JP2005083954A - Tomographic image system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To make a tomographic image similar to others in brightness, regardless of depth of a test body, and in particular, to reduce a risk of mistaking status judgments about deep sections, by changing light intensity of a light source in accordance with moving positions of a reference mirror, in order to mitigate the lowering of the amount of interference wave information which occurs under such a condition that the intensity of reflection light from an observation position is lowered as the depth of the test body is increased. <P>SOLUTION: A tomographic image system is provided with a control section 1 which changes the intensity of output light from the light source 10 in accordance with the moving positions of the reference mirror 42 so as to mitigate the lowering of the intensity of the reflection light from the observation position of the test body 31 caused by the change in the observation position of the test body 31 whose depth is increased. The control section 1 exponentially changes the intensity of the light source 10 in accordance with the moving positions of the reference mirror 42 corresponding to depth positions of the test body 31, based on a control table representing the light intensity of the light source 10. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention applies a technique based on optical coherence tomography (hereinafter referred to as OCT) configured by combining a light source that outputs light having a short coherence length and an equal optical path length type interferometer such as a Michelson interferometer. Alternatively, the present invention relates to a tomographic image apparatus used when obtaining a tomographic image of a subject in a field such as industry.

2. Description of the Related Art In recent years, devices that capture tomographic images of a subject using an OCT technique are known in the field of imaging a subject for medical use or industrial use, particularly in the field of electronic endoscopes.
Since this tomographic image apparatus using OCT uses light as a detection probe, there is no problem that the subject is exposed to X-ray irradiation unlike the conventional X-ray imaging apparatus, and in particular, the subject seems to be a human body. In this case, it is extremely preferable. In addition, a large-scale apparatus such as CT or MRI is not required, and the test of the subject can be easily performed. Therefore, the cost burden and physical burden on the subject can be reduced. preferable.

In addition, this tomographic imaging apparatus using OCT obtains interference wave information at each position in the depth direction of the subject by utilizing the low coherence of light having a broad spectrum width. Reflected light from the inside of the specimen can be detected with a spatial resolution on the order of μm, and the measurement resolution can be greatly improved as compared with a conventional X-ray imaging apparatus.
Such a tomographic image apparatus using OCT having many excellent characteristics is disclosed, for example, in Non-Patent Document 1 below.

FIG. 8 shows an outline of a conventional tomographic imaging apparatus. That is, the output from the low coherence light source 310 is made incident on the optical fiber 321. The light beam traveling in the optical fiber 321 is separated into two light beams by the 2 × 2 coupler 325, one of which is guided to the subject 331 side by the optical fiber 322, and the other is guided to the reference mirror 342 side by the optical fiber 323. It is burned.
An objective condensing lens 332 is provided at the subsequent stage of the light emitting end of the optical fiber 322, and the light beam is condensed on the subject 331 by the lens 332.

  On the other hand, the light emitted from the light emitting end of the optical fiber 323 is applied to the reference mirror 342 via the collimator lens 341. The reference mirror 342 is movable in the optical axis direction, and the optical fiber 322 is provided. The reference mirror 342 is located at a position where the optical path length from the light exit end of the optical fiber to the observation position in the depth direction of the subject 331 and the optical path length from the light exit end of the optical fiber 323 to the reference mirror 342 are equal to each other. It has been moved. As a result, a so-called Michelson interferometer capable of causing optical interference even with low coherent light is constructed, and interference wave information at each position in the depth direction of the subject 331 can be obtained.

  The reflected light from the observation position of the subject 331 and the reflected light from the reference mirror 342 travel back in their irradiation paths, and are combined by the 2 × 2 coupler 325 to interfere with each other, and the interference light is an optical fiber. The interference wave information is detected by the photodetector 352 by reaching the photodetector 352 via the line 324. Thereafter, the interference wave information detected by the photodetector 352 is converted into an electrical signal, which is input to the computer 365 via the amplifier 362, the band pass filter 363, and the A / D converter 364, and subjected to predetermined image processing. Will be.

