Kevin J. Chang MD, Presenter: Nothing to Disclose

Michael A. Heisler MD, Abstract Co-Author: Nothing to Disclose

Walter Huda PhD, Abstract Co-Author: Consultant, Siemens AG Research support, Siemens AG President, Huda's Physics in Medicine LLC Author with royalties, Wolters Kluwer nv Author with royalties, Medical Physics Publishing

Grayson L Baird MS, Abstract Co-Author: Nothing to Disclose

William W. Mayo-Smith MD, Abstract Co-Author: Royalties, Reed Elsevier Royalties, Cambridge University Press

Reducing kVp in CT colonography (CTC) without iterative reconstruction has been shown to decrease radiation dose at all patient sizes at a cost of increased image noise. The purpose of this study was to show the effect of patient size on radiation dose in exams using iterative reconstruction.

This retrospective study included 113 patients undergoing CTC. Each patient underwent a supine scan at 120 kVp and a prone scan at 100 kVp. All other scan parameters including automatic dose modulation (noise index) were unchanged. 63 patients had the exam performed without adaptive statistical iteration (ASiR) and 50 patients had 30% ASiR in the protocol. CT Dose Index (CTDIvol), Dose Length Product (DLP), and patient size (AP diameter at the level of the right renal hilum) were recorded at both 120 and 100 kVp. A general linear model with a logarithmic transform was used to compare CTDIvol and DLP versus patient size and kVp prior to and following incorporation of ASiR.

Scans with ASiR had significantly lower CTDIvol and DLP relative to scans without ASiR. As size increased, CTDIvol and DLP also increased non-linearly, regardless of ASiR and kVp (all p<0.0001). For scans without ASiR, CTDIvol and DLP were significantly higher (intercept) at 120 than at 100 kVp (p=0.009 and p=0.0001); however, the relationship between size and both CTDIvol and DLP was steeper at 100 than at 120 kVp (p=0.13 and p=0.02). However, with ASiR, while smaller patients had lower CTDIvol and DLP at 100 compared to 120 kVp (p=0.0023 and p=0.0004), the relationship between size and both CTDIvol and DLP at 100 kVp was steeper than at 120 kVp (p=0.0013 and p=0.0001) resulting in no significant dose reduction in larger patients. The relative amount of dose reduction with reducing kVp when using ASiR diminishes as patient size increases. Specifically, at 100 kVp, patients larger than 26 cm in AP dimension have a steeper increase in CTDIvol and DLP relative to 120 kVp with doses surpassing those at 120 kVp with increasing size.

The relationship between dose reduction and patient size in CTC exams using ASiR is complicated. Larger patients (over 26 cm) appear to have decreased dose reduction benefits from lower kVp exams when ASiR is employed.

While kVp reduction is a viable option to decrease radiation dose, dose savings may not be realized when combined with iterative reconstruction in larger patients.

Chang, K, Heisler, M, Huda, W, Baird, G, Mayo-Smith, W, Reducing Radiation Dose at CT Colonography: The Size-dependent Effect of Iterative Reconstruction on Reduced kV Imaging.  Radiological Society of North America 2013 Scientific Assembly and Annual Meeting, December 1 - December 6, 2013 ,Chicago IL. http://archive.rsna.org/2013/13044272.html