BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical coherence tomography apparatus, and more particularly, to an optical coherence tomography apparatus that can acquire a full-color optical coherence tomographic image. In addition, the invention relates to a measurement method using the optical coherence tomography apparatus.

2. Description of the Related Art

In the development of cosmetics or medicines and medical care, it is important to observe the internal state of the skin. In Europe, sales of cosmetics developed by animal experiments were banned entirely in the spring of 2013. Therefore, the necessity of a method for observing the human skin non-destructively and non-invasively is increasing.

An optical coherence tomography apparatus (hereinafter, referred to as an OCT apparatus in some cases) has been known as one of the non-destructive and non-invasive tomography methods. This is a tomography method using optical coherence, has been developed since the 1990's, is mainly applied to, for example, fundus examination, and uses near infrared light (1.3 μm or 1.5 μm).

A near infrared OCT apparatus which has been put into practical use has a depth resolution of about 20 μm. Therefore, the near infrared OCT apparatus is not suitable to observe the skin with high resolution. The layer structure of the skin includes keratin (a thickness of 10 to 30 μm), the epidermis (a thickness of 100 to 300 μm), and the dermis (a thickness of 1 mm or more) from the surface side and a resolution of 20 μm is not sufficient.

In contrast, a visible-light OCT apparatus that uses light from an ultraviolet range to a visible light range to improve resolution has been developed. An apparatus that can observe the aspect of the scattering of visible light in the skin is disclosed in, for example, JP2013-108766A, JP2015-163862A, and JP2007-523386A.

JP2013-108766A discloses a method in which super luminescent diode (SLD) light sources of each color generate low-coherent light of red (R), green (G), and blue (B) in the visible light range, foundation is applied to a skin replica, and the thickness of an uneven portion of the surface of the skin or a foundation layer is evaluated. However, since the RGB-OCT apparatus disclosed in JP2013-108766A is configured so as to comprise R, G, and B SLD light sources as the light sources in the visible light range, there are limitations in measurement at the wavelengths of the light sources provided in the apparatus and it is difficult to perform measurement in any color.

In contrast, since the OCT apparatus disclosed in JP2015-163862A has a light source unit including a white light source and a spectral shaping unit that cuts out any wavelength range, it is possible to acquire an optical coherence tomographic image using any wavelength in the visible light range.

In the field related to the human skin, a phenomenon called “yellowing” (Y. Ogura et al., J. Derm. Sci. 64 (2011) 45-52) in which the inside of the human skin turns yellow with advancing age has been known. It is considered that the phenomenon occurs due to, for example, the oxidation reaction of proteins in the dermis by the irradiation of the skin with ultraviolet rays. There is a demand for a measurement apparatus that can observe a color inside the skin.

There is a need to know a color inside an object not only in the field of the skin but also in various fields, such as systems including a vehicle body to which multiple layers of color materials are applied, art objects including paintings, food spoilage tests, medical diagnosis for the internal organs. Therefore, a strong demand for an apparatus that can acquire a full-color optical coherence tomographic image (hereinafter, referred to as an OCT image in some cases) of the inside of a measurement target and can be applied as a method for non-destructively quantifying a color inside a structure in a depth direction is expected.

However, for example, in JP2013-108766A and JP2015-163862A, the main intention is, for example, to compare simple OCT images to analyze the internal structure and the acquisition of a full-color image is not assumed.

In Francisco E. Robles, Christy Wilson, Gerald Grant & Adam Wax, “Molecular imaging true-color spectroscopic optical coherence tomography”, Nature Photonics 5, 744-747 (1 Dec. 2011), an OCT image is obtained by reproducing the color of the skin of a living mouse. Specifically, an OCT image is measured in a state in which the skin of the back of an albino mouse with an extremely thin pigment is thinly stretched and is interposed between chambers to be fixed and it is possible to acquire information at a relatively large depth greater than 130 μm.

In addition, JP2007-523386A discloses an apparatus that can acquire a full-color OCT image.

SUMMARY OF THE INVENTION

Measurement light emitted to the surface of a measurement target enters the measurement target, is scattered inside the measurement target, returns to the surface side, and is detected by a detector through an optical component such as a lens. In this case, in practice, the absorption of light by a pigment in the measurement target or the attenuation of light by scattering occurs. Therefore, even in a case in which a signal (interference light) based on scattered light from the inside of the measurement target can be detected, the color of the scattered light is a pseudo color including the influence of light absorption and is not a true color.

In Francisco E. Robles, Christy Wilson, Gerald Grant & Adam Wax, “Molecular imaging true-color spectroscopic optical coherence tomography”, Nature Photonics 5, 744-747 (1 Dec. 2011), the albino mouse with the skin which has a very thin pigment and is likely to transmit light is the measurement target and measurement is performed in a state in which the skin of the back of the mouse is stretched and fixed. Therefore, it is possible to capture an image with good quality. However, it is difficult to measure a deep region of, for example, the human skin or a coating film with a thick pigment, using the disclosed technique. In addition, it is considered that it is difficult to detect a true color. Further, in JP2007-523386A, the influence of, for example, the absorption of light by pigments in the measurement target is not considered. Therefore, even in a case in which a full-color OCT image can be acquired, the color of the full-color OCT image is just a pseudo color and it is difficult to observe a true color.

