Perfusion (blood flow)
Perfusion is the volume of blood flowing through certain mass (or volume) of tissue per unit time. Blood flow is usually given in units mL/(100 g * min) or mL/(mL * min).
Tissue perfusion can be measured noninvasively with positron emission tomography using the following general techniques:
- Clearance or uptake of an inert and diffusible tracer
- Equilibrium imaging of a short-lived, inert, and diffusible tracer
- Uptake of a labelled agent that is trapped in capillaries or diffusible tracer that is metabolically trapped in cells.
Tracers that remain in vascular space cannot be used to measure blood flow (Lassen, 1984). At its best, such tracer can provide an estimate of plasma or blood volume in the tissue, which may vary in the same direction as the plasma flow, and thus appear to correlate with blood flow.
The methods to measure perfusion with diffusible and inert tracers are based on the principle of exchange of inert gas between blood and tissues (Kety and Schmidt, 1945; Kety, 1985), and on the Fick’s principle. 133Xe clearance method has been widely used, but it will only give an estimate of the total blood flow (both nutritive and non-nutritive) from all the tissues that is drained by the vein, and the tissue-blood -partition coefficient must be known (but is highly variable).
The term blood flow commonly refers to the volume of blood passing through arteries and veins per unit time. Arterial bulk flow can end up in tissue capillaries (nutritive capillary blood flow), or flow into veins through shunts or arterio-venous anastomoses (nonnutritive blood flow). Depending on the blood flow measurement technique, and site of measurement, nutritive and nonnutritive flow may contribute to bulk blood flow with varied shares. Blood perfusion refers to nutritive capillary blood flow.
When perfusion is measured using diffusible PET tracers, such as [15O]H2O, the nonnutritive (noneffective) fraction of blood flow (blood flowing through shunts is not included in the perfusion estimate. The fraction of nonnutritive flow is especially high the in the skin. By definition, in shunts arterial and venous blood concentrations are equal, Ca - Cv = 0, and thus it has no effect on the concentration in tissue, based on the Fick’s principle.
However, nonnutritive blood flow will increase the estimate of arterial blood volume, because both the arterial and venous fraction of the shunt volume will have the same kinetics.
Microspheres of different diameters can be used in animal studies to measure nutritive and nonnutritive blood flow.
- Fick’s principle
- Analysis instructions by tracer
- Dynamic processes
- Compartmental model
- Compartmental model for radiowater
- ARG method
- Vascular system
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Created at: 2014-04-07
Updated at: 2018-12-10
Written by: Vesa Oikonen