Makoto Nokita

Performance of a novel 43-cm x 43-cm flat-panel detector with CsI:Tl scintillator

We have developed a novel flat-panel detector with CsI:Tl scintillator. The detector consists of a single piece 43cm x 43cm amorphous silicon thin-film transistor (TFT) array with MIS (metal-insulator-semiconductor) photoelectric converter having a pixel pitch of 160μm coated with a needle-like crystal CsI:Tl scintillator. Signal chain was totally revised from current detector utilizing an innovative sensor technology. The novel detector and current detector were equipped to a digital radiography system allowing a quantitative and comparative study. Results show that the novel detector has a linear response covering the radiographic exposure range. It has a moderate modulation transfer function (MTF) sufficient to the radiography tasks and effective to suppress the aliasing. The detective quantum efficiency (DQE) was almost twice than the current detector. The result of contrast-detail phantom exposed with a 1/2x dose level is equivalent to that of current detector with a 1x dose level. These results show that performance of novel detector is superior to and expected to reduce the patient dose in half than current detector due to higher DQE and innovative sensor technology.

A method to measure the presampling MTF using a novel edge test device and algorithm

We have developed a novel method to measure the presampling modulation transfer function (MTF) in digital radiography systems using a novel edge device and algorithm. It can simultaneously measure the presampling MTFs in horizontally and vertically by utilizing its four edges. Calculation algorithm is composed of six steps, which are detection of edge, determination of angle, differentiation, composition of line spread function (LSF), fast Fourier transform (FFT) and sinc correction, respectively. Verification of the accuracy was conducted comparing with the established slit method. The repeatability of the measurement and the dose dependence was also examined. The measured MTF of edge device was coincident to that of slit within 0.02 up to the Nyquist frequency (3.125 cycles/mm). The repeatability was within 0.002 up to the Nyquist frequency. It is also confirmed that the result is not affected by the alignment error against the x-ray axis. In conclusion, an accurate and feasible method to measure the presampling MTF was established using a novel edge test device and algorithm.