MR-PET offers new horizons for quantitative, specific assessment in animal models. Here the basic principles are described along with advantages and pitfalls.
Attendees will learn
1. Basic
principles of PET imaging
(positron emission, detection, attenuation correction, reconstruction)
2. Common tracers used in preclinical research, their use, advantages and limitations
3. How
to extract useful information from PET images
4. Pitfalls and problems in MR-PET
Historically, the largest obstacle to combining MR with PET has been the interference of the magnetic fields with the photomultiplier tubes used to the detect gamma photons emitted in the PET experiment. The advent of avalanche photodiodes and later silicon photomultiplier tubes has allowed detectors to be incorporated within or adjacent to MR magnets so that a range of combined machines are available: either adjoining or self-contained for simultaneous MR-PET acquisitions.
Although sequential systems do not require much compromise in the design of either machine, the real promise of MR-PET is the benefit gained by having truly coregistered data at the time of acquisition. For example, rapidly acquired MRI can be used for motion correction in the PET data. Cross-validation of techniques can be performed (e.g. ASL perfusion vs. PET perfusion in the brain; or late-gadolinium enhancement vs. FDG infarct in cardiac applications).
PET can be highly quantitative when proper corrections are made to the data. One of these corrections (important with larger animals) concerns signal attenuation which differs depending on the tissue type. It can be addressed by irradiating the subject with a known source that revolves around, thus giving a direct measure of the attenuation at each point. MR images acquired simultaneously can be used to model tissue attenuation based on MR contrast. The approach must be used carefully as some substances (in particular bone and air) have very different attenuating effects but similar MR contrast in most sequences.
MRI data can be used to reconstruct MR-PET data taking information into account that is not available during the PET scan. For example, co-injecting a gadolinium contrast agent with the PET tracer to obtain arterial input functions.
Modelling of the data seen to obtain information about biological processes can be complicated. A full model requires the solution of differential equations which can explain the biological interactions of the radiotracer injected with different tissues: either through receptor binding (e.g. in dopamine tracers) or via cell metabolism (FDG). Full kinetic analysis is time consuming and requires invasive blood sampling which may not be possible in small animals. Simple models are widely encouraged as an alternative, using standardised uptake values, perhaps with a reference tissue. The should be validated against a full model, however, to ensure they offer useful measures of the biological processes of interest.
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