Comparison of clinical myocardial blood flow estimation using dynamic SPECT and PET

Objectives The delineation of absolute myocardial blood flow (MBF) is a valuable parameter for characterization and monitoring of pathologies. Recent developments allowed the acquisition of kinetic data with fast SPECT systems. However, a direct comparison in patients with well-validated PET flow agents is lacking and is here evaluated for the first time.

Methods 10 patients (8m, 69±7y, 81±16kg) were imaged with PET (GE Discovery RX VCT; 720 MBq 13N-NH3) and SPECT (GE Discovery NM530c; 440 MBq 99mTc Tetrofosmin) with a delay of 11±10 days using pharmaceutical stress. SPECT data were generated with (AC) and without attenuation and scatter correction (NAC). In 10 patients rest only and in 7 rest and stress imaging was performed. Tracer injection was within the scanners and dynamic data was acquired for 15 minutes. Time activity curves (TAC) for myocardium and blood were generated using volumetric sampling. TACs were analyzed with the a 2 tissue compartment model including motion and partial volume correction. Mean PET MBF (ml/min/g) and SPECT K1 (1/min) values were used to estimate the extraction fraction, which was then applied to calculate MBF for SPECT data.

Results Table 1 shows rest and stress MBF and K1 values. MBF mean values show no significant difference. However, the variability within the rest/stress groups increases significantly between PET and both AC and NAC as well as K1 and MBF. The reason lies in the reduced image quality between the PET and the SPECT data when using short frames required for kinetic analysis.

Conclusions This direct comparison between PET and SPECT flow estimates in patients demonstrates its technical feasibility. However, it also confirms the rather low extraction fraction of standard SPECT flow agents. Although corrections can be computed from parallel acquisitions, those factors are high and further studies will clarify whether such an amplification yields stable, clinically valuable results.