JP2016209199A - Imaging apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a configuration of a clock generation unit suitable for comprehensively obtaining tomographic images in a desired depth range of a fundus oculus when a wide range of the fundus oculus is scanned in one pass.SOLUTION: An SS-OCT apparatus is configured as an interferometer in which a light path through which a portion of the light emitted from a light source unit pass is branched into a first light path and a second light path with the length different from the first light path. The apparatus comprises: a clock generation unit for generating a clock on which a conversion unit samples an analog signal; and a tomographic image acquisition unit for capturing a tomographic image of a fundus oculus on the basis of a digital signal which is converted from the analog signal by the conversion unit on the generated clock. The clock generation unit includes a correction unit for correcting the intensity of analog signals of the second interference light, which interference light is obtained by the interference between the light in the first light path and the light in the second light path, such that the difference in the intensity of the second interference light falls within a range which allows the analog signals of the second interference light to enter the conversion unit.SELECTED DRAWING: Figure 6

Description

  The present invention relates to an imaging apparatus that captures an optical coherence tomographic image.

  An imaging apparatus (hereinafter referred to as an OCT apparatus) using an optical coherence tomography (hereinafter referred to as OCT) has been developed (Patent Document 1). The OCT apparatus irradiates light on an object, changes the wavelength of the irradiated light, and causes interference between reference light and reflected light returning from different depths of the object. A tomographic image of the object can be obtained by analyzing the frequency component included in the time waveform of the intensity of the interference light (hereinafter abbreviated as interference spectrum). The OCT apparatus is used, for example, for fundus examination.

  Since many eye diseases are difficult to cure, it is important to detect lesions in the fundus early and to start treatment that delays the progression of the lesion to a wide area of the fundus. In particular, if the lesioned part progresses to the macula, it has a great effect on vision. Therefore, there is a demand for finding the lesioned part even if the lesioned part is sufficiently away from the macula. In order to meet this requirement, a wide angle of view of an OCT apparatus used for fundus examination is expected.

  Patent Document 1 discloses that a wide range of tomographic images are formed by connecting a plurality of tomographic images in order to widen the observation area of the fundus tomographic image. Patent Document 1 discloses an OCT apparatus (Swept Source OCT apparatus, hereinafter referred to as an SS-OCT apparatus) using a wavelength swept light source. Patent Document 1 exemplifies a fiber ring resonator and a wavelength selection filter as the wavelength swept light source.

JP 2012-115578 A

  However, in the method of Patent Document 1, it takes time and effort to perform image processing for continuously joining a plurality of acquired tomographic images. Therefore, it is preferable to acquire tomographic information over a wide range by one imaging. In this case, since the eyeball is a substantially spherical body, the optical path length of the irradiation light is greatly different between the central portion and the peripheral portion of the fundus. For this reason, with the configuration of the conventional OCT apparatus, it has been difficult to comprehensively acquire a tomographic image of a desired depth range of the fundus when one scan is performed over a wide range of the fundus.

  Here, in the SS-OCT apparatus, an optical path through which a part of the light emitted from the wavelength swept light source passes is branched into a first optical path and a second optical path having an optical path length difference with respect to the first optical path. It is known to use a clock generation unit that is configured as an interferometer and that generates a clock with which an A / D converter samples an analog signal.

  One of the objects of the present invention with respect to the above-described problem is that it is suitable for comprehensively obtaining a tomographic image of a desired depth range of the fundus when a single scan is performed over a wide range of the fundus. A configuration of a clock generation unit is provided.

One of the imaging ranges according to the present invention is:
The light emitted from the light source unit that sweeps the wavelength of the emitted light is branched into irradiation light that irradiates the fundus and reference light, and the reflected light from the fundus that is irradiated with the irradiation light interferes with the reference light An interference part to
A detection unit for detecting first interference light obtained by interference by the interference unit;
A conversion unit that converts an analog signal obtained by detecting the interference light by the detection unit into a digital signal;
An optical path through which a part of the light emitted from the light source unit passes is configured as an interferometer branched into a first optical path and a second optical path having an optical path length difference with respect to the first optical path, A clock generation unit for generating a clock for the conversion unit to sample the analog signal;
An imaging device comprising: a tomographic image acquisition unit that acquires a tomographic image of the fundus based on the digital signal obtained by the conversion unit converting the analog signal by the generated clock;
The clock generation unit is configured such that the difference in intensity of the analog signal of the second interference light obtained by interfering the light of the first optical path and the light of the second optical path is the analog signal of the second interference light. A correction unit that corrects the intensity of the analog signal of the second interference light so as to be within a range that can be input to the conversion unit is included.

  According to the present invention, it is possible to provide a suitable clock generator configuration for comprehensively acquiring a tomographic image of a desired depth range of the fundus when a single scan is performed over a wide range of the fundus. it can.

