Mechanisms of Localization:

Compartmental Localization

•  COMPARTMENTAL LOCALIZATION (Defined as the placement of a radiopharma-ceutical in a fluid space, and then imaging that fluid space)

   

     PULMONARY VENTILATION IMAGING

   

  • Mechanism: Compartmental localization of the inhaled radioactive gas within airways in the lungs

   

  • Radiopharmaceutical: Xe-133 gas- contains no particles

   

  • Immediate distribution to lungs; since Xe gas is lipophilic and can cross cell membranes, it dissolves in pulmonary capillaries and is circulated through blood stream, permitting cerebral blood flow studies.

   

  • t_biol of Xe-133 gas in lungs is <0.5 min in most patients.

   

     BLOOD POOL IMAGING

   

  • Mechanism: Compartmental localization of autologous radiolabeled red cells within the blood pool

   

  • Radiopharmaceutical: Tc-99m RBC's

   

  • Particle size: approximately 7 mm for Tc-RBC's

   

  • Immediate distribution within blood pool; ultimately Tc-99m dissociates from injected compounds and is cleared through the kidneys.

   

  • t_biol of Tc-99m RBC's, 15-25 hr

   

     CISTERNOGRAM

   

  • Mechanism: Compartmental localization of material injected directly into CSF via lumbar puncture.

   

  • Radiopharmaceutical: In-111 DTPA;  t_biol of In-DTPA is approximately 20 hr.

   

  • Particle size: none- compound is soluble.

   

  • Immediate distribution within CSF; ultimately bathes brain and brain stem; indicates presence of CSF leakage in nasopharynx; eventually cleared through the kidneys.
  
   

     VOIDING CYSTOGRAM

   

  • Mechanism: Compartmental localization of liquid infused into urinary bladder.

   

  • Radiopharmaceutical: Tc-99m SC

   

  • Particle size: 0.1-2.0 mm

   

  • Immediate localization in bladder; rapidly emptied via catheter into container for counting  of radioactivity.
    
     
  • t_biol is measured in minutes.