Purpose: In multi-slice CT, manufacturers have implemented automatic tube current modulation (TCM) algorithms.
These adjust tube current in the x-y plane (angular modulation) and/or along the z-axis (z-axis modulation) according to
the size and attenuation of the scanned body part. Current methods for estimating effective dose (ED) values in CT do
not account for such new developments. This study investigated the need to take TCM into account when calculating ED
values, using clinical data.
Methods: The effect of TCM algorithms as implemented on a GE BrightSpeed 16, a Philips Brilliance 64 and a Siemens
Sensation 64 CT scanners was investigated. Here, only z-axis modulation was addressed, considering thorax and
abdomen CT examinations collected from 534 adult patients. Commercially available CT dosimetry software (CT expo
v.1.7) was used to compute EDTCM (ED accounting for TCM) as the sum of ED of successive slices. A two-step
approach was chosen: first we estimated the relative contribution of each slice assuming a constant tube current. Next a
weighted average was taken based upon the slice specific tube current value. EDTCM was than compared to patient ED
estimated using average mA of all slices.
Results and Conclusions: The proposed method is relatively simple and uses as input: the parameters of each protocol,
a fitted polynomial function of weighting factors for each slice along the scan length and mA values of the individual
patient examination. Results show that z-axis modulation does not have a strong impact on ED for the Siemens and the
GE scanner (difference ranges from -4.1 to 3.3 percent); for the Philips scanner the effect was more important,
(difference ranges from -8.5 to 6.9 percent), but still all median values approached zero (except for one case, where the
median reached -5.6%), suggesting that ED calculation using average mA is in general a good approximation for EDTCM.
Higher difference values for the Philips scanner are due to a stronger current modulation in respect to the other scanners
investigated. It would be interesting to repeat the study by collecting patients in a prospective way, for whom the weight
and height are know and use a dedicated patient dosimetry software to calculate the dose. If the use of TCM has a larger
impact on calculated effective dose, appropriate correction factors should be used.