Investigations of CT Dose with Contrast Agent and Its Effects on the CTDI

Abstract

Purpose: Computed tomography is a leading imaging technique for head & neck and brain and most of these imaging protocols iodine-based contrast media are utilised. The chief aim of this research is to utilize the effects of the contrast media “CM” used in computed tomography “CT” which is used to enhance subject contrast on the delivered CT via its inclusion into the CT dose index “CTDI”, and to introduce a simple method to determine this effect via the available CT numbers at the imaged targets. Method: The CT dose increase is estimated theoretically and measured experimentally and then related to the average CT number in the volume of CM uptake. A factor dependent on CM concentration and beam energy is added to the CTDI equation to represent the increased dose burden. A simple holed Perspex phantom was built to measure the variation of imaged CT number. CT Gafchromic type film was alternately imaged in a reservoir of CM and water. The relative difference in the dose burden as obtained by scanning the two films represents the dose difference and hence the CM dependent increase. Results: Measured dose effects due to the inclusion of the CM varied depending on the concentration. The increase in dose is estimated to be about 17% for 20% contrast media in the target while that for 10% by volume is around 6.6%. These are estimated from the CT numbers. Patients’ data also shows influence of the CM on the CTDI values. Conclusion: The dosimetric effects of the contrast media are included into the CTDI and can be estimated by using the CT numbers obtained.