Optics 32 (4) (2003): Manabu Sato, Naohiro Tanno

  However, the above-described tomographic imaging apparatus using OCT uses low coherence light as measurement light in a Michelson interferometer, and based on reflection information on a tomographic boundary surface at each depth inside the subject. Since the reflected light intensity from each depth position to be observed attenuates exponentially according to the depth, the reflected light intensity from the deep part becomes extremely weak. Therefore, the interference wave information from the deep part is also very weak. For this reason, there is a possibility that the shallow part of the tomographic image is clear and the deep part is unclear. For this reason, it is difficult to determine a shallow portion and a deep portion of one tomographic image at the same brightness level, and there is a large risk of misjudging the state of a particularly deep portion, which is a problem.

  The present invention has been made in view of such circumstances, and enables one to determine a shallow portion and a deep portion with the same brightness level in one tomographic image, and particularly makes a state determination for a deep portion erroneous. An object of the present invention is to provide a tomographic imaging apparatus capable of reducing the risk.

The first tomographic imaging apparatus of the present invention is
A light source that emits a light beam having low coherence;
The light beam emitted from the light source is divided into two, one is irradiated on the subject, the other is irradiated on the reference mirror, and the light beams reflected from the subject and the reference mirror are made to interfere with each other, An optical path length type interferometer that obtains a light intensity distribution due to interference with a photodetector;
In the tomographic imaging apparatus in which the reference mirror is movable in the optical axis direction of the reference mirror so that interference wave information in the depth direction of the subject can be obtained,
Output from the light source according to the moving position of the reference mirror so as to mitigate a decrease in the amount of interference wave information due to a decrease in the amount of reflected light from the observation position as the depth of the observation position of the subject increases. And a light source light intensity control unit for changing the intensity of the light beam to be emitted.

The second tomographic imaging apparatus of the present invention is
A light source that emits a light beam having low coherence;
The light beam emitted from the light source is divided into two, one is irradiated on the subject, the other is irradiated on the reference mirror, the light beams reflected from the subject and the reference mirror are interfered with each other, and An optical path length type interferometer that obtains a light intensity distribution due to interference with a photodetector;
In the tomographic imaging apparatus in which the reference mirror is movable in the optical axis direction of the reference mirror so that interference wave information in the depth direction of the subject can be obtained,
In the photodetector according to the moving position of the reference mirror so as to mitigate a decrease in interference wave intensity due to a decrease in the amount of reflected light from the observation position as the depth of the observation position of the subject increases. A detector gain control unit for controlling the signal gain is provided.

  In the first and second tomographic imaging apparatuses described above, autofocusing is performed such that one of the light beams is converged to the observation position of the subject according to the movement of the reference mirror in the optical axis direction. It is preferable to have a function.

  According to the first tomographic imaging apparatus of the present invention, so as to mitigate a decrease in the amount of interference wave information due to a decrease in reflected light intensity from the observation position as the depth of the observation position of the subject increases. A light source light intensity control unit that changes a light intensity of a light beam output from the light source according to a movement position of the reference mirror, and the light source light intensity control unit causes the light source to move according to the movement of the reference mirror; Even if the light intensity output from the light source is changed and the depth of the observation position increases, the decrease in the amount of reflected light from the observation position can be mitigated. In addition, it is possible to reduce the risk of misjudging the state of a particularly deep part.

  Further, according to the second tomographic imaging apparatus of the present invention, the reduction in the interference wave intensity due to the reduction in the intensity of the reflected light from the observation position accompanying the increase in the depth of the observation position of the subject is alleviated. A detector gain control unit that changes a signal gain in the photodetector according to the movement position of the reference mirror, and the interference wave intensity of the observation position is increased even when the depth of the observation position is increased. Since the reduction can be mitigated, it is possible to determine the shallow portion and the deep portion with the same brightness level, and it is possible to reduce the risk of erroneous determination of the state of the particularly deep portion.

