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A method is described for measuring the regional cerebral-to-large vessel haematocrit ratio using inhalation of carbon-11-labelled carbon monoxide and the intravenous injection of carbon-11-labelled methyl-albumin in combination with positron emission tomography. The mean value in a series of nine subjects was 0.69. This is approximately 20% lower than the value of 0.85 previously reported. It is concluded that previous measurements of regional cerebral blood volume using a haematocrit ratio of 0.85 will have underestimated the value of regional cerebral blood volume by 20%.

Platelets labelled with 111In displayed similar survival curves after incubation with 111In-oxine in plasma, plasma-saline and dextrose saline media. The use of autologous red cells to cushion platelets during high-speed centrifugation facilitated platelet resuspension without greatly affecting the duration of the labelling procedure. Quantitative scanning after reinjection of labelled platelets in haematologically normal subjects showed that, initially, splenic indium amounted to about 35% of the injected dose and hepatic indium about 12%; these levels rose only slightly over the subsequent duration of the platelet life span. Subtraction of the signal from indium in platelets thought to be normally pooled within the spleen from the total indium signal gave splenic indium uptake curves which reflected splenic platelet destruction. Initially, the sum of indium levels in spleen, liver and blood equalled 100% of the dose. Thereafter, the sum fell progressively at a rate thought to be approximately equal to the rate of bone marrow uptake.

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