17 March 2007 Dark current and DQE improvements of mercuric iodide medical imagers
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A new TFT array has been developed specifically for mercuric iodide (HgI2) deposition. This new TFT array combined with a modified HgI2 Physical Vapor Deposition (PVD) process provides less than 10 pA/mm2 dark current at room temperature (22 °C) measured at 1 V/&mgr;m electrical field. This photoconductor (direct) imager was run at 10 fr/s framerate and gave a measured sensitivity of 19 μC/(R*cm2) 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 HgI2 imagers. MTF, NPS and DQE values were also evaluated on this 13 cm x 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 μm 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 HgI2 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 HgI2 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.
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G. Zentai, L. Partain, R. Pavlyuchkova, "Dark current and DQE improvements of mercuric iodide medical imagers", Proc. SPIE 6510, Medical Imaging 2007: Physics of Medical Imaging, 65100Q (17 March 2007); doi: 10.1117/12.713848; https://doi.org/10.1117/12.713848

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