SU‐E‐J‐51: The Impact of Off‐Center Position On Image Quality of Megavoltage CT

Purpose To quantify the impact of off‐center position on image quality of megavoltage CT in head and neck tumor patient and body tumor patient. Methods A Catphan 600 phantom and a Cheese phantom were imaged by TomoHD at the center and off‐center positions stepped by 5cm in AP and LR directions until the phantom edge approaching the imaging edge with fine, normal and coarse scan modes. Both phantoms incorporate image uniformity module, spatial resolution module and CT number linearity module. The CT number linearity module of Cheese phantom was set to simulate Catphan 600 phantom in a larger size. According to AAPM TG‐148 report, noise, image uniformity, spatial resolution and CT number variation were assessed for MVCT image quality. Centers of peripheral ROIs for noise and uniformity were about 4cm from phantom edge. Results Noise in all positions was within the criteria except the peripheral regions for Cheese phantom at off‐center scan positions. Image uniformity for most positions was acceptable, and all failed data were from the ROIs near imaging edge area. Spatial resolution degraded with off‐center scanning, and scan mode also made it worse with wider slice thickness, but just 1 level at most. CT number change with and without image uniformity correction for all positions were all less than tolerance, but it was close to the red line for water‐equivalent material. Conclusion This work indicates that if depth of lesions is below 4cm and MVCT are used for dose calculations, the ipsilateral body edge should be 5cm within imaging edge. Otherwise, image uniformity should be concerned.