JP2015114284A - Optical coherence tomography - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical coherence tomography (OCT) capable of acquiring tomographic information on a deep region of an object to be inspected without losing any tomographic information by using both an AD conversion unit with the high resolution and the low conversion speed and an AD conversion unit with the low resolution and the high conversion speed.SOLUTION: The OCT comprises a plurality of AD conversion units. The conversion speed of a first AD conversion unit is higher than the conversion speed of a second AD conversion unit, and the resolution of the first AD conversion unit is lower than the resolution of the second AD conversion unit.

Description

  The present invention relates to an optical coherence tomography used for medical diagnosis and the like.

  In optical coherence tomography (hereinafter abbreviated as OCT) using a wavelength tunable light source, the object is irradiated with light, the wavelength of the irradiated light is changed, and reflected light returning from different depths of the reference light and the object To interfere with. Then, by analyzing the frequency component contained in the time waveform of the intensity of the interference light (hereinafter sometimes abbreviated as interference spectrum), information relating to the tomography of the object, for example, a tomographic image is obtained. Further, when analyzing the frequency component of the interference spectrum, conversion from analog data to digital data (AD conversion) is performed using an AD board (Patent Document 1). Hereinafter, OCT using a wavelength tunable light source may be abbreviated as SS-OCT.

  Here, in SS-OCT, when acquiring interference spectrum data at equal wave number (equal frequency) intervals in order to acquire information on a fault at a deeper site, the sampling interval is increased to increase the resolution. It needs to be shortened.

JP 2010-14514 A

  In order to shorten the sampling interval, an AD board capable of high-speed AD conversion is required. However, in general, an AD board capable of high-speed AD conversion has a small number of bits, that is, a low resolution. When the number of bits that can be handled becomes small, even if the intensity of reflected light from an object is strong, data can be handled only within the range of the number of bits of the AD board, and there is a possibility that a part of tomographic information is lost. As a result, for example, the gradation of the luminance of the tomographic image becomes small, and there arises a problem that it is difficult to see the structure information that causes only a small gradation difference.

  On the other hand, AD conversion with a large number of bits takes a longer time to convert than AD conversion with a small number of bits, and thus it has been difficult to realize an AD board capable of high-speed conversion with a large number of bits.

  That is, it is difficult to achieve both high resolution (increase the number of bits) and high AD conversion speed.

  An optical coherence tomometer according to the present invention includes a light source unit that changes the wavelength of emitted light, light that irradiates an object with light from the light source unit, and reference light, and light that is irradiated to the object. Acquisition of information on the object based on an interference optical system that generates interference light by the reflected light and the reference light, a light detection unit that receives the interference light, and a time waveform of the intensity of the interference light The information acquisition unit includes a frequency separation unit that separates a time waveform of the intensity of the interference light into a plurality of different frequency components, and a frequency separated by the frequency separation unit. A first AD conversion unit that AD converts a component of the first frequency band among the components, and a second AD conversion that AD converts a component of the second frequency band that is lower in frequency than the first frequency band And the first AD conversion unit A calculation unit that obtains information of the object from the obtained data and the data obtained by the second AD conversion unit, and the conversion speed of the first AD conversion unit is that of the second AD conversion unit. It is faster than the conversion speed, and the resolution of the first AD converter is lower than the resolution of the second AD converter.

  According to the OCT according to the present invention, a high resolution (large number of bits) and low conversion speed AD conversion unit and a low resolution (small bit number) and high conversion speed AD conversion unit are used in combination. Information is not lost, and tomographic information from a shallow part to a deep part of the measurement target object can be acquired.

It is a schematic diagram which shows the structure of OCT which concerns on embodiment of this invention. It is a figure for demonstrating the frequency separation part in OCT which concerns on embodiment of this invention. In embodiment of this invention, it is a figure which shows the irradiation position of the irradiation light to the measurement object. It is a schematic diagram which shows the structure of OCT which concerns on Example 1 of this invention. It is a figure which shows the time change of the frequency of the light emitted from the wavelength sweep light source in Example 1 of this invention. It is a figure for demonstrating the acquisition method of the tomogram in Example 1 of this invention. It is a figure which shows the permeation | transmission characteristic of the interference signal of the frequency separation part in Example 1 of this invention. It is a schematic diagram which shows the structure of OCT which concerns on Example 2 of this invention. It is a figure which shows the frequency dependence of the intensity | strength of an interference signal in embodiment of this invention. It is a figure which shows the frequency dependence of the intensity | strength of an interference signal in embodiment of this invention. It is a figure which shows the frequency dependence of the intensity | strength of an interference signal in embodiment of this invention. It is a figure which shows the frequency dependence of the intensity | strength of an interference signal in embodiment of this invention.

  Although an embodiment of the present invention is described below, the present invention is not limited to this. FIG. 1 is a schematic diagram showing an apparatus configuration of an optical coherence tomography (OCT) according to the present embodiment.

  The OCT according to the present embodiment includes a wavelength swept light source 101 as a light source unit, an interference optical system 103, a light detection unit 104, and an information acquisition unit 105.

  Further, the interference optical system 103 in the OCT according to the present embodiment includes a reference mirror 107, an optical coupler 135, and a scanning mirror 111.

  In this embodiment, the reference mirror 107 is provided on the optical paths of the reference lights L5 and L5 ′. However, the reference mirror 107 is not necessarily required, and a fiber loop is formed so as to have an appropriate optical path length. It may be configured to be directly coupled.

