How is myocardial perfusion imaging performed?

Updated: Nov 20, 2018
  • Author: Richard S Krause, MD; Chief Editor: Barry E Brenner, MD, PhD, FACEP  more...
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Answer

Answer

There are two widely used agents for myocardial perfusion imaging. Thallium (Tl)-201 was the first agent widely used in clinical practice. It is a cation that acts similarly to potassium and is taken into viable cardiac myocytes. It has a half-life of 73 hours and distributes in cardiac tissue roughly in proportion to regional blood flow. In practice, Tl-201 is injected while the patient is at peak exercise or shortly after the pharmacologic stress agent is administered. Images are taken with a photon camera shortly thereafter and then again in 3-4 hours.

Defects on the initial image can represent regional ischemia or nonviable myocardium. After the cardiac stress is discontinued, the Tl-201 redistributes and fills in areas that were underperfused due to ischemia (reversible defect). Regions of the heart that have been irreversibly damaged by previous myocardial infarction (MI) do not demonstrate resolution of the defect on the delayed image (fixed defect). In this way, the test can discriminate between regions of inducible ischemia at risk for future MI and areas that have already been irreversibly damaged by prior MI.

The second widely used agent is technetium (Tc)-99m sestamibi, which acts as a calcium analog when taken up by the heart. It has a shorter (6 hour) half-life than thallium and redistribution does not occur as it does for the tl-201, although Tc-99m is taken up by the cardiac myocytes. Consequently, when performing a Tc-99m scan, a second injection is given at the time of the delayed image. Interpretation of the stress and delayed images is similar to that of Tl-201. Tc-99m tetrofosmin behaves similarly to Tc-99m sestamibi.

Images are obtained by a gamma camera that rotates around the body, obtaining a tomographic image. This is termed single-photon emission computed tomography (SPECT) scanning. Planar images are also available, but they are less accurate.  The images are interpreted qualitatively and can also be analyzed quantitatively by a variety of automated protocols.

Image quality can be improved by gating. This is a technique in which image acquisition is timed to only occur while the heart is in diastole, offering an image with greater resolution.


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