In all 4D to 3D registrations, user is offered a choice of currently selected timepoint as a Base, or instead determine timepoint of Maximal Information.  In PET to Anatomy registration it is always
Maximal Information.  Maximal information timepoint is assumed to have the best contrast between tissues and thus provide the best registration quality.

In all registrations, 3D and 4D,  implemented mechanism of tolerating the memory allocation failure.  Since Finetune is highly parallelized at many leveel and pet-thread memory requirements are high, for large images and high core count a memory allocation error occurs.  Old versions of FireVoxel either failed or lately the Parallel mechanism was disabled.  Now fully parallel veersion is ran,  the threads which failed are recorded into "Failed" thread array and then ran in serial mode.