U.S. patent application number 09/741774 was filed with the patent office on 2001-08-09 for ferromagnetic foreign body detection using magnetics.
Invention is credited to Avrin, William F., Czipott, Peter V., Kumar, Sankaran, Massengill, R. Kemp.
Application Number | 20010012915 09/741774 |
Document ID | / |
Family ID | 24690230 |
Filed Date | 2001-08-09 |
United States Patent
Application |
20010012915 |
Kind Code |
A1 |
Avrin, William F. ; et
al. |
August 9, 2001 |
Ferromagnetic foreign body detection using magnetics
Abstract
A probe instrument using room-temperature sensor(s) that can
measure magnetic susceptibilities variations. The instrument
includes room temperature magnetic sensors, and detects the sample,
that is the tissue response to an applied field coil. The sensors
that can be used include magnetoresistive, fluxgate and
magnetoinductive sensors. The applied field coil dimensions are
chosen so that the applied field is optimized for maximum response
from the item of interest while minimizing the effects due to the
overlying tissue and at the same time not unduly increasing the
sensitivity of the instrument to adjacent organs. To minimize noise
introduced in the sensor due to fluctuations in the applied field,
the applied field is canceled at the position of the sensor. To
overcome variations in the sensor output due to fluctuations in the
applied field, change in the ambient temperature and mechanical
relaxation of the instrument, the detector assembly is oscillated
while the examined patient remains stationary.
Inventors: |
Avrin, William F.; (San
Diego, CA) ; Czipott, Peter V.; (San Diego, CA)
; Massengill, R. Kemp; (Leucadia, CA) ; Kumar,
Sankaran; (San Marcos, CA) |
Correspondence
Address: |
Gerald W. Spinks
P.O. Box 2330
Port Orchard
WA
98366
US
|
Family ID: |
24690230 |
Appl. No.: |
09/741774 |
Filed: |
December 15, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09741774 |
Dec 15, 2000 |
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09135890 |
Aug 18, 1998 |
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6208884 |
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09135890 |
Aug 18, 1998 |
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08670393 |
Jun 25, 1996 |
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5842986 |
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Current U.S.
Class: |
600/424 |
Current CPC
Class: |
G01R 33/16 20130101;
A61B 5/055 20130101; A61B 5/242 20210101; G01V 3/08 20130101; A61B
5/416 20130101; A61B 5/05 20130101; A61B 5/4244 20130101 |
Class at
Publication: |
600/424 |
International
Class: |
A61B 005/05 |
Goverment Interests
[0002] The U.S. Government has a license on this invention under
the terms of National Institutes of Health Contract No. N
43-DK-7-2250.
Claims
We claim:
1. A method for noninvasive screening of a human body for the
presence of a ferromagnetic foreign body, said method comprising:
providing an instrument which includes at least one magnetic
sensor, an applied field coil and a current source connected to
said applied field coil and means for processing sensed signals
from said at least one magnetic sensor, wherein said instrument
functionally operates at room temperature and minimizes noise due
to temperature fluctuations at said magnetic sensor; positioning
said instrument external to a patient in proximity to a region of
interest; supplying said applied field coil with current, thereby
applying a magnetic field to said region of interest; sensing a
response from said region of interest with said instrument; and
outputting data corresponding to the magnetic susceptibility of a
ferromagnetic foreign body within said region of interest.
2. The method recited in claim 1, wherein said supplying of current
to said applied field coil comprises supplying alternating current
to said applied field coil.
3. The method recited in claim 2, wherein said supplying of
alternating current comprises supplying said alternating current at
a frequency between about 25 hertz and about 2000 hertz.
4. The method recited in claim 1, further comprising displacing
said magnetic sensor and said applied field coil simultaneously,
thereby compensating for noise in said sensed response.
5. The method recited in claim 4, wherein said displacing of said
magnetic sensor and said applied field coil has an amplitude of
between about one inch and about six inches.
