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Claims  |
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We claim:
1. An apparatus for detecting the location of a magnet associated with a
medical tube within the body of a patient, comprising:
means for sensing a first static magnetic field strength at a first
distance from the magnet and providing a first sensor signal which is a
function of the first static magnetic
means for sensing a second static magnetic field strength at a second
distance from the magnet and providing a second sensor signal which is a
function of the second static magnetic field strength, wherein the second
distance is greater than the first distance, wherein the means for sensing
the first static magnetic field strength and providing the first sensor
signal, and the means for sensing the second static magnetic field
strength and providing the second sensor signal comprise a static magnetic
field strength sensor driver for providing a driver signal, a first static
magnetic field strength sensor for receiving the driver signal and thereby
providing the first sensor signal, and a second static magnetic field
strength sensor for receiving the driver signal and thereby providing the
second sensor signal, wherein the static magnetic field strength sensor
driver comprises an oscillator and output transistors which are
alternately switchable by the oscillator and thereby provide the driver
signal, wherein the first static magnetic field strength sensor comprises
a first flux-gate toroidal sensor which includes a first excitation
winding for receiving the driver signal and a first detection winding for
providing the first sensor signal, and wherein the second static magnetic
field strength sensor comprises a second flux-gate toroidal sensor which
includes a second excitation winding for receiving the driver signal and a
second detection winding for providing the second sensor signal;
means for receiving the first sensor signal and providing a first detection
signal which is a function of the first sensor signal;
means for receiving the second sensor signal and providing a second
detection signal which is a function of the second sensor signal;
means for receiving the first and second detection signals and providing a
differential signal which is a function of the difference between the
first detection signal and the second detection signal; and
means for receiving and indicating a value for the differential signal.
2. An apparatus for detecting the location of a magnet associated with a
medical tube within the body of a patient, comprising:
means for sensing a first static magnet field strength at a first distance
from the magnet and providing a first sensor signal which is a function of
the first static magnetic field strength;
means for sensing a second static magnetic field strength at a second
distance from the magnet and providing a second sensor signal which is a
function of the second static magnetic field strength, wherein the second
distance is greater than the first distance;
means for receiving the first sensor signal and providing a first detection
signal which is a function of the first sensor signal;
means for receiving the second sensor signal and providing a second
detection signal which is a function of the second sensor signal;
means for receiving the first and second detection signals and providing a
differential signal which is a function of the difference between the
first detection signal and the second detection signal, wherein the means
for receiving the first and second detection signals and providing the
differential signal comprises a differential amplifier; and
means for receiving and indicating a value for the differential signal.
3. An apparatus for detecting the location of a magnet associated with a
medical tube within the body of a patient, comprising:
means for sensing a first static magnetic field strength at a first
distance from the magnet and providing a first sensor signal which is a
function of the first static magnetic field strength;
means for sensing a second static magnetic field strength at a second
distance from the magnet and providing a second sensor signal which is a
function of the second static magnetic field strength, wherein the second
distance is greater than the first distance;
means for receiving the first sensor signal and providing a first detection
signal which is a function of the first sensor signal;
means for receiving the second sensor signal and providing a second
detection signal which is a function of the second sensors signal;
means for receiving the first and second detection signals and providing a
differential signal which is a function of the difference between the
first detection signal and the second detection signal; and
means for receiving and indicating a value for the differential signal,
wherein the means for receiving and indicating a value for the
differential signal comprises a magnitude circuit for receiving the
differential signal and providing a magnitude signal which is proportional
to the magnitude of the differential signal, a visual display driver for
receiving the magnitude signal and providing a visual display signal, and
a visual display for receiving and visually indicating the visual display.
4. The apparatus of claim 3, wherein the visual display driver comprises a
light emitting diode bar array driver, and the visual display comprises a
light emitting diode bar array.
5. The apparatus of claim 3, wherein the means for receiving and indicating
a value for the differential signal further comprises a tone generator for
receiving the magnitude signal and providing a tone signal which is a
function of the magnitude signal, and a speaker for receiving and audibly
indicating the tone signal.
