Coronary artery calcium scoring at lower tube voltages - Dose determination and scoring mechanism

Population dose has been a concern with coronary artery calcium scoring CT since it is performed in adults with borderline risk. Lower tube voltage acquisitions are appealing but there are no agreed schemes for reduced dose determination. Moreover, conventional scoring cannot be used without changing the multiple Agatston thresholds.

By applying consistent calcium contrast-to-noise ratio to two anthropomorphic heart phantoms (medium and large) with 3-cm hydroxyapatite (HA) inserts, scanned using a dual-source CT, the relationship was derived between the volume CT dose index (CTDI_vol) at lower tube voltages and the baseline CTDI_vol at 120 kVp. The baseline CTDI_vol was obtained using the noise thresholds from the images acquired at 120 kVp. To preserve the conventional Agatston thresholds, down-scaling with the found factors was applied to images acquired at lower voltages with a dynamic heart module and 1.2-5 mm inserts (50-400 mg/cc) on the coronary tracks. Scores were evaluated on the scaled images by six readers.

The CTDI_vol at lower voltages was related to the baseline CTDI_vol following a power form of the voltage (index 1.246), regardless of the phantom size. The baseline CTDI_vol was 1.5 and 4.5 mGy, for the medium and large phantoms, respectively. Correspondingly, the reduced CTDI_vol at 100-70 kVp were 1.28-0.76 mGy, and 3.57-2.32 mGy. The downscaling factors were 0.88-0.63. The calcium scores at lower voltages were found within 12 % of the ground-truths.

A vendor-independent approach was established to obtain the reduced dose and correct coronary calcium scores at lower tube voltages.