Lingyun Chen, Presenter: Nothing to Disclose

Lifeng Yu PhD, Abstract Co-Author: Nothing to Disclose

Shuai Leng PhD, Abstract Co-Author: Nothing to Disclose

Cynthia H. McCollough PhD, Abstract Co-Author: Research grant, Siemens AG

CT Volume-of-Interest (VOI) scanning involves adding material (filters) into the x-ray beam to strongly attenuate x-rays at the patient periphery. This lowers the radiation exposure delivered to the region outside a VOI while maintaining the image quality inside the VOI. The purpose of this work was to determine for different patient sizes and anatomic regions how much the exposure outside the VOI can be reduced without affecting image quality within the VOI.

Raw projection data from 4 patients (lateral widths at the liver of 33, 38, 43 and 48 cm) who underwent a clinically-indicated routine CT scan of the chest, abdomen and pelvis were used to retrospectively simulate a VOI scan. A previously validated noise insertion software tool developed in our lab was used to simulate reduced-expsoure scans at 7 dose levels (5, 10, 15, 20, 30, 40, and 50% of the routine dose level). The reduced exposure projection data were used to replace the routine dose projection data outside a 10-cm diameter VOI centered in scan field to simulate projection data acquired with VOI filters of different thicknesses. A smooth transition between routine and reduced exposure projections was used to avoid streak artifacts. The modified projection data were transferred back to the CT scanner and images were reconstructed using parameters identical to those of the routine dose scan. The mean CT number and noise level within the VOI were measured in regions of homogenous tissue for the routine and VOI images for each patient.

For small size patients, the differences in mean CT number within the VOI between the routine dose level and VOI images were less than 4 HU for peripheral exposure great than 5% of the original dose in any anatomic region. This increased to 15% for chest and 40% for abdomen and pelvis in largest patient. The noise levels inside the VOI were identical to those obtained with routine dose scan.

CT number accuracy and noise level within the VOI can be preserved when the radiation exposure outside the VOI is reduced by as much as 95%. More aggressive filtration outside the VOI can be tolerated in the chest and small patients compared to in the abdomen or pelvis and large patients.

The radiation dose can be significantly reduced outside a VOI (up to 95%) using the VOI technique without sacrificing diagnostic information inside the VOI.

Chen, L, Yu, L, Leng, S, McCollough, C, CT Volume-of-Interest (VOI) Scanning: Determination of Radiation Reduction Outside the VOI.  Radiological Society of North America 2011 Scientific Assembly and Annual Meeting, November 26 - December 2, 2011 ,Chicago IL. http://archive.rsna.org/2011/11009195.html