Radioactively labelled microspheres in regional cerebral blood flow determinations. A study on monkeys with 15 and 35 mum spheres.

15 mum and 35 mum radioactive microspheres, differently labelled, were used simultaneously for rCBF-determinations in monkeys. Flow values calculated from data for 35 mum spheres were 27 +/- 7 g/min per 100 g higher for grey matter and 10 +/- 3 g/min per 100 g lower for white matter than flow values calculated from 15 mum spheres, while both sphere sizes resulted in similar values for the choroid plexus. A large number of 35 mum spheres were trapped in extracerebral pial arterioles. It is concluded that entrpment in relatively large arterioles of the 35 mum spheres explains the differences in flow values, and that this invalidates the use of 35 mum spheres, and larger, in rCBF-determinations.