Room: Exhibit Hall | Forum 1
Purpose: In AAPM Report No. 204, the CTDIvol-normalized average dose over the central scan plane of the patient is shown to decrease with increasing patient sizes. As patient organs are located at various locations in the height direction, this work was to examine the dependence of the dose to water phantom on its diameter at various locations across or beyond the scanned range.
Methods: A dose calculation algorithm and the published data by Li et al. [Med. Phys. 39, 5347-5352 (2012); 43, 5878-5888 (2016)] were used to calculate cross-sectional (or planar) average dose D_L(z) along the longitudinal axis of 10- to 50-cm diameter water phantoms undergoing constant mA scans. The relationship between dose and phantom diameter was examined at seven scan lengths (15, 20, 25, 30, 40, 50, 70 cm) and at eight longitudinal locations inside or beyond each scan range.
Results: At four locations inside scan range (midpoint z = 0, half-center point z = L/4, scan range average, 1 cm from a scan range edge), D_L(z)(water) monotonically decreased as phantom diameter increasing from 10 to 50 cm, and asymptotically increased as scan length increasing from 15 to 70 cm. But at four locations (5, 10, 20, 30 cm) away from scan range, D_L(z)(water) initially increased with phantom diameter; the metric approached the maximum near diameter at 20, 23, 28 and 32 cm, respectively; subsequently, the metric decreased at larger diameters.
Conclusion: This study may enable physicist to gain a more comprehensive view of CT dose dependence on subject size, and may also be insightful for patient organ dose evaluation in characterizing the dose dependence on patient size.