Cardiac Imaging with Multislice Spiral Computed Tomography

Abstract

Subsecond multislice spiral CT offers high potential for phase-selective and quantitative imaging of the heart and the coronary arteries. We have developed new electrocardiogram (ECG)-based reconstruction algorithms and applied them to data acquired with a Siemens SOMATOM Volume Zoom. Two classes of algorithms were investigated: 180°MCD (multislice cardio delta), a partial scan reconstruction of 180°+δ with δ< fan angle and 180°MCI (multislice cardio interpolation), a piecewise linear interpolation between successive spiral data segments, i.e., successive measured slices and gantry rotations. Effective scan times as low as 56 ms can be achieved with 0.5 s rotation time and M=4 simultaneously measured slices for pitch values that allow the coverage of the complete heart during one breath hold. To validate such algorithms, but also to compare arbitrary scanners and scan protocols, we specified and built dedicated quality control phantoms for cardiac CT. The phantom’s anthropomorphic body can hold different inserts for calibration purposes (calibration insert) and for the assessment of image quality with moving objects (motion insert). The calibration insert contains calcifications of different sizes and densities of calcium hydroxyapatite (HA). The motion insert consists of a small water tank with different objects attached to water-equivalent rods moving on realistic paths and velocities inside the tank. Calcium objects attached to the rod can be used to compare reproducibility in calcium scoring and to perform resolution tests for moving objects. Three-dimensional (3D) paths and velocities are freely programmable on a PC which also generates the appropriate ECG signal to be used for prospective cardiac triggering or to synchronize retrospective cardiac gating techniques, such as 180°MCD and 180°MCI. Effective scan times of below 100 ms have been verified for the Volume Zoom with 180°MCI in phantom experiments and patient scans. Even for tachycardic situations (>100 bpm) and irregular pulse frequencies, 180°MCI performs very well. The freely programmable motion phantom in combination with the calibration phantom provide excellent tools to compare results for different modalities. The reproducibility for coronary calcium scoring was found to be significantly improved with the SOMATOM Volume Zoom and 180°MCI relative to electron beam CT (EBCT). The tools described appear adequate; a consensus on test procedures and criteria is still necessary.