6. The method recited in claim 4, wherein said displacing of said
magnetic sensor and said applied field coil has a frequency of
between about 0.5 hertz and about 10.0 hertz.
7. The method recited in claim 1, wherein said outputting of data
corresponding to magnetic susceptibility comprises outputting of
data corresponding to the size of a ferromagnetic foreign body
within said region of interest.
8. The method recited in claim 1, wherein said outputting of data
corresponding to magnetic susceptibility comprises outputting of
data corresponding to the location of a ferromagnetic foreign body
within said region of interest.
9. An apparatus for noninvasive screening of a human body for the
presence of a ferromagnetic foreign body, said apparatus
comprising: a detector assembly that includes: at least one
magnetic sensor and an applied field coil for generating a magnetic
field; and a current signal generating source which connects to
said applied field coil; a means for processing signals from said
at least one magnetic sensor of observed magnetic susceptibility of
a ferromagnetic foreign body within a region of interest in a human
body; and a means for outputting data corresponding to said
magnetic susceptibility of said ferromagnetic foreign body; wherein
said detector assembly functionally operates at room temperature
and minimizes noise due to temperature fluctuations at said
magnetic sensor.
10. The apparatus recited in claim 9, wherein said current signal
generating source comprises an alternating current generating
source.
11. The apparatus recited in claim 10, wherein said alternating
current signal generating source provides current at a frequency
between about 25 hertz and about 2000 hertz.
12. The apparatus recited in claim 11, wherein said alternating
current signal generating source avoids power line frequencies and
harmonics thereof.
13. The apparatus recited in claim 9, wherein said means for
outputting data is adapted to output magnetic susceptibility data
corresponding to the size of a ferromagnetic foreign body within
said region of interest.
14. The apparatus recited in claim 9, wherein said means for
outputting data is adapted to output magnetic susceptibility data
corresponding to the location of a ferromagnetic foreign body
within said region of interest.
15. The apparatus recited in claim 9, wherein: said applied field
coil is adapted to create a first zone of a finite magnetic field
within said region of interest, and to create a second zone of
substantially zero magnetic field outside said region of interest;
and said at least one magnetic sensor is positioned within said
second zone of substantially zero magnetic field.
16. The apparatus recited in claim 9, wherein said at least one
magnetic sensor is mounted in a central region of said magnetic
field.
17. The apparatus recited in claim 9, wherein said applied field
coil is attached to a planar substrate.
18. The apparatus recited in claim 9, wherein said applied field
coil comprises: two parallel flat coils; and a connecting circuit
between said two parallel flat coils, said connecting circuit being
adapted to cause current to flow in identical directions at
corresponding locations in said two parallel flat coils, thereby
simulating parallel uniform sheets of current.
19. The apparatus recited in claim 9, wherein said applied field
coil comprises: at least two concentric coils; and a connecting
circuit between said at least two concentric coils, said connecting
circuit being adapted to cause current to flow in opposite
directions in said at least two concentric coils, thereby canceling
said magnetic field at a central region of said at least two
concentric coils.
20. The apparatus recited in claim 19, the largest of said at least
two concentric coils has a diameter between about 15 centimeters
and about 50 centimeters.
21. The apparatus recited in claim 19, further comprising at least
three of said concentric coils, wherein: at least two of the
outermost of said concentric coils are adapted to be alternatively
connected to said current source; and said signal processing means
is adapted to differentiate between magnetic susceptibilities deep
within said region of interest and magnetic susceptibilities near
the surface of said region of interest.
22. The apparatus recited in claim 9, wherein said detector
assembly comprises multiple stacked applied field coils.
23. The apparatus recited in claim 9, wherein said at least one
magnetic sensor is a fluxgate sensor.
24. The apparatus recited in claim 9, wherein said at least one
magnetic sensor is a magnetoinductive sensor.
25. The apparatus recited in claim 9, wherein said at least one
magnetic sensor is a magnetoresistive sensor.
26. The apparatus recited in claim 25, wherein said
magnetoresistive sensor is part of a Wheatstone bridge sensing
circuit.
