Automatic exposure control at single- and dual-heartbeat CTCA on a 320-MDCT volume scanner: Effect of heart rate, exposure phase window setting, and reconstruction algorithm

Purpose: To investigate whether electrocardiogram (ECG)-gated single- and dual-heartbeat computed tomography coronary angiography (CTCA) with automatic exposure control (AEC) yields images with uniform image noise at reduced radiation doses. Materials and methods: Using an anthropomorphic chest CT phantom we performed prospectively ECG-gated single- and dual-heartbeat CTCA on a second-generation 320-multidetector CT volume scanner. The exposure phase window was set at 75%, 70-80%, 40-80%, and 0-100% and the heart rate at 60 or 80 or corr80 bpm; images were reconstructed with filtered back projection (FBP) or iterative reconstruction (IR, adaptive iterative dose reduction 3D). We applied AEC and set the image noise level to 20 or 25 HU. For each technique we determined the image noise and the radiation dose to the phantom center. Results: With half-scan reconstruction at 60bpm, a 70-80% phase window- and a 20-HU standard deviation (SD) setting, the imagenoise level and -variation along the z axis manifested similar curves with FBP and IR. With half-scan reconstruction, the radiation dose to the phantom center with 70-80% phase window was 18.89 and 12.34mGy for FBP and 4.61 and 3.10mGy for IR at an SD setting SD of 20 and 25 HU, respectively. At 80bpm with two-segment reconstruction the dose was approximately twice that of 60bpm at both SD settings. However, increasing radiation dose at corr80bpm was suppressed to 1.39 times compared to 60bpm. Conclusion: AEC at ECG-gated single- and dual-heartbeat CTCA controls the image noise at different radiation dose.