PET has long been regarded as a quantitative imaging tool. That is, the voxel values of reconstructed images can be calibrated in absolute units of radioactivity concentration with reasonable accuracy and precision. The ability to accurately and precisely map the radiotracer concentration in the body is important for two reasons. First, it ensures that the PET images can be interpreted correctly since they can be assumed to be free of physical artefacts and to provide a true reflection of the underlying physiology. Second, it enables the use of tracer kinetic methodology to model the time-varying distribution of a labelled compound in the body and quantify physiological parameters of interest.
The reputation of PET as a quantitative imaging tool is largely based on the fact that an exact correction for attenuation of the signal due to absorption of photons in the body is theoretically achievable. However, accurate attenuation correction is not so easy to achieve in practice and there are many other factors, apart from photon attenuation, that potentially impact on the accuracy and precision of PET measurements. These include count-rate losses due to dead time limitations of system components, variations in detector efficiency, acceptance of unwanted scattered and random coincidences and dilution of the signal from small structures (partial volume effect). The ability to accurately measure or model these effects and correct for them, while minimizing the impact on signal-to-noise ratio, largely determines the accuracy and precision of PET images.
This chapter discusses the various sources of measurement error in PET. Methodological approaches to correct for these sources of error are described, and
their relative merits and impact on the quantitative accuracy of PET images are evaluated. The sequence of the following sections corresponds approximately to the order in which the various corrections are typically applied. It should be noted, however, that the particular sequence of corrections varies from scanner to scanner and depends on the choice of algorithms.
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