SU‐GG‐I‐78: How Well Does the CT MAs Predict Patient Dose?

Purpose: CT clinical protocols often specify technique factors in terms of x‐ray tube voltage and tube current‐rotation time product (mAs). In this study, we investigated variations in CTDI 100 per unit mAs value at a fixed x‐ray tube voltage (120 kV). Method and Materials: Data were obtained for CTDI 100 in head and body phantoms for 47 scanners from the four major imaging equipment vendors from the ImPACT web site. Data were analyzed as follows: (a) all CTDI 100 (mGy/mAs) data were plotted as a single histogram, and the 10 th , 50 th , and 90 th percentile values determined; (b) CTDI 100 for each vendor were averaged and compared with each other; and (c) CTDI 100 for each vendor were plotted as a function of the introduction of the CT scanner model under consideration. Results: The median CTDI 100 values were 0.18 mGy/mAs and 0.076 mGy/mAs for head and body phantoms respectively. The ratio of the 90 th percentile value to the 10% value was 2.0 for body phantoms, and 1.8 for head phantoms. Average vendor CTDI 100 ranged between 0.17 and 0.20 mGy/mAs for head phantoms, and 0.07 and 0.10 mGy/mAs for body phantoms. There were no evident trends in CTDI 100 as a function of time for any one of the four vendors investigated. Conclusion: Conclusions. At the same kV/mAs, patient doses can differ by up to a factor of approximately two, and clinical protocols should therefore be based on patient dose (CTDI100) rather than the selected mAs value.