Patent No. 3980076 Method for measuring externally of the human body magnetic susceptibility changes

 

Patent No. 3980076 

Method for measuring externally of the human body magnetic susceptibility changes (Wikswo, Jr, et al., Sep 14, 1976)

Abstract

A method for measuring externally of the human body magnetic susceptibility changes within the body particularly the human heart and interpreting these changes in order to quantify the flow of blood.

Notes:

GOVERNMENT CONTRACT

The Government has rights in this invention pursuant to Grant No. GI 34778 awarded by the National Science Foundation.

 

BACKGROUND OF THE INVENTION

This invention relates generally to a method for measuring blood flow by non-invasive measurement of magnetic susceptibility changes caused either by movement of blood or by variation in the magnetic susceptibility of the blood.

In the treatment of heart diseases it is important to determine the overall effectiveness of the heart as a pump, and to detect and quantify pathological conditions such as ventricular hypertrophy, stenotic or insufficient valves, and intra-cardiac shunts. The rate of cardiac volume change during contraction of the heart (systole) is related to the ventricular ejection velocity and can be used as a measure of myocardial contractility.

The prior art in cardiac output measurements can be divided into three general classes: invasive techniques requiring catheterization; invasive techniques not requiring catheterization; and non-invasive techniques.

In the invasive techniques requiring catheterization, a catheter is passed through an externally accessible vein or artery into the right or left chambers, respectively, of the human heart. Trans-septal venous catheterizations also provide access to the left chambers of the heart. In addition to being used to measure pressure and to withdraw blood samples from the heart, cardiac catheters may be used for measuring cardiac output.

One technique for measuring cardiac output relies upon the fact that the blood has a slight conductivity. As is known, when a conducting fluid moves in a direction perpendicular to the magnetic field an electromotive force is induced perpendicular to both the magnetic field and the direction of flow. This principle has been applied to catheter flowmeters including a self-contained catheter tip device and a catheter applied pair of sensing electrodes used in conjunction with an externally applied magnetic field. Induced voltages give an indication of the blood flow. The induced voltage depends upon the blood vessel diameter and the velocity and it is not possible to determine cardiac flow without knowledge of the vessel diameter.

Invasive techniques not requiring catheterization include fluorescence excitation in which a material is injected into the blood stream and the concentration of the material in the blood is periodically determined at different locations. Superparamagnetic fluid tracers have also been used wherein the patient is injected with a super-paramagnetic fluid and the concentration of the super-paramagnetic tracer is determined magnetically.

Among the non-invasive techniques are pulse echo ultra-sound, Doppler ultra-sound, pulse pressure measurements, ballistocardiograph and impedance plethysmography. In the latter, changes in total electrical thoracic impedance are measured by placing driving and sensing electrodes in relation to the heart-lung-diaphragm system. The measurement depends particularly upon electrode position, the current distribution through the thorax, the frequency of the driving signal, the rheologic properties of blood, and the fluid content of the lungs. Conventional impedance plethysmography has not been widely used for clinical measurements of cardiac output because of the major uncertainties in determining the above factors.

Another method using the same type of equipment is to make impedance measurements with an induction plethysmograph. This instrument operates on the principle that the magnitude of the eddy currents induced in a sample is proportional to the conductivity of the sample. This technique is subject to major limitations and has not yet been successfully applied.

In this method of induction plethysmography a transmitter coil is energized by sinusoidal current which creates a time varying magnetic field in its vicinity. A second coil located a fixed distance from the exciting coil acts as a receiver and the EMF induced in it is measured. An out-of-phase EMF is induced by the conductivity or eddy currents. In addition, an in-phase EMF is also induced in the receiver coil directly from the transmitter coil. This direct coupled or transformer voltage must be reduced to as low a value as possible by minimizing the mutual inductance and capacitance between the transmitter and receiver coils. A phasesensitive detector is utilized to ensure that only the out-of-phase conductivity signal is observed. The magnitude of the transformer component precludes observation of the in-phase susceptibility signal. Thoracic conductivity measurement using such an instrument might also be applied to cardiac output determinations. This technique has two serious limitations when compared to susceptibility measurements in accordance with the present invention. The magnitude of the susceptibility related signal is proportional to the volume of the heart, whereas the magnitude of the conductivity related signal is proportional to the five thirds power of the volume. This implies that for a given stroke volume a larger conductivity signal will be produced by a larger heart. Thus, it would be difficult to use the conductivity signal for absolute measurements of cardiac output. In addition, such a measurement would be affected by the anisotropy of the conductivity of cardiac muscles and the variations of blood conductivity with hemocrit.

OBJECTS AND SUMMARY OF THE INVENTION

It is object of the present invention to provide an improved non-invasive method for measuring blood flow.

It is another object of this invention to provide a method for measuring changes in the magnetic susceptibility of certain regions of the human body caused either by movement of the blood or by variation of the susceptibility of the blood.

It is a further object of the present invention to provide an improved method for measuring the changes in magnetic susceptibility within the human body and particularly in the heart region.

It is another object of the present invention to provide a method for measuring cardiac activity with a higher degree of accuracy than is now possible using non-invasive methods.

The foregoing and other objects of the invention are achieved by a method which applies a strong magnetic field to the region of the body in which the blood activity is to be measured and in which changes in the magnetic field are measured to thereby indicate the changes in susceptibility related to the movement of the blood and provide a measure of the blood flow.

----------------------------------

It seems likely that injected magnetic tracers could be of considerable clinical value and, in the low doses required with our sensitive magnetometers, of low toxicity. The ultimate usefulness of this technique as a clinical tool depends on the result of toxology studies preceding the use of the magnetic fluids in humans. The use of injected magnetic tracers promises, however, to be an important technique in animal studies. The MSF technique would permit determination of blood flow to the extremities, which would be useful in detection of occlusive peripheral vascular disease, and in the study of the effects of pharmacologic agents on peripheral circulation. If the susceptibility of the blood were altered, either invasively or non-invasively, it would be possible to determine regional blood flow and regional oxygenation to both the brain and certain visceral organs.