6. The apparatus of claim 3, wherein the means for receiving and indicating
a value for the differential signal further comprises a polarity circuit
for receiving the differential signal and providing a polarity signal
which is a function of the polarity of the differential signal, a polarity
display driver for receiving the polarity signal and providing a polarity
display signal, and a polarity display for receiving and visually
indicating the polarity display signal.
7. An apparatus for detecting the location of a magnet associated with a
medical tube within the body of a patient, comprising:
means for sensing a first static magnetic field strength at a first
distance from the magnet and providing a first sensor signal which is a
function of the first static magnetic field strength;
means for sensing a second static magnetic field strength at a second
distance from the magnet and providing a second sensor signal which is a
function of the second static magnetic field strength, wherein the second
distance is greater than the first distance;
means for receiving the first sensor signal and providing a first detection
signal which is a function of the first sensor signal;
means for receiving the second sensor signal and providing a second
detection signal which is a function of the second sensor signal;
means for receiving the first and second detection signals and providing a
differential signal which is a function of the difference between the
first detection signal and the second detection signal;
means for receiving and indicating a value for the differential signal; and
means for automatically controlling, monitoring and calibrating the means
for sensing the first static magnetic field strength and providing the
first sensor signal, the means for sensing the second static magnetic
field strength and providing the second sensor signal, the means for
receiving the first sensor signal and providing the first detection
signal, the means for receiving the second sensor signal and providing the
second detection signal, the and means for receiving and indicating a
value for the differential signal.
8. The apparatus of claim 7, wherein the automatic controlling, monitoring,
and calibrating means comprises a microprocessor.
9. A method of detecting the location of a magnet associated with a medical
tube within the body of a patient, comprising:
sensing a first static magnetic field strength at a first distance from the
magnet;
sensing a second static magnetic field strength at a second distance from
the magnet which is greater than the first distance;
providing a first sensor signal which is a function of the first static
magnetic field strength:
providing a second sensor signal which is a function of the second static
magnetic field strength;
receiving the first and second sensor signals and providing a differential
signal which is a function of the difference between the first static
magnetic field strength and the second static magnetic field strength;
receiving and indicating a value for the differential signal; and
determining the location of the medical tube by varying the first and
second distances until the greatest value for the differential signal is
indicated.
10. A method of verifying the location of a magnet associated with a
medical tube within the body of a patient, comprising:
sensing a first static magnetic field strength at a first distance from the
magnet;
sensing a second static magnetic field strength at a second distance from
the magnet which is greater than the first distance;
providing a first sensor signal which is a function of the first static
magnetic field strength;
providing a second sensor signal which is a function of the second static
magnetic field strength;
receiving the first and second sensor signals and providing a differential
signal which is a function of the difference between the first static
magnetic field strength and the second static magnetic field strength;
receiving and indicating the polarity of the differential signal; and
manipulating the magnet until the indicated polarity of the differential
signal changes.
11. The method of claim 10, wherein manipulating the magnet is accomplished
by rotation thereof.
12. An apparatus for detecting the location of a magnet associated with a
medical tube within the body of a patient, comprising:
a static magnetic field strength sensor driver for providing a driver
signal;
a first static field strength sensor coupled to the static magnetic field
strength sensor driver for receiving the driver signal and thereby
providing a first sensor signal which is a function of a first static
magnetic field strength at a first distance from the magnet;
a second static magnetic field strength sensor coupled to static magnetic
field strength sensor driver for receiving the driver signal and thereby
providing a second sensor signal which is a function of a second static
magnetic field strength at a second distance from the magnet, wherein the
second distance is greater than the first distance;
a first amplifier coupled to the first static magnetic field strength
sensor for receiving the first sensor signal and providing a first
amplified signal which is proportional to the first sensor signal;
a first integrator coupled to the first amplifier for receiving the first
amplified signal and providing a first detection signal which is a
function of the first sensor signal;
a second amplifier coupled to the second static magnetic field strength
sensor for receiving the second sensor signal and providing a second
amplified signal which is proportional to the second signal;
a second integrator coupled to the second amplifier for receiving the
second amplified signal and providing a second detection signal which is a
function of the second sensor signal;
a differential amplifier coupled to the first and second integrators for
receiving the first and second detection signal and providing a
differential signal which is a function of the difference between the
first detection signal and the second detection signal;
a magnitude circuit coupled to the differential amplifier for receiving the
differential signal and providing a magnitude signal which is proportional
to the magnitude of the differential signal;
a visual display driver coupled to the magnitude circuit for receiving the
magnitude signal and providing a visual display signal; and
a visual display coupled to the visual display driver for receiving and
visually indicating the visual display signal.
