JPH10332329A - Optical frequency sweeping type tomoimage measuring method and its device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical frequency sweeping type tomoimage measuring method and its device capable of providing tomoimage information in a wide space domain in a short time with high resolving power without using mechanical scanning. SOLUTION: A coherent light is generated from a light source 1 where optical frequency sweeping is possible, and after the coherent light is divided into a reference light and a measuring light, a specimen SP is irradiated by the measuring light, and interference light is generated by interference between a reflected object light and the reference light. The interference light is image- picked up using an image-picking up element 20 composed by arranging a plurality of light receiving picture elements in one dimension or two dimensions, and an image signal is outputted from the picking-up element 20 every fixed flame period during the sweeping term of the optical frequency. After the image signal is housed every flame in a flame memory 23, the image signal housed in the flame memory 23 is Fourier transformed every picture element on a time base to compute a tomoimage.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to an optical frequency sweep type tomographic image measuring method and apparatus for measuring a tomographic image of an object by interference between object light and reference light.

[0002]

2. Description of the Related Art Several studies for obtaining tomographic information inside a living body using near-infrared light which is relatively transparent to the living body have been published (Joseph A. Izatt, et al., "Ophth
almicDiagnotics using Optical Coherence Tomograph
y ", SPIE Vol. 1877, pp. 136-144, 1993), (Udo Haberland, e
t al., "Investigation of highly scattering mediausi
ng near-infrared continuous wave tunable semicondu
ctor laser ", SPIEVol.2389, pp.504-512, 1995), (Hideyuki Tajiri et al .:" Simultaneous precision measurement of refractive index and thickness by low coherence optical interference ", IEICE Technical Report OPE95-121, pp.7- 12,1996-01).

[0003] Methods for measuring internal information of a living body can be broadly classified into a transmission measurement method and a reflection measurement method. The former uses transmitted light such as straight-traveling light or quasi-straight-traveling light. If the transmitted light can be detected with high accuracy, a conventional X-ray CT algorithm can be applied as it is, and reconstruction of a tomographic image becomes easy.

However, a living body is a multiple scatterer for light, and the absorption coefficient of straight-ahead light in the near-infrared region is 0.1.
Although relatively small 01~0.1mm about ^-1, diffusion coefficient significant attenuation due to diffusion for very large as about 10 mm ^-1. For this reason, the straight traveling light is attenuated when passing through the living body by several mm, and becomes lower than the detection limit.

On the other hand, if quasi-straight light is used, the diffusion coefficient is equivalently about 1.0 to 3.0 mm ^-1 , and the detection limit can be expanded to the order of cm. However, in order to apply it to tomographic measurement of the human body, a detection distance of about 20 to 30 cm is required, so that it is difficult with current technology.

The latter reflection-type measurement method measures the reflectance at a certain depth position from the surface of a living body as a tomographic image, and is applicable to a large living body. As an example of this development, there is a coherence gate type interferometer, which uses a coherence length of a light source of 10 to 30 μm to obtain a high-resolution tomographic image.

[0007]

However, the coherence gate method has several problems. 1) To obtain a tomographic image at a certain depth, the interference optical system and the object must be relatively positioned. Scanning in the horizontal direction is required, and scanning in the depth direction is required to obtain a tomographic image in the depth direction. Therefore, mechanical three-dimensional scanning is required to obtain a stereoscopic image. 2) When a living body is irradiated with near-infrared rays, reflected light is generated from all depth positions, but the coherence gate method can only detect reflected light from a specific depth position. Therefore, the reflected light itself is weak and only a part of the reflected light contributes to the measurement, so that the light use efficiency is extremely low. 3) Since the light interference phenomenon is used, parallel light having excellent spatial coherence is required as measurement light. On the other hand, it is necessary for the light source to reduce the time coherence and increase the resolution of the tomographic image measurement. A super luminescent diode (SLD) has been developed as a light source that satisfies these two coherences, but has only a low output of several mW or less, and further has a light loss due to an optical system for satisfying the spatial coherence. This further reduces the amount of light available.