  A method for acquiring the information of the tomography of the object 108 by OCT according to the present embodiment will be described.

  First, the light L1 emitted from the wavelength swept light source 101 is demultiplexed by the optical coupler 134 into light (L2) traveling to the interference optical system 103 and light (L3) traveling to the clock generation unit 102 described later. The light L2 is demultiplexed by the optical coupler 135 into the irradiation light L4 and the reference light L5. The irradiation light L4 is reflected by the scanning mirror 111 and is applied to the object (subject) 108. Light L 4 ′ irradiated and reflected on the subject 108 enters the coupler 135. On the other hand, the reference light L5 is reflected by the reference mirror 107 and enters the coupler 135 as light L5 '.

  The light L4 ′ and the light L5 ′ interfere with each other by the coupler 104, and the generated interference light enters the light detection unit 104.

  Since the wavelength swept light source 101 sweeps the wavelength of the emitted light, that is, changes the wavelength of the emitted light with time, the light detection unit 104 acquires a time waveform of the intensity of the interference light. The time waveform of the intensity of the interference light is sent from the light detection unit 104 to the frequency separation unit 126 as an interference signal. In the frequency separation unit 126, the time waveform of the intensity of the interference light, that is, the interference signal is separated into a plurality of frequency components and sent to the information acquisition unit 105. Here, separating the interference signal into a plurality of frequency components means extracting the interference signal for each frequency component.

  In the OCT according to the present embodiment, when the information acquisition unit 105 acquires information on the subject 108 based on the time waveform of the intensity of the interference light, AD conversion is performed by a separate AD conversion unit for each frequency component.

  In FIG. 1, the frequency separation unit 126 includes a high-pass filter 122 that is a band-pass filter of a first frequency band component (high frequency component). In addition, the frequency separation unit 126 includes a Low-Pass filter 123 that is a bandpass filter of a second frequency band component (low frequency component) having a frequency lower than that of the first frequency component. In the present embodiment, the frequency band 202 transmitted by the high-pass filter 122 and the frequency band 203 of light transmitted by the low-pass filter 123 have an overlapping frequency band 201 (FIG. 2A). . FIG. 2A shows the transmission characteristic of the interference signal of the frequency separation unit, and the signal transmittance on the vertical axis of the graph of FIG. 2A shows the transmittance for the interference signal. In FIG. 2A, the signal transmittance of 202 is larger than that of 203, but this is an example, and may be reversed, or both may have the same signal transmittance.

  In the present embodiment, the frequency separation unit 126 shows an example in which the temporal waveform of the intensity of the interference light is separated into two different frequency components, but is separated into three or more different frequency components. May be. In the present embodiment, among the frequency components separated by the frequency separation unit 126, the first AD conversion unit 120 converts high-frequency components from analog data to digital data (AD conversion). Further, the low frequency component is AD converted by the second AD conversion unit 121. In the present embodiment, the AD conversion speed of the first AD conversion unit 120 is faster than the AD conversion speed of the second AD conversion unit 121, and the number of bits of the first AD conversion unit 120 is the second AD conversion speed. It is characterized by being smaller than the number of bits of the part 121. In other words, the resolution of the first AD converter 120 is lower than the resolution of the second AD converter 121. Details will be described below.

  Here, in the interference signal (interference spectrum time waveform) acquired in the SS-OCT apparatus, a signal from a deep part of the subject becomes a high frequency component, and a signal from a shallow part such as the vicinity of the object surface becomes a low frequency component. . In other words, if the time waveform of the interference spectrum is Fourier-transformed and the high frequency component is large, the reflection at the deep portion of the subject is large, and if the low frequency component is large, the reflection at the shallow portion is large.

  Further, light absorption and scattering exist in the living body, and the coherence length of light emitted from the wavelength swept light source is also finite. Therefore, in the normal measurement where the coherence gate, that is, the position where the optical path lengths of the reference light and irradiation light are equal, is placed slightly in front of the object surface, the signal intensity on the object surface is basically the strongest, and the signal intensity is deeper in the deeper part. become weak. When this is replaced with a tomographic image obtained by OCT (hereinafter sometimes abbreviated as an OCT image), the OCT image near the object surface has a higher luminance, and the deeper part has a lower luminance.

  Considering these, the interference signal from the vicinity of the surface of the object has a relatively large signal strength and a low frequency interference signal, and the signal from the deep part of the object tends to have a low signal strength and a high frequency interference signal. It can be said.

  In Patent Document 1, these signals are processed by one AD conversion element (AD board), and an AD conversion element capable of high resolution (large number of bits) and high speed conversion is required. However, high resolution (large number of bits) AD conversion takes a long time compared with low resolution (small number of bits) AD conversion with a general configuration AD conversion element. It was difficult to realize a possible AD conversion element.

  The OCT according to the present embodiment employs a configuration in which the above-described interference signal component having a large signal intensity and a low frequency and an interference signal component having a small signal intensity and a high frequency are processed by separate AD conversion elements. That is, an interference signal near the object surface is processed by a high-resolution AD conversion element with a low conversion speed, and a signal at a deep part of the object with a high conversion speed and a low-resolution AD conversion element. Process. An AD conversion element that processes a signal in a deep part of an object has a low resolution, but originally a signal intensity in a deep part of the object is small. Therefore, even when a low-resolution AD conversion element is used compared to a case where AD conversion from a high-frequency signal component to a low-frequency signal component is performed collectively with a single high-speed and large-bit AD conversion element, an OCT image is obtained. There will be no significant loss of tone.

