Can CT Shield Decrease Breast Tissue Dose Without Compromising Quality?

The objective of a recent study was to determine the effect on radiation dose and to measure image quality of images obtained using bismuth breast shields with automatic tube current modulation during pediatric chest multidetector computed tomography (CT).

The greatest dose reduction is achieved by placing the breast shield after obtaining the scout image to avoid Auto mA compensation due to the density of the shield.

Methodology
An anthropomorphic phantom representing a 5-year-old child was used. This phantom was scanned using the technique clinically used for the 5-year-old age-based chest CT protocol.

The phantom was first scanned with a fixed-tube current of 65 mA without a breast shield. The phantom was next scanned using the same parameters but with the addition of a 2-ply bismuth breast shield with a 1-cm-thick foam stand-off pad.

The shield measured 9 cm in the craniocaudal dimension and was designed to cover the chest to the midaxillary line.

The phantom was then scanned using z-axis automatic tube current modulation with 65 mA maximum, 10 mA minimum, and a noise index (measured in SD of Hounsfield units) of 12.0 H (SD) with the topogram (scout) frontal and lateral radiographs obtained with the bismuth breast shield in place.

Auto mA selects tube currents on the basis of patient density, size, and shape, as reflected in the localizer radiograph, to maintain the target noise index between maximum and minimum levels. The noise index approximates noise as measured in the center of a uniform phantom.

The phantom was finally scanned with z-axis automatic tube current modulation using the same parameters, but the scout radiographs were obtained before the breast shield was placed.

Results
The shield reduced the breast dose by 26%, and shielding and automatic current modulation reduced this dose by 52%.

Conclusions
“The greatest dose reduction is achieved by placing the shield after obtaining the scout image to avoid Auto mA compensation due to density of shield. With this technique, image noise increased but remained close to the target noise index.”

Author: : Basil Hubbi, MD

Reference:
Coursey C, Frush DP, et al. Pediatric Chest MDCT Using Tube Current Modulation: Effect on Radiation Dose With Breast. AJR; 2008; 190 (January): W54-W61