27. The apparatus recited in claim 26, further comprising: magnetic
sensor compensating electronics; and a feedback coil disposed about
said magnetoresistive sensor, said feedback coil being adapted to
lock an optimum operating point by applying a compensating
electrical current from said compensating electronics to said
feedback coil, thereby maintaining a constant measurement
sensitivity.
28. The apparatus recited in claim 9, wherein said detector
assembly further comprises an oscillating means for oscillating
said detector assembly.
29. The apparatus recited in claim 28, wherein: said detector
assembly is housed in a structure adapted to position said detector
assembly in proximity to said region of interest; and said
oscillating means comprises a motor with attached drive members,
said drive members being adapted to move said detector
assembly.
30. A magnetic susceptibility detector device comprising: an
applied field coil configured to connect to a current source, said
applied field coil being adapted to create a first zone of a finite
magnetic field, within a region of interest of a human body, and a
second zone of substantially zero magnetic field, outside said
region of interest, said applied field coil having at least two
concentric current carrying coils of conductor material; and a
sensing device including at least one magnetic sensor, said at
least one magnetic sensor being positioned within said second zone
of substantially zero magnetic field, said at least one magnetic
sensor being adapted to output data corresponding to magnetic
susceptibility of a ferromagnetic foreign body within said region
of interest.
31. The apparatus recited in claim 30, wherein said at least one
magnetic sensor is a fluxgate sensor.
32. The apparatus recited in claim 30, wherein said at least one
magnetic sensor is a magnetoinductive sensor.
33. The apparatus recited in claim 30, wherein said at least one
magnetic sensor is a magnetoresistive sensor.
34. An apparatus for noninvasive screening of a human body for the
presence of a ferromagnetic foreign body, said apparatus
comprising: a detector assembly that includes: at least one
magnetic sensor; and an applied field coil for generating a
magnetic field, said applied field coil being adapted to create a
first zone of a finite magnetic field within a region of interest
in the body, and a second zone of substantially zero magnetic field
outside said region of interest, said at least one magnetic sensor
being positioned within said second zone of substantially zero
magnetic field; a current signal generating source which connects
to said applied field coil; a means for processing signals from
said at least one magnetic sensor of observed magnetic
susceptibility of a ferromagnetic foreign body within said region
of interest; and a means for oscillating said detector assembly,
said detector assembly being attached to said means for oscillating
said detector assembly; wherein said detector assembly functionally
operates at room temperature and minimizes noise due to temperature
fluctuations at said magnetic sensor.
35. The apparatus recited in claim 34, wherein said current signal
generating source comprises an alternating current generating
source.
36. The apparatus recited in claim 34, wherein said means for
processing signals is adapted to output magnetic susceptibility
data corresponding to the size of a ferromagnetic foreign body
within said region of interest.
37. The apparatus recited in claim 34, wherein said means for
processing signals is adapted to output magnetic susceptibility
data corresponding to the location of a ferromagnetic foreign body
within said region of interest.
38. The apparatus recited in claim 34, wherein said applied field
coil comprises: two parallel flat coils; and a connecting circuit
between said two parallel flat coils, said connecting circuit being
adapted to cause current to flow in identical directions at
corresponding locations in said two parallel flat coils, thereby
simulating parallel uniform sheets of current.
39. The apparatus recited in claim 34, wherein said applied field
coil comprises: at least two concentric coils; and a connecting
circuit between said at least two concentric coils, said connecting
circuit being adapted to cause current to flow in opposite
directions in said at least two concentric coils, thereby canceling
said magnetic field at a central region of said at least two
concentric coils.
40. The apparatus recited in claim 34, wherein said detector
assembly comprises multiple stacked applied field coils.
41. The apparatus recited in claim 34, wherein said at least one
magnetic sensor is a fluxgate sensor.
42. The apparatus recited in claim 34, wherein said at least one
magnetic sensor is a magnetoinductive sensor.
