Guiding EP Ablations: How PET & PET/CT Can Help
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 positron emission tomographic scar map demonstrates good correlation between the epicardial voltage-defined scar and the positron emission tomographic-defined scar.
At the Bluhm Cardiovascular Institute at Northwestern Memorial Hospital in Chicago, clinicians perform PET exams prior to ventricular tachycardia (VT) ablation procedures. The clinicians started using this method 18 months ago in hopes of better defining scarring in defibrillator and other patient populations—where MRI is limiting.

"PET tells you which wall of the left ventricle has scarring so you are able to better focus on those areas, making the procedure more efficient," says Jason T. Jacobson MD, director of the Ventricular Arrhythmia Ablation Program at Northwestern. To date, almost a half dozen patients have undergone PET prior to an ablation.

"PET is very useful in patients when MRI does not provide a superior image," says Jacobson. However, he says "the growing acceptance and ability to do MRI in patients with defibrillators is going to relegate the use of PET for scar definition prior to VT ablation as a secondary status."

Combining PET with CT or MRI (even in dedicated devices in the near future) can provide a comprehensive assessment of the scar substrate, Dickfeld says. "We are moving towards an individualized, patient-tailored imaging approach that combines the top features of MRI, CT and PET imaging to obtain the best possible characterization for the individual patient," Dickfeld notes.

He and his colleagues evaluated the use of a contrast-enhanced cardiac MR (CE-CMR) in 22 patients with ICDs prior to VT ablation. Results showed the CE-CMR is safe in certain ICD patients and the method created detailed 3D scar maps for clinical mapping system and aided substrate-guided VT ablations (Circ Arrhythm Electrophysiol 2011;4(2):172-184.).

Likewise, at the Comprehensive Arrhythmia Research & Management (CARMA) Center in Salt Lake City, Nassir F. Marrouche, MD, electrophysiologist and director of the University of Utah's AF program, uses MRI to interpret the extension and progression of AF.

Marrouche and colleagues use MRI to decipher which patients are appropriate candidates for cardiac ablation, without subjecting patients to ionizing radiation. Performing 15 to 16 MRI exams in the EP lab per day, Marrouche says, "MRI is the most significant, non-radiation exposing, risk-free modality that allows for the characterization of tissue and cellular tissue levels up to 1 mm."

At CARMA, MRI is used peri- and post-procedurally to probe damaged tissue near the posterial left atrial (LA) wall, localize the LA thrombus and define the pulmonary veins. While the best way to identify the LA thrombi is screening via transesophageal echocardiogram (TEE), Marrouche says MRI can be a practical alternative.

He and his colleagues use MRI to probe damaged tissue near the posterial LA wall. Using delayed enhancement MRI (DE-MRI) can help score the amount of fibrotic or scarred tissue, which can help improve ablation success. While 5 to 12 percent of the AF patients seen at Utah are not good candidates for ablation, 40 percent with permanent AF are appropriate candidates. Researchers report a 99 percent success rate with DE-MRI.

What's the hold up?

While PET offers a supplemental metabolic scar characterization in addition to MRI or CT, the modality still has its pitfalls, says Jacobson. "The problem with PET is it can't tell you how deep within the heart wall the scar is."

MRI's higher resolution helps to detail scar location, how thick it is, how deep it is and which surface it's closest to, in addition to the scar's actual geometry, says Jacobson.  "While PET scanning allows you to see the area of the heart where scarring occurs, you may not see the fine details," he notes.

Yet, as more facilities begin adopting cardiac PET/CT into their EP labs, facilities must begin to focus on decreasing the radiation dose per exam, offers Mark Smith, PhD, nuclear medicine physicist at the University of Maryland. He says the answer may lie in time-of-flight (TOF) PET/CT scanners, which can use less tracer dose while at the same time improve image quality. Whereas conventional PET scanners create images by looking at the paths of coincident gamma rays produced by the radioisotopes injected into the patient, TOF scanners also measure time differences in the arrival times of these photons to improve the image.

Specialized software, Dickfeld notes, is necessary to import these PET, MRI and CT images into mapping systems. As commercial applications have been slow to emerge, facilities that want to use these techniques are frequently using homegrown software.

In conclusion, Dickfeld offers that "no single imaging technique is ideal for all patients." In fact, using a strategy to combine these types of imaging modalities—MRI and CT—with nuclear imaging modalities such as PET, can further enhance the accuracy of EP ablation procedures. Even if future cardiac devices become MRI compatible, abandoned leads, epicardial leads and new ICD implants will always be contraindicated. However, PET may be beneficial in these patients. Combining anatomic and metabolic imaging to more exactly map anatomy prior to or during ablation can provide a patient-tailored approach to EP procedures that may help facilitate better outcomes.