RU2184347C2 - Process generating images of internal structure of objects - Google Patents

Process generating images of internal structure of objects Download PDF

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RU2184347C2
RU2184347C2 RU2000122049A RU2000122049A RU2184347C2 RU 2184347 C2 RU2184347 C2 RU 2184347C2 RU 2000122049 A RU2000122049 A RU 2000122049A RU 2000122049 A RU2000122049 A RU 2000122049A RU 2184347 C2 RU2184347 C2 RU 2184347C2
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radiation
scattered
resolution
internal
plane
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RU2000122049A
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RU2000122049A (en
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Ю.Т. Мазуренко
Г.В. Папаян
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Мазуренко Юрий Тарасович
Папаян Гарри Вазгенович
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FIELD: optical instrumentation, specifically, coherent tomography of scattering biological objects with high spatial resolution. SUBSTANCE: coherent radiation of wide spectrum is split into reference and object beams. Object beam is focused inside object and backward radiation scattered by object is selected from focusing region by method of confocal filtration. Frequency shift is entered into reference beam. Reference and scattered radiations are spatially combined and as result of their interference summary radiation including light beats is obtained. Summary radiation is subjected to spatial spectral analysis and light beats are detected in each independent element of spectral analysis. Two-dimensional scanning of focusing point along focusing plane is carried out and three-dimensional image of internal structure of object is obtained after mathematical processing. EFFECT: increased productivity of recording of images of internal structures of objects. 2 cl, 9 dwg

Description

 The invention relates to optical instrumentation, in particular to a technique for obtaining images of the internal structure of three-dimensional objects (optical tomography) with extremely high spatial resolution, determined by the wave properties of radiation.
 Preferred fields of application of the invention are biological and medical microscopy, ophthalmoscopy, retinoscopy and endoscopy.
State of the art
For the development of tomography technique with a resolution determined by the wave properties of radiation, the method of confocal scanning microscopy is of fundamental importance (Patent US 03013467, publ. 1961). The principle of confocal microscopy is illustrated in figure 1. The radiation of the light source 1 is filtered by a point diaphragm 2 and a diffraction-limited radiation source is obtained (By a point diaphragm is meant a diaphragm whose aperture does not exceed the size of the diffraction spot of the filtered radiation). The most common source of diffraction-limited radiation is actually a laser. The radiation is sent to a beam splitter 3. The radiation reflected from the beam splitter 3 is focused by the lens 4 at point 5 inside the three-dimensional object 6. The secondary radiation emitted by the object is directed to the diaphragm 7 using the lens 4, while the center of the diaphragm is aligned with the image of the focus point 4. Secondary radiation emitted from the focal point 5 passes through the aperture 7 and enters the photodetector 8. The radiation emitted by the point secondary emitters of the object located at a distance from the point and focusing (for example, radiation from point 9) is more or less delayed by the diaphragm 6. As a result, the secondary radiation of the object emitted from a practically limited region of three-dimensional space, centered at the focal point 4, gets to the photodetector 8.
The size of this region in the transverse direction (x, y in FIG. 1) is equal to
d xy ≈0.9λ / 2A, (1)
and its size in the longitudinal direction (z in figure 1) is equal to
d z ≈1.8λ / (A 2 ). (2)
Here λ is the radiation wavelength,
A is the numerical aperture of the microscope objective 4.
 (E.g. Handbook of Biological Confocal Microscopy, Ed. J. B. Pawley, New York, Plenum, 1995).
The focus point 5 is moved in the three-dimensional space of the object (for example, when the object is moving) and the secondary radiation of the object is detected by the photodetector 8. Since Since the intensity of the secondary emitters depends on the density of the material of the object, the scan results in an image of the three-dimensional structure of the object. This image is characterized by a transverse resolution d xy and a longitudinal resolution d z .
 In particular, a confocal microscope that uses a heterodyne registration of radiation scattered by an object is described in US Pat. No. 3,780,217, class. H 04 N 1/26; H 04 N 7/18, publ. 1973. The use of confocal filtering of radiation scattered by an object in combination with heterodyne registration of radiation is an essential feature of the claimed invention.
One of the factors limiting the possibilities of confocal microscopy is that, according to formula (2), for small values of the numerical aperture A, the longitudinal resolution proportional to 1 / (A 2 ) is significantly worse than the transverse one. At the same time, small values of the numerical aperture are inevitable in the tasks of obtaining images at a great depth of the object, and are also characteristic of such important medical applications as endoscopy and ophthalmoscopy.
 The elimination of this drawback with the improvement of the longitudinal resolution was realized using the methods of low coherent interferometry in combination with confocal microscopy. The corresponding method for obtaining images of the internal structure of objects was called optical coherence tomography (OCT).