43. The apparatus recited in claim 34, wherein said at least one
magnetic sensor is a magnetoresistive sensor.
44. The apparatus recited in claim 43, wherein said
magnetoresistive sensor is part of a Wheatstone bridge sensing
circuit.
45. The apparatus recited in claim 44, further comprising: magnetic
sensor compensating electronics; and a feedback coil disposed about
said magnetoresistive sensor, said feedback coil being adapted to
lock an optimum operating point by applying a compensating
electrical current from said compensating electronics to said
feedback coil, thereby maintaining a constant measurement
sensitivity.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This is a continuation patent application of U.S. patent
application Ser. No. 09/135,890, filed on Aug. 18, 1998, and
entitled "Noninvasive Room Temperature Instrument to Measure
Magnetic Susceptibility Variations in Body Tissue", which is a
continuation-in-part application of U.S. patent application Ser.
No. 08/670,393, filed on Jun. 25, 1996, and entitled "Ferromagnetic
Foreign Body Screening Method and Apparatus", now U.S. Pat. No.
5,842,986, the disclosure of which is incorporated herein by
reference.
BACKGROUND OF THE INVENTION
[0003] 1. Field of the Invention
[0004] This invention relates generally to an instrument using
room-temperature sensors that measure magnetic susceptibility
variations in the body of a patient. In particular, the instrument
can noninvasively monitor ferromagnetic foreign bodies that may
become lodged in a patient.
[0005] 2. Background Art
[0006] There is a need for an accurate, noninvasive method to
detect the presence of ferromagnetic foreign bodies in a patient
who is being considered for magnetic resonance imaging.
[0007] As a matter of interest, biomagnetic susceptometry is a
diagnostic procedure that involves noninvasive, radiation-free,
direct, and accurate, measurement of the magnetic susceptibility of
organs and tissue within a human or animal body. For instance,
biomagnetic susceptometry can be used to measure human iron stores
contained in the liver, see Harris, J. W., et al. (1978),
Assessment of human iron stores by magnetic susceptibility
measurements, Clin. Res. 26, 540A.; Brittenham, G. M., et al.
(1993), Hepatic iron stores and plasma ferritin concentration in
patients with sickle cell anemia and thalassemia major, Amer. J.
Hematology 42, 85; Brittenham, G. M., et al. (1982), Magnetic
susceptibility of human iron stores, New England J. Med., 307, 167
1.; Fischer, R., et al. (1992), Liver iron quantification in the
diagnosis and therapy control of iron overload patients,
Biomagnetism: Clinical. Aspects, M. Hoke, et al., eds., Elsevier,
Amsterdam, p. 585., 1992; Fischer, R., et al. (1989), in Advances
In Biomagnetism, S. J. Williamson, et al., eds., Plenum, New York,
p. 501. Paulson. D. N., et al. (1991), Biomagnetic susceptometer
with SQUID instrumentation, IEEE Trans. Magnetics 27, 3249.; and
Nielsen, P., et al. (1995), Liver iron stores in patients with
secondary hemosideroses under iron chelation therapy with
deferoxamine or deferiprone, Br. J. Hematol. 91, 827.
[0008] Unfortunately, instruments based on Superconducting Quantum
Interference Devices (SQUIDs), are complex and expensive. They also
use liquid helium, leading to significant operating costs and
supply problems. Only a few such devices are in use worldwide
presently due to their complexity and expense.
[0009] SQUIDs based on the recently developed High-Temperature
Superconductors (HTS) could, in principle, reduce the cost of
magnetic suceptometry. HTS SQUIDs, which can operate at
liquid-nitrogen temperatures, would reduce operating costs, and
some of the equipment costs, compared to SQUID devices operating at
liquid helium temperatures. However, even at liquid-nitrogen
temperatures, the operating costs would be higher than those of
ordinary instruments operating at room temperature. Moreover,
HTS-SQUIDs are expensive to construct and use, because of the
difficulty and low yield of the fabrication process. The
difficulties, and the costs, are compounded because these devices
are vulnerable to moisture, thermal cycling, and static electrical
discharge. HTS-SQUIDs also require the same expensive electronics
as conventional SQUIDs.
