processors is based on the photodetector signals output by the photodetectors. In a firstexample, the photodetector signals are summed prior to step S400, such that the LDF signal can be computed from a single, combined photodetector signal. In a second example, each photodetector signal is processed independently of the other, and the resulting LDF signals are combined. The examples above describe computing a “raw” moment (equation 1), a normalized “raw” moment (equation 2) or a weighted moment (equation 3). Alternatively, a central moment iscomputed, e.g. by replacing the term ^^ in equations 1-3 by (^ − ^^)^, wherein ^^is a centralfrequency. The central frequency ^^ is for example computed as the normalized first rawmoment (equation 2).Fig. 10 is a schematic view of an exemplary computing device 1000 for implementing thecomputer-implemented method of any embodiment of the present disclosure. The computing device 1000 includes some or all of: a processor 1020 (e.g., a CPU), a memory 1030 (e.g., a solid state drive, or SSD), a communication interface 1040 (e.g., a wireless network communication interface and / or input / output interface e.g. for receiving a signal of a photodetector), and a power supply 1050 that are communicatively coupled together via a bus connection 1010. It will be understood that any type of non-transitory computer readable storage device may be used as the memory 1030 in addition or alternative to an SSD. The communication interface 1040 or the bus connection 1010 For example, computing device 1000 may be implemented by one or more instances (e.g., articles, pieces, units, etc.) of processing circuitry such as hardware including logic circuits; a hardware / software combination such as a processor executing software; or a combination thereof. For example, the processing circuitry more specifically may include, but is not limited to, a central processing unit (CPU), an arithmetic logic unit (ALU), a digital signal processor, a microcomputer, a field programmable gate array (FPGA), a System-on-Chip (SoC), a programmable logic unit, a microprocessor, application-specific integrated circuit (ASIC), or any other device or devices capable of responding to and executing instructions in a defined manner. In some example embodiments, the processing circuitry may include a non- transitory computer readable storage device, or memory (e.g., memory 1030), for example a solid state drive (SSD), storing a program of instructions, and a processor (e.g., processor 1020) that is communicatively coupled to the non-transitory computer readable storage device (e.g., via a bus connection 1010) and configured to execute the program of instructions to implement the functionality of some or all of any of the devices and / or mechanisms of any of the example embodiments and / or to implement some or all of any of the methods of any of the example embodiments. Embodiments 1. A system for performing a laser Doppler flowmetry measurement of a blood perfused tissue, the system comprising:- a light source configured to emit coherent light to the blood perfused tissue;- a photodetector configured to receive a portion of the coherent light scattered by the bloodperfused tissue and generate a photodetector output signal in response to receiving said portion; and- one or more processors configured to:- compute a power spectral density (Si(f)) of the photodetector output signal for a series oftime intervals (ti), thereby obtaining a series of power spectral densities;- compute a laser Doppler flowmetry, LDF, signal (<v>), comprising computing a moment(MN(i)) of each of the power spectral densities (Si(f)), wherein the moments (MN(i)) are computed over a selected frequency range (Fselected); and- output the LDF signal (<v>),characterized in that the one or more processors are configured to determine the selected frequency range Fselected, wherein determining the selected frequency range Fselected comprises: -for each respective frequency range Fj out of P different frequency ranges (Fj=1 to P):- computing a respective LDF signal (<vFj>), comprising computing moments(MN(i)) of the power spectral densities (Si(f)) over the respective frequency range (Fj); and -computing a measure of the amount of physiological information (Qj) in therespective LDF signal <vFj>, and -determining the selected frequency range (Fselected) as the frequency range (Fj) forwhich the computed measure of amount of physiological information (Qj) fulfills a predetermined criterion. 2. A system for performing a laser Doppler flowmetry measurement of a blood perfused tissue, the system comprising:- a light source configured to emit coherent light to the blood perfused tissue;- a photodetector configured to receive a portion of the coherent light scattered by the bloodperfused tissue and generate a photodetector output signal in response to receiving said portion; and- one or more processors configured to:- compute a power spectral density (Si(f)) of the photodetector output signal for a series oftime intervals (ti), thereby obtaining a series of power spectral densities;- compute a laser Doppler flowmetry, LDF, signal (<v>), comprising computing a moment(MN(i)) of each of the power spectral densities (Si(f)), wherein the moments (MN(i)) are computed over a selected frequency range (Fselected); and- output the LDF signal (<v>),characterized in that the one or more processors are configured to determine the selectedfrequency range (Fselected), wherein determining the selected frequency range Fselectedcomprises: -computing, for a number of different frequencies (f), a measure of the amount ofphysiological information (Qf) in the spectral densities (Si(f));- determining the selected frequency range (Fselected) as the range of frequencies (f) forwhich the computed measure of amount of physiological information (Qf) fulfills a predetermined criterion. 