Yoon Chul Nam RT, ARRT, Presenter: Nothing to Disclose

Moon Chan Kim RT, Abstract Co-Author: Nothing to Disclose

Dae Kyu Lee BS, Abstract Co-Author: Nothing to Disclose

Nam Soo Cho, Abstract Co-Author: Nothing to Disclose

Ji Hye Kim MD, PhD, Abstract Co-Author: Nothing to Disclose

Kyung Soo Lee MD, Abstract Co-Author: Nothing to Disclose

Myung Jin Chung MD, Abstract Co-Author: Nothing to Disclose

To quantify the number of overscanning rotation and overscanning length, and to assess their relatively serious contribution to dose in shorter scan range in 64 section body CT.

We evaluated the effect of overscanning length and number of overscanning rotation with change of scan parameters such as beam collimation and pitch factor with 64-section CT scanners of 4 major CT systems. We measured exposure doses for pediatric (120mm) and adult (400mm) phantom and calculated the proportion of over ranging dose in total dose. Wide beam collimations ranged from 28.8 to 40mm and narrow collimation, 16 to 25mm. Lower pitches ranged from 0.516 to 0.625, middle pitches, 0.906 to 1, and higher pitches, 1.375 to 1.4 in four scanner. Calculation method of overscanning length was as follow: We subtracted DLP of user planned from DLP of total exposed scan length, and divided it by CTDIVOL. Number of overscanning rotation was calculated through dividing the overscanning length by table speed.

Overscanning length ranged from 6.4 to 9.5 cm in using of higher pitches with wide beam collimation, from 2.3 to 4.1 cm in middle pitches with narrow beam collimation, and from 2.0 to 3.4 cm in lower pitches with narrow beam collimation, respectively. Overscanning rotation ranged from 1.16 to 2.15 in using of higher pitches with wide beam collimation, from 1.17 to 2.35 in middle pitches with narrow beam collimation, and from 1.71 to 2.57 in lower pitches with narrow beam collimation, respectively. The proportion of over ranging dose in total dose ranged from 35.3 to 44% in total DLP at pediatric scan, and from 13.8 to 19.2% at adult scan used wide beam with high pitch. In using of narrow beam with middle pitch, the proportion of over ranging dose in total dose ranged from 15.8 to 25.4% at pediatric scan and from 5.3 to 9.3% at adult scan.

The relative effect of overscanning was increased with short scan length, large collimation, and higher pitch. However, the degree of overscanning effects with increases in collimation and pitch were different between CT systems. Narrower collimation and lower pitch can minimize overscanning effect especially in pediatric examination.

In pediatric CT examination, overscanning effect is more seroius than in adult. Narrower collimation and lower pitch can minimize overscanning effect especially in pediatric examination.

Nam, Y, Kim, M, Lee, D, Cho, N, Kim, J, Lee, K, Chung, M, The Effect of Overscanning in Multidetector Helical CT Examination: Comparison between Four 64 Section CT Scanners.  Radiological Society of North America 2009 Scientific Assembly and Annual Meeting, November 29 - December 4, 2009 ,Chicago IL. http://archive.rsna.org/2009/8014088.html