Introduction of digital radiography systems and successive use of flat panel detectors revolutionized the field of diagnostic imaging. Wide dynamic range, high image quality, real-time image acquisition and processing, precise image recording, and ease of remote access are among the most prominent improvements. One of the decisive factors contributing to further advancements remains the continuous development of different X-ray detecting materials, from traditional phosphor screens in combination with secondary photodetectors for indirect detection to use of thin-film photoconductors in direct detection systems. The latter approach offers a two-fold benefit: simpler device structure resulting in lower manufacturing cost, and a high potential of providing images of superior contrast and sharpness due to inherently low signal spreading within the detector.
In the direct detection approach X-rays are absorbed by a photoconductor layer and converted to electron-hole pairs, which are then collected as electric charges on storage capacitors. Up to now amorphous selenium (a-Se) is the only photoconductor developed into direct detection type commercial medical imagers, for both general radiography and mammography applications. Detectors based on a-Se offer superior spatial resolution due to the simple conversion process. However, low atomic number and density (Z=34, ρ= 4.27 g/cm3), leading to low X-ray absorption, and high effective ionization energy (~50 eV) result in inadequate sensitivity, especially important for low exposure levels of fluoroscopic mode.
Materials of high atomic number and density have been investigated to replace a-Se. The purpose of this work is to evaluate polycrystalline Cadmium Telluride (CdTe) semiconductor material for application in large area diagnostic X-ray digital imaging in the direct detection configuration. Its high atomic number and density (Z=50, ρ= 5.86 g/cm3), low effective ionization energy (~5eV), as well as wide band gap, makes CdTe very attractive for room temperature radiation detection applications. Recent developments in large area photovoltaic applications of CdTe have moved this photoconductor to the frontiers of thin-film manufacturing and large area medical imaging.
The intrinsic image quality characteristics of the polycrystalline CdTe detector under diagnostic X-ray imaging have been investigated by Monte Carlo simulation using MCNP5 software package. The modulation transfer function (MTF), noise power spectrum (NPS), and detective quantum efficiency (DQE) of detectors of various thickness for diagnostic X-ray beams from 70 kVp to 120 kVp were determined. Thin film CdTe detector device operation was modeled with 1-D SCAPS (solar cell capacitance simulator) software package based on the energy deposition profiles obtained for diagnostic X-ray beams with Monte Carlo simulation. The sensitivity, linearity, and time response of prototype thin film CdTe detectors were measured. Electronic characteristics of a subset of thin detectors were verified against SCAPS simulation results allowing for model adjustments.
In this work we 1) calculate the diagnostic X-ray spectra of our Varian Ximatron simulator based on the measured output by tungsten anode spectral model using interpolation polynomials (TASMIP) technique, 2) study image quality characteristics, such as MTF, NPS, and DQE with MCNP5 Monte Carlo simulations, 3) investigate the device operation with SCAPS simulations, and 4) measure the device performance with a set of prototype devices. Based on our simulation and measurement results, we believe thin film polycrystalline CdTe is a promising material for direct detection large area digital medical imaging.