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Development and association of metrics of dose and image quality for comparing and optimizing protocols in CT imaging

Kolloquium der Abteilung 6

Nowadays, though Computed Tomography (CT) examinations only correspond to a small portion of medical imaging procedures (typically 10 % in France), they are credited with about 70 % of the total imaging collective dose. Reducing the dose due to CT examinations is therefore a major issue. However, decreasing the dose in CT imaging cannot be achieved at the expense of the image quality (IQ) needed to ensure a correct diagnosis.

Therefore, CT imaging needs to be described simultaneously using reliable metrics for delivered dose and IQ. However, such metrics are still lacking, especially for IQ, and we decided to develop novel ones. For IQ evaluation, the mathematical model observer (MO) Non Pre-Whitening Eye filter was implemented and validated, thanks to a clinical study involving a dozen of experienced radiologists. The MO calculated the Percentage of Correct answers (PC) on CT images acquired for various irradiation and reconstruction conditions on a home-made dedicated phantom, linked to lesion detection and discrimination clinical tasks.

For dose estimation, a complete Monte Carlo model of the GE Discovery CT750 HD scanner was developed with the PENELOPE code. All the elements were estimated by physical measurements. The modelling was validated with measurements in CTDI and anthropomorphic phantoms using ion chambers and Optically Stimulated Luminescence dosimeters.

Finally, the dose in the dedicated phantom was simulated in the various clinical study conditions and linked to the corresponding PC calculated by the MO. Some of the scanner standard protocols were placed on the curves after regression and compared from the double point of view of dose and IQ, in particular some protocols that integrate a Double Energy mode. This method paves the way for a standardized methodology enabling clinical physicists and radiologists to optimize protocols for defined clinical tasks while keeping the dose as low as possible.