Research and Development

Since the quantum noise of the X-ray is dominant as the image granularity of an X-ray image, use of a highly X-ray absorbent material as a phosphor is important to the improvement of the image granularity.

The cesium bromide (hereinafter referred to as "CsBr") has a high absorption coefficient in a wider X-ray energy area, and has excellent properties as a fluorescent material. Further, CsBr allows formation of a layer by vapor deposition. This is expected to ensure improvement of the fill factor -- another advantage of this technology.

Fig.1 Light guide effects of a pillar structure

In the conventional CR(Computed Radiography) plate using a granular type phosphor, the phosphor particles are dispersed on a resin binder, and layer formation is made by coating. When this plate produced by coating is used, light scattering occurs on the interface between the phosphor particles and resin, and image sharpness is reduced by the scattering of the scanning laser beam. Further, due to the light scattering inside the layer, a loss easily tends to occur to the photostimulable luminescence in response to the X-ray information captured by the lower layer of the plate, when detected by a light receiving section on the surface layer. By contrast, in the vapor deposition plate, the phosphor crystal is formed in the shape of a column. Thus, the intra-layer scattering of LD(Laser Diode) beam for scanning is reduced by the light guiding effect; further, this pillar-shaped structure ensures effective introduction of the photostimulable luminescence to the light receiving section. This arrangement is a very effective technological feature that improves the image quality of the CR plate (Fig. 1).

Fig.2 Sectional scanning electron micrograph view of CsBr phosphor

Fig. 3 Transmittance images of phosphor layer

In the layer formation of the phosphor, vapor deposition conditions as such as a degree of vacuum and substrate temperature are adjusted in such a way that a phosphor will be formed in a pillar-shaped crystal structure according to the vacuum vapor deposition. A heat-resistant 17 x 17-inch smooth substrate is installed in the large-sized chamber. While adjusting the vapor deposition conditions such as a degree of vacuum and substrate temperature, the vapor deposition material is heated whereby uniformity in the vapor-deposited area is achieved. The thickness of the phosphor layer is determined while adjusting the sharpness of the image performance and the granularity, in such a way that the layer thickness will be 1.5 times that of the coated plate formed of a granular phosphor.

To examine the crystal shape of the CsBr formed into a layer, the cross section of the phosphor layer was observed using a scanning electron microscope. It has been confirmed that the CsBr phosphor forms a pillar-shaped structure, as shown in Fig. 2.

Further, the phosphor layer was separated from the substrate and a spot light was applied from the back. Then it was examined from the front side. It has been revealed that a clearer image is provided by the vapor deposition plate than the coated plate (Fig. 3). This suggests a high degree of light guiding properties of the CsBr phosphor layer.