Optimal dose determination for coronary artery calcium scoring CT at standard tube voltage

Coronary artery calcium scoring (CACs) at 120 kVp is the standard practice. It is an important tool for preventative management of asymptomatic patients. However, the current dose delivery, albeit patient-size dependent, does not connect the CACs specific noise requirement to the dose, causing significant dose variations. We propose a new approach for optimal dose determination by incorporating the patient-size dependent noise threshold.

A polyethylene-based Mercury phantom of various diameters was scanned with a dual-source CT using CACs gating at different volume CT dose index (CTDI_vol). The relationship of noise to the diameter and CTDI_vol was obtained. The phantom diameter was then converted to the patient chest diameter through a retrospective analysis of a clinical cohort (N = 140). Finally, the patient-size dependent noise threshold was applied, and the optimal dose was derived. The prescribed doses were compared with those from a clinical CACs cohort (N = 262).

A power-exponential relationship was found for the noise versus CTDI_vol and phantom diameter (R² = 0.988). The phantom diameter versus the patient effective diameter was found to obey a linear relationship (R² = 0.998). Two noise threshold settings were made for dose options: one for more dose saving, and another for tighter noise constraint. Retrospective comparisons with clinical CACs studies showed an average dose reduction of 23% in 80.5% of the cases with option 1. The average dose reduction is 23% in 77.9% of the cases with option 2.

A new optimal dose scheme dictated by the target noise was established for CACs at 120 kVp. The proposed dose modulation can serve as the baseline from which further dose reduction is possible.