The schematic diagram which shows an example of the OCT apparatus which concerns on this invention The schematic diagram which shows an example of the scanning method of the irradiation light which the scanning part of the OCT apparatus which concerns on this invention performs Schematic diagram of the eyeball Diagram for explaining the problem of wide angle of view The figure for demonstrating the optical frequency change and k clock of a wavelength variable light source Schematic diagram of k clock generator Illustration for explaining the sampling theorem Schematic diagram of K clock generator using double-pass interferometer Schematic diagram of automatic gain control Schematic diagram of automatic gain control with excess gain suppression mechanism

  The imaging apparatus according to the present embodiment is an SS-OCT apparatus, and a detection unit (for example, a light receiving element) that detects first interference light obtained by interference by an interference unit (for example, an OCT interference unit 20 described later). ). In addition, the imaging apparatus according to the present embodiment includes a conversion unit (for example, an A / D converter 32 described later) that converts an analog signal obtained by the detection unit detecting the first interference light into a digital signal. In the imaging apparatus according to the present embodiment, an optical path through which a part of the light emitted from the light source unit passes is branched into a first optical path and a second optical path having a difference in optical path length with respect to the first optical path. The conversion unit includes a clock generation unit that generates a clock for sampling an analog signal. Here, the light source unit according to the present embodiment is an SS-OCT light source that sweeps the wavelength of emitted light, and is also referred to as a wavelength swept light source or the like. The clock generation unit according to the present embodiment is, for example, a k clock generation unit 80 described later. In addition, the imaging apparatus according to the present embodiment includes a tomographic image acquisition unit that acquires a tomographic image of the fundus oculi based on a digital signal obtained by the conversion unit converting an analog signal using the generated clock.

  In addition, the clock generation unit according to the present embodiment is configured such that the difference in the intensity of the analog signal of the second interference light obtained by interfering the light of the first optical path and the light of the second optical path is the analog of the second interference light. A correction unit that corrects the intensity of the analog signal of the second interference light so as to be within a range in which the signal can be input to the conversion unit is included. The correction unit is, for example, a correction circuit 84 described later. As a result, even if the difference in the intensity of the analog signal for each wavelength emitted from the light source section is larger than the input range in the A / D converter, the intensity of the analog signal is corrected to be within this range. can do. Thus, when a single scan is performed over a wide range of the fundus, a tomographic image of a desired depth range of the fundus can be comprehensively acquired.

  Here, when the optical path length difference of the interferometer of the clock generation unit is larger than the conventional one (for example, 22 mm or more in the air as will be described later), the light of the wavelength corresponding to the deep position of the second interference light. The strength is greatly attenuated. This is because, in the interferometer of the clock generation unit, the interference distance becomes much larger than the coherence length of the light source unit, so that the influence of attenuation is increased by roll-off. For this reason, when the optical path length difference of the interferometer of the clock generation unit is larger than the conventional one, the difference in intensity of the second interference light becomes large. In such a case, the clock generation unit according to this embodiment is used. Is desirable.

  In addition, it is ideal that the wavelength swept light source as the light source unit according to the present embodiment is configured so that light of each wavelength is emitted while changing linearly with time. However, the wavelength swept light source does not actually change exactly linearly but changes nonlinearly in general, and mode hop (a phenomenon in which the wavelength changes discontinuously at a certain timing) or the like also occurs. End up. That is, it is difficult for the wavelength swept light source to accurately sweep the wavelength as set. Therefore, the above-described clock generation unit is used to adjust the timing at which the conversion unit converts the analog signal into the digital signal. At this time, it is preferable that the clock generation unit generates the clock so that the conversion unit samples the analog signal at intervals of substantially equal wave numbers. Thereby, it is possible to easily convert from the wave number space to the real space without performing interpolation or the like. However, the present invention is not limited to this, and it is not necessary to sample at substantially equal wave number intervals. In this case, the wave number space can be converted to the real space by performing interpolation or the like.

  Moreover, in this embodiment, the light source part 10 will not be specifically limited if it is a light source which 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 20 in the present embodiment, for example, an external resonator type wavelength swept light source using a diffraction grating or a prism, or various external resonator type light sources using a Fabry-Perot tunable filter with a variable resonator length may be used. it can. Or SSG-DBR which changes a wavelength using a sampled grating, VCSEL (MEMS-VCSEL) of variable wavelength using a MEMS mechanism, 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 swept light source using a diffraction grating, a prism, etc., a diffraction grating is provided in the resonator to disperse the light, and a polygonal mirror or a striped reflection mirror is provided on a rotating disk. For example, a wavelength swept light source that continuously changes the wavelength of light emitted by using a light source. A VCSEL generally has a lower reflecting mirror, an active layer, and an upper reflecting mirror in this order, and includes an air gap between the active layer and the upper reflecting mirror, and the upper reflecting mirror and the lower reflecting mirror. It is configured as a surface emitting laser that changes the wavelength of emitted light by changing the position in the optical axis direction of at least one of the mirror.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. The following embodiments do not limit the present invention related to the scope of claims, and all combinations of features described in the present embodiments are essential for the solution means of the present invention. Is not limited. For example, the OCT apparatus according to the present embodiment is configured with a Mach-Zender interferometer, but the present invention is not limited thereto, and may be configured with a Michelson interferometer. The OCT apparatus according to the present embodiment is configured to change the reference optical path length, but the present invention is not limited to this, and is configured to change the optical path length difference between the reference light and the measurement light. Just do it. For example, the measurement optical path length may be changed by fixing the reference optical path length.

