Molecular imaging can be defined as ‘the noninvasive, real-time visualization of biochemical events at the cellular and molecular level within living cells, tissues, and/or intact subjects”. Our understanding of disease has been revolutionised by our understanding of the cell at the molecular level, leading to the design of targeted therapies.

Labelling of specific molecules with positron-emitting radioisotopes such as ¹¹C and ¹⁸F allows them to be detected at very low levels in vivo using positron emission tomography (PET). They can thus participate in or be transformed by biochemical processes without having an effect on the system that is being studied, and provide a readout of in vivo biology. PET thus allows ‘molecular imaging’, i.e. tissue physiology and biochemistry to be visualised and quantified noninvasively, indicating where this has been deregulated by disease (as the PET scanner detects only the PET isotope, understanding the metabolism of the PET tracer is necessary to interpret the resulting images).

Molecular imaging with PET lets us detect the molecular processes that underpin disease without the need for invasive surgery; this has the potential to allow therapy to be stratified to those patients who stand the best chance of responding. PET can also be used to detect whether patients are responding to therapy early on, allowing treatment to be altered quickly if it is of little or no benefit.

Above – ¹⁸F-FDG uptake in rat heart

PET also allows the determination of both the distribution of radiolabelled drug analogues (pharmacokinetics) as well as their therapeutic effects (pharmacodynamics).