3. A computer-implemented method for computing a laser Doppler flowmetry, LDF, signal, the method comprising:- receiving a photodetector output signal from an LDF system;- computing a power spectral density (Si(f)) of the photodetector output signal for a series oftime intervals (ti), thereby obtaining a series of power spectral densities;- computing a laser Doppler flowmetry, LDF, signal (<v>), comprising computing a moment(MN(i)) of each of the power spectral densities (Si(f)), wherein the moments (MN(i)) are computed over a selected frequency range (Fselected); and- outputting the LDF signal (<v>),characterized in that the method comprises a step of determining the selected frequencyrange Fselected, wherein determining the selected frequency range Fselected comprises:- each respective frequency range Fj out of P ranges (Fj=1 to P): -computing a respective LDF signal (<vFj>), comprising (Mi) of the power spectral densities (Si(f)) over the respective range ; and -computing a measure of the amount of physiological information (Qj) in therespective LDF signal <vFj>, and -determining the selected frequency range (Fselected) as the frequency range (Fk) forwhich the computed measure of amount of physiological information (Qk) fulfills a predetermined criterion. 4. A computer-implemented method for computing a laser Doppler flowmetry, LDF, signal, the method comprising:- receiving a photodetector output signal from an LDF system;- computing a power spectral density (Si(f)) of the photodetector output signal for a series oftime intervals (ti), thereby obtaining a series of power spectral densities;- computing a laser Doppler flowmetry, LDF, signal (<v>), comprising computing a moment(MN(i)) of each of the power spectral densities (Si(f)), wherein the moments (MN(i)) are computed over a selected frequency range (Fselected); and - outputting the LDF signal (<v>),characterized in that the method comprises a step of determining the selected frequency range (Fselected), wherein determining the selected frequency range Fselected comprises:- computing, for a number of different frequencies (f), a measure of the amount ofphysiological information (Qf) in the power spectral densities (Si(f)); -determining the selected frequency range (Fselected) as the range of frequencies (f) forwhich the computed measure of amount of physiological information (Qf) fulfills a predetermined criterion. 5. The system or method of embodiment 1 or 3, wherein the measure of the amount of physiological information (Qi) comprises a ratio between energy in a predetermined low frequency range of the respective LDF signal <vFj> and energy in a predetermined high frequency range respective LDF signal <vFj>. 6. The system or method of any one or more of embodiments 1, 3 or 5, wherein the measure of the amount of physiological information (Qi) comprises Shannon entropy of the respective LDF signal <vFj>. 7. The system or method of any one or more of embodiments 1, 3, 5 or 6, wherein the measure of the amount of physiological information (Qi) comprises a difference between the maximum and minimum points of the respective LDF signal <vFj>. 8. The system or method of any one or more of embodiments 1, 3, 5-7, wherein computing the measure of the amount of physiological information comprises determining a heart ratefrom the respective LDF signal <vFj>, and obtaining a reference heart rate, wherein themeasure of the amount of physiological information (Qi) is computed based on a difference or ratio between the heart rate determined from the respective LDF signal and the reference heart rate, wherein the reference heart rate is preferably obtained from a heart rate sensor. 9. The system or method of any one or more of embodiments 1, 3, 5-8, wherein computing the measure of the amount of physiological information in the respective LDF signal <vFj> comprises: subdividing the LDF signal <vFj> into individual LDF pulses, wherein preferably computing the measure of the amount of physiological information comprises computing the number of individual LDF pulses within a predetermined time span. 10. The system or method of embodiment 9, wherein computing the measure of the amount of physiological information comprises determining a central tendency of the individual LDF pulses to obtain an ensemble LDF pulse, wherein the measure of the amount of physiological information is computed from the ensemble LDF pulse. 11. The system or method of embodiment 9 or embodiment 10, wherein computing themeasure further comprises determining the average amplitude of the first N harmonics of theindividual LDF pulses, wherein N is an integer greater than 1.12. The system or method of embodiments 2 or 4, comprising:- obtaining one or more trigger signals indicative of a timing of individual LDF pulses;- using the one or more trigger signals, determining, from the power spectral densities (Si(f)),a spectrogram for each individual LDF pulse; and- determining, from the spectrograms of the individual LDF pulses, an ensemble spectrogram(XE(f,t)), comprising a central tendency of the spectrograms of the individual LDF pulses. 13. The system or method of embodiment 12, wherein the measure of the amount of physiological information comprises a dispersion d(f) of the ensemble spectrogram (XE(f,t)) as a function of frequency (f), wherein the predetermined criterion comprises a threshold criterion for the dispersion d(f). 14. The system of any one or more of the embodiments 1, 2, 5-13, comprising a wearable device comprising the light source and the photodetector. 15. A computer program comprising instructions that, when executed by one or more processors, cause the one or more processors to execute the method according to any one or more of the embodiments 3-14.