  In this manner, by dividing the signal by a plurality of AD conversion elements for each frequency band, it is not necessary to use a high-speed and high-resolution AD conversion element. Therefore, it is possible to realize an OCT apparatus capable of imaging from a shallow part to a deep part by using a low-speed and high-resolution AD conversion element and a high-speed and low-resolution AD conversion element that are both inexpensive.

  The interference signal output from the frequency separation unit 126 is subjected to AD conversion by each of the above-described AD conversion units, and then subjected to Fourier transform in the calculation unit 125, and information on a tomogram is obtained in each frequency band of the high-frequency component and low-frequency component of the interference signal. Is done. The information of the fault is typically a tomographic image, but may be an absolute value of a numerical value of reflected light intensity from each fault, or may be a normalized value, and is not particularly limited. In the following, the fault information will be described as a tomographic image.

  Further, the calculation unit 125 can connect a tomographic image generated from a high frequency component and a tomographic image generated from a low frequency component. The tomographic image generated from the high frequency component is an image of a deep part of the subject, and the tomographic image generated from the low frequency component is an image of a shallow part. And tomographic images of both shallow and shallow parts can be obtained.

  In the present embodiment, a tomographic image is acquired independently in each frequency band using each AD conversion unit, and a single deep tomographic image is constructed by connecting a plurality of images thereafter. Such a form is preferable because it is not necessary to adjust a difference in characteristics such as gain and jitter of each AD conversion unit at the time of signal acquisition in order to obtain a final tomographic image. Further, as described above, by considering the brightness adjustment and the optimum connection position when joining the images, even if there is a difference in the characteristics of each AD conversion unit, it is possible to construct a deep image.

  Note that, as described above, the calculation unit 125 performs Fourier transform on the data obtained by the first conversion unit 120 and the data obtained by the second conversion unit 121 to obtain respective tomographic images. It is not limited to the form in which the tomographic images are combined to form a tomographic image of the entire object 108. For example, the calculation unit 125 performs a Fourier transform on the data obtained by AD conversion by the first conversion unit 120 and the data obtained by AD conversion by the second conversion unit 121 to obtain a tomogram of the object 108. An image may be acquired.

  The above description is a process for irradiating a certain point of the subject 108 with light and obtaining a tomographic image in the depth direction at that point. This wavelength sweeping process for obtaining a tomographic image in the depth direction is called A scan. In order to obtain a two-dimensional tomographic image, the position where the light is irradiated is scanned by moving the scanning mirror 111. This process of scanning in the direction perpendicular to the depth direction is called B-scan. A process of scanning the scanning mirror in a direction perpendicular to both the A scan direction and the B scan direction is called C scan. By performing A, B, and C scans, a three-dimensional tomographic image of the subject can be obtained.

(AD converter)
In the first and second AD conversion units in the present embodiment, the AD conversion speed (sampling speed) of the first AD conversion unit is faster than the AD conversion speed of the second AD conversion unit, and the first AD conversion unit Is not limited as long as the resolution is lower than the number of bits of the second AD converter. Three or more AD conversion units may be provided. Note that each AD conversion unit may not be able to perform AD conversion of a DC component, that is, an interference signal in a frequency band including frequency 0.

  The OCT according to the present embodiment is also advantageous in that it is not necessary to adjust the difference in characteristics of the plurality of AD conversion units as compared to the high-speed AD conversion by the interleave operation using a plurality of AD conversion units. That is, in the interleaving operation, a plurality of AD converters having the same speed and the same resolution number are used, but adjustment for aligning the characteristics of the AD converters is complicated. The plurality of AD conversion units in the present embodiment need only satisfy the relationship between the conversion speed and the resolution as described above, and thus do not have to have the same characteristics.

(Frequency separation unit)
In the present embodiment, the frequency separation unit 126 separates the time waveform of the intensity of the interference light transmitted from the light detection unit 104, that is, the interference signal into a plurality of frequency components. In the above, the example in the case of having the frequency band 201 where the frequency components separated by the frequency separation unit overlap as shown in FIG. However, the first frequency band component (high frequency band component) 202 and the second frequency band component (low frequency band component) 203 overlap only at a certain frequency, and overlap at other frequencies. The structure which does not do may be sufficient. Moreover, the structure which isolate | separates into the 3 or more frequency band may be sufficient as the frequency separation part in this embodiment. In the following, a configuration that separates into two frequency bands will be described as an example, as described above. When separating into two frequency bands, for example, the interference signal can be separated into a high frequency component of 20 MHz or more and a low frequency component of 30 MHz or less.

  Next, the case where the frequency separation unit separates the interference signal so that the frequency components overlap each other will be described in more detail with reference to FIG. First, it is assumed that the tomographic image 204 generated from the low frequency component 203 of the interference signal and the tomographic image 205 generated from the high frequency component are shown in FIGS.

