Clinical comparison of standard-dose and 50% reduced-dose abdominal CT: effect on image quality

We hypothesized that radiation doses for abdominal CT could be reduced by adjusting the dose for a patient's weight and cross-sectional abdominal dimensions, with the resultant scans still being of diagnostic quality.

Using a multidetector CT scanner, we prospectively studied 39 patients who were 65 years and older who had a known history of cancer. After performing a diagnostic contrast-enhanced CT examination, we obtained four slices each (centered at the top of the right kidney) at a standard radiation dose (240-300 mA) and at a 50% reduced dose (120-150 mA) at a constant kilovoltage of 140. Scans were obtained during a single breath-hold, with a 2.5-mm detector configuration and a slice pitch of 6:1. Reconstructed slice thickness was 5 mm. In a blinded review, two radiologists rated the randomized CT scans for overall image quality and anatomic details of liver, spleen, adrenal glands, kidneys, pancreas, and abdominal wall, using a 5-point scale (1 = unacceptable, 2 = substandard, 3 = acceptable, 4 = above average, and 5 = superior). Patients' weight and abdominal circumference, area, and anteroposterior and transverse diameters were correlated with image quality of scans obtained at standard-dose and 50% reduced-dose CT. Statistical analysis of the data was performed using Wilcoxon's signed rank test.

Overall, the image quality score was significantly higher (p < 0.005) on the scans obtained with standard-dose CT. No statistically significant difference in image quality was noted in the 50% reduced- and standard-dose CT scans in patients who weighed less than 180 lb, or 81 kg, (p > 0.05) and who had a transverse abdominal diameter of less than 34.5 cm (p > 0.05), an anteroposterior diameter of less than 28 cm (p > 0.05), a cross-sectional circumference of less than 105 cm (p > 0.05), and a cross-sectional area of less than 800 cm(2) (p > 0.05). Good interobserver agreement (p > 0.5) was found between the two reviewing radiologists.

Abdominal CT scan quality appears to be acceptable even with a 50% reduction in radiation dose except in patients with large anthropometric measurements. A reduction in CT radiation dose is possible if the tube current is optimized for the patient's weight and abdominal dimensions.