Only few of the tracer molecules actually contain radioactive isotope; others contain non-radioactive ("cold") isotope. Molar activity (Am) and specific activity (As, SA) is the ratio between activity and mass of compound.
SA is always measured for a batch of radioligand; the unit of molar activity is usually MBq/µmol (or mCi/µmol), and the unit of specific activity is usually MBq/µg.
All radioactivity measurements, including SA, are corrected for physical decay to the time of injection (zero time). After decay corrections, radioactivity concentration in tissue and plasma (Bq/mL) is proportional to the chemical concentration of the ligand in picomoles per litre. SA is the proportionality constant for this conversion.
High SA is required to reach sufficient count level without injecting too high mass. Too high mass in the administered dose could cause unwanted pharmacological effects, or occupy target receptors leading to reduced uptake in the target regions (Keller et al., 2019). Long axial field of view PET scanners have markedly higher sensitivity, allowing comparable image quality with lower administered dose (Alberts et al., 2021; Slart et al., 2021). The mass effect is pronounced in preclinical studies with small animals (Jagoda et al., 2004; Kung & Kung, 2005; Wurzer et al., 2018).
Coenen HH, Gee AD, Adam M, Antoni G, Cutler CS, Fujibayashi Y, Jeong JM, Mach RH, Mindt TL, Pike VW, Windhorst AD. Open letter to journal editors on: International Consensus Radiochemistry Nomenclature Guidelines. Ann Nucl Med. 2018; 32(3): 236-238. doi: 10.1007/s12149-018-1238-z.
Vanessa Gómez-Vallejo, Vijay Gaja, Jacek Koziorowski and Jordi Llop (2012). Specific Activity of 11C-Labelled Radiotracers: A Big Challenge for PET Chemists. In: Positron Emission Tomography - Current Clinical and Research Aspects, Dr. Chia-Hung Hsieh (Ed.), ISBN: 978-953-307-824-3, InTech, DOI: 10.5772/31491.
Updated at: 2022-01-02
Created at: 2014-01-28
Written by: Vesa Oikonen