Guiding EP Ablations: How PET & PET/CT Can Help

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RVBfMDIyMDExLTA1LTIzIDE0OjIxOjAy.jpg - 3D reconstruction
Figure 1 (top): Three-dimensional reconstruction allows the visualization of epicardium but also from endocardium, which can be visualized through the mitral valve plane. (A) A left lateral view allows the visualization of the epicardium with obvious wall defect due to the myocardial scarring described previously (white arrow, B). A right anterior oblique view (B) the “clipping” of the left ventricle allows the visualization of the epicardium and endocardium and demonstrates the corresponding myocardial thickness (endocardial surface; epicardial surface). Myocardial scarring is well visualized as wall defect (white arrow, C). PET positron emission tomography. Figure 2 (bottom): Endocardial voltage map was unable to detect myocardial scar, and all wall segments revealed >1.5 mV voltage (left lateral view, A). However, 3-dimensional positron emission tomographic scar map (ocher shell) reveals a large inferobasal lateral wall defect signifying myocardial scar, which was consistent with the exit site of the presenting ventricular tachycardia (white arrow, B). An epicardial voltage map finally confirmed a large, nontransmural scar (red area,

Due to the commonality of arrhythmias and the number of procedures necessary to treat them, practitioners have turned to cardiac PET and PET/CT to help guide electrophysiology (EP) procedures and aid in pre-procedural planning.

These nuclear modalities allow clinicians a better understanding of a patient's anatomic structure and activity and may be advantageous due to their ability to better visualize and characterize scar tissue and guide ablations. Yet, acceptance remains sparse.

Research in the field increasingly depicts how MRI, CT, PET and PET/CT-guided procedures may be useful in the EP lab to assist with various procedures. Therefore, more clinicians may begin to realize the importance of the inclusion of these strategies.

Guidance of VT ablations with PET or PET/CT

Most ventricular arrhythmias are associated with scaring, which is currently detected by abnormal signals which are recorded with an intracardiac catheter. However, this method may not always give the correct assessment, says Timm-Michael Dickfeld, MD, PhD, an associate professor of medicine at the University of Maryland, Baltimore (UMD).

In fact, almost 4 percent of the points recorded in the heart using this method had a false-low voltage or pseudo low voltage because the catheter could not be placed properly within the inferior or in the mid-anterior wall.

Dickfeld and researchers at the University of Maryland Medical Center (UMMC) in Baltimore who are part of the facility's Maryland Arrhythmia and Cardiology Imaging Group (MACIG), an interdisciplinary forum including several specialties from radiology to engineering, have combined PET/CT capabilities that provide both metabolic (via PET) and morphologic (via CT) information.

Looking for a comprehensive approach to correct for scar tissue with nuclear medicine techniques, the group uses Rubidium-82 and 18F-FDG coupled with PET/CT to evaluate the metabolic properties of the heart.

Dickfeld and colleagues create 3D scar maps to look at segmental analyses to quantitatively compare EP voltages and PET/CT inferred tissue properties.

The team has found that PET/CT accurately assesses left ventricular (LV) scar and border zones and is useful in understanding the scar characterization that may not be attained through EP voltage maps—the current "gold standard" [J Am Coll Cardiol Img 2008; 1:73-82].

Using PET/CT, MRI or CT, a 3D model of the heart or scar can be constructed and uploaded into the clinical mapping system to help define scar and border zones to guide ablations.

"We now have the ability to cut the heart into 720 small sections and register the metabolic data from the PET exam to the voltage data from the electrical data we obtain during a procedure," says Dickfeld. "By having certain reference points, we are able to overlay those very accurately so we can prescribe the electrical data and certain metabolic values and look at the areas where we can successfully terminate the arrhythmias."

And while SPECT systems may be less expensive, the better spatial resolution of PET scanning allows clinicians to better visualize scarring, Dickfeld notes. PET's spatial resolution is twice than that of SPECT.

Is two better than one for EP?

Figure 1 (top): Three-dimensional reconstruction allows the visualization of epicardium but also from endocardium, which can be visualized through the mitral valve plane. (A) A left lateral view allows the visualization of the epicardium with obvious wall defect due to the myocardial scarring described previously (white arrow, B). A right anterior oblique view (B) the “clipping” of the left ventricle allows the visualization of the epicardium and endocardium and demonstrates the corresponding myocardial thickness (endocardial surface; epicardial surface). Myocardial scarring is well visualized as wall defect (white arrow, C). PET positron emission tomography.
 
Figure 2 (bottom): Endocardial voltage map was unable to detect myocardial scar, and all wall segments revealed >1.5 mV voltage (left lateral view, A). However, 3-dimensional positron emission tomographic scar map (ocher shell) reveals a large inferobasal lateral wall defect signifying myocardial scar, which was consistent with the exit site of the presenting ventricular tachycardia (white arrow, B). An epicardial voltage map finally confirmed a large, nontransmural scar (red area, <0.5 mV) in the inferobasal lateral location (white arrow, C). Registration of the epicardial voltage map and 3-dimensional