Claims

CLAIMS 1. A system for performing a laser Doppler flowmetry measurement of a blood perfused tissue, the system comprising:- a light source configured to emit coherent light to the blood perfused tissue;- a photodetector configured to receive a portion of the coherent light scattered by the bloodperfused tissue and generate a photodetector output signal in response to receiving said portion; and- one or more processors configured to determine a selected frequency range (Fselected) forcomputing a laser Doppler flowmetry, LDF signal, wherein determining the selected frequencyrange (Fselected) comprises: -computing a power spectral density (Si(f)) of the photodetector output signal for aseries of time intervals (ti), thereby obtaining a series of power spectral densities; -for each respective frequency range Fj out of P different frequency ranges (Fj=1 to P):- computing a respective LDF signal (<vFj>), comprising(MN(i)) of the power spectral densities (Si(f)) over the respective range (Fj); and -computing a measure of the amount of physiological information (Qj) in therespective LDF signal <vFj>, and -determining the selected frequency range (Fselected) as the frequency range (Fj) forwhich the computed measure of amount of information (Qj) fulfills apredetermined criterion, wherein the one or more processors are further configured to output an LDF signal computed using the selected frequency range.

2. A system for performing a laser Doppler flowmetry measurement of a blood perfused tissue, the system comprising:- a light source configured to emit coherent light to the blood perfused tissue;- a photodetector configured to receive a portion of the coherent light scattered by the bloodperfused tissue and generate a photodetector output signal in response to receiving said portion; and- one or more processors configured to determine a selected frequency range (Fselected) forcomputing a laser Doppler flowmetry, LDF signal, wherein determining thefrequencyrange Fselected comprises:- computing a spectrum (Si(f)) of the photodetector output signal for a series of timeintervals (ti), thereby obtaining a series of spectra;- computing, for a number of different frequencies (f), a measure of the amount ofphysiological information (Qf) in the spectra (Si(f)); -determining the selected frequency range (Fselected) as the range of frequencies (f) forwhich the computed measure of amount of physiological information (Qf) fulfills a predetermined criterion, wherein the one or more processors are further configured to compute an LDF signal using the selected frequency range, and to output the LDF signal.

3. A computer-implemented method for computing a laser Doppler flowmetry, LDF, signal, the method comprising:- receiving a photodetector output signal from an LDF system;- determining a selected frequency range (Fselected) for computing an LDF signal, whereindetermining the selected frequency range Fselected comprises: -computing a power spectral density (Si(f)) of the photodetector output signal for aseries of time intervals (ti), thereby obtaining a series of power spectral densities; -for each respective frequency range Fj out of P different frequency ranges (Fj=1 to P):- computing a respective LDF signal (<vFj>), comprising computing moments(Mi) of the power spectral densities (Si(f)) over the respective frequency range (Fj); and -computing a measure of the amount of physiological information (Qj) in therespective LDF signal <vFj>, and -determining the selected frequency range (Fselected) as the frequency range (Fk) forwhich the computed measure of amount of physiological information (Qk) fulfills a predetermined criterion,the method comprising: outputting an LDF signal computed using the selected frequencyrange.

