GB2621991A

GB2621991A - Interferometric near infrared spectroscopy system

Info

Publication number: GB2621991A
Application number: GB2212435.8A
Authority: GB
Inventors: Parker William; Borycki Dawid; Lotay Amrit; Dragojevic Tanja
Original assignee: Comind Technologies Ltd
Current assignee: Comind Technologies Ltd
Priority date: 2022-08-26
Filing date: 2022-08-26
Publication date: 2024-03-06
Also published as: GB202212435D0; WO2024042338A1

Abstract

Interferometric near-infrared spectroscopy (iNIRS) system 10 comprises light source 20, which emits wavelength-swept light, and detector(s) 30. Reference channel 26 delivers light emitted from the source to the detector(s). Each of multiple optical channels comprises one or more sample delivery channels 25 and one or more sample receiving channels 35. The delivery channels deliver light emitted from the source, via sample delivery probes 25’, to object 2; the receiving channels, via sample receiving probes 35’, deliver light that has travelled through the object to be received at the detector(s). The detector(s) combine light from the reference channel(s) with light from the receiving channel(s) to output an electrical signal based on beat frequency components in the optically combined signal. Delay line(s) 352a are associated with the sample delivery channel(s) and/or sample receiving channel(s), the delays chosen to that there is no overlap between beat frequency components in the electrical signal due to different optical channels.

Description

Interferometric Near Infrared Spectroscopy System

Technical Field

The present disclosure relates to the field of interfenDmetric near infrared spectroscopy ('iNIRS') systems. For example, such an iNIRS system may be provided for neuroimaging and analysis.

Background

Near infrared spectroscopy ('NIRS') is a spectroscopic method which uses the near infrared region of the electromagneticspectrum (e.g., between 700 and 2500 nm). NIRS systems can be used to provide non-invasive monitoring of scattering and absorption properties of certain media for example some human tissues. Radiation at NI R wavelengths is less easily absorbed by human skin (and also bones) than visible light of shorterwavelength, and so NIR radiation may penetrate both skin and skull, and penetrate into brain tissue. NIRS may be used as a technique for non-invasive imaging of human brain tissue by monitoring scattering and absorption properties of the NIRS radiation within the brain tissue.

NIRS comprises sophisticated optical and electronic circuitry. It is thus desirable to provide technological improvements that may reduce the hardware required.

Summary

Aspects of the disclosure are set out in the independent claims and optional features are set out in the dependent claims. Aspects of the disclosure may be provided in conjunction with each other, and features of one aspect may be applied to other aspects.

An interferometric near infrared spectroscopy ON IRS) system and method may be disclosed, comprising a light source configured to provide an emission of light, where the emitted light is wavelength swept. One or more detectors may be provided in the system. Further, one or more reference channels may be arranged to deliver the emitted light from the light source to the one or more detectors. The iNIRS system may comprise a plurality of optical channels, comprising one or more sample delivery channels with one or more associated sample delivery probes to deliver the emitted light from the light source to an object, and one or more sample receiving channels with one or more associated sample receiving probes to receive light comprising emitted light that travelled through the object to the one or more detectors. The iNIRS system may comprise one or more delay lines associated with the one or more sample delivery channels and/or the one or more sample receiving channels. At the detectors, the one or more detectors are arranged to combine into an optically combined signal: a) emitted light received via the one or more reference channels; and b) received light received via the one or more sample receiving channels. The one or more detectors are further arranged to output an electrical signal based on beat frequency components comprised in the optically combined signal. The delays of the one or more delay lines are selected so that there is no substantial overlap between the beat frequency components due to different ones of the plurality of optical channels in the electrical signal.

According to various embodiments of the invention, a portion of one optical channel of the plurality of optical channels may share a common sample delivery channel and/or common sample receiving channel with a second optical channel of the plurality of optical channels. For example, a controller may be enabled to generate a frequency spectrum for each of the different ones of the plurality of optical channels from the electrical signal. At the controller, the frequency spedra associated with the plurality of optical channels may be autocorrelated and combined. The controller may compute path delays associated with components of the frequency spectrum for each of the different ones of the plurality of optical channels from the electrical signal, or for path delays associated with components of the combined frequency spectra. Further, the controller may align the frequency spectra for each of the different ones of the plurality of optical channels based on the autocorrelation. In accordance with various embodiments of the invention, the controller may be enabled to compute a time-of-flight distribution based on the frequency spedra and/or path delays. The selected delays between different ones of the plurality of optical channels may be chosen at least as large as the expected delay spread.

Figures Some examples of the present disclosure will now be described, by way of example only, with reference to the figures, in which: Fig. 1 shows a schematic diagram of an example iNIRS system.

Fig. 2 shows a graph depicting example time of flight data for incident photons.

Figs. 3a to 3c show schematic diagrams of example iNIRS systems.

In the drawings like reference numerals are used to indicate like elements.

Specific Description

The present disclosure relates to a interferometric near-infrared sprectroscopy (iNI RS) system for an object to be imaged. The object may be a biological object or biological tissue/matter, for example skin, bone, muscle, fat, and/or brain, and comprise e.g., body fluids such as blood. For this, the system is arranged for light to be directed towards the object. A portion of the directed light will traverse the object, also sometimes referred to as sample. The portion of the light that traverses the sample may be received at one or more detectors, suitably equipped to process the portion of light in the form of optical and/or electrical signals. These signals may be used for various signal and data processing in the system.

The system may be arranged for light to be directed towards the object such that the light along one or more sample delivery optical channels, and some of that light to be received from the object and directed towards an optical detector along one or more sample receiving optical channels. The system may be arranged so that there are at least two different optical paths between a source and the object and/or at least two different optical paths between the object and the detector. The system may be arranged such that these at least two paths and/or channels are of different length. Light may therefore travel along a different length of optical channel when travelling from the light source to the detector (via the object to be imaged) depending on which of the at least two channels that light travels through. This difference in optical channel length is of a sufficient length such that, when the received light from the at least two optical channels is received and/or processed at the detector, there will be minimal, or substantially no, spectral and/or temporal overlap between signals associated with a first optical path length through the optical channels and a second optical path length through the optical channels. Two differenttime of flight distributions may therefore be obtained through one digitiser channel, as will be further explained below with reference to the figures.

An example of an interferometric Near Infrared Spectroscopy ('iNIRS') system will now be described with reference to Fig. 1.

Fig. 1 shows a schematic diagram of an example interferometric Near Infrared Spectroscopy ('iNIRS') system 10. The iNIRS system 10 includes a light source 20, at least one detector 30, and a controller 40. The system 10 may include a plurality of detectors 30 although only two are shown in Fig. 1. Inset A shows a more detailed view of the light detector 30 shown in the dashed box listed A in Fig. 1.

The iNIRS system 10 may include a light source modifier22, and light splitters24, 28. The iNIRS system 10 includes a sample delivery channel 25 and a reference delivery channel 26. The iNIRS system 10 is shown coupled to an object 2 to be imaged, which, in this example, may be a subjects head 2 (e.g., for providing neuroimaging). The iNIRS system 10 includes a sample delivery probe 25' and a plurality of sample receiving probes 35', 351', 352'. In the example of the object being a subject's head 2, the sample delivery probe 25' and the sample receiving probes 35', 351', 352' may be for coupling optical channels to the scalp of the subject head 2. There is shown a first sample receiving channel 351 and a second sample receiving channel 352. There is shown a delay line 352a.

Two exemplary light detectors 30 are shown in Fig. 1. Each light detector 30 has an associated reference receiving channel 36. The reference receiving channel 36 isf or receiving reference light from the light source 20, for example coupled via light splitters 24, 28 and via reference delivery channel 26 as illustrated in Fig. 1. The other light detector 30 shown is coupled to the scalp of subjects head 2 via one optical channel: sample receiving channel 35 (with associated sample receiving probe 35') The light source 20 may comprise a laser enabled to emit light conforming to certain properties, some of which may be controlled by the light source modifier 22. In accordance with various embodiments of the invention, the laser may be a high coherence laser. For example, the laser may be a Distributed Feedback laser ('DFB') or a MEMS-Vertical Cavity Surface Emitting laser ('MEMS-VCSEL'). Other types of suitable laser include a Distributed Bragg Reflector laser ('DBR'), a Fourier Domain Mode Locking laser ('FDML'), a Vertical Cavity Surface-Emitting laser (VCSEL'), an external cavity diode laser (ECDL), feedback-stabilised laser, or line-locked laser In accordance with various embodiments, a Master Oscillator Power Amplifier (MOPA) configuration may be used. As will be clear to the person skilled in the art, various laser configurations may be used for the present invention, comprising e.g., LiDAR lasers, lasers for coherent telecommunications or Optical Coherence Tomography (OCT) lasers. Additionally, or alternatively, a pulsed supercontinuum laser may be used in combination with a pulse stretching mechanism, such as a grating or GRISM pulse stretcher or length of dispersive optical fibre. For example, such an arrangement may be configured to temporally separate the wavelengths in the pulse such that a frequency chirped pulse is created (e.g., for ultimately providing an interferogram when sample and reference pulses are compared).

The light source modifier 22 may comprise a source for providing a variable electrical control signal (e.g., a variable current orvoltage provider) to the light source 20. The light source modifier 22 is coupled to the light source 20. The light source modifier 22 may be electrically connected to the light source 20 to provide a variable current/voltage thereto, enabled to control the light source 20 to desirably adjust properties of the light emitted by the light source 20. Specifically, the light source modifier22 may interactwith the light source 20 to vary the wavelength of the light emitted from light source 20. This may be referred to as wavelength swept emission of light or frequency sweeping.