 The OCT method uses non-monochromatic radiation with a wide spectrum (for example, 50 nm) and, therefore, with low temporal coherence. In case of two-beam interference of nonmonochromatic radiation, the range of the path difference within which interference is possible is limited. This range is determined by the coherence time of the radiation and the narrower it is, the wider the spectrum of the radiation (the lower its coherence time). This property was used to obtain high longitudinal resolution when observing light-scattering objects (patent EP 0659383, class A 61 3/12, publ. 1995; patent US 5459570, class G 01 D 9/02. Publ. 1995; patent WO 9838907, class A 61 B 5/00, publ. 1998; patent WO 9732182, class G 01 B 11/12; G 01 B 9/02; publ. 1997; patent US 5321501, class G 01 B 9 / 02, publ. 1994; D. Huang et al. "Optical Coherence Tomography", Science, Vol. 254, pp. 1178-1181, 1991; JM Schmitt, "Optical Coherence Tomography (OCT): A. Review", IEEE Journal of Selected Topics in Quantum Electronics, Vol. 5, No. 4, p. 1205-1215, 1999; Optical Coherence Tomograph, a description of a device manufactured by Zeiss-Humphrey.
The specified method is illustrated in figure 2. The radiation from a point source of nonmonochromatic radiation 1 (in fact, the most commonly used source is a superluminescent diode emitting diffraction-limited radiation with a fairly wide spectrum) is split by a beam splitter 3 into reference radiation 10 and object radiation 11. The object radiation is focused at point 5 inside object 6. Moreover, sources of secondary scattered radiation appear in the object. The radiation scattered by the object in the opposite direction is directed to the diaphragm 7. The reference radiation is reflected from the reflector 12, the beam splitter 3, and it is also focused on the diaphragm 7. Thus, in this example, a Michelson interferometer is implemented, in which the object under study acts 6. The reference radiation and the scattered radiation emitted by the secondary sources of the object interfere in the plane of the diaphragm 7. In this case, interference is possible only for those secondary emitters, which are located within the plane layer 13 perpendicular to the lens optical axis (z axis). The position of the middle plane of this layer corresponds to the equality of two optical paths (a) from the source to the specified plane and from the specified plane to the diaphragm 7 and (b) from the source to the reflector 12 and further to the diaphragm 7. The layer thickness 13 d c determines the longitudinal resolution of the OCT method . This thickness is equal to half the coherence time times the speed of light in the medium. In turn, the coherence time is equal to the reciprocal of the radiation frequency band. The expression describing the longitudinal resolution of the OCT method has the form:
d c ≈0.44λ 2 / Δλ. (3)
Here λ is the average wavelength of the radiation source,
Δλ is the width of the spectrum of the radiation source in the wavelength scale
(e.g. JM Schmitt, "Optical Coherence Tomography (OCT): A Review", IEEE Journal of Selected Topics in Quantum Electronics, Vol. 5, 4, p. 1205-1215, 1999).
The value of d c may be significantly less than the resolution of d z in the method of confocal microscopy defined by formula (2).
In addition to the radiation emitted by layer 13, there is scattered radiation coming from other points of the object and also incident on the photodetector 8. To select the radiation of layer 13 (capable of interference with reference radiation) from radiation scattered by other layers of the object (not capable of interference with reference radiation), the reflector 12 is continuously moved in the longitudinal direction. In this case, a Doppler frequency shift of the reflected reference radiation occurs. In the interference of the reference radiation and the scattered radiation emitted by the layer 13, intensity beats occur at the frequency of the Doppler shift. These beats are detected by the photodetector 8. As the reflector 12 moves, the layer of interfering secondary emitters 13 also moves inside the object. Thus, the object is scanned in the longitudinal direction and its internal structure is simultaneously recorded in the form of a dependence of the beat intensity on the z coordinate of the position of the layer 13c with a resolution d c defined by formula (3).
 In order to obtain an image of the three-dimensional structure of the object, the focus point is also moved in the transverse directions (x and y). In this case, the transverse resolution is determined by formula (1).
 A comparison of optical coherence tomography with confocal microscopy, which is important to justify the advantages of the claimed invention, is shown in FIGS. 3 and 4. These figures show a more detailed picture of the focusing of radiation inside an object, taking into account the wave properties of the radiation. The radiation structure near the focal point 5 of the lens 4 has the form of a constriction. The hatched ellipse 14 in FIG. 3 depicts a region of space defining spatial resolution in confocal scanning. 4, the hatched region 15 defines the spatial resolution of the OCT method. As can be seen from a comparison of FIG. 3 and FIG. 4, the use of OCT can significantly improve the longitudinal resolution compared to the confocal method.
 The described OCT methods have the disadvantage that they use mechanical scanning of the reflector of the interferometer. This limits the speed of measurement or leads to very complex designs of a moving reflector.
 Another important drawback is that the performance of these methods is significantly lower than the fundamentally possible. This is due to the fact that of all, in principle, available for measuring the scattered radiation emitted from the volume limited by the confocal resolution body 14 (Fig. 3), only the radiation emitted from the relatively small region 15 of Fig. 4 is used. These disadvantages are eliminated in the claimed invention.
 In another optical coherence tomography method (patent WO 9835203, class G 01 B 9/02; H 01 S 3/085, publ. 1998; patent US 5956355, class H 01 S 3/10, publ. 1999) instead of non-monochromatic The source uses a monochromatic radiation source, the radiation frequency of which is quickly tuned to a given frequency range. The photodetector registers the dependence of the beat signal on the radiation frequency of the source. The results of detecting the interference signal depending on the frequency are converted into the dependence of the reflected signal on the longitudinal coordinate z in the object under study using the Fourier transform. The disadvantage of this method is the difficulty of implementing a laser device with a sufficiently fast frequency scan in a fairly wide frequency range. In fact, implementations of this method use a mechanical scan of a mirror placed in a laser cavity for frequency tuning (SR Chinn, EA Swanson, JG Fujimoto, "Optical coherence tomography using a frequency-tunable optical source". Optics Letters, Vol. 22, 5, pp . 340-342, 1997). The claimed invention also uses spectral decomposition, but without frequency scanning.
