Medium Half-Life Inorganic Radionuclides for Pet Imaging

Abstract

In vivo radiotracer imaging continues to grow in importance and consideration is constantly given to the relative merits of the different techniques of radionuclide imaging and how they complement each other. The aim of this review is to highlight the role of inorganic medium-lived radionuclides within the field of Positron Emission Tomography (PET). PET has evolved around the use of short-lived radioactive tracers of natural organic elements such as 11C (20 min), 13N (10 min.) and 15O (2 min). Although fluorine is not a commonly natural constituent of organic molecules, 18F (110 min), as a hydrogen analogue, is the radionuclide most commonly used and 18F-FDG is currently the most widely used PET radiotracer. This is due at least in part to the longer half-life of 18F which offers both practical and technical advantages to study prolonged biochemical interactions that are not possible even with the longest half-life organic radionuclide (11C, 20 min.). This review therefore focuses on exploiting the use of medium half-life radionuclides in PET imaging. This expansion could aid in realising the true potential of PET by combining the sensitivity and quantitative accuracy of the technique with a large number of specific tracers developed for single photon imaging using for example, 99mTc, 123, 131I, 111In, 67Ga and 67Cu radionuclides. Here, the positron-emitters 94mTc, 120,124I, 66,68In, 66,68Ga and 61,64Cu could be used as the PET imaging analogues. The abundance of inorganic positron-emitting radionuclides span a wide range of half-lives (∼ 1–100 hours) and spectrum of chemical properties ranging from the alkali metals to transition elements to lanthanides and the halogens (although not strictly ’inorganic’ elements, the halogens are included for completion.