SELECTION OF ANTI-SCATTER GRIDS FOR DIFFERENT IMAGING TASKS - THE ADVANTAGE OF LOW ATOMIC-NUMBER COVER AND INTERSPACE MATERIALS

A Monte Carlo computer program has been developed for the study of ant i-scatter grids used in diagnostic radiology. The program estimates th e scatter from soft tissue phantoms representative of either adult or paediatric examinations and uses dose increase, signal-to-noise ratio improvement and contrast improvement factors to study grid performance . It has been used to quantify the advantage of replacing grids with a luminium covers and interspaces by grids using materials of low atomic number for these components. Two approaches are used. First, the alum inium and low atomic number alternatives are compared for five grid ra tios at fixed strip density and width and for tube potentials of 50, 7 0, 100 and 150 kV. Second, 44 commercially available grids are compare d for three different imaging Situations (lumbar spine, chest and paed iatric). The results demonstrate that grids made with carbon fibre cov er and cotton fibre interspace result in greater improvements in contr ast and signal-to-noise ratio, and lower dose increase factors, than d o grids made with aluminium. The dose reduction varies with irradiatio n conditions and is generally larger at lower tube potentials, higher grid ratios and lower strip densities. A typical reduction in mean abs orbed dose in the patient is 30% in an adult lumbar spine (AP view) at 70 kV with a grid with 36 strips per centimetre and ratio 12.