4. A computer-implemented method for computing a laser Doppler flowmetry, LDF, signal, the method comprising:- receiving a photodetector output signal from an LDF system;- determining a selected frequency range (Fselected) for computing an LDF signal, whereindetermining the selected frequency range Fselected comprises:- computing a spectrum (Si(f)) of the photodetector output signal for a series of timeintervals (ti), thereby obtaining a series of spectra;- computing, for a number of different frequencies (f), a measure of the amount ofphysiological information (Qf) in the spectra(Si(f)); -determining the selected frequency range (Fselected) as the range of frequencies (f) forwhich the computed measure of amount of physiological information (Qf) fulfills a predetermined criterion, the method further comprising: computing an LDF signal using the selected frequency range, and outputting the LDF signal.

5. The system or method of claim 1 or 3, wherein the measure of the amount of physiological information (Qi) comprises a ratio between energy in a predetermined low frequency range of the respective LDF signal <vFj> and energy in a predetermined high frequency range respective LDF signal <vFj>.

6. The system or method of any one or more of claims 1, 3 or 5, wherein the measure of the amount of physiological information (Qi) comprises Shannon entropy of the respective LDF signal <vFj>.

7. The system or method of any one or more of claims 1, 3, 5 or 6, wherein the measure of the amount of physiological information (Qi) comprises a difference between the maximum and minimum points of the respective LDF signal <vFj>.

8. The system or method of any one or more of claims 1, 3, 5-7, wherein computing the measure of the amount of physiological information comprises determining a heart rate fromthe respective LDF signal <vFj>, and obtaining a reference heart rate, wherein the measure ofthe amount of physiological information (Qi) is computed based on a difference or ratio between the heart rate determined from the respective LDF signal and the reference heart rate, wherein the reference heart rate is preferably obtained from a heart rate sensor.

9. The system or method of any one or more of claims 1, 3, 5-8, wherein computing the measure of the amount of physiological information in the respective LDF signal <vFj> comprises: subdividing the LDF signal <vFj> into individual LDF pulses, wherein preferably computing the measure of the amount of physiological information comprises computing the number of individual LDF pulses within a predetermined time span.

10. The system or method of claim 9, wherein computing the measure of the amount of physiological information comprises determining a central tendency of the individual LDF pulses to obtain an ensemble LDF pulse, wherein the measure of the amount of physiological information is computed from the ensemble LDF pulse.

11. The system or method of claim 9 or claim 10, wherein computing the measure furthercomprises determining the average amplitude of the first N harmonics of the individual LDFpulses, wherein N is an integer greater than 1.

12. The system or method of claims 2 or 4, wherein determining the selected frequency range (Fselected) comprises:- one or more trigger signals indicative of a timing of individual LDF pulses;- using the one or more trigger signals, determining, from the spectra (Si(f)), a spectrogramfor each individual LDF pulse; and- determining, from the spectrograms of the individual LDF pulses, an ensemble spectrogram(XE(f,t)), comprising a central tendency of the spectrograms of the individual LDF pulses.

13. The system or method of claim 2 or 4 or 12, wherein the measure of the amount of physiological information comprises a dispersion d(f) of the spectra, as a function of frequency (f), and preferably the predetermined criterion comprises a threshold criterion for the dispersion d(f).

14. The system or method of the combination of claim 12 and 13, wherein the measure of theamount of physiological information comprises a dispersion d(f) of the ensemble spectrogram (XE(f,t)) as a function of frequency (f), wherein preferably the predetermined criterion comprises a threshold criterion for the dispersion d(f).

15. The system of any one or more of the claims 1, 2, 5-14, comprising a wearable device comprising the light source and the photodetector.

16. A computer program comprising instructions that, when executed by one or more processors, cause the one or more processors to execute the method according to any one or more of the claims 3-15.

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