Computed tomography quality indexes: evaluation experience

Aim of this work was to identify proper figures of merit (FoM's) to quantitatively and objectively assess the whole acquisition process of a CT image and to evaluate which are more significant. Catphan® phantom images where acquired with a 64 slices computed tomography system, with head and abdomen protocols. Automatic exposure modulation system was on, with different settings. We defined three FoM's (Q, Q1 and Q2) including image quality parameters and acquisition modalities; two of them (Q and Q 1) include also a radiation dose quantity, the third (Q2) does not. Then we drew from these the comparable FoM's (CNR, Q1 Q2), that do not have dose in their definitions, in order to investigate how they depend on perceived image quality. The FoM's were evaluated for each series. At the same time, expert observers evaluated the number of low contrast inserts seen in the phantom' images. The considered CNR, Q1 , Q2 FoM's are linearly related to the perceived image quality for both the acquisition protocols (head: r2=0.91;0.94;0.91; abdomen: r2=0.93;0.93;0.85). Q and Q1 values analysis shows that these FoM's can distinguish between different acquisition modalities (head or abdomen) with statistically significant difference (p>0.05). The studied FoM's can be usefully used to quantitatively and objectively assess the whole CT image acquisition process. Those FoM's including also radiation dose (Q, Q 1) can be used to objectively quantify the equilibrium between image quality and radiation dose for a certain acquisition modality.