Phagocytosis involves the physical entrapment of colloidal particles by Kupffer cells in the reticulothelial system following an intravenous injection. Colloidal suspensions contain particles in the range of approximately 0.05 to 4 µm (see Figure 5) and may include things as diverse as Tc-SC and cigarette smoke in air. The most commonly used phagocytic agents, Tc-sulfur colloid and Tc-microaggregated albumin, typically have particle sizes ranging from approximately 0.1-2.0 µm. The smaller the particles, the greater the bone marrow uptake; larger particles tend to localize in the liver and spleen. Due to the small size of the colloid compared to the diameter of the average capillary, which is 7 µm, capillary blockade does not occur. Distribution in the RES is typically 85% in the liver, 10% in the spleen, and 5% in marrow. In severely diseased livers, the ratio may change significantly with increased uptake in the spleen. The tbiol of Tc-sulfur colloid in the liver is infinitely long; by comparison, tbiol of microaggregated albumin is 6-12 hr. The t½ of clearance from the blood for these agents is approximately 2.5 min, so in 10 min only approximately 6% remains in the blood stream. Imaging may therefore begin as early as 5-10 min post injection.
|Stephen Karesh, PhD.||
Last Updated: August 14, 1996