SU‐GG‐I‐65: The Effect of Measurement Location On In‐Air CTDI Values

Abstract

Purpose: To experimentally investigate the variation of CTDIair with radial distance from the scanner isocenter. Method and Materials: Radiation dose measurements were obtained on a Siemens Sensation 16 CT scanner operated with both the head and body beam filtration filters. Measurements were made of CTDIair (mGy/mAs) as a function of the radial distance r from the CT scanner isocenter. The radial profile CTDIair(r) was fit to a third order polynomial which permitted the average CTDI over any radius to be obtained analytically. Results: At r = 8 cm, values of CTDIair relative to the isocenter were 0.824 and 0.838 for the head and body filters, respectively; at r = 16 cm, the values of CTDIair relative to the isocenter were 0.521 and 0.546 for the head and body filters, respectively. Values of CTDIair averaged over an 8 cm radial distance were ∼0.91 × CTDIair(0), and averaged over a 16 cm radial distance were ∼ 0.74 × CTDIair(0). Conclusion: Quantification of variations of CTDIair(r) are of interest because: (a) this parameter quantifies the performance and effectiveness of beam shaping filters; (b) CTDIair data can be used to obtain Tissue Air Ratios that can estimate organ doses in CT.