1-Tissue Compartment Model (Alpert)

This model is intended to calculate the parameters of a 1-tissue compartment model. It is just a slightly modified version of the classical implementation of Alpert's time-weighted integral approach for brain perfusion.

There are two changes:

1. The parameters are expressed as K₁, k₂, and Vt (rather than f, f/p, p).
2. Total blood activity can be subtracted for spillover correction.