Image Reconstruction for ECG-Triggered and ECG-Gated Multislice CT

Abstract

Computed tomography (CT) imaging of the heart and the coronary anatomy requires high temporal resolution to avoid motion artifacts and to achieve sufficient spatial resolution—at best submillimeter—to adequately visualize small anatomical structures such as the coronary arteries. Furthermore, the complete heart volume has to be examined within the time of one breath-hold. First attempts to use single-slice spiral CT systems for cardiac scanning were not convincing because of poor temporal resolution and insufficient volume coverage with thin slices (1,2). Since 1999, 4-slice CT systems with higher volume coverage speed and improved temporal resolution thanks to faster gantry rotation (rotation time down to 0.5 s) have been clinically used for electrocardiogram (ECG)-triggered or ECG-gated multislice CT (MSCT) examinations of the cardiac anatomy (3–9). Coverage of the entire heart volume with thin slices (4 × 1-mm/4 × 1.25-mm collimation) within one breath-hold period became feasible, allowing for new applications such as high-resolution CT angiographies of the coronary arteries (6–9). First clinical studies have demonstrated the ability of MSCT to characterize lipid, fibrous, and calcified coronary plaques (10). Despite all promising advances, challenges and limitations remain for cardiac MSCT with 4-slice detectors. Spatial resolution is still not sufficient to clearly depict stents or severely calcified coronary arteries (6,7). Temporal resolution is not yet adequate for patients with higher heart rates, and a diagnostic outcome cannot be guaranteed in these cases despite careful selection of the reconstruction interval (11,12). The scan time of about 40 s required to cover the entire heart volume (approx 12 cm) with 4 × 1-mm, 4 × 1.25-mm collimation is at the limit for a single breath-hold scan. In 2000, a shorter scan time was realized with 8 × 1.25-mm collimation, cardiac MSCT, which enables scan times of about 20 s. In 2001, a new generation of MSCT systems was introduced. With simultaneous acquisition of up to 16 submillimeter slices and gantry rotation times down to 0.4 s, spatial resolution in the transverse direction and temporal resolution are further improved, while examination times are considerably reduced (13,14). Sixteen-slice systems have the potential to overcome the limitations of established 4-slice/8-slice CT scanners, and first clinical studies have already demonstrated enhanced clinical performance (15,16). While system properties such as gantry rotation time and detector slice width determine the intrinsic temporal and spatial resolution of the data, dedicated scan and image reconstruction techniques are needed to optimize the outcome of cardiac CT examinations. In this chapter, we present the basics of ECG-triggered and ECG-gated MSCT scanning. We give an overview on image reconstruction techniques, starting with single-segment partial scan reconstruction and ending with multisegment approaches. We discuss the pros and cons of single- and multisegment reconstruction. We demonstrate the properties of reconstruction algorithms with patient scans, and end with a short summary and discussion.