Stress nuclear is also called myocardial perfusion imaging, and is a non-invasive approach to assess regional blood flow in the heart and possible infarction (dead tissue). This technique uses a radioactive tracer, usually technetium sestamibi, as a blood flow marker. Once injected, this agent flows into the heart, and gets trapped in the cells of the heart. Tissue with normal coronary blood flow will receive a normal amount of tracer uptake. On the other hand, regions supplied by an obstructive heart vessel will have less coronary blood flow and have less tracer uptake. The nuclear camera is then used to image the heart by detecting the radioactive tracer.
The test requires two days: one for images during rest and another for imaging with exercise, or stress. This study is able to determine the degree and location of reduced blood flow to the heart as well as pumping function and existence of scarred heart tissue.
In the picture below, radioactive tracer is detected by the nuclear camera and is portrayed in a gradient of a color spectrum: purple for low count, bright yellow for high count. During exercise, or during a stress stage, the heart physiologically tries to increase coronary blood flow by dilating all of its vessels. The normal vessel (right) accommodates by dilating appropriately to meet its increased demand. The obstructed vessel (left) also dilates to increase blood supply, but is insufficient to satisfy the increased requirements. As a result, there is a disparity in the tracer uptake between regions supplied by the normal vessel versus those supplied by obstructed vessel seen only during the stress stage (and not the rest stage). This provides the basis for identify areas of ischemia (on the left). Ischemia is a condition in the heart where there is an inadequate blood supply to meet its demand, which can cause chest pain.