 Also known is a tomography method (T. Mitsui, "Dynamic range of optical reflectometry with spectral interferometry", Japanese Journal of Applied Physics, Vol. 38, Part. 1, 10, pp. 6133-6137, 1999), which uses spectral decomposition the sum of the reference and scattered radiation. As a result of spectral decomposition, the intensity distribution of the sum of the reference and scattered radiation over the spectrum (spectral hologram) is obtained. This distribution contains interference bands. The spectral hologram is recorded with a panoramic receiver by direct detection. Then the hologram is decrypted by known methods. The disadvantage of this method is the low sensitivity of direct detection of a weak useful scattering signal against the background of significant spurious illumination. In the claimed invention, also using spatial spectral decomposition, this disadvantage is eliminated.
 The closest analogue (prototype) of the claimed invention are the method and device described in patent WO 9961865, class. G 01 B 9/02, publ. 1999. The aim of this invention is to obtain images of the cross-section of objects with a longitudinal resolution that is significantly better than confocal longitudinal resolution.
The essential features of this invention are illustrated in FIG. The radiation source 1 is diffraction-limited (for example, a laser) and nonmonochromatic, with a certain spectrum width Δλ. The source radiation is split by a beam splitter 3 into reference radiation 10 and object radiation 11. The reference radiation is passed through a phase modulator 16, as a result of which all its frequency components acquire the same frequency shift. The object radiation is focused at a point inside the object 6. The radiation scattered by the object and the reference radiation are spatially combined into a single total radiation 17, which is incident on the photodetector 8. As in the confocal microscopy method (see Fig. 1), the scattered radiation is spatially filtered with a diaphragm on a plane conjugated to the focus plane of the object radiation. Using the photodetector 8 and the processing device 18, a beat signal of the reference and object radiation caused by a shift in the frequencies of the reference radiation and interference of the reference and scattered radiation is recorded. Only scattered radiation emitted by a thin layer of the object is involved in the interference, as shown in FIG. The position of this layer is determined by the conditions for equal path lengths in the two arms of the interferometer, and its thickness d c is determined by formula (3). The value of d c is chosen substantially less than the longitudinal confocal resolution d z . Therefore, a situation is realized in which the transverse resolution is determined by diffraction according to formula (1), and the longitudinal resolution is determined by formula (3). As a result, a body of volume resolution 15, shown in FIG. 4, is realized.
 When scanning the focus point of the object radiation in the focusing plane x, y and simultaneously registering the beat signal, an image of the cross section of the object with a longitudinal resolution determined by formula (3) is obtained. When the focus plane is shifted in the longitudinal direction, it is possible to obtain images of any other cross-sections of the object with the same longitudinal resolution.
 The considered prototype of the claimed invention has the disadvantage that its performance is significantly lower than possible. This is due to the fact that of all, in principle, available for measuring the scattered radiation emitted from the volume limited by the confocal resolution body 14 (Fig. 5), only the radiation emitted from the small region 15 of Fig. 5 is used.
SUMMARY OF THE INVENTION
The main objective of the invention is to increase the performance of recording images of the internal structure of objects.
 A method that provides a solution to the specified technical problem is as follows (see also Fig.6, which shows an example of a functional diagram).
 The diffraction-limited non-monochromatic radiation 19 emitted by the source 1 is split into a reference radiation 10 and an object radiation 11 (in this example, using a beam splitter 3). The beam of object radiation using the lens 4 is focused at a point 5 located inside the investigated three-dimensional object 6. In this case, the incident object radiation is scattered on the inhomogeneities of the object. The radiation scattered by an object can be described as the radiation of a large number of secondary radiation sources belonging to the object. Secondary sources generally arise in the entire area of an object illuminated by object radiation. Part of the radiation 20 scattered by the object and falling into the aperture of the lens 4, is used to direct the beam in the reverse direction of the rays to the plane 21 conjugate to the focus plane of the object radiation 22. In order to limit the area of the object from which the scattered radiation can be observed, the scattered radiation incident to the plane 21, they are spatially filtered using a pinhole 7 (Other spatial filtering methods are also possible, for example, transmitting radiation through a single-mode optical fiber). Using spatial filtering, the scattered radiation is selected (cf. FIG. 1) and thereby the region from which the scattered radiation is observed is limited to the confocal resolution region shown in FIG. 3. The longitudinal and transverse dimensions of this region are determined by formulas (1) and (2).
 Using a phase modulator, a frequency shift is introduced into the reference, object, or scattered radiation. In this example, a frequency shift is introduced into the reference radiation 10 using a phase modulator 16. Then, the reference radiation 10 is spatially combined (for example, using a beam splitter 3) with the scattered radiation 20 and the total radiation 17 is obtained. In the example shown in Fig. 6, the reference the radiation 10 is spatially filtered, as is the scattered radiation, however, this is not necessary and the combination of the reference and scattered radiation into the total radiation can be performed after spatial filtering of the scattered radiation.
