Room: Exhibit Hall
Purpose: To characterize the four bow tie filters (BTF) (two head and two body) on a dual-source Siemens FORCE CT scanner through non-invasive means by using a real-time dosimeter.
Methods: The Characterization of Bowtie Relative Attenuation (COBRA) method, developed by JM Boone and further elaborated by BR Whiting, was augmented with additional with beam quality measurements. A 0.6 cc ion chamber was positioned near the periphery of the scan field of view for each of the two CT x-ray sources (tube A and tube B); the detector remained stationary while the tubes were separately activated and rotated around the detector; the tube collimation was open to its maximum so that the x-ray cone-beam encompassed the dosimeter throughout its rotation. Measurements were made at 80 and 120 kVp. The acquired dose-rate waveforms were distance corrected and then fitted to a polynomial equation for smoothing purposes. Additional stationary tube exposure measurements were obtained at three different fan angles in order to better characterize the quality of the x-ray beam (HVLs of Al) through different amounts of bow tie material. The bow tie filter thickness filter was then determined in mm of aluminum-equivalent thickness for the four BTFs at various fan angles relative to the center (0o) of the filter.
Results: The shapes of the bow tie filter profiles determined for the two energies had the expected shape and were in close agreement with each other (e.g., the max thickness deviation between the two profiles for the Tube B head filter was 0.06 mm).
Conclusion: We have determined the shape and relative thickness of four BTFs (two head and two body) in a Siemens FORCE CT scanner into aluminum-equivalent thicknesses. Length-wise measurements of HVLs along the fan beam was an effective method to account for varying beam hardening that occurs.