Aggregation of microspheres in blood flow measurements

The aggregation of NEN-TRAC radionuclide-labeled 15 microns resin microspheres was studied in vitro and in vivo. In vitro, the control group not agitated at all was compared with groups subjected to agitation with a Vortex Shaker or agitation with first a Vortex shaker and then a sonification bath. Loading the microspheres in a catheter made agitation unnecessary (5.6 +/- 1.7 aggregates/100 spheres and 2.3 +/- 0.5 microspheres/aggregate). Sonification increased aggregation when the solution was drawn into 1 cc syringes (13.7 +/- 2.6 aggregates/100 spheres and 2.8 +/- 1.2 microspheres/aggregate). In vivo less aggregates were observed. Loading the microspheres into PK 50 catheters and then injecting them into the left ventricle of the heart after agitation in a Vortex shaker for 1 min minimized aggregation (1.6 +/- 1.0 aggregates/100 spheres and 2.2 +/- 0.4 spheres/aggregate). Sonification increased both the number of aggregates and the number of spheres in each aggregate significantly (10.0 +/- 3.1 aggreates/100 spheres and 2.8 +/- 1.3 spheres/aggregate). Aggregates of 10 to 40 microspheres were occasionally observed. When the microspheres were injected into the aorta at the level of the diaphragm, one large aggregate (approximately 50 spheres) was seen. The handling of the microsphere solution is important to prevent aggregation and the treatment recommended by the manufacturer does not seem to be the optimal. Large aggregates, which are rare, might embolize the precapillary arteries. Small aggregates are more common and they might result in disproportionately fewer microspheres in vessels than their actual blood flow. This would make the microsphere method less reliable in blood flow measurements.