A novel flat-panel imager for medical x-rays

Abstract

The last decade has seen rapid development and clinical adoption of active-matrix flat-panel imagers (AMFPIs) for diagnostic x-ray imaging. AMFPIs provide better image quality than traditional screen films and computed radiography,1, 2 but further improvement is desirable, especially in spatial resolution and low-dose performance. Existing AMFPIs use a two-dimensional array of thin-film transistors to read out a charge image generated by an x-ray image sensor. In ‘direct conversion’ this sensor is an x-ray photoconductor, while in ‘indirect conversion’ the sensor is a scintillator coupled with discrete photodiodes. These methods have two main limitations. First, in low-dose applications such as fluoroscopy (real-time x-ray imaging), electronic noise degrades the imaging performance behind dense tissues. Second, the smallest pixel size currently used for digital mammography, 70μm, may not be adequate in some cases. For example, characterizing the shape of microcalcifications has been shown to be compromised with pixel size del = 100μm, while del = 50μm can preserve the needed information.3 To improve the detector low-dose performance with high resolution, we are investigating a new, indirect-conversion flatpanel imager with avalanche gain and a field-emitter array (FEA), which is referred to as SAPHIRE (scintillator avalanche photoconductor with high-resolution emitter readout). The concept of SAPHIRE is shown in Figure 1. It consists of a needle-structured cesium iodide (CsI) scintillator, optically coupled (for example, through fiber optics) to a uniform thin layer of amorphous selenium (a-Se) photoconductor, with thickness dSe ∼ 4 − 25μm. The selenium layer is operated in avalanchemultiplication mode, and is called HARP (high avalanche rushing amorphous photoconductor).4 Figure 1. Cross-sectional schematic of the SAPHIRE detector (thickness not to scale). Light generated by x-rays striking the cesium iodide (CsI) scintillator passes through the transparent indium tin oxide (ITO) electrode to generate electrons and holes (charge carriers) in the amorphous selenium layer, designated HARP. A high voltage (HV) causes avalanche multiplication as the holes traverse the HARP layer, and the amplified signal is detected using an electron beam emitted by a field emitter array (FEA).