Abstract:
A method for analyzing a diffusing sample comprises steps of: a) illuminating the sample with temporally coherent and wavelength-modulated incident light beam; b) producing a succession of interference signals with the diffuse light; c) combining each interference signal with a reference signal (Ref(t,τ 0 )) generating a time port; d) extracting the continuous component of each signal characteristic of the predetermined delay (τ 0 ) derived form c); applying a nonlinear function on each of the continuous components of the signals characterizing the predetermined delay (τ 0 ); and f) averaging each of the images, with the non-linear function, of the continuous components of the signals characterizing the predetermined delay (τ 0 ). The invention is useful for observing organs of the human body.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method of analyzing a scattering sample by time resolved measurement of the light scattered within this sample. 
     BACKGROUND OF THE INVENTION 
     In the medical field, it is known to use light, especially with wavelengths located in the infrared region, in order to study in some depth the nature of biological tissue. 
     However, biological tissue is highly scattering at optical wavelengths, which tends to blur all information relating to its internal structure. Now, this structure influences the data and it may be necessary to assess this influence. For this, it is beneficial to study the dynamics of light propagation in biological tissue. To this end, it is known to send a very short light pulse to the medium to be studied and to carry out time resolved measurements of the scattered light. Time resolution provides valuable information concerning the structure of the medium. 
     The measurements may be carried out in reflectance or in transmittance. 
     The reflectance measurements are used especially in order to determine the oxygenation level of the blood hemoglobin. 
     The transmittance measurements are used especially for detecting cancerous tumors of the breast. The images obtained by time resolved transmittance measurement of the scattered light produce high contrast images making it possible for a tumor to be clearly detected therefrom. However, the spatial resolution of the images is relatively mediocre. 
     This limitation arises from the technology used for implementing the measurement. This is because the time resolution needs to be carried out over a very short time of about a few tens of picoseconds. Thus, such resolution requires highly sophisticated and expensive technologies. 
     In the known observation devices, the source producing the light pulse is adapted to provide a very brief pulse. 
     For example, in the device described in document U.S. Pat. No. 5,692,511, the light source used is a titanium-sapphire laser. The latter is very expensive. Furthermore, the detection is awkward to implement. The solution utilized in the device described in this document uses an avalanche photodiode connected to an electronic time gate in order to determine the number of photons transmitted during a particular time period. The avalanche photodiode has a relatively long time response which degrades the potential contrast of the time gate. 
     More generally, in devices for analyzing scattered light by time resolved measurement, it is common to use a source making it possible to produce brief pulses such as pulsed lasers, which are expensive. 
     Furthermore, means used for detecting the light present problems specific to each of them which do not allow satisfactory use of the device. 
     From these means of detecting the signal, it is possible to distinguish:
         fast detectors with electronic processing of the signal coming from the detector, the relatively long response time of which considerably degrades the contrast of the time gate;   time amplitude converters in photon counting mode which require decreasing the signal in order to be placed in a suitable mode, which is not compatible with the short acquisition time;   optical gates with a nonlinear effect which require powerful lasers with low rates of about 10 Hz, which considerably limits the sensitivity;   slit scanning cameras which have very high time resolution (about 2 ps), but the dynamic range of which is not very high; and   ultrafast intensified cameras, the time resolution of which is not very high (about 80 ps) for a low repetition rate (about 1 kHz).       

     Thus, currently available technologies do not make it possible to obtain good spatial resolution, especially with scattering tomography. 
     OBJECT OF THE INVENTION 
     The aim of the invention is to provide a device for analyzing a scattering sample by time resolved measurement of the scattered light, the implementational cost of which is modest. 
