Rubidium demonstrates ammonia-tracer equivalence in blood-flow PET

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Although it would be nice to have, not every healthcare facility can afford (or house) a cyclotron. Quantification of myocardial blood flow (MBF) and coronary flow reserve (CFR) using PET and 13N-ammonia and 15O-water tracers delivers a very sensitive evaluation; however, the tracers have short half lives of 10 and 2 minutes, respectively, and require an on-site cyclotron for their production.

The tracer 82-rubidium (82Rb) has an even shorter half life, 76 seconds, than 13N-ammonia or 15O-water—but it can be developed and delivered via a countertop strontium generator with an automated infusion system, making it more accessible for cardiac PET studies in the clinical setting. As such, it is the PET imaging tracer most commonly used for assessing myocardial perfusion in patients with known or suspected coronary artery disease (CAD).

Adding MBF quantification capabilities to 82Rb assessments of myocardial perfusion could help better estimate the extent of underlying CAD. According to research published in the July issue of the Journal of Nuclear Medicine, absolute quantitation of MBF with 82Rb dynamic cardiac PET is reproducible and accurate compared with 13N-ammonia.

A team of researchers from Massachusetts General Hospital, Shriners Hospital for Children and Brigham and Women’s Hospital in Boston and the Faculty of Medicine at Holy Spirit University of Kaslik in Mount Lebanon, Lebanon, determined the reproducibility of MBF estimates with 82Rb PET as well as the intra- and interobserver reliability of these quantitative measures. In addition, they assessed the accuracy of 82Rb PET by comparing measures of MBF obtained with it to those obtained with 13N-ammonia.

Transverse, coronal, and sagittal slices as well as short-axis, long-vertical-axis, and horizontal-axis images of 82-Rb (A) and 13N-ammonia (B) stress studies in same subject. Polar maps of relative perfusion (center) and absolute hyperemic MBF (right) are also shown (bottom right); white corresponds to highest values in color scale. Image and caption courtesy of SNM.

“The absolute quantification of MBF has been proposed as a possible solution to the underestimation of jeopardized myocardium by stress  myocardial perfusion imaging,” the authors wrote.

Reproducibility was evaluated in 22 subjects undergoing dynamic rest and dipyridamole stress 82Rb-PET studies at a two-week interval. The inter- and intraobserver variability of MBF quantitation with dynamic 82Rb PET was assessed with four repeated estimations by each of four observers.

Accuracy was evaluated in 20 subjects undergoing dynamic rest and dipyridamole stress PET studies with 82Rb and 13N-ammonia, respectively, also at a two-week interval.

All 82Rb studies were performed in the 2D mode on a Discovery STE Lightspeed 64 (GE Healthcare) PET/CT system. The 13N-ammonia studies were conducted in 2D mode on a PC-2048 Scanditronix (GE) PET system.

The researchers reported that the reproducibility of the MBF estimates in repeated 82Rb studies was very good at rest and during peak stress as was the reproducibility of the CFR estimates. The slope of the correlation line was very close to one for the estimation of MBF (0.986) and CFR (0.960) in repeated 82Rb studies.

The intraobserver reliability was less than 3 percent for the estimation of MBF at rest and during peak stress as well as for the estimation of CFR, and interobserver reliabilities were 0.950 at rest and 0.975 at peak stress. The correlation between myocardial flow estimates obtained at rest and those obtained during peak stress in 82Rb and 13N-ammonia studies was also determined to be very good.

The scientists noted that their cohort represented only healthy subjects and those at risk for or with known CAD, defined by the presence of fixed or reversible perfusion defects on clinically indicated PET.

“Despite our best efforts, we had few patients with documented CAD and none with evidence of stress perfusion defects,” the authors wrote. “Furthermore, our extraction fraction model was not validated in a pathologic situation. Thus, our findings regarding the reproducibility and accuracy of MBF estimates with 82Rb cannot be readily extrapolated to areas of low or very low blood flow (e.g., myocardial infarction).”