To shield or not to shield: shielding may have unintended effects on patient dose in CT

Abstract

Objectives: The aim of the patient out-of-plane shield is to reduce the patient radiation dose. Its effect on tube current modulation was evaluated with the out-of-plane shield visible in the localizer but absent in the scan range in chest CT with different CT scanners.

Methods: An anthropomorphic phantom was scanned with six different CT scanners from three different vendors. The chest was first scanned without any shielding, and then with the out-of-plane shield within the localizer but outside the imaged volume. All pitch values of each scanner were used. The tube current values with and without the out-of-plane shield were collected and used to evaluate the effect of overscanning and tube current modulation (TCM) on patient radiation dose.

Results: The highest increase in cumulative mA was 217%, when the pitch was 1.531. The tube current value increased already 8.9 cm before the end of the scanned anatomy and the difference between the tube current of the last slices (with and without the out-of-plane shield in the localizer) was 976%.

Conclusion: Applying an out-of-plane shield outside the scanned volume but visible in the localizer images may increase the patient dose considerably if the scanner's TCM function is based only on localizer images.

Clinical relevance statement: The use of an out-of-plane shield in CT may strongly increase the tube current modulation and thus provide the patient with a higher radiation dose.

Key points: • Applying an out-of-plane shield outside the scanned volume but visible in the localizer images may increase patient radiation dose considerably. • The effect is visible with scanners that use solely localizer-based tube current modulation. • Features like overscanning may be difficult for the user to notice when planning the scanning, and yet they may affect tube current modulation and through it to patient dose.

Keywords: Chest; Computed tomography; Phantom; Radiation dose.

Fig. 1

The contact shield placement 1 cm below the imaged volume on the Kyoto phantom. Shield was visible in localizer images but not included in the scanned volume

Fig. 2

Tube current as a function of the slice, imaged using TCM and with all pitch values available at each scanner. Here we present the most notable differences in average mA values per image measured in our study. Location 0 cm represents a superior direction of a patient. (a) GE Optima 660, (b) Canon Aquilion Prime

Fig. 3

The tube current increase visualized over phantom anatomy. Blue horizontal lines represent the anatomical area in the phantom, which was exposed over 20% difference between the average tube current of the slice without and with an out-of-field shield in the localizer. The total length of the imaged volume was 32 cm (between vertical lines) and the earliest increase was detected 9.7 cm before the end of the scan