A comparison of PET imaging characteristics of various copper radioisotopes

Purpose PET radiotracers which incorporate longer-lived radionuclides enable biological processes to be studied over many hours, at centres remote from a cyclotron. This paper examines the radioisotope characteristics, imaging performance, radiation dosimetry and production modes of the four copper radioisotopes, ⁶⁰Cu, ⁶¹Cu, ⁶²Cu and ⁶⁴Cu, to assess their merits for different PET imaging applications. Methods Spatial resolution, sensitivity, scatter fraction and noise-equivalent count rate (NEC) are predicted for ⁶⁰Cu, ⁶¹Cu, ⁶²Cu and ⁶⁴Cu using a model incorporating radionuclide decay properties and scanner parameters for the GE Advance scanner. Dosimetry for ⁶⁰Cu, ⁶¹Cu and ⁶⁴Cu is performed using the MIRD model and published biodistribution data for copper(II) pyruvaldehyde bis(N⁴-methyl)thiosemicarbazone (Cu-PTSM). Results ⁶⁰Cu and ⁶²Cu are characterised by shorter half-lives and higher sensitivity and NEC, making them more suitable for studying the faster kinetics of small molecules, such as Cu-PTSM. ⁶¹Cu and ⁶⁴Cu have longer half-lives, enabling studies of the slower kinetics of cells and peptides and prolonged imaging to compensate for lower sensitivity, together with better spatial resolution, which partially compensates for loss of image contrast. ⁶¹Cu-PTSM and ⁶⁴Cu-PTSM are associated with radiation doses similar to [¹⁸F]-fluorodeoxyglucose, whilst the doses for ⁶⁰Cu-PTSM and ⁶²Cu-PTSM are lower and more comparable with H₂¹⁵O. Conclusion The physical and radiochemical characteristics of the four copper isotopes make each more suited to some imaging tasks than others. The results presented here assist in selecting the preferred radioisotope for a given imaging application, and illustrate a strategy which can be extended to the majority of novel PET tracers.