Hereinafter, a tomographic imaging apparatus according to an embodiment of the present invention will be described with reference to the drawings.
FIG. 1 is a conceptual diagram showing a tomographic image apparatus according to an embodiment of the present invention.
The tomographic imaging apparatus according to the present embodiment is applied to a medical endoscope, and includes a light source unit, an interferometer unit, and a signal processing unit.

The light source unit includes a low coherence light source 10. Further, the interferometer unit as a whole constitutes a so-called unbalanced Michelson interferometer, and includes four optical fibers 21, 22, 23, 24, a 2 × 2 coupler 25, and a front stage of the subject 31. An objective condenser lens 32, a collimator lens 41, a reference mirror 42, and a photodetector 52 that captures an interference wave image carrying subject information.
The signal processing unit includes an amplifier 62, a band pass filter 63, an A / D converter 64, an image processing unit 65, and an image display unit 66.

Hereinafter, the operation of the above-described embodiment apparatus will be described.
The low coherence light source 10 is a light source that emits light having a wide spectrum width (wide wavelength band) in the near infrared region (hereinafter also referred to as low coherence light), and is, for example, an SLD (Super-luminescent diode) or ASE. (Amplified Spontaneous Emission) It consists of a light source. The light emitted from the low coherence light source 10 is collected on the incident end face of the optical fiber 21 by a condenser lens (not shown) and transmitted to the 2 × 2 coupler 25 through the optical fiber 21.

  The transmitted low coherence light is divided into two by the 2 × 2 coupler 25, one being transmitted to the optical fiber 22 and the other being transmitted to the optical fiber 23. The low coherence light transmitted by the optical fiber 22 is emitted from the emission end face of the optical fiber 22 and is condensed and irradiated onto the subject 31 (human body) by the objective condenser lens 32.

  The low coherence light irradiated to the subject 31 is reflected from each position in the depth direction of the subject 31, travels backward on the irradiation path, and reaches the 2 × 2 coupler 25. On the other hand, the low coherence light reflected from the reference mirror 42 also travels in the irradiation path in the reverse direction and reaches the 2 × 2 coupler 25. In this way, the low coherent light (hereinafter referred to as object light) reflected at each position in the depth direction of the subject 31 and the low coherent light (hereinafter referred to as reference light) reflected by the reference mirror 42 are used. The 2 × 2 coupler 25 combines the light beams, but the combined light beams have extremely short coherence lengths, so that only object light having substantially the same optical path length as the reference light causes interference.

  That is, since the interference wave information of only the position in the depth direction of the subject 31 corresponding to the position of the reference mirror 42 in the optical axis direction (specifically, the range of the coherence length of the low coherence light) is obtained. By moving the reference mirror 42 in the optical axis direction (arrow B direction), interference wave information at each position in the depth direction of the subject 31 is obtained in time series.

  Thereafter, the interference light obtained by the 2 × 2 coupler 25 reaches the photodetector 52 through the optical fiber 24, and the interference wave information is detected by the photodetector 52 as a change in light intensity. become. The interference wave information detected by the photodetector 52 is converted into an electrical signal, which is input to the image processing unit 65 via the amplifier 62, the band pass filter 63, and the A / D converter 64, and a tomographic image of the subject 31 is obtained. The generated tomographic image is displayed on the image display unit 66.

  In the image processing unit 65, envelope processing is performed on each one-dimensional reflected light intensity signal output from the photodetector 52 and carrying information in the depth direction of the subject 31. Yes. That is, since each reflected light intensity signal appears in the form of an AC signal, the negative component is folded and converted into a signal having only a positive component, and then the envelope is extracted to obtain a continuous reflected light intensity signal. I am doing so.

  Thereafter, image signal processing is performed to connect each reflected light intensity distribution in the continuous depth direction (z direction) with respect to each point in the y direction to generate a two-dimensional tomographic image signal, which is output to the image display unit 66. A two-dimensional tomographic image of the subject 31 is displayed.

