1. Field of the Invention
The present invention relates to a radiation phase contrast imaging apparatus using a grating.
2. Description of the Related Art
A method for detecting a variation of wavefront arising from a difference in refractive index of media in a transmission path of an X-ray incident on a subject using an X-ray Talbot interferometer to generate an image of the subject is proposed as described, for example, in U.S. Pat. No. 7,180,979.
The method described above has a potentiality to utilize a corn beam X-ray source or the like, as opposed to other phase imaging methods that uses a crystal analysis body which basically requires parallel monochrome X-rays.
The method using a Talbot interferometer uses, for example, a microfocus X-ray source because it requires a high degree of spatial coherence. Such an X-ray source can yield quite a small amount of X-rays in a short exposure time of several seconds, so it necessitates a longer time of X-ray exposure in comparison with a conventional radiography technique. Thus, the method poses a problem that a favorable phase image can not be obtained due to mechanical instability, body movement of the subject, or the like during the exposure period for detecting a subtle variation of the wavefront.
Consequently, in order to solve the problem, a method that uses a Talbot-Lau interferometer, which is a modification of the method using a Talbot interferometer, is proposed as described, for example, in F. Pfeiffer et al., “Phase retrieval and differential phase-contrast imaging with low-brilliance X-ray sources”, Nature Physics 2, pp. 258-261, 2006. The method secures a sufficient X-ray yield in a short time by disposing a multi-slit unit just after the focus of a relatively large focus X-ray source to form a new array of microfocus X-ray sources with each slit as an independent radiation point.
In the method using a Talbot-Lau interferometer, however, it is necessary to make the focus size of the X-ray source sufficiently small relative to the pitch of the first grating in order to ensure a high degree of spatial coherence. Consequently, it is necessary to form a very small slit using a member having a low X-ray transmittance, and it has been very difficult to manufacture such multi-slit units.
When a Talbot-Lau interferometer is configured by disposing a first grating and a second grating from the X-ray source side, the multi-slit unit and gratings need to have high dimensional accuracy and each optical element needs to be geometrically arranged highly accurately.
In view of the circumstances described above, it is an object of the present invention to provide a radiation phase contrast imaging apparatus capable of acquiring a radiation phase contrast image without using a multi-slit unit like that described above.
It is a further object of the present invention to provide a radiation phase contrast imaging apparatus capable of acquiring a radiation phase contrast image without requiring highly precise gratings and geometrical arrangement of each optical element.