As described above, when the coherence gate method is applied to a living body, the tomographic measurement from the surface to several hundred μm is the limit, and the application field such as the cornea of the eye is limited.

An object of the present invention is to provide an optical frequency sweep type tomographic image measuring method and apparatus capable of obtaining tomographic image information in a wide space area in a short time and with high resolution without performing mechanical scanning. is there.

[0010]

According to the present invention, a coherent light is generated from a light source capable of sweeping an optical frequency, the coherent light is divided into a reference light and a measuring light, and an object is irradiated with the measuring light. The interference between the reflected object light and the reference light generates interference light, and the plurality of light receiving pixels are one-dimensional or two-dimensional.
Using the imaging device arranged in a three-dimensional manner, the interference light is imaged, and an image signal is output from the imaging device every predetermined frame period during the optical frequency sweep period. An optical frequency sweep type tomographic image measuring method characterized in that a tomographic image is calculated by Fourier transforming the image signal stored in the image memory on a time axis for each pixel after storing the image signal in the image memory.

According to the present invention, a light source generates coherent light having a continuously changing optical frequency and irradiates the object with the coherent light, and reflects the object light (straight light and quasi-straight light) reflected inside the object using an interferometer. ) And the reference light, and by measuring the intensity distribution of the interference light, and measuring the change in the intensity distribution corresponding to the change in the optical frequency, a tomographic image along the depth direction can be constructed.

The principle will be described in detail. When the lengths of the two arms in the interferometer, that is, the distance to the reference mirror and the distance to the object surface are equal, the beat frequency of the interference light becomes zero because the optical frequencies of the reference light and the object light are the same. . Next, when the object light is reflected light from a certain depth position inside the object, the length of the arm on the object light side becomes longer. At this time, since the optical frequency of the light source is continuously changing, the optical frequency of the reference light and the optical frequency of the object light are different, so that a beat is generated in the interference light, and the beat frequency is equal to the length of the two arms. Corresponding to the difference. Since the reflected light occurs at all the depth positions along the measurement light path in the object, the object light is a mixture of the reflected lights at different depth positions. Furthermore, since the length of the arm of the interferometer differs according to the depth of the reflection position, the interference light has a different beat frequency. Therefore, with the reference mirror fixed, the reflection coefficient at the depth position corresponding to the beat frequency can be calculated by frequency-analyzing the intensity change of the interference light, and the spatial distribution of the reflection coefficient is obtained as a tomographic image. Can be measured.

Further, in the present invention, the interference light is imaged by using an image sensor having a plurality of light receiving pixels arranged one-dimensionally or two-dimensionally. This makes it possible to capture data relating to a one-dimensional or two-dimensional imaging area at a time. For data in the depth direction,
It can be distinguished by analyzing the frequency of the interference light. Therefore,
A tomographic image can be obtained without performing mechanical scanning, and high-precision measurement can be performed in a short time without being affected by a feed error or vibration.

Further, during the sweeping period of the optical frequency, an image signal is output from the image sensor at a predetermined frame period, and the image signal is stored in the image memory for each frame. Each time, a tomographic image is calculated by performing a Fourier transform on the time axis. This makes it possible to analyze the frequency of the image signal stored for each frame for each pixel, and to easily construct a tomographic image.

Further, according to the present invention, the optical frequency can be swept,
A light source for generating coherent light, the coherent light from the light source is split into reference light and measurement light for irradiating an object, and interference light obtained by interference between the object light reflected by the object and the reference light is generated. And an image sensor for imaging the interference light from the interferometer, and a predetermined frame during the optical frequency sweep period. An image memory for storing an image signal output from the image sensor in each cycle for each frame, and a Fourier transform on the time axis of the image signal stored in the image memory for each pixel to calculate a tomographic image. An optical frequency sweep type tomographic image measuring apparatus comprising a signal processing unit.

According to the present invention, as described above, a coherent light having a continuously changing optical frequency is generated from a light source to irradiate the object, and the object light reflected straight inside the object using an interferometer (straight forward). Light and quasi-straight light) and the reference light, thereby measuring the intensity distribution of the interference light, and measuring the change in the intensity distribution corresponding to the change in the optical frequency to obtain a tomographic image along the depth direction. Can be constructed.

