Toshio Kameshima

Novel large-area MIS-type x-ray image sensor for digital radiography

We have developed a brand new, large-area X-ray image sensor for Digital Radiography System (DRS). The sensor utilizes a thin film transistor (TFT)/metal insulator semiconductor (MIS)-type photoelectric converter array made from hydrogenated amorphous silicon (a-Si:H). The sensor has 2688 X 2688 pixels at a pitch of 160 micrometer. The active area is 17 inch X 17 inch. The sensor utilizes scintillator coupled to the array. The light generated by X-rays is detected by the MIS-type photoelectric converters, and the resultant signals are scanned out by switching the TFTs. The a-Si TFT/MIS-type photoelectric converter array is characterized by high signal to noise ratio (SNR) and simple fabrication process. We will describe the principle and the performance of the sensor. In addition, we will present some X-ray images of a human subject obtained with this sensor. Dynamic range of the sensor covers most of the exposure range for radiography. SNR is limited almost only by the X-ray photon noise. MTF is sufficient for digital chest radiography. X-ray images have good contrast. The experimental results and obtained images show that the brand new sensor has great advantages for replacing X-ray film. The simple fabrication process of the sensor promises high productivity and low cost of DRS.

Development and evaluation of a portable amorphous silicon flat-panel x-ray detector

ABSTRACT The design, development and evaluation of a portable x-ray detector are described. The completed detector has a pixelpitch of 100 2m, an active imaging area of 22.5 x 27.5cm 2 (9x11inch), package outer dimensions of 32.5 x 32.5cm 2 (13x13inch 2 ), a thickness of only 20mm, and a weight of around 2.8kg. A number of significant advances in the designand production processes were needed to produce such a compact detector with such a small pixel pitch, while maintainingthe image qualityachieved a current detector (CXDI-22) which has a 160 m pixel pitch. These include the developmentof a low power readout IC, advances in detector packaging design, concentrating on lightweight and strong components,and redesign of the pixel structure to improve the fill-factor.Acomparison is made of the imaging characteristics of this new detector with the CXDI-22 detector, and it is shown thatthe new detector demonstrates improved spatial resolution (CTF), and noise equivalent quanta (NEQ) particularly athigher spatial frequencies. The new detector is also shown to demonstrate superior performance in a contrast-detailphantom evaluation.This newdetector should be particularlyuseful for limb and joint examinations as it offers high spatial resolution,combined with the same freedom in positioning provided byconventional screen-film cassettes.Keywords: x-raydetector, portable detector, digital radiography, amorphous silicon (a-Si:H), thin film transistor (TFT)

Sensor gain and noise requirements for fluoroscopic applications

The low x-ray exposures used in fluoroscopic applications (0.1 - 10mR at the sensor surface) mean that the requirements for sensor gain and noise are particularly strict. The achievable DQE is determined by a number of factors, including the sensor quantum efficiency, x-ray absorption Swank factor, secondary quanta conversion efficiency, internal gain (e.g. the number of electrons collected per visible photon produced in the phosphor), and additive noise. The influence of these factors is examined for three direct detection x-ray sensors (PbI2, a-Se and GaAs), and one indirect detector sensor (CsI). Although the characteristics of these sensors are very different, it is demonstrated that all are appropriate for use in fluoroscopic applications as a replacement for current image intensifier based systems.