The light source 20 is coupled to the light splitter 24. The light splitters 24, 28 have an input for receiving light from a light source and comprise suitable circuitry and/or hardware that is enable to output light on two or more outputs onto channels. Light splitters 24, 28 of the present disclosure may comprise fibre-optic splitters. The light splitter 24 outputs, for example, are coupled to the reference delivery channel 26 and to the sample delivery channel 25. The splitter is configured so that the majority of the light is directed towards the subject's scalp via sample delivery channel 25. For example, the splitter may be a 90:10 splitter, or a 99:1 splitter.. The light splitter 28 input is coupled to one output of the light splitter24 via the reference delivery channel 26. The outputs of light splitter 28 are coupled to the reference receiving channels 36, which in turn couple the detectors 30 to the splitter 28 and indirectly to the light source 20. In other words, each light detector 30 may be connected to the light source 20 indirectly to receive reference lighttherefrom (via one or more ref erencechannels). The sample delivery channel 25, ref erencedelivery channel 26, reference receiving channels 36, the delay line 352a, the sample receiving channel 35, the first sample receiving channel 351, and the second sample receiving channel 352 are enabled to communicatively couple optical devices by enabling light transmission, typically implemented as optical fibres. Some or all of the optical channels of the iNI RS system 10 may be provided by optical fibres. The iNI RS system 10 may include optical devices such as lenses, reflection and/or refraction devices for beam steering, as relevant.

The sample delivery channel 25 couples the light splitter 24 to the sample delivery probe 25'. The sample delivery probe 25' will be placed at a location on the scalp of subject'shead 2. The sample delivery probe 25' is enabled to couple light received to the object/subject's head 2. Similarly, sample receiving probes 35, 351', 352' are enable to couple to an object 2 to receive light. For example, the sample delivery probe 25' may include one or more lenses for spatially distributing sample light from the sample delivery channel 25 towards the subject's brain tissue. As another example, one or more of the sample receiving probes 35, 351', 352' may include a lens for focussing received light into its associated sample receiving channel 35, 351, 352, 352a (as connected to that sample receiving probe). As another example, the probes 25', 352', 351', 35' may be bare optical fibres which have been cleaved and/or polished.

Each sample receiving channel 35, 351, 352, 352a may comprise a single-mode fibre ('SMF') or a few-mode fibre ('FMF'). Each sample receiving channel may be coupled to an additional fibre in the form of a multi-mode fibre ('MMF') or an FMF. Each sample receiving channel may be coupled to that additional fibre and arranged so that each sample receiving channel (e.g., each SMF or FMF) excites its own unique mode within the additional fibre 35, 351, 352, 352a. The reference channel(s) 36 may be provided by an SMF or a FMF (typically an SMF). The reference channels 36 may be coupled to the additional fibre, e.g., to excite a fundamental mode of the additional fibre. The additional fibre may be the arranged to combine the light in a FMF or MMF fibre coupler. Alternatively, sample light in a FMF/MMF could be combined with the reference light using one or more free-space beam combining elements such as a beamsplitter cube.

The object 2 may be a subject's head, for example for neurobiological imaging. The object 2 may generally be any biological object or biological tissue/matter, for example skin, bone, muscle, fat, and/or brain, and comprise e.g., body fluids such as blood.

Each sample receiving probe 35', 351', 352' may be placed on the scalp of the subject head 2. Each sample receiving probe 35', 351' 352' may be coupled to an associated sample receiving channel 35, 351, 352a, 352 that is coupled to an associated light detector 30. In other words, each light detector 30 is connected for indirectly receiving sample light from the light source 20, where the sample light has travelled from the sample delivery probe 25 through the subject heal 2 to the sample receiving probes 35', 351', 352'.

A second sample delivery probe 352' is coupled to the second sample receiving channel 352 via a delay line 352a. The delay line 352a is enabled to delay an optical input signal in time before it is emitted at its output. A delay line 352a may be implemented by an additional optical channel length, for example by a length of optical fibre. Other forms of optical delay lines, for example based on optical cavities, may be used.

The detectors 30 are enabled to receive and process optical input signals. Further, the detectors 30 comprise suitable logic, circuity and/or code that is enabled to suitably convert and process the received optical input signals to electrical signals. The detectors 30 generate an electrical output signal that may be coupled to an input of a controller 40, illustrated by a dashed line. For example, the detector 30 may receive light that has travelled through the object 2 via the sample receiving probes 351', 352', 35', and viathe delay line 352a in some instances, and via the sample receiving channels 351, 352, 35. The detector 30 may also receive light from light source 20 that has not travelled through the object 2 via the reference delivery channel 26, reference receiving channel 36 and the light splitters 24, 28. Further details of detectors 30 will be discussed with reference to Inset A below.

The controller 40 may comprise suitable logic, circuitry and/or code with data receiving and processing functionality. For example, the controller 40 may include at least one Application Specific Integrated Circuit ('ASIC'). Other examples for the controller 40 may include a Field Programmable Gate Array ('FPGA') and/or a Data Acquisition module ('DAQ'). The controller may also comprise a microcontroller or microprocessor. The controller 40 is coupled to each of the detectors 30 electrically, as illustrated by the dashed line. The controller 40 may be connected to each detector 30 via a wired connection (for receiving electrical signals indicative of detection therefrom), and/or the connection may be wireless (for receiving transmitted data indicative of detection therefrom). The output of the controller 40 is coupled to the light source modifier 22.

This connection may be wired or wireless. Correspondingly, based on processing of input signs received from at least the detectors 30, the controller 40 may control the function of the light source modifier 22.

With reference to Inset A of Fig. 1, there is shown an exemplary configuration for a detector 30, which according to various embodiments of the invention, may be used to convert and process optical signals to electrical signals. The exemplary detector 30 may be configured to convertthree optical outputs into a discrete-time/digitized, i.e., sampled, electrical signal.

Referring to Inset A of Fig. 1, a detector 30 may comprise a light combiner and splitter 301, a balanced photodetector 303, and analogue to digital converter ('ADC') 306. An exemplary balanced photodetector 303 may comprise detection photodiodes 310, 312, a transimpedance amplifier (TIA) 304, and an amplifier 305. The ADC 306 is arranged to provide a digital signal output307, which may correspond to an electrical output of detector30, as illustrated by a dashed line in Fig. 1. The digital signal output 307 may also be referred to as an interferogram, as will be explained below.

The optical inputs to the light combiner and splitter 301 of detector 30 are coupled to reference receiving channel 36 and sample receiving channels 351, 352. Similarly, the optical inputs to the light combiner and splitter 301 of detector 30 may be coupled to reference receiving channel 36 and a single sample receiving channel 35, as illustrated in Fig. 1.

One example of an arrangement for converting received light signals into digital data is shown in Inset A of Fig. 1. Inset A shows an arrangement of components that may be used as part of a light detector 30 of the present disclosure. As shown in the iNIRS system 10 of Fig. 1, the detector is arranged to receive three inputs: (i) reference light which has travelled along reference delivery channel 26 and reference receiving channel 36, (ii) first sample light which has been received through the first sample receiving channel 351, and (iii) second sample light which has been received through the second sample receiving channel 352.

The light combiner and splitter 301 is arranged to additively combine the electric fields of the light signals received via channels 36, 351, 352. For example, the channels may couple into a beam combination element. This combination could be achieved using e.g., fused fibre couplers, bean splitter cubes, diffraction gratings or more other splitting/combining optical elements such as and fibre to free-space to fibre multiplexing optics. The combined optical signal, sometimes referred to as the mixed signal, is then optically split in light combiner and splitter 301 and output to a first light channel 302a and a second light channel 302b. The mixed signal energy may be split 50:50, for example, between the first light channel 302a and the second light channel 302b. In accordance with various embodiments of the invention, the proportions of how the mixed signal energy is split between the channels 302a, 302b may be adapted to the specific configuration of the detector 30.

For example, the light combiner and splitter 301 may receive light signals at its inputs that may be represented as electric fields E36(t), E3s/(t), and E352(t) for each of the channels 36, 351, and 352, respectively. The light combinerand splitter301 may to generate output signals proportiona to: E302,(0 = E36 E (t) , E302b (t) = E36(t)-E(t) (11 where E) = E351 E352 (0 may be a signal comprising an electric field proportional to a sum of the electric fields from the sample receiving channels 351, 352 of light signals that have traversed the sample object 2. The signals 302a and 302b will typically be provided so that the signals E(t) on each photodiode 310, 312 are 1800 out of phase with each other. The balanced photodetector 303 may be arranged to output a current corresponding to the difference bet/wen the incident optical energy received at the photodiodes 310, 312. The balanced photodetector 303 may remove any unwanted DC/common terms from this signal, such as slow fluctuations emanating from the light source 20 or other common-mode effects such as noise.

In accordance with various embodiments of the invention, the combiner and splitter 301 may be implemented as one device or may be implemented as multiple devices. For example, in accordance with the above equations, the combining of the received sample signals E35/(t) and E352(t) into E(t) may be implemented in one unit/stage, and the combinations of &(t) and E.36(0, may be performed in a second unit/stage.

From the above explanation, it is clear to the skilled person that a light combiner and splitter 301 may comprise two or more inputs, as illustrated below in Fig. 3a-c, depending on the number of sample receiving channels 351, 352, 353 and/or reference channels 26, 26a, 26b. Correspondingly, the detector 30 comprises two or more inputs.

