Dear Milo, 

Check the references to these papers out: 

http://www.usp.br/bioterio/Artigos/Procedimentos%20experimentais/Routeadministration-4.pdf

Article Imaging of intravitreal injected solution dispersion

In this study, radio-labeled dopamine was injected intracerebral (IC) at various volumes into the brainstem and then radioactivity was measured from punches in series of coronal sections at two different time points (1 and 15 min post-injection). Then they plotted the proportion of radioactivity in the lateral, horizontal, and anterior-posterior planes, relative to the injection site. 

Gives a sense of dispersion - and was helpful.

Thanks Milo,

However, it is still pretty hard to know what is the final concentration of injected drug. You have a volume but it has both celllular and intercellular spaces.