Marcel Van Straten PhD, Presenter: Nothing to Disclose

Paul Deak, Abstract Co-Author: Nothing to Disclose

Willi A. Kalender PhD, Abstract Co-Author: Consultant, Siemens AG Founder, VAMP GmbH Scientific Advisor, VAMP GmbH Shareholder, VAMP GmbH

While AEC adapts the tube current to the patient size, dose estimates are generally carried out for standard-sized patients. In this study we investigate the effect of AEC on the organ doses for patients of varying size.

Monte Carlo organ dose calculations (ImpactMC, VAMP GmbH, Erlangen, Germany) were performed for abdominal multi-slice spiral CT scan using several versions of a voxelized, whole-body patient model [Zubal et al., Med Phys. 1994 Feb; 21(2): 299-302] and typical scanner geometry. The size of the model was varied in two ways: (1) by scaling the dimensions of all voxels, keeping the body mass index constant; (2) by adding subcutaneous fat all-around. Attenuation values as a function of projection angle and table position were derived from each model. Using these values, the tube current was determined such that a constant image noise was obtained at the center for all table positions and patient sizes at a minimum exposure (i.e. for modulation with table position and projection angle).

As an example, we here report mean organ doses for the liver; other organs showed similar results. The dose for the original model (23 cm in anterior-posterior direction) was taken as a reference. A size increase of 5% (by scaling) resulted in an increase of 32% in the average tube current and an increase of 20% in dose. A size decrease of 5% (by scaling) resulted in a decrease of 25% of the average tube current and a decrease of 22% in dose. An increase of 3.6 cm (by adding fat) resulted in an increase of 134% of the tube current and 83% in organ dose.

When keeping the noise level constant with AEC, relatively small changes in patient size result in large changes of exposure levels. Organ doses are affected accordingly, but to a lesser degree.

This study increases the awareness of the dependence of organ doses on patient size, especially at CT scanners with automatic exposure control.

Van Straten, M, Deak, P, Kalender, W, Patient-Specific Organ Dose Determination for Patients of Varying Size on CT Scanners with Automatic Exposure Control (AEC).  Radiological Society of North America 2006 Scientific Assembly and Annual Meeting, November 26 - December 1, 2006 ,Chicago IL. http://archive.rsna.org/2006/4437319.html