  Here, in the present embodiment, the light source unit 10 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 20 in the present embodiment, for example, an external resonator type wavelength swept light source using a diffraction grating or a prism, or various external resonator type light sources using a Fabry-Perot tunable filter with a variable resonator length may be used. it can. Or SSG-DBR which changes a wavelength using a sampled grating, VCSEL (MEMS-VCSEL) of variable wavelength using a MEMS mechanism, 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 swept light source using a diffraction grating, a prism, etc., a diffraction grating is provided in the resonator to disperse the light, and a polygonal mirror or a striped reflection mirror is provided on a rotating disk. For example, a wavelength swept light source may be used by continuously changing the wavelength of light emitted from the light source. A VCSEL generally has a lower reflecting mirror, an active layer, and an upper reflecting mirror in this order, and includes an air gap between the active layer and the upper reflecting mirror, and the upper reflecting mirror and the lower reflecting mirror. It is configured as a surface emitting laser that changes the wavelength of emitted light by changing the position in the optical axis direction of at least one of the mirror.

(Configuration of SS-OCT system)
FIG. 1 is a diagram illustrating a configuration example of an imaging apparatus (OCT apparatus) using an optical coherence tomography method according to an embodiment of the present invention. The OCT apparatus includes a light source unit 10 in which an emitted optical frequency is swept, an OCT interference unit 20 that generates interference light, a detection unit 30 that detects interference light, and the fundus of the subject 100 based on the interference light. And an information acquisition unit 40 for acquiring the information. The information acquisition unit 40 also functions as a tomographic image acquisition unit (image generation unit) that acquires (generates) a tomographic image of the fundus. Further, the OCT apparatus has a measurement arm 50 and a reference arm 60.

  The OCT interference unit 20 includes couplers 21 and 22. First, the coupler 21 branches the irradiation light that irradiates the fundus with the light emitted from the light source unit 10 and the reference light. The irradiation light is irradiated to the subject 100 via the measurement arm 50. More specifically, the irradiation light incident on the measurement arm 50 is emitted as spatial light from the collimator 52 after the polarization state is adjusted by the polarization controller 51. Thereafter, the irradiation light is applied to the fundus of the subject 100 via the X-axis scanner 53, the Y-axis scanner 54, and the focus lens 55. Note that the X-axis scanner 53 and the Y-axis scanner 54 are scanning units having a function of scanning the fundus with irradiation light. The irradiation position of the irradiation light to the fundus is changed by the scanning unit. Then, backscattered light (reflected light) from the fundus is emitted from the measurement arm 50 via the focus lens 55, the Y-axis scanner 54, the X-axis scanner 53, the collimator 52, and the polarization controller 51 again. Then, the light enters the coupler 22 via the coupler 21.

  On the other hand, the reference light enters the coupler 22 via the reference arm 60. More specifically, the reference light incident on the reference arm 60 is adjusted in the polarization state by the polarization controller 61 and then emitted from the collimator 62 as spatial light. Thereafter, the reference light passes through the dispersion compensation glass 63, the optical path length adjustment optical system 64, and the dispersion adjustment prism pair 65, enters the optical fiber through the collimator lens 66, exits from the reference arm 60, and enters the coupler 22.

  The reflected light of the subject 100 that has passed through the measurement arm 50 at the coupler 22 interferes with the light that has passed through the reference arm 60. Then, the interference light is detected by the detection unit 30. The detection unit 30 includes a differential detector 31 and an A / D converter 32. First, in the detection unit 30, the interference light that has been demultiplexed immediately after the interference light is generated by the coupler 22 is detected by the differential detector 31. The OCT interference signal converted into an electric signal by the differential detector 31 is converted into a digital signal by the A / D converter 32. Then, the digital signal is sent to the information acquisition unit 40, and frequency information such as Fourier transform is performed on the digital signal, whereby fundus information is obtained. The obtained fundus information is displayed as a tomographic image by the display unit 70.

  Here, the imaging apparatus according to the present embodiment preferably further includes an analysis unit that segments a plurality of layers by analyzing the acquired tomographic image. For example, the information acquisition unit 40 functions as the analysis unit. Also good. At this time, it is preferable to further include an image generation unit that generates a planar image along any one of the plurality of layers based on the analysis result of the analysis unit. For example, the information acquisition unit 40 functions as an image generation unit. You may let them. Then, the planar image and the tomographic image are displayed on the display unit 70 in a state in which the positional relationship between the macular and optic disc of the fundus included in the planar image and the macular and optic disc of the fundus included in the tomographic image are associated with each other. It is preferable to further include a control unit. For example, the information acquisition unit 40 may function as a display control unit. As a result, a planar image along any one of the plurality of layers can be observed within a wide field angle range, so that diagnostic efficiency and diagnostic accuracy can be improved. It is also preferable to further include a calculation unit that calculates fundus curvature information including the macular and optic disc of the fundus using the tomogram of the macular and optic disc of the fundus. For example, the information acquisition unit 40 functions as the calculation unit. You may let them. Thereby, the curvature of the fundus in the range of a wide angle of view can be quantitatively evaluated, so that diagnostic efficiency and diagnostic accuracy can be improved. In addition, when acquiring a tomographic image including the macular and optic disc of the fundus, the scanning unit may be controlled so that the irradiation light is applied to the macula and the optic disc in one scan. Further, after acquiring a three-dimensional tomographic image of the fundus, a tomographic image including the macula and the optic nerve head may be reconstructed from the three-dimensional tomographic image.

  In the OCT apparatus of FIG. 1, the sampling timing of the interference light is performed at equal optical frequency (equal wave number) intervals based on a k clock signal transmitted from a k clock generation unit 80 provided outside the light source unit 10. In addition, a coupler 90 is provided to branch a part of the light emitted from the light source unit 10 to the k clock generation unit 80. Note that the k clock generation unit 80 and the coupler 90 may be incorporated in the light source unit 10.

