Ideal tracer kinetics is important in understanding the strengths and limitations of current and future tracers.
Thallium-201 has excellent tracer kinetics but is limited by low-energy emission level and long half-life, and therefore is not recommended for routine clinical use.
Technetium-based tracers offer improved image quality due to higher emission level and shorter half-life and are the radiopharmaceuticals of choice for single-photon emission computerized tomographic nuclear cardiology.
The most common positron emission tomography tracer, 82Rb, is generator-derived radiopharmaceutical, with excellent blood flow characteristics and low radiation, making it suitable for most clinical practices.
Myocardial perfusion scintigraphy has significantly evolved since its introduction more than four decades ago. In 1972, the only radiotracer available for nuclear cardiology was 201Tl.1–3 Although this agent had impressive characteristics for the assessment of myocardial blood flow, its long half-life (72 hours) and the multiple emission energy photons (numerous Auger photons center at 72 keV and photons at 137 keV and 167 keV) made scatter and attenuation correction difficult.4,5 By the early 1990s, investigations were underway with using 99mTc.6–12 This isotope has the advantage of a single-emission photon at 141 keVand a more ideal half-life of 6 hours. Thus image quality would be improved and radiation exposure reduced over thallium.12 This agent could be prepared at a centralized radiopharmacy and delivered for use to nearby nuclear cardiology laboratories. Alternatively, 99mTc could be produced from a molybdenum generator located at the radiopharmacy. 99mTc sestamibi, 99mTc tetrofosmin, and 99mTc teboroxime were subsequently developed to assess myocardial blood flow.13,14 Sestamibi and tetrofosmin are still in widespread use today, while teboroxime is not, due to clinical limitations. The development of technetium-based tracers for nuclear cardiology led to a more widespread use of myocardial perfusion imaging (MPI) that has continued through the present time.
Positron emission tomography (PET) radiotracers were also undergoing revolutionary changes. Positron emitting isotopes of oxygen, carbon, nitrogen, and fluorine could all be used for producing metabolically useful molecules. In 1996, Medicare approved reimbursement for glucose analog, 18F- fluorodeoxyglucose (FDG), for the assessment of metabolically active tumors. This stimulated the development of cyclotron networks for producing the 18F and marked improvements in PET scanners opened the door to high-quality PET MPI using a generator-based potassium analog, 82Rb.15–18 This allowed for on-site production of a short-lived positron emission agent without the need for an expensive cyclotron.
This chapter will review important characteristics of the various single-photon emission computerized tomographic (SPECT) and PET MPI radioactive agents used in clinical practice, with an increasing focus on quantitative myocardial blood flow measurement. The emphasis of this chapter is to provide the nuclear cardiologists with practical knowledge of these agents and a high-level understanding of the direction of radiopharmaceutical development.
IDEAL PHYSIOLOGIC CHARACTERISTICS OF PERFUSION IMAGING AGENTS