1. Field of the Invention
The present invention relates to particularly an optical coherence tomography apparatus including an interference optical system which is used in the medical field, an optical coherence tomography method, an ophthalmic apparatus, a method of controlling the ophthalmic apparatus, and a storage medium.
2. Description of the Related Art
Currently, various types of ophthalmic apparatuses using optical devices are used. Such apparatuses include, for example, an anterior ocular segment imaging apparatus, a fundus camera, and a scanning laser ophthalmoscope (SLO). Among them all, an optical coherence tomography (OCT) apparatus (to be referred to as an “OCT apparatus” hereinafter) is an apparatus capable of obtaining a high-resolution tomogram of an object to be examined. This OCT apparatus has been becoming an indispensable apparatus for dedicated retinal outpatient clinics.
For example, the OCT apparatus disclosed in Japanese Patent Laid-Open No. 11-325849 uses low-coherent light as a light source. Light from the light source is split into measurement light and reference light through a splitting optical path such as a beam splitter. Measurement light is light to irradiate an object to be examined such as the eye through a measurement light path. Return light of this light is guided to a detection position through a detection light path. Note that return light is reflected light or scattered light containing information associated with an interface relative to the irradiation direction of light on the object. On the other hand, reference light is light to be guided to the detection position through a reference light path by being reflected by a reference mirror or the like. It is possible to obtain a tomogram of an object to be examined by causing interference between this return light and reference light, collectively acquiring wavelength spectra by using a spectrometer or the like, and performing Fourier transform of the acquired spectra. An OCT apparatus which collectively measures wavelength spectra is generally called a spectral domain OCT apparatus (SD-OCT apparatus).
In an SD-OCT apparatus, a measurement depth Lmax is represented, as an optical distance Lmax, by a pixel count N of the image sensor of a spectrometer and a spectrum width ΔK of the frequency detected by the spectrometer according to equation (1). Note that the spectrum width ΔK is represented by a maximum wavelength λmax and a minimum wavelength λmin. The pixel count N is often an even number, and is generally the factorial of 2, that is 1024 or 2048.
                                                                                          L                  max                                =                                  ±                                      N                                          4                      ⁢                                                                                          ⁢                      Δ                      ⁢                                                                                          ⁢                      K                                                                                                                                                                Δ                  ⁢                                                                          ⁢                  K                                =                                                      1                                          λ                      min                                                        -                                      1                                          λ                      max                                                                                                          }                            (        1        )            
If, for example, a central wavelength of 840 nm, a band of 50 nm, and a pixel count of 1024 are set, λmax=840+50/2=840+25=865 nm, λmin=840−50/2=840−25=815 nm, and N=1024. In this case, optical distance Lmax=3.6 mm. That is, it is possible to perform measurement up to about 3.6 mm on the plus side relative to the coherence gate. The coherence gate is the point at which a reference light path coincides with an optical distance in a measurement light path. When a desired region (a distance in the depth direction) is sufficiently smaller than 3.6 mm (for example, 1 mm or less), the measurement depth can be reduced by decreasing the pixel count of the spectrometer. Decreasing the pixel count is important in order to speed up processing and reduce the data amount. This is because, when measuring a three-dimensional image of the retina, it takes much measurement time and produces a large amount of data. When an object to be examined is a moving object like the eye, in particular, it is required to further shorten the measurement time.
On the other hand, changing the pixel count of a spectrometer is equivalent to changing the resolution of the spectrometer. A problem in this case will be described with reference to FIG. 1. FIG. 1 is a graph obtained by plotting, for each spectrometer resolution, the light intensity measurement results obtained when the position of the coherence gate is moved while a mirror is located at the position of an object to be examined. The ordinate corresponds to the light intensity, and the abscissa to the distance. With an increase in distance from the coherence gate, light intensity attenuation called Roll-Off occurs. The degree of attenuation of a light intensity Int mainly depends on the resolution of a spectrometer and the pixel count of an image sensor. Letting x be a distance variable and a be a coefficient proportional to the resolution of the spectrometer, the degree of attenuation is proportional to a sinc function given by
                    Int        ∝                              sin            ⁢                                                  ⁢            2            ⁢                                                  ⁢            π            ⁢                                                  ⁢            x            ⁢                                                  ⁢            α                                π            ⁢                                                  ⁢            x                                              (        2        )            
As is obvious from FIG. 1, as a value indicating a resolution increases (from 0.1 nm to 0.2 nm, 0.5 nm, and 1.0 nm), the cycle in which plotted points approach zero is shortened. As described above, images formed from spectrum data from spectrometers having different resolutions differ in light intensity in the depth direction. Differences in light intensity are differences in image contrast. This makes images in the same region look different. That is, with spectrometers having different resolutions, obtained images look different.
In consideration of the above problems, the present invention provides a technique of correcting the contrast differences between images which are caused when wavelength resolutions differ (spectrometers differ in resolution in the case of an SD-OCT) in an FD-OCT apparatus such as an SD-OCT apparatus.