The first light channel 302a may be coupled to a first detection photodiode 310 and the second light channel 302b may be coupled to a second detection photodiode 312. The photodiodes being comprised in the balanced photodetector 303. The detection photodiodes 310 and 312 are arranged to receive two light signals and generate an output current that is proportional to the difference in light intensity between the input signals. Correspondingly, the output current of the detection photodiodes 310, 312 is coupled into the transimpedance amplifier 304 and may be proportional to: 1(t) oc 1E302a(012 -IE3o2b(t)12 = 4 Re[E36(0E;(0] ( 2) Where* denotes the complex conjugate operation and Re[.] may denote the real part of a complex quantity. The detector 30 is arranged to combine reference light E36(t) with sample light Es(t) (as part of an interferometer). The iNIRS system 10 may be arranged to determine one or more properties of the subject's brain tissue based on this combination of reference light and sample light (as will be described in more detail below).

The transimpedance amplifier TIA 304 comprises suitable logic, circuitry and/or code to convert an input current /(t) to a proportional output voltage, i.e., the transimpedance amplifier is arranged as a current to voltage converter. The voltage output of the TIA 304 may be coupled to amplifier 305 for further amplification. In some embodiments, an amplifier 305 may not be necessary, depending on the particular configuration of the photodetector303.

The amplifier 305 may be used to scale the output signal to the full range of the ADC 306 and limit the electronic frequency of the circuit to further maximise the SNR. This amplified voltage is then provided to the ADC 306 to be digitised. The ADC 306 comprises a digitiser having sufficient bandwidth so that the full signal bandwidth containing time of flight information may be digitised without attenuation. As will be appreciated in the context of the present disclosure, and as described in more detail below, the ADC may have a digitisation bandwidth which is far greater than that of a single DTOF. By effectively providing more DT0Fs in each interferogram to be digitised, the iNIRS system 10 may utilise a greater proportion of the digitisation bandwidth for each ADC 306 of the system 10.

The output of the amplifier 305 may be coupled to an ADC 306. The ADC 306 shall be configured to convert a continuous-time electric input signal to a discrete-time electric output signal, sampled at a desirable sampling rate. This discrete-time signal is output over digital signal output channel 307 and is coupled to controller 40, for suitable processing.

Each detector 30 may provide part of an interferometer in system 10, such as a Mach-Zehnder interferometer (when receiving sample and reference light from the light source). Each of the different light detectors 30 may be coupled to the same light source 20 (each via one or more reference channels 36). The light detectors 30 may be spatially separated from the light source 20. The sample receiving probes 351', 352' may also be spatially separated from one another or they may be co-located on a sufficiently similar region of tissue that the received signals can be averaged together. For reference light to reach a light detector(s) 30 from the light source 20, the reference light will travel along one or more reference channels 36. For sample light to reach a light detector 30 from the light source 20, the sample light will travel indirectly via the subject's head 2 brain tissue. The sample light is directed towards the scalp of the subject's head 2 via one or more sample delivery channels 25'. The sample light may then pass through the subjects brain tissue and travel into a receiving channel and into the optical detector. A first optical path may be the optical path from e.g., splitter 24 via sample delivery probe 25', through the subjects head 2, sample receiving probe 352', delay line 352a, and sample delivery channel 352 to detector 30. Asecond optical path may be the optical path from e.g., splitter24 via sample delivery probe 25', through the subjects' head 2, sample receiving probe 351' and sample delivery channel 351 to detector 30. The first optical path and the second optical path generally are of different length, in accordance with various embodiments of the invention as illustrated in exemplary Fig. 1, Fig 3a -Fig. 3c. The illumination of the subject's brain tissue may thus occur using a different light channel to the detection of light from the subject's brain tissue.

In accordance with various embodiments of the invention, the iNI RS system 10 may be completely or partially housed within a garment (not shown) forthe subject's head 2. Forexample, the iNIRS system 10 may be provided in a hat/cap that may be worn by the subject on their head 2. The garment may also be a headband with attachment, or any other suitable fixture to couple at least parts of the iNI RS system 10 to the subject head 2. The head garment may be arranged to hold the light source 20 and detectors 30 in a fixed arrangement relative to the scalp of the subject head 2. For example, the head garment may include a plurality of sample receiving probes 35', 351', 352' and/or sample receiving channels 35, 351, 352 and/or detectors 30. The controller40 may be separate to the head garment (e.g., and connected wirelessly or by wire) or it may also be provided as part of the head garment (e.g., by an ASIC within the head garment which may be wire-coupled to the detectors 30 and/or light source modifier 22). For example, the garment may be configured to comprise the sample delivery probes 25' and channels 25, and the sample receiving probes 35', 351', 352' and channels 35, 351, 352, with the other components of the system 10 located elsewhere.

The sample delivery probe 25', and sample receiving probes 351', 352', 35' are arranged to be positioned on the subject's head 2 to provide imaging of a selected region of their brain. At least some of the probes 25', 351', 352', 35' are arranged to be spatially separated from the light source 20. One or more probes 25', 351', 352', 35' may be arranged to be sufficiently spaced apart from the light source 20 so that at least some of the photons of sample light from the light source 20 which is received at the light detector 30 will have penetrated into the subjects brain tissue.

A portion of the light that is delivered to the subject's head 2 via the sample delivery probe 25' is received at the sample receiving probes 35', 351' 352'. The light will have travelled through the subjects head 2. The light will be scattered by the brain tissue, resulting in delay and/or attenuation. The scattering transmission channel for the subject's head 2 may be modelled as a sum of delayed and attenuated signals that may be received at the sample receiving probes. For example, )1(r) = wi YO. 3) Where y(t) may be an exemplary received signal at a probe, x(t) may be the transmitted light signal, i may denote the i-th delay path with delay; and wi may be the i-th attenuation coefficient In some instances, the sample receiving probes may be spatially proximal to each other, as illustrated in Fig. 1 for sample receiving probes 351' and 352'. In other instances, the sample probes are not substantially collocated and may be separated by a distance.

For the iNIRS system 10 of Fig. 1, the detector 30 which is coupled to the scalp of the subject's head 2 via only one sample receiving channel 35 and associated sample receiving probe 35' need not be provided. This detector 30 is shown as an example to illustrate that multiple detectors may be used in the iNI RS system 10, and not all of the detectors need to be coupled to two or more sample receiving channels. However, for the following description, reference will predominantly be made to the detector 30 which is coupled to the scalp of the subject's head 2 via two or more sample receiving channels 351, 352 and sample receiving probes 351', 352'.

The detector 30 is coupled to the light source 20 via the reference receiving channel 36 (and reference delivery channel 26). The detector 30 is also coupled to the subject's scalp via two separate optical channels: the first sample receiving channel 351 and the second sample receiving channel 352. Each of these sample receiving channels may be coupled to the subjects head 2 via sample receiving probes 352', 352' (first and second sample receiving probes respectively). The first sample receiving channel 351 is of a different length to the second sample receiving channel 352. For this, the second sample receiving channel 352 includes delay line 352a (as shown in Fig. 1). In view of the delay line 352a, the distance light has to travel from the sample receiving probe 351' to the detector 30 through the first sample receiving channel 351 is less than the distance that lightwould have to travel through the second sample receiving channel 352.

The first sample receiving probe 351' may be coupled to the subject's scalp at a location adjacent to the location at which the second sample receiving probe 352' is coupled to the subject's scalp.

The first sample receiving channel 351 may therefore be arranged so that it is imaging a substantially similar (e.g., the same) region of the subject's brain to the second sample receiving channel 352. That is, on average, the light which enters the first sample receiving channel 351 and travels to the detector 30 is likely to be representative of the same region of the subject's brain as the light which enters the second sample receiving channel 352 and travels to the detector 30. The difference between the two being that the light which travels to the detector 30 via the second sample receiving channel 352 will travel a longer distance between the subjects scalp and the detector due to the delay line 352a.

From here on in, sample light which is received at the detector 30 from the object 2 to be imaged (e.g., the subject's brain), and which travelled along the first sample receiving channel 351 will be referred to as 'first sample light'. Similarly, sample light which is received at the detector 30 but which travelled along the second sample receiving channel 352 will be referred to as 'second sample light'. The second sample light will take a longer time to travel from the subject's scalp to the detector 30 than the first sample light due to the delay line 352a for the second sample receiving channel 352. Reference light is light which does not travel through the subject head 2 but is coupled to the light source 20 via a reference receiving channel 36 The light source 20 is configured to provide wavelength swept emission of light. Forthis, the light source 20 may be configured to produce a series of emissions of pulses of light. During each pulse, the wavelength of light may be "swept" through a range of wavelengths. For example, the sweeping may be in the form of a chirped pulse. Light will be emitted at a plurality of different wavelengths during one pulse. For example, the wavelength may continually increase or decrease during one pulse (the rate of change of wavelength may be constant, or it may be variable). The series of chirped pulses may be contiguous (e.g., with a zero inter-pulse time interval). The light source 20 may be configured to successively emit a series of pulses, with each pulse having a wavelength sweep. However, it will be appreciated that the light source 20 need not provide continuous sweeping. For example, the light source could be tuned in steps rather than continuously, such that the light source 20 emits light at different wavelengths in different time intervals (e.g., discrete time intervals for emission at each of a plurality of wavelengths). The light source 20 may sweep unidirectionally (e.g., only increasing or decreasing in wavelength during one wavelength sweep), or it may sweep bidirectionally (e.g., both increasing and decreasing in wavelength during one wavelength sweep). Unidirectional sweeping can be beneficial as it increases the number of detected photons per sweep.

The controller 40 may be configured to selectively control the wavelength sweeping of the light source 20 via the light source modifier 22. The wavelength sweeping of the light source 20 may be controlled by using the light source modifier 22 to apply a corresponding electrical signal to the light source 20. The controller 40 may be arranged to control application of a current/voltage to the light source 20 using the light source modifier 22 to provide a selected pattern for the wavelengths of light emitted by the light source 20.

