Korean J Radiol.  2009 Apr;10(2):156-163. 10.3348/kjr.2009.10.2.156.

In-Plane Shielding for CT: Effect of Off-Centering, Automatic Exposure Control and Shield-to-Surface Distance

• Kalra MK
• Dang P
• Singh S
• Saini S
• Shepard JA

• KMID: 1088727
• DOI: http://doi.org/10.3348/kjr.2009.10.2.156

Abstract

OBJECTIVE
To assess effects of off-centering, automatic exposure control, and padding on attenuation values, noise, and radiation dose when using in-plane bismuth-based shields for CT scanning.
MATERIALS AND METHODS
A 30 cm anthropomorphic chest phantom was scanned on a 64-multidetector CT, with the center of the phantom aligned to the gantry isocenter. Scanning was repeated after placing a bismuth breast shield on the anterior surface with no gap and with 1, 2, and 6 cm of padding between the shield and the phantom surface. The "shielded" phantom was also scanned with combined modulation and off-centering of the phantom at 2 cm, 4 cm and 6 cm below the gantry isocenter. CT numbers, noise, and surface radiation dose were measured. The data were analyzed using an analysis of variance.
RESULTS
The in-plane shield was not associated with any significant increment for the surface dose or CT dose index volume, which was achieved by comparing the radiation dose measured by combined modulation technique to the fixed mAs (p > 0.05). Irrespective of the gap or the surface CT numbers, surface noise increased to a larger extent compared to Hounsfield unit (HU) (0-6 cm, 26-55%) and noise (0-6 cm, 30-40%) in the center. With off-centering, in-plane shielding devices are associated with less dose savings, although dose reduction was still higher than in the absence of shielding (0 cm off-center, 90% dose reduction; 2 cm, 61%) (p < 0.0001). Streak artifacts were noted at 0 cm and 1 cm gaps but not at 2 cm and 6 cm gaps of shielding to the surface distances.
CONCLUSION
In-plane shields are associated with greater image noise, artifactually increased attenuation values, and streak artifacts. However, shields reduce radiation dose regardless of the extent of off-centering. Automatic exposure control did not increase radiation dose when using a shield.

Keyword

Tomography scanners; Computed tomography (CT); Radiation protection; Radiation dosage

MeSH Terms

Analysis of Variance
Artifacts
Phantoms, Imaging
Radiation Dosage
Radiation Protection/*methods
*Tomography, X-Ray Computed

Figure

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  Fig. 1 Transverse CT image and surface plot of phantom without shield. Three regions of interest were drawn on CT images at same level and position in anterior (A), central (C) and posterior (P) regions of phantom. As expected, vertebra has highest CT numbers on surface plot image.

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  Fig. 2 Original transverse CT images of phantom (*) acquired at 0 cm (Ao), 1 cm (Bo), 2 cm (Co), and 6 cm (Do) offsets (black arrows) and their corresponding CT numbers' contour maps (A, B, C, D). Compared to image acquired with 6 cm offset (D), images with 0-2 cm offsets have considerably higher CT numbers and lighter shade of gray on surface plots (A, B, C). Also note difference in CT numbers in anterior and posterior aspects of phantom.

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  Fig. 3 Off-centering of shielded phantom below gantry isocenter did not cause any substantial change in CT numbers (HU ant/center in anterior or central regions of the phantom), noise, and average surface radiation dose compared to scanning with appropriate phantom centering.

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  Fig. 4 Combined modulation technique (AEC) and plane shielding. A. Compared to images acquired without shield, CT numbers in anterior (HU ant) and central (HU center) regions of phantom are substantially higher with shield, with and without combined modulation, whereas CT numbers are similar in posterior region (HU post) of phantom. B. Compared to images acquired without shield, image noise in anterior (SD ant) and central (SD center) regions of phantom are substantially higher with shield, with and without combined modulation, whereas image noise is similar in posterior region (SD post) of phantom. In particular, combined modulation scanning was associated with greater image noise compared to fixed tube current scanning when phantom was scanned with shield for both techniques. C. Comparison of images acquired with or without shield, with both conditions having constant fixed tube current, combined modulation technique resulted in significantly lower individual (doses 1, 2, 3, 4) and average surface radiation doses (AVG DOSE).

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