 During interference of the reference and scattered radiation, light beats appear, which create oscillations in the intensity of the total radiation at a frequency equal to the introduced frequency shift. The detection of light beats allows with high sensitivity and noise immunity to register radiation scattered by the object.
 To determine the dependence of the radiation scattered by the object on the x, y coordinates, on the plane 22, a positional scan of the focus point of the object beam in the focus plane is performed, repeating for each position the detection of the variable component of the intensity of the total radiation.
 These signs related to the prototype. Significant differences of the claimed invention from the prototype are as follows.
 The total radiation is subjected to spatial spectral decomposition, which can be performed, for example, using a device such as spectrograph 23. As a result of spectral decomposition, an image of the spectrum of the total radiation is formed on plane 24. In real spatial spectral decomposition, the frequency resolution of the spectral device is limited. Therefore, at each point in the image of the spectrum contains a certain range of frequencies equal to the frequency resolution of the spectral device. Approximately the same frequency range is contained in the portion of the spectrum image, the width of which is equal to the inverse dispersion of the spectral decomposition, multiplied by the frequency resolution. Thus, the resulting image of the spectrum consists of independent spectral components, each of which has a frequency bandwidth determined by the frequency resolution of the device.
 As noted, the total radiation is formed as a result of interference of the reference and scattered radiation. This also applies to each independent spectral component of the total radiation emitted during spectral decomposition. Since a frequency shift is introduced into the reference radiation, the interference is unsteady. Therefore, each independent spectral component contains intensity oscillations at a frequency equal to the frequency shift introduced by the phase modulator.
The condition for obtaining such oscillations is the excess of the frequency band contained in a separate spectral component over the magnitude of the frequency shift introduced by the phase modulator 16. This condition is easily satisfied for all cases of interest. The frequency resolution of the device is not required better than the reciprocal of the travel time of the confocal resolution region d z with light. In the wavelength scale, this value is determined by formula (2). For example, with d z = 2 mm, the frequency resolution is 10 12 Hz. On the other hand, the frequency shift that can be introduced into the radiation by phase modulation does not exceed 100 MHz. Thus, this condition is met, and each element of the spectral resolution contains light beats.
 The amplitude and phase of the variable component of the intensity of the total radiation are measured simultaneously for all independent spectral components isolated during spectral decomposition. This can be done, for example, using a line of photodetectors 25. The phase is measured relative to the phase of the alternating electrical signal used to phase modulate the radiation. Thus, heterodyne detection of scattered radiation for each of its spectral components is carried out.
 As is known (for example, ND Ustinov, IN Matveev, VV Protopopov, "Methods of processing optical fields in a laser location", Moscow, 1985), the amplitude of an alternating electric signal obtained by heterodyning scattered (reflected) light is proportional to the amplitude scattering coefficient multiplied by the amplitude of the incident light, and the phase of this signal corresponds to the phase shift during scattering. When heterodyning all spectral components of the scattered radiation, the dependence of the amplitude and phase of the scattered radiation on the frequency of the incident radiation is obtained.
 For each frequency, the amplitude of the scattered radiation is normalized to the square root of the intensity (i.e., amplitude) of the reference radiation at that frequency. In this case, the amplitude-phase (complex) frequency response of the scattering is obtained, independent of the spectrum of the radiation source.
Subsequent processing of measurement results is based on the theory of linear signal conversion (for example, A. Papulis, Theory of Systems and Transformations in Optics, Moscow, 1971, pp. 54-57). The obtained amplitude-phase scattering characteristic is a complex frequency response of a linear transformation of the input signal. In this case, the input signal is the reference radiation, and a linear transformation is the process of radiation scattering. Following the theory of linear signal conversion, Fourier transform of the measured frequency response is performed using processing device 18 and the resulting impulse scattering characteristic is obtained:
h (t) = ∫H (ν-ν 0 ) exp [-i2π (ν-ν 0 ) t] d (ν-ν 0 ). (4)
where ν is the radiation frequency,
ν 0 is the central radiation frequency,
t is the time
i is the imaginary unit
H (ν-ν 0 ) is the complex frequency response of scattering
h (t) is the impulse response of the scattering or the amplitude shape of the impulse of the scattered radiation that occurs after the object is illuminated by a pulse whose duration is substantially less than the duration of the scattering pulse,
integration is carried out over the entire measured spectrum.
Then calculate the impulse response of the scattering in units of intensity:
I (t) = | h (t) | 2 . (5)
Here I (t) is the dependence of the intensity of the scattered radiation on time after illumination of the object with a sufficiently short pulse.
The application of the impulse scattering characteristic I (t) to obtain a one-dimensional image of the internal structure of an object is illustrated in Fig. 7. A short radiation pulse I 0 (t) falls on the scattering object 6, the thickness of which is significantly greater than the geometric length of the pulse. In the process of propagation of this pulse inside the object, secondary emitters arise and disappear, corresponding to different values of the z coordinate. When registering the intensity of backscattered radiation in a certain plane 26, the signals corresponding to different coordinates z come with a delay
t = 2z / ν,
where ν is the speed of light in the medium.
The scattering intensity corresponding to the z coordinate is proportional to the density of scattering centers P (z). Therefore, the dependence of the density of scattering centers P (z) on the z coordinate is:
P (z) = const • I (2z / ν). (6)
The distribution P (z) can be called a one-dimensional longitudinal image of the structure of the object.
