TU‐EF‐204‐08: Dose Efficiency of Added Beam‐Shaping Filter with Varied Attenuation Levels in Lung‐Cancer Screening CT

Abstract

Purpose:Added filtration such as tin filter has the potential to improve dose efficiency of x‐ray beam in lung‐cancer screening CT. However, dose efficiency with added beam filtration is highly dependent on patient attenuation level. In this phantom study, we evaluated the image quality at different tube voltages with and without added tin filter when attenuation level varies.Methods:A 30 × 20 cm anthropomorphic thorax phantom with three added extension rings were used to simulate small (S), medium (M), large (L), and extra‐large (XL) adult patients. These phantoms were scanned on a 192‐slice CT scanner (Force, Siemens) at 100 and 120kV without tin filtration, and 100 and 150 kV with tin filtration (100Sn and 150Sn), at multiple dose levels at each kV. Images were reconstructed using iterative reconstruction (ADMIRE, Siemens). Radiation dose was measured with a 0.6 cc ion chamber in the middle and peripheral areas of the phantom. Image quality was assessed using mean image noise at uniform areas in the central region and lung. Radiation dose that is required for each kV to match the noise in a routine lung‐cancer CT screening technique (120kV, 25 quality reference mAs) was calculated.Results:At each of the four phantom sizes, 100Sn had the lowest noise in both soft tissue and lung. Compared with 120 kV, 100Sn saved 39%–60% dose for the same noise, depending on phantom size. For the XL phantom (50 by 40 cm), 150Sn provided images with the least beam‐hardening artifact in peripheral region.Conclusion:For thoracic CT, added tin filtration can provide considerable dose reduction compared with 120 kV. 100Sn provides better dose efficiencies for all phantom sizes, while 150Sn provides better image quality in peripheral region for extra‐large patients.Drs.Joel G. Fletcher and Cynthia H. McCollough receive research support from Siemens Healthcare.