Determination of the optimal conditions for dental subtraction radiographyusing a storage phosphor system

Objectives: To determine the optimal kVp and exposure conditions for digita l subtraction radiography system using a storage phosphor system. Method: Signal-to-noise (SNR) measurements were acquired using a Digora sys tem (Soredex, Helsinki, Finland) of large area, low contrast, clinically re alistic details against varying degrees of background attenuation for both single exposure unsubtracted and subtracted images. These results were comb ined with a measure of estimated thyroid dose to derive a figure of merit ( FOM) for the unsubtracted and subtracted images. Results: For both unsubtracted and subtraction radiography, an exposure at 50 kVp and 250 mu Gy produce the best overall FOM. However, using the syste m at the 60 kVp maximum a FOM at 1000 mu Gy for unsubtracted radiography an d 500 mu Gy for subtraction radiography gave the best SNR performance. Conclusion: Operating parameters have been derived which allow the user to choose between optimising SNR and dose (50 kVp, 250 mu Gy for unsubtracted and subtracted radiography) or SNR alone (60 kVp, 1000 mu Gy for unsubtract ed and 60 kVp, 500 mu GY for subtracted), for the visualisation of clinical ly representative details using the Digora system.