13. The apparatus of claim 12, wherein the static magnetic field strength
sensor driver comprises an oscillator and output transistors which are
alternately switchable by the oscillator and thereby provide the driver
signal, wherein the first static magnetic field strength sensor comprises
a first flux-gate toroidal sensor which includes a first excitation
winding for receiving the driver signal and a first detection winding for
providing the first sensor signal, and wherein the second static magnetic
field strength sensor comprises a second flux-gate toroidal sensor which
includes a second excitation winding for receiving the driver signal and a
second detection winding for providing the second sensor signal.
14. The apparatus of claim 12, wherein the visual display driver comprises
a light emitting diode bar array driver, and the visual display comprises
a light emitting diode bar array.
15. The apparatus of claim 12, further comprising a tone generator coupled
to the magnitude circuit for receiving the magnitude signal and providing
a tone signal which is a function of the magnitude signal, and a speaker
coupled to the tone generator for receiving and audibly indicating the
tone signal.
16. The apparatus of claim 12, further comprising a polarity circuit
coupled to the differential amplifier for receiving the differential
signal and providing a polarity signal which is a function of the polarity
of the differential signal, a polarity display driver coupled to the
polarity circuit for receiving the polarity signal and providing a
polarity display signal and a polarity display coupled to the polarity
display driver for receiving and visually indicating the polarity display
signal.
17. The apparatus of claim 12, further comprising a microprocessor coupled
to the static magnetic field strength sensor driver, the first amplifier,
the second amplifier, the differential amplifier, and the visual display
driver for automatically controlling, monitoring and calibrating the
static magnetic field strength sensor driver, the first amplifier, the
second amplifier, the differential amplifier, and the visual display
driver. |
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Claims |
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Description |
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TECHNICAL FIELD
This invention is generally directed to an apparatus and method for
detecting the location of a medical tube within the body of a patient and,
more specifically, to detecting the location of a medical tube using a
detection apparatus which senses a static magnetic field strength gradient
generated by a magnet associated with the medical tube.
BACKGROUND OF THE INVENTION
There are many instances in clinical medicine where detecting the location
of a medical tube within a patient is important. For example, when
positioning feeding tubes through the mouth or nose of a patient, it is
essential that the end of the feeding tube pass into the patient's
stomach, and that it does not "curl up" and remain in the esophagus. If
the end of the feeding tube is not properly positioned within the stomach,
aspiration of the feeding solution into the patient's lungs may occur. In
addition to feeding tubes, a variety of other medical tubes require
accurate positioning within a patient's body, including dilating tubes to
widen an esophageal stricture, tubes for measuring pressure waves in the
stomach and esophagus of a patient who is suspected of having esophageal
motor disorders, Sengstaken-Blakemore tubes in the stomach and esophagus
of a patient to control bleeding from varicose veins in the esophagus,
colonic decompression tubes in the colon of a patient to assist in
relieving distention of the colon by gas, urologic tubes in the bladder,
ureter or kidney of a patient, and vascular tubes in the heart or
pulmonary arteries of a patient.
Currently, the location of a medical tube within the body of a patient is
routinely detected by the use of imaging equipment, such as a chest or
abdominal X-ray. However, such a procedure requires transportation of the
patient to an X-ray facility or, conversely, transportation of the X-ray
equipment to the patient. This is both inconvenient and costly to the
patient, and is particularly stressful in those instances where the
patient repeatedly and inadvertently removes a medical tube, such as a
feeding tube, thus requiring repeated reinsertion and X-rays.