The described process corresponds to the well-known method of pulsed location, which allows to determine the distribution of scattering centers along the direction of the beam according to the shape of the scattering pulse. However, taking into account the time resolution required in these applications (up to 10 -13 s), the use of the pulsed optical location method does not make sense. The claimed invention allows to obtain a result completely equivalent to the result of a pulsed location using stationary heterodyne spectral measurements.
An important condition for the implementation of the proposed method is the correct choice of the required number of independent elements of spectral decomposition. The magnitude of the longitudinal resolution d c , as in the other described coherent methods, is determined by formula (3). On the other hand, with a fixed position of the focus point of the object radiation, the maximum accessible for observation range of the image depth of the object d z is determined by formula (2), which determines the confocal resolution. The ratio d z / d c gives the number N of independent elements of longitudinal resolution:
N = d z / d c (7)
From the theory of Fourier transforms (for example, A. Papulis, Theory of Systems and Transformations in Optics, Moscow, 1971) it is known that if a complex signal is limited in time and spectrum, then the number of independent elements in time dependence is almost equal to the number of independent elements in the spectrum . Thus, the number of independent spectral elements in the spatial spectral decomposition of the total radiation should not be less than the expected number of elements of the longitudinal resolution of the object N. For example, the confocal longitudinal resolution can be 300 μm, and the longitudinal resolution associated with the use of low coherent interferometry can be 10 μm. In this case, the number of independent elements of spectral decomposition (and, accordingly, independent photodetectors 24 of FIG. 6) should be at least 30.
Thus, when the focus point is stationary, a one-dimensional longitudinal image of the object is obtained with a depth equal to the focus depth d z defined by formula (2) and with a longitudinal resolution d c defined by formula (3).
 When scanning the focus point 5 in the focus plane 22, the described process of registering a one-dimensional image of the internal structure of the object is repeated for each scan position. In this case, in the General case, get the image of the internal three-dimensional structure of the object.
 In particular, the following types of scanning are possible.
The focus point moves along a certain curve on the focus plane 22. In this case, the composition of one-dimensional images relating to all points of the curve forms an image of a longitudinal section of an object with a cylindrical surface of general form. The generatrix of this surface is the scanning curve. Moreover, the depth of the obtained section is equal to the depth of focus d z defined by formula (2). In particular cases, the scan curve may be a straight line or a circle. Then the dissecting surface is, respectively, a plane parallel to the z axis, or a circular cylinder.
When two-dimensional scanning, covering a certain two-dimensional region on the focus plane 22, a three-dimensional image of the internal structure of the object with a depth of d z and a base area determined by the region of the plane 22, which is covered by scanning, is obtained.
 If necessary, to expand the results of registration of the internal structure to a larger depth range, the focus plane 22 is shifted by the depth of focus and the described measurement process is repeated.
 Consider the new technical results that can be obtained by carrying out the claimed invention.
 The main field of application of the coherent interferometric methods discussed above for obtaining images of the internal structure of objects, as well as the claimed invention, is the study of biological tissues, including tissues of a living organism. Therefore, when considering the advantages and disadvantages of these methods, it is necessary to keep in mind the combination of the optical properties of a living biological environment and the requirements for interferometric measurement methods.
 A living biological environment experiences natural involuntary displacements. On the other hand, carrying out interferometric measurements requires the immobility of the investigated object with an accuracy of fractions of the wavelength. Therefore, an important requirement is the speed of measurements. In addition, in medical applications, the speed of measurements is essential for real-time monitoring (in particular, in television mode).
 The high measurement speed is hindered by another property of real biological tissues - a very low conversion coefficient of incident radiation into backscattered radiation and, accordingly, a very low intensity of scattered radiation. Another factor limiting the intensity of scattered radiation is sanitary standards (especially strict in ophthalmoscopy), which limit the intensity of incident radiation and, as a consequence, scattered radiation.
 These circumstances require the use of extremely sensitive methods for measuring the weak interfering part of the radiation against the background of strong side scattering of light by the object and stray scattered radiation. A known method in optics is heterodyne registration of radiation (for example, VV Protopopov, ND Ustinov, "Laser heterodyning", Moscow, 1985).
 The use of mechanical scanning of optical elements, in particular, when obtaining longitudinal resolution, is also a hardware factor limiting the speed of measurements. An increase in the speed of mechanical scanning leads to more complicated and expensive equipment (for example, A.M. Rollins et al., "In vivo video rate optical coherence tomography". Optics Express, Vol. 3, 6, pp. 219-229, 1998).
 Thus, the most important characteristic of the considered class of coherent methods of optical tomography of biological objects is the performance of recording images of the internal structure of objects.
 The main technical result that can be obtained by implementing the claimed invention is to significantly increase the productivity of recording images of the internal structure of objects.
The set of distinctive features from the prototype of the features that ensure the receipt of the specified technical result, which applies to the requested amount of legal protection, is as follows:
1. For each scanning position, the focus point of the reference radiation on the plane of its focusing, the total radiation is subjected to spatial spectral decomposition.
 2. At the same time, for all spectral components isolated during spectral decomposition, the amplitude and phase of the variable component of the intensity of the total radiation are measured.
 3. Perform mathematical processing of the results of these measurements and get a one-dimensional image of the internal structure of the object along the axis of focusing of the object radiation.
