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How Localizer Orders Impact Image Quality and Radiation Dose in Computed Tomography - A Phantom Study

J Jimenez, J Rong, C Jensen, A Jones, X Liu*, UT MD Anderson Cancer Center, Houston, TX

Presentations

(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: To investigate the effects of localizer order on image quality and radiation dose in computed tomography.


Methods: A semi-anthropomorphic liver phantom was scanned on a Siemens SOMATOM Force and a GE Discovery CT750 HD CT scanner using a diagnostic abdominal CT protocol at different table heights to simulate patient off-centering. Scanner reported CTDIvol was compared while varying the order of paired localizer with tube current modulation (TCM) enabled. The contrast-to-noise ratio (CNR) was computed for a simulated hyperdense liver nodule and the figure-of-merit (FOM), defined as CNR²/CTDIvol, was calculated to facilitate optimization of low contrast performance.


Results: CTDIvol varies little for the Force regardless of localizer order or table height. CTDIvol increased or decreased steeply with increasing off-centering of the phantom in the vertical (table height) direction when using lateral first localizers for CT750 HD, however using AP or PA first localizers reduced the impact of vertical off-centering on CTDIvol. The FOM was observed to be the highest with phantom centered with iso-center regardless of localizers order used for the Force. The FOM was observed to be the highest with phantom slightly lower (~30mm) than iso-center for CT750 HD.


Conclusion: The impact of localizer order and table height on image quality and radiation dose may vary, and depend on the model of system. The table height does not have influence on radiation dose for Force. Optimal image quality, defined as the maximum FOM, always occurred when the phantom was centered regardless localizer order. However FOM decreased with increasing off-centering. The table height has strong influence on radiation dose for CT750 HD when lateral first paired localizers used. Maximum FOM occurred when phantom was nearly centered and FOM decreased with increasing off-centering. It is recommended to use AP/PA first paired localizers for CT750 HD in protocol optimization.

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