[0010] The instant invention obviates the need for cryogenically
cooled SQUIDs by providing operational use at room temperature,
making for much less expensive fabrication and use. The invention
allows, generally, for measurements of variations of magnetic
susceptibility in a patient and, in particular, for an accurate and
inexpensive way of detecting areas of increased magnetic
susceptibility in patients. In addition, certain improvements
introduced in this invention are applicable to all types of
magnetic susceptibility measurements.
BRIEF SUMMARY OF THE INVENTION
[0011] Broadly speaking, this invention provides a practical method
and apparatus for measuring variations of magnetic susceptibilities
in a patient, and, in particular, preferably localized areas of
increased magnetic susceptibility. The probing instrument's distal
end assembly includes a room temperature functioning magnetic
sensor that can detect the characteristic magnetic response from
tissue to a magnetic field supplied by an applied-field coil that
is also part of the instrument's distal end assembly. The applied
field coil can be an alternating current (AC) coil. The magnetic
susceptibility measurements have sufficient resolution to monitor
small variations in magnetic susceptibility within the patient,
when the instrument is placed external to the patient.
[0012] The magnetic sensor can be, but is not necessarily limited
to, a magnetoresistive sensor (including giant magnetoresistive and
spin-dependent tunneling sensors), a fluxgate magnetometer, or a
magneto-inductive sensor. The applied field coil dimensions are
such that an applied field is optimized for maximum response from
localized areas of interest in the body. In particular, the
instrument of the present invention is preferably designed for
detecting the presence of ferromagnetic foreign bodies (FFBs) in a
patient. For this application, the applied field coil dimensions
are optimized to maximize the magnetic susceptibility response from
the item of interest and minimize effects caused by the overlying
tissue, while not unduly increasing the sensitivity of the probe
instrument due to an organ being in close proximity to the item of
interest. To minimize noise introduced in the magnetic sensor due
to fluctuations in the applied field, the applied field is canceled
at the position of the sensor. Both the real and imaginary parts of
the applied field are canceled. To overcome variations in the
sensor output caused by changes in ambient temperature and
mechanical relaxation of the instrument, the sensor-sample distance
is modulated by oscillating the detector assembly. In contrast with
conventional biomagnetic measurement instruments that use SQUID
sensors, where a patient is moved relative to the instrument, the
proposed invention's magnetic sensor is moved relative to the
patient. The instrument's detector assembly has an applied field
coil fabricated on a printed circuit (PC) board that is attached to
a solid nonmetallic support base which in turn attaches to an
oscillatory member which displaces the detector assembly when used
for examining a patient.
[0013] The probe instrument's distal end detector assembly has a
geometrically designed applied field coil using either multiple
parallel-sheet coils or a substantially coplanar applied field coil
of concentric design. The magnetic sensor is preferably a
magnetoresistive (MR) sensor. When an MR sensor is used, a feedback
coil is mounted on the sensor, which "locks" the sensor at its
optimum operating point by applying a compensating field to cancel
changes in the ambient field, thus maintaining a constant
sensitivity of the detector assembly.
[0014] The probing instrument's magnetic sensor control
electronics, a motor/crank rod arrangement for oscillatory movement
of the instrument's distal end detector assembly, an applied field
source signal generator, a lock-in amplifier, an audio amplifier,
and an FFT spectrum analyzer or equivalent computer device for
signal analysis can all be incorporated in a single medical
instrument housing for field use.
[0015] A physician uses the probing instrument by positioning the
probe's distal end adjacent to an area of interest, and the
instrument's detector assembly is preferably oscillated over the
area of interest. The probe instrument then analyzes the observed
signal, and outputs data corresponding to material of interest.
