Optimization of beam parameters for dual-energy digital subtraction angiography

Dual-energy digital subtraction angiography (DSA) is immune to the misregistration artifacts which plague conventional temporal subtraction DSA. However, since the signal-to-noise ratio (SNR) of dual-energy DSA is lower than temporal subtraction it is important to optimize the dual-energy technique with respect to the SNR. This study investigates the optimization of x-ray exposure parameters for the low- and high-energy x-ray beams, drawing upon our experience with dual-energy DSA studies in 20 cardiac patients. It is shown that the SNR is optimized, with respect to patient exposure, when the image intensifier (II) exposure for the high-energy beam is approximately 5 times higher than the low-energy beam. The unequal distribution of exposure between the low- and high-energy beams is achieved by selecting different video apertures for the two beams, and is therefore referred to as the "dual-aperture" technique. The dual-aperture technique achieves the same SNR as the single-aperture technique (which uses equal II exposures for the two beams) with 45% less patient exposure. In addition, the dual-aperture technique enhances the ability to maintain an optimum low-energy kVp, which increases the dual-energy signal amplitude. The optimum high-beam filter thickness for use with the dual-aperture approach is determined. The optimum filter thickness, which is dependent upon the desired tube loading, is found to range from 1.3 to 2.0 mm Cu for tube loads ranging from 188 to 688 J/beam pair.