Room: AAPM ePoster Library
To evaluate techniques and strategies for minimizing reconstruction artifacts, such as image distortion and incorrect HU values, in the extended FOV (eFOV) of Siemens CT scanners used for treatment planning.
An anthropomorphic body phantom with added fat layers was scanned on a Siemens Definition Edge using 5 different protocols with varied pitch, slice thickness, acquisition configuration, and scan direction. Four phantom positions (2 inside the scan FOV (sFOV), 2 outside the sFOV) were examined to determine the impact of eFOV reconstruction on preserving image integrity and HU values. Distortion of the phantom external contour was investigated by evaluating the phantom diameter from axial slices. ROIs were placed in sections of fat, soft tissue, and bone for computation of mean HU. All values were compared to measurements made with the phantom at isocenter and reported as a function of phantom position and protocol.
The differences in diameter relative to isocenter, averaged over all phantom positions, were minimized for protocols with a lower pitch; the maximum error observed, 41.6mm, was reduced to 25.9mm. Little dependence was observed for acquisition configuration or reconstructed slice thickness. The observed errors were greatest for the slice nearest to the transition between air and fat, which changed with scan direction. Mean HU values for positions outside the sFOV were found to differ greatly from those at ioscenter, with little dependence on protocol. The average HU value difference for tissue, across all protocols, from isocenter to maximum offset position, was 58±3.9.
Selection of pitch has the greatest impact on reducing eFOV image distortion. Sharp transitions between air and tissue amplify the distortions, however can be partially reduced by changing scan direction (tissue-air vs. air-tissue). HU values have little dependence on protocol parameters. Future work includes investigating effect of reconstruction kernel on HU value.
Not Applicable / None Entered.