Surface dosimetry in a CT scanner using MOSFET detectors and Monte Carlo simulations

Abstract

Diagnostic imaging with CT procedures is responsible for significant radiation doses to patients. To enable individual patient dose estimates, a combination of MOSFET detectors and Monte Carlo (MC) simulations was investigated for the determination of patient surface dose. The behaviour of MOSFETs in kV x-rays from a CT scanner was investigated with experiments and MC simulations with a CT scanner model. A dose reproducibility of 5% and a mean loss of sensitivity with accumulated dose of about 10% was noted for the MOSFETs. Beam energy increase from 80-140 kVp resulted in a response decrease of 10%. The MOSFET detectors were calibrated in terms of absolute surface dose with the aid of MC simulations. Good agreement was achieved between measured and calculated surface dose on a cylindrical Lucite phantom. Experiments with a stationary x-ray tube and contiguous axial scanning led to differences limited by 8%. Surface dose in helical scanning was investigated with measurements with radiological film and an array of five MOSFET detectors, leading to good agreement. It is concluded that an array of MOSFET detectors, calibrated in terms of surface dose, is a valuable tool to assess individual patient surface dose. In combination with MC simulations this may lead to estimations of effective dose.