 To justify the specified technical result, we compare the registration of the image of the internal structure of the object by the method described in the prototype with the method described in the claimed invention, at a fixed focus point of the object radiation. This comparison is illustrated in FIG. 3 and FIG. 8. Figure 3 corresponds to the prototype. Within the region 14 allocated by confocal filtering, a single resolution element 15 is detected. Fig. 8 corresponds to the claimed invention. Ceteris paribus, within the same region of confocal resolution 14 using the proposed method simultaneously detect many resolution elements 15. Thus, the performance of the proposed method is increased by the number of times equal to the number of resolution elements within the depth of focus. The number of these elements is determined by formula (7) and in reality can be several tens.
In a more general formulation, the performance of the measurement process is determined by the bandwidth available at the desired signal-to-noise ratio. For heterodyne measurements, the signal-to-noise power ratio is described by the expression
SNR = ηP / (hν • ΔF). (8)
Here SNR is the signal-to-noise ratio,
η is the quantum efficiency of the photocathode,
P is the power of the detected radiation,
h is Planck's constant,
ν - is the radiation frequency,
ΔF is the recording bandwidth.
 (for example, ND Ustinov, IN Matveev, VV Protopopov, "Methods of processing optical fields in a laser location", Moscow, 1985).
Other things being equal in the prototype, the scattered radiation is collected from the volume of the medium, the thickness of which is equal to the resolution of the coherent method d c (see 14 in FIGS. 3 and 8). In the inventive method, the radiation is collected with a volume depth corresponding to the confocal resolution d z (see 15 in FIGS. 4 and 8). These volumes differ in the number of times
N = d z / d c ,
equal to the number of elements of longitudinal resolution. Thus, the power of the detected radiation P in the claimed invention is the same number of times N = d z / d c . Therefore, according to formula (8) with the same signal-to-noise ratio, the available bandwidth and, accordingly, the recording performance in the claimed invention can be tens of times higher.
 Thus, the implementation of the claimed invention allows to increase the productivity of registration of the internal structure of objects tens of times compared with the prototype.
 Compared with other analogues described above, in which, as in the claimed invention, the registration of the spectrum of scattered radiation is used, the claimed invention also creates a technical effect in the form of an increase in the recording performance. This effect follows from the following features distinguishing the claimed invention from other analogues discussed above.
 1. The difference of the claimed invention from the method described in patents WO 9835203, class. G 01 B 9/02; H 01 S 3/085 publ. 1998 and US 5956355, cl. H 01 S 3/10, publ. 1999 lies in the fact that there is no mechanical or other scan upon receipt of the emission spectrum.
 2. The difference of the claimed invention from the method described in T. Mitsui, "Dynamic range of optical reflectometry with spectral interferometry", Japanese Journal of Applied Physics, Vol. 38. Part. 1, 10, pp. 6133-6137, 1999, consists in the use of heterodyne registration of the amplitude and phase of the spectral components as a method that provides the highest signal-to-noise ratio when detecting weak radiation under conditions of strong background noise.
In addition to the described technical result, there are other technical results in specific forms of carrying out the invention:
1. Simultaneous registration of many elements of the longitudinal resolution of the object. To detect many elements of the longitudinal resolution of the object in the prototype, longitudinal scanning is necessary. In the claimed invention, this registration occurs simultaneously for all elements of the longitudinal resolution (cf. FIGS. 3 and 8).
 2. Image acquisition of two-dimensional longitudinal section of an object during one-dimensional scanning. If the focus point on the x, y plane is scanned along a certain curve, then as a result of registration, an image of the longitudinal section of the object is obtained with a cylindrical surface based on the scanning curve. In a particular case, an image of a flat longitudinal section is obtained. This result is not possible for the prototype.
 3. Obtaining a three-dimensional image of the internal structure of the object during two-dimensional scanning. When scanning, filling a certain area on the x, y plane, a three-dimensional image of the structure of the object is obtained. This result is not possible for the prototype.
List of figures
Figure 1. Confocal microscopy fashion illustration.
 FIG. 2. Illustration of the method of optical coherence tomography with scanning the difference in stroke of the interferometer.
 Figure 3. Spatial resolution with confocal microscopy.
 FIG. 4. Spatial resolution of the optical coherence tomography method.
 FIG. 5. Illustration of the method of heterodyne confocal microscopy with a broadband radiation source.
 6 Illustration of the claimed invention.
 FIG. 7. Illustration of the relationship of the image of the longitudinal structure of the object with the shape of the scattering pulse.
 FIG. 8. Illustration of evidence of increased productivity of registration of the internal structure of an object when implementing the claimed invention.
 FIG. 9. Illustration of information confirming the possibility of carrying out the invention.
