Functional evaluation of the blood‐brain barrier using positron emission tomography

Positron emission tomography is capable of making measurements of the permeability–surface‐area (PS) product of the blood‐brain barrier, if modeling constraints can be met. A strategy for measuring the PS product of ammonia is presented that can be applied to any compound that is taken up by the brain and trapped before back‐diffusion occurs. A second strategy for measuring the PS product of compounds that are not metabolically trapped is also presented. The flexibility of positron emission tomography and the variety of appropriately labeled compounds suggest that this technique will be very important in the characterization of the blood‐brain barrier in humans in health and in disease.