     SUMMARY OF THE INVENTION 
     To this end, the subject of the invention is a method of observing a scattering sample of the aforementioned type, characterized in that it comprises the steps of: 
     a) illuminating the sample with a temporally coherent and wavelength-modulated incident light beam; 
     b) producing a succession of interference signals, recorded over a time interval, by superposition of the scattered light obtained at the output of the sample and of light taken from the incident beam illuminating the sample; 
     c) combining each interference signal with a reference signal generating a time gate centered on a predetermined delay in order to produce a signal characteristic of the predetermined delay; 
     d) extracting the d.c. component (“continuous component”) of each signal characteristic of the predetermined delay; 
     e) applying a nonlinear function to each of the d.c. components of the signals characteristic of the predetermined delay; and 
     f) carrying out a linear combination of each of the images, with said nonlinear function, of the d.c. components of the signals characteristic of the predetermined delay. 
     According to particular embodiments, the method comprises one or more of the following characteristics:
         said nonlinear function is a function chosen from the group consisting of square functions, and the absolute value function;   said nonlinear function is a function of at least two variables taken from the d.c. components of the signals characteristic of the predetermined delay obtained from the succession of interference signals recorded over the time interval;   for the combination of each interference signal with said reference signal, the incident beam is additionally amplitude modulated by means of an amplitude modulation introducing the predetermined delay;   for an amplitude modulation equal to the sum of said reference signal and of a reference signal with zero delay, said linear combination comprises a calculation subtracting from each of the images, with said nonlinear function, d.c. components of the signals characteristic of a zero delay;   said linear combination comprises calculating the mean of each of the images, with said nonlinear function, of the d.c. components of the signals characteristic of the predetermined delay;   when producing the succession of interference signals, a relative displacement is imposed between the incident beam and the scattering sample, the frequency of the imposed displacement being less than the frequency of the wavelength modulation of the light of the incident beam; and   said reference signal comprises a sinusoidal function of the product of the predetermined delay on which the time gate is centered and of the wavelength modulation function of the incident beam.       

     The subject of the invention is also an apparatus for analyzing a scattering sample by time resolved measurement of the light scattered within this sample, characterized in that it comprises: 
     a) means for illuminating the sample with a temporally coherent and wavelength-modulated incident light beam; 
     b) an interferometer for producing a succession of interference signals, recorded over a time interval, by superposition of the scattered light obtained at the output of the sample and of light taken from the incident beam illuminating the sample; 
     c) means for combining each interference signal with a reference signal generating a time gate centered on a predetermined delay in order to produce a signal characteristic of the predetermined delay; 
     d) means for extracting the d.c. component of each signal characteristic of the predetermined delay; 
     e) means for applying a nonlinear function to each of the d.c. components of the signals characteristic of the predetermined delay; and 
     f) means for carrying out a linear combination of each of the images, with said nonlinear function, of the d.c. components of the signals characteristic of the predetermined delay. 
     The invention also relates to equipment for measuring the content of a particular component in part of a living organism, comprising an apparatus for analyzing a scattering sample as described above, and means for calculating said content of a particular component as a function of the result of the analysis supplied by said apparatus. 
     According to a particular embodiment, said analysis apparatus is suitable for producing interference signals from the light backscattered by an organ of a human being or of an animal, and said calculating means are suitable for calculating the oxygenation level of this organ. 
     Finally, the subject of the invention is imaging equipment comprising:
         as described above, an analysis apparatus which is suitable for producing and processing several successions of interference signals obtained from the light scattered in several adjacent regions of the sample and thus to supply an analysis result for each of the adjacent regions of the sample; and   means for producing an image of the sample from the analysis result supplied for each of the adjacent regions of the sample.       

     According to a particular embodiment, said analysis apparatus is suitable for producing interference signals from the scattered light transmitted through an organ of a human being or of an animal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will also be better understood on reading the following description, given solely by way of examples, and made with reference to the drawings in which: 
         FIG. 1  is a schematic view of an analysis apparatus according to the invention; 
         FIG. 2  is a flow chart illustrating the structure of the data processing unit of the detector of the apparatus of  FIG. 1 ; 
         FIGS. 3 ,  4 ,  5  and  6  are curves illustrating the shape of signals which can be obtained at various points of the data processing unit of  FIG. 2 , in the presence and in the absence of a scattering medium; 
         FIG. 7  is a schematic view of equipment for measuring by reflectance the oxygenation level of hemoglobin implementing a method according to the invention; 
         FIG. 8  is a schematic view of imaging equipment in transmittance implementing a method according to the invention; 
         FIG. 9  is a view similar to that of  FIG. 2  illustrating a variant of the data processing unit of the detector of the apparatus of  FIG. 1 ; and 
         FIG. 10  is a schematic view of an alternative apparatus according to the invention associated with a flow chart of a type similar to that of  FIG. 2  illustrating the structure of the unit for processing data detected by this alternative apparatus. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The apparatus shown in  FIG. 1  illustrates the principle of the analysis method according to the invention. 