Prior attempts at detecting the location of medical tubes within a patient
have met with only limited success. For example, in U.S. Pat. No.
5,099,845 to Besz et al., a transmitter is located within a catheter, and
an external receiver, tuned to the frequency of the transmitter, is used
to detect the location of the catheter within the patient. This approach,
however, requires either an external or internal power source to drive the
transmitter. An external power source adds significant risk associated
with shock or electrocution, and requires that electrical connections be
made prior to positioning of the catheter within the patient. An internal
power source, such as a battery, must be relatively small and can only
provide power to the transmitter for a limited time. This precludes
long-term detection of the catheter's location, and poses additional risks
associated with placing a battery internally in a patient, such as the
risk of battery leakage or rupture. In addition, the transmitter is
relatively complex, and requires an active electronic circuit (either
internal or external to the catheter), as well as the various wires and
connections necessary for its proper function. Lastly, the signal produced
by the transmitter is attenuated differently by different body tissues and
bone. This attenuation requires adjustments in the transmitter's signal
strength and frequency depending on the location of the catheter within
the patient's body.
A further attempt at detecting the location of medical tubes within a
patient is disclosed in U.S. Pat. No. No. 4,809,713 to Grayzel. There, an
electrical cardiac-pacing catheter is held in place against the inner
heart wall of a patient by the attraction between a small magnet located
in the tip of the pacing catheter and a large magnet located on (e.g.,
sewn into) the patient's chest wall. An indexed, gimbaled,
three-dimensional compass is used to determine the best location for the
large magnet. The compass' operation relies upon the torque generated by
the magnetic forces between the small magnet and the magnetized compass
pointer in order to point the compass towards the small magnet. However,
this compass will simultaneously try to orient itself to the earth's
ambient magnetic field. Because of this, the forces between the small
magnet and the magnetized compass pointer at distances greater than
several centimeters are not strong enough to accurately orient the compass
towards the small magnet. Furthermore, although the compass aids
positioning of the large magnet, positioning of the small magnet, and
hence the pacing catheter, still requires the use of imaging equipment,
such as X-ray or ultrasound.
For the foregoing reasons, there is a need in the art for an apparatus and
method for detecting the location of a medical tube within the body of a
patient which avoids the problems inherent in existing techniques. The
apparatus and method should provide for the detection of the medical tube
at distances ranging from several centimeters to several decimeters,
should not require the medical tube to have an internal or external power
source, and should obviate the need to independently verify positioning of
the medical tube with imaging equipment.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide an
apparatus and method for detecting the location of a medical tube within
the body of an animal patient (including humans) without the aid of
imaging equipment, particularly X-ray. It is a further object to detect
the location of the medical tube without relying upon torque generated by
the magnetic forces between the medical tube and the detection apparatus.
Yet, a further object is to detect the location of the medical tube while
dynamically nulling sensing of the earth's ambient magnetic field, and to
thereby allow detection distances suitable for locating a wide variety of
medical tubes at any location within the body of the patient.
The present invention satisfies these objectives by providing an apparatus
and method for detecting the location of a magnet associated with a
medical tube within the body of a patient. In one aspect of this
invention, the apparatus of this invention comprises a first and second
means for sensing a first and second static magnetic field strength,
respectively, at first and second distances from the magnet, respectively,
where the second distance is greater than the first; means for providing a
first detection signal, which is a function of the first static magnetic
field strength; means for providing a second detection signal, which is a
function of the second static magnetic field strength; means for providing
a differential signal, which is a function of the difference between the
first and second detection signals; and means for indicating a value for
the differential signal.
The first and second sensing means also provide, respectively, a first
sensor signal, which is a function of the first static magnetic field
strength, and a second sensor signal, which is a function of the second
static magnetic field strength. The means for providing the first
detection signal receives the first sensor signal, and the means for
providing the second detection signal receives the second sensor signal.