  The tomographic image 204 and the tomographic image 205 are obtained by converting the interference signal of each frequency region into data of equal wave number intervals and then Fourier transforming, and the tissue structure 211 and the like are imaged. In OCT, since the interference signal from a deeper part has a higher frequency, the frequency components on the horizontal axis of the OCT image shown in FIGS. 2B to 2D correspond to the distance in the depth direction of the OCT image. The vertical axis represents the position of the point on the observation target object from which the interference signal is acquired. In other words, a schematic diagram of an image obtained by arranging interference signals obtained at each point on the observation target object, converting the data at equal wave number intervals and performing Fourier transform, and arranging them vertically according to the irradiation position on the object is shown. Yes. That is, the signal acquisition position on the vertical axis in FIGS. 2B to 2D is a position where light is irradiated by the B-scan.

  The tomographic image 204 and the tomographic image 205 are integrated as a final image 206 having all image information from a shallow part to a deep part by image processing. In FIG. 2, the tomographic images 204 and 205 are connected at the connection frequency 207.

(Clock generator)
In the OCT according to the present embodiment, it is preferable to use the clock generation unit 102 so that the sampling of the interference signal from the interference optical system 103 is at an equal wave number interval. The clock generation unit 102 includes an optical system that transmits or reflects light at equal wavenumber intervals, and a signal transmission unit that detects the transmitted or reflected light with a photodetector or a differential photodetector and transmits a clock signal. Hereinafter, the clock generation unit 102 may be referred to as a k clock (k-clock) system.

  When a part of the light emitted from the wavelength swept light source 101 is guided to the k clock system 102, a clock signal (S1) is generated at a constant wave number interval peculiar to the k clock system as the wavelength of the light emitted from the wavelength swept light source 101 changes. Is done. It should be noted that the clock signal does not have to be strictly at the same wave number interval.

  By sampling the interference signal sent from the light detection unit 104 based on this clock signal, it is possible to acquire the interference signal at equal wave number intervals. Therefore, it is preferable to perform AD conversion in the first AD converter and the second AD converter based on the clock signal generated from the clock generator 102.

  In the OCT according to the present embodiment, the clock signal (S1) is divided into, for example, two hands, and one signal (S2) is directly input to the first AD conversion unit 120 that performs AD conversion at high speed. The other signal (S3) is input to the second AD conversion unit 121 that performs AD conversion at a low speed after the frequency is reduced through the frequency adjustment unit 124 having a function of thinning out the clock signal, for example.

  Further, the method of adjusting the frequency of the clock signal for input to the second AD converter 121 is not limited to the method of thinning out the high-frequency clock signal as described above. For example, it is possible to generate a new low-frequency clock signal by reducing the frequency of the clock signal itself using a circuit that converts the frequency of the clock signal, and to generate a low-frequency clock signal based on this. .

(Information acquisition unit)
The information acquisition unit 105 acquires information about the subject 108 based on the time waveform of the intensity of the interference light received by the light detection unit 104. Specifically, the information of the subject 108 is acquired by performing frequency analysis such as Fourier transform in the calculation unit of the information acquisition unit 105. Sampling timing in the time waveform of the intensity of the interference light is performed at equal frequency (equal wave number) intervals based on the clock signal transmitted from the previous clock generation unit 102.

  Here, some examples of processes in which the information acquisition unit 105 acquires a plurality of tomographic images for each frequency band of the interference signal and combines them will be described.

  Fourier transform the data obtained by the first AD conversion unit to obtain a first tomogram corresponding to the high frequency component of the interference signal, Fourier transform the data obtained by the second AD conversion unit, Consider a case where a second tomographic image corresponding to a low frequency component of an interference signal is acquired. Further, the frequency separation unit divides the interference signal (the time waveform of the intensity of the interference light) into each frequency component so as to have a frequency band where the first frequency band and the second frequency band overlap. Then, a connection frequency is determined within the overlapping frequency band, and two tomographic images are synthesized (connected) at the connection frequency.

  Here, since the first AD conversion unit and the second AD conversion unit have different gains, even the intensity of the same interference signal may be converted into different numerical values. This corresponds to the fact that, when a tomographic image is obtained, images with different luminance can be produced even at the same connection frequency.

  Therefore, it is preferable that the calculation unit synthesizes the luminance to match at the position where the first tomographic image and the second tomographic image are synthesized. Moreover, it is preferable to synthesize the first and second tomographic images so that the gradient of the luminance change is minimized at the position where the first tomographic image and the second tomographic image are synthesized. That is, when considering the luminance distribution of the tomographic image obtained by combining the first tomographic image and the second tomographic image, the derivative of the function of the luminance position is obtained at the position where the tomographic images in the overlapping frequency band are combined. It is preferable to combine both tomographic images so as to be the smallest.

  Hereinafter, the process of synthesizing tomographic images will be described in detail with reference to FIGS. FIG. 3 is a view of the object 108 of FIG. 1 as viewed from the light irradiation direction.

  In FIG. 3, when generating a tomographic image in the depth direction along a certain straight line 310 on the object 108, the connection frequency can be selected to a different value for each position on the straight line 310. This is because, for example, when the tomographic structure in the depth direction of the object is different for each point such as the point 302, the point 303, and the point 304 where the light is irradiated, the suitable connection frequency is different for each point.

  FIG. 2D shows an example of the tomographic structure 208 at the point 302, the tomographic structure 209 at the point 303, and the tomographic structure 210 at the point 304.

