As an imaging apparatus for nondestructively observing the inside of a sample by using X-rays, there are an absorption contrast X-ray imaging apparatus using changes in an intensity caused by a sample as contrast, and a phase contrast X-ray imaging apparatus using phase-shift caused by a sample as contrast. The former absorption contrast X-ray imaging apparatus mainly comprises an X-ray source, a sample positioning mechanism and a detector. The X-rays emitted from the X-ray source are irradiated on a sample positioned by the sample positioning mechanism, and detected by the detector after passing through the sample. Thus an image constituting the changes in the intensity caused by absorption of the sample as contrast was obtained. Since a measuring principle and a structure of the apparatus are relatively simple, this imaging technique is widely used in many fields including a medical diagnostic field. In the case of two dimensional observation, it is called roentgen, and in the case of three dimensional observation by a Computed Tomography (CT) scanner, it was called X-ray CT.
On the other hand, there are an apparatus described in the Non-Patent Document 1 (Appl. Phys. Lett. 6, 155 (1965)), an apparatus disclosed in the Patent Publication Document 1 (JP-A-10-248833) and the like as the latter phase contrast X-ray imaging apparatus. In general, the phase-shift caused by a sample is remarkable large as compared with changes in the intensity, so the phase contrast X-ray imaging apparatus has an advantage in that the sensitivity is higher than that of the absorption contrast X-ray imaging apparatus. For this reason, even if an imaging target is biological soft tissue composed mainly of light elements such as oxygen and carbon, which have been difficult to observe by the absorption contrast X-ray imaging apparatus, the phase contrast X-ray imaging apparatus makes it possible to observe the inside structure of it with low X-ray exposures and without using contrast agents nondestructively and in a state of high-sensitivity.
The phase contrast X-ray imaging apparatus as described above is constituted by adding an X-ray interferometer such as a Bonse-Hart interferometer (described in Non-Patent Document 1 (Appl. Phys. Lett. 6, 155 (1965)), or an interferometer (described in Non-Patent Document 2 (J. Appl. Cryst. 7, 593 (1974)) which is composed by two crystal blocks, to an X-ray source, a sample positioning mechanism and a detector. The Bonse-Hart interferometer, as shown in FIG. 1, has three wafers (beam splitter 1, mirror 2 and analyzer 3) arranged in parallel and at equal intervals on a crystal block fabricated from a perfect crystal ingot. An incident X-ray 4 is split into two beams such as a beam 5 and a beam 6 by the beam splitter 1, the beams are reflected by the mirror 2 and are combined on the analyzer 3 to form two interference beams 7 and 8. When a sample 9 is positioned in an optical path of the beam 5 or the beam 6, the phase shift caused by the sample 9 appears as the changes in the intensities of interference beams 7 and 8 by the principle of superposition (interference pattern). The phase map (spatial distribution of phase-shift caused by the sample) was calculated from the interference patterns detected by an image detector.
Furthermore, an imaging apparatus enabling the three dimensional nondestructive observation by combining the phase contrast imaging method and the method for normal X-ray CT, is disclosed in Patent Publication Document 2 (JP-A-4-348262) etc. In this case, the X-ray is irradiated on the sample from a plurality of directions different from each other in the same way as that of the normal X-ray CT, and a cross sectional image of the sample is reconstructed from the phase contrast projection images obtained for respective projections.
The X-ray is approximately transparent for the light elements such as oxygen and carbon, and almost all of incident X-rays are passed through the object. Therefore, the change in the intensity caused by absorption of a sample is extremely small, and it is difficult to perform the fine observations of a sample mainly composed by the light elements such as biological soft tissues and organic material etc. by the absorption contrast X-ray imaging apparatus. In order to improve the sensitivity, the contrast agents and/or an extension of exposure time is tried to be used. However, in this case, some problems that a position can be observed is limited and/or an X-ray exposure is increased, has been occurred.
On the other hand, though the sensitivity of the phase contrast X-ray imaging method is satisfactorily sufficient, the phase-shift α generated by the sample is detected as a wrapped value α′ (α′=α−Int(α/2π)*2π) in its region of 0−2π, as shown in FIG. 2. Therefore, a complicated calculation called a phase unwrapping method (described in JP-A-2001-153797) is required to obtain the true phase-shift α. Furthermore, in a region where a shape and an internal structure of the sample is complicated and its density is rapidly varies spatially, an optical path deviates from an original optical path by X-ray diffraction, the visibility of an interference pattern is lowered or an interference fringes are disappeared. As a result, the unwrapping processing cannot be performed normally and the phase-shift α cannot be obtained accurately. To avoid this problem, a method that the sample is placed in a sample cell filled liquid to reduce a difference of density between the sample and its periphery, is disclosed in JP-A-7-209212. An influence of the shape of the sample can be reduced by this method, but, the influence of the rapid change in the density inside the sample is not avoidable.
As evident from the above description, a sensitivity range of conventional absorption X-ray imaging and a sensitivity range of the phase contrast X-ray imaging are separated into both ends as shown in FIG. 3. For the sample including an organ of large density change such as the bone or the lung and an organ of small density change such as biological soft tissue, neither of the imaging methods can observe the sample with satisfactory sensitivity.
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