Over the last 18 months, more manuscripts have been published on cardiac ablation lesion imaging than in the 18 years before that. So it is quite an opportune time to have a quick look at how we can visualize ablation lesions in VT ablation. First of all, the bad news: we would be doing much better if there were no blood in the heart and if the heart would not be moving! Now, the good news: despite all that, there are still several imaging modalities that can provide some feedback.

The easiest way is by using plain, normal light. As the wavelength of visible light does not penetrate blood well enough, blood will have to be replaced. This is normally done by either an air- or fluid-filled balloon or an open, constantly irrigated transparent plastic hood, which are placed against the myocardial wall to allow direct visualization via an integrated fiber optic scope. The disadvantage is that color images can only be acquired of the endocardial surface, but not from the intramyocardial wall.

Extending the wavelength to infrared (1500 nm) allows one to look “through” blood, but only black-and-white images can be seen. Black-and-white display also occurs with OCT, which has a great spatial resolution of 10 micrometers, but only penetrates about 1-2 mm in the myocardial tissue.

Clinical intracardiac ultrasound often demonstrates increased brightness in the area of ablation, but this is somewhat inconsistent and partially due to tissue edema. Myocardial contrast echocardiography has been evaluated in animal models with a good lesion delineation, but requires intracoronary contrast delivery. With high-frequency ultrasound (25-35 Mhz), recent animal studies show promising real-time evaluation using M-mode imaging. Additionally, using the different elastic properties of ablated tissue, acoustic radiation force impulse imaging has demonstrated some promise in delineating lesions.

Rotational contrast-enhanced C-arm CT fluoroscopy is able to demonstrate an early contrast void followed by delayed enhancement in LV lesions in animal models. This is similar to the enhancement pattern seen on contrast-enhanced MRI and similar diffusion characteristics that might be expected given the similar molecular weight of iodine contrast and gadolinium of about 800 Da.

MRI is, of course, a very attractive candidate with contrast-enhanced imaging (showing the above-mentioned enhancement pattern and “very late enhancement"), but also non-contrast enhanced T1 and T2-weighted imaging. The latter is currently being evaluated as a possible candidate for the emerging field of real-time MRI, as it allows lesion visualization during ablation without the need for repeated contrast injection. Given a temperature-dependent predictable change in the proton resonance frequency shift, MRI is also able to noninvasively measure the intramyocardial temperature in animal models.

In summary, several technologies are currently being evaluated and improved upon to ultimately provide the physician with feedback on if and how much tissue has been ablated. This would be a welcome supplementation to the current surrogate markers of ablation lesions such as impedance measurements or even catheter contact force.

I have no doubt that in ten years from now we will have direct visual feedback when we tackle complex ablations. This will provide urgently needed information about when we have burned enough and where possible gaps might be hiding.

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).