Quantitative Myocardial Blood Flow and Reserve with Low Dose Rest/High Dose Stress Regadenoson PET/CT-MPI: The Utility of Residual Activity Correction With N-13 Ammonia

Objectives: Tracer dose reduction is a desirable societal goal for myocardial perfusion imaging (MPI). Residual activity subtraction methodology may allow for aggressive dose reduction in some instances and is the intent of studies designed to measure its effect on quantification of myocardial blood flow metrics in a variety of compressed dose adjusted protocols. We studied its utility of one such protocol using N13-ammonia. Methods: We studied 150 consecutive patients comparing quantitative myocardial blood flow and flow reserve using a unique low dose resting (6mCi) and high dose stress (16mCi) strategy with regadenoson PET/CT-MPI utilizing N-13 ammonia as the tracer both with and without residual activity subtraction methodology (Cedars QGS:QPS vs. Cedars QGS:QPS Residual Activity Beta Software). A four-ring detector, fully digital, GE Discovery MI PET/CT scanner was employed. All patients confirmed their abstinence of caffeine for a least 12 hours prior to the exam. There was a 10-15-minute delay between the resting and stress acquisitions. Dynamic data sets were acquired in gated list-mode and a 28-frame strategy for 10 minutes both at rest and with stress. CT free-breathing attenuation scans were performed prior to rest and post stress. A 60 second delay was built into the protocol between the completion of the regadenoson infusion and the hand injection of N-13 tracer over 20 seconds, each followed by a 20ml saline flush.

Results: There were 76 female and 74 male patients studied. For the entire group, the global myocardial peak stress blood flow mean was 2.74 ml/g/min, SD=0.81, range between 0.62-5.21 ml/g/min, and median=2.65 ml/g/min. With residual activity subtraction the peak stress blood flow mean=2.62 ml/g/min, SD=0.79, range=0.54-5.14 ml/g/min, and median=2.55 ml/g/min. The mean difference between individual studies was 0.12 ml/g/min with SD=0.07 or about 4%. The measures were highly correlated with linear-fit R-squared=0.99. For myocardial blood flow reserve the global mean=3.48, SD=1.15, range=0.72-8.41 and median=3.4. The residual activity subtracted metrics measured: mean=3.32, SD=1.12, range=0.62-7.98, and median=3.23. Mean difference between the measures=0.14, and SD=0.09. By linear-fit the R-squared=0.99. Conclusion: Using the prescribed low dose rest/high dose stress regadenoson N-13 ammonia PET/CT-MPI protocol, residual activity subtraction results in only a modest downward adjustment to the measured flow and reserve metrics of 4% or less. More aggressive dose reduction with N13-ammonia for the stress dose trace activity will be feasible using residual activity subtraction methodology. Acknowledgements: We would like to thank Angelina Aviles for data entry.