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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

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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Figure 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).

Mentions: The effects of a combined modulation technique on the CT numbers, quantitative image noise, and surface radiation dose are summarized (Fig. 4). No significant difference in the CT numbers was noted between the images obtained from scanning the shielded phantom at constant tube current and the combined modulation technique (p = 0.2). However, image noise was considerably higher for images acquired with the combined modulation technique compared to the constant tube current scanning (p < 0.0001). Likewise, surface radiation dose was also significantly less for the combined modulation technique compared to the constant tube current technique (p < 0.0001).


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)

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

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

Figure 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).
Mentions: The effects of a combined modulation technique on the CT numbers, quantitative image noise, and surface radiation dose are summarized (Fig. 4). No significant difference in the CT numbers was noted between the images obtained from scanning the shielded phantom at constant tube current and the combined modulation technique (p = 0.2). However, image noise was considerably higher for images acquired with the combined modulation technique compared to the constant tube current scanning (p < 0.0001). Likewise, surface radiation dose was also significantly less for the combined modulation technique compared to the constant tube current technique (p < 0.0001).

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