PET studies

see , e.g., Mol Pharm. 2021 Jan 4; 18(1): 416–428.

Yes, it's possible. As Gert said, you can perform PET scans. Another method is contrast-agent (gadolinium) enhanced MRI, which is also usually applied in patient diagnostics for BBB damage. We had good experiences with this in a model of radiation-induced brain injury (https://doi.org/10.3389/fonc.2020.598360).

Gadolinium has a size of around 0.5 kDa. In case you are interested in the permeability of bigger proteins or want to measure size-specific break-down, you can perform in vivo optical imaging using (conjugated) Cy5.5

The protocol is described here: https://link.springer.com/protocol/10.1007%2F978-1-61779-191-8_29.

Gert Fricker Suckert Theresa Thank you so much for your suggestions. They are of great help indeed.

Use in vitro techniques

https://visikol.com/services/in-vitro/blood-brain-barrier/#:~:text=The%20BBB%20is%20permeable%20to,critical%20molecules%20across%20the%20membrane.&text=This%20assay%20service%20allows%20for,and%20function%20of%20the%20BBB.