A look over the studies published in 2011 evaluating the use of MRI for ischemic VT ablation demonstrates an encouraging development: their number nearly equals all the studies published previously. Thus, I thought it would be appropriate to use the year's end to take a look back.

In 2008, Codreanu et al(1) published the first study comparing cardiac MRI with voltage maps in ten patients. This study found that 1.54mV was the best bipolar cut-off to differentiate between MRI scar and non-scar, confirming that the frequently used clinical cut-off of 1.5mV established in a swine model by investigators at the University of Pennsylvania seemed to stand the test of time. Interestingly, while MRI scar and voltage scar correlated in general quite well, a clear mismatch between the two modalities was seen in one-third of patients. A similar number of mismatches was found in 2011 by Winjmalen et al(2) in 15 post-infarct patients. Indeed, work in our lab(3) in 2011 showed that a small rim (~2 mm) of surviving myocardium can mask underlying scar or that thin layers (<25% transmurality) of endocardial scar may not be detected due to large far-field signals of adjacent myocardium. While a general good correlation between voltage and MRI scar has been seen in all studies, they also demonstrate that a perfect 1:1 prediction between MRI and voltage are rarely biological reality. In most human studies, the MRI-defined scar area approximates the area of abnormal voltage (<1.5mV) rather than the area of voltage-defined scar (<0.5mV). An important and consistent finding brought forward by Desjardins et al(4) and since then confirmed by all other investigators is that the successful ablation sites for reentrant, post-infarct VT always demonstrate areas of delayed enhancement.

Two studies published in 2011 attempt to integrate the concept of “grey zone” (thought to represent heterogenous areas consisting of scar and alive myocardium) into VT ablations. Perez-David et al(5) found channels of “grey zone” in 88% of patients with monomorphic VT compared to only 33% of patients without VT. These “grey zone” channels corresponded in several of the patients to voltage-map channels. Andreu et al(6) used the voltage map as the gold standard and demonstrated that a full-width-half method at 60% of the voxel maximum was best to approximate the 0.5mV/1.5mV findings. Applying these settings, the investigators were able to identify 81% of the voltage channels. These findings raise the possibility that with good quality MRI data, one might be able to identify channels relevant for reentrant VT. On the other hand, some definite differences between MRI and voltage maps, even after applying these careful matching algorithms, seem to confirm that technique-intrinsic differences will persist. Voltage mapping has evolved as the gold standard for scar delineation in the field of electrophysiology, while MRI has established itself as the most sensitive gold standard for fibrosis imaging in radiology. We should expect some differences between the techniques. The slight difference in information may not have to be a disadvantage: both modalities may actually provide supplementary information that may give more insight in the arrhythmia process rather than have both techniques duplicate the exact same data.

References
1. Codreanu A, Odille F, Aliot E, et al. Electroanatomic characterization of post-infarct scars comparison with 3-dimensional myocardial scar reconstruction based on magnetic resonance imaging. J Am Coll Cardiol 2008;52:839-842.
2. Wijnmaalen AP, van der Geest RJ, van Huls van Taxis CF, et al. Head-to-head comparison of contrast-enhanced magnetic resonance imaging and electroanatomical voltage mapping to assess post-infarct scar characteristics in patients with ventricular tachycardias: real-time image integration and reversed registration. Eur Heart J 2011;32:104-114.
3. Dickfeld T, Tian J, Ahmad G, et al. MRI-Guided ventricular tachycardia ablation: integration of late gadolinium-enhanced 3D scar in patients with implantable cardioverter-defibrillators. Circ Arrhythm Electrophysiol 2011;4:172-184.
4. Desjardins B, Crawford T, Good E, et al. Infarct architecture and characteristics on delayed enhanced magnetic resonance imaging and electroanatomic mapping in patients with postinfarction ventricular arrhythmia. Heart Rhythm 2009;6:644-651.
5. Perez-David E, Arenal A, Rubio-Guivernau JL, et al. Noninvasive identification of ventricular tachycardia-related conducting channels using contrast-enhanced magnetic resonance imaging in patients with chronic myocardial infarction: comparison of signal intensity scar mapping and endocardial voltage mapping. J Am Coll Cardiol 2011;57:184-194.
6. Andreu D, Berruezo A, Ortiz-Pérez JT, et al. Integration of 3D electroanatomic maps and magnetic resonance scar characterization into the navigation system to guide ventricular tachycardia ablation. Circ Arrhythm Electrophysiol 2011;4:674-683.

Timm Dickfeld, MD, PhD is the Chief of Electrophysiology at the VA Baltimore, Associate Professor of Medicine at the University of Maryland, and Founder of the Maryland Arrhythmia and Cardiology Imaging Group (MACIG).