  For example, when the point 302 and the point 303 have a gradual change in luminance in the depth direction, that is, when the frequency of the interference signal is different, the connection frequency 207 is set to a different value for the point 302 and the point 303 within the frequency band 201. It is preferable to do. This corresponds to providing a different connection frequency 220 for each part instead of the same connection frequency 207 for each part as a connection frequency. Such an operation is suitable, for example, when the connection frequency 904 corresponding to the point 922 where the change in the signal strength of the frequency dependency 901 of the interference signal strength is gradual in FIG. Since the tomographic structure is different for each part in the living tissue, the shape of the frequency dependence 901 of the interference signal intensity obtained for each measurement part is different. For example, the interference signal at a certain measurement site is 910, while the interference signal at another measurement site is 911. In FIG. 9, points where the signal intensity is gentle are, for example, point 920 and point 921. As described above, since the frequency at which the intensity change of the interference signal is gradual and the frequency at which the intensity change is abrupt are different for each measurement target part, it is preferable to change the connection frequency for each measurement part as described above. The curve 901 indicating the frequency dependence of the interference signal intensity may be simply referred to as an interference signal here. In the present embodiment, setting the connection frequency 207 to a frequency at which the brightness of one bit in the tomographic image obtained for each frequency component is as equal as possible is more accurate as an image after connection. This is preferable because an image of luminance information can be generated. The vertical axis in FIG. 9 represents the signal intensity. However, when a tomographic image is generated based on the signal intensity, the vertical axis can be paraphrased as luminance. The same applies to FIGS. 10, 11 and 12 below.

  For example, in the case of a configuration in which the maximum luminance of the tomographic image of the low frequency component is AD-converted with 12 bits, the connection frequency at which the maximum luminance of the tomographic image of the high frequency component is 1/4 of the maximum luminance of the tomographic image of the low frequency component Set. This corresponds to processing with 10 bits that is ¼ of the AD conversion bit number of the interference signal of the high frequency component and 1/4 of AD conversion of the interference signal of the low frequency component.

  FIG. 9 is a graph 901 showing the relationship between the frequency of the interference signal and the signal intensity at a certain point on the subject 108 obtained by the OCT according to the present embodiment. That is, it can be said that the figure shows the relationship between the frequency (depth information) of the interference signal and the signal intensity (information about the intensity of reflection at that depth). With respect to the interference signal 901, a tomogram of a region 905 having a frequency lower than the connection frequency 904 is generated via the low-speed AD converter 121, and a tomogram of the high-frequency region 906 is generated via the high-speed AD converter 120. Shall. In FIG. 9, the maximum value 902 of the signal strength in the low frequency region 905 and the maximum value 903 of the signal strength in the high frequency region 906 are set.

  For example, it is assumed that the maximum value 902 is four times the maximum value 903, the low-speed AD conversion unit 121 has 12 bits, and the high-speed AD conversion unit 120 has 10 bits. This is preferable because the signal intensity per bit expressing the maximum value 902 and the signal intensity per bit expressing the maximum value 903 are aligned. Therefore, it is preferable to combine the first AD converter and the second AD converter so that the intensity widths of the interference signals for 1 bit match.

  In FIG. 2B, the position on the image connecting the tomographic image 204 obtained from the low frequency component of the interference signal and the tomographic image 205 obtained from the high frequency component, that is, the connection frequency 207 is the above-described overlapping frequency band 201. Any place inside is acceptable. It is preferable that the overlapping frequency bands 201 have a certain width because the degree of freedom in setting the connection frequency 207 increases. That is, it is preferable that the first frequency band component (high frequency component) and the second frequency band component (low frequency component) that are lower in frequency than the first frequency band overlap.

  For example, as the position on the image where a plurality of images are connected, that is, the connection frequency, it is preferable to select a connection frequency at which the luminance change of the image in the depth direction becomes gentle. For example, in FIG. 9, it is preferable to avoid the vicinity of the frequency 907 where the intensity change of the interference signal 901 is abrupt and to set the connection frequency 904 where the change of the signal intensity is gentle. Furthermore, when connecting (synthesizing) tomographic images by setting the connection frequency within the overlapping frequency band 201, the brightness of both tomographic images is corrected by image processing so that the brightness of both tomographic images at the connection frequency is uniform. It is also preferable to do. Details will be described with reference to FIG. 10 showing an example of the relationship between the frequency of the interference signal and the signal intensity.

  For example, consider a situation in which an interference signal 1001 in a low frequency region 1007 and an interference signal 1002 in a high frequency region 1008 are connected to each other at a connection frequency 1009 as shown in FIG. The signal may be corrected so that the signal intensity 1005 and the signal intensity 1006 of the points 1003 and 1004 on the respective interference signals corresponding to the connection frequency 1009 coincide with each other or become as close as possible. In other words, before connection processing, one or both of the interference signal 1001 and the interference signal 1002 can be corrected by multiplying the signal strength by a certain factor.

  It is preferable that a tomographic image with less sense of incongruity can be generated by smoothly connecting the signal intensity of the interference signal at a position where the images are connected to each other.

(Light source)
In the present embodiment, the light source unit is not particularly limited as long as it is a light source that changes the wavelength of light. In order to obtain object information using the OCT apparatus, it is necessary to continuously change the wavelength of light emitted from the light source unit.

  As the light source unit in the present embodiment, for example, a surface emitting laser, an external resonator type wavelength swept light source using a diffraction grating or a prism, and various external resonator type light sources using a Fabry-Perot tunable filter with a variable resonator length are used. Can be. Or SSG-DBR which changes a wavelength using a sampled grating, MEMS-VCSEL of wavelength variable, etc. can also be used. A fiber laser can also be used. The fiber laser may be a dispersion tuning method or a Fourier domain mode lock method.

