Systematic investigation of the signal properties of polycrystalline HgI₂detectors under mammographic, radiographic, fluoroscopic and radiotherapy irradiation conditions

Abstract

The signal properties of polycrystalline mercuric iodide (HgI2) film detectors, under irradiation conditions relevant to mammographic, radiographic, fluoroscopic and radiotherapy x-ray imaging, are reported. Each film detector consists of an approximately 230 to approximately 460 microm thick layer of HgI2 (fabricated through physical vapour deposition or a screen-print process) and a thin barrier layer, sandwiched between a pair of opposing electrode plates. The high atomic number, high density and low effective ionization energy, W(EFF), of HgI2 make it an attractive candidate for significantly improving the performance of active matrix, flat-panel imagers (AMFPIs) for several x-ray imaging applications. The temporal behaviour of current from the film detectors in the presence and in the absence of radiation was used to examine dark current levels, the lag and reciprocity of the signal response, x-ray sensitivity and W(EFF). The results are discussed in the context of present AMFPI performance. This study provides performance data for a wide range of potential medical x-ray imaging applications from a single set of detectors and represents the first investigation of the signal properties of polycrystalline mercuric iodide for the radiotherapy application.

Signal behavior of polycrystalline HgI2at diagnostic energies of prototype, direct detection, active matrix, flat-panel imagers

“…The two major types of barrier layer used, labeled A and D, were based on different polymers and correspond to those reported in an earlier study of the signal properties of simple, non-pixelated, film detectors. (Su et al 2005) The variations in barrier type D (designated by number codes following the letter, see table 2) correspond to deliberate changes in conductivity. (The manufacturer declined to disclose further information about the barrier layers for proprietary reasons.…”

“…These behaviors are similar to those reported in earlier studies involving simple polycrystalline HgI 2 detectors. (Su et al 2005) Since higher dark current more rapidly consumes pixel signal capacity, as well as increases shot noise, the selection of electric field strength therefore generally involves a tradeoff between high x-ray signal and low dark current. In the present study, the value of electric field strength was generally chosen based on consideration of this tradeoff.…”

“…With a high atomic number and mass density, relatively high detection efficiencies at diagnostic energies can be achieved with film thicknesses of only 300 to 500 μm. (Su et al 2005) Moreover, recent investigations of polycrystalline HgI 2 have demonstrated that this material offers an attractively low effective work function, W EFF -(In the present context, W EFF is defined as the average amount of absorbed x-ray energy required to generate one unit of detected pixel charge.) Values for W EFF of ~5 eV have been reported for polycrystalline HgI 2 (Su et al 2005, Street et al 2002 at far lower electric field strengths (less than 1 V/μm) than the ~10 V/μm required to obtain a W EFF of ~50 eV for a-Se.…”

Performance evaluation of polycrystalline photoconductors for radiation therapy imaging

“…͑This differs considerably from the situation for the earlier study conducted under diagnostic conditions where dark current considerations did limit the choice of field strength, with the result that pixel signal extraction for the 20 prototypes was as low as 81%.͒ 43 Unconstrained by concerns about dark current, the PIB and PVD prototypes were operated at the highest electric field strength that each array would support ͑ϳ1.3 and ϳ1.0 V / m, respectively͒ without inducing temporal spikes in the dark current. Maximization of the field strengths helped to reduce the known effects of polarization and charge trapping in PIB and PVD detectors 43,44 while also serving, by virtue of high signal response, to reduce the relative contribution of noise from the acquisition electronics. These field strengths were used for all measurements of sensitivity, lag, MTF, and NPS.…”

“…In addition, relatively thick layers of HgI 2 are conceivable, as suggested by an early study of polycrystalline HgI 2 samples as thick as 1.8 mm. 39 Finally, the electric field strength required to extract signal from polycrystalline HgI 2 at high levels of sensitivity is only on the order of ϳ0.5 to 1 V/ m, [40][41][42][43][44] much lower than the ϳ10 V / m required for a-Se detectors. 45 In this article, an initial investigation of the signal and noise properties of prototype MV AMFPIs incorporating polycrystalline HgI 2 is reported.…”