The light source 20 may be controlled to wavelength sweep according to a selected pattern for the sweeping. For example, the light source 20 may sweep through a selected range of wavelengths of light and/orthe light source 20 may sweep through wavelengths of light according to a selected sweep profile (e.g. linear increasing, sinusoid, triangular etc.). For example, the light source 20 may sweep according to a selected sweeping rate, or a selected total sweeping time. The light source 20 is configured to wavelength sweep light so that during one wavelength sweep, light will be directed towards the subject's brain tissue through the sample delivery channel (and to the detectors via the reference channels) at each of a plurality of different wavelengths. The wavelength of light emitted by the light source 20 will vary over time. As such, an indication of the time at which lightwas emitted from the light source 20 may be determined based on a wavelength of that light.

For example, if the light emitted at sample delivery probe 25 may change its frequency linearly over some period of time as f (t) = fo + St, then observing a frequency of light fi received via a sample receiving probe (e.g. 351') may be used to estimate the delay introduced by the channel through the subject head 2 from r =-rnt°, for this particular frequency sweeping pattern.

Such a frequency difference may be obtained from the balanced photodetector 303. As was explained above, the balanced photodetector303 generates a signal proportional to /(t) = 4 Re[E36(0E; (t)] (4) For example, at a particular instant in time, the reference signal E36(t) may be cos (27tf0 0 and the sample received signal may be frequency shifted by Af<<fo, i.e. Es(t)= cos (2m(f0 + ant) due to a delay introduced by the channel through the subject head 2 in combination with the sweeping in frequency, then by trigonometric identity, 1(t) occos cos (27tAf t) + cos (2rc(2f0 + ADO. The measured intensity /(t) comprises a high frequency term that may be low-pass filtered out and a frequency component at the offset frequency Af. This may be referred to as beat frequency.

The light source 20 may be configured to sweep through a selected wavelength range. For example, the light source 20 may be configured to sweep in optical frequency over a range of 50 GHz. For example, this may enable the light source 20 to emit modulated light at a plurality of different wavelengths between e.g., 829.94 nm and 830.06 nm when centred on 830nm for example or between 1309.857 nm and 1310.143 nm when centred on 1310 nm for example. The light source 20 may be configured to sweep through a wavelength range of at least 0.025 nm, such as at least 0.05 nm, such as at least 0.075 nm, such as at least 0.1 nm, such as at least 0.11 nm (e.g., about a wavelength on which it is centred). The light source 20 may have a high output power, a long coherence time, and broad mode-hop free wavelength tuning. The light source 20 may have a relatively narrow linewidth and a longer coherence length, e.g., because the light source 20 will not sweep over particularly large bandwidths.

Light sources 20 of the present disclosure may be configured to provide emission of high coherence light, e.g., substantially coherent light. It will be appreciated that the light source 20 may not both emit perfectly coherent light and also provide wavelength swept emission of light, e.g., because light at different wavelengths will change phase at different rates. Light sources of the present disclosure may be controlled to sweep through a wavelength range which is relatively narrow compared to their absolute wavelength. Correspondingly, typically Af «h. In otherwords, the difference between the maximum and minimum wavelengths for one wavelength sweep will be relatively small compared to those absolute wavelengths. Each light source 20 may be configured to emit light (i.e., an electric field) which does not have much change in its phase over time.

The iNIRS system 10 of the present disclosure will receive first sample light, second sample light, and reference light all of which originated from the same light source 20. The light sources of the present disclosure are configured to provide wavelength swept emission of sufficiently coherent light, such that sample and reference light, as received at the optical detector 30, will be in relatively constant relative phase to each other. In other words, the coherence length of the light source may be such that the multiple scattering in the tissue will not reduce the coherence or fringe contrast below a noise floor forthe measurement.

For example, each light source 20 of the present disclosure may comprise a laser. The laser may be selected based on its coherence length.ln other words, the iNI RS system 10 may be arranged so that a maximum expected time of flight delay for sample light photons (received at the optical detector 30 which have travelled through the subject's brain tissue of the subject head 2 and received at a probe 352' coupled to a delay line 352a), e.g. which will be for the second sample light (as described in more detail below), relative to reference light photons (received atthe optical detector which have travelled along the one or more reference channels) is within a coherence time period for the laser (e.g. the difference in optical path length between the sample and reference light is within the coherence length of the laser). Within this coherence time period, the phase of light emitted by the laser is approximately constant relative to the phase of the light received via the sample receiving probes 35', 351', 352' (despite changes in the wavelength of light being emitted). It may be advantageous that the longest paths travelled by the received sample light, including the additional optical length 352a, at the detector 30 may be much smaller than the laser coherence length. As will be appreciated, if the phase between the reference channels 36 and the sample received channels 35, 351,352 are not substantially constant relative to each other over a certain processing time, such changing phase would introduce additional error in the estimation of the path delays through the subject head 2. The additional optical path length 352a is arranged such that the difference in optical path length between the first sample receiving channel 351 and the second sample receiving channel 352 means that first sample light photons will have shorter times of flight than second sample light photons.

For example, the iNIRS system may be configured to have a coherence length or range of approximately 50 m in air -e.g., the light sources may be selected which have a coherence length of between 50 and 100 m (a coherence time period of between 166 ns and 333 ns). It will be appreciated that this particular range is not intended to be limiting, rather it is illustrative of the approximate range for the light source. The light source may be selected so that it has a coherence length which is two or more times greater than the maximum expected optical path length difference, e.g., the coherence length may be three or four or more times greater. Having a light source with a coherence length which is much greater than the optical path length may increase accuracy for measuring sample light photons which have undergone a large number of scattering interactions within the subject's brain tissue.

The iNIRS system 10 is arranged so that the source-detector path lengths for reference and sample light are different. In other words, the iNIRS system 10 is arranged so that an average, or expected, optical path length for light travelling from the light source 20 to each detector 30 via the subject's brain tissue will be different to the optical path length for light travelling from the light source 20 to said detector 30 via reference channel(s) 36.

The iNI RS system 10 is arranged so that sample light may travel along two or more different paths of optical channels when travelling from the source 20 to the detector 30. As such, sample light may be able to travel along different paths through the optical channels On addition to travelling through different paths through the subject's head 2). The iNIRS system 10 includes at least two different routes of optical channels through which sample light could travel to the detector3Ofrom the light source 20 (in addition to travelling through the subject's brain). In the example shown in Fig. 1, the different optical channel paths are those extending from the object 2 to be imaged (e.g. the subject's scalp) to the detector 30. That is, sample light may be directed towards the object 2 from the light source 20 (along sample delivery channel 25) and that sample light may travel through the object 2 and to the detector 30, where the route from the object 2 to the detector 30 is either along the first sample receiving channel 351 or the second sample receiving channel 352. In other words, the iNIRS system 10 is arranged to provide two or more different source-detector optical path lengths for sample light (irrespective of whichever route that sample light takes when travelling through the object 210 be imaged).

As will be appreciated in the context of the present disclosure, photons of sample light which are directed towards the subject's brain tissue may travel from the light source 20 to a light detector via a practically infinite number of different paths through the object 2. A photon of sample light may undergo a large number of scattering events between the sample delivery probe 25' and the sample receiving probe 35', 351', 352'. The iNIRS system 10 may be arranged to provide neuroimaging and analysis based at least in part on activity in the subject's brain tissue, which may affect the sample light received. The time of flight fora sample light photon from light source to light detector 30 will of course increase as the path length it takes increases. As such, a photon which travels a longer path, and penetrates deeper into the subject's brain tissue, will take longer to arrive at the light detector 30 than a photon that has travelled a shorter path. The longer the time of flight for a sample light photon, the deeper that photon is likely to have penetrated into the subject's brain tissue. Sample light photons received via the sample receiving probes 35', 351', 352' will have longer times of flight than reference light via reference channels 36.

As will be appreciated in the context of the present disclosure, the path which each individual sample light photon travels through the object 2 to be imaged (between sample delivery probe 25' and sample receiving probe 35', 351', 352) cannot be predicted. However, where there are a great number of these sample light photons, the overall time of flight distribution for such sample light photons may be modelled statistically. As such, for a given source-detector probe pair, one or more expected properties for a time-of-flight distribution for sample light may be known. For instance, for each source-detector probe pair, an expected time difference between the shortest time of flight photons and the longest time of flight photons may be known. This difference mw be referred to as the delay spread of the channel through the object 2. For example, this may be based on previous observable signals for the earliest arriving detectable photons and the latest arriving detectable photons. In other words, for any given source-detector pair, there may be a known maximum expected delay for a resulting time of flight distribution for sample light photons.

The iNIRS system 10 may effectively provide two source-detector pairings between the same source 20 and detector 30. In the example of Fig. 1, this is due to the two different optical paths that are provided between the object 2 and the detector 30 (e.g. due to the first and second sample receiving channels 351, 352). There will be a known expected width for the time of flight distribution for the first sample light photons (e.g. a known time interval spanning between the shortest and longest times of flight for first sample light photons). For example, this width may be somewhere in the region of 0.5 to 2 ns. The delay line 352a may add an additional distance of optical channel along which second sample light photons have to travel to reach the detector 30, where that additional distance corresponds to at least the expected width for the first sample light photons. For example, the delay line 352a may have a distance which is at least as long a distance corresponding to a combination of both: (i) the expected delay spread for the first sample light, and (ii) an additional buffer time period (selected so that anomalously long first sample light photon times of flight are still reaching the detector before any of the second sample light photons).

In other words, the delay line 352a adds an additional distance of optical channel of such a length that the duration of time it takes for second sample light photons to travel along that additional length of optical channel will be at least as long as the expected width of the first sample light photon time of flight distribution. The delay line 352a is arranged so that detectable photons of first sample light may mostly have shorter times of flight than detectable photons of second sample light. For example, the photons of first sample light with the longest times of flight through the object to be imaged and that are detected by the detector 30 will still have shorter overall times of flight from source 20 to detector 30 than the photons of second sample light which have the shortest times of flight through the object to be imaged. This will be further discussed with reference to Fig. 2.