Further, an interference light is imaged by using an image pickup device having a plurality of light receiving pixels arranged one-dimensionally or two-dimensionally. This makes it possible to capture data relating to a one-dimensional or two-dimensional imaging area at a time. The data in the depth direction can be distinguished by analyzing the frequency of the interference light. Therefore, a tomographic image can be obtained without performing mechanical scanning, and high-precision measurement can be performed in a short time without being affected by a feed error or vibration.

Further, during the sweeping period of the optical frequency, an image signal is output from the image sensor at a predetermined frame period, and the image signal is stored in the image memory for each frame. Each time, a tomographic image is calculated by performing a Fourier transform on the time axis. This makes it possible to analyze the frequency of the image signal stored for each frame for each pixel, and to easily construct a tomographic image.

Further, the present invention is characterized in that the image pickup device has a low-pass filter characteristic having a cutoff frequency equal to a frame frequency which is a reciprocal of a frame period.

According to the present invention, the image pickup device generally has a low-pass filter characteristic, but the cutoff frequency substantially matches the frame frequency. When the length of the arm of the reference mirror in the interferometer is set to a constant value and the optical frequency of the light source is continuously changed, the beat frequency is detected as a time-series signal that fluctuates at a low frequency. Since the beat frequency corresponds to the depth inside the object, a certain depth range corresponding to the range of the measured beat frequency is the measurement range. Therefore, since signals outside the depth range to be measured are superimposed as noise, an image signal having a high S / N ratio can be obtained by removing noise having a frequency higher than the cutoff frequency of the image sensor.

Further, the invention is characterized in that the light source is constituted by a semiconductor laser device.

According to the present invention, since the information in the depth direction is obtained by the frequency analysis of the beat frequency, a strict restriction on the time coherence is not required. Therefore, a semiconductor laser device capable of increasing the output can be used instead of the SLD light source, so that it is possible to measure a deep part of the object.

Further, the present invention provides a condensing lens provided between the interferometer and the image pickup device, for condensing interference light from the interferometer.
It is characterized by including a spatial filter composed of a pinhole for passing only a condensing spot and a collimating lens for collimating light passing through the pinhole.

According to the present invention, when the measurement light passes through the object, a considerably strong diffused light is generated in addition to the object light composed of the straight light and the quasi-straight light. By providing a filter, harmful diffused light can be removed, and an image signal with a high S / N ratio can be obtained.

[0025]

FIG. 1 is a block diagram showing an embodiment of the present invention. The present apparatus is configured as a Michelson-type interferometer formed by a light source 1, a beam splitter 5, a reference mirror 6, a sample SP to be measured, and an image sensor 20 for light detection. The present invention can be applied to interferometers other than the Michelson type.

The light source 1 is composed of a semiconductor laser element (LD) that generates coherent light, and the drive circuit 1a modulates the injection current to sweep the optical frequency. For example, the injection current changes in a sawtooth shape. And the optical frequency changes linearly and continuously. The modulation amplitude of the optical frequency is
The laser oscillation is set in a range where mode pop does not occur, and the modulation cycle is set by the control circuit 25 to the image pickup device 20.
Is controlled so as to be synchronized with the frame scanning of the frame. In addition, the wavelength of the light source 1 is 700 nm
Near-infrared light of 900 nm is preferred.

A collimating lens 2 is provided on the light emitting side of the light source 1.
Is adjusted so that the coherent light from the light source 1 becomes parallel light. Between the light source 1 and the beam splitter 5, a return light preventing element composed of a polarizing element 3 and a quarter-wave plate 3 is provided to stabilize laser oscillation of the light source 1.

The beam splitter 5 splits the coherent light from the light source 1 into reference light and measurement light. The reference light is reflected by the reference mirror 6 and returns to the beam splitter 5 again. The measurement light irradiates the sample SP, and a part of the sample SP
When the light is reflected by P, it becomes an object light and the beam splitter 5 again.
Return to Thus, the reference light and the object light interfere with each other.

