Roy Marcus MD, Presenter: Nothing to Disclose

Fabian Bamberg MD, MPH, Abstract Co-Author: Speakers Bureau, Bayer AG Speakers Bureau, Siemens AG Research Grant, Bayer AG Research Grant, Siemens AG

Klement Neumaier, Abstract Co-Author: Nothing to Disclose

Maximilian F. Reiser MD, Abstract Co-Author: Nothing to Disclose

Konstantin Nikolaou MD, Abstract Co-Author: Speakers Bureau, Siemens AG Speakers Bureau, Bracco Group Speakers Bureau, Bayer AG

Thorsten R. C. Johnson MD, Abstract Co-Author: Nothing to Disclose

Proper CT imaging of morbidly obese patients remains an imaging challenge. The necessary increase in tube voltage and current results in dose length products (DLP) with high extrapolated effective dose estimates. However, actual equivalent dose exposition is presumably lower as an effect of the shielding of the adipose tissue layer. Thus, the aim of this study was to assess the association between conventionally estimated and measured radiation dose in morbidly obese patients.

The study consisted of an ex- and an in-vivo part. In the ex-vivo experiment, an Alderson Phantom was equipped with 108 thermo-luminescent detectors (TLD) throughout the lower chest, the abdomen and pelvis and scanned on a Dual Source CT (DSCT): (I) Slim phantom with automatic potential and current modulation and (II) Obese phantom embedded in a circumferential 30 cm layer of pork fat, simulating a patient with a BMI>35, with 2x140kVp and current modulation. In the in-vivo study, 7 patients (BMI > 35) referred for abdominal imaging were scanned on a DSCT with 2x120kVp and automatic current modulation. Effective dose was derived according to IRCP-103 (TLD ex-vivo), based on DLP with standard conversion factor k (DLP-based; ex and in-vivo), and using a Monte-Carlo-Simulation (MCS; ex- and in-vivo).

TLD, MCS and DLP based dose values did not show any differences in the ex-vivo setting simulating lean body habitus (I: 3 vs. 3.2 vs. 3 mSv). In the ex-vivo setting simulating obese body habitus (II), TLD and MCS based values did not a show a significant difference; however, both were significantly lower than DLP-based value (9.52 vs. 11.6 vs. 34.2 mSv, p<0.001). Applying the derived MCS-based model to the human CT acquisitions similar differences were observed (15.4±4.1 vs. 58.2±24.2 mSv). A significantly lower k value in the amplitudes of 0.0055-0.0075 provided more accurate estimates of effective radation dose in these patients.

Our results indicate that estimated and measured radiation dose in obese patients undergoing CT differs significantly with falsely documented high dose estimates in this population (up to 4-fold). Thus, a weight adapted k value of 0.0055-0.0075 for such patients may provide more accurate effective dose estimates.

Currently reported dose values in obese patients undergoing CT do not provide adequate estimates of radiation dose and should be evaluated carefully.

Marcus, R, Bamberg, F, Neumaier, K, Reiser, M, Nikolaou, K, Johnson, T, Differences of Radiation Dose Estimates Compared with Direct Measurements in Morbidly Obese Patients undergoing Abdominal Computed Tomography: An Experimental Ex-Vivo and Patient-based Study.  Radiological Society of North America 2014 Scientific Assembly and Annual Meeting, - ,Chicago IL. http://archive.rsna.org/2014/14014221.html