The iNIRS system 10 is arranged so that the detector 30 may provide part of an interferometer assembly (in combination with the first and second sample channels and the reference channel) configured to combine reference and sample light to obtain an interference pattern (an interferogram). The obtained interference pattern may comprise a contribution associated with the first sample light (and first sample receiving channel 351) and a contribution associated with the second sample light (and second sample receiving channel 352).

The sample light received at the detector 30 will include photons of first sample light and photons of second sample light. These received photons will be at different wavelengths due to different delays and the frequency-swept nature of the light signal at the sample delivery probe 25'. Photons of first sample light received at the detector 30 will have taken different, unique, paths through the object to be imaged. As such, these photons of first sample light received at the detector 30 will have different times of flight to each other, and thus they will be at different wavelengths to each other (due to the wavelength swept emission from the light source 20). Similarly, the photons of second sample light received at the detector 30 will have had different times of flight through the object, and thus these photons will be at different wavelengths to each other.

The additional optical path length 352a is arranged so that the photons of first sample light received at the detector 30 will contain photons in a first wavelength range, and the photons of second sample light received at the detector 30 will contain photons in a second wavelength range. The additional optical path length 352a may have a length selected so that there is substantially no overlap between the firstwavelength range and the second wavelength range.

The resulting interferogram obtained by the detector 30 will thereforecontain a plurality of different beat frequencies (due to the various differences in wavelength). The detected second sample light will be at different beat frequencies than the detected first sample light (as the photons of second sample light will have had longer times of flight). Forexample, the frequency-sweeping at the light source 20 may be such, that the frequency is monotonically increasing over a certain time interval. In this case, if a first sample light is detected with a delay to the reference signd within this certain time interval, but earlier in time than a second detected second sample light, and that detected sample light is also detected with a delay to the reference signal within this certain time interval, then the second detected sample light will be at a higher beatfrequency thm the detected first sample light. Within the range of beat frequencies obtained for each of the first and second sample light, the higher beat frequencies may correspond to photons with longer times of flight (e.g., deeper penetrating photons). It is to be appreciated that, in this example, the reference light will travel a shorter distance to reach the detector than the sample light. Of course, the alternative could be provided in which the sample light travels less far than the reference light -in which case, the higher beat frequencies will be associated with the shortest times of flight However, in this example, the reference light will be assumed to be travelling less far than the sample light.

It is appreciated that in general the mapping between path delays and beat frequencies depends on the frequency sweeping patterns used, i.e. the nature of the wavelength swept emitted light at the light source 20. In some instantiations, further signal characteristics of the mixed signal, i.e. the interferogram, may be used, such as signal phase.

For a single path through the object 2, the intensity received at the photodetector 303 may be /(t) cc (DTA" 0 at beat frequency 6, f. Correspondingly, for an arbitrary number of paths through the object 2, the intensity may be similar to 1(t) at cos(2aAft ( 5) Where ai are amplitudes for path i for the frequency at Afi. Due to the delay line 352a of appropriate length, the frequencies corresponding to the paths received through probe 351' generally form a non-overlapping cluster with the frequencies due to the paths through probe 352', as explained above.

When the probes 351' and 352' are sufficiently closely located to each other on the object 2, for example as illustrated in Fig. 1, the signals received at the probes 351' and 352' may be very similar and may be modelled as identical, except for an extra attenuation A and delay due to the delay line 352a, leading to a frequency offset t9. Then, the intensity may be approximated as 1(0 'a cos(2mAft + Acti cos(27(Aft +19)0 ( 6) Where a are amplitudes for the frequency at An, A may be an attenuation factor introduced by the delay line 352a, and t9 may be a frequency offset introduced by the delay of delay line 352a.

In other words, the first summation may represent the signal received by probe 351' via the sample receiving channel 351. The second summation term may represent the signal received by probe 352' via the sample receiving channel 352.

The iNIRS system 10 may be configured to obtain a digital representation of each resulting interferogram at the digital signal output 307. For example, the detector 30 may include an analogue to digital converter ('ADO') 306 configured to obtain discrete-time interferogram data from each interferogram provided by the detector 30. Each obtained interferogram may be Fourier analysed (e.g., using an FFT or IFT) for obtaining an indication of a distribution of time of flight ('DTOF') for sample light photons incident on the light detector 30. This operation may be implemented in the controller 40, for example. Each determined DTOF may provide a distribution showing the time of flight for all sample light photons which were incident on the light detector 30 at a given moment in time. The DTOF may contain an ensemble average representing a large number of incident photons (in each of a plurality of different time of flight, 'TOE', bins). The intensity for each TOF bin will provide an indication of the number of incident photons in that TOF bin (e.g., for all incident photons having a time of flight within a range of time of flights covered by that TOF bin). A phase of a TOF bin (e.g., obtained using a Fourier analysis) may represent m average phase for all of the photons arriving in that TOF bin.

As illustrated above, the digital output signal 307 will be a discrete-time signal proportional to 1(t) L at (27rAfit) . Taking the Fourier Transform of this at controller 40 will provide a signal spectrum approximately: 711(01 cc ai [5(f -aft) + cl(f + 61)] ( 7) where the ai are proportional to the energy found in each TOE bin associated with ri (aft), where the determination of the delays vi depends on the specific sweeping pattern used, as explained above. For simplicity, this has been shown in the continuous domain, but may be similar for discrete-time signals, using a discrete-time Fouriertransform (DFT).

In the example of Fig. 1, the detector 30 will obtain an interferogram which effectively contains two separate DT0Fs: one for photons of first sample light which have travelled through the object to be imaged, and one for photons of second sample light which have travelled through the object to be imaged. An example of such obtained interferogram data after an FFT at controller 40 is shown in Fig. 2.

Fig. 2 shows a plot 400 of amplitude versus time of flight for photons of sample light received at the detector 30 and processed at controller 40. The data shown in Fig. 2 may have been obtained by performing an FFT on obtained interferogram data, as explained above. The amplitude may provide an indication of the total number of photons detected at each time of flight. The plot 400 includes: first sample light data 401 containing photons of first sample light received via e.g., probe 351' in a firsttime of flight range 401a, and second sample light data 402 containing photons of second sample light received via probe 352' in a second time of flight range 402a. The data shown in Fig. 2 may have been obtained using a single digitiser channel of the ADC. Because of a suitably chosen delay line 352a, the interferogram as shown in Fig. 2 may be such that the first light data 401 and the second light data 402 do substantially not overlap. As can be seen in Fig. 2, the first sample light data 401 is contained in a much lower time of flight range than the second sample light data 402. The plot 400 effectively contains two separable DT0Fs: one for the first sample light and one for the second sample light.

One advantage of the using such a suitable delay line 352a leading to substantially not overlapping light data On time / frequency) sets 401 and second light data 402, is that a single data processing pipeline may be used to effectively sequentially process the light data 401 and the light data 402. As will be appreciated, the light data sets 401 and 402 would require two processing pipelines if they were to be processed at the same time and/or may interfere with each other. Effectively, it would not be possible to distinguish the data arriving from probe 351' and the data arriving from probe 352' if the data were not separated in time.

Each DTOF follows a similar distribution. This distribution includes an initial sharp peak followed by a gradually diminishing tail. The ends of the DTOF may be defined by the points at which any signal associated with that DTOF is no longer distinguishable from background noise. In other words, the points at which a DTOF starts and ends for detected sample light are the points where the number of received sample light photons are sufficiently small (or non-existent)that a detected amplitude at those beat frequencies (i.e., times of flight) is not significantly (e.g. statistical significance) greater than a detected amplitude associated solely with background noise (rather than incident sample light photons).

The two DT0Fs shown in the plot 400 of Fig. 2 are separated from each other by a separation time 410. The separation time 410 is a temporal offset between the first DTOF (for first sample light received at probe 351') and the second DTOF (for second sample light received at probe 352') that are contained within the FFT of the digital output 307 data, mapped onto appropriate delays/delay bins. In Fig. 2, this separation time 410 is shown as a temporal offset between the peak value of the first sample light data 401 and the peak value of the second sample light data 402. However, it is to be appreciated that the separation time 410 may be a temporal offset between other suitable points in each time-of-flight distribution, such as between the lowest time of flight for f irst and second sample light, the longest time of flight for first and second sample light, and/or an average value for f irst and second sample light time of flights (e.g., a mean time of flight for first and second sample light). In other words, the separation time 410 represents a difference in detected times of flight for the first and second sample light photons.

The separation time 410 corresponds substantially to the delay line 352a associated with the second sample light received at the detector 30. The separation time 410 is longerthan the delay spread 401a for the first sample light data 401 (e.g., the separation time 410 is greater than the amount of time separating the start and end of the first time of flight range 401a). As can be seen in Fig. 2, there is no overlap between the first sample light data 401 and the second sample light data 402 and thus the first sample light data 401 may be separable from the second sample light data 402. In other words, two separate DT0F5 may extracted from the obtained interferogran (one for f irst sample light data 401 and one for second sample light data 402). Each of these separate, extractable, DT0Fs comprises substantially no data which corresponds to photonsfrom the other sample light data.

The controller 40 of the iNI RS system 10 may be configured to process the interferogram data.

