Purpose or Case Report: To review the basics of CT dose calculation, in particular the nuances of dose calculation / estimation in cardiac CT and the many and varied methods of dose reduction in cardiac and cardiovascular CT.
Methods & Materials: We summarise the main themes covered in the CT physics portion of our advanced cardiovascular CT fellowship program.
Results: There is increasing evidence from recent studies that medical radiation exposures during childhood are associated with increased risks of subsequent malignancies, however, CT imaging can play a key role in the assessment of the heart and coronary and great vessels in neonates and young children who are too unwell or anatomically complicated for adequate assessment via echocardiography or cardiac MRI.

Measures of dose:
We explain the difference between absorbed and effective dose, define CTDIvol and how this is measured, define DLP and the significance of patient weight and height on effective dose achieved per unit DLP, and define size-specific dose estimates and how these might be useful for understanding paediatric CT doses.

We explain the origin of the conversion factors used to estimate effective dose from DLP and the significance of historic use of a standard chest CT conversion factor approximately half the more recently calculated cardiac CT specific conversion factor and the significance this has on reported dose in the literature.

Dose reduction techniques:
Topics discussed include:
1) Optimization of tube voltage (kV) and current (mA) and the benefit of lowering the kV to increase the conspicuity of contrast medium.
2) Adaptive dose control and the effects of automatic exposure controls on end doses achieved.
3) High pitch and dual source scanning
4) The effect of iterative reconstruction on image quality and the potential for lower diagnostic doses
5) Modes of ECG gating (retrospective gating with and without ECG controlled dose modulation, prospective gating in end-diastolic and end-systolic phases and ultra-high pitch single shot acquisitions (FLASH)
Conclusions: There have been significant advances over the past 5-10 years in dose reduction, but a basic understanding of CT physics and dose calculation is vital in realising the potential of these techniques and in assessing future innovations. It is important that we as imaging experts are able to clearly explain to our referrers and patients, the risks associated with CT, the benefits of CT in the correct situation and the measures taken to reduce dose to as low as is reasonable achievable.