  In this way, by using the low coherence light as the inspection light and by using the equal optical path length type Michelson interferometer as a basic configuration, the wavelength widths that interfere with each other can be extremely shortened. The spatial resolution in the depth direction can be set very short, and for example, a spatial resolution on the order of μm can be obtained.

  Further, the objective condensing lens 32 that condenses and irradiates the subject 31 with the low coherent light is configured such that a part of the lens is movable in the optical axis direction (arrow C direction) and can be focused on the observation position. Accordingly, it is possible to prevent the subject image from being out of focus at the observation position.

  Further, the light emitting end portion of the optical fiber 22 and the objective condenser lens 32 constitute a tip probe portion to be used through the forceps channel of the endoscope. By moving the tip probe portion in the direction of arrow A, Tomographic image information corresponding to each point in the lateral direction (y direction) of the specimen 31 can be obtained. That is, two-dimensional tomographic image information of the subject 31 can be obtained by moving the tip probe portion in the y direction.

  By the way, in general, when the subject 31 is a living body, if the irradiation light intensity is constant, the reflected light intensity from the depth position of each tomographic boundary surface is exponentially attenuated according to the depth. To do. In other words, the depth information of the subject 31 is represented by the intensity ratio of the reflected light to the irradiated light.

  Therefore, when the subject 31 has a layer configuration perpendicular to the depth direction as shown in FIG. 2, light reflection occurs at the boundary of each layer, and thus the subject information carried by each reflected light interferes. It will be detected as a wave signal.

  For this reason, conventionally, when a tomographic image signal is detected for a subject 31 as shown in FIG. 2, the peak value of the signal intensity decreases as it goes deeper as shown in FIG. Since the difference in brightness between the images in a sheet is large, it is very difficult to see, and the image in the deep part is particularly unclear.

  Therefore, in the tomographic imaging apparatus of the present embodiment, as shown in FIG. 1, interference due to a decrease in reflected light intensity from the observation position of the subject 31 as the depth of the observation position of the subject 31 increases. A control unit 1 for controlling the light source light intensity that changes the light intensity of the low coherent light output from the light source 10 in accordance with the movement position of the reference mirror 42 is provided so as to alleviate the decrease in wave intensity. The control unit 1 uses a relational expression table corresponding to an inverse function (exponential function) of the function representing the relationship between the depth position of the subject 31 and the reflected light intensity attenuated according to the depth position. It is provided as a control table. Based on this light source light intensity control table, the control unit 1 smoothly changes the intensity of the light source 10 exponentially according to the movement position of the reference mirror 42 corresponding to the depth position of the subject 31. That is, the light intensity of the low coherent light output from the light source 10 changes exponentially according to the movement position of the reference mirror 42 as shown in FIG. 4, and the peak intensity of the obtained interference wave signal is As shown in FIG. 5, the depth is substantially constant regardless of the depth position of the observed object 31.

As a result, the tomographic image displayed on the image display unit 66 is easy to see because there is almost no difference in brightness in one image, and particularly, a deep image can be clearly displayed.
Note that the position of the reference mirror 42 may be detected using a known position sensor.

Further, in the apparatus according to the present embodiment, a predetermined lens (focal length f) of the objective condenser lens 32 is applied to the light based on an instruction signal from the control unit 1 in accordance with the movement position of the reference mirror 42 in the optical axis direction. Focusing is performed by moving in the axial direction (in the direction of arrow C), and by providing an autofocus function for converging one of the light beams to the observation position of the subject 31, the focus adjustment of the observation position is performed manually. Therefore, the operator's labor can be reduced and the measurement time can be shortened.
The predetermined lens is driven by using a known lens driving actuator.

  Further, in order to improve the contrast of the interference wave signal, for example, an ND filter capable of adjusting the transmitted light amount is provided in front of the reference mirror 42, and the light amount of the object light and the light amount of the reference light according to the change in the light amount of the object light. Are preferably adjustable so that they are at the same level. In this case, the position adjustment of the ND filter may be performed based on the instruction signal from the control unit 1.

