Yoshihiro Izumi

Experimental evaluation of a-Se and CdTe flat-panel x-ray detectors for digital radiography and fluoroscopy

Described are two types of direct-detection flat-panel X-ray detectors utilizing amorphous selenium (a-Se) and cadmium telluride (CdTe). The a-Se detector is fabricated using direct deposition onto a thin film transistor (TFT) substrate, whereas the CdTe detector is fabricated using a novel hybrid method, in which CdTe is pre-deposited onto a glass substrate and then connected to a TFT substrate. The detector array format is 512 X 512 with a pixel pitch of 150 micrometer. The imaging properties of both detectors have been evaluated with respect to X-ray sensitivity, lag, spatial resolution, and detective quantum efficiency (DQE). The modulation transfer functions (MTFs) measured at 1 lp/mm were 0.96 for a- Se and 0.65 for CdTe. The imaging lags after 33 ms were about 4% for a-Se and 22% for CdTe. The DQE values measured at zero spatial frequency were 0.75 for a-Se and 0.22 for CdTe. The results indicate that the a-Se and CdTe detectors have high potential as new digital X-ray imaging devices for both radiography and fluoroscopy.

Large-area deposition of a polycrystalline CdZnTe film and its applicability to x-ray panel detectors with superior sensitivity

This paper describes our investigation of the X-ray detective characteristics of a thick polycrystalline CdZnTe film deposited on a large-area substrate. We deposited a polycrystalline CdZnTe film on a 9 inch by 9 inch substrate, and investigated its quality. It was verified to be quite uniform within the substrate. We also cut the film and connected it to a 3 inch by 3 inch TFT panel for evaluating the X-ray imaging performance. The TFT array format was 512 by 512 pixels with a pixel pitch of 150 micrometers . The thickness of the CdZnTe film was about 300 micrometers after lapping and polishing, and the film density per unit area was higher than 170 mg/cm2. The average sensitivity was 1.5E9 e-/mR/mm2; the beam condition was 80 kV with 26-mm Al filtration. The MTF measured at 1 lp/mm was 0.82. The time response and uniformity of X-ray sensitivity were not still adequate, and further improvements are in progress. In conclusion, we have demonstrated the applicability of the polycrystalline CdZnTe film to a large-area detector, although further investigations and improvements are needed.