  As an external resonator type wavelength sweep light source using a diffraction grating, a prism, etc., a resonator is provided with a diffraction grating to disperse light, and a polygon mirror or a striped reflection mirror is provided on a rotating disk. A wavelength swept light source or the like can be used by continuously changing the wavelength of the emitted light using.

(Light detector)
In the light detection part in this embodiment, if the intensity | strength of interference light is converted into the intensity | strength of electricity, such as a voltage, it will not specifically limit. Information on the time waveform of the intensity of the interference light is converted into information on the time waveform of the received light voltage by this light detection unit.

(object)
In the present embodiment, the object is a measurement target by the OCT apparatus according to the present embodiment, and the type is not particularly limited. Examples include living bodies such as eyeballs, skin, blood vessels, and teeth.

(Display section)
In the OCT according to the present embodiment, the object information acquired by the information acquisition unit is a tomographic image, and may include a display unit that displays the acquired tomographic image.

(Use)
The OCT according to the present embodiment can be used for medical diagnosis such as ophthalmic imaging, skin imaging, angiography, and dental imaging for obtaining a tomographic image of the fundus.

(Example 1)
Hereinafter, OCT according to an embodiment of the present invention will be described with reference to FIG. 4, but the present invention is not limited to this.

  In this embodiment, it is assumed that the object (subject) to be observed is the fundus. The OCT according to the present embodiment includes a light source system 401, a reference light optical path fiber 402 that forms the optical path of the reference light, a fiber coupler 403 that forms an interference unit, and a reflection mirror 404. The OCT according to the present embodiment includes an inspection light optical path fiber 405, an irradiation condensing optical system 406, and an irradiation position scanning mirror 407 as a scanning mirror, which constitute a measurement unit of the subject. The interference optical system 400 indicates a portion surrounded by a dotted line in FIG. 4A, and generates interference light using light emitted from the wavelength swept light source 415.

  In addition to these, the OCT according to the present embodiment includes a light receiving fiber 408, a photodetector 409 as a light detection unit, an irradiation fiber 410, a signal processing unit 411, and an image output monitor 413 as a display unit.

  In the OCT according to the present embodiment, the signal processing unit 411 includes a frequency separation unit 440 and an information acquisition unit 450.

  The frequency separation unit 440 transmits the component of the high frequency band (first frequency band) of the interference signal through the High-Pass filter 431 and the component of the low frequency band (second frequency band) through the Low-Pass filter 434. Permeate. The information acquisition unit 450 includes a high-speed AD conversion element 435 as a first AD conversion unit and a low-speed AD conversion element 436 as a second AD conversion unit. The resolution of the high speed AD conversion element 435 is lower than that of the low speed AD conversion element 436. The high-speed AD conversion element 435 AD-converts the high-frequency component of the interference signal that has passed through the high-pass filter 431, and the low-speed AD conversion element 436 AD-converts the low-frequency component of the interference signal that has passed through the low-pass filter 434. .

  The information acquisition unit 450 includes the Fourier transformer 432 and the calculation unit 433. However, instead of providing the Fourier transformer, the calculation unit 433 may have a function of performing Fourier transform.

  Next, details of the configuration of the light source system 401 will be described with reference to FIG. FIG. 4B is a schematic diagram showing in detail the configuration of the light source system 401 in FIG.

  The light source system 401 includes a wavelength swept light source 415 as a light source unit and a k clock system 460 as a clock generation unit. The k clock system 460 includes an interferometer 417, a light source light amount monitor unit 418, and a k trigger generation circuit 419. The interferometer 417 in the k clock system 460 transmits the light emitted from the wavelength swept light source 415 at equal wave number intervals, and the k trigger generation circuit 419 generates a clock signal at the timing when the light is incident. Hereinafter, the generated clock signal may be referred to as a k trigger signal.

  One of the generated clock signals is sent to the circuit 430 that thins out the k trigger signal, and the other is sent to the high-speed AD conversion element 435. Based on the k trigger signal from the k trigger generation circuit 419, the information acquisition unit 450 samples the interference signal. That is, at the timing when the k trigger signal is received, the interference signal is AD converted by each AD conversion element. Reference numeral 414 denotes a subject as an object. Reference numerals 420 and 421 denote collimators for coupling light into the fiber and collimating light from the fiber. The fiber constituting the interference optical system 400 is a single mode fiber in this embodiment, but the configuration of the present invention is not limited to this configuration.

  In this embodiment, a system that introduces light from a Michelson interferometer in which the optical path length difference between both arms of the interferometer 417 is 8 mm to the differential photodetector is used as the k clock system 460. The interference signal output from the interferometer 417 generates a k-trigger signal for each light frequency of 18.737 GHz. The wavelength sweep light source 415 has a wavelength sweep frequency of 50 kHz (with a wavelength sweep period of 20 us), and a wavelength sweep range of 1000 nm to 1090 nm. Therefore, the number of generations of the k trigger signal per wavelength sweep of the wavelength sweep light source 415 is 1321 times. The sweep speed of the wavelength swept light source 415 is controlled to be constant, and the light emission frequency 501 of the light source changes with time as shown in FIG. At this time, the generation frequency of the k trigger signal is 66 MHz.

  A control signal is input from the light source control unit 412 to the wavelength swept light source 415.

  The wavelength swept light source 415 is controlled by the light source control unit 412 in terms of its oscillation wavelength and intensity and its change over time.