  FIG. 7 shows an example of a two-dimensional tomographic image of the subject 31 obtained by the present embodiment. In both the images shown in FIG. 7, the vertical direction matches the z direction (upper surface is the surface of the subject 31), and the horizontal direction matches the y direction. Further, in the image of FIG. 7, the range of 2 mm of the tissue of the subject 31 is shown in both the vertical direction and the horizontal direction.

  According to FIG. 7, the two-dimensional tomographic image obtained by the present embodiment has a substantially uniform level of brightness over the entire range in the depth direction, and a clear image can be obtained even in the deep part. It is also clear that the spatial resolution is excellent.

<Change of mode>
In the above embodiment, the control unit 1 for controlling the light source light intensity for changing the light intensity of the low coherence light output from the light source 10 according to the moving position of the reference mirror 42 is provided. Instead, the reference mirror 42 is moved so as to mitigate a decrease in interference wave intensity due to a decrease in reflected light intensity from the observation position of the subject 31 as the depth of the observation position of the subject 31 increases. It is also possible to provide a control unit 101 for controlling the detector gain, which controls the signal gain in the photo detector 52 in response (for the sake of simplicity of the drawing, the control unit 1 for controlling the light source light intensity is detected in FIG. It is described to be diverted to the control unit 101 for controlling the gain of the generator).

  That is, the control unit 101 for controlling the detector gain corresponds to an inverse function (exponential function) of a function representing the relationship between the depth position of the subject 31 and the reflected light intensity attenuated according to the depth position. The relational expression table is provided as a detector gain control table. Based on the detector gain control table, the control unit 101 smoothly changes the signal gain of the photodetector 52 exponentially in accordance with the movement position of the reference mirror 42 corresponding to the depth position of the subject 31. Let That is, the signal gain of the photodetector 52 also changes exponentially according to the moving position of the reference mirror 42 as shown in FIG. 4, as in the case of the light intensity of the low coherence light output from the light source 10. In other words, the peak intensity of the obtained interference wave signal is substantially constant regardless of the depth position of the observed object 31 as shown in FIG.

  As a result, the tomographic image displayed on the image display unit 66 is easy to see because there is almost no difference in brightness in one image, and in particular, a deep image can be clearly displayed.

Further, although the apparatus shown in FIG. 1 uses an unbalanced Michelson interferometer, a balanced Michelson interferometer as shown in FIG. 6 may be used instead. The processing used in the unbalanced Michelson interferometer can be used in the same manner in the balanced Michelson interferometer. Therefore, in FIG. 6, members corresponding to those in FIG. 1 are denoted by reference numerals obtained by adding 200 to the reference numerals appended to the members in FIG. However, the balanced Michelson interferometer requires a path that was not used in the unbalanced Michelson interferometer, and is provided with two 2 × 2 couplers 212 and 213, an optical isolator 214, and the like. Further, in the two optical outputs from the 2 × 2 coupler 213, in each of the signals I ₊ and I ₋ , the interference wave component has a positive / negative reverse sign, while the bias component has the same sign. Accordingly, by taking the difference between the two output signal intensities from the photodetectors 252A and 252B using a comparator or the like, the bias component including the fluctuation of the light source is canceled out, and only the interference wave signal component is theoretically calculated. It can be obtained as 2 times.

This will be explained using mathematical formulas. The optical signal intensity from the reference mirror 242 is I _r , the optical signal intensity from the subject 31 is I _s, and the phase difference between the two interfering light waves is φ, and the combined signals are combined. When the later interference wave signal intensity is I _p , the interference wave signal intensity I _p by the unbalanced Michelson interferometer is expressed by the following equation.