The reference mirror 6 is held by the fine movement stage 7 driven by the mirror drive circuit 7a, and is fixed at the time of optical frequency sweep, but moves when the measurement range of the sample SP is switched.

The sample SP has optical characteristics equivalent to those of a living body. Here, a sample in which an intralipid aqueous solution (concentration: 0.1%), which is a standard sample, is placed in a glass container is used. In order to give a reflection coefficient, a target TA having a pattern of chromium vapor deposited films having a light reflectance of 30% and 1% is prepared, and is held in an aqueous solution so as to be perpendicularly reflected with respect to measurement light. doing.

The interference light from the beam splitter 5 enters the image sensor 20 and is converted into an image signal according to the intensity distribution of the interference light.

A spatial filter 11 is provided between the beam splitter 5 and the image sensor 20. Spatial filter 1
1 includes a condensing lens 8 for condensing the interference light in a spot shape, a pinhole plate 9 for passing only the condensing spot, and a collimating lens 10 for collimating the light passing through the pinhole plate 9, and is a telecentric system. The pinhole plate 9 is arranged on the focal plane of the double lens imaging system arranged in the above. The spatial filter 11 has the following functions: 1) a function of forming an imaging optical system having a large depth of focus by means of a pinhole to increase the imaging allowable depth of the sample SP; and 2) high intensity of return light from the sample SP. The diffused light is removed by the pinhole plate 9 as much as possible, and the function of extracting only the straight light and the quasi-straight light from the sample SP is achieved.

The image pickup device 20 is composed of a high-sensitivity CCD (charge-coupled device) sensor or the like in which a plurality of light-receiving pixels are arranged one-dimensionally or two-dimensionally. Second frame period Tf
Output one screen as an image signal in time series. The image signal from the image sensor 20 is converted into a digital value by an A / D (analog-digital) conversion circuit 21 and then stored in a frame memory 23 in the computer. The frame memory 23 has a memory capacity for storing the number of frame images obtained by dividing the measurement time Tm by the frame period Tf, where one sweep time Ts of the optical frequency is set equal to the frame period Tf and the measurement time is Tm. And stores image data for 2 ÷ (1/30) = 60 sheets, for example, with a measurement time Tm = 2 seconds. If the image sensor 20 is a one-dimensional sensor, it outputs a one-dimensional frame image, and if it is a two-dimensional sensor, it outputs a two-dimensional frame image.

The signal processing section 22 in the electronic computer performs various arithmetic processing according to a predetermined program. In this case, the image signal stored in the frame memory 23 is subjected to a Fourier transform on a time axis for each pixel, thereby obtaining a frequency. Perform analysis. The Fourier frequency matches the beat frequency of the interference light,
Furthermore, since it can be associated with the depth position where the object light is reflected, it is possible to calculate a tomographic image in the depth direction after all.

When a one-dimensional frame image is Fourier-transformed on the time axis, a tomographic image in a one-dimensional imaging region is obtained.
The image can be directly displayed on a display device 24 such as a CRT (cathode ray tube). When the two-dimensional frame image is Fourier-transformed on the time axis, a tomographic image in the two-dimensional imaging region is obtained. Therefore, by specifying a linear display position, the tomographic image at the display position can be displayed on the display device 24. .

FIG. 2 is a graph showing a sweep waveform of an optical frequency. The horizontal axis represents time, and the vertical axis represents the optical frequency of the light source 1.
For example, frame period Tf = 1/30 second, sweep time Ts
= 1/30 second, and when the sweep width (fb-fa), which is the difference between the upper limit value fb and the lower limit value fa of the optical frequency, is set to 50 GHz, the cutoff frequency of the image sensor 20 is about 30 Hz.
And an interference signal having a beat frequency over a depth range of about 2 mm corresponding to a sweep width of 50 GHz is stored in the memory. For this reason, when the measurement time is set to Tm = 2 seconds and the stored data is subjected to Fourier analysis to obtain a tomographic image for each Fourier frequency, the tomographic image interval in the depth direction is 2 mm / 60 = 33 μm. Further, by moving the position of the reference mirror 6, the depth range that can be measured is shifted as a whole.
Therefore, by performing measurement a plurality of times while moving the measurement range in units of 2 mm, tomography over 0 cm to several cm can be realized with high resolution.

