Image mottle in abdominal CT

RATIONALE AND OBJECTIVES. To investigate image mottle in conventional CT images of the abdomen as a function of radiographic technique factors and patient size. METHODS. Water-filled phantoms simulating the abdomens of adult (32 cm in diameter) and pediatric (16 cm in diameter) patients were used to investigate image mottle in CT as a function of x-ray tube potential and mAs. CT images from 39 consecutive patients with noncontrast liver scans and 49 patients with iodine contrast scans were analyzed retrospectively. Measurements were made of the mean liver parenchyma Hounsfield unit value and the corresponding image mottle. RESULTS. For a given water phantom and x-ray tube potential, image mottle was proportional to the mAs^-0.5. Increasing the phantom diameter from 16 cm (pediatric) to 32 cm increased the mottle by a factor of 2.4, and increasing the x-ray tube potential from 80 kVp to 140 kVp reduced the mottle by a factor of 2.5. All patients were scanned at 120 kVp, with no correlation between patient size and the x-ray tube mAs. The mean mottle level was 7.8 ± 2.2 and 10.0 ± 2.5 for the noncontrast and contrast studies, respectively. An increase in patient diameter of 3 cm would require ~65% more mAs to maintain the same level of image mottle. CONCLUSIONS. The mottle in abdominal CT images may be controlled by adjusting radiographic technique factors, which should be adjusted to take into account the size of the patient undergoing the examination.