  The light emitted from the wavelength swept light source 415 is demultiplexed into the reference light optical path fiber 402 and the inspection light optical path fiber 405 by the fiber coupler.

  Further, a collimator 421 is attached to the tip of the reference light path optical fiber 402 so that parallel light is irradiated onto the reflection mirror 404. Then, the light is reflected by the reflection mirror 404 and introduced into the light receiving fiber to reach the photodetector 409.

  At the same time, the light introduced into the inspection light path optical fiber 405 by the fiber coupler 403 is irradiated to the subject 414, and backscattered light is generated from the inside and the surface of the test object. The backscattered light is condensed from the fiber coupler to the photodetector 409 through the irradiation condensing optical system 406.

  The light received by the photodetector 409 is converted into a spectrum signal by the signal processing unit 411, and further, the tomographic information of the subject 414 is obtained by Fourier transforming the converted signal.

  A method for acquiring a tomographic image using OCT in the present embodiment will be described with reference to FIGS. FIG. 6 is a diagram for explaining a tomographic image acquisition method according to the present embodiment. The horizontal axis of each graph in FIG. 6 is the wave number of light, which can be obtained by multiplying the frequency of light by 2π divided by the speed of light. FIG. 6A is a graph showing an example of the k trigger signal, and FIG. 6B is a part of the point (zero cross point) that intersects the horizontal axis in the k trigger signal of FIG. 6A. FIG.

  First, the point 302 on the subject 310 is irradiated with light. At this time, the k-clock system 417 generates the k trigger signal 601. Next, the k trigger generation circuit 419 generates the k trigger signal at the zero-cross point of the rising slope of the k trigger signal, for example, the point 602, the point 603, the point 604, the point 605. This is a high-speed k trigger signal for processing a high-frequency signal component, and the generation rate of the k trigger signal is 66 MHz.

  Further, the k-trigger signal is thinned in half by the circuit 430, thereby generating the low-speed k-trigger signal points 606, 607, and so on. The generation rate of the low-speed k trigger signal is 33 MHz.

  An interference signal 610 obtained by extracting a high-frequency component from the signal obtained by the photodetector 409 by the high-pass filter 431 is acquired based on k trigger points 602, 603, 604, 605... (FIG. 6C). )). Points obtained on the interference signal 610 are point 611, point 612, point 613, point 614, and so on.

  Further, the interference signal 620 from which the low-frequency component is extracted by the low-pass filter 434 is acquired based on the points 606, 607,... Of the low-speed k trigger signal (FIG. 6D). The points on the obtained interference signal 620 are point 621, point 622, and so on.

  Acquiring an interference signal based on a k-trigger signal with a generation rate of 66 MHz corresponds to acquiring the interference signal at intervals of optical frequency every 18.737 GHz.

  The high-speed AD conversion element 435 has 8 bits and a speed of 100 MHz, and the low-speed AD conversion element 436 has 10 bits and a speed of 50 MHz.

  Here, the transmission characteristics of the interference signal of the high-pass filter 431 and the low-pass filter 434 will be described in detail with reference to FIG. The signal transmittance on the vertical axis in FIG. 7 represents the transmittance with respect to the interference signal.

  In this embodiment, the High-Pass filter 431 transmits a signal of 20 MHz or higher (702 in FIG. 7). On the other hand, the Low-Pass filter transmits DC, that is, a signal from 0 MHz to 30 MHz (703 in FIG. 7). Of the frequency components of the signal transmitted by both filters, the overlapping frequency band is 20 MHz to 30 MHz as indicated by 701. Therefore, the connection frequency 704 is set to 25 MHz. In FIG. 7, 702 has a higher signal transmittance than 703, but this is an example, and may be reversed, or both may have the same signal transmittance.

  Next, details of a method for obtaining a tomographic image using the OCT according to the present embodiment will be described with reference to FIGS. FIG. 11 shows an example of an interference signal obtained at a certain point of the subject when the OCT according to the present embodiment is used.

  The high-frequency interference signal 610 and the low-frequency interference signal 620 are each Fourier-transformed to obtain tomographic information including the frequency component 1101 of the interference signal in the low-frequency region 1107 and the frequency component 1102 in the high-frequency region 1108 (FIG. 11).

  Next, both images of the frequency component 1101 and the frequency component 1102 are connected to the tomographic information at the frequency 1109 (25 MHz) in FIG. Here, the value of the signal intensity 1103 of the tomographic information in both frequency bands at the connection frequency is 120.0, and the value of 1104 is 50.0. The final frequency component, which is a tomographic image, is obtained by multiplying the signal strength of the frequency component 1102 by 2.4 so as to match both signal strengths and connecting the frequency component 1110 to the frequency component 1101 in the low frequency region. Signal 1111 is obtained.

  Next, a case where the irradiation position with respect to the object is displaced by 10 μm on the straight line 310 and measured at the point 303 will be described with reference to FIGS. FIG. 12 is an example of an interference signal obtained at a certain point (point 303) of the subject when the OCT according to the present embodiment is used.

