Ratio of time-activity curves
Tissue-to-reference tissue ratio (SUV ratio) and tissue-to-blood ratio are simple analysis methods often well-suited for diagnostic semi-quantitative PET studies, but that can also be applied to quantitative studies for instance in brain receptor studies, especially for analysis of steady-state and bolus+infusion PET studies.
Reference tissue must not be affected by the studied phenomenon, for instance a disease process, or in brain receptor studies it must not contain the receptors that the radioligand binds to. Instead, the reference tissue should reflect the input function, so that inter-individual differences in concentration of the radiopharmaceutical in the arterial plasma are cancelled out from the ratio. In diagnostic studies a healthy tissue region from the same organ can be used as reference. If even such reference tissue is not found, a large region is sometimes used, for instance the whole brain.
Ratios are dependent on the time of the measurement, and suitable ratio calculation time (and scan time) for the clinical use must be determined by first conducting a series of dynamic PET studies. In addition, studying the kinetics of the ratios can provide valuable and comprehensible information on the properties of a new radiopharmaceutical for model development.
Let us simulate some typical PET time-activity curves
(TACs) to see how the ratios could be utilized:
(to be added).
- Late-time tissue-to-plasma ratio
- Standardized uptake value (SUV)
- Fractional uptake rate (FUR)
- Asymmetry index (AI)
- Perfusion ratio
- Bmax and KD
- Retention index (RI)
- Static late scan
- Fitting TTACs
Carson RE. PET physiological measurements using constant infusion. Nucl Med Biol. 2000; 27(7): 657-660. doi: 10.1016/S0969-8051(00)00138-4.
Updated at: 2020-05-13
Created at: 2019-03-31
Written by: Vesa Oikonen