How does magnification affect image quality and patient dose in digital subtraction angiography?

Digital subtraction angiography (DSA) images were obtained of a phantom containing 1 mm diameter vessels. The iodine concentrations ranged from 5 to 50 mg/cc, which permitted the determination of the detection threshold concentration of iodine. The source to image receptor distance was 105 cm, and image magnification was varied between 1.15 and 2.0. One experiment was performed at an input exposure of 1 μGy per frame, and a second experiment was performed at 4 μGy per frame. Surface (skin) doses were measured using an ionization chamber, and the corresponding values of energy imparted were determined from the exposure area product. Increasing the radiation exposure by a factor of four reduced the threshold iodine concentration by ~30%. The average detection threshold iodine concentration for a magnification of 1.15 was 13.6 mg/cc. Detection performance improved linearly with magnification, with an average value of 7.4 mg/cc at a magnification of 2.0. Values of energy imparted were essentially independent of image magnification, whereas surface doses increased by a factor of four when image magnification was increased from 1.15 to 2.0. Magnification offers improved imaging performance at no additional patient risk provided that surface doses do not exceed the dose threshold for deterministic effects such as skin burns and epilation.