GB2446880A

GB2446880A - Imaging fluid flow properties using Doppler optical coherence tomography

Info

Publication number: GB2446880A
Application number: GB0703657A
Authority: GB
Inventors: Rulkang K Wang
Original assignee: Cranfield University
Current assignee: Cranfield University
Priority date: 2007-02-24
Filing date: 2007-02-24
Publication date: 2008-08-27
Also published as: GB0703657D0

Abstract

A Doppler optical coherence tomography (DOCT) system is disclosed in which one or two OCT systems are used to produce an interference signal which is processed in combination with measured Doppler characteristics to measure the localised fluid-flow velocity in a scattering sample 40 with a micrometre-scale resolution. The system is able to describe the complete quantification of 2D or 3D fluid flow in a sample to yield tomographic images of fluid flow and structure. The present system may be implemented with optical fibres or in free space.

Description

Method and Apparatus for Imaging Fluid-flow Using Optical Coherence

Tomography

Field of Invention

This invention relates to optical coherence tomography or optical low-coherence reflectometry that measures the two arid three dimensional velocity vectors of fluid-flow, with a micrometer-scale localisation resolution

Background of the Invention

The accurate determination of location and flow velocity of moving particles in scattering media is important in a number of areas. Some examples of areas are listed below: * Medical diagnostics where the accurate quantification of the flow of blood and/or other fluids is critical for medical diagnostics, for example to detect retinal and *. : : peripheral blood perfusion, to measure patency in small vessels, and to evaluate S... * . I...

tissue necrosis. * .*.

* Fluid rheology where visualisation of structural and flow of complex fluids is S..

critically important in the characterisation of liquid crystals, polymers, gels and **..

* elastomers, lipid and surfactant systems. I..

* Industrial processes that involve mixing, for example supercritical fluid mixing in pharmaceutical production.

* Flow mixing in combustion engines, and pipe flow characterisation.

* Air flow characterisation in aerodynamics.

Optical coherence tomography (OCT) is a technique that allows for the non-invasive, cross-sectional optical imaging of scattering samples, with high resolution and sensitivity.

OCT is an extension of low-coherence, or white-light, interferometry in which a low temporal coherence light source is utilised to obtain a precise localisation of internal reflection to a probed structure, along an optic axis. The extension of OCT to measure the velocity of fluid flow has been previously reported. Wang et al, "Charactensation of fluid flow velocity by optical coherence tomography", Optics Letters, Vol 20, No ii, 1995. In addition, a US patent 5991697 (Chen et al) described an Optical Doppler Tomography system and method that used an optical coherence tomography system in combination with the Doppler effect to measure the axial profiles or tomographic images of fluid flow velocity in a scattering sample. However, a disadvantage of these systems is that they cannot provide a measure on the actual velocity of the moving particle unless * : :* the Doppler angle, defined as the angle between the detection beam direction and the particle moving direction, is precisely known. For many applications the precise estimation of the Doppler angle is difficult, particularly when the flow is embedded S...

*. within a highly scattering medium: for example in vivo blood flow monitoring, which often gives rise to a problem in determining an accurate estimation of the real flow . velocity. Another difficulty arises from the effects of the turbulent flow where, unlike the case with laminar flow, the velocity direction is a time function at a location; the Doppler angle is thus not a constant value. International patent W098/55830 (Lzzat et at) also describes an imaging system that combines OCT with the Doppler effect to obtain tomographic images of fluid flow velocity in a scattering sample. Unfortunately, the above-mentioned disadvantages are also evident with this method, although this approach

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includes a variance indicator of the interference signals, indicating the strength of the turbulence in the fluid-flow in the scattering sample. However, the variance of interference signal, they described in the patent does not necessarily indicate the turbulence in the fluid flow. This will be described in the context of the current invention. Moreover, the inventors of this method did not disclose how the variance of the interference signal was related to the actual velocity of the probed fluid flow.

As described above, knowledge of the Doppler angle a priori in conventional systems remains a serious obstacle to the practical application of this method. In order to resolve this problem, many groups have proposed either angle or velocity measurements perpendicular to the detection beam direction. A recent report [D.P. Dave, T.E. Miler, Opt. Lett. 25, 1523 (2000)1 proposed a method that used a dual channel optical coherence tomography. The system employed a Wollaston prism in the sampling arm for the * :* ::* accurate measurement of the Doppler angle in scattering media. However, this method is * restricted to 2-dimensional situations. Furthermore, it is only possible to obtain the velocity projected to the probing beam plane. Thus, the final velocity determined does * not (necessarily) represent the actual velocity of the moving particle. Another recent report [H. Ren, K.M. Brecke, Z Ding, Y. Zhao, J.S. Nelson and Z. Chen, Opt. Left. 27, *. 409 (2002)] proposed a method that used Doppler bandwidth, induced by the moving particles passing through a sharp focused sampling beam, to determine the transverse flow velocity. However, for this technique to be valid it has to employ the focus tracking method, in order to match the focus-spot of the sampling beam with the scanning mirror.

