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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

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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Figure 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.

Mentions: The effect of off-centering on CT numbers, quantitative image noise, and surface radiation dose for the shielded phantom is summarized (Fig. 3). As expected, the quantitative image noise increased with increasing off-centering distance, but there was no change in the CT numbers with changes in the off-centering distance. Moreover, no significant trend or statistical difference was noted in either of these three outcome variables (p > 0.08). The severity of streak artifacts did not change with increase in off-centering distance (score = 1 at all off-centering distances).


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)

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

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

Figure 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.
Mentions: The effect of off-centering on CT numbers, quantitative image noise, and surface radiation dose for the shielded phantom is summarized (Fig. 3). As expected, the quantitative image noise increased with increasing off-centering distance, but there was no change in the CT numbers with changes in the off-centering distance. Moreover, no significant trend or statistical difference was noted in either of these three outcome variables (p > 0.08). The severity of streak artifacts did not change with increase in off-centering distance (score = 1 at all off-centering distances).

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