Masaaki Akahane MD, Presenter: Nothing to Disclose

Shuhei Komatsu, Abstract Co-Author: Nothing to Disclose

Kenji Ino PhD, Abstract Co-Author: Nothing to Disclose

Jiro Sato MD, Abstract Co-Author: Nothing to Disclose

Akira Hagiwara MS, Abstract Co-Author: Employee, General Electric Company

Kuni Ohtomo MD, Abstract Co-Author: Nothing to Disclose

The purpose of this study was to compare the image quality of usual-dose chest CT reconstructed with three different algorithms: retrospective ECG-gated half-scan, conventional non-gated full-scan, and newly developed non-gated overscan.

Eight patients underwent single chest CT scan using 64-slice scanner (LightSpeed VCT, GE Healthcare, Milwaukee, WI). First, dummy protocol of non-gated helical scan at a pitch of 0.969 was planned from the apex to the base of the lung with a noise index of 12 for automatic exposure control, and estimated dose-length product (DLP) was recorded, but this dummy protocol was not actually performed. Next, retrospective ECG-gated protocol at a pitch of 0.24 was planned for the same scan range as the dummy, and tube current was adjusted so as to make the estimated DLP equivalent to the dummy protocol. After the scan was performed, obtained raw data were reconstructed with 1) retrospective ECG-gated half-scan, 2) conventional non-gated full-scan, and 3) non-gated overscan. Non-gated overscan helical reconstruction is a newly developed algorithm that utilizes projection data from four rotations to make a single image. Acquired images were displayed on the monitor with lung and mediastinal window. Blurring, noise and overall image quality were assessed on a slice-by-slice basis by using a three-point scale. Statistical analysis was performed using Wilcoxon signed-rank test.

Mean DLP of the patients was 274mGy-cm, which was comparable to the average DLP (i.e. 400mGy-cm) of chest CT according to NRPB-W67 after correction for body weight; thus the scan protocol in this study could be regarded as usual-dose. Blurring was significantly lower on the images with retrospective ECG-gated half-scan. Noise was significantly lower on the images with non-gated overscan. Regarding overall image quality, retrospective ECG-gated half-scan was best for lung window; whereas non-gated overscan was best for mediastinal window.

Combination of retrospective ECG-gated half-scan with lung window and non-gated overscan with mediastinal window would be better than conventional helical scan with equivalent dose.

ECG-gated scan with overscan reconstruction algorithm can improve the image quality of chest CT without dose penalty.

Akahane, M, Komatsu, S, Ino, K, Sato, J, Hagiwara, A, Ohtomo, K, Image Quality of Usual Dose Chest CT: Comparison of Retrospective ECG-gated Half Scan, Conventional Nongated Full Scan, and Nongated Overscan Reconstructions.  Radiological Society of North America 2009 Scientific Assembly and Annual Meeting, November 29 - December 4, 2009 ,Chicago IL. http://archive.rsna.org/2009/8015345.html