This invention relates to an optical tomography image forming method for generating optical tomographic images by using OCT (Optical Coherence Tomography) measurement.
Recently, as an endoscope apparatus for observing the inside of a body cavity of a living body, electronic endoscope apparatuses which obtain an image of a living body based on reflected light reflected from a living body which is irradiated by an illuminating light, and display the image on a monitor, or the like, have come into wide use in various field. Also, many endoscope apparatuses include a forceps port, and via a probe introduced into the body cavity through this forceps port, the biopsy and treatment of tissues in the body cavity can be performed.
As the above-described endoscope apparatus, an ultrasonic tomographic imaging apparatus that uses an ultrasonic wave and the like are also known. Further, as an example, an optical tomographic imaging apparatus that employs light interference of low coherence light may also be used. In those optical tomographic imaging apparatuses, a low coherence light emitted from a light source is split into a measuring light and a reference light. Thereafter, reflected light, which is the measuring light reflected by a measured object when the measuring light is irradiated onto the measured object, is guided to a light multiplexing means. Meanwhile, the reference light is guided to the light combining after the optical path length thereof is changed. Then, the reflected light is combined with the reference light via the light combining means, and the interference light produced by the mixing of the reflected light with the reference light is measured via heterodyne detection or the like.
Further, when the measuring light is irradiated onto the measured object, a probe is used, the probe which is inserted into the body cavity from a forceps through a forceps channel. The probe includes an optical fiber for guiding the measuring light and a rotatable mirror which is provided at the tip of the optical fiber for reflecting the measuring light in the orthogonal direction. The measuring light is irradiated on the measured object in the body cavity from the probe, and the reflected light reflected from the measured object is guided to the light combining means again through the optical fiber of the probe. Here, by utilizing the fact that interference light is detected when the optical path lengths of the measuring light and the reflected light equate to the optical path length of the reference light, the measuring position (the depth of measurement) in the measured object is changed, by changing the optical path length of the reference light. This is a so-called OCT measurement (refer to Patent Document 1).
In this OCT measurement, information on the depth direction of a measured object is obtained from the difference in the optical path length of the optical path length of the measuring light and the optical path length of the reference light. However, because human tissue, which is a measured object of OCT measurement, or the like, is generally different from air in refractive index, there is a possible problem in that, when the measuring light passes through the interior of the measured object, the optical path length varies in accordance with the refractive index. Thereby, when a tomographic image is formed by using the measured OCT values themselves, the actual distance does not match the optical path length due to differences in refractive index, resulting in a distortion on the formed tomographic image compared to the actual tissue.
The present invention has been achieved in consideration of the above-described problem, and an object of the present invention is to provide an optical tomographic image forming method capable of obtaining a tomographic image closer actuality.
The optical tomographic image forming method described in Claim 1, in which a low coherence light emitted from a light source is split into a measuring light and a reference light, and an optical tomographic image of a measured object is formed by detecting the interference light that is obtained by superposing reflected light; reflected from said measured object when said measuring light is irradiated onto said measured object via a condenser lens; and reflected light, reflected from a reference mirror, being positioned a predetermined optical path length away from the splitting position, when said reference light is irradiated onto the reference mirror, wherein, a refractive index of said measured object is inputted, and the optical tomographic image is corrected in accordance with the inputted refractive index of said measured object, and outputted.
According to the present invention, more real optical tomographic image with less distortion can be acquired in such a manner that a refractive index of said measured object is inputted, and said optical tomographic image is corrected in accordance with the inputted refractive index, and then outputted.
The optical tomographic image forming method described in Claim 2 as set forth in Claim 1, wherein a known datum is inputted as the refractive index of said measured object The term “known datum” is datum of the refractive index of tissue closer to that of the measured object, and other refractive indexes which have been obtained via experiments, or the like.
The optical tomographic image forming method described in Claim 3 as set forth in Claim 1 or 2, wherein, in a case in which said measuring light is obliquely incident upon a first reflecting surface of said measured object, an optical path length, through which said measuring light passes, of a case in which said measuring light is refracted at said first reflecting surface and then reflected at a second reflecting surface of said measured object, is set as the optical path length between said first reflecting surface and said second reflecting surface, assuming that said measuring light travels in a straight line inside said measured object regardless of the incident angle of said measuring light, and said optical tomographic image is corrected in accordance with the refractive index between said first reflecting surface and said second reflecting surface, and then outputted. Note that, in this specification, “the second reflecting surface” is positioned inside the measured object so as to be inwardly positioned behind “the first reflecting surface”, and includes the surface of the measured object.