  The above is a process for acquiring information about a tomography at a certain point of the subject 100. Obtaining information about a tomography in the depth direction of the subject 100 in this way is called A-scan. Further, the information about the tomography of the subject in the direction orthogonal to the A-scan, that is, the scanning direction for acquiring a two-dimensional image is orthogonal to the scanning direction of B-scan, and further, the A-scan and B-scan. Scanning in the direction is called C-scan. This is because, when acquiring a three-dimensional tomographic image, when performing two-dimensional raster scanning within the fundus, the high-speed scanning direction is B-scan, and the low-speed scanning direction in which B-scan is arranged in the orthogonal direction is C- Call it scan. A two-dimensional tomographic image can be obtained by performing A-scan and B-scan, and a three-dimensional tomographic image can be obtained by performing A-scan, B-scan and C-scan. B-scan and C-scan are performed by the X-axis scanner 53 and the Y-axis scanner 54 described above.

  The X-axis scanner 53 and the Y-axis scanner 54 are configured by deflection mirrors that are arranged so that their rotation axes are orthogonal to each other. The X-axis scanner 53 performs scanning in the X-axis direction, and the Y-axis scanner 54 performs scanning in the Y-axis direction. The X-axis direction and the Y-axis direction are directions perpendicular to the eyeball direction of the eyeball and perpendicular to each other. Further, the line scanning direction such as B-scan and C-scan may not coincide with the X-axis direction or the Y-axis direction. For this reason, the B-scan and C-scan line scanning directions can be appropriately determined according to a two-dimensional tomographic image or a three-dimensional tomographic image to be imaged.

  Further, various scanning is possible by driving both the X-axis scanner 53 and the Y-axis scanner 54 and changing the angle of the deflection mirror. For example, raster scanning as shown in FIGS. 2A and 2B may be used, or a method of passing a point of the eyeball (for example, the macula) a plurality of times as shown in FIG. In addition, as shown in FIG. 2D, scanning may be performed in a spiral manner around one point (for example, macular) of the eyeball.

(Scanning angle)
By the way, in the fundus examination, there is a demand for imaging the macula and the optic disc by the same scanning. A range (scanning angle) of scanning with irradiation light of the OCT apparatus required to realize this will be described with reference to FIG. FIG. 3 is a schematic diagram when the eyeball is assumed to be a sphere. There is a macula on the opposite side of the pupil center of the eyeball. In addition, the optic disc is located slightly away from the macula. This macular and optic nerve head are particularly important sites in the fundus.

  In a standard adult fundus, the distance D encompassing the macula and the optic disc is about 5.75 mm. The irradiation light is swung around the pupil center of the eyeball and scans the fundus. When imaging the range including the optic disc with the same scan centering on the macula, taking into account individual differences, the length (imaging range) L of the shortest curve connecting the macula and the optic disc needs to be about 14 mm. Here, let α be the deflection angle of the measurement light swiveled around the center of the pupil corresponding to this imaging range. Since the average diameter of an adult eyeball is about 24 mm, the deflection angle α needs to be 33.4 degrees or more in order to make the imaging range L 14 mm or more. This angle is expressed by arcsin (1.38 × sin (33) using the fact that the average refractive index in the eyeball is 1.38, and expressed by the deflection angle β of irradiation light incident on the pupil center in the air. .4 degrees / 2)) × 2≈47 degrees. In other words, in order to image the macula and the optic disc simultaneously, focusing on the macula, when the fundus is scanned linearly with irradiation light, the angle range for scanning the fundus should be 47 degrees or more in terms of air. That's fine. In the following description, a field angle is a range obtained by converting an angle range for scanning the fundus when the fundus is scanned linearly with irradiation light. That is, let the deflection angle β be the angle of view.

Next, problems that occur when scanning with the above-described deflection angle β will be described with reference to FIG. FIG. 4 is a schematic diagram when the eyeball is assumed to be a sphere as in FIG. 3. The broken line in FIG. 4 represents the scanning locus. As shown in FIG. 4, the physical distance from the center of the pupil to the outer wall of the eyeball, that is, the fundus is a + b in the macula, and is a in a portion away from the macula (portion of the angle θ / 2). a and b are expressed by the following equations using the length T of the axial axis and the deflection angle θ in the eyeball.
a = T × cos (θ / 2) Equation 1
a + b = T Equation 2
Thus, the distance from the pupil center to the macula and the distance from the pupil center to a position away from the macula are different by b. Since this b becomes larger as the angle θ becomes larger, in the OCT apparatus for fundus examination with a wide angle of view, the optical path length from the pupil center to the macula and the distance from the pupil center to the peripheral position away from the macula Will be very different. The axial length T of an adult has a large individual difference, and the range of the axial length T in which 95% of people are included is 21 mm or more and 28 mm or less. Here, as the value of the axial length T, 28 mm which is the maximum value of the range is used, and when the deflection angle θ in the eyeball is 33.4 degrees, Equations 1, 2 and b are about 1.2 mm.

  Further, the fundus tissue observed by the OCT apparatus for fundus examination is the retina near the surface of the fundus and the choroid behind it. Since the retina is about 0.50 mm at the thickest part and the choroid is about 0.30 mm, the OCT device for fundus examination needs to image to a depth of at least 0.80 mm. That is, a distance difference of 0.8 mm occurs between the fundus surface and the choroid.

