Compartmental localization is defined as the placement of a radiopharmaceutical in a fluid space and maintaining it there long enough to image that fluid space. Since a fluid is defined as a liquid or a gas, the airways of the lungs qualify as a fluid space. The use of Xe-133 gas, Xe-127 gas, or Kr-81m gas as a ventilation agent is therefore a good example of this mechanism. Immediate distribution is to the lungs; since Xe-133 is lipophilic and can cross cell membranes, the gas passively diffuses into pulmonary capillaries and the activity is circulated through the blood stream, permitting cerebral blood flow studies. The tbiol of all these gases in the lungs is <0.5 min in most patients. Ultimately the activity is cleared from the body through the lungs.
Another example of compartmental localization is blood pool imaging using autologous Tc-99m labeled red cells or Tc-99m Human Serum Albumin within the blood pool. The immediate distribution is within the blood pool; ultimately the Tc-99m dissociates from these compounds and is cleared through the kidneys. The tbiol of Tc-99m HSA in the blood pool is approximately 1-2 hr; for Tc-99m RBC's, The tbiol is approximately 20 hr.
One can perform a cisternogram following injection of In-111 DTPA directly into the cerebrospinal fluid (CSF). This use of compartmental localization involves early and delayed views, permitting tracing of the kinetics of CSF. Immediate distribution is entirely within CSF; ultimately the activity bathes the brain and brain stem and indicates the presence of CSF leakage into the nasopharynx. The tbiol of In-DTPA is approximately 20 hr; radioactivity is eventually cleared through the kidneys.
Even an "artificial" localization such as the infusion of a dilute solution of Tc-99m pertechnetate or a suspension of Tc-99m sulfur colloid into the urinary bladder in a voiding cystogram qualifies as compartmental localization. The immediate localization is in the bladder; the activity is rapidly emptied via catheter with a tbiol measured in minutes. This study provides significant clinical information while conferring a minimal radiation dose to the patient due to the short retention time of the radiopharmaceutical in the bladder.
|Stephen Karesh, PhD.||
Last Updated: August 14, 1996