A QUANTUM ACCOUNTING AND DETECTIVE QUANTUM EFFICIENCY ANALYSIS FOR VIDEO-BASED PORTAL IMAGING

The quality of images generated with radiographic imaging systems can be degraded if an inadequate number of secondary quanta are used at an y stage before production of the final image. A theoretical technique known as a ''quantum accounting diagram'' (QAD) analysis has been deve loped recently to predict the detective quantum efficiency (DQE) of an imaging system as a function of spatial frequency based on an analysi s of the propagation of quanta. It is used to determine the ''quantum sink'' stage(s) (stages which degrade the DQE of an imaging system due to quantum noise caused by a finite number of quanta), and to suggest design improvements to maximize image quality. We have used this QAD analysis to evaluate a video-based portal imaging system to determine where changes in design will have the most benefit. The system consist s of a thick phosphor layer bonded to a I mm thick copper plate which is viewed by a T.V. camera. The imaging system has been modeled as ten cascaded stages, including: (i) conversion of x-ray quanta to light q uanta; iii) collection of light by a lens; (iii) detection of light qu anta by a T.V. camera; (iv) the various blurring processes involved wi th each component of the imaging system; and, (v) addition of noise fr om the T.V. camera. The theoretical DQE obtained with the QAD analysis is in excellent agreement with the experimental DQE determined from p reviously published data. It is shown that the DQE is degraded at low spatial frequencies (<0.25 cycles/mm) by quantum sinks both in the num ber of detected x rays and the number of detected optical quanta. At h igher spatial frequencies, the optical quantum sink becomes the limiti ng factor in image quality. The secondary quantum sinks can be prevent ed, up to a spatial frequency of 0.5 cycles/mm, by increasing the over all system gain by a factor of 9 or more, or by improving the modulati on transfer function (MTF) of components in the optical chain. (C) 199 7 American Association of Physicists in Medicine.