Furthermore, the authors did not disclose how the actual velocity could be obtained. In addition, this technique did not consider spectrum broadening (due to the dispersion of "-the light source caused by the flow) into consideration; this may give rise to an inaccurate estimation of the flow-velocity vector.

Therefore, what is needed is a method and apparatus that can be used to accurately determine the real flow-velocities, in two or three dimensions, within a turbid or scattering medium.

Description of the Invention

The present invention is a method where a combination of the Doppler effect on the interference signal between the reference and sample beams and Doppler spectrum bandwidth is used to accurately measure the localised flow velocity within scattering media: when optical coherence tomography imaging is being performed. It does not require knowledge of the Doppler angle a priori; consequently this technique is Doppler angle independent. Notwithstanding, the present invention is also capable of providing * : . the Doppler angles, if it is required. This could be useful with the current existing *: Doppler optical coherence tomograhy system, the velocities of fluid flow to be *..* * * determined without significant modification of the system configuration.

The optical coherence tomography system described, permits imaging of the fluid flow *. within a scattering medium in both 2 or 3 dimensions, as shown in Figure 1. Optical coherence tomography is an imaging technique that uses a Michelson interferometer with a partially coherent light source, generally denoted by reference numeral 10, to form optical sections of scattering materials. The preferred embodiment 10 of the present invention captures interference signals formed between the light retro-reflected from a scanning system 32 in the reference arm and the light back-scattered from the scattering medium 40 in the sampling arm. From this arrangement, microstructures and the fluid-flow velocities within the scattering medium are simultaneously obtained.

As shown in Figure 1, the optical coherence tomography system used with the present invention includes a low coherence light source 11, such as a super luminescent diode (SLD), fibre optic couplers 21, 22 and 23, reference arm scanning assembly 32 and sample probe 30. Other (known) low coherence light sources include, a semiconductor optical amplifier, a rare-earth doped fibre amplifier (commonly used in optical communications), and a femto-second laser. Light from the source 11 and an aiming beam of visible light 34, generated from a laser, (usually a helium neon laser operating at a wavelength of 633nm), are coupled into the fibre optic interferometer 10 via the 2X1 optic couplers 21 and 22, respectively. The light source 11 is split into a reference 25 and sample 24 arms respectively, of the interferometer 10 via a 2X2 optic coupler 23. The *: : :* light intensity in the reference arm can be manipulated in order to reduce the photon *::* access noise and to increase the signal-to-noise ratio of system 10. Polarisation controlling plates are used to match the poiarity of the reference and sample beams, in anns 24 and 25 respectively, to optimise the interference fringe contrast in the system 10.

The light in the reference arm 25 from the optic fibre 26 is directed to an optics 31 device *...., :. that is used to deliver the light to the scanning assembly 32. The light reflected back from 32 is received by the optic fibre 26, via the optics 31, and transmitted back to the fibre coupler 23. The light in the sample arm 24 is coupled to a sample probe 30 adapted to focus light on a scattering sample 40 and to receive the light reflected back from the sample 40. The reflected light received back from the sample is transmitted back to the fibre coupler 23. Preferably, the sample probe 30 has an adjustable focal length, thus allowing adjustment of the focusing beam waist, focal spot size, working distance and depth of focus.

The reflected light received by the fibre coupler 23, back from both the reference and sample arms, is combined and transmitted to the photo-detectors. There are a number of ways to transmit the combined light received at 23 to the photo-detector. A simple method is to split the combined light by fibre coupler 23, and transmit one part of the split light directly to a photo-detector via the optic fibre 29. An alternative method, illustrated in Figure 1, is described below. The combined light is split by a fibre coupler 23 into two parts. One part is transmitted to the photodetector 51 via the fibre coupler 21; another part to goes the photodetector 52 via fibre coupler 22. The light detected by 51 and 52 respectively is sent to a differential amplifier 53 to be differentially amplified.

Because a low coherence light source is used, the combined light from both the reference * ::* and sample arms will form interference fringes (or interference signal), only when the * " optical path lengths of in both the reference and sample arms are matched to within the * coherence length of light source used. The interference signal is a function of the * distance difference between the optical path lengths in the sample and reference arms.

The scanning assembly 32, in the reference arm, is used as one of the methods to provide *:. mterferogram data.