  Therefore, in order to image the macula and the optic disc with the same scan and obtain information about the surface of the optic disc and the choroid behind the macula, a distance of 2 × (b + 0.80) ≈4.0 mm Difference is required. This distance difference corresponds to 4.0 mm × 1.38≈5.5 mm in terms of the optical path difference in the air. That is, in order to realize an OCT apparatus that can obtain tomographic information even when the angle of view is 47 degrees or more, an optical path length difference of 5.5 mm in air is required.

  In Fourier domain OCT including SS-OCT, acquired interference signal data is subjected to Fourier transform processing in wave number space, and distance information is output. In the case of SS-OCT, data acquisition is performed in the time domain using an A / D converter. Here, if the optical frequency emitted from the wavelength tunable light source changes accurately linearly with respect to time, data at equal frequency intervals, that is, equal wave number intervals can be obtained by sampling at equal time intervals. However, as schematically shown in FIG. 5A, the optical frequency of the wavelength tunable light source is generally nonlinear with respect to time because the wavelength is swept by changing the resonator length by the drive mechanism. . Therefore, even if the Fourier transform process is performed by sampling at equal time intervals, the distance information cannot be acquired because the data is not data at equal wave intervals. Therefore, in SS-OCT, data acquisition is generally performed using k clocks, which are sampling clocks with equal wave number intervals.

(Clock generator)
Next, the k clock generator will be described with reference to FIG. The reference numerals in FIG. 6 correspond to those in FIG. For example, a part of the light emitted from the light source is branched by the coupler 90 having a branching ratio of 95: 5 and the branched light is incident on the k clock generation unit. The branched light is further branched into two optical paths by the coupler 81, and the branched optical paths are configured as a first optical path and a second optical path. The first optical path and the second optical path are caused to interfere with each other by the coupler 83 with an optical path length difference 82 provided. Thereby, a k-clock interferometer can be configured. Further, it includes a correction circuit 84 that receives an interference signal from the k-clock interferometer, converts it to an electrical signal, and corrects the amplitude. Here, the optical path length difference 82 of k clock corresponds to the frequency of k clock described later.

In the second optical path having an optical path length difference with respect to the first optical path, for example, a substance (gas or the like) whose refractive index can be changed may be provided. Moreover, it can also be realized by changing the optical distance between the fiber ends in a configuration in which light is once emitted from the fiber into the air and returned to the fiber. Further, in the above-described configuration in which light is emitted once out of the fiber, a method of changing the optical path length difference by using a plurality of folding mirrors and moving the folding mirror provided on the movable stage in the optical axis direction may be used. Here, a mechanism for realizing these methods is called a changing unit. At this time, it is preferable to further include a control unit that controls a changing unit that changes the optical path length difference in accordance with the scanning angle. For example, when the scanning angle is increased, unnecessary imaging in the depth range can be reduced by increasing the optical path length difference. For this reason, imaging time can be shortened. At this time, the scanning unit is preferably configured to be able to change from a first angle at which the scanning angle is 47 degrees or more to a second angle that is less than 47 degrees. Moreover, it is preferable that the clock generation unit is configured to be able to change from a first optical path length difference having an optical path length difference of 22 mm or more to a second optical path length difference having a length of less than 22 mm.
Moreover, it is preferable that the imaging device according to the present embodiment further includes a selection unit that selects a plurality of shooting modes with different scanning angles. At this time, the control unit may control the scanning unit and the changing unit so as to change the scanning angle and the optical path length difference according to the selected photographing mode. For example, the imaging mode that captures a tomographic image that includes both the macula and the optic nerve head has a larger scanning angle than the imaging mode that captures either the macula or the optic nerve head as a tomographic image, so the optical path length difference It is preferable to lengthen the length. The selection unit may be configured to be able to select the distance of the tomographic image in the depth range. At this time, the selection unit is preferably configured to be selectable from a first distance in which the distance is 4.0 mm or more in the eyeball to a second distance that is less than 4.0 mm in the eyeball. At this time, if this distance becomes shorter, it is preferable to change so that the optical path length difference is also shortened. Here, the selection unit may be configured to be able to select any one of a plurality of photographing modes including a photographing mode for photographing so as to include the vitreous body of the eye, the retina, and the choroid. At this time, when a photographing mode for photographing so as to include the vitreous body of the eye, the retina, and the choroid is selected, for example, a tomographic image of the fundus at a distance of 4.0 mm or more in the eyeball in the depth range is acquired. It is preferable. This is because a distance of 4.0 mm or more in the eyeball in the depth range is required for tomographic imaging so that the vitreous body of the eye, the retina, and the choroid are sufficiently included. It should be noted that the clock generation unit can obtain the same effect even if it is configured to artificially change the number of samplings and the frequency of the clock, which will be described later, instead of changing the optical path length difference. For example, when the distance of the tomographic image in the depth range is shortened, the clock generator is preferably configured to reduce the number of samplings and the clock frequency.

