64-slice Cardiac PET/CT: Slice Count Does Matter

Emory Crawford Long Hospital in Atlanta, Ga., ranks among the busiest cardiac PET sites in the world. In 2006, the community-based, acute-care teaching facility performed more than 3,000 cardiac PET studies. Last year, the hospital deployed a new solution in the cardiac imaging arsenal, installing Siemens Medical Solutions Biograph 64 PET/CT scanner. “Biograph 64 brings some key benefits to the cardiology practice,” reports Randy Patterson, MD, director, Cardiovascular Imaging, who has been beta testing the new system since last April. “It’s great to have structure and function in a single view.”

Cardiac PET/CT angiography images; Siemens

Currently, the system’s chief use is cardiac PET scanning with the CT scan used for attenuation correction. “The 64-slice PET/CT system has sped up the cardiac PET process and provides us with outstanding PET images. We’re able to save about 20 minutes per patient with the system,” explains Patterson. In addition, PET/CT scanners provide higher quality images and shorter acquisition times than stand-alone PET systems. Most patients are imaged in less than 30 to 35 minutes, rather than 50 to 60 minutes on a conventional PET camera. That shortened scan time translates into better service for patients and referring physicians because the department can readily accommodate more outpatient and inpatient nuclear cardiology studies as well as five to six emergency department cases daily.

While the initial applications are exciting, the new scanner also holds the ticket to a number of promising future applications. For example, the cardiology department, working with the radiology department, plans to use the scanner to provide more CT angiography (CTA) and calcium scoring studies. The primary roadblock is financial. “PET with CT angiography is an insurance issue,” says Patterson. “Very few patients can be approved for both studies. Although not all patients need both studies, many could benefit from both coronary CTA [structure] and PET perfusion imaging [function].”

Nevertheless, the department has turned to the scanner for quite a few CT angiography cases. CTA is especially helpful in patients after bypass graft surgery; the study helps surgeons determine the location of the myocardium jeopardized by stenotic arteries that need bypass. In other cases, surgeons refer to the study to locate a patent graft that they do not wish to disturb as they plan their surgical approach. In addition, a CT with calcium scoring can provide physicians with additional diagnostic confidence in patients with a low risk of coronary artery disease when there is some clinical question after initial testing. Many patients can avoid invasive coronary arteriography if the coronary calcium score is zero and they meet appropriate age criteria.

The ultimate role of PET/CT CTA, however, may be in patients with intermediate probability of coronary artery disease. “CTA can be problematic,” Patterson says. “In some cases, it’s clear that the artery is either normal or completely occluded. If it’s an in-between severity of coronary stenosis, it becomes much harder to render an accurate diagnosis. The addition of PET is a tremendous help if the artery is 30 to 70 percent narrowed. We can look at perfusion on PET and determine whether or not the narrowing of the coronary artery is significant.”

PET/CT also aids in other non-textbook cases. Take for example patients with high coronary calcium. Coronary calcium or coronary stents can make it difficult to visualize the dye stream as it passes through the coronary arteries on CT images. Once again, the functional view of perfusion to the heart muscle provided by the PET results helps the physician diagnose the patient.

Patterson envisions additional applications for 64-slice PET/CT. “We hope that 64-slice PET/CT will be helpful in patients with normal PET results who show 30 to 40 percent narrowing with shaggy, irregularly shaped plaque. This vulnerable plaque suggests thrombus, which might be treated with more specific antiplatelet drugs in addition to aspirin therapy. This is an area that needs a large scale clinical study,” Patterson says.

In the meantime, the hybrid scanner is pulling its weight with other Emory research initiatives. For example, researchers are using it to examine the graft patency of coronary bypass patients. The researchers also are using the scanner to complete five-year follow-up studies on patients whose surgeries were performed with heart-lung machines versus without heart-lung machines in an attempt to determine if outcomes of surgeries performed on a beating heart are comparable to those performed on relaxed heart provided by the heart-lung bypass machine. They also are involved in several other studies.

Although 64-slice cardiac PET/CT is in its infancy, leading-edge research and use demonstrates its potential. The hybrid systems combine anatomy and function to produce superior quality scans in a shorter time enabling adopters to accommodate a higher patient volume, better meet the needs of referring physicians and allowing some patients to avoid invasive procedures and potentially receive more appropriate treatment.