The output from the deferential amplifier is amplified by an amplifier 54 and digitised by an analogue-to-digital converter 55. The output from 55 is then transferred to a computer workstation 57 or a purposely-designed digital signal processing hardware 58, providing a tomographic image of the scanned scattering sample. This is then displayed in a display unit 59. The workstation also provides the controls, via data acquisition and

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scanning control unit 56, to the scanning assembly 32, the sample probe 30 and the data acquisition of pomt 55.

Phase modulation is provided by the scanning assembly 32 in the reference arm 25, but it may be preferential to provide phase modulation in both the reference and sample arms 24 and 25, or just in the sample arm 24. Other methods for providing phase modulation include optical phase modulators or piezoelectric cylinders, installed in the reference or sample arm. The simplest scanning assembly 32 can be a reference mirror mounted on a translation stage, controlled by a computer workstation 57. It will be apparent to those of ordinary skills in the art that there are many known methods and/or mechanisms for providing scanning in the reference arm, other than a mirror mounted onto the translation stage; all of which are within the scope of the present invention.

Two dimensional tomographic images are formed by continuous axial scans through the scanning assembly 32 and through sequential lateral scans by a sampling probe 30, both * of which are controlled by the scanning control unit 56. One cycle of scanning, in the scanning assembly 32, is referred to as collecting an A scan. The A-scan data provides a *.** *. one-dimensional profile of backscattering intensity versus depth, within the scattering sample. * *

The preferred embodiment of the present invention evaluates the Doppler shift, 4t of the interference signal with respect to the modulated frequency, Jo, and the Doppler bandwidth a of the interference signal: from which the velocity of fluid flow in the scattering sample is obtained, in what follows, the formulation is derived in the time/spatial domain. The scan geometry is illustrated for the case where a fluid flow comprises the moving particles (with velocity of V), flowing through the sample beam focused into the fluid sample (Figure 2). The coherence length of the low coherent light source and the focusing optics that is used to focus the probe beam into the scattering medium limits the sampling volume. As depicted in Figure 2, the velocity can be decomposed into two orthogonal vectors, i.e. one component projected in the direction of probing beam, V, and the another component projected in the transverse plane, relative to the probing beam, V,. Thus, the frequency of the interference signalf is shifted from the modulated frequency,fo, by an amount: Af=f-f0=2V/20 (1) The projection velocity of V is thus V = 4f,% /2.

The moving particle and the scanning velocity in the reference arm will induce spectral broadening in the interference signal. In the present invention, four factors that influence the spectral broadening are considered: 1) the velocity from the scanning system, 2) the *:*::* projection velocity, V, from the moving particles, 3) the projection velocity, V, in the transverse plane relative to the probing beam, and 4) the sampling rate of the sampling *.S.

device within the system 10 of the present invention. S..

* The spectral broadening of the interference signal induced by the scanning system is: f0A2 (2) The spectral broadening of the interference signal induced by the projection velocity, V, of the moving particles to the probing beam is: (3) A0 A The spectral broadening of the interference signal induced by the projection velocity, V,, of the moving particles to the transversal plane relative to the probing beam is: = (4) where u is the beam waist of the focusing beam that is directed into the scattering medium.

The last factor being considered, that contributes to the spectral broadening of the interference signal, is an artefact caused by the sampling rate of a sampling device to the interference signal. When FFT of the sampled interference signal of duration is Tjpy', the resultant power spectrum will be broadened by an amount proportional to 1/ but independent of f If the data window is rectangular, the analysis time broadening is exactly:

I O4 =

* Note: if some of the post-FFT spectral smoothing is perfonned to reduce the speckle, then Equation (5) should be modified to reflect the effective spectral window width. * S., Thus S...

04= (5) : ** wheTe k is a constant factor for the correction of spectral broadening, induced by the * sampling rate of a sampling device.

Therefore in general, by a combination of the four factors above, the total amount of spectral broadening becomes: 0 0i + 0 + 0 + (74 (6) With Equations (2)-(6), one can easily calculate the absolute velocity term, J V,j: IC) I 1= -i/2 -(-L2 + (7) The last tenn in the parenthesis of Equation (7) is independent of the velocity of moving particles within the scattering medium, and can be estimated accurately from the known scanning velocity in the reference arm and the sampling frequency of the sampling device. Thus, with Equation (1) and (7), the absolute velocity of the moving particles can be expressed as: v=ç2+v22 foJ2+2ff2w2[a_f_)]2 (8) Note that all the variables in the right hand side of Equation (8) are readily available from the system design parameters, light source parameters, and values from the interference signal either through the FFT method or by other similar mathematical methods. Thus, as * : :* described above, it is obvious that the determination of velocity magnitude of the moving particles does not require knowledge of the Doppler angle a priori. * *..