     This apparatus makes it possible to analyze, in transmittance, a scattering sample  10  by time resolved measurement of the scattered light. It uses an interferometric technique to carry out the measurement and to this end, comprises an interferometer denoted by the general reference  11 . 
     It is possible, by means of the apparatus, to reconstruct the time variation of the scattered energy with time resolutions of about 50 ps to 20 ps. 
     More specifically, the apparatus uses a temporally coherent and wavelength-modulated light source  14  so as to simulate, by suitable processing of the detected interference signal, an incoherent source. 
     The source  14  comprises a laser cavity  16  connected to a control unit  18  suitable for modulating the wavelength of the beam produced by the laser cavity  16 . The laser cavity  16  is, for example, a laser diode or an extended cavity laser diode. It is suitable for producing a temporally coherent monochromatic light beam. 
     The central emitting wavelength is chosen depending on the applications. According to a particular embodiment of the method, several different central wavelengths are successively used, thus making it possible to carry out spectroscopic measurements. The wavelengths are, for example, equal to 780 nm and 850 nm for analysis of the oxygenation level of the tissue. 
     The control unit  18 , whose practical embodiment is within the scope of a person skilled in the art, is suitable for periodic modulation of the wavelength of the beam produced by the diode  16 . This modulation is advantageously a sinusoidal modulation. The amplitude δλ of this modulation is of the order of a few hundredths of a nanometer. 
     The temporal resolution Δt obtained for a modulating amplitude δλ of a few hundredths of a nanometer is about Δt=2λ 2 /(πcδλ), that is to say a few tens of picoseconds. 
     Advantageously, the light produced by the diode  16  is modulated continuously, thereby avoiding mode jumps. 
     The modulating frequency f is chosen to be fast enough so that the scattering medium can be considered as immobile during the modulating period f −1 . The modulating frequency is advantageously between 0.1 and 10 kHz and is, for example, chosen to be 1 kHz. 
     A lens  20  may be provided at the output of the diode  16  in order to adjust the width of the incident beam produced by the diode. The incident beam, denoted  22 , is sent toward the interferometer  11  which comprises, at the input, a semireflecting mirror  23  which separates the incident beam  22  into two beams  24  and  26 . These two beams  24 ,  26  propagate respectively along a reference arm  24 A of the interferometer and along another arm  26 A containing the sample. 
     In the arm  26 A, the beam  26  is first sent into an optical system, known per se, which makes it possible to adjust the step difference between the optical paths of the two arms of the interferometer. At the output of the adjustment system  28 , the beam  26  is sent via a mirror  30  onto the sample  10  forming the scattering medium. A lens  32  may advantageously focus the beam onto a given point of the sample. 
     Means  34  for collecting the scattered beam, such as a convergent lens, collect the beam at the output of the sample  10 . The scattered light is then sent, via a mirror  36 , to a semireflecting mirror  38  which superimposes the scattered beam and the beam  24  propagated along the reference arm of the interferometer. 
     Advantageously, the interferometer  11  comprises means making it possible for the position of the beam  26  arriving on the sample  10  to be continuously slightly modified. The position of the beam is modified during the analysis, to increase the accuracy thereof, as will be explained below. 
     These means comprise, for example, components for moving the mirror  30  and/or the convergent lens  32 , so as to modify the position of the beam on the sample and/or the angle of incidence of the beam with respect to the sample. 
     Similarly, the interferometer advantageously comprises, downstream of the sample  10  in the arm  26 A, means making it possible to modify the region collecting the scattered light, on the sample. These means comprise, for example, components for moving the collection means  34  or the mirror  36 . 
