MONTE-CARLO STUDY OF GRID PERFORMANCE IN DIAGNOSTIC-RADIOLOGY - FACTORS WHICH AFFECT THE SELECTION OF TUBE POTENTIAL AND GRID RATIO

A Monte Carlo computational model has been developed for the study of the performance of anti-scatter grids in diagnostic radiology. It is u sed here to estimate the scatter in the image plane from soft tissue p hantoms (representing the patient) and to calculate image contrast and the mean absorbed dose in the phantom. Different scattering condition s, representative of various examinations, have been investigated: adu lt lumbar spine; small field radiography and fluoroscopy; adult chest and paediatric pelvis and chest. For each scattering condition, the co mbinations of tube potential and grid ratio have been found which, for a well designed grid, result in the lowest mean absorbed dose in the phantom for a fixed contrast level. In examinations which generate lar ge amounts of scatter, the use of high grid ratios in combination with high tube potentials is favourable with regard to both mean absorbed dose in the phantom and tube charge. When less scatter is generated, e ither the grid ratio or the tube potential can be varied to achieve th e desired contrast level. High grid ratios require shorter exposure ti mes, but need careful alignment in the beam to prevent primary radiati on cut-off. It is shown that the air gap technique can be used to redu ce patient dose in examinations with small amounts of scatter, but in combinations with a lower tube potential than when a grid is used.