Reference tissue in receptor studies
Reference tissue is a region where
- there is no specific uptake of the radioligand,
- uptake is not affected by disease process or treatment, and
- nonspecific binding is similar than in the regions of interest
- (and it has to be located near the tissue of interest so that it can be found in the same PET image).
Reference tissue can be used as a substitute of arterial plasma curve as input function in quantitative analysis of PET studies, applying compartmental models or multiple-time graphical analyses, or in semi-quantitative analysis applying tissue-to-reference tissue ratio. Ex vivo studies are needed to validate that reference tissue does not express the target. Ex vivo and in vivo blocking or displacement studies can verify that the region is suitable as reference region for a specific radioligand. In addition, the in vivo human studies should be able to demonstrate that the distribution volume (VT) of the radioligand is low and similar in the reference region in patient and healthy volunteer groups.
Estimation of the VT requires a metabolite-corrected plasma input function, and application of compartmental model or Logan plot. In optimal situation the ratio of reference tissue and plasma input curves reaches a steady level, and may even be used to estimate the fractions of parent radiopharmaceutical and labelled metabolites in the plasma (Wong et al., 1986). In some cases, low and unchanged SUV in the proposed reference region could be used for verification (de Vries et al., 2021).
|Reference tissue||Arterial plasma|
Specific binding in reference tissue may lead to severe underestimation of receptor occupancy.
If label-carrying metabolite passed the blood-brain barrier, then reference tissue input model may lead to severe bias in results, and plasma input models may be preferable (Zoghbi et al., 2006). Specific model needs to be developed, where the concentrations of the metabolite in plasma is used as second model input (Matsubara et al., 2014).
For certain radiopharmaceuticals it may be possible that the concentration of labelled metabolite at late time points is minimal and can be ignored (Kim et al., 1999).
When no suitable cerebral reference region is available, it may be possible to quantify brain receptors using muscle as reference region (Le Foll et al. 2007). However, there are several caveats in using this approach:
- Radioactive label carrying metabolites that can not penetrate the blood brain barrier may well enter extracellular tissues and prevent their use as reference region
- Blood flow is much lower in muscle than in the brain, therefore becoming the limiting factor for K1 in the reference tissue
- Furthermore, blood flow in muscle is highly variable in awake state
- Non-displaceable distribution volume is probably different in the brain and in extracerebral reference region; therefore it has to be measured, and only if proven to be constant the binding potentials can be corrected for it (Le Foll et al. 2007)
- Additional requirements may be set by the model, for example, SRTM requires that reference region kinetics can be reasonably well described by 1-tissue compartment model
In oncology, the concentration in tumour is frequently divided by
concentration in normal tissue, which often is muscle (T/N or T/M).
This mimics visual analysis, but gives a numerical value to the target-to-background difference.
Problem is that radioligand uptake in muscle may multiply if patient is nervous or position is hard to keep.
If radioligand uptake is very high in a certain brain region, it may take up all ligand that is available in the plasma. In that case the tissue curve is proportional to the AUC (integral) of arterial plasma curve. "Positive" reference tissue can be found and used with few radioligands, including certain AChE radiopharmaceuticals.
- Tissue-to-reference tissue ratio
- Reference tissue input compartmental models
- Model calculations for regional data
- Calculation of BPND images
- Reference region methods in radiowater studies of the brain and kidneys
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Updated at: 2021-11-30
Written by: Vesa Oikonen