The controller 40 may be configured to process the interferogram data obtained from the detector (and ADC) to obtain DTOF data forthe first sample light and DTOF data forthe second sample light. As set out above, this DTOF data may be indicative of beat frequency data, which is itself indicative of the path delays ri through the object 2. The controller40 may be configured to extract first DTOF data 401 and second DTOF data 402. This may comprise separating out the two DT0Fs contained in the interferogram. The controller 40 may be configured to separate based on an indication of time of flight and frequency in the Fourier transform of the discrete-time output signal obtained from detector 30. The indication of time of flight may comprise an indication of a value for the beat frequency (e.g., prior to processing this data to obtain DTOF data), or an indication of time of flight (e.g., as obtained from the processed DTOF data).

The controller 40 may be configured to process the interferogram data to obtain two different DT0Fs. The controller 40 may be configured to perform imaging for the object 2 (e.g. neuroimaging of a subject's brain) based on both of the two different DT0Fs obtained from the same interferogram data as illustrated in Fig. 2. The controller 40 may be configured to first separate out the two different DT0F5 401, 402 from the interferogram data 400. This may comprise identifying the first DTOF data 401 as all data points below a first threshold time of flight (and optionally above a lower threshold time of flight, which is less than the first threshold time of flight), as well as identifying the first DTOF data 402 as all data points above a second threshold time of flight (and optionally below an upperthreshold time of flight, which is more than the second threshold time of flight). The first and second threshold time of flight may be the same time, or they may be different (with the second threshold time of flight being greater than the first time of flight).

The controller 40 may be configured to compensate for this difference in times of flight between the first and second sample light. For this, the controller 40 may be configured to align the first DTOF (for the first sample light 401) with the second DTOF (for the second sample light 402). This may be achieved by correlating data 401 and 402. In such an instance, the appropriately matched first and second DTOF 401,402 may be added. Because noise terms are uncorrelated between 401 and 402, a signal power gain may be achieved, improving the Signal-to-Noise-Ratio (SNR) in the combined signal, compared to using either the first DTOF 401 or second DTOF 402 on its own. This is particularly true when the sample receiving probes 351' and 352' are spatially close, i.e., nearby. In that case, the DTOF signals 401 and 402 tend to be more similar.

As will be appreciated, alternatively by deliberately spacing the sample receiving probes 351' aid 352' spatially apart, more diversity will be present in the sample received signal which may permit a larger volume of the object 2 to be investigated.

The controller 40 may be configured to align the first DTOF data with the second DTOF data. For example, the controller 40 may apply a temporal offset to one or both of the first and second DTOF data. The temporal offset may be selected so as to remove the influence of the delay line 352a on the time-of-flight data for the second DTOF. For example, the controller40 may align the first and second DTOF so that they both only contain information about photon time of flight through the object (rather than also through channel(s) between light source and object, and/or through channel(s) between object and detector). The controller 40 may be configured to align the first and second DTOF by applying a fixed temporal offset associated with the difference in duration of time taken for light to travel along the first and second sample receiving channels. The controller 40 may be configured to align the first and second DTOF by applying a temporal offset based on aligning one or more features in the first time of flight data with corresponding features in the second time of flight data, such as by aligning based on a peak TOF value, an average TOF value, and/or a maximum or minimum TOF value for each of the two DT0Fs.

As such, the controller40 of the iNIRS system 10 may be configured to obtain two pieces of DTOF data (e.g., first and second DTOF data 401, 402) using one light source 20 and one light detector 30, using multiple probes 351', 352'. The controller 40 may obtain these two pieces of DTOF data 401, 402 using only one ADC 306, e.g., which digitised both DTOF distributions from the sane interferogram obtained by the controller40. The iNI RS system 10 may therefore be configured to obtain twice as much data for each data processing pipeline. The first and second sample receiving probes 351', 352' may be arranged proximal to each other on the object to be imaged. The controller 40 may be configured to average the first and second DTOF 401, 402 data for the object 2, when the data are suitably matched to correlate, as described. For example, the iNIRS system 10 may be configured to sequentially obtain a series of interferograms (and thus interferogram data 400). For each item of interferogram data 400, the controller 40 may be configured to obtain first and second DTOF data 401, 402. The controller 40 may be configured to provide imaging of the object (e.g., neuroimaging of a subject's brain) based on a temporal evolution of the DTOF data 400 (e.g., based on how the time-of-flight distributions evolve over time). For this, the first and second DTOF data in each item of interferogram data 400 may be combined over two or more separate interferogram data 400 to provide averaged DTOF data, and the imaging of the object may be based on temporal evolution of sequential items of averaged DTOF data.

In accordance with various embodiments of the invention, the amplifier 305 of detector 30 may be used to scale the output signal to the full range of the ADC 306 and limit the electronic frequency of the circuit to further maximise the SNR. This amplified voltage is then provided to the ADC 306 to be digitised. The ADC 306 comprises a digitiser having sufficient bandwidth so that the full signal bandwidth containing time of flight information may be digitised without attenuation. As will be appreciated in the context of the present disclosure, and as described in more detail below, the ADC may have a digitisation bandwidth which is far greater than that of a single DTOF 401, 402. By effectively providing more DT0Fs in each interferogram to be digitised, the iNI RS system 10 may utilise a greater proportion of the digitisation bandwidth for each ADC 306 of the system 10.

For example, the ADC 306 bandwidth may be at least as large as the bandwidth forthe combined light signal data 401, 402 (e.g. the digitiser bandwidth may be sufficiently large to process a plurality of different DTOF 401,402 distributions included in one interferogram 400). Forexample, the ADC 306 may be selected to have a sampling rate high enough so that the Nyquist criterion is met for the bandwidth of the signal to be processed. The ADC 306 may be provided as part of each light detector 30, or the digitiser may be part of the controller 40, and the controller40 may be coupled to each of the light detectors 30 to receive continuous time electrical interference signals therefrom which are to be digitised.

The iNIRS system 10 is configured to obtain a plurality of digital signal outputs 307 in time indicative of a plurality of received sample lighton light detectors 30For example, for each light detector 30, a time series of digital signal outputs307 may be obtained, wherein each subsequent digital signal output 307 is for a subsequent point in time at which a combined light signal was obtained. As described above, each digital signal output 307 may be in the form of interferogra-n data indicative of an interferogram obtained using the detector 30 and the controller 40 (from which two or more sample light DT0F5 401,402 may be obtained for that point in time at that detector 30).

In the examples described herein, the iNIRS system 10 is configured to obtain interferogram data 410 which effectively contains an indication of two separate DT0Fs (one for the first sample light 401 and one for the second sample light 402). In the example of Fig. 1, two sample receiving channels 351,352 are provided, with a delay line 352a in one of those sample receiving channels, so that one source 20 and one detector 30 may be used for obtaining two separate DT0Fs 401, 402. However, it will be appreciated in the context of the present disclosure that similar functionality may be provided by introducing a delay into one of two or more sample delivery channels On addition to, or as an alternative to, introducing adelay into one of two or more sample receiving channels). An example of such an arrangement will now be described with reference to Fig. 3a.

Fig. 3a shows an iNI RS system 10. As with the system of Fig. 1, there is a light source 20, a detector 30, and an object 2 to be imaged. There is furthershown a reference delivery channel 26, a delay line 252a, and a light splitter 24. In Fig. 3a, there is a first sample delivery channel 251 and a second sample delivery channel 252. Each sample delivery channel 251, 252 couples the light source 20 to the object 2 to be imaged. The iNIRS system 10 also includes a sample receiving channel 35 and a sample receiving probe (not shown). The detector 30 is arranged to combine reference light carried on the reference channel 26 with sample light carried on the sample receiving channel.

In Fig. 3a, a delay line 252a is provided in the second sample delivery channe1252. The additional optical channel length 252a shown in Fig. 3a is arranged to provide the same functionality as the delay line 352a shown in Fig. 1. That is, the additional optical channel length 252a is arranged so that some of the sample light incident at the detector will have a substantially longer time of flight as compared to other sample light incident at the detector (where that additional time of flight is due to the sample light travelling along an additional length of optical channel). In Fig. 3a, the light source 20 is configured to direct first sample light towards the object 2 through the first sample channel 251 and second sample light towards the object 2 through the second sample channel 252. Both the first and second sample light may be received at the same sample receiving probe coupled to the sample receiving channel 35. Each of the first and second sample light may then be combined with reference lightto provide first and second beat frequencies in a detector 30. As with the example of Fig. 1, the additional optical channel length 252a is selected to inhibit overlap between received sample signals on a common sample receiving channel 35 associated with the first sample light and received sample signals associated with the second sample light. The light source 20 may be coupled to a light splitter 24 which is configured to split light from the light source 20 between: (i) first sample light to be directed towards the object along the first sample delivery channel 251, (ii) second sample light to be directed towards the object along the second sample delivery channel 252 (and through the additional optical channel length 252a), and (iii) reference light to be directed towards the detector 30 along one or more reference channels 26.

As illustrated in Fig. 3a, such an exemplary configuration may achieve a similar function to Fig. 1. Namely, the temporal separation between two sample delivery paths 251, 252 may be provided with a delay line 252a on the delivery side, i.e., light may be delayed before it is delivered to the object 2, instead of on the receiving side.

Another example iNIRS system 10 will now be described with reference to Fig. 3b. Fig. 3b shows an iNI RS system which includes a first light source 20a and a second light source 20b. There is further shown light splitters 24a, 24b, object 2, sample delivery channels 25a, 25b, reference delivery channels 26a, 26b, sample receiving channels 351, 352, delay line 352a, and detector 30. The first light source 20a is coupled to: (i) the object 2 via a sample delivery channel 25a, and (ii) the detector 30 via a reference channel 26a. The second light source 20b is coupled to: (i) the object via a sample delivery channel 25b, and (ii) the detector 30 via a reference channel 26b.

