Mechanisms of Localization:

Active Transport

•  MECHANISM: ACTIVE TRANSPORT (Defined as utilization of a normally active, energy-dependent metabolic pathway in the body to move a radiopharmaceutical across a cell membrane and into the cell.

   

     THYROID UPTAKE/SCAN

   

  • Mechanism: Active Transport. Iodide is trapped, undergoes intermediate syntheses involving a thyroglobulin intermediate, and is ultimately organified into T₃ and T₄.

   

  • Radiopharmaceutical: I-123 sodium iodide or I-131 sodium iodide. Particle size: none- compound is soluble

   

  • Initial localization in thyroid, stomach, and parotids; ultimately stored in thyroid or cleared through the kidneys

   

  • t_biol in thyroid: 24 days.

   

  MYOCARDIAL PERFUSION

   

  • Mechanism: Active Transport of radiothallium. This involves utilization of the normally operative energy dependent metabolic pathway for handling potassium since thallous ion (Tl¹⁺) and Rb ion (Rb1+) are potassium analogs and are therefore included in the well documented ATPase-driven Na/K pump mechanism.

   

  • Radiopharmaceutical: ²⁰¹TlCl, ⁸²RbCl. Particle size: none. Compound is soluble

   

  • Initial localization in heart, liver, muscle; ultimately recycled so very little is cleared through the kidneys

   

  • t_biol in whole body: 10 days.

   

     RENAL IMAGING

   

  • Mechanism: active transport if compound is processed by tubular secretion.

   

  • Radiopharmaceutical: Tc-MAG3 for tubular secretion studies. Particle size: none- compounds are soluble

   

  • Approximately 80+% of MAG3 by tubular secretion, the remainder by GFR.
    
     
  • Imaging begun immediately post injection and acquisition is divided into 3 min frames, permitting generation of renogram curves.