After integrating time delay parameter, a new fitting result.

Sokoloff model:   Integrating the "Time Delay" fitting parameter. At the very least, this time delay parameter should be in [0,FrameTimeDelta] (=[0,15] sec) as this is the temporal resolution of TAC.

Updating the Sokoloff model.  During the demo to Fernando, observed that in "naive convolution" calculation upsampling time to 1 sec makes a big positive precision difference.  But this is computationally expensive and additional Upsampling Interval parameter will add to parameter list.
So instead attempting to upsample only the IRF ( Impulse Response Function) and integrate fast convolution technique from:

Fast and accurate voxel-by-voxel perfusion imaging using convolution models.  Artem Mikheev, Henry Rusinek.  Image Analysis - Parametric Mapping Session at the ISMRM 2008 Totonto, CANADA.