BPnd (SRTM Ref): Simplified Reference Tissue Model

The BPnd (SRTM Ref) method is based on the simplified reference tissue model (SRTM) developed by Lammertsma et al. [28] which relies on the following assumptions:

1. The distribution volume is the same for the tissue of interest and the reference tissue: K₁/k₂=K₁'/k₂'.
2. The kinetics in the receptor-rich tissue of interest is such, that it is difficult to distinguish between the specific and the free/non-specific compartment; ie. the TAC can be fitted by a 1-tissue compartment model. This assumption may not be valid for all tracers.

Gunn et al [34] transformed the SRTM model into a solution which better suited for pixel-wise application. It is based on a set of basis functions which are generated by convolving the reference TAC with decaying exponentials. The exponents employed should cover a range which is reasonable for the tracer considered. To calculate the binding potential of a TAC a least squares fit is performed with each of the basis functions. That fit with minimal deviation between the TAC and the model curve is regarded as the solution, and the binding potential is calculated from the set of fit parameters.