Giulia Zamboni MD, Presenter: Grant, Toshiba Corporation

Darren D. Brennan MBBCh, Abstract Co-Author: Nothing to Disclose

Carol J Wilcox, Abstract Co-Author: Nothing to Disclose

Vassilios D. Raptopoulos MD, Abstract Co-Author: Research grant, Toshiba Corporation, Tustin, CA

The purpose of this study is to assess the impact of z-axis automatic dose modulation on mAs and signal-to-noise ratio (SNR).

We retrospectively (IRB waiver) examined 55 oncology patients who had undergone sequential MDCT examinations at our institution before and after the introduction of automatic z-axis dose modulation. For each patient, we compared a scan performed with a fixed-mA (selected by the technologist), performed on 4- to 64-row MDCT scanners, and a subsequent scan performed on a 16-row scanner using dose modulation (Noise Index: 11.57; mA range: 100–500). From each scan, four levels were selected (celiac trunk, left renal vein, iliac crest, and acetabulum roof) to log the mAs used, calculate SNR (SNR=HU/SD), and measure the image noise (standard deviation (SD) in the air).

The SNR was found to be significantly higher in the dose modulation group compared to the fixed-mA group, in both the upper abdomen (liver SNR: 8.4±2.1 vs. 7.4±2.8; p=0.0001) and the pelvis (psoas SNR: 4.2±0.9 vs. 3.6±1.1; p=0.004). The relationship between the SNR in the liver and the psoas was found to be consistent in the dose-modulation group (p<0.0001), while there was no correlation in the fixed-mA group. Image noise was significantly lower in the dose modulation group than in the fixed-mA group in both the upper abdomen and the pelvis (5.4±1 vs 8.9±3.7; p<0.0001). However, mean mAs used were 57.15 higher in the dose modulation group (p<0.0001).

The use of z-axis dose modulation decreases noise, improves SNR, and produces more homogeneous in quality studies. Further studies evaluating various noise indexes are warranted to achieve optimal dose reduction–image quality studies.

Z-axis dose modulation produces homogeneous in quality studies.

Zamboni, G, Brennan, D, Wilcox, C, Raptopoulos, V, Impact of Z-Axis Dose Modulation on CT Exam Image Quality and Choice of mA.  Radiological Society of North America 2006 Scientific Assembly and Annual Meeting, November 26 - December 1, 2006 ,Chicago IL. http://archive.rsna.org/2006/4438179.html