Intra-Individual Variability of Myocardial Blood Flow and Flow Reserve Assessed by [¹⁵O]H₂O-PET in Patients with...

Background/Objectives: Myocardial blood flow (MBF) and myocardial flow reserve (MFR) are key measurements in myocardial perfusion imaging (MPI), with [¹⁵O]H₂O-PET considered the reference standard. To further establish clinical and research utility, we investigated intra-individual variability of MBF and MFR over time in patients with angina, but no obstructive coronary disease. Methods: In a routine clinical setting, we prospectively studied 21 patients, >50 years with normal left ventricular function and no known coronary stenosis, undergoing clinically indicated PET MPI. Scan and re-scan were conducted within 30 days. Analyses were conducted by general clinical staff and re-evaluated by an expert reader. Reproducibility was assessed using paired t-tests, Bland-Altman analysis, repeatability coefficient (RC), within-subject coefficient of variation (CV) and intraclass correlation coefficient (ICC). Results: Twenty-one patients (mean age 70 ± 8.2 years; 48% female) underwent repeated scans with a median interval of 21 days. Resting MBF was 0.91 ± 0.24 vs. 0.92 ± 0.22 mL/min/g (r = 0.87; RC 0.23 mL/min/g; CV 9%; ICC 0.87). Hyperaemic MBF averaged 3.06 ± 0.9 vs. 2.97 ± 0.78 mL/min/g (r = 0.83; RC 0.98 mL/min/g; CV 11.6%; ICC 0.81). MFR showed only moderate reproducibility (3.47 ± 1.23 vs. 3.23 ± 0.92; RC 1.90; CV 21%; ICC 0.60). Neither expert re-evaluation nor rate-pressure product correction of the resting MBF improved the reproducibility. Variability was largely unaffected by atrial fibrillation and diurnal variation. Conclusions: Resting and hyperaemic MBF are reproducible, whereas MFR shows moderate variability, which should be considered when interpreting changes in individual patients.