Christoph Suess PhD, Presenter: Nothing to Disclose

Rainer Raupach PhD, Abstract Co-Author: Nothing to Disclose

Jean Gauchiran, Abstract Co-Author: Nothing to Disclose

Dominique Sirinelli MD, Abstract Co-Author: Nothing to Disclose

The CT dose index (CTDI) is typically determined using standard phantoms for body and head according to IEC 60601-2-44. It can be used to compare the applied dose for different modes or scanners or to compare to reference dose levels. But in its current definition it does not well relate to the absorbed dose for a specific patient. Especially in paediatric examinations, the dose measured in the 32 cm CTDI phantom significantly underestimates the patient dose.

CTDI values were measured for a series of 4 water equivalent oval phantoms with size between 8 cm x 12 cm and 24 cm x 36 cm. The patient’s size and density distribution are automatically evaluated from the axial CT image by a software tool implemented on the scanner’s console. Evaluation is done automatically after the CT examination. The patient adapted CTDI corresponding to the selected slice is calculated by interpolation between the measured sample points, where both shape and in-plane attenuation information is taken into account.The estimation has been verified using a state-of-the-art Monte Carlo simulation on the basis of the reconstructed CT volume data set.

The dose for 200 paediatric patients was evaluated with this technique. Particularly for small patients, the patient adapted CTDI was up to a factor of 2.5 larger than the standard CTDI displayed on the CT console. The calculated CTDI values were in good agreement with the dose calculated by Monte Carlo simulations. Errors were less than 10%.

The displayed CTDI is not always a reasonable estimate for the patient dose. Based on the axial CT images, a patient adapted CTDI can be calculated, which gives a realistic estimation of the patient’s absorbed dose.

D.S.: Employee of CHU ToursR.R.,C.S.,J.G.: Employee of Siemens Medical Solutions

Suess, C, Raupach, R, Gauchiran, J, Sirinelli, D, Estimation of the Patient Dose by Size Adapted CT Dose Index for Pediatric Examinations.  Radiological Society of North America 2004 Scientific Assembly and Annual Meeting, November 28 - December 3, 2004 ,Chicago IL. http://archive.rsna.org/2004/4413852.html