On the other hand, since the interference wave signal intensity _Ip by the balanced Michelson interferometer is expressed as a difference between both output signal intensity I ₊ and I ₋ from the photodetectors 252A and 252B as described above, It is represented by

As is clear from the comparison of the above two formulas, with the interference wave signal intensity _Ip by the balanced Michelson interferometer, the bias component in the detection signal is canceled and only the necessary information component regarding the subject 231 is extracted. become. For this reason, even if the gains of the photodetectors 252A and 252B are increased, the dynamic range of the photodetectors 252A and 252B is not exceeded, and the intensity of the detection signal can be increased.

  The tomographic image apparatus of the present invention can be modified in other forms. For example, a low coherence light source such as a normal diode or a high-pressure mercury lamp can be used instead of the above-described light source. Can be used.

  In the above embodiment, the Michelson type is used as the equal optical path length type interferometer, but other equal optical path length type interferometers such as the Mach-Zehnder type are used instead. Also good.

  Further, the subject is not limited to the human body, and may be various other tissues in which light enters the inside and the reflected light is obtained from each position inside.

The conceptual diagram which shows the tomographic image apparatus which concerns on embodiment of this invention Schematic diagram showing the layer structure of the subject perpendicular to the depth direction The graph which shows the interference signal strength with respect to the depth position of the subject in the prior art The graph which shows the light intensity of the low coherence light output from the light source with respect to the depth position in embodiment of this invention The graph which shows the interference signal strength with respect to the depth position of a subject in embodiment of this invention The figure which shows the partial change aspect of embodiment shown in FIG. The figure which shows an example of the two-dimensional tomographic image of the subject obtained by embodiment shown in FIG. Conceptual diagram showing a tomographic imaging apparatus according to the prior art

Explanation of symbols

1, 101, 201 Control unit 10, 110, 210 Low coherence light source 21, 22, 23, 24, 221, 222, 223, 224,
321, 322, 323, 324 Optical fiber 25, 212, 213, 225, 325 2 × 2 couplers 31, 231, 331 Subject 32, 232, 332 Objective condenser lens 42, 242, 342 Reference mirror 52, 252 A, 252 B , 352 Photodetector 41, 241, 341 Collimator lens 62, 262, 362 Amplifier 63, 263, 363 Band pass filter 64, 264, 364 A / D converter 65, 265, 365 Image processing unit 66, 266, 366 Image display Part

Claims (3)

A light source that emits a light beam having low coherence;
The light beam emitted from the light source is divided into two, one is irradiated on the subject, the other is irradiated on the reference mirror, and the light beams reflected from the subject and the reference mirror are made to interfere with each other, An optical path length type interferometer that obtains a light intensity distribution due to interference with a photodetector;
In the tomographic imaging apparatus in which the reference mirror is movable in the optical axis direction of the reference mirror so that interference wave information in the depth direction of the subject can be obtained,
Output from the light source according to the moving position of the reference mirror so as to mitigate a decrease in the amount of interference wave information due to a decrease in the amount of reflected light from the observation position as the depth of the observation position of the subject increases. A tomographic imaging apparatus comprising a light source light intensity controller that changes the intensity of a light beam to be emitted. A light source that emits a light beam having low coherence;
The light beam emitted from the light source is divided into two, one is irradiated on the subject, the other is irradiated on the reference mirror, and the light beams reflected from the subject and the reference mirror are made to interfere with each other, An optical path length type interferometer that obtains a light intensity distribution due to interference with a photodetector;
In the tomographic imaging apparatus in which the reference mirror is movable in the optical axis direction of the reference mirror so that interference wave information in the depth direction of the subject can be obtained,
In the photodetector according to the moving position of the reference mirror so as to alleviate a decrease in interference wave intensity due to a decrease in the amount of reflected light from the observation position as the depth of the observation position of the subject increases. A tomographic imaging apparatus comprising a detector gain control unit for controlling a signal gain. 2. An autofocus function for focusing so that one of the light beams converges on an observation position of the subject according to a movement position of the reference mirror in an optical axis direction. 2. A tomographic imaging apparatus according to item 2.

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