Technical Principles of Cardiac Image Acquisition

Abstract

To virtually freeze cardiac motion and to avoid motion artifacts, very short exposure time is needed for the acquisition of transaxial slices. High temporal resolution is particularly important for imaging the coronary arteries as they are located very closely to the heart muscles and show strong movement during the cardiac cycle. The intensity of movement varies within the cardiac cycle. The strongest movement is present during contraction of the atria and the ventricles in systole. Imaging should be performed during the diastolic phase of the cardiac cycle as less movement is present during the filling phase and the least amount of blurring due to motion artifact is to be expected. Thus, image acquisition and reconstruction needs to be synchronized as accurately as possible with the movement of the heart, i.e. by using ECG information that is recorded in parallel with the CT scan acquisition. The duration of the phase with lowest cardiac motion during diastole narrows with increasing heart rate (Fig. 4.1). According to a rough estimation, a temporal resolution of about 250 ms is appropriate for motion-free imaging up to a heart rate of about 70 bpm, whereas 150 ms is required for clinically usual heart rates up to 100 bpm. Image acquisition during other phases of the cardiac cycle (e.g. systole) based on ECG information can be useful for evaluation of functional information. However, motion-free imaging during other phases than diastole requires about a temporal resolution of 50 ms (Stehling et al. 1991) and will usually not be possible with state-of-the-art CT systems. Thus, only larger or not rapidly moving cardiac anatomy should be assessed during phases of high cardiac motion.