Dynamic contrast-enhanced CT studies: balancing patient exposure and image noise

To establish the essential basis for balancing the dose versus noise trade-off in dynamic contrast-enhanced (DCE) CT by means of a phantom study.

Measurements were performed at a 64-section dual-source system, using the default protocols for DCE imaging (40 scans) of the trunk (current-time product per scan, 100 mAs; voltage, 120 kVp; pixel size, 0.9 × 0.9 × 8 mm3; CTDIvol per examination, 264 mGy) and head (270 mAs, 80 kVp, 0.45 × 0.45 × 8 mm3, 429 mGy). For 3 representative sections of an anthropomorphic phantom (head, upper abdomen, pelvis) transaxial dose distributions were measured by thermoluminescent dosimeters. The image noise was determined for 5 values of the current-time product (but otherwise identical parameter settings) and 4 pixel resolutions at a water-filled trunk and head phantom.

Highest exposures occurred at the periphery of the trunk and head with maximum skin entrance doses of about 300 mGy. Effective doses related to the 3 exposure scenarios were between 4 and 20 mSv, but were not at all predictive of local exposure levels. The image noise was inversely proportional to the square root of the current-time product and, with restrictions, to the pixel size. Noise levels determined for the standard settings were 13.8 HU (trunk) and 4.4 HU (head) and thus comparable with the contrast enhancement typically detected in tumors and ischemic brain tissues, respectively.

The opposing requirements of risk and noise limitation in DCE-CT cannot be balanced without substantially reducing the spatial resolution. But even so, local radiation exposures are rather high for a diagnostic procedure. Indications to perform a DCE examination should thus be strictly limited to patients who really benefit from it.