  The interference signal is acquired in the same manner as described above, and the fault information at this point is acquired in the same manner. Note that the frequency component in the low frequency region obtained at the point 303 is 1201, the frequency component in the high frequency region is 1202, and the frequency components in the point 304 are 1203 and 1204, respectively (FIG. 12). The connection frequency 1210 of the frequency components 1201 and 1202 is 24 MHz, and the connection frequency 1211 of the frequency components 1203 and 1204 is 27 MHz. The final frequency component signal obtained at point 303 is 1220 and at point 304 is 1221. The signal components of the frequency component 1202 and the frequency component 1204 are changed at the connection frequency so that the intensity at the connection frequency matches the frequency component 1201 and the frequency component 1203, respectively.

  Through a series of these operations, tomographic information 1220, 1221,... At each point on the straight line 310 can be obtained, and a tomographic image on a specific plane can be obtained.

(Example 2)
In the first embodiment, the configuration shown in FIG. 4 is used as the interference measurement system. However, the configuration is not limited to such a configuration. For example, as shown in FIG. 8, an interference signal is acquired using a differential detector. You may comprise.

  The OCT according to this embodiment includes a light source system 801 having a light source unit, an isolator 802, a reference light path optical fiber 806 constituting a reference unit, a polarization controller 818, a fiber coupler 805 constituting an interference unit, and a reflection mirror 807. Have. The OCT according to this embodiment includes a collimator 820 and a collimator 821. As in FIG. 4, the light source unit 801 has a k clock system and an interferometer (not shown).

  Furthermore, the OCT according to the present embodiment includes an inspection light optical path fiber 814, a polarization controller 819, an irradiation condensing optical system 815, and an irradiation position scanning mirror 808 that constitute an analyte measurement unit.

  In addition, the OCT according to the present embodiment includes a fiber coupler 803, a fiber coupler 804, a light receiving fiber 816, a light receiving fiber 817, and a balance photodetector 810 as a light detection unit. Furthermore, it has a signal processing unit 811 constituting an image processing unit and an image output monitor 813 as a display unit. The configuration of the signal processing unit 811 is the same as that of 411 in FIG. A circuit 822 for thinning out the k trigger signal is disposed in front of the signal processing unit 811.

  The OCT according to the present embodiment includes a light source control unit 812 that controls the light source unit in the light source system 801. Note that reference numeral 809 denotes a subject.

  In the OCT according to the present embodiment, a high-frequency component interference signal is processed by an AD conversion element that is high speed and small bits, and a low-frequency component interference signal is processed by an AD conversion element that is low speed and high bits. This eliminates the need for a high-speed and large-bit AD converter.

101 Light source (wavelength swept light source)
DESCRIPTION OF SYMBOLS 103 Interference optical system 104 Photodetection part 105 Information acquisition part 120 1st AD conversion part 121 2nd AD conversion part 125 Operation part 126 Frequency separation part

Claims (12)

A light source unit that changes the wavelength of the emitted light;
An interference optical system that divides the light from the light source unit into irradiation light and reference light for irradiating the object, and generates reflected light of the light irradiated to the object and interference light by the reference light;
A light detector that receives the interference light;
Based on the time waveform of the intensity of the interference light, an information acquisition unit that acquires information of the object;
An optical coherence tomograph having
The information acquisition unit separates the time waveform of the intensity of the interference light into a plurality of different frequency components, and AD-converts the component of the first frequency band among the frequency components separated by the frequency separation unit And a second AD converter that AD converts a component of the second frequency band having a frequency lower than that of the first frequency band,
A calculation unit that obtains information on the object from the data obtained by the first AD conversion unit and the data obtained by the second AD conversion unit;
The conversion speed of the first AD converter is faster than the conversion speed of the second AD converter,
The resolution of the first AD converter is lower than the resolution of the second AD converter;
Optical coherence tomography characterized by. The computing unit is
Fourier transform the data obtained in the first AD conversion unit to obtain a first tomographic image, Fourier transform the data obtained in the second AD conversion unit to obtain a second tomographic image, The optical coherence tomography device according to claim 1, wherein the acquired tomographic image of the object is acquired by synthesizing the acquired first tomographic image and the second tomographic image.   3. The optical interference according to claim 1, wherein the frequency separation unit separates the time waveform of the intensity of the interference light so as to have a frequency band in which the first frequency band and the second frequency band overlap. Tomometer.   The said calculating part synthesize | combines said 1st tomographic image and said 2nd tomographic image based on the luminance information of said 1st tomographic image and said 2nd tomographic image. The optical coherence tomography according to any one of the above.   The optical coherence tomograph according to claim 4, wherein the arithmetic unit synthesizes the first tomographic image and the second tomographic image so that the luminances coincide at a position where the first tomographic image and the second tomographic image are synthesized.   The optical coherence tomograph according to claim 4, wherein the calculation unit combines the first tomographic image and the second tomographic image so that the gradient of luminance change is minimized.   The optical coherence tomography further includes a clock generation unit that generates a clock signal at equal wavenumber intervals in accordance with a change in wavelength of light emitted from the light source unit, and the first AD conversion based on the clock signal The optical coherence tomography according to claim 1, wherein AD conversion is performed in the first and second AD conversion units.   The said calculating part is synthesize | combined so that the intensity width of the interference signal for 1 bit may correspond with said 1st AD conversion part and said 2nd AD conversion part. Optical coherence tomography as described.   The optical coherence tomography device according to any one of claims 1 to 8, wherein the object is a living body.   The optical coherence tomography device according to claim 9, wherein the living body is an eyeball.   The optical coherence tomometer according to claim 9, wherein the living body is a blood vessel.   The optical coherence tomometer according to claim 1, wherein the light source unit is a surface emitting laser.