Relationship between chest lateral width, tube current, image noise, and radiation exposure associated with coronary artery calcium scanning on 320-detector row CT

Background: The relationship between chest lateral width, tube current, image noise, and radiation exposure on 320-detector row CT has not been reported. Objective: We investigated the relationships between chest lateral width, estimated radiation exposure (DLPe), and image noise in 300 patients undergoing clinical coronary calcium scanning. Methods: Patients undergoing coronary calcium scanning with 320-detector row CT (prospective, volumetric mode, 120 kV of tube voltage, 100–550 mA of tube current, 0.5-mm detector width) were grouped by chest lateral width (small, medium, and large) from anteroposterior topograms and 100 consecutive patients were selected from each group (n = 300). Tube current, DLPe, and noise were compared among groups with Kruskal-Wallis or one-way ANOVA. Phantom experiments were performed to evaluate the accuracy of calcium quantification as a function of size and tube current. Results: Median tube current in small, medium, and large patients was 130, 200, and 250 mA, respectively (P < 0.0001). Despite the use of higher tube current settings, noise levels also increased with size (20.2 ± 4.5 HU, 22.0 ± 3.9 HU, and 25.1 ± 4.9 HU, respectively; global P < 0.001]). DLPe was significantly higher with increasing size (54, 83, and 104 mGy · cm, respectively; P < 0.0001]). Phantom experiments showed that 50–100 mA, 150–200 mA, and approximately 300 mA in small, medium, and large phantoms were associated with stable estimate of calcium. Conclusions: Increasing chest lateral width is associated with increasing radiation exposure and image noise. The use of 50–100 mA in small and 150–200 mA in medium patients is associated with acceptable noise and stable estimate of coronary calcium scan. In large patients, precise identification of individual calcified lesions remains difficult despite increasing tube current and radiation exposure.