Investigation of avalanche multiplication in amorphous selenium for use in digital fluoroscopy.
Published: 2005
Language: eng
Number of pages: 116
Description: Fluoroscopy delivers a high dose to the patient, but the exposure per image is low. A low exposure rate places stringent requirements on the tolerable level of electronic noise of the flat-panel detector (FPD). Flat-panel detectors have high resolution and exquisite image quality; however, theoretical calculations indicated they did not posses the necessary noise characteristics to perform well in fluoroscopy. First, the fluoroscopic imaging performance of the FPD was investigated experimentally.Two problems with avalanche multiplication were investigated: (i) the intrinsic noise of avalanche multiplication and (ii) the fluctuation in depth over which the charge undergoes impact ionization. Theoretical calculations and experiments examining the noise properties of an indirect and a direct imaging system implementing avalanche multiplication were performed. The results showed that the first problem (i) was solved because each X ray is represented by thousands of avalanche multiplications rather than just one, significantly averaging out the intrinsic noise. The second problem (ii) was solved in the indirect approach. In the direct approach the detector must be separated into two distinct layers: one for conversion, and the other for avalanche multiplication. Avalanche multiplication in a-Se is a viable approach that can be used to achieve a quantum noise limited FPD in fluoroscopy.The imaging performance of an amorphous selenium (a-Se) FPD was experimentally evaluated at fluoroscopic exposure rates, (&sim;0.1--10 muR/frame, 75 kV). The detective quantum efficiency (DQE) was largely exposure rate independent but became exposure rate dependent at low exposure rates due to the electronic noise. Therefore, the electronic noise did not permit quantum noise limited performance over the fluoroscopic exposure rate range. More gain in the form of avalanche multiplication was investigated as a solution.Current active matrix systems introduce noise that inhibits present direct and indirect FPD from producing optimal fluoroscopic images. Avalanche multiplication in a-Se was investigated to determine whether a gain sufficient to overcome the noise limitations of the active matrix could be achieved. The result showed a 25 fold increase in avalanche gain was available. This is sufficient to permit quantum noise limited operation; however, avalanche noise could mitigate any advantage.
Notes: Source: Dissertation Abstracts International, Volume: 66-10, Section: B, page: 5352.

Thesis (Ph.D.)--University of Toronto, 2005.

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