When X-rays transmit through an object, an X-ray image is formed (Fig. 1). This is the principle of X-ray imaging, and is called the absorption contrast On the other hand, because X-rays are electromagnetic waves similar to visible light rays, when X-rays transmit through an object, the X-ray phase changes. This phase change is generally observed as a refraction or interference. The X-ray intensity difference or image contrast based on this phase change is the phase contrast.
You can see that this phase contrast image has better sharpness compared to the images taken by the conventional contact photographing. You can see a black line at the boundary with the air, and the outer edges of air bubbles inside the tube are shown in white.
It has bee said so far that the phase contrast is generated by X-ray interference. With the X-ray tubes used for medical treatments, it was considered that the phase contrast disappears by geometrical unsharpness in macrophotography that keeps the X-ray detector apart form the subject during photographing.
Under the assumption that x rays do not interface, we have found from a geometric optical theoretical analysis that the phase contrast can be obtained with a small focal point of 0.1mm of a Mo anode X-ray tube for mammography, when the distance from that focal point to the subject is 0.5m or more, and the distance from the subject to the X-ray detector is 0.25m or more.
Fig. 3 shows an example of a clinical image (provided by Shiga University of Medical Science). When we compare the PCM image (left) with the conventional SF image (right) photographed using the same subject dose, the breast structure is shown much clearly in the PCM image. And, as a result of examining many clinical images, the representation of fine calcification images--so called the mission of mammography--was evaluated to be better than conventional SF images.
We strongly hope that this system will contribute to early detection of breast cancers in the future.
A system developed by applying the phase contrast technology is the digital PCM system. It consists of a mammography unit (MERMAID), a computed radiography unit (REGIUS190), and a dry silver salt image printer (DRYPRO793).
This system uses the phase contrast technology first time in the world. Output images have the similar resolution as that of the conventional SF system. Furthermore, images are output to dry film with the maximum density of 4.0.