Chapter 7: New Developments in Cardiac PET Radionuclide Production: Tracers, Cyclotrons, Generators, and Delivery Systems for Cardiac PET Imaging

INTRODUCTION

The field of cardiovascular PET imaging is constantly evolving, and this includes all aspects of tracers, production of tracers, and delivery to the patient. This chapter discusses the new developments in cardiac PET tracers, cyclotrons, and delivery systems.

PHYSICS OF TRACERS

Positrons are the stable antiparticle of the electrons. Despite being stable, they are extremely rare in nature. When positrons combine with electrons, they quickly annihilate, leaving behind nothing but photon energy. The positrons that are used in nuclear cardiology originate from the decay of radionuclides, the most common of these being fluorine-18 (¹⁸F), carbon-11 (¹¹C), oxygen-15 (¹⁵O), nitrogen-13 (¹³N), and rubidium-82 (⁸²Rb). These radionuclides an excess number of protons; when the proton is converted to a neutron, an energetic positron is ejected from the nucleus of the atom.

PHYSICS OF POSITRONS

Positron emission tomography (PET) relies on the annihilation of a positively charged particle (an antielectron or positron) with a conventional electron to produce two high-energy gamma rays traveling 180 degrees from each other. The existence of positrons was first proposed by Paul Dirac in 1928,¹ and ultimately discovered in 1932 by Carl D. Anderson.² Positrons, though stable in vacuum, annihilate almost immediately when they come in contact with electrons, producing a complete conversion of the mass of the two particles (511 keV/c²) into energy. This unique property of positrons, the complete conversion of the positron-electron pair into two photons of identical energies (511 keV), was recognized as potentially revolutionary for medical imaging. The first application of positron annihilation medical imaging was first explored by Brownell et al in 1953 for imaging brain tumors.³ This technique was later expanded to tomographic imaging in 1971.⁴

Before a positron can annihilate, it needs to come to rest in the medium (thermalization). This thermalization drift from the parent atom reduces the overall resolution of the image. For ¹⁸F, the maximum energy of the emerging positron is 633 keV, leading to a thermalization length of 0.239 cm.⁵ However for ⁸²Rb, the maximum energy of the positron is 3.148 MeV, leading to a maximum thermalization length of 1.561 cm (Table 7-1).