(1) Field of the Invention
The present invention relates to X-ray imaging apparatuses and methods, or more particularly, to an apparatus and method for non-destructively examining the inside of an entity.
(2) Description of the Related Art
Imaging apparatuses to be used to non-destructively observe the inside of a sample using X-rays include an absorption contract X-ray imaging apparatus that utilizes a change in the intensity of X-rays caused by the sample and a phase contrast X-ray imaging apparatus that utilizes a change in the phase (phase shift) of X-rays.
The former absorption contrast X-ray imaging apparatus is composed mainly of an X-ray source, a sample positioning mechanism, and a detector. X-rays emitted from the X-ray source are irradiated to a sample positioned by the sample positioning mechanism, and X-rays transmitted the sample are detected by the detector. The absorption contrast X-ray imaging apparatus produces an image in which a change in the intensity of X-rays caused by absorption of the sample is shown as an image contrast. Since the principles of measurement and the configuration of the apparatus are relatively simple, the absorption contrast X-ray imaging apparatus is widely used in many fields including a field of medical diagnosis by the name of an X-ray system in which projection data is acquired for two-dimensional observation and by the name of an X-ray CT system in which computed tomography (CT) is performed for three-dimensional observation.
On the other hand, the latter phase contrast X-ray imaging apparatus requires, in addition to the above components, a means for detecting a phase shift. Compared with the absorption contrast X-ray imaging apparatus, the phase contrast X-ray imaging apparatus offers very high sensitivity. Therefore, the phase contrast X-ray imaging enables observations of biological soft tissues without usage of contrast agents and without harmful X-ray exposure. This is because phase shift cross section of a light element is approximately one thousand times larger than absorption cross section
Means for detecting a phase shift include, as described in Physics Today (vol. 53, 2000, 23), (1) a method that is disclosed to employ an X-ray interferometer in Japanese Patent Application Laid-Open Nos. 4-348262 and 10-248833, (2) a method that is described to detect an angle of refraction of X-rays using an analyzer crystal in the brochure for the PCT International Publication WO95/05725 and Japanese Patent Application Laid-Open No. 9-187455, and (3) a method utilizing Fresnel diffraction. Among the methods, the method (1) directly detects a phase shift and therefore offers the highest sensitivity. The method (1) having relation to the present invention will be described below.
Japanese Patent Application Laid-Open No. 4-348262 describes a configuration including an X-ray source, a sample positioning mechanism, a detector, and an X-ray interferometer such as a Bonse-Hart interferometer (described in Appl. Phys. Lett. (vol. 6, pp. 155, 1965)) or an interferometer having the Bonse-Hart interferometer divided into multiple crystal blocks (described in J. Appl. Cryst. (vol. 7, pp. 593, 1974)).
FIG. 1 is a perspective view schematically showing the configuration of a Bonse-Hart interferometer. The Bonse-Hart interferometer is formed as one crystal block cut out from a single-crystal ingot monolithically, and that has three wafers equidistantly juxtaposed in parallel with one another (serving as a beam splitter 1, a mirror 2, and an analyzer 3). Incident X-rays 4 are split into two beams of a beam 5 and a beam 6 by the first wafer (beam splitter 1), reflected from the second wafer (mirror 2), and recombined by the third wafer (analyzer 3). Consequently, two interference beams 7 and 8 are formed. If a sample 9 is positioned in the path of the beam 5 or 6, a change in the phase of the beam caused by the sample changes in the intensities of the interference beams 7 and 8 due to superposition (interference) of the waves. By taking advantage of the principle, an image showing a change in a phase (a phase contrast image) can be obtained by the changes in the intensities of the interference beams 7 and 8 are detected by an image detector or the like.
Imaging apparatuses in which the phase contrast imaging method and an ordinary X-ray CT technique are combined in order to enable three-dimensional non-destructive observation, include an imaging apparatus described in Japanese Patent Application Laid-Open No. 4-348262. Similarly to ordinary X-ray CT, X-rays are irradiated to a sample in multiple directions, and a phase contrast tomographic image of the sample is reproduced by computing using respective projection data sets.
Light elements, such as oxygen or carbon, are nearly transparent to X-rays, and almost incident X-rays are not absorbed by the light element. Therefore, a change in an intensity derived from absorption by a subject is very small, and observation with high sensitivity of biological soft tissues or organic materials are difficult by using absorption contrast X-ray imaging method. In efforts to compensate insufficient sensitivity, a contrast agents is used or an exposure time is extended. However, this induces a problem in that a region capable of being imaged is limited or an exposure increases.
On the other hand, the sensitivity of phase contrast X-ray image is sufficiently high, but complex computation called phase unwrapping is needed to obtain phase contrast images. As shown in FIG. 2, a change α in a phase caused by a sample is detected as a value α′ rounded off (wrapped) to fall within 0 to 2π (α′=α−Int(α/2π)×2π). Therefore, it is required a process for restoring the true change a in a phase (phase unwrapping) using a calculation method described in, for example, Japanese Patent Application Laid-Open No. 2001-153797. Furthermore, X-rays are refracted in a region of a sample in which the density is spatially abruptly varied caused by the complex shape or the internal structure of the sample. The X-rays are deviated from their original optical path, and superposed on a beam other than a reference beam as described later with reference to FIG. 4B and FIG. 4C. Since the coherence length of X-rays is so short as to range from several micrometers to several tens of micrometers, this deviation induces a decrease in sharpness (visibility) of an interference pattern or disappearance of interference fringes. And the unwrapping cannot be performed normally, and the change α cannot be accurately restored.
In efforts to avoid the foregoing problem, an example like the one described in Japanese Patent Application Laid-Open No. 7-209212 may be immersed in a liquid in order to decrease the difference in density between the sample and its surroundings. In this case, the influence of a shape can be minimized but an rapid change in density inside the sample cannot be coped with. Moreover, an object to be measured is limited to a specific one.
FIG. 3 is an explanatory diagram of observable regions of the conventional absorption, phase contrast X-ray imaging methods, and the present invention. According to the conventional absorption and phase contrast X-ray imaging methods, a region of a density change in a sample to which each of the X-ray imaging methods is sensitive is limited to an extreme region of large or small values. This indicates that the X-ray imaging methods cannot enable observation at a high density resolution of a sample in which a region exhibiting a large density change, for example, bones and lungs, and a region exhibiting a small density change, such as, biological soft tissues are mixed.