myocardial perfusion imaging

Myocardial perfusion imaging maps where blood actually reaches the heart muscle by using a tiny, harmless radioactive tracer that travels in the bloodstream and lodges in living muscle in proportion to its blood supply. A special camera then detects the tracer's signal and builds a picture of the heart wall: well-supplied muscle glows evenly, while a region fed by a narrowed artery shows up as a relatively dim, cooler spot.

The test is done in two phases, at stress and at rest, and the comparison is what counts. The tracer is injected at peak exercise or pharmacologic stress and again later at rest, with images taken each time. A spot that looks dim under stress but fills in normally at rest signifies reversible ischemia — living muscle being starved when pushed, the target for treatment. A spot that stays dim in both signifies a fixed defect, usually old scar from a previous infarction. SPECT cameras are common; PET offers higher resolution and can quantify flow.

Perfusion imaging shines at detecting the functional impact of coronary disease, localizing the territory at risk, and judging how much muscle is viable. Its drawbacks are a modest dose of radiation, cost, and some pitfalls — overlying tissue can mimic a defect, and balanced narrowing of all arteries can paradoxically look uniform. It complements, rather than replaces, anatomical imaging of the arteries themselves.