MO-FG-CAMPUS-IeP2-02: Effects of Metal On CT Size-Specific Dose Estimates

Abstract

Purpose: To evaluate the accuracy of size-specific dose estimates (SSDE) in CT in the presence of simulated metal prostheses. Methods: Radiation dose in tissue (f-factor = 0.94) was measured at various chamber positions in a conventional nested CTDI phantom with nominal 0.5 inch metal rods inserted to simulate the presence of prosthetic implant(s). An average weighted tissue dose (AWTD) was calculated in a manner similar to CTDIw. Subsequent scans were performed with varying phantom diameter, number of metal rods and type of metal. The scan acquisition parameters were fixed for all such measurements (i.e. CTDIvol was constant). Axial CT images reconstructed both with and without a metal artifact reduction algorithm (SEMAR) were used to calculate the water-equivalent diameter (Dw) per AAPM TG Report 220. The Dw values were subsequently used to determine the SSDE from the known CTDIvol. In addition SSDE was also calculated from the effective diameter per AAPM TG Report 204. Accuracy of the calculated SSDE values were assessed by comparing to the AWTD measurements. Results: In the 32-cm diameter phantom the SSDE calculations from Dw (TG-220) were within ±1% of the AWTD measurements regardless of type of metal and number of metal rods while SSDE calculated from effective diameter (TG-204) overestimated the AWTD by 7–10%. In the 16-cm diameter phantom the SSDE calculations from Dw (TG-220) were within ±4% of the AWTD measurement. The Dw calculations used to determine SSDE varied by less than 0.2% between the images reconstructed with and without the metal artifact reduction algorithm. Conclusion: The TG-220 SSDE method can provide an accurate estimation of tissue dose in the presence of metal while TG-204 SSDE method overestimates the dose. The determination of Dw is independent of reconstruction algorithm.