The system includes first and second sample receiving channels 351, 352 of the type described above in relation to Fig. 1 (e.g., with a delay line 352a in the second optical channel 352). The iNIRS system 10 may be arranged so that the first sample receiving channel 351 may receive first sample light from the first light source 20a and first sample light from the second light source 20b, and the second sample receiving channel 352 may receive second sample light from the first light source 20a and second sample light from the second light source. 20b In particular, one advantage with the system of Fig. 3b (as compared to that of Fig. 1) is that, if more light sources 20a, 20b are added to the system, these light sources 20a, 20b may combine with the existing two sample receiving channels so that first and second sample light may be provided to the detector 30 for each light source (where the second sample light has an optical delay as compared to the first sample However, light sources 20a, 20b advantageously do not send sample light to the object 2 simultaneously, as the plurality of light sources 20a, 20b might otherwise interfere with each other on the receiving side. In this case, the light sources 20a, 20b may be operated in a time-multiplexing fashion, or, an additional delay line may be introduced in either sample delivery path 25a or 25b (before the light splitters 24a, 24b, not illustrated). The additional delay line, and the delay line 352a for the second sample receiving channel 352 may be selected so that all four received signal path combinations (source 20 to sample receiving channels 351,352; source 20b to sample receiving channels 351, 352) may combine on the same interferogram without overlap. Additionally, or alternatively, the system may be configured to provide temporal multiplexing of the operation of the two light sourcesso that only one light source is operated at a time (and thus the additional optical channel length 352 may itself inhibit spectrd overlap between in the interferogram).

It may be observed that by suitable choice of transmit light signals via sample delivery channels 25a, 25b, it may be possible to emit light simultaneously via light sources 20a, 20b and still disentangle the light source contributions at the detector 30, for example by introducing some form of orthogonality between the light sources 20a, 20b.

Fig. 3c shows another example iNIRS system 10. In the iNIRS system 10 of Fig. 3c, there is one light source 20 and one detector 30. The light source 20 is coupled to the object 2 via a sample delivery channel 25, and to the detector 30 via a reference channel 26. There is furtiershown a light splitter 24, a first sample receiving channel 351, a second sample receiving channel 352 aid associated delay line 352a. The system of Fig. 3c is similar to that of Fig. 1, exceptthat the system of Fig. 3c includes a third sample receiving channel 353 and associated delay line 353a. The third sample receiving channel 353 includes an additional optical channel length 353a, which is of a different length to the delay line 352a of the second optical channel 352. The three sample receiving channels 351, 352, 353 are arranged to be of different lengths so thatthree sample light received signals may be contained within one interferogram without overlap. For example" one of the additional optical channel lengths 352a, 353a may be significantly bigger than the other to provide a further increase in time of flight to sample light photons travelling along that channel. As will be clear to the person skilled in the art, the same concept may be extended directly to en arbitrary plurality of sample receiving channels and associated delay lines.

It is to be appreciated in the context of the present disclosure that the different examples may be combined with each other. For example, an iNIRS system may be provided in which there are two sample delivery channels (of different lengths) and two sample receiving channels (of different lengths). These channels may be arranged (e.g., of selected lengths) so that all of the combinations of different sample light channel paths will fit in their own region on the resulting interferogram. That is, the channels may be arranged to stack all of the processing signals within one interferogram without overlap between the distributions. Similarly, multiple sources may be used where some, or all, of those sources are coupled to the object via two or more sample delivery channels. Likewise, there may be three or more sample delivery channels and/or sample receiving channels.

The present disclosure may therefore provide iNIRS systems 10 in which sample light may travel from source to object, and from object to detector, along two or more different optical paths, and may be combined through suitable selection of delay lines to combine in a non-overlapping manner. This arrangement may enable more data to be obtained without needing to increase the number of digitisers ADC 306 in the system. For example, iNIRS systems of the present disclosure may be arranged to maximise the number of different beat frequency distributions provided within one interferogramto be digitised. Thatis, fora given digitiserbandwidth, the iNIRS system may be arranged with the maximum number of different distributions present in one interferogram without overlap between those distributions in that interferogram.

In each of the examples described herein, each interferogram may provide an indication of the sample light photons which were incident on the detector 30 (or photodetector 303) at a given moment in time. Within that interferogram, there may be two or more different beat frequency distributions associated with different sample light incident times on the detector (first/second etc.). A controller 40 may be configured to extract a DTOF for each of these sample light beat frequency distributions. Each said determined DTOF may provide a distribution showing the time of flight for relevant sample light photons which were incident on the light detector 30 at a given moment in time. Each DTOF may contain an ensemble average representing a large number of incident photons (in each of a plurality of different TOF bins). The intensityfor each TOF bin will provide an indication of the number of incident photons at that TOF. A phase of a TOF bin (e.g. obtained using a Fourier analysis) may represent an average phase for all of the photons arriving in that TOF bin. As described in more detail below, numerous properties of the subject's brain tissue may be determined based on such DTOF data obtained for the subject's brain tissue.

The iNIRS system 10 may be configured to obtain a plurality of time-ordered DT0Fs (e.g., 401, 402) from the light detector 30 and the controller 40. This may include averaged DT0Fs (where the simultaneously obtained first and second sample light DT0Fs 401, 402 were averaged). The digitiser 306 may provide a discrete-time/digital output indicative of the different measurements to the controller 40, and this digital output may optionally be processed in a number of ways to provide and/or use such DTOF data in the controller 40. Examples of such steps will now be described.

The controller 40 may be configured to receive raw digital interferogram data (e.g., data representative of the interferogram obtained by converting the combined light signal into digital data). This raw interferogram may be divided into individual sweeps for the wavelength swept emission from the light source 20. For example, the sweep rate of the light source 20 and a time at which the first sweep commenced may be used to determine the sweep cycles. The data may then be divided into groups, where each group representsan individual sweep. An optional Hilbert transform may be performed on the data at this stage. Data windowing may be performed (e.g., with a Hann or Blackman-Harris window) to reduce sidelobes in the data. A Fourier analysis may be performed on the data, either inverse or normal. For instance, an inverse Fourier Transform may be performed. The Fourier analysis may be performed for each wavelength sweep of the light source 20. The resulting data may be in the form of a series of Temporal Point Spread Functions ('TPSF'), with each TPSF corresponding to an associated wavelength sweep. The TPSF data may be processed to remove an Instrument (Impulse) Response Function ('IRF) therefrom to provide the DTOF data. For example, the I RF may be filtered out (e.g., deconvolved and/or subtracted) using post-processing.

In other words, the iNIRS system 10 may be configured to determine a time-ordered series of time-of-flight distributions for sample light photons incident on one or more detectors. Where the object 2 to be imaged is a subject's brain, this DTOF data may be processed to provide information relating to a number of different physical properties of the subject's brain tissue. The DTOF data may be used to determine optical properties of the medium through which the sample light has travelled. Each TOF bin in a DTOF may represent a selected volume within the subjects brain, and each DTOF represents atotal volume of tissue probed by the photons (e.g. each DTOF may represent a weighted average of properties of the brain tissue, as well as other tissues through which the photons have travelled such as scalp, skull etc.). The optical properties include scattering and absorption properties for the subject's brain tissue. The DTOF data may be used to determine dynamic properties of the subject's brain tissue, such as how particular properties vary over time. This includes how the optical properties evolve over time, as well as properties indicative of movement within the subject's brain tissue (e.g., due to the flow of blood). For example, the changes of the DTOF over time may be observed.

The controller 40 may store data which correlates time of flight fora sample light photon (or fora TOF bin) to an indication of average path trajectoryfor that photon. This may include an indication of the depth of the penetration into the subject's brain tissue forthat photon, and/or an indication of the region(s) of the subject's brain tissue through which that photon travelled from light source 20 to light detector 30. The controller 40 may be configured to process the DTOF data by dividing this data up into selected TOF bins. Within each TOF bin, the data may provide depth-resolved evolution data for the subject's brain tissue. That is, as the TOF may be associated with certain penetration depths or regions, each TOF bin may contain data showing properties associated with a certain penetration depth or region. The evolution of data within each TOF bin may therefore provide an indication of how one or more properties of the subject's brain tissue are evolving. For example, where the evolution suggests a change in movement (e.g., a flow of blood), that movement may be identified, as may the region in which that movement is occurring. For this, a TOF-resolved decay slope may be used to identify how the curve is decaying overtime for specific TOFs (e.g., for specific penetration depths/regions).

In other words, the iNIRS system 10 may be configured to perform an autocorrelation in which DT0Fs for successive wavelength sweeps are combined to assess fluctuations in the light field at the light detector 30 over time. The fluctuations may be quantified due to relevant fluctuations in DT0Fs over time. The fluctuations may also be depth-resolved, by identifying the relevant TOFs at which those fluctuations are occurring (and thus the relevant penetration depths/regions).

The controller 40 may be configured to process the data received from the light detector(s) 30 to provide time of flight information with depth-resolved autocorrelations for the subject's brain tissue. The controller 40 may be configured to use this information to obtain an indication of a plurality of different properties of the subject's brain tissue, such as intracranial pressure CI CP'), blood flow index, artery elasticity, etc. These properties of the subject's brain tissue may be used for a plurality of different forms of neuroimaging and analysis, such as in a brain-computer interface, f or imaging regions of the brain to identify potential localised injury or strokes, and/or to monitor neuro responses to substances, such as drugs.

