X-ray detectors that we developed utilizing polycrystalline CdZnTe films exhibited superior sensitivity, but inadequate temporal response and output uniformity for medical imaging purposes. In order to improve those deficiencies, we tested new procedures for deposition and post-deposition chemical-heat treatment of polycrystalline CdZnTe films, in addition to investigating new device structures. We doped the polycrystalline CdZnTe films with Cl in a new manner so as to achieve effective grain boundary passivation. Polycrystalline CdZnTe films that were Cl-doped by our new procedure were found to have a finer and more uniform grain structure. We fabricated and evaluated devices with a replaced barrier layer against charge injection under negative bias. All these measures helped reduce the temporal lag of a 300 μm thick polycrystalline CdZnTe film exposed to X-ray irradiation. Moreover, utilizing this film in a detector reduced the detector's output uniformity. We have succeeded in improving the X-ray temporal response and output uniformity of a 300 μm thick CdZnTe film. This study will also discuss countermeasures against a number of problems that were encountered, including MTF degradation and short range image lag.
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.
We made a prototype flat-panel X-ray detector with a polycrystalline CdZnTe film, and evaluated its imaging performance with respect to leakage current, X-ray sensitivity, MTF, DQE and image lag. The detector incorporates a novel hybrid technique in which zinc-doped CdTe is pre-deposited onto a ceramic substrate and then connected to a TFT circuit substrate. We carefully selected the material for the sensor substrate in order to avoid both incident x-ray attenuation in the substrate and micro-cracks in CdZnTe film. The film thickness was approximately 300 micrometers . The imaging area is composed of 512 X 384 pixels, with a pixel pitch of 150 micrometers .
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.
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