Comparison of Microsphere-Equivalent Blood Flow (15O-Water PET) and Relative Perfusion (99mTc-Tetrofosmin SPECT) in Myocardium Showing Metabolism-Perfusion Mismatch

Myocardial perfusion imaging with 99mTc-tetrofosmin is based on the assumption of a linear correlation between myocardial blood flow (MBF) and tracer uptake. However, it is known that 99mTc-tetrofosmin uptake is directly related to energy-dependent transport processes, such as Na+/H+ ion channel activity, as well as cellular and mitochondrial membrane potentials. Therefore, cellular alterations that affect these energy-dependent transport processes ought to influence 99mTc-tetrofosmin uptake independently of blood flow. Because metabolism (18F-FDG)-perfusion (99mTc-tetrofosmin) mismatch myocardium (MPMM) reflects impaired but viable myocardium showing cellular alterations, MPMM was chosen to quantify the blood flow-independent effect of cellular alterations on 99mTc-tetrofosmin uptake. Therefore, we compared microsphere-equivalent MBF (MBF_micr; 15O-water PET) and 99mTc-tetrofosmin uptake in MPMM and in “normal” myocardium. Methods: Forty-two patients with severe coronary artery disease, referred for myocardial viability diagnostics, were examined using 18F-FDG PET and 99mTc-tetrofosmin perfusion SPECT. Relative 18F-FDG and 99mTc-tetrofosmin uptake values were calculated using 18 segments per patient. Normal myocardium and MPMM myocardium were classified using a previously validated 99mTc-tetrofosmin SPECT/18F-FDG PET score. In addition, 15O-water PET was performed to assess kinetic-modeled MBF (MBF_kin), the water-perfusable tissue fraction (PTF), and the resulting MBF_micr (MBF_kin·PTF), which is comparable to tracer uptake values. 99mTc-tetrofosmin uptake and MBF_micr values were calculated for all normal and MPMM segments and averaged within their respective classifications. Results: Mean relative 99mTc-tetrofosmin uptake was 86% ± 1% in normal myocardium and 56% ± 1% in MPMM, showing a significant difference (P