Michael A. Heisler MD, Presenter: Nothing to Disclose

Kevin J. Chang MD, Abstract Co-Author: Nothing to Disclose

Jason T. Machan PhD, Abstract Co-Author: Nothing to Disclose

William W. Mayo-Smith MD, Abstract Co-Author: Nothing to Disclose

Reducing kVp in CT colonography (CTC) results in a decrease in radiation dose at a cost of an increase in image noise. The purpose of this study was to assess the impact of the use of iterative reconstruction to mitigate the increase in noise associated with a lower kVp.

Following a standard cathartic bowel preparation and diatrizoate fluid tagging for CTC, 20 patients underwent a supine CT scan at 120 kVp and a prone CT scan at 100 kVp after automated CO2 insufflation. All other scan parameters including automatic dose modulation were unchanged. Supine and prone datasets were reconstructed at 1.25 mm slice thickness with filtered back projection (FBP) as well as increasing percentage blends of adaptive statistical iterative reconstruction (ASIR) at settings of 30%, 60%, and 90%. Multiple bilateral regions of interest (ROI) were traced in air, psoas muscles, and tagged colonic or enteric fluid. Noise was measured as the mean standard deviation of air attenuation outside the patient. The mean attenuation of tagged fluid was recorded at all 4 reconstruction settings at both 120 and 100 kVp. Mean soft tissue attenuations (psoas) were also recorded to calculate a contrast to noise ratio (CNR). Noise, tagged fluid and soft tissue attenuation, and CNR were compared using two-tailed paired t-tests.

Decreasing kVp from 120 to 100 results in a significant increase in image noise and tagged fluid attenuation at all ASIR levels (p<0.001 for all measures). Increasing ASIR levels from 0% (FBP) to 30%, 60%, and 90% resulted in significant increases in CNR at both 120 and 100 kVp (p<0.001 for all comparisons). When compared to a benchmark noise level of 22.7 HU on the 120 kVp 0% ASIR dataset, an ASIR level of 30% at 100 kVp yielded no significant difference in image noise (24.6 HU, p=0.07). 100 kVp images reconstructed with ASIR levels of 60% and 90% yielded significantly lower noise levels of 20.4 HU (p=0.02) and 16.1 HU (p<0.001).

ASIR can be used to decrease image noise when using low dose 100 kVp CTC exams. An ASIR level of 30% on 100 kVp exams had comparable noise to 120 kVp exams without ASIR.

Iterative reconstruction can adequately compensate for the increase in image noise associated with low dose low kV imaging while improving CNR.

Heisler, M, Chang, K, Machan, J, Mayo-Smith, W, Radiation Dose Reduction for CT Colonography: Using Iterative Reconstruction to Decrease Noise at Lower kVp.  Radiological Society of North America 2012 Scientific Assembly and Annual Meeting, November 25 - November 30, 2012 ,Chicago IL. http://archive.rsna.org/2012/12030985.html