Imaging fundamentals

Comprehensive reviews of imaging fundamentals and procedures are available.1 ,2 The two most commonly used isotopes for myocardial perfusion imaging are thallium-201 and technetium-99m. Thallium-201 is generated by a cyclotron. It is then transported as a finished product to the location where it is used, which is feasible because it has a half life of 73 hours. The isotope decays by a reasonably complex scheme, but most of the photons have an energy of about 80 keV, which is a low energy. Technetium-99m is bound to other compounds for the purposes of myocardial perfusion imaging. It is formed on site by elution from a molybdenum-99 generator. Technetium-99m is a meta-stable compound which is constantly formed from molybdenum-99 within the generator. Technetium-99m has a half life of about six hours, and emits photons with a 140 keV energy. This energy is much higher than the emissions of thallium, but much lower than the 511 keV emissions of PET radiopharmaceuticals. The differences in physical properties between thallium-201 and technetium-99m are relevant to the choice of radiopharmaceutical, which will be discussed later.

Both thallium-201 and technetium-99m radiopharmaceuticals are most commonly imaged using single photon emission computed tomography (SPECT). This technique employs many of the same back projection techniques that have been applied to conventional radiographs for CT scanning. Although multiple view planar images were first employed for myocardial perfusion imaging, they have been largely replaced by SPECT, which is superior from the standpoint of localisation, quantification, and image quality. Regardless of the radiopharmaceutical …