Finally, the means for providing the differential signal receives the
first and second detection signals, and the means for indicating the
differential signal's value receives the differential signal.
By sensing the static magnetic field strength of the magnet associated with
the medical tube, the present invention obviates the need for imaging
equipment, such as X-ray, to verify positioning of the medical tube. Also,
by sensing the magnet's field strength at two different distances (i.e.,
the first and second distances) from the magnet between which the magnet's
field strength will have a gradient and the earth's field strength will
not, and by indicating the gradient to the user, the present invention
dynamically nulls sensing of the earth's ambient magnetic field. This
nulling allows the magnet to be sensed at distances ranging from several
centimeters to several decimeters, which makes the detection apparatus
suitable for locating the medical tube at any location within the
patient's body.
In one embodiment of this invention, the first and second sensing means
comprise a static magnetic field strength sensor driver, and first and
second static magnetic field strength sensors. The driver provides a
driver signal which causes the sensors to provide the first and second
sensor signals. In a preferred embodiment, the driver comprises an
oscillator and output transistors, wherein the output transistors are
alternately switched by the oscillator and are thereby caused to provide
the driver signal. The sensors each comprise a flux-gate toroidal sensor,
which includes an excitation winding which receives a driver signal, and a
detection winding which provides the respective sensor signal. By
providing a driver signal which causes the sensors to provide the first
and second sensor signals, the present invention does not need to rely
upon magnetic forces between the magnet and the apparatus for detecting
the location of the medical tube.
In another embodiment, the detection apparatus further comprises a means
for automatically controlling, monitoring, and calibrating (a) the first
and second means for sensing the first and second static magnetic field
strengths; (b) the means for providing the first detection signal; (c) the
means for providing the second detection signal; (d) the means for
providing the differential signal; and (e) the means for indicating the
differential signal's value. In a preferred embodiment, the automatic
controlling, monitoring, and calibrating means is a microprocessor.
In another aspect of this invention, the apparatus of this invention
comprises the static magnetic field strength sensor driver, the first and
second static magnetic field strength sensors, first and second
amplifiers, first and second integrators, a differential amplifier, a
magnitude circuit, a visual display driver, and a visual display.
The first amplifier receives the first sensor signal and provides a first
amplified signal which is proportional to the first sensor signal.
Similarly, the second amplifier receives the second sensor signal and
provides a second amplified signal which is proportional to the first
sensor signal.
The first and second integrators receive the first and second amplified
signals, respectively, and provide the first and second detection signals,
respectively. The differential amplifier receives the first and second
detection signals and provides the differential signal.
Further, the magnitude circuit receives the differential signal and
provides a magnitude signal which is proportional to the magnitude of the
differential signal. The visual display driver receives the magnitude
signal and provides a visual display signal. The visual display receives
and visually indicates the visual display signal.
In a preferred embodiment, the visual display driver comprises a light
emitting diode bar array driver, and the visual display comprises a light
emitting diode bar array.
In another preferred embodiment, the apparatus further comprises a tone
generator for receiving the magnitude signal and providing a tone signal
which is a function of the magnitude signal, and a speaker for receiving
and audibly indicating the tone signal.
In still another preferred embodiment, the apparatus further comprises a
polarity circuit for receiving the differential signal and providing a
polarity signal which is a function of the polarity of the differential
signal, a polarity display driver for receiving the polarity signal and
providing a polarity display signal, and a polarity display for receiving
and visually indicating the polarity display signal.
In still another preferred embodiment, the apparatus further comprises the
microprocessor for automatically controlling, monitoring and calibrating
the static magnetic field strength sensor driver, the first amplifier, the
second amplifier, the differential amplifier and the visual display
driver.
In still another aspect of this invention, a method for detecting the
location of a magnet associated with a medical tube within the body of a
patient comprises the following steps: sensing the first and second static
magnetic field strengths at the first and second distances; providing the
first and second sensor signals; receiving the first and second sensor
signals and providing the differential signal; receiving and indicating
the value of the differential signal; and determining the location of the
medical tube by varying the first and second distances until the greatest
value is indicated.
