Portal imaging with a direct-detection active matrix flat panel imager.

摘要

The problem of charge creation by x-rays in amorphous selenium (a-Se) is studied. A quantitative theory is developed which includes collective and single electron-hole pair excitations by a passing electron. This theory is incorporated into a Monte Carlo code to calculate track structures in a-Se. The initial positions of the electron-hole pairs along these tracks are used to study the fraction of pairs which recombine versus incident x-ray energy and applied electric field. The experimentally-observed energy dependence of recombination is attributed to a spur size which is dependent on the velocity of the ionizing electrons. The theory and simulations agree with available experimental data in the energy range from 20 keV to 10 MeV.; The use of an a-Se based direct-detection active matrix flat-panel imager (AMFPI) is explored at megavoltage energies for use in the verification of radiotherapy treatments. As with most other megavoltage detectors, a metal front plate is used to reduce patient scatter and to act as a buildup layer. The Modulation Transfer Function (MTF), Noise Power Spectrum (NPS), and Detective Quantum Efficiency (DQE) are measured. The DQE for the direct detection AMFPI is compared with the published DQE of an indirect detection AMFPI for portal imaging. The direct detector has a lower DQE at zero frequency, but there is a cross-over at approximately 0.3 cycles/mm after which it has a higher DQE.; A theoretical expression for the DQE of medical imaging detectors with non-elementary cascade stages is derived. This formalism can be used in conjunction with Monte Carlo techniques to evaluate the DQE of megavoltage imaging detectors. The predictions of the theory agree with the experimental DQE results for the direct-detection AMFPI and also for published results for the DQE of both a metal/phosphor detector and an indirect-detection AMFPI.; The effect of scatter on image quality is modeled in terms of the scatter fraction (SF) and scatter-to-primary ratio (SPR) using Monte Carlo techniques. To validate these simulations, the SF is measured experimentally using a prototype a-Se detector which uses an electrostatic probe to measure the a-Se surface potential. The simulations are used, along with the DQE simulations, to study the effect of metal/a-Se or metal/phosphor thicknesses on image quality in direct and indirect AMFPIs at megavoltage energies. It is found that for a-Se or phosphor thicknesses less than about 300 μm, a front plate of about 1 mm copper is optimal whereas for larger a-Se/phosphor thicknesses a front plate of about 0.4 mm may in some situations lead to better image quality.