Nuclear Medicine and PET Imaging

Nuclear medicine imaging, including positron emission tomography (PET), displays the distribution of an administered radiotracer rather than anatomy directly. Because the signal originates from the tracer's biochemical behaviour, these techniques map physiological and metabolic function; positron emitters in particular allow tomographic reconstruction of tracer uptake. PET is therefore primarily a functional modality, often fused with CT or MRI for anatomical localisation.

Definition

Nuclear medicine imaging produces images from radiation emitted by an administered radiotracer; in positron emission tomography the tracer emits positrons, whose annihilation yields paired photons detected in coincidence to reconstruct a tomographic map of tracer distribution.

Scope

The topic covers how radiotracers generate the imaging signal, the coincidence detection of annihilation photons that underlies PET, the reconstruction of emission tomograms, the use of standardised acquisition protocols for quantification, and the way functional images are combined with cross-sectional anatomy. It is a reference on how these images are generated, not clinical guidance.

Core questions

• How does an administered radiotracer create the imaging signal in nuclear medicine?
• How does coincidence detection of annihilation photons enable PET reconstruction?
• Why is functional imaging usually fused with anatomical imaging such as CT or MRI?
• How are acquisition and quantification standardised so uptake values are comparable?

Key concepts

• Radiotracer principle
• Positron annihilation and coincidence detection
• Emission tomography reconstruction
• Functional and metabolic imaging
• Standardised uptake quantification
• Hybrid PET/CT and PET/MRI
• Theranostics

Mechanisms

A radiopharmaceutical labelled with a radionuclide is administered and distributes according to its biochemical target; the radiation it emits is detected externally to map that distribution. In PET, the radionuclide emits a positron that annihilates with a nearby electron, producing two 511 keV photons travelling in opposite directions; detecting these in coincidence localises the annihilation along a line, and many such lines are reconstructed into a tomographic image. Iterative statistical reconstruction, such as maximum-likelihood expectation maximisation, models the emission process to improve image quality (Shepp & Vardi, 1982). Because the signal reflects function rather than structure, PET is usually acquired as hybrid PET/CT or PET/MRI so that uptake can be localised to anatomy, and standardised acquisition supports quantitative comparison (Boellaard et al., 2014).

Clinical relevance

Nuclear medicine and PET add a functional and metabolic dimension to anatomical imaging, and recommendations on their appropriate use and standardised performance support consistent interpretation (Fletcher et al., 2008; Boellaard et al., 2014). The pairing of diagnostic tracers with therapeutic radionuclides — theranostics — is described as a growing field (Turner, 2018). This entry describes how these images are generated and is not a basis for individual diagnostic or treatment decisions.

History

Nuclear medicine grew from the use of radioactive tracers and the gamma camera in the mid-twentieth century. Positron emission tomography emerged as a tomographic method for positron-emitting tracers, and statistical reconstruction approaches such as maximum-likelihood expectation maximisation improved emission-image quality (Shepp & Vardi, 1982). The introduction of hybrid PET/CT, and later PET/MRI, combined functional and anatomical imaging, while standardised guidelines codified quantitative practice (Boellaard et al., 2014).

Key figures

• Lawrence Shepp
• Yehuda Vardi

Seminal works

• shepp-vardi-1982

Frequently asked questions

How is PET different from CT or MRI?

CT and MRI map anatomy directly, whereas PET maps the distribution of an administered radiotracer and therefore shows function or metabolism; PET is commonly fused with CT or MRI so that functional findings can be located in the anatomy.

What is detected to form a PET image?

The tracer emits positrons that annihilate with electrons, each annihilation producing two photons in opposite directions; detecting these photons in coincidence localises the event and allows the tracer distribution to be reconstructed tomographically.