Study On The Quality Control For Computed-tomography Simulators

The CT-simulator is an auxiliary device for radiotherapy. It can not only give thedoctors and medical physicists information on tumor and vital organs, but also for thetreatment planning verification. The CT-simulator plays a very important role inradiotherapy. It is very necessary to establish a comprehensive QA program.The quality control for CT-simulators can be divided into three parts, dose index,imaging performance and geometric positioning accuracy. The three parts can beoperated at the same time, or in the daily, weekly and monthly cycle. The experimentsused the catphan-500phantom and diagnostic level dosimeters. This study analyzed therelationship of scanning parameters and the performance for CT-simulators. Thesescanning parameters include tube voltage，thickness，standard reconstruction algorithm，and so on. In these scanning conditions，a qualified CT-simulator must meet thefollowing specifications:1). Dose index. The CTDIWfor head is not more than60mGy and the CTDIWforabdomen is not more than60mGy.2). Imaging performance. The deviation of the CT number at the center and at theedge should be less than4HU; The noise level should be less than0.35%; Formulti-slice CT, The deviation of the CT number between different slices should be lessthan6HU; The CT number linear deviation generally does not exceed5%; The slicethickness deviation should be not more than1.0mm; The spatial resolution should begreater than5.0Lp/cm; The low contrast should be not more than1%.3). Geometric positioning accuracy. The error is not more than1.0mm.From the localization of tumor, the design of treatment plan, the calculation of dosedistributions, to simulation and operate for the treatment plan, The CT-simulator is usedthroughout the whole process of the radiotherapy, its performance is directly related tothe quality of radiotherapy. This paper describes in detail the QC device, QC methodand quantitative index. The purpose is to provide the medical physicist with aframework and guidance for establishment of a comprehensive QA program forCT-simulator.