A figure of merit comparison between bremsstrahlung and monoenergetic x-ray sources for angiography

A figure of merit (FOM) has been developed which embodies parameters related to image quality in the numerator and radiation integral dose to the patient in the denominator. In this manner, maximizing image quality and minimizing radiation dose amounts to maximizing the FOM. Furthermore, the FOM is designed to be independent of x-ray exposure (number of photons used), and this eliminates one important parameter in an optimization scenario. Monoenergetic x-ray beams (0% bandwidth) are compared with conventional Bremsstrahlung x-ray sources from a tungsten target, for angiographic imaging systems using 144 mg/cm² CsI image intensifiers as the detector. Thus the results are applicable to both digital subtraction angiography (DSA) and digital fluoroscopic procedures involving iodine-based contrast (e.g., roadmapping). The results demonstrate improvement factors (the ratio of the best FOM of the monoenergetic beam over the best FOM of the polyenergetic beam) ranging from 2.3 to 1.4. The improvement factors averaged over four iodine contrast thicknesses (50, 100, 500, and 1000 mg/cm²) were 1.61 (σ = 0.159) for the 10 cm thick patient, 1.68 (σ = 0.172) for the 20 cm thick patient, and 1.82 (σ = 0.186) for the 30 cm thick patient. The conclusions are that monoenergetic x-ray beams are capable of delivering the same image quality at about half the radiation dose to the patient compared to conventional x-ray tubes.