In practice, the Doppler bandwidth of the interference signal for a data window is S...

evaluated, which includes the Doppler spectral broadening and the intrinsic spectral * bandwidth of the low coherence light source used. Therefore, the final Doppler spectral *.ee.. * .

*:. broadening a is obtained from a subtraction of the intrinsic bandwidth of the low coherence light source from the evaluated Doppler bandwidth of the interference signal.

In addition, as it shown in Figure 2, the Doppler angle can also be evaluated. It is: a = arctan(._) (9) The above outlines the evaluation of parameters representing fluid-flow velocity within the sampling volume as shown in Figure 2, or in other words within the processing data window TFFT of a pixel of the tomographic image. Because the scattering medium is continuously scanned by the system 10, the evaluation of fluid-flow is also continuously performed at each data window, with the elapse of scanning time. The known technique of short time Fourier transforms can be used as a means to obtain time-dependent (or depth-dependent) parameters. However, other time-frequency analysis techniques can also be used for this purpose. All the time-frequency analysis techniques that are used for obtaining depth dependent velocity parameters are covered by the current invention.

The tomographic images are then formed by collection of all the evaluated values at each pixel, in accordance with the designed scanning method. The tomographic image of the fluid-flow in the scattering sample can be displayed only by the use of the magnitude values of the velocity evaluated by Equation 8, at each pixel. Such tomographic images provide the flow-velocity strength, indicative of fluid flow at localised positions in the scattering sample. Another method for displaying the tomographic images of fluid flow * is to combine the magnitude of velocities and corresponding Doppler angles.

The method for tomographic imaging of a fluid flow in a scattering medium is S..

:. diagrammatically depicted in the flow chart Figure 3: the method starts at step 500. The radiation from a low coherence light source with a central wavelength of 2. and S * *. bandwidth of z12 is phase modulated in an iriterferometer 10 at a modulation frequencyfo in step 501. A scattering sample within which there is flowing scattering fluid is continuously scanned with the source at step 502, in the sample arm where the source is focused into the sample with a beam waist of diameter c. Interference signals from the radiation between the light retroreflected from the reference arm and light backscattered from the sample into the interferometer are then detected at step 503. The interference signals are data processed at step 504 within each data window TFF'T to obtain the Doppler frequency f Doppler frequency shift 4f power spectrum P(fo) at the modulation frequency Jo and Doppler bandwidth o The fluid-flow velocity parameters are then obtained at each data window for continuous scans at step 505. Tomographic images of the fluid flow within and the structure of the scattering sample are finally formed from the processed data at step 506.

The above description outlines the method to calculate the spectral broadening of the interference signal, from which a tomographic image of fluid-flow in the scattering medium can be obtained. However, it is obvious that it needs a longer computation time.

It is therefore not suitable for real time tomographic imaging of fluid-flow within the scattering medium. Here, an alternative method to evaluate the real time fluid-flow velocity, thereby tomographic imaging of the fluid sample, is disclosed. This disclosed * * method uses correlation functions to calculate the bandwidth of interference signal which * .*.

*,*, can realize real-time tomographic fluid-flow imaging without significantly affecting the system architectures. ***.

** Variance of the Doppler signal is used to approximate the square of the Doppler bandwidth. Suppose P(f) is the Doppler power spectrum, then the variance is defmed as: f(ff)2p(f)dJ Variance = (10) J P(.f)4f if P(f) is Gaussian distributed and a-is the Doppler bandwidth, then P(/) can be written as: P( ) = exp[,r(3r0)2] (11) By combining Equations (10) and (11) one can find that the variance of the velocity is proportional to the square of the bandwidth, that is Variance=--(12) 2,r When the Doppler power spectrum is not Gaussian, even though analysis of the expression of the bandwidth in terms of variance is not easy to obtain, the variance is still approximately proportional to the square of the bandwidth.

The method for calculating the variance using correlation functions can be found in the following reference S.M. Kay, Modern Spectral Estimation, Prentice Hall, 1988. If S(t) is the Doppler signal received by the imaging system 10, the autocorrelation function R(t) can be defined as: RQ) = ISQ + r)SQ)dz-(13) Since P(/) is the Fourier transform of R(t), the Doppler frequency shift corresponding to * the projection of the average projected velocity is: ***.

= r JP(f)df = -j R'(O) (14) f P(f)df R(0) Furthermore, from the previous definition of the variance one can obtain S..

2 1 JR(T)I1 Variance-i1-(15) T2[ R(0)j Therefore, the bandwidth can be calculated from R(0) and R(T).

