Chapter 11
Cardiac Positron Emission Tomography
1. What is positron emission tomography (PET)?
A positron, as its name implies, is a positively charged particle that is ejected from the nucleus of an unstable atom. It is identical in mass to an electron. Strictly speaking, it is “antimatter” and very shortly after leaving the nucleus it collides with an electron in what is called an annihilation reaction. This reaction generates two 511 keV gamma photons that are emitted almost diametrically opposite from each other. The energy of these photons is captured by a special (PET) scanner and through a sophisticated network of electronics as well as computer software and hardware, it is transformed into an image.
2. Which are the two most common PET radiopharmaceuticals used for myocardial perfusion imaging (MPI)?
Rubidium-82 (Rb-82) and nitrogen-13 (N-13) ammonia.
3. What are the characteristics of Rb-82?
Rb-82 is a monovalent cation analog of potassium. It is commercially available as a strontium-82 generator. Its physical half-life is 75 seconds. It is extracted with high efficiency by myocardial cells through the Na+,K+-ATPase pump. The adult radiation dose from a Rb-82 MPI varies from 1.75 to 7.5 mSv (Fig. 11-1).
Figure 11-1 Abnormal Rb-82 myocardial perfusion imaging demonstrating inducible ischemia of left circumflex vascular territory. The figure shows stress and rest images displayed in three planes: short axis (apex to base), vertical long axis (septum to lateral wall), and horizontal long axis (from inferior to anterior). The rest images demonstrate good perfusion throughout the left ventricle. The stress images demonstrate decreased tracer in the lateral wall (arrows). (Modified with permission from Branch KR, Caldwell JH, Soine LS, et al: Vascular [humoral] cardiac allograft rejection manifesting as inducible myocardial ischemia on nuclear perfusion imaging, J Nucl Cardiol. 12(1):123-124, 2005.)
4. What are the characteristics of N-13 ammonia?
N-13 Ammonia is an extractable myocardial perfusion tracer which, due to its 10 minute half-life, requires an on-site cyclotron. It is retained in myocardial tissue as N-13 glutamine by the action of glutamine synthetase. The adult radiation dose from an N-13 ammonia MPI is approximately 1.4 mSv.
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