     The beam denoted  40 , obtained by superposition, at the output of the interferometer  11 , is sent to detection and analysis means  42 . 
     These means  42  comprise a detector  44  connected to a data processing unit  46 , the main means of which are described with respect to FIG.  2 . 
     The detector  44  is, for example, a photodiode. 
     Advantageously, the characteristics of the optical systems placed in the arm  26 A of the interferometer, downstream of the sample  10 , are chosen so that the coherent surface of the scattered beam at the detector  44  is of the same order of magnitude as the active surface of this detector. 
     In the embodiment described, the data processing unit  46  is formed from an assembly of analog electronic circuits suitable for providing the functions described with respect to FIG.  2 . 
     At the input, the unit  46  comprises a high-pass filter  50  in order to filter the signal collected by the detector  44 . The cut-off frequency of the filter  50  is of the order of one to ten times the modulation frequency f of the light emitted by the diode  16 . In the present case, the cut-off frequency is of the order of one kilohertz. 
     The main objective of the high-pass filter  50  is to remove the d.c. component of the signal. It also eliminates low-frequency interference and possible interference effects related to the wavelength modulation of the diode  16 . The interference signal obtained at the output of the filter  50  has a much higher frequency than the modulating frequency. 
     The signal obtained at the output of the filter is advantageously amplified by an optional amplifier  51 . 
     Although the filter  50  is advantageously used, the latter may be omitted. 
     A multiplication stage  52  is provided at the output of the amplifier  51 . It multiplies the filtered signal by a reference signal Ref(t,τ 0 ). The signal Reft,τ 0 ) comes from a device  54  for producing this signal. This device is, for example, a programmable signal generator or an analog electronic circuit consisting of a set of oscillators, the characteristic parameters of which can be adjusted. 
     The output of the multiplication stage  52  is connected to a stage  56  for extracting the d.c. component (or “continuous component”) of the signal, that is to say, for calculating its mean value. 
     The signal Ref(t,τ 0 ) is such that the d.c. component of the product of the reference signal Ref(t,τ 0 ) and of the signal measured by the detector  44 , in the absence of scattering medium in the second arm  26 A of the interferometer, is essentially zero, except if the delay δt between the two arms of the interferometer is approximately equal to τ 0 , to within Δt, where Δt is the time resolution. The delay δt corresponds to the difference in optical path length between the two arms of the interferometer  11  in the absence of a scattering sample, divided by the velocity of light. 
     The combination of the interference signal and of the reference signal Ref(t,τ 0 ) acts as a time gate centered on the delay τ 0 , this gate selecting the component of the interference signal corresponding to a delay τ 0  with respect to the time for travelling through the reference arm  24 A of the interferometer. 
     The signal Ref(t,τ 0 ) is, for example, expressed in the form:
 
 Ref ( t,τ   0 )= A  sin n (2 πf t )cos(δλ 0 τ 0  cos(2 πf t )+φ)
 
for modulation of the light emitted by the diode  16  being expressed in the form δλ(t)=δλ 0  cos(2πf t).
 
     Several values of A, n and φ are possible. For example, the following values are suitable: A=1, n=4 and φ=0. 
     According to a variant embodiment, the signal Ref(t,τ 0 ) is produced using an optical interferometer without a sample and having a delay τ 0  between the two arms, by filtering the d.c. component of the signal detected at the output of the interferometer and by multiplying the result by another function generated by an analog electronic device, this function canceling itself out at the extrema of the modulation δλ(t). This other function will, for example, be sin n (2πft) 
     In a stage  56 , the extraction of the d.c. component of the product calculated in stage  52  is obtained, for example, by using a low-pass filter. In this case, the signal extracted is a d.c. signal whose variation with time is a slow variation associated with movements of the sample  10  or with movements of the optical systems  30 ,  32  and/or  34 ,  36 . 
     As a variant, the extraction of the d.c. component is provided by an integrating device which integrates the signal obtained at the output of stage  52  for exactly a whole number of half-periods f −1 /2. In this case, the signal from stage  56  is sampled after each integration of this type. 