In practice, the correlation function is a discrete function and is calculated within a finite time. Suppose I is the time interval between two successive data windows, or two successive pixels of the tomographic image; if the number of sample points within the data window is N, R(O) and R(T) are expressed as: J(O) = !S(i. T)S(i. T) (16) I(T) = S'((i +l)*T)S(i.T) (17) The number of sample points affects the accuracy of the correlation function. Since a negative variance might occur with these formulas, one may use a modified formula to obtain R(O): = T) +IS((i +1).T)2] (18) Therefore, after subtraction of the intrinsic spectral bandwidth of the low coherence light source used, the evaluated Doppler bandwidth can be plugged into Equation 7 or *:*::* Equation 8 to obtain the velocity parameters of a fluid flow in the scattering sample.

The method for tomographic imaging of a fluid flow in a scattering medium using the correlation is diagrammatically depicted in the flow chart Figure 4. The method starts at step 600. Radiation from a low coherence light source with a central wavelength of 2o and bandwidth of AA is phase modulated in an interferometer 10, at a modulation * frequency Jo in step 601. A scattering sample within which there is flowing scattering fluid is continuously scanned with the source at step 602 in the sample arm, where the source is focused into the sample at a beam waist of diameter c. Interference signals of the radiation between the light retroreflected from the reference arm and the light back scattered from the sample into the interferometer are then detected at step 603. The interference signals are data processed by the correlation method of the present invention / L) at step 604; within each data window Tppj' to obtain the Doppler frequency f Doppler frequency shift 4f, power spectrum P(f) at the modulation frequency f0 and the Doppler bandwidth a The fluid-flow velocity parameters are then obtained at each data window for continuous scans at step 605. Tomographic images of the fluid flow within and the structure of scattering sample are then formed from the processed data at step 606.

The methods disclosed so far are able to obtain the velocity magnitude VI and Doppler angle a of the moving particles flowing within the scattering medium. However, there are still difficulties found in determining the actual velocity vector in 3-D space because an ambiguity of circular symmetry relative to the probe beam exists between the velocity vector direction and the Doppler angle, as determined by the above-described methods.

An alternative Doppler optical coherence tomography system able to solve this problem is now described (illustrated in Figure 5). In this arrangement two probe beams i.e. 301 **::* and 302 coming from two separate OCT systems 101 and 102 respectively, are directed towards the scattering sample, within which there are the scattering fluid flow or moving particles. The preferred OCT systems are shown in 10; however, any other variations of the OCT system can also be used. The requirement is that the two beams must intersect in space at their beam waists. The two beams can be at any angle except 00 or 1800.

These can be met by using precision mechanics when installing the system. The preferred angle between the two beams is 900, because it involves the minimum effort to obtain the fluid flow velocity vector in space. With regard to Figure 5, the two probe beams are at an angle of 900 the velocity vector is derived below.

Referring to Figure 5, it can be seen that the co-ordinate system of the scattering sample is determined by the beams 301 and 302, with z axis passing through the probe beam 301 towards the focusing lens, x axis passing through the probe beam 302 towards the focusing lens and the y axis orthogonal to the plane passing through the beams 301 and 302. As a result, the vector of fluid flow V can be completely determined by V, i',j, a and flrespectively, as shown in Figure 5. Assuming that the OCT system 101 is identical to the previously described system 10, then when the probe beam 301 is continuously scanned, the projection velocity onto and the transverse velocity relative to the beam 301 can be obtained: l'ç=4f20/2 (19) V, J&%_(1iL2+__)J (20) And the Doppler angle is (v a = arctan --I (21) * ** V) * * S * ** * When the probe beam 302 is continuously scanned, the projection velocity onto the beam 5:.. 302isthen S...

*:. V=4f't/2 (22) * .. where A is the central wavelength of the low coherence light source in system 102, and

S S..

* 4f' is the Doppler shift of the interference signal with respect to the modulated frequency,f'o, for the system 102. Therefore, the angle can be determined: p = arccos(!-J (23) Thus, the fluid-flow vector v in the defined co-ordinate system can be represented by ii lvi = V22 + a = arctan[..J. = arccosJ (24) The above derivation of fluid-flow vector uses only the projection velocity of the moving particles onto the probe beam 302. However, the other parameters of fluid-velocity within the context of system 102 can also be obtained, (for example) the transverse velocity and Doppler angle relative to the probe beam 302. These parameters can be selected and used together with the parameters obtained within the context of system 101, to obtain the complete representation of fluid-flow velocity of scattering medium in space. It is therefore understood that any combination of the parameters obtained, within the context of systems 101 and 102 respectively, to find the complete representation of fluid-flow velocity, is covered by the present invention.

