Dark Current and DQE Improvements of Mercuric Iodide Medical Imagers

摘要

A new TFT array has been developed specifically for mercuric iodide (HgI_2) deposition. This new TFT array combined with a modified HgI_2 Physical Vapor Deposition (PVD) process provides less than 10 Pa/mm~2 dark current at room temperature (22℃) measured at 1 V/μm electrical field. This photoconductor (direct) imager was run at 10 fr/s framerate and gave a measured sensitivity of 19 μC/(R~*cm~2) using a RQA5 radiation quality x-ray beam (70kVp x-ray with 21 mm Al filtering). This sensitivity value is higher than the sensitivity reported by our group for any previous HgI_2 imagers. MTF, NPS and DQE values were also evaluated on this 13 cm × 13 cm size imager with 127 μm pixel pitch. The MTF value is higher than 40% at the Nyquist frequency (3.9 lp/mm). This is much better than the MTF of a 600 urn CsI scintillator/photodiode (indirect) imager, which is only 16% (Varian internal data) and it is similar to the MTF value of the a-Se (another photoconductor) imagers. The first frame image lag is less than 8% when the imager was run at a 10 fr/s framerate. The low dark current and some noise reduction in the detector electronics, made it possible for the DQE to be measured down to low fluoroscopic dose levels ( < 4 μR/fr). The DQE(0) value is over 50% at a dose of 35 μR/fr and still about 40% at 3.76 μR/fr. The 270 μm thick PVD HgI_2 layer only absorbs less than 75% of the ～51 keV mean energy X-ray photons (70 kVp RQA5 filtered beam). This means that if the thickness of the HgI_2 layer is increased to 500 μm (increasing the absorption up to over 90%) the DQE(0) should then increase to about 60- 65% (assuming everything else remains unchanged). This value is close to the 65 - 70 % DQE(0), measured for the indirect (CsI) imagers at higher doses. Such a high DQE value makes this material competitive both for fluoroscopic and for radiographic applications.