Further, by making the cut-off frequency of the image sensor 20 coincide with the frame frequency, noise from outside the depth range to be measured can be removed, so that an image signal with a high S / N ratio can be obtained.

[0038]

As described above in detail, according to the present invention, by using an image pickup device in which a plurality of light receiving pixels are arranged one-dimensionally or two-dimensionally, data relating to a one-dimensional or two-dimensional imaging region is obtained. At a time, and no mechanical scanning is required. Therefore, high-precision measurement can be performed in a short time without being affected by a feed error or vibration.

Further, during the optical frequency sweep period, an image signal is output from the image sensor at predetermined frame periods, and the image signal is stored in the image memory for each frame. Each time, a tomographic image is calculated by performing a Fourier transform on the time axis. This makes it possible to analyze the frequency of the image signal stored for each frame for each pixel, and to easily construct a tomographic image.

Also, by utilizing the fact that the cutoff frequency of the image sensor substantially coincides with the frame frequency, it is possible to remove noise from a range other than the depth range to be measured, thereby obtaining an image signal with a high S / N ratio. Can be.

Further, by using a semiconductor laser element capable of increasing the output, it is possible to measure a deep part of an object.

By providing a spatial filter between the interferometer and the image sensor, harmful diffused light can be removed, and an image signal with a high S / N ratio can be obtained.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing one embodiment of the present invention.

FIG. 2 is a graph showing a sweep waveform of an optical frequency.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 light source 2 collimating lens 3 polarizing element 4 波長 wavelength plate 5 beam splitter 6 reference mirror 11 spatial filter 20 image sensor 21 A / D conversion circuit 23 frame memory 22 signal processing unit 24 display device SP sample

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Keiichi Yamada, Inventor Keiichi Yamada 4-36 Hamadera-Funao-cho, Sakai-shi, Osaka Daiken Medical Instruments Co., Ltd. 4-36, Daiken Medical Device Co., Ltd.

Claims (5)

[Claims] An optical frequency sweepable light source generates coherent light, divides the coherent light into reference light and measurement light, irradiates an object with measurement light, and reflects reflected object light and the reference light. An interference light is generated by interference with light, and the interference light is imaged using an image sensor configured by arranging a plurality of light receiving pixels one-dimensionally or two-dimensionally. An image signal is output from the image sensor every predetermined frame period, the image signal is stored in the image memory for each frame, and the image signal stored in the image memory is Fourier-transformed for each pixel on the time axis to obtain a tomographic image. An optical frequency sweep type tomographic image measuring method, comprising calculating an image. 2. A light source capable of sweeping an optical frequency to generate coherent light, a coherent light from the light source being split into a reference light and a measuring light for irradiating the object, and an object light reflected by the object An interferometer for generating interference light obtained by interference with the reference light, and an image sensor configured to arrange a plurality of light receiving pixels one-dimensionally or two-dimensionally, and for imaging interference light from the interferometer And an image memory for storing, for each frame, an image signal output from the image sensor every predetermined frame period during the optical frequency sweep period, and a time axis for storing the image signal stored in the image memory for each pixel. And a signal processing means for calculating a tomographic image by performing a Fourier transform on the tomographic image. 3. The optical frequency sweep type tomographic image measuring apparatus according to claim 2, wherein the image sensor has a low-pass filter characteristic having a cutoff frequency equal to a frame frequency which is a reciprocal of a frame period. 4. The optical frequency sweep type tomographic image measuring apparatus according to claim 2, wherein said light source is constituted by a semiconductor laser device. 5. A condensing lens provided between an interferometer and an image sensor for condensing interference light from the interferometer, a pinhole passing only a condensing spot, and collimating light passing through the pinhole. 3. An optical frequency sweep type tomographic image measuring apparatus according to claim 2, further comprising a spatial filter constituted by a collimating lens.