In practice, the velocity vectors represented by a tomographic image are obtained from the tomographic images from the two OCT systems, where the images from two systems ::: have one-by-one correspondence in pixels. *4 I

The method for tomographic imaging of a vector representation of fluid flow in a scattering medium indicative of velocity-vector information is diagrammatically depicted in the flow chart Figure 6. The method starts at step 700. The radiation sources from the * sampling arms of two interferometers 101 and 102, respectively, are directed into the scattering medium. At step 701, the radiation with a central wavelength of 2 and bandwidth of z123 from the interferometer 101 is phase modulated at a modulation frequency fj. Radiation with a central wavelength of A.2 and bandwidth of zU2, from the interferometer 102, is phase modulated at a modulation frequencyJ2. A scattering sample within which there is flowing scattering fluid is continuously scanned with the sources frominterferometers of 101 and 102, respectively, at step 702 in the sample arms; where the sources are focused into the sample. In each interferometer, interference signals from the radiation between the light retroreflected from the reference arm and light backscattered from the sample are then detected at step 703. The interference signals are data processed by the methods described above at step 704, within each data window T to obtain the Doppler frequencyfo, (/) and Doppler frequency shift 4fj (4J), power spectrum P(f1) (P(J1)) at the modulation frequencyf, () and Doppler bandwidth o, (o,).

The fluid-flow velocity-vector parameters are then obtained at each data window for continuous scans at step 705. Tomographic images of the fluid flow within and the structure of scattering sample are fmally formed from the processed data at step 706.

Insubstantial changes from the claimed subject matter, as viewed by a person with ordinary skill in the art, now known or later devised, are expressly contemplated as being equivalently within the scope of claims. Therefore, obvious substitutions now or later * ** *:* known to one with ordinary skill in the art are defmed to be within the scope of the defined elements.

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Claims

The Claims 1. A method for tomographic imaging of a fluid flow in a

scattering medium using an optical coherence tomography system, the optical coherence tomography system including an interferometer having a reference arm and a sample arm. The method comprises the following steps: a) Continuously scanning a fluid flow sample with a radiation from a low coherence light source through the interferometer, said fluid flow sample having a fluid flow therein and a structure in which said fluid flow is defined; b) Detecting interference fringe signals of said radiation formed between the radiation backscattered from said sample and the radiation reflected from the reference arm in said interferometer.

c) Data processing said detected interference fringe signals to obtain the Doppler power spectrum at the modulated frequency, Doppler frequency shift with respect * ** to the modulated frequency, and the Doppler bandwidth and the Doppler angle of *S*.

said interference signal at each pixel of a scanned image. SI..

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* d) Obtaining a tomographic image of the fluid flow in said scanned fluid sample by S. S

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the use of said Doppler shift, said Doppler bandwidth and said Doppler angle at I. S.s* each pixel of said scanned image. **.

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e) Obtaining a tomographic image of the structures in said scanned fluid sample by the use of said Doppler power spectrum at each pixel of said scanned image.

1 The method of claim I wherein the steps (d) and (e) are reversed.

2 The method of claim 1 wherein scanning said fluid flow sample with a focused light beam, with a known focal spot size, working distance and focus depth, from a source * 10 of at least partially coherent radiation comprises scanning fluid sample with a source of low coherence radiation.

3 The method of claim 1 wherein the said optical coherence tomography interferometer is a Michelson interferometer 4 Said interferometer of Claim 3 is a fibre optic version of any type.

Said interferometer of Claim 3 is a free space version of any type.

6 The method of claim 1 wherein the interference signals are detected by a single photo-detector.

7 The method of claim I wherein the interference signals are detected by two photo-detectors differentially amplified, or simply by a differential detector.

8 The method of claim 1 wherein said interferometer has a reference beam and a sample beam and scanning said fluid flow scattering sample through said interferometer further comprises matching poiarity of said reference and sample beams to optimise the interference fringe contrast in said interferometer. *e..

9 The method of claim 1, wherein said data processing of interference signal includes a *S*.

* step of performing a time-frequency analysis on said interference signals to extract S..

spectral infonnation from the interference signal at each data window as a function of *...

* depth. I..

S

The method of claim 9 wherein the time-frequency analysis step includes the step of performing a short-time Fourier transform on the said interference signal data, and wherein time-frequency analysis step can be any time-frequency analysis methods: wavelet transformation on the said interference signal data for example. * 2'

ii The method of claim 9 wherein the data window is defined as a certain time duration, within which the sampled data points are available for data processing to obtain values for one pixel of the scanned tomographic image. Said time duration defmes the depth or distance that a pixel in said tomographic image represents.

12 The method of claim 9 wherein said spectral information includes a Doppler power spectrum at the modulated frequency, Doppler frequency shift with respect to the modulated frequency, and the Doppler bandwidth and the Doppler angle of said interference signal at each pixel of a scanned image, and wherein the velocity of said fluid-flow is determined in accordance with equations: IvI=v2 +v2 = /(j2 and a =arctanI 2) L,% T. j 13 The method of claim 12 wherein * ** a) The processing step is performed at each data window to produce a corresponding * ** power spectrum at the said modulation frequency with which the structural **.

tomographic image of said scattering sample is formed. S...

b) The processing step further includes a step of calculating depth-dependent velocity values for each pixel with which the velocity tomographic image of said S.....

scattering sample is formed.

