Comparison of CMOS and a-Si flat panel imagers for X-ray imaging

CMOS X-ray imagers are gaining importance in the field of small area X-ray imaging. The paper gives a comparison of CMOS and a-Si flat panel imagers for X-ray imaging applications. Advantages of the CMOS imagers include the higher readout speed and lower noise due to the much higher electrical charge mobility in crystalline versus amorphous silicon. The lower noise provides a wider dynamic range in CMOS even when the total pixel storage capacitance is the same as in a-Si. Also, because the noise floor is lower, the low dose Detective Quantum Efficiency (DQE) is significantly higher and the X-ray detection is quantum noise limited down to very low dose levels. The higher readout speed provides faster CT scanning. This is important when the patient has to hold their breath during a scan such as during breast CT exams. Besides the lower noise and higher speed, the pixel size can be much smaller because active components made of crystalline silicon are smaller than active components made of a-Si. Small pixels are advantageous for mammography, dental and other very high resolution X-ray imaging applications. However, there are also some drawbacks to CMOS X ray imagers. One is the limited size. Another drawback of CMOS technology is the limited radiation hardness. Amorphous silicon is highly resistive to X-ray radiation. This paper will provide technical data and examples of CMOS X-ray imagers and present further information on how we can improve the radiation hardness and overcome the size limitations of CMOS imagers.