The effect of x-ray beam quality and geometry on radiation utilization efficiency in multidetector CT imaging.

This study has the following objectives: To measure the geometric efficiency (GE) of a multidetector CT (MDCT) system and investigate its dependence on several exposure parameters and to correlate GE with radiation dose burden of patients undergoing MDCT imaging. Dose profiles for all available beam collimations, tube voltage values, focal spot sizes, and modes of operation were determined for a modern MDCT scanner using an array of thermoluminescent chip dosimeters positioned side by side. Dose profiles measured free-in-air and at the center and periphery of the standard polymethyl methacrylate (PMMA) phantoms were used to estimate GE. The standard free-in-air and a new measure of GE determined using the standard PMMA phantoms were correlated with volume computed tomography dose index normalized to the z-axis coverage per rotation which is directly related to patient radiation dose burden. GE was found to be from 30% to 88% as beam collimation was changed from 1.2 to 24 mm, with thin beam collimations corresponding to higher "wasted" dose. For the same tube voltage and beam collimation, the use of small focal spot was associated with higher GE compared to the large focal spot. Besides, beam quality was found to have a much weaker effect on the GE value. In comparison to free-in-air, a weighted GE determined using the standard PMMA phantoms was found to have significantly better correlation with patient radiation burden (p < 0.05). Overbeaming strongly depends on the beam collimation and focal spot size, while the impact of beam quality on GE is less pronounced. Thin beam collimations are associated with a GE of as low as 30%.