Magnetic resonance versus radionuclide pharmacological stress perfusion imaging for flow-limiting stenoses of varying severity

Background - Although magnetic resonance first-pass imaging (MRFP) has potential advantages in pharmacological stress perfusion imaging, direct comparisons of current MRFP and established radionuclide techniques are not available. Methods and Results - Graded regional differences in coronary flow were produced during global coronary vasodilation in chronically instrumented dogs by partially occluding the left circumflex artery. Regional differences in full-thickness flow quantified using microspheres were compared with regional differences obtained with MRFP and radionuclide SPECT imaging ( ^99mTc-sestamibi and ²⁰¹Tl). Relative regional flows (RRFs) derived from the initial areas under MRFP signal intensity-time curves were linearly related to reference microsphere RRFs over the full range of vasodilation (y=0.93x+4.3; r²=0.77). Relationships between ^99mTc-sestamibi and ²⁰¹Tl RRFs and microsphere RRFs were curvilinear, plateauing as flows increased. The high spatial resolution of the MRI enabled transmural flow to be evaluated in 3 to 5 layers across the myocardial wall. Reductions in subendocardial flow were visually apparent in MRFP images for ≥50% reductions in full-thickness flow. Endocardial-to- epicardial gradients in MRFP flow increased progressively with stenosis severity, whereas transmural flow patterns in remote normally perfused myocardium remained normal. Flow reductions of ≥50% not identified by radionuclide imaging were apparent in MRFP full-thickness and transmural analyses. Conclusions - High-resolution MRFP can identify regional reductions in full-thickness myocardial blood flow during global coronary vasodilation over a wider range than current SPECT imaging. Transmural flow gradients can also be identified; their magnitude increases progressively as flow limitations become more severe and endocardial flow is compromised increasingly.