It will be appreciated in the context of the present disclosure that examples described herein ae not intended to be limiting. Instead, examples describe certain potential ways of implementing the claimed technology. Forexample, the iNI RS system 10 is described with a series of optical cables providing channels and probes for coupling those channels to the subject's scalp. However, it will be appreciated that the probes themselves may be part of the optical channels, or probes may not be provided at all. Similarly, the arrangement of reference channels is just intended to show that reference light is delivered from the light source to the light detectors via optical channels (ratherthan via the subject's brain tissue). For example, where multiple light sources are provided, each light source may include one reference channel for each light detector, where that reference channel directly connects the light source to the light detectorAdditionally, oralternatively, where multiple light sources are used, reference light may be transmitted on a common reference opticsi channel, where some of that reference light is taken from the common reference optical channel to each of the optical detectors. The light source may also be arranged to deliver light to one of a plurality of different locations on the subject's scalp. For example, the light source may be coupled to a plurality of different sample delivery channels, each extended towards the subjects scalp (e.g., from a light splitter).

It will be appreciated that the particular arrangement shown for signal processing circuitry of the detector need not be considered limiting. Each light detecting arrangement may be configured to combine first sample, second sample light and reference light to provide combined light signals with components at one or more first beat frequendes and one or more second beat frequencies and to process those combined light signals to determine one or more properties of the subjects brain tissue. Any suitable signal processing and/or conversion circuitry could be used for this. For example, a transimpedance amplifier may not be needed (e.g., depending on the photodetector or ADC, no current to voltage conversion may be needed, orthis may be performed in a different way). Similarly, a balanced photodetector need not be used, and instead a single photodetector, such as a photodiode, could be used. Similarly, the arrangement with the ADC shown in the Figs. need not be considered limiting. For example, multiple ADCs may be used (e.g., one for each detector output stream), or all detector output streams may be fed into one common ADC.

It will be appreciated from the discussion above that the examples shown in the figures are merely exemplary, and include features which may be generalised, removed or replaced as described herein and as set out in the claims. With referenceto the drawings in general, itwill be appreciated that schematic functional block diagrams are used to indicate functionality of systems and apparatus described herein. In addition, the processing functionality may also be provided by devices which are supported by an electronic device. It will be appreciated however that the functionality need not be divided in this way, and should not be taken to imply any particular structure of hardware otherthan that described and claimed below. The function of one or more of the elements shown in the drawings may be further subdivided, and/or distributed throughout the apparatus of the disclosure. In some examples the function of one or more elements shown in the drawings may be integrated into a single functional unit.

As will be appreciated by the skilled reader in the context of the present disclosure, each of the examples described herein may be implemented in a variety of different ways. Any feature of ay aspects of the disclosure may be combined with any of the other aspects of the disclosure. For example, method aspects may be combined with apparatus aspects, and features described with reference to the operation of particular elements of apparatus may be provided in methods which do not use those particular types of apparatus. In addition, each of the features of each of the examples is intended to be separable from the features which it is described in combination wEth, unless it is expressly stated that some other feature is essential to its operation. Each of these separable features may of course be combined with any of the other features of the examples in which it is described, or with any of the other features or combination of features of any of the other examples described herein. Furthermore, equivalents and modifications not described above may also be employed without departing from the invention.

Certain features of the methods described herein may be implemented in hardware, and one or more functions of the apparatus may be implemented in method steps. It will also be appreciated in the context of the present disclosure that the methods described herein need not be performed in the order in which they are described, nor necessarily in the order in which they are depicted in the drawings. Accordingly, aspects of the disclosure which are described with reference to products or apparatus are also intended to be implemented as methods and vice versa. The methods described herein may be implemented in computer programs, or in hardware or in any combination thereof. Computer programs include software, middleware, firmware, and any combination thereof. Such programs may be provided as signals or network messages and may be recorded on computer readable media such as tangible computer readable media which may store the computer programs in non-transitory form. Hardware includes computers, handheld devices, programmable processors, general purpose processors, application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), and arrays of logic gates.

Any controller of the present disclosure may be implemented with fixed logic such as assemblies of logic gates or programmable logic such as software and/or computer program instructions executed by a processor. The controller may comprise a central processing unit (CPU) and associated memory, connected to a graphics processing unit (GPU) and its associated memory. Other kinds of programmable logic include programmable processors, programmable digital logic (e.g., a field programmable gate array (FPGA), a tensor processing unit (TPU), an erasable programmable read only memory (EPROM), an electrically erasable programmable read only memory (EEPROM), an application specific integrated circuit (ASIC), or any other kind of digital logic, software, code, electronic instructions, flash memory, optical disks, CD-ROMs, DVD ROMs, magnetic or optical cards, other types of machine-readable mediums suitable for storing electronic instructions, or any suitable combination thereof. In particular, any controller of the present disclosure may be provided by an ASIC.

Other examples and variations of the disclosure will be apparent to the skilled addressee in the context of the present disclosure.

Claims

Claims 1. An interferometric near infrared spectroscopy, iNIRS, system comprising: a light source configured to provide an emission of light, where the emitted light is wavelength swept; one or more detectors; one or more reference channels arranged to deliver the emitted light from the light source to the one or more detectors; the iNIRS system comprising a plurality of optical channels comprising: one or more sample delivery channels with one or more associated sample delivery probes to deliver the emitted light from the light source to an object; and one or more sample receiving channels with one or more associated sample receiving probes to receive light at the one or more detectors, the received light comprising emitted light that travelled through the object; and wherein each of the plurality of optical channels comprises one of the one or more sample delivery channels and one of the one or more sample receiving channels; wherein the iNIRS system comprises one or more delay lines associated with the one or more sample delivery channels and/or the one or more sample receiving channels; wherein the one or more detectors are arranged to combine into an optically combined signal: emitted light received via the one or more reference channels; and received light received via the one or more sample receiving channels; wherein the one or more detectors are arranged to output an electrical signal based on beat frequency components comprised in the optically combined signal; wherein the delays of the one or more delay lines are selected so that there is no substantial overlap between the beat frequency components due to different ones of the plurality of optical channels in the electrical signal.
2. The iNIRS system of claim 1, wherein a portion of one optical channel of the plurality of optical channels shares a common sample delivery channel and/or common sample receiving channel with a second optical channel of the plurality of optical channels.
3. The iNIRS system of any preceding claim, comprising a controller enabled to generate a frequency spectrum for each of the different ones of the plurality of optical channels from the electrical signal.
4. The iNIRS system according to claim 3. wherein the controller is enabled to autocorrelde and combine the frequency spectra for each of the different ones of the plurality of optical channels.
5. The iNIRS system according to preceding claims 3 to 4, wherein the controller is enabled to compute path delays associated with components of the frequency spectrum for each of the different ones of the plurality of optical channels from the electrical signal.
6. The iNIRS system according to claim 4, wherein the controller is enabled to compute path delays associated with components of the combined frequency spectra
7. The iNIRS system according to any of the preceding claim, wherein the one or more delay lines comprise a length of optical fibre or an optical cavity.
8. The iNIRS system according to any claim 4 to 7, wherein the controller is enabled to align the frequency spectra for each of the different ones of the plurality of optical channels based on the autocorrelation.
9. The iNIRS system according to claim 3 to 8, wherein the controller is enabled to compute a time-of-flight distribution based on the frequency spectra and/or path delays.
10. The iNIRS system of any preceding claim, wherein the selected delays between different ones of the plurality of optical channels are at least as large as the expected delay spread.
11. An interferometric near infrared spectroscopy, iNIRS, method comprising: providing an emission of light from a light source, where the emitted light is wavelength swept; configuring one or more detectors to process received light; delivering the emitted lightfrom the light source to the one or more detectors along one or more reference channels; generating a plurality of optical channels by: delivering the emitted light from the light source to the object via one or more sample delivery channels with one or more associated sample delivery probes; and receiving light at the one or more detectors via one or more sample receiving channels with one or more associated sample receiving probes, the received light comprising emitted light that travelled through the object; and one or more sample receiving channels with one or more associated sample receiving probes to receive light at the one or more detectors, the received light comprising emitted light that travelled through the object; and wherein each of the plurality of optical channels is selected to comprise one of the one or more sample delivery channels and one of the one or more sample receiving channels; Combining into an optically combined signal at the one or more detectors: emitted light received via the one or more reference channels; and received light received via the one or more sample receiving channels; generating an electrical output signal at the one or more detectors based on beat frequency components comprised in the optically combined signal; selecting delays of one or more delay lines so that there is no substantial overlap between the beat frequency components due to different ones of the plurality of optical channels in the electrical signal, and wherein the iNIRS system comprises one or more delay lines associated with the one or more sample delivery channels and/orthe one or more sample receiving channels.
12. The iNIRS method of claim 11, whereby a portion of one optical channel of the plurality of optical channels is shared through a common sample delivery channel and/or common sample receiving channel with a second optical channel of the plurality of optical channels.
13. The iNIRS method of claim 11 or 12, whereby a frequency spectrum isgeneratedforeach of the different ones of the plurality of optical channels from the electrical signal at a controller.
14. The iNIRS method according to claim 13, whereby the frequency spectra are autocorrelated and combined for each of the different ones of the plurality of optical channels at the controller.
15. The iNIRS method according to preceding claims 13 to 14, whereby path delays are computed associated with components of the frequency spectrum for each of the different ones of the plurality of optical channels from the electrical signal at the controller.
16. The iNIRS method according to claim 14, whereby path delays are computed associated with components of the combined frequency spectra at the controller.
17. The iNI RS method according to any of claims 11 to 16, whereby delays are generated in the one or more delay lines by selecting a desirable length of optical fibre or an optical cavity.
18. The iNIRS method according to any claim 14 to 17, whereby the frequency spectra for each of the different ones of the plurality of optical channels are aligned based on the autocorrelation at the controller.
19. The iNIRS method according to claim 13 to 18, whereby atime-of-flight distribution based on the frequency spectra and/or path delays is computed.
20. The NIRS method of any of claims 11 to 19, whereby the delays between different ones of the plurality of optical channels are selected so that they are at least as large as the expected delay spread.