[0016] The novel features of this invention, as well as the
invention itself, will be best understood from the attached
drawings, taken along with the following description, in which
similar reference characters refer to similar parts, and in
which:
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0017] FIGS. 1, 2 and 3 show features of a magnetic susceptibility
detector, which includes the applied field coil(s) with a magnetic
sensor(s), as disclosed in parent U.S. patent application Ser. No.
08/670,393, now U.S. Pat. No. 5,842,986;
[0018] FIGS. 4, 5 and 6 show preferred features of the detector
assembly, which include the preferred circular applied field coils
and center mounted magnetoresistive sensor used in the probing
instrument;
[0019] FIG. 7 shows a block diagram with the preferred applied
field current source and analyzing components used in the probing
instrument; and
[0020] FIG. 8 shows an exemplary perspective view of the probing
instrument.
DETAILED DESCRIPTION OF THE INVENTION
[0021] Provided herein is a room-temperature medical probing
instrument that measures variations of magnetic susceptibility. The
invention's probe instrument can make magnetic susceptibility
measurements with a very small degree of uncertainty.
[0022] The noise of the room-temperature instrument is small
compared to the uncertainties that are caused mainly by the
magnetic response of tissues overlying the area of interest. For a
room temperature system, as for existing instruments based on SQUID
sensors, this overlying tissue effect, and not the noise in the
magnetic sensors, determines the precision of the measurement.
Because the crucial limitation in the sensitivity of the magnetic
susceptibility measurement is imposed by the overlying tissue
response, rather than the noise of the sensor itself, the somewhat
higher noise of the room temperature functioning magnetic sensor
compared to a SQUID is not a limiting factor in the performance of
the instrument.
[0023] Performance of the room-temperature instrument depends on
two critical issues:
[0024] a) The instrument has to be sensitive enough to see the
small magnetic signals produced by the magnetic susceptibility of
the item of interest; and
[0025] b) The magnetic susceptibility of the item of interest has
to be determined accurately in the presence of the interfering
signal produced by the slight magnetic susceptibility of the
overlying tissue and other surrounding tissues, such as an
organ.
[0026] In magnetic susceptibility measurements, a magnetic field is
applied, inducing a magnetization in the area of interest. A small
magnetic field produced by this sample magnetization is then
detected using a magnetic sensor. At low applied field, the sample
magnetization is proportional to the intensity of the applied field
and to the magnetic susceptibility of the sample.
[0027] In magnetic susceptometry, very weak susceptibilities are
sometimes encountered. For comparison, in liver susceptometry, the
difference in magnetic susceptibility between the liver and
surrounding tissue is proportional to the liver iron concentration.
The main iron compound stored in the liver has a susceptibility of
approximately 1.6.times.10.sup.-6 (in SI units) per milligram of
iron per cubic centimeter of liver. Typical patients with iron
overload have several milligrams of iron per cubic centimeter of
liver. The instrumental noise of existing SQUID biosusceptometers
corresponds to an uncertainty of about 20 micrograms per cubic
centimeter in liver iron concentration. Factors including
uncertainty in the magnetic susceptibility of surrounding tissues
contribute sources of systematic uncertainty in clinical liver
measurements. Clinical measurements with existing SQUID-based
instruments achieve uncertainties in the range of 0.2-0.5
milligrams of iron per gram of liver, which corresponds to a
magnetic susceptibility resolution of (3-7).times.10.sup.-7 (SI
Units).
[0028] To detect a weak magnetic response, there are two technical
issues:
[0029] a) Minimization of noise in the detector's magnetic-field
sensors (and, to a lesser extent, the background noise from the
environment) so that detection of the magnetic response can be
performed without applying excessively large fields; and
[0030] b) Ensuring that the spurious signals due to the applied
fields are small compared with the desired magnetic susceptibility
signal.
