SU‐D‐103‐04: KERMA Ratios (Rk) in Adult Head Volume Mode CT Scans

Purpose: To measure the ratio of in‐phantom KERMA to isocenter KERMA (KERMA Ratio, Rk) for adult head CT scans acquired on a 320 slice MDCT scanner operated in volume mode. Methods: An adult anthropomorphic phantom (Rando) was modified to accept OSL dosimeters. Ten slices of the phantom (head and neck) had approximately 80 dosimeters placed. The phantom was scanned in volume mode (16 cm nominal beam width) on a 320 slice MDCT scanner at 120 and 80 kV tube potentials. Scans were also acquired in scout mode for subtraction of the scout KERMA. 24 OSLs were scanned in‐air to measure the isocenter KERMA. The isocenter KERMA was scaled to the mAs used for the phantom scans and the Rk value for each dosimeter in the phantom was calculated. Results: At 120 kVp, the mean, median, and range of the Rk values for locations in the primary beam were 0.48, 0.49, and 0.41 to 0.57, respectively. At 80 kV, the equivalent values were 0.29, 0.29, and 0.20 to 0.39, respectively. For locations outside the primary beam at 120 kV, the values were 0.05, 0.03, and 0.01 to 0.17, respectively, and at 80 kV the values were 0.04, 0.02, and 0.01 to 0.19, respectively. Previously published organ locations were used to calculate the Rk values for the brain, orbits, oral mucosa, and thyroid. At 120 kV, the RK values for these organs were 0.49, 0.5, 0.06, and 0.02, respectively. At 80 kV, the organ Rk values were 0.30, 0.39, 0.05, and 0.02, respectively. Conclusion: Rk values provide a convenient means of estimating organ dose, only requiring knowledge of the technique used and the scanner isocenter output. Head CT Rk values for a 320 slice MDCT scanner using volume mode are similar to previously published values for 16 and 8 slice MDCT scanners.