     The data processing unit further comprises a stage  58  for applying a nonlinear function to the d.c. component of the signal obtained at the end of stage  58 . This function is, for example, a square function, or a function raised to any other even power, or else the application of the absolute value function. 
     The application of such a function leads to considering just the amplitude of the signals, independently of their sign and to allowing their mean not to be zero. 
     The result of applying the time gate used during the multiplication carried out by stages  52  to  58  is illustrated in  FIGS. 3 and 4 . The signal represented in these figures is the d.c. component obtained at the output of stage  58 , as function of the value of the predetermined delay τ 0 . 
     The signal shown in  FIG. 3  is obtained from the apparatus of  FIG. 1 , in which no sample is placed in the arm  26 A. 
     In the example considered in this figure, the top of the peak corresponds to a delay substantially equal to 2080 ps, this value corresponding to the delay δt between the two arms of the interferometer. 
     In contrast, the signal shown in  FIG. 4  is obtained, at the output stage  56 , in the presence of a scattering medium placed in the branch  26 A of the interferometer. 
     On this curve, the presence of a nonzero signal is noticed in the vicinity of the value δt of the delay between the two arms of the interferometer. 
     In this figure, a first signal close to the time origin is also noticed, this signal corresponding solely to the scattered light coming from the arm  26 A, independently of interference with the reference coming from the arm  24 A. This signal is not used for implementing the method. 
     The following stage denoted  60  of the unit  46  is suitable for establishing the mean of the signals obtained at the output of stage  58  for a particular duration. The mean calculated by stage  60  is determined for a duration of about a second. To this end, stage  60  comprises, for example, a low-pass filter. 
     As a variant, calculation of the mean is replaced by simply adding sampled signals, coming from stage  58  or any other form of linear combination carried out on these signals. 
     The signal obtained at the end of stage  60  for a given value of the predetermined delay τ 0  is proportional to that part of the scattered energy which has a delay τ 0  with respect to the time of travel through the reference arm  24 A of the interferometer, to within Δt. 
       FIG. 5  shows an example of the signal obtained at the output of stage  60 , as a function of the predetermined delay τ 0 . This figure clearly shows a signal for values of the predetermined delay τ 0  greater than the delay δt between the two arms of the interferometer. 
     Because of the mean calculated at the end of applying a nonlinear function to the signals obtained at the output of the time gate, the profile of the curve of  FIG. 5  shows a clear profile representing the structure of the scattering medium. 
     The delay τ 0  makes it possible to determine the time needed for the beam to pass through the sample, for example, by simply subtracting, from the delay τ 0 , the delay time δt between the two arms of the interferometer, and by adding a correction associated with the sample thickness. 
     The signal of  FIG. 5  is shown on a larger scale in  FIG. 6 , where the time origin has been corrected for the delay δt between the two arms of the interferometer. 
     The time variation of the calculated mean value is associated with a transformation of the relevant parameters in the medium studied. 
     According to a particular embodiment, the apparatus comprises several data processing units  46 , each one associated with a delay value τ 0 , this making it possible to obtain measurements of the scattered energy for different delays τ 0 . 
     The data processing unit  46 , described above, consists of an analog electronic device. However, as a variant, this data processing unit comprises an analog high-pass filter  50 , at the output of which an analog/digital converter is provided. All the processing carried out by stages  52  to  60  is then carried out in a completely digital manner by implementing a suitable algorithm in a computer. In this case, the different number of values τ 0  for which the calculation is carried out may be very large. 
     During the measurement, the signal obtained at the output of stage  58  fluctuates over the same time scale as the signal obtained at the output of stage  56 . These fluctuations are due to the movements in the scattering medium itself when this medium is a biological medium. These movements induce fluctuations over time scales of about a millisecond. 
     Now, these fluctuations are desirable in order that the mean value obtained at the output of stage  60  is as accurate as possible. This is because these fluctuations make it possible to measure various values in order to take the mean thereof. This mean will be closer to its theoretical value, which is the time resolved scattered intensity, the larger the number of different values obtained by the fluctuations. 