14 The method of claim I wherein the tomographic image of fluid-flow in scattering sample is displayed by the use of only magnitude values of velocity at each pixel.

The method of claim I wherein the tomographic image of fluid-flow in scattering sample is displayed by a combined use of magnitude values of velocity and corresponding Doppler angles at each pixel. i2

16 A method for tomographic imaging of a fluid flow in a scattering medium using an optical coherence tomography system, the optical coherence tomography system including an interferometer having a reference arm and a sample arm. The method comprises the following steps: a) Continuously scanning a fluid flow sample with radiation from a low coherence light source through the interferometer, said fluid flow sample having a fluid flow therein and a structure in which said fluid flow is defined; b) Detecting interference fringe signals of said radiation formed between the radiation backscattered from said sample and the radiation reflected from the reference arm in said interferometer.

c) Data processing said detected interference fringe signals, by the use of the correlation method, to obtain the Doppler power spectrum at the modulated frequency, Doppler frequency shift with respect to the modulated frequency, and the Doppler bandwidth and the Doppler angle of said interference signal at each *S..

pixel of a scanned image. ***.

* d) Obtaining a tomographic image of the fluid flow in said scanned fluid sample by *.* the use of said Doppler shift, said Doppler bandwidth and said Doppler angle at *.*.S* each pixel of said scanned image. **.

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e) Obtaining a tomographic image of the structures in said scanned fluid sample by the use of said Doppler power spectrum at each pixel of said scanned image.

17 The method of claim 16 wherein the steps (d) and (e) are reversed.

18 The method of claim 16 wherein the said interferometer comprises measuring automatically the velocity and the Doppler angle of fluid-flow sample by the Doppler principle, wherein said interferometer further comprises a correlation processing based on the relation between the Doppler angle and the Doppler bandwidth and using variance to approximate the square of the Doppler bandwidth for measuring the angle of motion of fluid-flow sample.

19 The method of claim 18 wherein said variance of said correlation function method can be a temporal average, a spatial average, or a proper combination of both the temporal and spatial averages.

An apparatus for optical tomographic imaging of fluid flow within scattering sample comprising an optical coherence tomography system wherein an optical coherence tomography system comprises: a) An interferometer including an optical source of low coherence radiation, a sample arm and a reference arm, the interferometer generating an interference signal between the light retroreflected from the reference arm and light * *.

backscattered from the scattering sample in the sample arm. *..

b) A scanning system in the reference arm to generate the depth dependent *.*.

* interference signal between the between the light retroreflected from the reference arm and light backscattered from the scattering sample in the sample arm, and a scanning system in the sampling arm to change the scanning position of the S..

S

reference arm.

c) A data processing system, coupled to the interferometer, processing the interference signal to generate the depth-dependent velocity parameters of the fluid-flow scattering sample obtained from spectral information evaluated from the interference signals, processing the interference signals to generate values of 2'k depth dependent positions of the scattering in the scattering sample, assigning the processed data to form the tomographic images of both the structural and fluid-flow velocities the of scattering sample.

21 The apparatus of claim 20, wherein the data processing system includes: a) A data sampling device coupled to the output of the interferometer; b) A time-frequency analysis unit coupled to the output of the data-sampling device to obtain the Doppler power spectrum at the modulated frequency, Doppler frequency shift with respect to the modulated frequency, the Doppler bandwidth and the Doppler angle of said interference signal at each pixel of a scanned image.

c) A depth dependent velocity and position estimation unit coupled to the output of the time-frequency analysis unit.

22 The apparatus of claim 20, wherein the data processing system includes: a) A data sampling device coupled to the output from the interferometer; * S. b) A correlation-processing unit coupled to the output of the data-sampling device to obtain the Doppler power spectrum at the modulated frequency, Doppler *...

* frequency shift with respect to the modulated frequency, the Doppler bandwidth and the Doppler angle of said interference signal at each pixel of a scanned image.

c) A depth dependent velocity arid position estimation unit coupled to the output of the correlation-processing unit.

