The application of electromagnetic induction to the measurement of blood flow and blood velocityin vivo is reviewed. The electronics requirements are discussed and the associated problems are surveyed. An account is given of Bevir's virtual-current theory and its application to various flow and velocity measuring devices. The devices at present known to be in use or under development are reviewed under the following headings: invasive local-magnetic-field devices, which include cannular and perivascular flowmeters; 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

Blood flow and blood velocity measurement in vivo by electromagnetic induction.

A drive system for an electromagnetic flowmeter wherein the fluid to be metered is conducted through a flow tube to intersect a transverse magnetic field created by an electromagnet having an excitation coil. The resultant voltage induced in the fluid is transferred to a pair of electrodes mounted at diametrically-opposed points on the tube to yield a signal representing flow rate. The drive system, which is powered from an a-c line, includes a full-wave rectifier whose output voltage is applied periodically to the excitation coil at a rate which is low relative to the line frequency. In order to maintain the level of current through the coil at a substantially constant level during the excitation period without, however, consuming an undue amount of energy, the coil is shock-excited by a high voltage surge derived from the rectifier to generate a current flow therein having a substantially constant current level, thereby producing a steady-state flux field minimizing the presence of unwanted components in the output signal of the flowmeter.

High-voltage impulse driver for electromagnetic flowmeter

A square-wave signal generator circuit for generating a rectangular waveform signal particularly for application to the field coil of an electromagnetic flow meter. A coil excitation current from a direct current source is switched to provide a current flow in alternate directions through the field coil. During the transient time of the coil excitation current following the switching operation, a higher output voltage from the source is used to shorten the current rise and fall times while a lower voltage from the source is used during a steady state time of the excitation current to minimize power losses. A switching circuit using a switching transistor is used to select the higher voltage while a back-biased diode is used to immediately introduce the lower voltage when the excitation current has reached a steady state. A voltage comparator is used to compare the voltage generated by the excitation current with a reference voltage to produce an output signal for operating the switching transistor between its on and off states during the transient and steady state current conditions, respectively.

Square wave signal generator

The invention relates to a method for operating an electromagnetic flowmeter (1) and to a flowmeter for carrying out the method. A measuring tube (2) is provided with a coil arrangement (3, 4) for producing a magnetic field at right angles to the direction of flow. The direction of current flow in the coil arrangement (3, 4) is altered periodically, and the current is kept essentially constant in one period after a rise to a predetermined value. In this case, the voltage applied to the coil arrangement (3, 4) and/or its derivative is determined and compared with an associated reference value. If this voltage or derivative does not fall below the reference value in a desired minimum time within a measurement period, an error message can be emitted in order to indicate a fault. If the voltage or derivative falls below the reference value in good time, it is ensured that the magnetic field is sufficiently stable and measured values for a voltage which can be tapped off at measurement electrodes (5, 6), if need be, are subject to a slight offset error. As a result, a more reliable measurement result with improved accuracy is achieved.

Method for operating an electromagnetic flowmeter and electomagnetic flowmeter

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.

Blood flow and blood velocity measurement in vivo by electromagnetic induction

An electrical system for measuring the speed of flow in a body of a fluid includes a current generator for energizing a coil and establishing a magnetic field in the body of the fluid. A current switching circuit connected between the current generator and the coil, cyclically reverses direction of current flow through the coil to reverse the magnetic field. The switching circuit includes energy storage capacitors for storing energy released by the coil during one half of each cycle and for returning the stored energy to the coil during the next half cycle. A control pulse generator supplies control pulses for cyclically controlling the reversal of current flow and the storage and release of energy in the capacitors.

Electrical system for electromagnetically measuring flow of fluid

A combination of deformable flow probes of negligible lateral dimensions with an electronic circuit capable of providing a prolonged plateau of dB/dt = 0 and of sampling the flow signal at the end of this interval permits electromagnetic measurement of blood flow with a reliable zero base line secured by switching off the magnet. An extracorporeal magnet provides the magnetic field. The flow transducer is introduced into the vascular system percutaneously through a standard angiographic catheter by conventional technique. The idea of the current generator can be described as “principle of interrupted resonance.” The current wave form can be described as a sequence of disconnected bisected sine waves joined at the apices by horizontal current plateaus where di/dt is strictly zero.

A Constant-Field Interrupted Resonance System for Percutaneous Electromagnetic Measurement of Blood Flow

The “interrupted resonance” circuit proved effective in combination with an extracorporeal magnet in permitting electromagnetic blood flow measurements to be made in branch arteries of dogs by application of perivascular cuffs equipped with pick-up electrodes contacting the vessel wall. A four-turn coil, acting as a transformer secondary measures the amplitude of the magnetic field component which is effective in inducing the flow signal and thus permits calibrations for arbitrary orientations of the perivascular cuff relative to the magnetic field of the extracorporeal magnet. Recordings of mean and phasic blood flow in dogs' arteries provide illustrations of effectiveness in pharmacological studies and exhibit the reliability of the non-occlusive zero-flow base line obtained by de-energizing the magnet.

James S. Imai

Papers

Electrical system for electromagnetically measuring flow of fluid

A Constant-Field Interrupted Resonance System for Percutaneous Electromagnetic Measurement of Blood Flow

Extracorporeal magnet perivascular electromagnetic flow meters.

A system for electromagnetic blood flow measurement with extracorporeal sinusodial field magnet.