Imaging of the heart has played an important role in defining cardiac structures and characterising the arrhythmic substrates. Among the available imaging modalities, magnetic resonance imaging (MRI) has led to a better understanding of the mechanisms of cardiac arrhythmias and hence to improved ablation results. Felipe Bisbal, Juan Fernández-Armenta and Josep Brugada, all from the Arrhythmia Unit, Cardiology Department, Hospital Clinic, Barcelona, Spain, have published an article on the use of MRI in electrophysiology procedures in the journal Heart. They speak to Cardiac Rhythm News on the subject.
What is the role of imaging in the management of cardiac rhythm disorders?
Imaging of the heart is crucial in the invasive treatment of cardiac rhythm disorders. Fluoroscopy is the standard imaging modality to guide catheter positioning in most simple arrhythmias such as supraventricular tachycardias and typical atrial flutter. In this setting, a rough anatomical definition of the cardiac silhouette is usually enough to effectively and safely treat these arrhythmias. However, when approaching more complex substrates such as atrial fibrillation, atypical atrial flutters or ventricular tachycardias, a more comprehensive assessment of cardiac anatomy and better tissue characterisation is required. Importantly, the improvement of imaging modalities, specially cardiac magnetic resonance imaging (MRI), has led to a better understanding of the mechanisms of cardiac arrhythmias, and hence to improved ablation results.
What are the most common imaging modalities used in electrophysiology currently and which of those are the most advantageous in this field?
Currently, several imaging techniques are available in the setting of heart rhythm disorders; the most commonly used are echocardiography (transthoracic, transesophageal or intracardiac), cardiac computed tomography (CT) and MRI. Imaging can assist the selection of patients suitable for ablation, accurately define the cardiac anatomy and assess the arrhythmogenic substrate, as well as assess the outcome of the procedure in terms of improvement in cardiac remodelling and function. Each of the image modalities can provide information on some of the topics; however, MRI is probably the only all-in-one modality able to provide anatomy, function and tissue characterisation.
What is the role of MRI in ablation of cardiac arrhythmias?
Cardiac MRI has shown the ability to provide crucial information in the setting of ablation of complex substrates. As commented previously, this image modality can evaluate cardiac anatomy, function and importantly, characterise the myocardial tissue in a non-invasive manner. These data have been proven useful stratifying the risk of patients with rhythm disorders, ie. to determine which patients at risk of life-threatening arrhythmias will benefit from an implantable cardioverter defibrillator or which patients with atrial fibrillation will improve after ablation. Additionally, initial data show that the ablation of different substrates (ventricular tachycardia and atrial fibrillation) can be successfully treated under direct MRI guidance by integrating the post-processed images into the navigation system.
What cardiac arrhythmias (with ablation procedures) benefit most using this imaging modality and why?
Probably, scar-related ventricular tachycardia is the arrhythmogenic substrate that benefits most from MRI to guide ablation procedures. By localising areas of scar, MRI can help limit the mapping to areas of interest. MRI can show myocardial scars that are not identified by electroanatomic mapping. The Leiden group, in a paper published in the European Heart Journal (2011), showed that MRI allowed the visualisation of small or intramyocardial scars that were undetectable in the electroanatomical map, and helped in the ablation of ventricular tachycardia. Recently, we have published a series of 80 patients with non-idiopathic ventricular arrhythmias (Andreu et al, Eur Heart J 2014; in press). The clinical arrhythmia was successfully abolished in 96% of patients, all of them showing hyperenhancement on MRI. The presence of subepicardial late enhancement in the successful ablation site had suitable values of sensitivity and specificity in predicting an epicardial origin of the arrhythmia. In this paper, we proposed an algorithm to identify the epicardial origin of a ventricular arrhythmia based on the presence and distribution of hyperenhancement on MRI.
MRI is also especially useful in the setting of atrial fibrillation ablation. The non-invasive quantification of the atrial fibrosis by MRI (staging of atrial remodeling) seems to be a strong predictor of procedural success (Marrouche NF et al, DECAAF trial. JAMA 2014). On the other hand, in those patients with unsuccessful ablation, MRI can help to identify the areas with incomplete ablation (gaps) and guide the catheter positioning during a repeat atrial fibrillation ablation procedure to successfully re-isolate the pulmonary veins (Bisbal F et al, JACC Cardiovascular Imaging 2014; in press).
Are there any limitations with the current MRI systems for ablation of these cardiac arrhythmias?
Cardiac MRI has rapidly evolved and provides information of the anatomy of cardiac structures and an accurate tissue characterisation. However, there are still important limitations precluding the generalisation of the technique in this setting. The spatial resolution is still insufficient to accurately delineate millimetre conducting channels responsible for the ventricular tachycardia and to reliably assess the fibrosis in the thin left atrial wall. Acquisition times are still long, which ultimately impact image quality. On the other hand, MRI is still a relatively expensive and time-consuming technique that requires specially trained professionals. These limitations could delay the generalisation of its use in patients with cardiac arrhythmias.
Is the current MRI technology safe for patients and physicians?
MRI technology is based on strong magnetic fields and radiowaves and no radiation is used to obtain images. Thus, MRI is considered a safe imaging technique. Special attention must be taken in patients with implantable cardiac devices. The presence of implantable cardiac devices may create artefacts, hindering the interpretation of the images. On the other hand, although this image modality has been safely performed in patients with pacemakers or implantable defibrillators, the general consensus is to avoid MRI in this population. However, recent development of new MRI-compatible devices will allow patients to safely undergo an MRI.
