Conventionally, as a radiation image capturing device for imaging an internal structure of an object by making radiation transmit through the object, various devices have been proposed. A commonly-used radiation image capturing device is configured to capture a radiation projection image by irradiating an object with radiation to make the radiation transmit through the object. In such a projection image, shading appears depending on the ease of permeation of radiation, which represents the internal structure of the object.
With such a radiation image capturing device, it only can capture an image of an object having properties capable of absorbing radiation to some extent. For example, soft biological tissues hardly absorb radiation. Even if it is tried to capture an image of such a tissue with a general device, nothing is reflected on the projection image. When trying to capture an image of an internal structure of an object that does not absorb radiation as described above, there is a theoretical limit in a general radiation image capturing device.
Under the circumstances, a radiation phase-contrast image capturing device for imaging an internal structure of an object by utilizing a phase-contrast of transmitted radiation has been proposed. Such a device is configured to image an internal structure of an object by utilizing Talbot interference.
FIG. 17 illustrates a radiation phase-contrast image capturing device. The radiation phase-contrast image capturing device is provided with a radiation source configured to irradiate radiation, a multi-slit configured to align phases of the radiation, a phase grating with a pattern of a streak form, and a detector configured to detect radiation. In the device of FIG. 17, an object may be positioned in between the phase grating and the detector. The multi-slit is configured such that vertically extended slits are arranged in a lateral direction. The phase grating is configured such that vertically extended shielding lines which are less likely to transmit radiation are arranged in a lateral direction.
The principle of a radiation phase-contrast image capturing device will be briefly explained. When phase-matched radiation is irradiated to the phase grating, a self-image of the phase grating appears at a position away from the phase grating by a specific distance (Talbot distance). The detector is adjusted in position with respect to the phase grating so that the self-image is reflected. This self-image looks like an image in which a shadow of the phase grating is projected. However, it should be noted that the self-image is not a simple projection but results from an interference fringe caused by radiation interference.
When an object is placed between a phase grating and a detector, the radiation emitted the phase grating will transmit through the object before being detected by the detector. The self-image appearing on the detector at this time is slightly disturbed by transmitting through the object. This disturbance is due to the phase shift caused while the radiation transmits through the object.
By detecting the distorted self-image with the detector and subjecting the self-image to predetermined image processing, an image showing the phase-contrast distribution of the radiation transmitted through the object can be generated. Such an image is referred to as a transparent image. According to the radiation phase-contrast image capturing device, a transparent image representing the internal structure of the object can be generated even for objects that do not absorb radiation.
A detector used for a radiation phase-contrast image capturing device is expensive. This is because the self-image has a very fine in pattern and therefore the detection elements of the detector are required to be miniaturized in order to capture the self-image. The self-image has a stripe pattern composed of regularly arranged dark lines, but the arrangement pitch of the dark lines cannot be freely changed. The self-image is an image caused by interference of light. Therefore, the arrangement pitch of shielding lines arranged in a phase grating is determined by the wavelength of radiation. Considering the necessity to make radiation transmit through the object, it is necessary to set the wavelength of radiation to be considerably short, and the arrangement pitch of shielding lines in the phase grating becomes small correspondingly. Therefore, the arrangement pitch of the dark lines of the self-image becomes narrow. In order to detect such a fine image, a detector higher in the spatial resolution is required. Such a detector is expensive.
Under the circumstances, in order to make the detector less expensive, devices that perform scanning image capturing have been proposed. That is, as shown in FIG. 18, a self-image of an object may be obtained by performing image capturing a plurality of times while moving the detector with respect to the object. In FIG. 18, the detector is too small to capture the entire image of the object in the image capturing visual field in one image capturing. However, if one self-image is obtained by performing image capturing, for example, three times while moving the detector and combining three pieces of the self-image obtained at this time, even if the size of the detector is reduced, a self-image of the absorption grating covering the entire object can be obtained. If the size of the detector is small, the production cost of the device can be suppressed accordingly.
However, with the configuration as shown in FIG. 18, unnecessary exposure to the object occurs. The spread of radiation output from the radiation source is wide so that the radiation is able to reach the entire area of the object. However, as shown in FIG. 18, in the case of scanning image capturing, it is impossible to perform image capturing of the entire area of the object at one time, so the image capturing is performed three times separately. For example, in the case of the 1st image capturing, as shown in FIG. 19, the radiation necessary for capturing the self-image is only a part of the radiation which spreads from the radiation source downward in the figure and is incident on the detector. The other part of the radiation which spreads from the radiation source upward and is shown with hatched lines in FIG. 19 will not be detected with the detector. Therefore, the radiation indicated with the hatched lines is the so-called useless radiation irradiated to the object even though it does not contribute to the image capturing. In cases where the object has physical properties which deteriorate due to radiation irradiation or in cases where the object is an organism, such unnecessary radiation irradiation should be avoided.
In order to avoid that the object is irradiated with unnecessary radiation during the scanning image capturing, it is only necessary to provide a collimator that absorbs radiation (see, for example, Japanese Unexamined Patent Application Publication No. 2012-24339, hereby incorporated by reference).
Even in the case of not performing scanning image capturing, there is a case in which it is better to provide a collimator. There is a case in which it is desirable to provide a mode for capturing an image of only a part of the object. In such cases, it is better to provide a collimator so that the part not related to the image capturing is not irradiated with radiation.
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