A computer model of an image intensifier system working under automatic brightness control

A computer model of a fluoroscopy unit operating under Automatic Brightness Control (ABC) has been constructed. Although the model specifically simulates a Siemens Siremobil 2000 unit it is general in nature and can easily be applied to other systems. The model is divided into two stages: the first simulates the ABC features of the unit, finding the appropriate kVp/mA operating point for any assortment of attenuators in the beam; the second employs Monte Carlo techniques to account for scatter and generates the image displayed on the TV screen. The first stage is developed by breaking the operation of the unit into its constituent parts then implementing each part in a module of code. In order to calculate entrance surface dose rates (ESDRs) for water and PMMA phantoms, backscatter factors for these materials are generated using Monte Carlo techniques at a variety of kVps and field sizes. Expected ESDRs are then calculated for different thicknesses of water and PMMA phantoms. The predictions of the model are in excellent agreement with experimental measurements. In the second stage the image intensifier is modelled as a 256×256 grid of CsI voxels and the energy deposited in each voxel is calculated. An image is then generated from the dose distribution. Images of simple contrast objects are produced to demonstrate the operation of this stage. The model can be applied to the evaluation of dose reduction techniques, particularly the investigation of the balance between patient dose and image quality