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Dose Determination at Lower Tube Voltages for Coronary Calcium Scoring Based On Consistent Contrast-To-Noise Ratio

D Zhang*, Y Zhou, W Paz, Cedars-Sinai Medical Center, Beverly Hills, CA


(Sunday, 7/12/2020) 1:00 PM - 2:00 PM [Eastern Time (GMT-4)]

Room: Track 1

Purpose: Coronary calcium scoring (CCS) has been often used in cardiac risk prognostication. As such, dose as low as reasonably achievable is desired. Lower tube voltage is proposed for this purpose due to the intrinsic calcium enhancement. However, the dose reduction practice has been empirical and inconsistent. We intended to obtain the dose at lower tube voltages for consistent contrast-to-noise ratio (CNR) as at the standard 120 kVp.

Methods: Two pairs of 800 mg/cc and 200 mg/cc hydroxyapatite inserts were placed inside a medium size heart phantom (CIRS), with the air cavity filled with soft tissue rods. The phantom was scanned on a Siemens Force CT using the dual-source axial mode and manual techniques at various combinations of kVp and CTDIvol (CTDIvol: 0.5 to 15 mGy, kVp: 70 - 120). Each scan was repeated 3 times and the images were reconstructed to a typical slice thickness (3 mm) without iterative reconstruction. The CNR of the inserts was measured among all slices and the mean values were fitted to a function of CTDIvol at each kVp. The results were further fitted to a function of kVp. A residual analysis was performed to demonstrate the fitting accuracy.

Results: The CNR was fitted to a power relationship with CTDIvol and kVp (r2 > 0.958). The power indices were found to be 0.427 and 0.554, respectively. The average residual was less than 3%. Based on the findings, in order to maintain the same CNR of calcium as at 120 kVp, the corresponding CTDIvol for 100 kVp, 90 kVp, 80 kVp, and 70 kVp was reduced by factors of 0.89, 0.79, 0.69, 0.59, and 0.50, respectively.

Conclusion: CNR has a power relationship with CTDIvol and kVp. The CTDIvol can be obtained for low kVps in order to maintain the same CNR as for 120 kV.


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