23 A method for tomographic vector-imaging of a fluid flow in a scattering medium comprised of the following: a) Providing two optical coherence tomography systems each with a source of at least partially coherent radiation; b) Positioning the sampling beams from the two said interferometers at the same plane with an angle other than 00 and 1800 c) Phase modulating of said radiation in said interferometers at a modulation frequency in each interferometer; d) Continuously scanning a fluid flow sample with said sources through said interferometers, said fluid flow sample having a fluid flow therein and a structure in which said fluid flow is defmed.

e) Detecting interference fringe signals of said radiation backscattered from said sample into said interferometers separately; f) With each interferometer, data processing said detected interference fringe signals to obtain the Doppler frequency shift with respect to the modulated frequency and the Doppler bandwidth of the interference signal at each pixel of a scanned image.

g) Forming a tomographic vector-image of the fluid flow in said scanned fluid * *.

sample by the use of said Doppler shifts and said Doppler bandwidths from each interferometer at each pixel of said scanned image. *S.. S...

* f) Obtaining a tomographic image of the structures in said scanned fluid sample by the use of said Doppler power spectrum at each pixel of said scanned image from *Io..S * . * one of the said interferometer. *I.

24 The method of claim 23 wherein the steps (g) and (f) are reversed.

The method of claim 23 wherein the above claims from 2 to 13 apply to each said interferometer.

26 The method of claim 23 wherein, within each interferometer, said data processing of detected interference fringe signals, by the use of the correlation method, to obtain the Doppler power spectrum at the modulated frequency, Doppler frequency shift with respect to the modulated frequency, the Doppler bandwidth and the Doppler angle of said interference signal at each pixel of a scanned image.

27 The method of claim 26 wherein the above claims of 17 and 18 apply to each interferometer.

28 The method of claim 23 wherein the tomographic vector imaging of fluid flow scattering sample is generated from any combination of the velocity parameters evaluated from each interferometer.

29 An apparatus for optical tomographic vector imaging of fluid flow within scattering sample comprising two optical coherence tomography systems, each optical coherence tomography system comprising: a) An interferometer including an optical source of low coherence radiation, a * sample arm and a reference ann, the interferometer generating an interference * *, signal between the light retroreflected from the reference arm and light **..

backscattered from the scattering sample in the sample arm. **S. * S...

* b) A scanning system in the reference arm to generate the depth dependent interference signal between the between the light retroreflected from the reference S. 5.5.

arm and light backscattered from the scattering sample in the sample arm, and a *I.

scanning system in the sampling arm to change the scanning position of the reference arm.

c) A data processing system, coupled to the interferometer, processing the interference signal to generate the depth-dependent velocity parameters of fluid-flow scattering sample obtained from spectral information evaluated from the 2) interference signals, processing the interference signals to generate values of depth dependent positions of scattering in the scattering sample, assigning the processed data to form the tomographic images of structures of the scattering sample.

The apparatus of claim 29 wherein the sampling beams from said two interferometers are arranged in the same plane with an angle other than 00 and 1800 31 The apparatus of claim 29 further comprises a data processing unit that combines the parameters evaluated from each mterferometer, to obtain a tomographic vector-image of fluid-flow in the scanned scattering sample.

32 The apparatus of claim 29, wherein the data processing system in each said interferometer includes: a) A data sampling device coupled to the output of the interferometer; b) A time-frequency analysis unit coupled to the output of the data-sampling device, * *.

to obtain the Doppler power spectrum at the modulated frequency, Doppler frequency shift with respect to the modulated frequency, and the Doppler 4**.

* bandwidth and the Doppler angle of said interference signal at each pixel of a scanned image.

I. *I* c) A depth dependent velocity and position estimation unit coupled to the output of S..

the correlation-processing unit.

33 The apparatus of claim 29, wherein the data processing system includes: a) A data sampling device coupled to the output of the interferometer; b) A correlation-processing unit coupled to the output of the data-sampling device to obtain the Doppler power spectrum at the modulated frequency, Doppler frequency shift with respect to the modulated frequency, and the Doppler bandwidth and the Doppler angle of said interference signal at each pixel of a scanned image.

c) A depth dependent velocity and position estimation unit operatively coupled to the output of the time-frequency analysis unit.

34 The apparatus of claim 29, wherein said two optical coherence tomography systems comprising two separate optical coherence tomography systems operating at either different wavelengths or at the same wavelength of low coherence light sources.

The apparatus of claim 29, wherein the said two optical coherence tomography systems comprising one optical coherence tomography system operating at two different wavelengths of low coherence light sources, wherein this optical coherence tomography system has two sample probes from the sample arm.

36 An optical coherence tomography system comprising: radiation from a low coherence * ** light source; a reference scanning assembly; means for scanning the sample probe; *** means for directing the source radiation to a sample; means for directing the source 4*S radiation to the reference scanning assembly; means for combining reflected radiation from the scattering sample and from the reference scanning assembly to produce the a.

interference signal from the combined reflected radiation; means for generating the tomographic structural image of scattering sample; means for processing the interference signal to produce the depth dependent velocity parameters in the scattering sample