Purpose: Coronary artery calcium scoring (CACs) CT is performed at 120-kVp in current practice. The dose becomes a concern because of CACs’ screening nature. The dose delivery is empirical and may not be optimal. We aimed to find the optimal dose for different patient sizes. The result can also serve as the baseline for further dose reduction for CACs at lower kVp.
Methods: A surrogate noise (SN) was constructed as a function of the patient effective diameter and the volume CT dose index (CTDIvol). For the specified threshold noises at two known patient sizes covering the concerned range, the corresponding CTDIvol values were experimentally determined. To derive the CTDIvol as a function of the patient size within the range, we assumed that the threshold noise, thus SN, varies linearly with the patient size. The experiment was conducted using two thoracic phantoms (medium and large) with a moving heart module. They were scanned at 120 kVp using a prospectively gated CACs with a Siemens Force CT at different CTDIvol (0.76 -6 mGy). Its motion was driven by an ECG of 60 beats-per-minute. Noise measurement were made in the heart blood pool. A constant noise threshold (20 HU) and the recommended variable noise thresholds (20 HU for medium and 23 HU for large subjects) from Society of Cardiovascular CT (SCCT) were applied separately. The results from the model were further compared retrospectively with 33 clinical cases using dose modulation with a reference effective mAs of 80.
Results: According to the model, the doses to achieve the constant noise threshold (20 HU) were found more consistent with the observed patient doses, but switching to the SCCT variable thresholds resulted in a dose reduction of up to 25%.
Conclusion: A quantitative model was established to obtain the patient-size dependent optimal dose for CACs scans.
Not Applicable / None Entered.