(Optical path length difference of interferometer of clock generator)
The k clock interference signal becomes a sine wave as the optical frequency changes with time. Since the optical frequency changes nonlinearly with time, the period of this sine wave also changes with time. However, they are equally spaced in the frequency domain. That is, since the zero-cross point or peak point of the k clock interference signal has an equal wave number interval, if sampling is performed using the zero-cross point or peak point as the clock position as shown in FIG. 5B, an OCT interference signal in the wave number space is acquired. it can. The acquired k-clock interference signal is subjected to amplitude correction using an amplifier or the like to an amplitude and voltage that can be applied to the A / D converter to generate a k-clock. Since the k clock is a sampling clock, it must be based on the sampling theorem. For example, when the frequency of the OCT interference signal is 1/2 or less of the frequency of the k clock as shown in FIG. 7A, the original signal can be reproduced, but as shown in FIG. If this happens, a false signal will be acquired. Therefore, the sampling theorem is that the frequency of the k clock must be at least twice the frequency of the OCT interference signal.

  By the way, the interference phenomenon occurs when the difference between the two optical path lengths is an integer n times the wavelength λ and nλ. Therefore, since the interval between the interference fringes is reduced in proportion to the optical path length difference, the frequency as a signal is increased. That is, in order to set the frequency of the k clock to at least twice the frequency of the OCT interference signal, the optical path length difference 82 may be set to at least twice the distance that is the limit in the depth direction. That is, in consideration of the sampling theorem, the optical path length difference 82 of k clocks needs to be at least twice the required tomographic depth range. As described above, when the depth range of the tomographic image is 5.5 mm or more in the air (4.0 mm or more in the eyeball), the optical path length difference 82 of the k clock needs to be 11 mm or more in the air. There is.

  In addition, the sample optical path of the OCT interferometer is usually configured by a double path including an optical path where the irradiation light irradiates the fundus and an optical path where the reflected light returns from the fundus. On the other hand, as shown in FIG. 6, the optical path of the k-clock interferometer is composed of a single path that splits the light and then combines it again by providing an optical path length difference without reflection. It is common. Therefore, when the k-clock interferometer is configured with a single path, the optical path length difference 82 of the k-clock needs to be further doubled from the above-mentioned 11 mm or more to 22 mm or more in the air. That is, when the k-clock interferometer is configured with a single path, the optical path length difference 82 of the k clock needs to be four times or more the required depth range of the tomographic image. Thereby, the depth range of the tomographic image can be set to 4.0 mm or more in the eyeball. The k-clock interferometer can also be configured with a double-path interferometer as shown in FIG. The reference numerals in FIG. 8 correspond to those in FIGS. 1 and 6. In this way, when the k-clock interferometer is configured with a double path, the optical path length difference 82 of the k-clock should be at least twice the required depth range of the tomographic image in consideration of the sampling theorem. good.

  In general, the optical path length difference of the k-clock interferometer is measured at the choroid-sclera boundary when the fundus scanning angle is converted to about 40 degrees in the air. The distance in the depth direction of the tomographic image) is obtained. Therefore, it becomes a depth range of about 3.7 mm in terms of air. For this reason, the optical path length difference of the single-pass k-clock interferometer is 3.7 mm × 4 = 14.8 mm when converted in air, and is designed to be about 15 mm. However, when the optical path length difference of the k-clock interferometer is about 15 mm when converted in the air, there arises a problem that the image is folded around the fundus when the angle of view is widened. That is, the optical path length difference of the k-clock interferometer needs to be 22 mm or more in order to obtain a depth range of 5.5 mm or more in the air in which tomographic information can be obtained at an angle of view of 47 degrees or more.

In the example of FIG. 6, the k clock generation unit is outside the light source, but may be inside the light source. By configuring the k clock generation unit inside the light source, the configuration of the imaging apparatus can be simplified. Further, in order to realize a 22-mm optical path length difference of k clocks, a coherent length of 14 mm or more is preferable. As described above, by making the optical path length difference of the interferometer of the clock generation unit 22 mm or more, tomographic images in a desired depth range of the fundus are covered when a single scan is performed over a wide range of the fundus. Can be obtained.
Here, if the depth range (measurement distance) Δz of the tomographic image, the center wavelength λc, and the sweep wavelength width Δλ are set, the number of samples N, which is the number of sampling the entire depth range of the tomographic image at a time, is (4 × It is derived from Δz × Δλ) / λc ^2. On the other hand, assuming that the frequency sweep frequency fA and the duty ratio (period during which light emission is effective as OCT during one sweep) d, the frequency fs of the k clock is derived from (N × fA) / d. In the light source according to the present embodiment, for example, λc = 1040 nm, Δλ = 110 nm, fA = 100 kHz, and d = 0.446. At this time, considering the case where the depth range of the tomographic image according to the present embodiment is 5.5 mm in the air, that is, 4.0 mm in the eyeball, the number of samples N is (4 × 5.5 × 10 ^6). X110) / 1040 ² = 2237. In this case, the frequency fs of the k clock is derived as (2237 × 100 × 10 ³ ) /0.446=501.57 MHz. Considering a case where the conventional depth range is 2.8 mm in air, that is, 2.0 mm in the eyeball, the number of samples is (4 × 2.8 × 10 ⁶ × 110) / 1040 ² = 1139. Is derived. In this case, the frequency fs of the k clock is derived as (1139 × 100 × 10 ³ ) /0.446=255.38 MHz. As described above, in order to comprehensively acquire a tomographic image of a desired depth range of the fundus when performing a single scan over a wide range of the fundus, the clock generation unit It is preferable that the number of sampling at one time is configured to be about 2200 times or more. In addition, when a single scan is performed over a wide range of the fundus, the clock generator has a clock frequency of about 500 MHz or more in order to comprehensively acquire a tomographic image of a desired depth range of the fundus. It is preferable to be configured as described above.
Note that the present invention is not limited to a single scan performed over a wide range of the fundus. That is, the present invention is not limited to the case where the scanning unit is configured so that the angle at which the irradiation light is scanned on the fundus is 47 degrees or more when converted in the air. When acquiring a tomographic image of the fundus at a distance of 4.0 mm or more in the eyeball in the depth range regardless of the scanning angle, the clock generator corresponds to a distance of 4.0 mm or more in the eyeball. What is necessary is just to be comprised so that it may become an optical path length difference. At this time, as described above, the clock generation unit is preferably configured so that the optical path length difference is 22 mm or more in the air if the second optical path is configured with a single path. Moreover, it is preferable that the clock generation unit is configured so that the optical path length difference is 11 mm or more in the air if the second optical path is configured with a double path. The clock generation unit can obtain the same effect even if it is configured to change the number of samplings and the clock frequency in a pseudo manner instead of setting the optical path length difference to these lengths.

