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.
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