[0031] With respect to sensor noise requirements, in order to
measure a given magnetic susceptibility, the applied field must be
large enough and the noise from the magnetic sensor must be low
enough so that the magnetic susceptibility response is much greater
than the sensor noise. In practice, using a room-temperature
instrument, the applied field is limited by the need to avoid
excessive ohmic heating in the applied field coils of the detector
assembly. Excessive heat loads can induce thermal drifts in the
geometry of the applied field coils. As discussed below, such
drifts could affect the ability to suppress spurious signals due to
the applied field. However, an applied magnetic field of roughly
10.sup.-3 T to an area of interest does not incur excessive thermal
drift effects.
[0032] If a field of 10.sup.-3 T is applied, and the magnetic field
due to the response of the sample is 10.sup.-7 times the applied
field, then the magnetic sensor noise must be less than 10.sup.-10
Tesla. Such noise requirements can readily be met using
room-temperature functioning magnetic sensors. The preferred sensor
for the invention is a magnetoresistive (MR) sensor with very low
noise. Such sensors are commercially available from Honeywell,
Phillips, and other companies. The MR sensor operates on the
principle that the resistance of particular magnetic materials
(such as permalloy, an alloy of nickel and iron) is a function of
the ambient magnetic field. Changes in the magnetic field result in
changes in sensor resistance, which can be measured and quantified.
MR sensors developed by Kodak have noise spectral densities below
30 pT/Hz.sup.1/2 at frequencies above 20-30 Hz. Similar noise
levels are achieved by MR sensors commercially available from
Honeywell. With a measurement bandwidth of 0.1 Hz (three seconds of
data averaging) these sensors exhibit an RMS sensor noise of
10.sup.-11 Tesla. This noise level is ten times below a
representative signal of 10.sup.-10 Tesla. A variety of other
sensor types could also meet the requirements of the present
invention, including other sensors based on magnetoresistance (such
as giant magnetoresistance sensors and spin dependent tunneling
sensors), as well as fluxgate magnetometers and magnetoinductive
sensors.
[0033] To measure magnetic signals below 100 pT, care is required
to reject magnetic noise from the environment. The requirements for
noise rejection are less stringent in the present invention than in
the existing SQUID biosusceptometers. The SQUID systems use dc
magnetic fields, and produce a dc magnetic susceptibility response.
These systems convert this dc magnetic response into a time-varying
magnetic signal by moving the patient up and down. However, even
with this modulation, the measurement takes place at a rather low
frequency. At such frequencies, the background noise in many
environments is quite large.
[0034] The invention's room-temperature system preferably applies
an AC magnetic field at a frequency between around 25 and 2,000
hertz, and detects the magnetic response at the same frequency. At
these frequencies, environmental background fluctuations are
usually small, as long as noise peaks at harmonics of the
power-line frequency are avoided. Magnetic signal measurements
needed for the probe instrument are 10.sup.7 times smaller than the
field applied to a patient's body. In making such a measurement,
technical issues include the stability of the applied magnetic
field, the stability of the magnetic sensors, and the geometrical
stability of the magnetic-field coils and sensor array.
[0035] The proposed invention's instrument is designed so that
fluctuations of the current in the applied-field coil have only a
negligible effect on the magnetic measurements. The invention uses
a detector assembly whose applied field coil is geometrically
configured such that almost no magnetic field occurs at a location
where the magnetic sensor is positioned in relation to the applied
field coils. If the magnetic sensor were exposed to the full
amplitude of the applied field, then the current in the field coils
would have to be stable to at least one part in 10.sup.7 to resolve
the weak magnetic signals observed in magnetic susceptibility
measurements. However, if the detector's sensor observes only
10.sup.-4 of the field applied to the sample, the coil current can
vary by as much as one part in 10.sup.4, and the corresponding
variations in the magnetic measurements are then only 10.sup.-8 of
the field applied to the sample.