The optical tomographic image forming method described in Claim 4 as set forth in Claim 1 or 2, wherein, in a case in which said measuring light is obliquely incident upon a first reflecting surface of said measured object, an optical path length, through which said measuring light passes, of a case in which said measuring light is refracted at said first reflecting surface and then reflected at a second reflecting surface of said measured object, is determined in accordance with: the incident angle of said measuring light upon said first reflecting surface; the refractive index of incident side of said first reflecting surface; and the refractive index between said first reflecting surface and said second reflecting surface, and said optical tomographic image is corrected and then outputted.
The optical tomographic image forming method described in Claim 5 as set forth in any one of Claims 1 through 3, wherein an refractive index of the said measured object is obtained from the difference between a first position and a second position, the first position in which said measured object; or said condenser lens and said reference mirror are moved so as to maximize the intensity of an interference light of reflected light, reflected from said first reflecting surface, and reflected light from said reference mirror, and the second position in which said measured object or said condenser lens and said reference mirror are moved so as to maximize the intensity of an interference light of a reflected light, reflected from said second reflecting surface, and a reflected light from said reference mirror.
The optical tomographic image forming method described in Claim 6 as set forth in any one of Claims 1 through 5, wherein coherency distance Δ1 of the low coherence light, which is expressed by the following equation, is less than or equal to 30 μm.
Δ1=(2ln (2λo2))/(πΔλ)
where
Δ1: Center wavelength of said low coherence light
λo: Bandwidth of said low coherence light (range of intensity more than half of the maximum intensity)
According to the present invention, it becomes possible to provide an optical tomographic image forming method capable of obtaining a tomographic image closer to actuality.
a is a schematic diagram of a cross-sectional image of actual tissue of measured object S, and
a is a schematic diagram of a cross-sectional image of actual tissue of a measured object, and
Hereinafter, the preferred embodiment of the present invention will be described in detail with reference to the drawings.
Here, light source SLD is composed of, for example, a laser light source which emits low coherent light such as SLD (Super Luminescent Diode), ASE (Amplified Spontaneous Emission), or the like. Note that because the optical tomographic image measuring apparatus is to obtain a tomographic image of measured object S which is a living body, such as the inside of a body cavity, or the like, a light source, which is capable of minimizing attenuation of light due to scattering and absorption when light propagates through the interior of measured object S, is used. As an example, an ultra-short pulse laser light source with a wide spectral range, and of which the center wavelength is 1.3 μm while propagating through a living body, or the like, is preferably used. Also, the coherency distance Δ1 of low coherence light is preferably not more than 30 μm because the spatial resolution of measurement is preferred to be no more than 30 μm in case of the measurement of a living body.
Δ1=(2ln (2λo2))/(πΔλ)
where
Δ1: Center wavelength of low coherence light
λo: Bandwidth of low coherence light (range of intensity more than half of the maximum intensity) Beam splitting means BS is composed of, for example, an optical fiber coupler of 2×2, and it is so configured that low coherent light L, guided from light source SLD via optical fiber FB, is split into measuring light L1 and reference light L2.
Reference light L2 is reflected at reference mirror RAM, and is incident upon beam splitting means BS again as reflecting light L4. Also, reflecting light L3, which is measuring light L1 reflected from the boundary between the refractive indices of internal tissue of measured object S, is incident upon beam splitting means BS again. When a total of optical path lengths of measuring L1 and reflecting light L3, and a total of optical path lengths of reference L2 and reflecting light L4, are approximately the same, reflecting light L3, having been incident upon beam splitting means BS, and reflecting light L4 are superposed and interfered. Superposed light L5, having been superposed, is incident upon detector DT and converted into electric signals, and detected.
An operation of optical tomographic image measuring apparatus 1 will be described. In
Here, a diagram of a cross-sectional image of actual tissue of measured object S is schematically shown in
Thereby, when an optical tomographic image of the measured object is formed by obtaining the thickness of each layer t1 through t3 directly from an interval between peak values of the tomographic signal shown in
Particularly, as shown in
Further, in another modified example, in a case in which at least two reflection surfaces (a first reflecting surface and a second reflecting surface) in the direction of depth of measured object S, the refractive index of measured object S may be obtained from the difference between a first position; in which said measured object, or said condenser lens and said reference mirror are moved so as to maximize the intensity of interference light of reflected light L3, reflected from the first reflecting surface of measured object S, and reference light L; and a second position, in which said measured object or said condenser lens and said reference mirror are moved so as to maximize the intensity of interference light of said measuring light, reflected from the second reflecting surface of said measured object, and said reference light. In this case, image correction by refractive indexes can be simultaneously carried out by also measuring tomographic images while measuring the refractive index via the OCT apparatus.