(Correction part)
Here, since the wavelength variable light source generally has a Gaussian spectrum shape, the intensity is weak near the start and end of the wavelength sweep, and the maximum intensity is near the center. Therefore, the k clock interference signal also has a spindle-shaped waveform. Further, when the optical path length difference of k clock is increased, the interference intensity is also greatly attenuated, so that the difference of interference intensity in the k clock interference signal is increased. On the other hand, the minimum voltage and the maximum voltage that can be input are defined for the external clock input unit of the A / D converter. At this time, the difference in the interference intensity in the k clock interference signal may be larger than the range of voltage that can be input in the A / D converter. Therefore, it is necessary to correct the k clock interference signal to an amplitude and voltage that can be applied to the A / D converter using an amplifier or the like.

(Comparator and gain controller)
As a correction method, automatic gain control (hereinafter referred to as AGC) is performed by the correction unit configured to feed back the comparison result of the comparison unit to the gain control unit. FIG. 9A shows a schematic diagram of AGC in the correction circuit 84 which is an example of a correction unit. Here, the AGC includes a variable gain amplifier (hereinafter referred to as VGA) that is an example of a gain control unit, and a voltage detector that is an example of a comparison unit, and an amplitude having a constant voltage level from an input signal that varies in amplitude. To automatically adjust to the signal. At this time, the comparison unit compares the intensity of the analog signal of the second interference light (interference light obtained by interference with the interferometer of the clock generation unit) with a threshold value. The gain controller attenuates the intensity of the analog signal of the second interference light when the intensity of the analog signal of the second interference light is larger than the threshold. Further, the gain control unit amplifies the intensity of the analog signal of the second interference light when the intensity of the analog signal of the second interference light is smaller than the threshold value. That is, when an amplitude variation signal is input, the voltage detector outputs a gain control signal based on the reference voltage, and negative feedback control is performed to change the gain of the VGA.

  In SS-OCT, a k-clock interference signal is also output in a time zone in which the wavelength is swept, but no signal is output in a time zone without a sweep. Therefore, a time zone without a signal is generated. FIG. 9B shows a typical circuit example of the correction circuit 84 when AGC is performed using the k clock interference signal, and FIG. 9C schematically shows the signal at that time. The voltage detector defines the gain control signal by comparing the voltage level of the amplitude variation signal with a reference voltage.

(Delay part and suppression part)
Further, in order to prevent oscillation caused by switching of VGA gain in the AGC loop, a method of connecting a phase compensation capacitor, which is an example of a delay unit, to reduce the response speed is used. That is, the delay unit delays the speed at which feedback is performed. Here, in the time zone where there is no signal, since the amplitude variation signal is lower than the reference voltage, the gain control signal continues to operate to output the gain to the maximum, and electric charge is accumulated in the phase compensation capacitor. It will be. Therefore, when the signal enters a certain time zone, the accumulated charge is output as a gain control signal, so that the gain that should be originally not increased and a signal with an excessive amplitude may be output. is there. Therefore, in the present embodiment, as illustrated in FIG. 10A, it is preferable to provide an excess gain suppression mechanism that is an example of a suppression unit in the output unit of the gain control signal in the correction circuit 84. In other words, the suppressing unit suppresses that the intensity of the analog signal is amplified more than a predetermined value when the speed of performing feedback is delayed by the delay unit.

  At this time, in the present embodiment, as a mechanism for adjusting the gain control signal in the correction circuit 84, a voltage limiter that is an example of an excessive gain suppression mechanism may be installed. FIG. 10B shows a schematic state of this circuit example and signals. The voltage limiter is used to output an arbitrary waveform above or below a certain voltage level. As a result, an excessive voltage of the gain control signal due to the charge accumulated in the phase compensation capacitor is limited, and a k clock having a constant amplitude can be generated. The voltage limiter is preferably provided in the phase compensation capacitor unit in order to minimize the influence of the response speed. That is, it is preferable that the suppressing unit is configured to release the electric charge accumulated in the capacitor in the delay unit.

