What is the primary mechanism of localization for Tc exametazime (HMPAO)?

• A. Active transport
• B. Simple diffusion
• C. Facilitated diffusion
• D. Receptor-mediated endocytosis

Answer: (B)

The primary mechanism of localization for Technetium-99m exametazime (HMPAO) is simple diffusion. This radiopharmaceutical is lipophilic, which allows it to readily cross the cell membranes of brain tissues after intravenous injection. Once it penetrates the cells, it rapidly undergoes conversion to a more hydrophilic compound, which prevents it from leaving the cells. This property is essential for its use in imaging brain perfusion because it allows for the accurate assessment of regional cerebral blood flow.

Understanding that HMPAO operates through simple diffusion highlights the importance of lipophilicity in the pharmacokinetics of radiopharmaceuticals used in diagnostic imaging. This mechanism enables the radiopharmaceutical to localize in areas of the brain with adequate blood supply, providing crucial information regarding cerebral perfusion.