A method for obtaining the refractive index of measured object S will be described more concretely.
Firstly, the first step is explained. As shown by the alternate long and short dashed line in the figure, the position of condenser lens CNL or the position of measured object S is controlled by using an actuator, which is not shown in the figure, so that the outgoing light emitted light from condenser leans CNL is condensed onto spot A on the boundary surface between the first layer and air. When the outgoing light emitted from condenser lens CNL is condensed upon spot A, the light intensity of reflected light L3, returning to optical fiber FB, is maximized, and therefore, personal computer PC controls the position of condenser lens CNL or the position of measured object S by monitoring the light intensity of reflected light L3, returning to optical fiber FB.
Secondly, as shown by the solid line in the figure, the position of condenser lens CNL or the position of measured object S is controlled by using the actuator, which is not shown in the figure, so that the outgoing light emitted light from condenser leans CNL is condensed onto spot B on the boundary surface between the first layer and the second layer. When the outgoing light emitted from condenser lens CNL is condensed upon spot B, the light intensity of reflected light L3, returning to optical fiber FB, is maximized, and therefore, personal computer PC controls the position of condenser lens CNL or the position of measured object S by monitoring the light intensity of reflected light L3, returning to optical fiber FB. In such a way, personal computer PC obtains Δd after condensing the outgoing light, emitted from condenser lens CNL, onto spots A and B.
Here, converging angle of the light emitted from condenser lens CNL is set to θ, the refractive index of first layer S1 is set to “n”, and the thickness of first layer S1 is set to Δx. The distance between the boundary of first layer S1 and air, and the converging spot of the light emitted from condenser lens CNL, in a case in which the refractive index of first layer S1 is 1, is set to Δd. The radius of light diameter of the light, emitted from condenser lens CNL, at the boundary between first layer S1 and air, is set to Δy. By assuming that θ has a sufficiently small value, the conversing angle inside first layer S1 is indicated as θ/n as shown in the figure. From the foresaid assumption, the formulas [Δx=Δy/tan (θ/n)≈Δy/(θ/n)] and [Δy=Δd×tan θ≈Δd×θ] can be obtained. From these two formulas, the formula of “Δx=n×Δd” can be obtained. Note that Δd is a known amount which personal computer PC can obtain as mentioned above.
Next, the second step will be described. The outgoing light, emitted from condenser lens CNL, is condensed onto spot A, for example. Then, under this condition, reference mirror RAM is moved so as to maximize the light intensity of superposed light L5. The position of reference mirror RAM is memorized by personal computer PC. Because the outgoing light emitted from light source SLD is a low coherent light, the light intensity of superposed light L5 is maximized only when the optical path lengths of reflecting lights L3 and L4 become equal.
Next, the outgoing light, emitted from condenser lens CNL, is condensed onto spot B. Under this condition, reference mirror RAM is moved so as to maximize the light intensity of superposed light L5, and that position is memorized by personal computer PC. Then, personal computer PC obtains distance Δd′, between those two positions, having been memorized. This Δd′ satisfies the following formula: Δd′=n×Δx. From the formula: Δd′=n×Δx and the previously-obtained formula: Δx=n×Δd, the formula: n=(Δd′/Δd)1/2 is obtained.
Thereby, by obtaining Δd′ and Δd, the refractive index “n” can be calculated.
Next, a correction of tomographic image will be described in a case in which the surface of measured object S is a curved-surface, or the like, and the measuring light is obliquely incident upon the surface.
More specifically, because formula: [n0×sin θ=n1×sin θ′] is satisfied, in
According to the embodiment, the positions of spot SP1 and spot SAP2 are corrected in accordance with refractive index n0 of air, which is on the incidence side of surface SS1, and refractive index n1 of first layer S1. More specifically, image correction is carried out via personal computer PC assuming that the measuring light is reflected at spot SP2, not at position (X) where the measuring light travels distance d1′ from spot SP1 in a direction of straight line. Here, distance d1′ becomes a length closer to the configuration of actual measured object, and calculated as: d1′=D1×(n0/n1). In such a way, by obtaining optical path length d1′ which is closer to actual measured object, and by moving image surface SSA2 closer to boundary surface SS2 over the entire scanning range, a tomographic image, which is closer to actual tissue, can be displayed.
It is needles to say that the present invention can be applied to any one of TD (Time Domain)−OCT measurement and FD (Fourier Domain)−OCT measurement.
Number | Date | Country | Kind |
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2008-278511 | Oct 2008 | JP | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/JP2009/065488 | 9/4/2009 | WO | 00 | 4/25/2011 |