Cardiac Motion

Cardiac Motion, a major obstacle for obtaining adequate coronary MRA images, can be divided into two types: motion related to intrinsic cardiac contraction/relaxation and motion due to superimposed diaphragmatic and chest wall movement during respiration. Since the extent of each motion supercedes the diameter of the coronary artery, blurring artifacts of the coronary lumen occur unless adequate motion suppression techniques are used. To account for intrinsic cardiac motion, ECG gating is absolutely essential. However, considerable ECG signal degradation occurs because of ra-diofrequency field and gradient-switching noise. To overcome this, a vector ECG approach has been found to be very robust for R-wave detection as compared to alternate gating strategies such as peripheral pulse detection. However, under the influence of a strong static magnetic field, the so called magnetohydrodynamic effect is enhanced and an artifactual voltage overlaid to the T-wave of the ECG results. This artifactual augmentation of the T-wave may frequently mislead the R-wave detection algorithm so that triggering is performed on the T-wave instead of the R-wave. This results in serious artifacts on coronary MRA and coronary vessel wall images. Since this artifact increases with field strength, this presents a major challenge for MRA, particularly at higher field strengths such as 3 Tesla. However, by analyzing the ECG vector in 3D space [4], the true T-wave can be separated from the artifactual T-wave augmentation. Moreover, reliable R-wave detection has recently been shown to be feasible even at higher field strength [5].

Another issue lies with actual coronary artery motion which occurs in a triphasic pattern during the cardiac cycle. Hence, mid-diastolic diastasis has been identified as the preferred time for image acquisition as it also coincides with the interval of rapid coronary filling. This period is inversely related to the heart rate and can be determined using a heart rate dependent formula. However, because of considerable interpatient variation, a recommendation is to determine a patient specific dias-tasis period which can be achieved by acquiring a cine image perpendicular to the long axis of the proximal/mid right coronary artery (RCA).

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