  In the present embodiment, a reference voltage control circuit may be installed in the correction circuit 84. FIG. 10C shows a schematic example of this circuit example and signals. The reference voltage is a voltage that defines an amplitude voltage level in AGC. Since the gain control signal is output by comparing the voltage level of the reference voltage and the amplitude variation signal, the output of the gain control signal can be controlled by controlling the reference voltage. In SS-OCT, since the wavelength sweep cycle is constant, the timing of the signal generation point can be grasped. That is, by inputting a control voltage signal for controlling the reference voltage immediately before the signal is generated and raising the reference voltage for an arbitrary period, there is an effect of forcibly lowering the gain control signal in that time zone. Furthermore, it can be adjusted to an arbitrary gain control signal by changing the time of the control signal. Thereby, a k clock having a constant amplitude can be generated. That is, it is preferable that the suppressing unit is configured to change the threshold value compared with the intensity of the analog signal in the comparing unit at a predetermined timing.

  As described above, for example, even when the optical path length difference of the interferometer of the clock generation unit is larger than before (for example, 22 mm or more in air), a tomographic image of a desired depth range of the fundus is obtained. It is possible to provide a suitable clock generator configuration for comprehensive acquisition.

(Other embodiments)
The present invention can also be realized by executing the following processing. That is, software (program) that realizes the functions of the above-described embodiments is supplied to a system or apparatus via a network or various storage media, and a computer (or CPU, MPU, or the like) of the system or apparatus reads the program. It is a process to be executed.

DESCRIPTION OF SYMBOLS 10 Light source part 20 OCT interference part 30 Detection part 40 Information acquisition part 53 X-axis scanner 54 Y-axis scanner 80 k clock generation part 82 k clock optical path length difference

Claims (15)

The light emitted from the light source unit that sweeps the wavelength of the emitted light is branched into irradiation light that irradiates the fundus and reference light, and the reflected light from the fundus that is irradiated with the irradiation light interferes with the reference light An interference part to
A detection unit for detecting first interference light obtained by interference by the interference unit;
A conversion unit that converts an analog signal obtained by detecting the first interference light by the detection unit into a digital signal;
An optical path through which a part of the light emitted from the light source unit passes is configured as an interferometer branched into a first optical path and a second optical path having an optical path length difference with respect to the first optical path, A clock generation unit for generating a clock for the conversion unit to sample the analog signal;
An imaging device comprising: a tomographic image acquisition unit that acquires a tomographic image of the fundus based on the digital signal obtained by the conversion unit converting the analog signal by the generated clock;
The clock generation unit is configured such that the difference in intensity of the analog signal of the second interference light obtained by interfering the light of the first optical path and the light of the second optical path is the analog signal of the second interference light. An imaging apparatus comprising: a correction unit that corrects the intensity of the analog signal of the second interference light so that the input signal can be input to the conversion unit.   The correcting unit compares the intensity of the analog signal of the second interference light with a threshold value, and the second unit when the intensity of the analog signal of the second interference light is larger than the threshold value. A gain control unit for attenuating the intensity of the analog signal of the interference light and amplifying the intensity of the analog signal of the second interference light when the intensity of the analog signal of the second interference light is smaller than the threshold; The imaging apparatus according to claim 1, wherein the imaging apparatus is configured to feed back a comparison result by the comparison unit to the gain control unit.   The imaging apparatus according to claim 2, further comprising a delay unit that delays a speed at which the feedback is performed.   The imaging apparatus according to claim 3, further comprising: a suppressing unit that suppresses an intensity of the analog signal from being amplified more than a predetermined value when a speed at which the feedback is performed is delayed by the delay unit. . The delay unit is configured to include a capacitor,
The imaging device according to claim 4, wherein the suppression unit is configured to release the electric charge accumulated in the capacitor.   The imaging apparatus according to claim 4, wherein the suppression unit is configured to change the threshold value at a predetermined timing. The clock generation unit is configured to have the optical path length difference corresponding to a distance of 4.0 mm or more in the eyeball.
The tomographic image acquisition unit is configured to acquire a tomographic image of the fundus at a distance of 4.0 mm or more in the eyeball in a depth range. The imaging device according to item.   8. The clock generation unit according to claim 1, wherein the second optical path is configured by a single path, and the optical path length difference is configured to be 22 mm or more in the air. The imaging device according to item.   8. The clock generation unit according to claim 1, wherein the second optical path is configured by a double path, and the optical path length difference is configured to be 11 mm or more in the air. The imaging device described in 1.   The said clock generation part is comprised so that the number which samples the whole depth range of the said tomogram at a time may be 2200 times or more, The one of Claim 1 thru | or 9 characterized by the above-mentioned. Imaging device.   The imaging apparatus according to claim 1, wherein the clock generation unit is configured such that a frequency of the clock is 500 MHz or more.   12. The imaging apparatus according to claim 1, wherein the clock generation unit generates the clock so that the conversion unit samples the analog signal at intervals of substantially equal wave numbers.   The light source unit includes a lower reflecting mirror, an active layer, and an upper reflecting mirror in this order, and includes an air gap between the active layer and the upper reflecting mirror, and the upper reflecting mirror and the lower reflecting mirror are provided. The imaging according to any one of claims 1 to 12, wherein the imaging is a surface emitting laser that changes a wavelength of emitted light by changing a position of at least one of the mirrors in an optical axis direction. apparatus. A scanning unit that scans the irradiation light on the fundus;
14. The scanning unit according to claim 1, wherein an angle at which the irradiation light is scanned on the fundus is 47 degrees or more when converted in the air. Imaging device.   The imaging apparatus according to claim 14, wherein the scanning unit is configured to scan the irradiation light in a range of 14 mm or more of the fundus.

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