A new technique to characterize CT scanner bow tie filter attenuation and applications in human cadaver dosimetry simulations.
PURPOSE To present a noninvasive technique for directly measuring the CT bow tie filter attenuation with a linear array x ray detector. METHODS A scintillator based x ray detector of 384 pixels, 307 mm active length, and fast data acquisition (model X Scan 0.8c4 307, Detection Technology, FI 91100 Ii, Finland) was used to simultaneously detect radiation levels across a scan field of view. The sampling time was as short as 0.24 ms. To measure the body bow tie attenuation on a GE Lightspeed Pro 16 CT scanner, the x ray tube was parked at the 12 o'clock position, and the detector was centered in the scan field at the isocenter height. Two radiation exposures were made with and without the bow tie in the beam path. Each readout signal was corrected for the detector background offset and signal level related nonlinear gain, and the ratio of the two exposures gave the bow tie attenuation. The results were used in the geant4 based simulations of the point doses measured using six thimble chambers placed in a human cadaver with abdomen/pelvis CT scans at 100 or 120 kV, helical pitch at 1.375, constant or variable tube current, and distinct x ray tube starting angles. RESULTS Absolute attenuation was measured with the body bow tie scanned at 80 140 kV. For 24 doses measured in six organs of the cadaver, the median or maximum difference between the simulation results and the measurements on the CT scanner was 8.9% or 25.9% respectively. CONCLUSIONS The described method allows fast and accurate bow tie filter characterization.
Medical physics