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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 - Korean J Radiol (2009)

Bottom Line: 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).In-plane shields are associated with greater image noise, artifactually increased attenuation values, and streak artifacts.

View Article: PubMed Central - PubMed

Affiliation: Department of Radiology, Massachusetts General Hospital, Boston, MA 02114, USA. mkalra@partners.org

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.

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Related in: MedlinePlus

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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Figure 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.

Mentions: The surface plots of the CT numbers showed a substantial increase in the anterior aspect of the phantom with no offset of the shield, which decreased with increasing shield to surface offset (Fig. 2).


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 - Korean J Radiol (2009)

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.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC2651436&req=5

Figure 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.
Mentions: The surface plots of the CT numbers showed a substantial increase in the anterior aspect of the phantom with no offset of the shield, which decreased with increasing shield to surface offset (Fig. 2).

Bottom Line: 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).In-plane shields are associated with greater image noise, artifactually increased attenuation values, and streak artifacts.

View Article: PubMed Central - PubMed

Affiliation: Department of Radiology, Massachusetts General Hospital, Boston, MA 02114, USA. mkalra@partners.org

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.

Show MeSH
Related in: MedlinePlus