SU-D-103-06: Bismuth Shield Usage in MDCT Thoracic Scans: Organ Dose Vs. Image Quality

Abstract

Purpose: To study Bismuth (Bi) shielding for the breast, organ dose and image quality were compared under the following conditions: (1) tube current modulation (TCM), (2) TCM with a Bi shield placed after topogram, and (3) manually reduced tube current (RTC) with no Bi. Methods: All measurements were performed with a 64‐slice scanner at 120 kVp. Organ dose was measured with MOSFETs using an adult male anthropomorphic phantom with supine breast attachments. The reference exposure and reduced exposure (with 4‐ply Bi shield) was measured with an ion chamber located at the level of the breast. The mA was reduced by normalizing the reference mA to the ratio of the reduced exposure to the reference exposure. Image quality was measured using a high contrast insert placed in the lung. Regions of Interest (ROIs) were drawn in thoracic organs to measure signal‐to‐noise ratio (SNR), percent contrast (%Contrast), noise, and HU values. Results: Organ doses (mGy) for the three scans (TCM, TCM with Bi, and RTC) were 11.1, 6.89, and 6.04 to the breast; 8.83, 8.01, and 7.62 to the lung; and 8.20, 7.36 and 8.40 to the heart, respectively. HU increase was greatest in the TCM with Bi scan for organs closest to the shield. The SNRs were 37.6, 34.1, and 43.3 and the %Contrast values were 349.2, 326.3, and 354.3 with TCM, TCM with Bi, and RTC, respectively. Conclusion: For thoracic CT, this RTC method provides a dose reduction to the breast similar to that of the TCM with Bi. Decrease in SNR and %Contrast in the TCM with Bi scan was expected due to decrease in photons reaching the detectors and beam hardening from the shield, respectively. Increased SNR in RTC scan was due to the increased mA compared to TCM scan at the level of measurement. U.S. NRC Health Physics Fellowship Grant No. NRC‐HQ‐12‐G‐38‐0022