6.1 Basics

In nuclear medical examinations, radionuclides are introduced into human metabolism. Radionuclides are nucleotides that emit radioactive energy and undergo spontaneous transformation to nuclei of another element. Radionuclides are also called tracers. The radiation they emit can be measured using special detectors. This yields images of the tracer uptake in various regions of the heart.

The two nuclear medicine techniques primarily used in cardiology are:

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  SPECT (single photon emission computed tomography), used in myocardial perfusion imaging

  
  
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  PET (positron emission tomography), gold standard for assessing myocardial vitality

6.2 Myocardial Perfusion Imaging

For myocardial perfusion imaging, a tracer is injected intravenously. This tracer accumulates in perfused areas of the myocardium, while nonperfused areas show no uptake. This allows perfusion of the myocardium to be visualized. Myocardial perfusion imaging is indicated primarily for assessing and visualizing the extent of myocardial ischemia.

The tracers usually used are thallium-201 chloride or technetium-99m methoxyisobutylisonitrile (MIBI). Thallium-201 chloride is a potassium analog that, like potassium, enters cardiomyocytes via the Na,K-ATPase proportional to regional blood flow. After a delay, thallium-201 chloride is then cleared out of the system. The tracer is washed out more slowly in poorly perfused areas. However, due to the exposure to radiation, which is almost double that of technetium-99m MIBI, thallium-201 chloride should not be used in children.

Technetium-99m MIBI is a lipophilic substance that diffuses into cardiomyocytes through the cell membrane, but it does not accumulate in the affected area of the heart if perfusion is insufficient. Unlike thallium-201 chloride, the examination using technetium-99m MIBI is not based on washout, but on measuring tracer uptake.

However, since even severe coronary stenoses allow sufficient perfusion of the heart at rest (coronary reserve), the scan is performed both at rest and after exercise. A bicycle ergometer or medication (e.g., dobutamine or adenosine) can be used for exercise testing.