Clinicians question utilization of 18F-FDG imaging alone for myocardial viability

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Myocardial tissue is morphologically homogenous, yet the heterogeneous uptake of 18F-FDG in the myocardium of healthy subjects is frequently observed. A team of Italian researchers, in a study published this month in the Journal of Nuclear Medicine, sought to determine if this impacts the proposed use of 18F-FDG distribution alone for the purpose of determining myocardial viability.
The team hypothesized that the spatial and temporal variability of myocardial 18F-FDG uptake might result in a significant prevalence of transient severe defects on PET images that could be mistaken for myocardial scar in the heart of subjects free of any cardiac disease.
To test their hypothesis, the scientists retrospectively analyzed the spatial and temporal variability of global and regional myocardial 18F-FDG uptake in a series of noncardiac patients who submitted to whole-body PET for the characterization of a neoplastic disease.
The database from the nuclear medicine department at the University of Eastern Piedmont in Novara, Italy, was searched and found 49 consecutive patients (31 male and 18 female with a mean age of 56) with three or more PET scans who fit the criteria for the study. The mean time interval between scans one and two was 147 days; the mean time between scans two and three was 137 days.
“Patients with history of diabetes, chest pain, syncope, or dyspnea were excluded as were patients with abnormal findings on resting electrocardiography or echocardiography,” the authors noted.
All images for the study were acquired on a Biograph 16 PET/CT system (Siemens Medical Solutions) with Pico-3D digital electronics. The CT portion of the exam was used both for attenuation correction of the PET data and for localization of the 18F-FDG uptake in PET images. The maximum standardized uptake value (SUV) of the left ventricle (LV) myocardium and average SUV of LV blood flow and the liver were calculated for all patients, and a semiquantitative analysis of regional myocardial 18F-FDG uptake was performed using Cedars QPS software.
The application was used to generate polar maps that were subdivided into 20 segments according to the software’s protocol.
“Myocardial segments provided by quantitative polar map analysis were grouped for regional analysis according to coronary supply vessel (left anterior descending), left circumflex, right coronary artery, myocardial walls (anterior lateral posterior, septum), and relative position of territories with respect to heart base (proximal, medial, distal),” the authors wrote.


Serial myocardial 18FDG PET results of 58-year-old male patient with non-Hodgkin's lymphoma who submitted three times to whole-body PET/CT. Short-axis, horizontal long-axis, and vertical long-axis slices are displayed together with polar maps of the semiquantitative QPS software. Overlays of 20 myocardial segments, coronary artery territories, and myocardial walls are represented on top, middle, and bottom 2D polar maps, respectively. Patient was asymptomatic and completely free of any sign of cardiovascular disease during observation period. After 18F-FDG injection in fasting conditions, with blood glucose levels never exceeding 100 mg/dL, a marked variability of regional myocardial uptake over time is evident.
Image and caption copyright © by the Society of Nuclear Medicine Inc. from "Spatial and Temporal Heterogeneity of Regional Myocardial Uptake in Patients Without Heart Disease Under Fasting Conditions on Repeated Whole-Body 18F-FDG PET/CT," Eugenio Inglese, Lucia Leva, Roberta Matheoud, Gianmauro Sacchetti, Chiara Secco, Patrizia Gandolfo, Marco Brambilla, and Gianmario Sambuceti, October 2007 Journal of Nuclear Medicine.

The team found that global myocardial uptake of 18F-FDG displayed a significant variability over time. They noted a poor temporal reproducibility of LV myocardium SUV on serial PET scans; and an even lower reproducibility of LV blood flow and liver SUVs as well as on regional uptake.
They stated that the data indicate that metabolic propensity to glucose use largely varies across different regions at the same time and at different times in the same region, even in the presence of a relatively stable average glucose consumption in the whole heart.
“This observation has at least two major implications: first, on a pathophysiologic basis, it indicates that myocardial metabolism is influenced not only by systemic variables (such as serum concentrations of metabolite and related hormones), but also by largely unknown variables that are specific to each myocardial cell and to each time point, even in patients without cardiac disease; second, on a clinical basis, it suggests a word of caution on the use 18F-FDG imaging alone as a diagnostic tool for the identification of myocardial viability,” they wrote.
The researchers noted that the retrospective nature of the study prevented them from indentifying any possible mechanism underlying the large variability in myocardial metabolism that they witnessed. Also, they observed that the study cohort were not true control subjects, but were cancer patients.
“However, these data still indicate the need for a cautious interpretation of increased regional 18F-FDG uptake as a marker of ischemia if not associated with a previous documentation of hypoperfusion,” they concluded.