X-RAY SCATTER IN MEGAVOLTAGE TRANSMISSION RADIOGRAPHY - PHYSICAL CHARACTERISTICS AND INFLUENCE ON IMAGE QUALITY

The physical characteristics of x rays scattered by the patient and re aching the imaging detector, as well as their effect on verification ( portal) image quality, were investigated for megavoltage (0.1-20 MeV) x-ray beams. Monte Carlo calculations and experimental measurements we re used to characterize how the scatter and primary fluences at the de tector plane were influenced by scattering geometry and the energy spe ctrum of the incident beam. The calculated scatter fluences were diffe rentiated according to photon energy and scattering process. Scatter f ractions were measured on a medical linear accelerator (Clinac 2100c, 6 MV) for a typical imaging geometry using an ionization chamber and a silicon diode. After correction for the energy dependence of the cham ber and diode, the scatter fractions generated by the Monte Carlo simu lations were found to be in excellent agreement with the measured resu lts. In order to estimate the effect of scatter on image quality, the scatter and primary signals (i.e., energy deposited) produced in five different types of portal imaging detectors (lead plate/film, storage phosphor alone, lead plate/storage phosphor, Compton recoil-electron d etector, and a copper plate/Gd2O2S phosphor) were calculated. The resu lts show that, for a specified geometry, the scatter fraction can vary by an order of magnitude, depending on the sensitivity of the imaging detector to low-energy (<1 MeV) scattered radiation. For a common por tal imaging detector (copper plate/Gd2O2S phosphor), the scattered rad iation (i) reduced contrast by much as 50% for a fixed display-contras t system, and (ii) decreased-the differential-signal-to-noise ratio (D SNR) by 10%-20% for a quantum-noise-limited portal imaging system. For currently available TV-camera-based portal imaging systems, which hav e variable display contrast, the reduction in DSNR depends on the ligh t collection efficiency and the noise characteristics of the TV camera . Overall, these results show that scattered radiation can reduce cont rast significantly in portal films while deteriorating image quality o nly moderately in on-line systems.