     Thus, advantageously, if the fluctuations of the medium are insufficient or too slow, additional fluctuations are induced by moving the beam  26  by displacing the mirror  30 , the lens  32 , the collection means  34  and/or the mirror  36 . 
     The movement is controlled such that the time scale of the time fluctuations is greater than the modulation period f −1 . Thus, the frequency of beam displacement is less than that of the wavelength modulation of the incident light. 
     An observation apparatus using an interferometer, and means of emitting a temporally coherent and wavelength-modulated light beam, together with processing means as described above, may advantageously be used for measuring, by reflectance, the oxygenation level of the hemoglobin of a human being. 
     To measure this oxygenation level, two wavelengths λ 1  and λ 2  are necessary. 
     To this end, the equipment for measuring the oxygenation level illustrated in  FIG. 7  comprises a first laser diode  100  and a second laser diode  102 , the central emitting wavelengths of which are respectively λ 1  and λ 2 . These two diodes are each controlled by a control unit  104  and  106  making it possible to modulate the wavelength of the emitted light, as described in the previous embodiment. 
     An automatic shutter  108  is placed at the output of the laser diodes  100  and  102  in order to selectively illuminate an interferometer denoted  109  with one or other of the light beams emitted by these diodes. 
     The output of the automatic shutter  108  is coupled to an optical fiber  110  by suitable coupling means  111 . The fiber  110  is a monomode optical fiber. 
     The fiber  110  routes the light to a 50/50 coupler denoted  112  making it possible to separate the incident light and to direct the latter along the two arms denoted  114  and  116  of the interferometer. 
     The reference arm  114  is formed by a monomode optical fiber  118 . 
     The second arm  116  comprises a monomode optical fiber  120  forming an emitting fiber making it possible to direct the incident beam to the sample to be studied. In the present case, the end of the optical fiber is placed on the arm of a patient. 
     Downstream of the sample to be studied, the second arm  116  of the interferometer comprises a second monomode optical fiber, denoted  122 , constituting an optical fiber for collecting the scattered light. Its free end is placed close to the free end of the emitting fiber  120  so as to collect the backscattered scattered light. The optical fibers  118  and  122  are connected to each other by a 50/50 coupler, denoted  124 , making it possible to superimpose the light beams transmitted in the two arms of the interferometer. 
     The output of the coupler  124  is connected to detection and analysis means  126  similar to the means  42  of the embodiment of FIG.  1 . 
     Advantageously, an optional vibrator  130  is applied to the emitting optical fiber  120  and/or the receiving fiber  122  so as to cause a slight displacement of the latter during analysis in order to cause fluctuations of the signal and thus to improve, as explained above, the accuracy of the measurement carried out. 
     Preferably, for each wavelength λ 1 , λ 2 , the scattered light is measured for various delays τ 0 , as described with respect to the embodiment of FIG.  1 . 
     The equipment comprises, connected to the detection and analysis means  126 , a unit  132  for calculating the oxygenation level of the hemoglobin. 
     The oxygenation level of the hemoglobin is deduced from the absorption coefficients of the medium calculated at the two wavelengths λ 1 , λ 2  specific to the diodes  100  and  102 . The absorption coefficient is calculated, as a function an equation known per se, from the exponential decay of light scattering as a function of time, this decay being defined from various measurements carried out at different delays τ 0 . 
     While the apparatuses illustrated in  FIGS. 1 and 7  only allow a time resolved measurement to be carried out at any point in question of the sample, the apparatus illustrated in  FIG. 8  is imaging equipment making it possible to produce a two-dimensional image of the sample studied. 
     This equipment adopts a structure substantially similar to that of the apparatus of FIG.  1 . Thus, elements identical or similar to those of this embodiment are denoted by the same reference numbers. 
     In this imaging equipment, the arm  26 A of the interferometer, in which the sample is placed, comprises, upstream of the sample, a telescope  150  enabling the beam  26  propagating along the arm to be enlarged. Thus, most of the sample surface is illuminated by the incident beam. 
     Similarly, the reference arm  24 A of the interferometer has a telescope  152  also enlarging the diameter of the beam  24  propagating along this arm. 
