Bloody PET

Blood accounts for ∼8% of total body weight, or ∼70 mL/kg of lean body weight. Blood contains "formed elements" (blood cells) suspended in plasma. Specific gravity (mass density) of blood is 1.057±0.007 g mL-1 and densities of plasma and serum are 1.035±0.005 and 1.018±0.003 g mL-1, respectively (Schneck, 2015, in Biomedical Engineering Fundamentals). Viscosities of blood and plasma are 3-6 and 1.5-2 times that of water, respectively. Blood cells include erythrocytes (red blood cells, ∼95%), thrombocytes (∼5%), and leukocytes. All blood cells are produced in red bone marrow. Plasma contains electrolytes, lipids, glucose, urea, and other compounds, and ∼6-8% of its mass are proteins. The administered radiotracer can be dissolved or bound to any of the constituents of the blood, which may affect its availability for transport across the endothelial barrier into tissues, and its distribution volume in tissues. Most of the radioligands are metabolized in the body and in the blood. For quantitative analysis the concentration of radiotracer in the blood or plasma must be measured to obtain an input function, which usually requires blood sampling.

The rate that PET tracer equilibrates between blood cells and plasma determines whether plasma or whole blood should be used as the input function in data analysis. Usually the transport rate between plasma and red blood cells is slow enough that its effects on exchange between capillary blood and tissue can be ignored (Knudsen et al., 1994). Even if plasma can be considered as the correct input, blood curve may need to be converted to plasma curve in case blood curve is derived from dynamic PET image or automatic blood sampling system was used.

Erythrocyte cell membranes contain many adhesive proteins, which may bind PET tracers, and affect plasma-to-blood ratio, together with membrane lipids and binding to plasma proteins (Paixao et al, 2009). Adherence to RBC membrane and/or intracellular uptake will affect plasma pharmacokinetics of the tracer.

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Updated at: 2019-12-31
Created at: 2019-02-10
Written by: Vesa Oikonen