Computed Tomography (CT) Automatic Exposure Controls (AEC) Testing Protocol Using a CelT Phantom

The purpose of this research was to set-up a protocol for using the CeIT elliptical test phantom to test the performance of Automatic Exposure Control (AEC) systems on the CT scanners in use at Aberdeen Royal Infirmary (ARI).  These are the GE Lightspeed, GE Optima 6600 and Siemens Somaton Definition in Radiology and the Philips Brilliance in Radiotherapy treatment planning. The variation of image noise and the tube current-time product (mAs) were studied from images obtained from each scanner. Noise was measured using the standard deviation of five selected regions of interest in the images of the phantom obtained from the CT scanners. Normalised percentage noise (noise %) was then calculated to compare how the scanners dealt with image noise with relation to the mAs. The results showed an increase in mAs values (increase in dose) with the phantom and regulation of the noise leading to acquisition of quality images from all three scanners. Off-centering, using the AP scout increased the dose to the phantom when the patient table was above the isocenter and reduced the dose to the phantom when the table was below the isocenter. This shows the importance of patient centering for effective AEC system’s dose regulation.  Different SPRs also affected the operations of the AEC systems differently, with PA giving more dose followed by LAT and AP in the GE and Philips scanners which were studied on this aspect. The Philips D-DOM modulation kept almost constant dose across the scanning process regardless of the phantom size, hence D-DOM should be used with care. CeLT phantom was useful in studying dose regulation by different AEC systems, hence it is useful in quality control (QC) tests of AEC systems on CT scanners as a testing protocol was formulated from this study.

Author(s): Keolathile Diteko, Rebecca Duguid and Lee Hampson

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