     The localized sensor  44  used in the embodiment is in this case replaced by an array of independent sensors or pixels  154 . This array comprises a set of sensors distributed in lines and in columns. Each detection sensor is associated with an integrated circuit which forms the high-pass filter  50  and amplifies the signal. The signal from each sensor filtered and amplified in this way is then multiplied by the reference signal Ref(t,τ 0 ) then integrated over a whole number of half periods in order to extract the d.c. component therefrom. After this, the steps of calculating a nonlinear function and the mean of the results of this function are advantageously carried out digitally for each sensor. 
     The set of results obtained for each sensor is sent to a suitable unit  156  in order to produce an image of the sample studied as a function of the results received. 
     The value of the delay τ 0  is advantageously adjusted so as to obtain a high-contrast image by selecting the scattered light at short times. 
     Advantageously, the telescope  150  and the collector system  34 ,  36  are combined with means making it possible to alter their inclination with respect to a reference axis so as to obtain images in transmittance in different directions and thus to allow a three-dimensional reconstruction of the medium from images obtained of the sample for various positions of the telescope  152  and of the collector system  34 ,  36 . 
     The equipment of  FIG. 8  is designed, in particular, for observing an organ of a human being or animal by transmittance. It is especially used for searching for cancerous tumors of the breast. 
     The apparatuses described here and the method which they implement make it possible to analyze a scattering sample by time resolved measurement of the light scattered in this sample for a very low cost. 
     This is because the light source used can be formed from a single laser diode. Similarly, the detection and analysis means may be produced at low cost. 
     Furthermore, with the wavelength-modulated light used, the electromagnetic fields are much less intense than in a pulsed regime, as is the case in the prior art. 
     Finally, great sensitivity may be obtained by virtue of the interferometric nature of the analysis. 
     Even with a single photodiode as detector, very high sensitivity and a very dynamic range may be obtained. 
       FIG. 9  illustrates a variant of the analysis method according to the invention designed to obtain a time resolved measurement of a quantity relating to the light scattered in the sample to be analyzed. This variant is distinguished from the method described with respect to  FIGS. 1  to  6  solely by the structure of the data processing unit  46  at its stages  56  and  58 . 
     The output from the multiplication stage  52  is connected to a stage  56 ′ for extracting respective d.c. components of the signals obtained at the output of stage  52  for a particular duration, substantially equal to that retained for determining the mean calculated by stage  60  in the method described with respect to FIG.  2 . The d.c. components thus obtained in stage  56 ′ are denoted S i , where i denotes the i th  signal output from stage  52  during the aforementioned duration. The means implemented to obtain each d.c. component S i  are similar to those explained above. 
     The output from this stage  56 ′ is connected to a stage  58 ′ for applying a nonlinear function g to at least two variables taken from the d.c. components S i  from stage  56 ′. It should be noted that the term “nonlinear” is in this case considered in a broad sense, that is to say, in the case of functions with several variables, only functions which are linear with respect to their variables taken overall in vectorial form, such as the function “sum of two vectors” for example, are excluded. 
     This nonlinear function is, for example, a product function applied to two distinct values S i . Thus, it is possible to consider the function g(S i , S i+α )=S i ×S i+α  where α is a positive nonzero integer. Such a function is suitable for studying the time variation of the value S i+α  with respect to the value S i . This because, as has been explained above, a biological medium is the source of many movements and, in spite of practical precautions especially consisting in choosing a modulation frequency of the laser source  14  which is high enough to consider the scattering medium  10  as immobile during the modulating period, the signal S i+1  recorded following a signal S i  is slightly different from this signal S i . The signal S i+2  is even more so. More generally, a decorrelation of the signals S i+α  and S i  is noted for α≧1. The time variation of this decorrelation as a function of α and of τ 0  directly provides useful data relating to the nature and intensity of movements in the analyzed medium  10 . 