The invention has been made in view of the above-mentioned problems and an object of the invention is to provide an optical coherence tomography apparatus that can acquire a true color of the inside of a measurement target and a measurement method using the optical coherence tomography apparatus.

According to the invention, there is provided an optical coherence tomography apparatus comprising: a light source unit that emits low-coherent light with a red wavelength, low-coherent light with a green wavelength, and low-coherent light with a blue wavelength at the same time; a light splitting unit that splits the low-coherent light emitted from the light source unit into measurement light and reference light; a measurement light emission optical system that irradiates a measurement target with the measurement light; a multiplexing unit that multiplexes the reference light and light reflected from the measurement target in a case in which the measurement target is irradiated with the measurement light; an interference light detection unit that detects interference light of the reflected light and the reference light multiplexed by the multiplexing unit; and an image generation unit that generates an optical coherence tomographic image of the measurement target from the interference light detected by the interference light detection unit. The image generation unit calculates an attenuation related value related to attenuation of signal intensity of the interference light of the red wavelength, the green wavelength, and the blue wavelength in a first depth region, corrects the signal intensity of a second depth region deeper than the first depth region according to the attenuation related value to calculate a correction signal for the interference light, and generates a full-color optical coherence tomographic image using the correction signals calculated for each of the red wavelength, the green wavelength, and the blue wavelength.

In the optical coherence tomography apparatus according to the invention, the image generation unit may comprise: an attenuation constant calculation unit that calculates, as the attenuation related value, an attenuation constant for the signal intensity of the interference light of the red wavelength, the green wavelength, and the blue wavelength in the first depth region; and a signal correction arithmetic unit that corrects the signal intensity in the second depth region with the attenuation constant obtained by the attenuation constant calculation unit to calculate the correction signal.

The optical coherence tomography apparatus according to the invention may further comprise a spectral reflectance measurement unit that measures spectral reflectance of a surface of the measurement target. The image generation unit may comprise: a pigment concentration calculation unit that calculates a concentration of a pigment included in the first depth region from the spectral reflectance; and a signal correction arithmetic unit that calculates an amount of attenuation of light by the pigment as the attenuation related value on the basis of the concentration of the pigment obtained by the pigment concentration calculation unit and corrects the signal intensity in the second depth region to calculate the correction signal.

Here, the pigment concentration calculation unit may calculate a concentration of melanin as the pigment.

In the optical coherence tomography apparatus according to the invention, preferably, the red wavelength is 612 nm, the green wavelength is 537 nm, and the blue wavelength is 448 nm. Here, the wavelength is a peak wavelength of each color of the low-coherent light emitted from the light source unit.

In addition, the low-coherent light of each color emitted from the light source unit has a spectrum with a substantial Gaussian distribution shape having the peak wavelength as the center.

In the optical coherence tomography apparatus according to the invention, preferably, the interference light detection unit comprises an optical detector that detects interference light with the red wavelength, an optical detector that detects interference light with the green wavelength, and an optical detector that detects interference light with the blue wavelength, the optical detectors being separately provided.

The optical coherence tomography apparatus according to the invention may be a spectral-domain type or a time-domain type and is preferably the spectral-domain type in order to reduce the measurement time.

In particular, preferably, the optical coherence tomography apparatus according to the invention is the spectral-domain type having the following configuration. A first cylindrical lens that irradiates the measurement target with the measurement light in a linear shape is provided as the measurement light emission optical system. The optical coherence tomography apparatus further comprises a second cylindrical lens that is provided between the multiplexing unit and the interference light detection unit such that axes of cylinders of the first and second cylindrical lenses are orthogonal to each other. The interference light detection unit spectroscopically detects the interference light and the image generation unit converts a signal based on the interference light spectroscopically detected by the interference light detection unit into depth information using Fourier transform.

According to the invention, there is provided a measurement method using the optical coherence tomography apparatus according to the invention. The measurement method comprises: irradiating a measurement target with the measurement light; detecting the interference light of reference light and light reflected from the measurement target; generating an optical coherence tomographic image of the measurement target; displaying the optical coherence tomographic image on an image display device; calculating optical features on a surface of the measurement target or inside the measurement target from the interference light; and displaying the optical features on the image display device.

The optical features include, for example, intensity of reflected light at any position on the surface of the measurement target or inside the measurement target, a profile of the intensity of the reflected light in a depth direction, or an attenuation constant. Here, the intensity of the reflected light includes light caused by scattered light.

Examples of the measurement target include coating films, the human skin, plants, printed matters, paints which are not capable of being destroyed, and precious antique art objects.

Preferably, in a case in which the measurement target is the human skin, the optical coherence tomographic images of the human skin are generated before and after any cosmetic or medicine is applied to the human skin. Preferably, the optical features are calculated and the optical features and the optical coherence tomographic images before and after the application are displayed on the image display device. In addition, since it is difficult to fix the human skin so as not to move in micron order within the measurement time, it is preferable to perform measurement with one shot.

According to the optical coherence tomography apparatus of the invention, the image generation unit calculates the attenuation related value related to the attenuation of the signal intensity of the interference light of the red wavelength, the green wavelength, and the blue wavelength in the first depth region, corrects the signal intensity in the second depth region deeper than the first depth region according to the attenuation related value to calculate the correction signal for the interference light, and generates the full-color optical coherence tomographic image using the correction signals calculated for each of the red wavelength, the green wavelength, and the blue wavelength. Therefore, it is possible to acquire a true color of the inside of the measurement target.

