What property of a PET tracer determines its ability to enter the brain?

The ability of a PET tracer to enter the brain is significantly influenced by its lipophilicity. Lipophilicity refers to the affinity of a compound for lipid environments, which is crucial for crossing the blood-brain barrier. The blood-brain barrier is composed of tightly packed endothelial cells that restrict the passage of many substances, particularly those that are hydrophilic.

Compounds that are lipophilic can more easily diffuse through the lipid membranes of these cells because they can interact favorably with the fatty components of the membranes. This property allows lipophilic tracers to penetrate into the brain tissue and reach their targets, facilitating effective imaging and diagnosis in PET scans.

While factors such as ionic charge, hydrophilicity, and molecular weight can also play roles in the transport dynamics, lipophilicity is the primary determinant for the successful entry of PET tracers into the brain, making it the correct answer for this question.