What is the mechanism of localization for Tc HDP and Tc MDP?

• A. Chemisorption
• B. Physisorption
• C. Diffusion
• D. Absorption

Answer: (A)

The localization mechanism for Technetium-99m hydroxymethylene diphosphonate (Tc HDP) and Technetium-99m medronate (Tc MDP) primarily involves chemisorption. This process is characterized by the strong interaction between the radiopharmaceutical and the mineral component of the bone, specifically hydroxyapatite.

In the case of Tc HDP and Tc MDP, they are composed of diphosphonate compounds that have a high affinity for bone tissue, particularly in areas with increased osteoblastic activity, such as sites of inflammation or tumors. Chemisorption occurs when there is a chemical binding due to the affinity of the diphosphonate for the calcium ions in hydroxyapatite, leading to localization at these specific sites in the bone.

This chemical interaction is crucial for the imaging properties of these radiopharmaceuticals, as it allows them to concentrate in the targeted bone areas, making them visible during scintigraphy. The strong binding ensures that the radiopharmaceutical remains in place for imaging, which is essential for accurate diagnostics in conditions like bone metastases or osteomyelitis.