This article is a DOCTOR 2 DOCTOR discussing Cardiac PET. Now available in clinical practice and known as the GOLD STANDARD in stress testing, I thought you would enjoy reviewing the basics involved as well as the indications for Cardiac PET.

Cardiac Positron Emission Tomography (PET)

Technical and Scientific Background

Positron emission tomography (PET) has been the “silent gold standard” for years in the field of cardiac imaging. Due to limited availability and high costs, it has remained a research tool until recently. Secondary to new technology, especially the ability to bring the isotopes to a facility not equipped with a cyclotron, has changed the field of outpatient cardiac imaging. In addition, cardiac specific PET scanners, reasonably priced, allow for a cost effective way to detect and assess CAD. Physicians now have the ability to test appropriate patients using the most sensitive and specific imaging tool in the cardiologists arsenal, Cardiac PET.

Rubidium-82 generators are delivered to the PET center on a monthly basis. This allows the nuclear technician the ability to scan an almost unlimited number of patients over the next 3-4 weeks. Rubidium has a half life of 75 seconds, making pharmacologic imaging necessary. The patient lies comfortably, with their head outside of the scanner, for 30 minutes. Rubidium-82 is used to obtain cardiac PET images before and after dipyridamole, the pharmacologic agent of choice at this time. Aminophylline is given at the end of the procedure to minimize side effects, which are minimal.

PET methodology utilizes Beta (+) decay of a nucleus resulting in emission of a positron, which rapidly annihilates with an electron, giving off two 511-keV photons that travel opposite each other (180 degrees). Images are produced as the two photons are detected simultaneously in the ring shaped scanner. Spatial resolution is presently in the range of 4-6 mm, making PET superior to conventional nuclear imaging. Temporal resolution is also superior and clinical studies have consistently shown increased sensitivity and specificity with PET compared to conventional nuclear testing.

Clinical Uses for Cardiac PET Imaging

Rubidium-82 is used to detect ischemia and/or scar in patients. A second isotope, 18 F-FDG is a glucose analogue used with PET imaging to assess for myocardial viability. Patients with known disease thought to be scar tissue, but having anginal symptoms, will benefit from 18 F-FDG imaging to assess the amount of viable myocardium in the supposed area of scar tissue. Cardiologists can then determine if intervention would be helpful.

PET offers additional benefits in comparison to nuclear testing. As mentioned, the total procedure takes 30 minutes compared to 3-4 hours for nuclear testing. Technetium, the primary isotope in conventional nuclear testing, has become difficult to obtain and recently most labs are limited in their ability to test patients. PET using rubidium-82 has less radiation exposure to the patient and staff. Research in molecular and nanotechnology is within years of taking Cardiac PET imaging and its benefits to a new level.

Clinical Recommendations for Appropriate PET Testing

Clinically, PET appropriateness criteria are in development. I have discussed this issue with experts during my recent training as well as medical directors for insurance companies as we pre-certify PET studies. The following is a summary of my findings to be used as a starting point.

  • Patients with non-diagnostic or equivocal stress echo that were tested for ischemic symptoms would qualify for a PET scan.
  • Patients with non-diagnostic or equivocal nuclear testing that are being assessed for ischemic symptoms qualify for a PET scan.
  • Patients that can not exercise and need pharmacologic testing that have ischemic symptoms or need clearance for major surgery would qualify for a PET scan.
  • Patients with BMI over 40 and in some cases over 35, which are being assessed for ischemic symptoms, can go straight to a PET scan.
  • Patients with breast implants or large pendulous breasts, undergoing assessment for ischemic symptoms, can undergo PET imaging.
  • Patients with a high CAC, viewed as over 300 or 400, can often go to PET imaging. Symptoms are needed to pre-certify.
  • Patients with a Coronary CTA showing borderline obstructive plaque, usually in the 50-70% range, can go directly to PET imaging. Again, symptoms make it easier.
  • Patients with new findings of CHF and/or a cardiomyopathy can usually proceed directly to PET testing.
  • Patients requiring a decision regarding the need for revascularization, either from the ischemic or viability standpoint.

IT SHOULD BE NOTED THAT THE ACC/ASNC, IN RESPONSE TO THE NEED TO LOWER PATIENT EXPOSURE TO RADIATION, RECOMMEND PET IMAGING AS THE STARTING POINT FOR PATIENTS UNDERGOING ASSESSMENT FOR ISCHEMIC HEART DISEASE WHEN PET IS AVAILABLE.

Please contact me with any questions or recommendations.
Thank you

James S. Rellas, MD, FACC