     To this end, the output from stage,  58 ′ is connected to stage  60  for finding the mean of signals obtained at the output of stage  58 ′, which mean is a function, on the one hand, of the delay τ 0 , as previously, and on the other hand, of the value of the integer α. A means for studying the decorrelation of signals S i  from each other consists in following the time variation of this mean from stage  60 , for increasing α and fixed τ 0 , for example. 
     In the case of measuring the oxygenation level of the hemoglobin of a human being, illustrated in  FIG. 7 , the data relating to the decorrelation of signals S i  provide information on the blood circulation in tissues such as the muscle capillaries, in addition to measuring the oxygenation level as explained with respect to FIG.  7 . The time resolved acquisition of these data makes it possible, for example, to isolate the contribution from the blood circulation in such capillaries with respect to that in larger blood vessels. 
     In the case of searching for cancerous tumors illustrated in  FIG. 8 , data relating to the decorrelation of signals S i  provides information on the vascularization state of a tumor observed by the equipment of FIG.  8 . Generally, such tumors are over-vascularized. The contrast of the image of the sample studied may thus be improved by using these decorrelation data. 
       FIG. 10  illustrates a second variant of the analysis method according to the invention in which the means of combining the interference signal with the reference Ref(t,τ 0 ) consist of means for modulating the amplitude of the laser source  16 . More specifically, this variant of  FIG. 10  is only differentiated from the embodiment described with respect to  FIGS. 1 and 2  by the following. 
     First of all, the control unit  18  is replaced by a control unit  18 ′ adapted so that it can both wavelength-modulate the beam produced by the laser cavity, in a manner substantially similar to the unit  18  of  FIG. 1 , and amplitude-modulate this laser beam, in an independent manner, from an external amplitude modulation signal. Producing such a control unit  18 ′ is within the scope of a person skilled in the art, for example, in the case of using an extended cavity laser diode  16 . 
     According to the invention, this additional amplitude modulation is capable of introducing, into the analysis apparatus, the reference signal Ref(t,τ 0 ). To this end, a device  54 ′ for producing a signal Ref(t,τ 0 )+Ref(T, 0 ) is provided, of a type substantially similar to the device  54  of FIG.  2 . This device  54 ′ is connected to the control unit  18 ′ in order to supply the external amplitude modulation signal. 
     At the output of the filtering  50  and amplification  51  stages, the recorded interference signal is then presented directly in the form of a combination, especially a product, comprising on the one hand a first term of a type similar to that from stages  50  and  51  of  FIG. 1 , that is to say a first term carrying the wavelength-modulated information and, on the other hand, a second term of a type similar to that from the device  54  of  FIG. 1 , that is to say a second term carrying the delay τ 0 . The combined signal thus obtained is characteristic of the delay τ 0 . 
     Moreover, the stage  60  of finding the mean of signals from stage  58  of  FIG. 1  is replaced by a stage  60 ′ adapted both to produce such a mean, and to subtract from the result obtained, the mean value corresponding to the processing by stages  56  and  58  of a signal characteristic of a delay τ 0 =0, that is to say a zero delay. This stage  60 ′ therefore only produces a linear combination of the results from stage  58 . 
     In this way, the results at the output of stage  60 ′ are identical to those from stage  60  of FIG.  2 . 
     This variant makes it possible to simplify the unit  56  for processing interference signals. For example, for the imaging equipment of  FIG. 8 , this variant makes it possible to simplify the array of sensors  154 , each sensor being associated with an integrated circuit which filters, amplifies and extracts the d.c. component, with acquisition over a half period, of each interference signal. It is then possible to use a CCD camera as means  156  for producing an image of the sample studied. 
     Another variant (not shown) of the analysis method according to the invention consists in combining the two variants of  FIGS. 9 and 10  together and, especially in the variant of  FIG. 10 , in replacing steps  56 ,  58  by steps  56 ′,  58 ′. 
     Moreover, various layouts of the interferometric assembly  11  can be envisioned, in so far as they arise from the knowledge of a person skilled in the art. For example, the superposition mirror  38  may produce two beams  40 , each of these two beams being detected by a respective sensor  44 . The difference obtained between these two detected beams gives information similar to that previously coming from the single sensor, but advantageously with less noise.