This editorial refers to ‘Real-time MRI guided cryoablation of the pulmonary veins with acute freeze-zone and chronic lesion assessment’ by J. Lichter et al. on pages .
Recently, the first clinical applications of interventional cardiac magnetic resonance (iCMR)-based cardiovascular interventions have been published.1,2 In these series radiofrequency (RF) ablations of typical atrial flutter were performed actively tracking specially designed catheters using cardiac magnetic resonance (CMR) guidance. The number of available devices that can be used in this iCMR-setting is very limited—even obtaining femoral access had to be transferred to a non-CMR environment because of a lack of suitable, MR-compatible devices such as needles and wires. Hereby, one of the major limitations of iCMR-guided cardiovascular procedures becomes obvious: the portfolio of products is still rather limited, and devices that we use in fluo labs on a routine and daily basis are still not available such as cardioverter-defibrillators or a 12-lead electrocardiograms (ECGs).
In this issue of EP-Europace, Lichter et al.3 describe their initial experience in using MR-compatible cryoballoons and tip catheters in interventional CMR. These devices have been technically modified from their original characteristics: the circular mapping catheter, which is normally placed distal to the balloon was removed and some features such as the temperature sensor because of their non-MR-compatibility. After establishing access to the left chambers eight canines have been transferred into a 3T iCMR suite.
One of the key questions when performing ablation procedures using iCMR is to be able to assess the acute lesion injury and especially to predict the chronic lesion size. Lichter et al.3 demonstrated that despite acute 100% circumferential ablation lesions the percentage decreased to 95% at 3 months of follow-up. In focal ablations the size of the acute lesion was overestimated by CMR imaging almost by three-fold compared with chronic lesion estimation by enhancement imaging and gross pathology.
Why interventional cardiac magnetic resonance?
The creation of transmural and durable lesions in ablation procedures has been one of the major challenges in achieving permanent arrhythmia control. In theory, intraprocedural evaluation of the lesion that has been created, and interaction with neighbouring tissues, could provide extremely valuable information to guide ablation procedures. The usefulness of iCMR-guided procedures (especially ablations) is nicely demonstrated in the study by Lichter et al.3: visualizing the lesion growth in a ‘live’ manner, allowing energy titration according to the lesion size that is quantified by imaging. Furthermore several technical tools will become available and will only be available in iCMR modality: thermometry in the tissue, detailed visualization of neighbouring structures (oesophagus etc.), and acute lesion assessment. Today, there is little doubt that the CMR obtained before the procedure and post-processed by a dedicated software, is extremely useful in guiding ablation for ventricular tachycardia, allowing precise estimation of diseased ventricular tissue and localizing channels of slow conduction.4 CMR could also demonstrate its usefulness in guiding re-do procedures5 or first ablation procedures6 at the atrial level: DECAAF (Delayed-Enhancement MRI Determinant of Successful Radiofrequency Catheter Ablation of Atrial Fibrillation) II study will analyse the utility of CMR to provide an individualized approach to guide ablation in persistent atrial fibrillation (AF).
Where do we go?
Bringing ablation tools into the CMR scanner is a potentially valuable idea: why shouldn’t we ablate right where we detect the diseased tissue? No export of images, no registration, no mapping of anatomical structures that are already available as 3D reconstructions. We probably have to rethink our current strategies especially related to energy sources and the way of application: CMR-compatible needle devices for injection of ethanol have been shown to be effective and maybe cryoenergy will play an important role in iCMR procedures. The study by Lichter et al.3 contributes significantly in pushing that field forward, improving our understanding and sending the warning to all of us: acute lesion dimensions will not necessarily translate into the same size of chronic lesions—at least in cryo-energy, but the observation of overestimation has also been made in early RF cases. If we succeed in predicting the permanent lesion dimensions in the acute setting, iCMR could for the first time overtake the conventional approach in a very crucial aspect: reducing re-do procedures by inducing the correct amount of energy in the initial procedure. Titration of energy is still probably the major problem in RF ablations, but maybe also in cryoablation cases: all attempts to create permanent isolation failed: adenosine testing, contact force with sophisticated algorithms, pace-and-ablate etc. did not succeed in improving the results of AF ablation procedures significantly.
Reconstruction of the left ventricle demonstrating the three-dimensionality of the scar and predicting possible channels for ventricular tachycardia.
iCMR—5 years from now
The increase of groups publishing in the field of iCMR-guided interventions clearly indicates the great interest of implementing the potential advantages of this approach into clinical practice. The more scientists will start their iCMR programmes, the easier it will be to push the development of urgently required technical innovations forward: industry has to provide more CMR-compatible tools like wires, needles, trans-septal access kits, defibrillators, 12-lead-ECG, 3D-mapping systems etc. The study by Lichter et al.3 for the first time demonstrates the usability of cryo devices in iCMR setting and nicely showed the potential but also the limitations of this technology.
Conflict of interest: P.S. has received travel support and research grants from Imricor. Member of advisory board of Abbott, Biosense Webster. L.M. has received compensation for lecturing and research grants from Abbott, Biosense Webster, Boston Scientific, Livanova, Medtronic. Stockoptions from Galgo-Medical.
Footnotes
The opinions expressed in this article are not necessarily those of the Editors of Europace or of the European Society of Cardiology.
