Fractional uptake rate (FUR)
Fractional uptake rate was previously sometimes called retention index (Ri). FUR is a simple PET quantifier, calculated as a ratio of tissue activity at time T and integral of plasma activity from 0 to T [Ishizu et al. 1994, 1995]. Thus the unit of FUR is min-1 [Thie 1995]. FUR can be calculated from a single late PET scan, like SUV, but blood sampling from the injection to the scan time is required.
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| Figure 1. For calculation of FUR, plasma TAC is integrated from 0 (injection time) to the middle time of PET frame (or middle point of PET time range when PET scan consists of several frames). Note that integration to the end of PET data would be a mistake. Mean tissue concentration is then divided by plasma integral to get the FUR value. |
FUR is an approximation to the Gjedde-Patlak plot (MTGA for irreversible uptake) slope Ki, to the extent that at large T (late time after injection) the effective distribution volume term in Gjedde-Patlak analysis is not important (y axis intercept is assumed to be 0). FUR and SUV are proportional, related by plasma clearance rate and a dimensionless initial distribution volume (Thie 1995).
Metabolic rate
When the PET tracer is an analog of glucose (e.g. [F-18]FDG) or fatty acids (e.g. [F-18]FTHA) or other native substrate in the tissue, and it is metabolically trapped in tissue during the PET scan, the FUR estimate, like influx constant (Ki) from Gjedde-Patlak plot, can be used to calculate the metabolic rate of the native substrate. For example, in [F-18]FDG the FUR can be multiplied by concentration of glucose in plasma, and divided by the appropriate lumped constant, to get an estimate of glucose uptake rate.
Naturally, the FUR-based metabolic rates contain the same bias as FUR itself when compared to metabolic rates based on Ki.
See also:
- Calculation of FUR image
- Calculation of regional FUR
- Standardized Uptake Value (SUV, DUR)
- Multiple-time graphical analysis for irreversible tracer uptake (Gjedde-Patlak plot)
