Grids or air gaps for scatter reduction in digital radiography: a model calculation

The relative advantages of grids and air gaps for scatter reduction in a digital radiography system were investigated using a theoretical model. In this model the properties of the scatter reduction device are described by primary transmission and selectivity. The signal-to-noise (SNR) improvement factor for fixed exposure to the patient was used as a performance indicator. The results show that the SNR improvement depends strongly on the local scatter fraction; for all practical configurations, however, it stays below a factor of 2. For high scatter fractions, an air gap of 20 cm has about the same effect on SNR improvement as a highly selective grid; for low and medium scatter conditions the air gap performs better than any grid. Additive system noise reduces the SNR improvement factor compared to the case with quantum noise only, the reduction being more pronounced for the grids than for the air gap. The results suggest that the use of an air gap instead of a grid is advantageous in digital radiography systems.