In still another aspect of this invention, a method of verifying the
location of a magnet associated with the end of a medical tube within the
body of a patient comprises the following steps: sensing the first and
second static magnetic field strengths at the first and second distances;
providing the first and second sensor signals; receiving the first and
second sensor signals and providing the differential signal; receiving and
indicating the polarity of the differential signal; and manipulating the
magnet until the indicated polarity changes.
These and other features of the present invention will be better understood
with reference to the following detailed description, appended claims and
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1(a) and 1(b) are block diagrams illustrating the structure and
operation of a representative detection apparatus of this invention.
FIG. 2 is a block diagram illustrating an embodiment of the first and
second sensor, as well as the sensor driver.
FIG. 3 illustrates an embodiment of a detection apparatus of this
invention.
FIG. 4 illustrates the location of a magnet fixed to the end of a medical
tube positioned within the body of a human patient using the detection
apparatus of FIG. 3.
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides an apparatus and method for detecting the
location of a medical tube within the body of a patient. As used herein,
the term "medical tube" means any type of tube or device which may be
inserted into a patient's body, including (but not limited to) catheters,
guide wires, and medical instruments. For example, catheters include such
items as feeding tubes, urinary catheters, guide wires and dilating
catheters, as well as nasogastric tubes, endotracheal tubes, stomach pump
tubes, wound drain tubes, rectal tubes, vascular tubes,
Sengstaken-Blakemore tubes, colonic decompression tubes, pH catheters,
motility catheters, and urological tubes. Guide wires are often used to
guide or place dilators and other medical tubes. Medical instruments
include endoscopes and colonoscopes. In short, the location of any foreign
object within a patient's body is a suitable device for detection by the
present invention, and is encompassed within the term "medical tube".
The present invention detects the location of the medical tube by sensing
the static magnetic field strength gradient produced by a permanent magnet
associated with the medical tube. As used herein, the term "associated
with" means permanently fixed, removably attached, or in close proximity
to, the medical tube. In one embodiment, such as a feeding tube, the
magnet is associated with the end of the medical tube. In another
embodiment, such as a Sengstaken-Blakemore tube, the magnet is associated
with the medical tube at a location above the gastric balloon. Preferably,
the magnet is a small, cylindrical, rotatably attached, rare-earth magnet.
Suitable magnets include rare earth magnets such as samarium cobalt and
neodymium iron boron, both of which generate high field strengths per unit
volume. While magnets which generate a high field strength for their size
are preferred, weaker magnets such as Alnico or ceramic may also be
utilized.
Since the magnet of this invention is permanent, it requires no power
source. Accordingly, the magnet maintains its magnetic field indefinitely,
which allows long-Germ positioning and detection of medical tubes without
the disadvantages associated with an internal or external power source. In
particular, by avoiding the use of a power source, the undesirable
electrical connections necessary for the use of a power source are
avoided. Thus, there is no risk of shock to (or possible electrocution of)
the patient. Furthermore, the magnet's static magnetic field passes
unattenuated through body tissue and bone. This property allows the use of
the present invention to detect the medical tube at any location within
the patient's body.
The magnet, and hence the medical tube, is detected using a detection
apparatus which contains at least two static magnetic field strength
sensors configured geometrically to null detection of ambient, homogeneous
magnetic fields (e.g., the earth's field), while still detecting the
magnetic field strength gradient produced by the magnet. The detection
apparatus is an active, electronic instrument, and can detect the
relatively small magnetic field strength gradient produced by the magnet
at distances ranging from several centimeters to several decimeters, and
preferably from about 2 centimeters to about 3 decimeters. It also
indicates the value of the gradient, thus allowing the user to accurately
determine the location of the magnet, and hence the medical tube. In a
preferred embodiment, the detection apparatus indicates the value of the
gradient as both a magnitude and a polarity. By manipulating the magnet
until the indicated polarity changes, detection of the location of the
medical tube can be verified. Such manipulation of the magnet can be
accomplished either by means of an attached guide wire, or by rotating the
medical tube itself.
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