Chapter 3: Radiopharmaceuticals for Cardiac Imaging

INTRODUCTION

This chapter reviews the most important characteristics of the various single-photon emission computed tomography (SPECT) and positron emission tomography (PET) radiopharmaceuticals that are currently used in clinical practice. Myocardial perfusion scintigraphy has significantly evolved¹ since its introduction more than four decades ago. Three major factors have specifically contributed to this evolution: (1) technical improvements in scintigraphic data acquisition and analysis, (2) introduction of new technetium-99m (Tc-99m)-labeled MPI radiopharmaceuticals with different properties than thallium-201 (Tl-201), and (3) availability of PET radiotracers and PET-dedicated cameras. This chapter reviews the most important characteristics of the various SPECT and PET MPI radioactive agents that are currently used in clinical practice. The knowledge of the characteristics and kinetics of the various radiotracers is clinically relevant and essential for designing and implementing optimized imaging protocols.² The optimal timing of imaging after injection either at stress or at rest is determined by rate of uptake in the heart and adjacent organs, as well as the residence time of radiotracers within the myocytes. The efficiency of myocardial extraction over a wide range myocardial blood flows is relevant for reliable detection of obstructive coronary artery disease and absolute quantification of regional myocardial blood flow. Therefore, knowledge of the basic characteristics of MPI radiotracers is essential for optimal clinical use of these agents. In addition, a section on ^99mTc-pyrophosphate cardiac imaging is included, given its role in the diagnosis of cardiac amyloidosis, while other more specific agents such as ¹²³I-MIBG and ¹²³I-IPPA are covered in another chapter (Chapter 23). Radiation safety has become a more sensitive concern more recently. Therefore, data on radiation dosimetry are also discussed for each radiopharmaceutical.

SPECT RADIOPHARMACEUTICALS

General Physiologic Characteristics of Perfusion Imaging Agents

Although many different classes of radioactive MPI agents exist, they should all present a minimum of common basic characteristics.⁴

1. The myocardial uptake of the radiotracer must be proportional to the regional myocardial blood flow over a relatively wide range of blood flows.

2. The myocardial uptake should be high enough to allow for detection of regional inhomogeneity by external gamma scintigraphy.

3. The initial myocardial distribution of the radiotracer at the time of injection must remain stable during the acquisition time of the images.

4. The effect of blood flow on myocardial transport of the radiotracer must be predominant to the effect of metabolic cellular alterations.

5. Finally, the agent should be labeled to a radionuclide having adequate physical characteristics to provide high photon flux and optimal counting statistics.

Information on basic properties of radionuclide MPI agents is generally obtained from cultured myocardial cells, isolated perfused hearts, or in vivo animal models.⁵ Precise measurements of cellular or capillary-tissue tracer kinetics are usually obtained from cell cultures and isolated perfused heart models, whereas regional tracer distribution and uptake in other organs are studied with in vivo animal models. The two most important ...