Yotaro Ishihara MS, Presenter: Employee, General Electric Company

Roy A. Nilsen, Abstract Co-Author: Employee, General Electric Company

Grant M. Stevens PhD, Abstract Co-Author: Employee, General Electric Company

Dominic Crotty PhD, Abstract Co-Author: Employee, General Electric Company

Mary Sue Kulpins, Abstract Co-Author: Employee, General Electric Company

Motoki Watanabe, Abstract Co-Author: Employee, General Electric Company

The AAPM Task Group 204 demonstrated SSDE, a patient size-specific radiation dose index. While SSDE could be more accurate than CTDIvol, SSDE based on the effective diameter (D_eff) could have errors for non-uniform objects. The purpose of this study is to compare SSDE calculated from D_eff and from "Water – Equivalent Diameter" (D_w).

D_w is calculated using normalized summation of all pixel values on an axial image. PMMA phantoms (32, 24, 16 and 8cm diameter) were scanned and D_w and D_eff were measured and compared. Oval PMMA phantoms with voids to simulate upper and middle lung level were scanned on the cradle at the ISO-center of an MDCT system (Optima660CT, GE Healthcare, UK). These simulated lung phantoms also have a feature to insert a dosimeter probe (9015, RADCAL, CA). SSDE values from D_eff were measured based on Task Group 204 report, and SSDE values from D_w were calculated with the correlation between D_w and CTDI100 (both center and peripheral) for these simulated lung phantoms. In addition, the CTDIvol were calculated using the dosimeter measurements in the simulated lung phantoms. SSDE values based on both D_eff and D_w were compared with the measured CTDIvol values.

The calculated D_w and D_eff values of the PMMA phantoms (32, 24, 16 and 8cm diameter) were (D_eff, D_w) = (32.0, 33.7), (24.0, 25.2), (16.0, 16.7) and (8.0, 8.0) (cm). The resulting SSDE from D_eff were both 39.1 mGy at middle and upper lung level. SSDE from D_w were 40.7 and 37.7 mGy at middle and upper lung level. Measured CTDIvol were 40.5 and 38.3 mGy at middle and upper lung level. Comparison of the calculated SSDE with the dose measurements yielded errors in SSDE from D_eff were -3.5% and +2.0%, and errors in SSDE from D_w were 0.4% and -1.6% at middle and upper lung level respectively.

SSDE calculated from Water – Equivalent Diameter may be more accurate than the effective diameter method. In addition, this Water – Equivalent Diameter method calculated from an axial image or scout image has a potential possibility to be automatically determined on a CT scanner.

This Water - Equivalent Diameter method enables a CT scanner to calculate the more accurate SSDE automatically.

Ishihara, Y, Nilsen, R, Stevens, G, Crotty, D, Kulpins, M, Watanabe, M, A Phantom Study of Size-specific Dose Estimates (SSDE) Based on Automatically Calculated Water - Equivalent Diameter.  Radiological Society of North America 2014 Scientific Assembly and Annual Meeting, - ,Chicago IL. http://archive.rsna.org/2014/14012040.html