Information confirming the possibility of carrying out the invention
The inventive method is characterized by the use of well-known means of optical technology, optoelectronics and computer technology. In particular, a significant part of such technical means is used in research and development in the field of coherent tomography (for example, D. Huang et al., "Optical Coherence Tomography", Science, Vol. 254, pp. 1178-1181, 1991; JM Schmitt, Optical Coherence Tomography (OCT): A. Review, IEEE Journal of Selected Topics in Quantum Electronics, Vol. 5, 4, p. 1205-1215, 1999; Optical Coherence Tomograph, a description of a device manufactured by Zeiss-Humphrey, and also described in analogues of the claimed invention (patent EP 0659383, class A 61 3/12, publ. 1995; patent US 5459570, class G 01 D 9/02, publ. 1995; patent WO 9838907, class A 61 B 5/00, publ. 1998; patent WO 9732182, class G 01 B 11/12; G 01 B 9/02; publ. 1997; patent US 5321501, class G 01 B 9/02, publ. 1994; WO 9961865, class G 01 B 9/02, publ. 1999) In particular, optical coherence tomography devices are described in detail in the review by JM Schmitt, Optical Coherence Tomography (OCT): A. Review ", IEEE Journal of Selected Topics in Quantum Electronics, Vol. 5, 4, p. 1205-1215, 1999.
 Consider in more detail the possibility of carrying out the invention on the example of a device that implements the inventive method for obtaining images of the internal structure of objects. This device is illustrated in Fig.9.
 The inventive method can be implemented using spatial optics or using integrated and waveguide optics. The diagram of FIG. 9 covers both options. Methods and devices for coupling waveguide and spatial systems are well known in the optical communication technology and in OCT (JM Schmitt, "Optical Coherence Tomography (OCT): A. Review", IEEE Journal of Selected Topics in Quantum Electronics, Vol. 5, 4, p . 1205-1215, 1999).
The device includes a source of diffraction-limited non-monochromatic radiation 1. Such a source - a device can be a laser with a wide spectrum of radiation or a superluminescent diode, the radiation of which is subjected to spatial filtering in order to limit the angular divergence of radiation to the diffraction limit. Usually a superluminescent diode is used, emitting at a wavelength of 0.8 μm, which corresponds to a good transmission of biological tissues. The spectral width of a superluminescent diode is usually 40 nm, which allows, according to formula (3), to obtain a longitudinal resolution of about d c ≈10 μm. We will consider these characteristics in an example implementation.
 The radiation from source 1 is directed to an optical system that splits the primary radiation into reference radiation 10 and object radiation 11. In this example, such an optical scheme is a mirror-beam splitter; in waveguide implementations, a directional coupler, well known in fiber optics, is used as a beam splitter.
The object radiation passes through the scanning device 27 and the lens 4. The lens 4 is designed to focus the beam of the object radiation at a point located inside the three-dimensional object under study, and to collect in the reverse rays of radiation scattered by the object. This lens can be an ordinary microscope lens. The numerical aperture of the lens determines according to formulas (1) and (2) the transverse resolution and the depth of focus (confocal longitudinal resolution). Let us take as an example the numerical aperture of the lens A = 0.07. Then we obtain the transverse resolution d xy ≈5 μm and the focus depth d z ≈300 μm.
According to formula (7), the number of independent elements of longitudinal resolution at a fixed position of the focus point (see also Fig. 8) is
N = d z / d c = 30.
 Scanning device 27 is either in the form of fast-rotating mirrors or in the form of oscillating galvanometric mirrors (JM Schmitt, "Optical Coherence Tomography (OCT): A Review", IEEE Journal of Selected Topics in Quantum Electronics, Vol. 5, 4, p. 1205-1215, 1999). It is designed to quickly change the tilt of parallel optical beams of object radiation incident on the lens 4. This leads to a scan of the focus point on the focus plane. In general, the scanning device provides two-dimensional positioning of the focal point on the focus plane.
 The beam of scattered radiation 20, passing in the reverse direction of the rays through the lens 4 and the scanning device 27, becomes stationary.
 The second lens 31 projects the scattered radiation onto a plane 21 conjugate to the focus plane of the object radiation. On the plane 21 at a point corresponding to the image of the focus point of the object beam, a point diaphragm 7 is placed, which carries out confocal spatial filtering of scattered radiation. In waveguide variants, confocal spatial filtering is provided automatically when scattered radiation passes through a single-mode waveguide.
 In the considered example of implementation, the phase modulator 16 is placed in the channel of the reference radiation 10. After passing through the phase modulator, the reference radiation acquires a coherent shift of all its constituent frequencies by a fixed value. As a phase modulator, for example, an acousto-optic modulator, well known in acousto-optics (for example, L. N. Magdich, V. Ya. Molchanov, “Acousto-optical devices and their application”, Moscow, 1978) can be used. The shift of the radiation frequency after passing through the phase modulator may be, for example, 10-100 MHz.
 The scattered and reference radiation are combined into a single total radiation 17 using the appropriate optical system. This system in the considered example consists of a beam splitter (or directional coupler) 28 and mirrors 29, 30. Thus, a two-beam interferometer is actually realized in which the input beam splitter is mirror 3 and the output beam splitter is mirror 28.
 A pair of mirrors 29, 30 provides for their simultaneous movement of the adjustment of the optical path length in the channel of the reference radiation with an accuracy sufficient to obtain interference of the reference and scattered radiation.
 After passing through the diaphragm 7, the total radiation is subjected to spatial spectral decomposition. This can be done, for example, using a spectrograph 23. The input slit of the spectrograph of the device is optically aligned with the diaphragm 7.
 In this example of implementation, the width of the radiation spectrum is 40 nm, and the number of independent elements of longitudinal resolution is 30. As noted, this number is equal to the minimum number of independent elements in spectral decomposition. Therefore, the required spectral resolution is ≈1 nm. Such permission may well be implemented, because corresponds to a spectral instrument of the middle class (IV Skokov "Optical spectral devices", Moscow, 1984). As a result of the action of the spectrograph in its output plane, a spectrum scan of the total radiation with the specified resolution is obtained.
