The Challenging Role & Approaches to Cardiac MR in Arrhythmia
Dana Peters1

1Radiology and Biomedical Imaging, Yale University, New Haven, CT, United States

Synopsis

This material covers the basic and advanced approaches for preserving image quality in patients with arrhythmia, for cine, flow and late gadolinium enhancement techniques. Furthermore, it presents some advanced MRI methods for evaluating patients with arrhythmic disease, namely atrial fibrillation, ventricular arrhythmias, and arryhthmogenic right ventricular cardiomyopathy (ARVC).

Highlights

  • During arrhythmia, cardiac function and flow can be measured using prospective ecg-triggering or real-time imaging.
  • For most arrhythmias, 2D LGE is usually of good quality, although single-shot imaging may help. Several methods have shown improved quality for high resolution 3D LGE and T1 mapping, using pulse sequence modifications.
  • Cardiac MRI provides important information on scar patterns and ventricular and atrial function in these patients.

Target Audience

Those performing cardiac MRI of patients with arrhythmias.

Outcome and Objectives:

This material covers the basic and advanced approaches for preserving image quality in patients with arrhythmia, for cine, flow and late gadolinium enhancement techniques. Furthermore, it presents some advanced MRI methods for evaluating patients with arrhythmic disease, namely atrial fibrillation, ventricular arrhythmias, and arryhthmogenic right ventricular cardiomyopathy (ARVC).

Purpose:

Cardiac MRI requires exact synchronization of the data acquisition with the contraction of the heart, obtained with an electrocardiogram (ECG) signal of the MRI subject. ECG-gating is used to provide sharp imaging of cardiac function and scar/fibrosis and evaluation of cardiac flow. Patients imaged during arrhythmia have an irregular ECG waveform, leading to reduced image quality. Several MRI approaches exist to improve quality. Furthermore, in these patients, new methods are being developed directed towards evaluating the arrhythmic substrate itself, for diagnosis, prognosis and treatment.

Methods and Results:

Approaches to Cardiac MRI: When imaging patients who are experiencing arrhythmia, the RR interval changes from short to long and back. Mild or infrequent arrhythmia usually will not reduce quality. Solutions are to shorten the RR interval selected on the scanner, to the shortest potential RR interval that might be encountered during the imaging. Furthermore, some “arrhythmia” is an artifact of a poor quality ECG. For infrequent arrhythmias, arrhythmia rejection is available on the scanner. For coronary MRI, this has been shown to improve quality, by rejecting the arrhythmic beats (1). For cine imaging of left ventricle (LV), right ventricle (RV) and atrial function, prospective ecg-gating will provide better quality than retrospective during arrhythmia, because retrospective ecg-gating relies on each beat’s overall similarity, while prospective ecg-gating relies on the similarity of only matched frames (2,3). However, prospective gating only acquires images from systole and early diastole, so that the function during late diastole is not measured (i.e. the atrial kick). For cine imaging to evaluate LV chamber sizes and ejection fraction, it has been observed that each arrhythmic beat generates a different stroke volume compared to that in normal rhythm, depending on the subsequent beat (4). Therefore, arrhythmia rejection improves the image appearance, but may disguise the influence of arrhythmia on average ejection fraction or volumes. Real-time imaging is possible, and can provide acceptable quality, typically 3 x3 x 8mm resolution with <50 ms temporal resolution. For imaging of late gadolinium enhancement (LGE), segmented 2D LGE often performs well, because contrast is preserved using 2RR intervals between inversion pulses, even with arrhythmia. However, in some cases single-shot LGE may provide more information compared to segmented LGE (5). For high resolution 3D LGE, with 1RR between inversion, the image quality is variable, sometimes being surprisingly diagnostic, but other times showing ghosts and “fog” as well as motion blur. New solutions for 3D LGE have been demonstrated to improve quality (6-8). Patients with arrhythmia might often have devices. Improved LGE quality can be obtained using sequences less sensitive to off-resonance artifacts, e.g. wide band inversion pulses for LGE (9). T1-mapping of subjects during arrhythmia is also possible using modified methods (10-12), and imaging during systole may improve quality (13). For flow imaging with frequent arrhythmias, real-time phase contrast is possible, providing 40ms frames and 3 x 3mm2 spatial resolution (14).

Role of Cardiac MRI: MRI is capable of imaging the arrhythmic substrate, i.e. fibrosis. The most common arrhythmia is atrial fibrillation; these patients are often sent for contrast-enhanced MRA, for pulmonary vein isolation planning. These benefit from high resolution 3D LGE (15), to evaluate left atrial fibrosis, which may predict outcome of therapy (16). They also benefit from measurement of left atrial volumes and ejection fractions. Atrial strain may also be informative, but has not yet been evaluated for its clinical impact (17,18). Therapies for patients with ventricular arrhythmias resulting from LV scar also greatly benefit from 2D or 3D LGE (19,20), since the goal of these therapies is to “homogenize” the scar using ablation. In diagnosis of ARVC, MRI is often indicated, due to the complexity of this disease (21).

Discussion and Conclusions:

Many patients sent for cardiac MR have arrhythmia, and further improvement in cardiac MR protocols is needed to preserve image quality for these subjects, who benefit from evaluation by MRI.

Acknowledgements

No acknowledgement found.

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Proc. Intl. Soc. Mag. Reson. Med. 25 (2017)