Transactions of the Institute of Measurement and Control

The application of electromagnetic induction to the measurement of blood flow and blood velocity in vivo is reviewed, The electronic requirements are discussed and the associated problems are surveyed. A brief account is given of Bevir's virtual current theory and its application to flow and velocity measuring devices. The devices at present known to be in use or to be under development are reviewed under the following headings: invasive local magnetic field devices which include cannular and perivascular flowmeters, also intravascular velometers and flowmeters, invasive extracorporeal field perivascular and intravascular flowmeters; and noninvasive systems which either utilise skin electrodes and a separate extracorporeal magnet or comprise an integral extracorporeal flowmeter. There is a brief discussion of the present state of development.

Ultrasonic fields can be used to measure the complex dynamic flow patterns in a wide range of intact blood vessels with calibres ranging from fractions of a millimetre to several centimetres. Many of these vessels may be examined non-invasively from outside the body. Ultrasonic flowmeters have developed from the early transit-time instruments through the continuous-wave Doppler velocimeters to the complex multigate pulsed Doppler and conposite duplex B-mode/pulsed Doppler imaging systems which are capable of measuring true volume flowrates. The natural constraints associated with the fundamental methods, the various techniques, and the signal-processing stages are all considered in relation to the quantitative measurement of blood-flow velocity and volume flow rate with particular reference to potential sources of error.

This paper reviews the fundamental basis of radionuclide tracer methods used in the investigation of the circulation. The various Xenon-133-clearance methods for the measurement of cerebral blood flow are described, and their advantages and limitations are indicated. An outline is also given of the use of corresponding tracer-clearance techniques in the determination of blood flow in liver and muscle. The potential advantages of short-lived positron-emitting radionuclides and positron-emission tomography are considered, and quantitative and qualitative non-diffusible radionuclide tracer methods are also discussed.

This paper outlines a simplified theory to explain the behaviour of photoelectric plethysmographs used for the measurement of blood flow in the skin. The paper reports the finding that light diffusing through blood, from a source outside a blood vessel, can diffuse preferentially in the direction of motion of the blood, and that this preferential diffusion is likely to arise from a combination of the plasma skimming which occurs at the vessel wall and from the orientation effects of the erythrocytes in motion.

A new principle for the measurement of microcirculatory blood flow is described. The laser Doppler flowmeter u tilises the fact that a laser light beam incident on tissue is scattered in static structures as well as in moving red cells. Light beams scattered in moving red cells undergo a frequency shift according to the Doppler effect, while beams scattered in static tissue alone remain unshifted in frequency. If shifted and unshifted light is mixed on a photodetector surface, beat notes are produced. The power spectral density of these beat notes is determined by red cell concentration and velocity. From the alternating portion of the photocurrent, an output signal that is related to flux of red cells in a specific tissue volume can be calculated. Some applications of the method are presented.