Breast shields, tube current modulation drive down pediatric CT doses

The combination of tube current modulation and bismuth shields during pediatric chest CT scanning can reduce radiation dose with a minimal impact on image quality, according to a study by researchers from Duke University and the University of Arkansas. Proper implementation of these techniques could be key to safer diagnostic scans.

The number of pediatric CT scans performed annually in the U.S. is fast approaching the 10 million mark, according to the clinical literature. This factor, amplified by an increasing awareness about the high radiation doses delivered by CT, has led pediatric imagers to tighten radiation protection measures using both new and old ideas.

Automatic tube current modulation and bismuth shielding have been shown to be effective to lower CT radiation dose. But the effect of their combined use on dose reduction and image quality has never been assessed, according to coauthor Dr. Donald P. Frush, chief of pediatric radiology at Duke.

On paper, combining both techniques did not seem effective, since the scanner might increase the dose to penetrate the shield. To get around the problem, the investigators tested shields on a childlike phantom after the scout CT scan used for tube current modulation. They found the technique lowered total dose by more than 50%, with only minimal image noise.

The researchers published results of their study in the January issue of the American Journal of Roentgenology (AJR 2008;190:W54-W61).

Frush and colleagues performed chest CT scans on an anthropomorphic phantom that represented a five-year-old child. They made two scans in each of four sequences: with a bismuth breast shield, without it, and with automated tube current modulation before and after placement of the shield.

The shield reduced radiation dose to the chest by 26%. Shielding plus tube current modulation boosted dose reductions by 52%. Multiple organ doses were lowest when the shield was placed after the scout radiograph. Mean noise in the range of shielding increased from 11.4 to 13.1 Hounsfield units in the superior mediastinum and from 10 HU to 12.8 HU in the heart. Although the difference was significant (p<0.01), the result was near the target noise index of 12 HU.

The study has several limitations, above all the use of a single type of CT scanner, since not all automatic tube current modulation products used in clinical practice work under scout radiograph guidance. The anthropomorphic phantom cannot make up for the respiratory/cardiac motion and age-related variables distinctive to human patients.

Findings, however, suggest a benefit for the management of radiation dose in pediatric patients and could help radiologists better understand the capabilities and constraints of their equipment before using shielding, Frush said.

"It's important for radiologists to limit dose in CT, especially to children, and especially to radiosensitive regions such as breast tissue," Frush said. "Radiologists should be familiar with strategies to do this."

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