 To register the distribution of the amplitude and phase of the scattered radiation along its spectrum, a line of photodetectors is placed in the spectrum plane. The line is placed in the direction of dispersion so that the number of independent elements of the spectrum corresponds to the same number of photodetectors. In this example, the number of photodetectors should be at least 30. The line of photodetectors can consist of a set of high-frequency photodiodes.
 The processing device 18 contains electronic phase-sensitive detection systems connected to each photodetector included in the line. Phase sensitive detection systems emit electrical signals with a frequency equal to the frequency shift introduced by the phase modulator 16. These systems are synchronized with the phase modulator 16 using an electrical connection 32.
 As a result of phase-sensitive detection, electrical signals are formed that determine the amplitudes and phases of the oscillations of the intensity of the total radiation (beat signal) for each independent element of spectral decomposition.
 In the processing device, these signals are converted from analog to digital. Then the set of digital signals corresponding to all photodetectors is processed using a computing device. In this case, each signal is normalized to the amplitude of the reference radiation for each given frequency and calculations are performed according to formulas (4), (5), (6). The result is a density distribution of the scattering centers of the object along the focus axis within the focus depth.
 When scanning the focus point along the focus plane, different types of images of the internal structure of the object are obtained in the form of one-dimensional or three-dimensional images, as described above. These results are output to the information display device 33, which may be, for example, a personal computer.
 Consider the example of the described implementation issues of sensitivity and performance of the proposed method.
A typical backscatter coefficient for biological objects such as skin is ≈0.1 mm -1 / steradian (JM Schmitt, G. Kumar, Applied Optics, Vol. 37, 13, pp. 2788-2797, 1998). This coefficient takes into account the thickness of the scattering layer and the solid angle of collection of the scattered radiation. Considering that the solid angle is determined by the aperture of lens 4, and the thickness of the scattering layer by the depth of focus of the lens, we obtain that the total backscattering coefficient is ≈4 • 10 -4 .
Assuming that the power of the source is 1 mW, the quantum efficiency of the photodetector is 0.5, the radiation loss due to beam splitters and spurious reflections is 0.9, we obtain from formula (8) for this example:
SNR≈10 11 / (ΔF).
With the frequency bandwidth ΔF = 1 MHz, which is characteristic of the television standard, we obtain SNR≈10 5 .
 Under the same conditions, the use of OCT with scanning the path difference in the interferometer (Fig. 2) would have a signal-to-noise ratio of about 30 times worse.
More stringent conditions are characteristic of retina tomography. The limitations on retina exposure to light are 200 μW (E. Swanson et al. Optics Letters, Vol. 18, 21, pp. 1864-1866, 1993). The retina reflection coefficient in confocal ophthalmoscopy is 10 -4 -10 -5 (K. Kobayashi et al., Optical Engineering, Vol. 34, 3, pp. 717-726, 1995). Using a lower value, we get
SNR≈10 9 / (ΔF).
For ΔF = 1 MHz, SNR ≈ 10 3 . In an OCT method using longitudinal scanning, the signal-to-noise ratio is approximately 30 times worse under the same conditions.
 In practice, obtaining 1 frame of the longitudinal section of the retina by the traditional OCT method takes 1 s (E. Swanson et al. Optics Letters, Vol. 18, 21, pp. 1864-1866, 1993, Optical Coherence Tomograph, description of the device manufactured by Zeiss-Humphrey. The gain in the signal-to-noise ratio allows the implementation of the claimed invention to go to the television mode.

Claims (3)

 1. The method of obtaining images of the internal structure of objects, which consists in the fact that diffraction-limited non-monochromatic radiation is split into reference and object radiation, the object radiation beam is focused at a point located inside the three-dimensional object under study, the radiation scattered by the object is projected onto a plane, optically conjugated to the focus plane of the object radiation beam, confocal spatial filtering of the scattered radiation is carried out, the hour is shifted From the reference, or object, or scattered radiation, the reference and scattered radiation are spatially combined with the formation, as a result of interference, of the total radiation, a variable component of the total radiation intensity is detected at a frequency equal to the frequency shift of the reference, object or scattered radiation, scanning of the focus point of the object beam in the focusing plane, repeating for each scan position the detection of a variable component of the intensity of the total radiation , characterized in that the total radiation is subjected to spatial spectral decomposition, and in the process of detection, the amplitude and phase of the variable component of the intensity of the total radiation are measured simultaneously for all spectral components extracted during spectral decomposition, mathematical processing of the measurement results is performed and a one-dimensional image of the internal structure of the object along the axis is obtained focusing the beam of object radiation for each scan position.
 2. The method according to p. 1, characterized in that they scan the focal point of the object beam along the line and get a two-dimensional image of the object on a cylindrical surface, based on the scan line.
 3. The method according to p. 1, characterized in that perform a two-dimensional scan of the focus point of the object beam and get a three-dimensional image of the internal structure of the object.
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WO2016013955A1 (en) * 2014-07-25 2016-01-28 Шлюмберже Холдингс Лимитед Method for evaluating material sample structural changes resulting from acting upon sample
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