Optimal Cardiac MR scanning techniques in patients with arrhythmia
Consultant Cardiothoracic Radiologist, Department of Radiology and imaging services, Kovai Medical Centre & Hospital, Coimbatore, Tamil Nadu.
“Cardiac MR is a specialized scan requiring unique skills to perform and obtain good quality images. Presence of arrhythmia during scanning is a major stumbling block in this and we shall see the ways this can be overcome with diagnostic quality images.”
Cardiac MR is done using ECG gating to overcome the inherent continuous physiologic movement of the heart. The ECG gating can be prospective or retrospective. As a thumb rule, cine and flow images are usually done using retrospective gating and morphological images are done using prospective gating. A single cardiac MR image is obtained by collecting data through multiple cardiac cycles or R-R intervals and using these multiphasic data to reconstruct an image. When the heart rate is regular and stable, there is minimal sampling error across multiple R-R intervals resulting in good quality images. However, when the heart rate is irregular, it results in spatial and temporal inconsistencies in images acquired over multiple R-R intervals. This results either in prolonged scan duration or unacceptable artifacts. A few modifications that can be done to improve image quality/ reduce scan duration in patients with arrhythmia are as follows.
Functional cine imaging:
- Arrhythmia rejection when we encounter only few premature contractions to discard data from abnormal cardiac cycle.
- Preferable to try retrospective gating as the first resort. Reduce the number of phases per R-R interval to reduce scan duration – Minimum of 12 phases per cardiac cycle to obtain reasonable quality functional dataset for volume and wall motion assessment.
- Reduce the number of segments per cycle according to the shortest R-R interval encountered – Will increase scan duration without compromising scan quality.
- Reduce the phase percentage to reduce scan duration – Not less than 50%.
- R-R window percentage – Higher value accepts greater variation in HR at the expense of cardiac blurring.
- Using free breathing cine sequence with respiratory gating to overcome prolonged scan duration with arrhythmia.
- Prospective triggering with minimum number of phases corresponding to the shortest R-R interval encountered. The downside is loss of late diastolic data of cardiac cycles with prolonged R-R interval.
- Use pulse gating instead of ECG gating, as in some patients not all ECG variations are conducted to pulse and will result in less arrythmia related blurring.
- Arrhythmia with short R-R interval may prolong scan duration due to insufficient trigger delay in a single cardiac cycle. A free breathing single shot imaging technique for late gadolinium enhancement can be used. But, it results in reduction in spatial and contrast resolution.
- Respiratory gated free breathing 3D PSIR sequence with navigator for tracking diaphragmatic movement can be used. This being independent of breath hold duration, can give good images with prolonged scan duration.
- In tachyarrhythmia, pulsing every second R wave to allow sufficient time for recovery of inversion pulse.