Cardiac Imaging with PET/CT is a unique and powerful modality in the assessment of cardiac disease.
This technique offers dynamic assessment of the blood flow to the myocardium, which can be quantified in milliliters/gram myocardium/minute.
The video above depicts the first seconds after injection with the perfusion radiotracer, N-13 Ammonia (top- short axis, bottom- horizontal long axis and vertical long axis). Radiotracer activity is seen sequentially in the right ventricle, lungs, and left ventricle. In the latter half of the video the tracer is gradually taken up and fixed in the myocardium, as activity in the lungs simultaneously decreases. Using regions of interest for the myocardium and input from the cardiac blood pool, compartment models can calculate the blood flow to the entire left ventricle, vascular territories, or individual myocardial segments.
There are 3 commonly used indications for cardiac PET/CT:
1. Suspicion of Coronary Ischemia
PET/CT assessment of coronary ischemia is performed using a perfusion radiotracer (N13-Ammonia or Rubidium-82) with image acquisition at rest and after pharmacologic vasodilation. PET may be more commonly used after inconclusive/nondiagnostic assessment with another modality (e.g. limited SPECT due to body habitus) or with suspicion of balanced ischemia (triple-vessel disease or proximal left main stenosis).
Though data varies widely from study to study, the table below is an estimate of the diagnostic accuracy of various imaging modalities for coronary artery disease.
Schuijf JD, et al. Heart 2005;91:1110-1117. Dowsley T, et al. Can J Cardiol 2013;29:285-296. Hamirani YS, et al. J Comput Assist Tomogr 2010;34:645-651. Gaemperli O, et al. Radiology 2008;248:414-23. Ziessman HA, O’Malley JP, Thrall JH, eds. Nuclear Medicine, the Requisites, 4th ed. Philadelphia: Elsevier, 2014.
Compared to SPECT/CT, cardiac PET/CT has been generally shown to have slightly higher sensitivity (e.g. due to detection of balanced ischemia) and slightly higher specificity (e.g. due to higher resolution and fewer attenuation artifacts).
2. Evaluation of Myocardial Viability
In many centers, metabolic PET evaluation using F18-fluorodeoxyglucose has replaced Thallium-201 SPECT for assessment of viable myocardium in patients with cardiac ischemia/infarct. Ischemic portions of the heart which remain viable (“hibernating myocardium”) have greater levels of glucose metabolism (and thus FDG uptake) compared to those which are normally perfused. Myocardial glucose uptake is typically maximized by oral glucose loading and insulin administration.
Patients in whom PET imaging demonstrates viable myocardium have a significantly improved survival with revascularization compared to medical therapy (96.8% versus 84% annual survival, 75% versus 30% 4-year survival). Di Carlo MF, et al. J Thorac Cardiovasc Surg 1998;116:997-1004. Allman KC, et al. J Am Coll Cardiol 2002;39:1151-1158.
3. Evaluation of Cardiac Sarcoidosis
In patients with known or suspected cardiac involvement with sarcoidosis, combined perfusion and metabolic imaging with PET/CT provides an assessment for diagnosis and/or therapy response.
For effective detection of inflammatory FDG activity in the myocardium, physiologic myocardial glucose uptake should be minimized. This is accomplished using a non-carbohydrate diet prior to the exam and/or a period of fasting.
At some centers cardiac PET/CT is used for other less common indications, such as assessment of patients with hypertrophic cardiomyopathy. More on this coming soon…