[0036] FIGS. 1, 2 and 3 show the applied field coil and magnetic
sensor design and system for determining FFB object(s) as disclosed
in parent U.S. patent application Ser. No. 08/670,393 of Avrin et
al., entitled "Ferromagnetic Foreign Body Screening Method and
Apparatus", now U.S. Pat. No. 5,842,986. The detector assembly 10
makes use of the technical principles discussed above. This
detector assembly 10 provides magnetic susceptibility measurement
information available for the detection of retained ferromagnetic
foreign body (FFB) object(s), that is metallic objects inside human
tissue, as a way of screening patients prior to magnetic resonance
imaging (MRI) or other medical procedures. This detector assembly
10 can also be used in the invention herein.
[0037] The invention herein teaches of another detector assembly
with design improvements that improve the noise of the magnetic
susceptibility measurements and optimize response from the item of
interest with respect to an interfering signal from overlying
tissue or an adjacent organ. System components also include
equipment for using magnetic measurement signals from the sensors
to detect and locate ferromagnetic objects, and for distinguishing
the signals of the target objects from other interfering magnetic
fields.
[0038] Below, the detector assembly in the parent U.S. patent
application Ser. No. 08/670,393 is first described, followed by a
description of the instant invention's exemplary instrument design
for improved detection characteristics.
[0039] FIGS. 1, 2, and 3 collectively show a detector assembly 10
which is intended to be placed near the body region to be screened.
The applied field coils 18, when supplied with a current from the
current signal generator 22, generate a time-varying applied
magnetic field to the body. The magnetic material in the body
region responds, providing a small magnetic field that is detected
by a sensor 24 (shown in FIG. 3) or array of sensors (not shown)
positioned adjacent to the body region. Together, the applied field
coils 18 and the sensor(s) 24 allow measurement of anomalies in the
magnetic susceptibility of the body region being screened. In
particular, the geometry of the applied field coils and the
placement of the magnetic sensor(s) is such that the interfering
applied field experienced by the magnetic sensor(s) 24 is as small
as possible. As discussed earlier, this arrangement reduces the
interfering signal produced by the varying magnetic field. The
detector assembly consisting of the sensor(s) 24 and applied field
coils 18 can be stationary, or can be movable to generate a
magnetic susceptibility anomaly map over the body part being
screened. The intensity and the time dependence or frequency
dependence of the magnetic susceptibility anomaly can be
interpreted rapidly by a signal processor to reveal the location
and size of ferrous metallic objects retained within the screened
body region.
[0040] The applied magnetic field may be several orders of
magnitude larger than the signal of the FFB object(s). One
arrangement of the device 10 is to configure the applied field
coils 18 so that the applied field is nearly canceled out in
regions within the device 10, within which the magnetic sensors 24
are positioned and attached (FIG. 1). The applied field coil
element 12 is laid out on the surfaces of two printed circuit (PC)
boards 14, 16. The two PC boards 14, 16 are placed parallel to each
other, with the magnetic sensors placed between the two PC boards
14, 16. Each PC board 14, 16 accommodates a multiplicity of
parallel, evenly spaced current paths 19 traveling in one direction
in the center region of the board, with return paths 20 along the
outer edges of the board, approximating two spiral patterns. The
spiral patterns on one PC board are connected in series so that,
when a current is passed through them, the resulting electric
current distribution approximates a uniform sheet of current
flowing in the y-direction as shown, over a substantial region near
the center of the board. This region of the board is roughly
defined by the area between the markers A-A and between the markers
B-B. This current distribution produces a magnetic field that is
nearly uniform over a region of space near the center of the board.
The two boards 14, 16 of this design are placed parallel to each
other, with this relationship being shown. The PC boards 14, 16 are
separated by a distance S which is small compared with the length
and width of the central region of uniform current. The two PC
boards 14, 16 are mounted so that the current paths 19 on one board
are oriented parallel to the corresponding current paths 19 on the
other board. The current paths on the two boards 14, 16 are then
connected in series to an AC signal generating power supply 22, so
that the current flows in the same direction on both boards, the
y-direction in the arrangement shown. The source 22 can be equipped
with a control device, as is known in the art, to cause the field
to be pulsed, to vary in frequency, or to have a waveform with
multiple frequencies. These time variations in the applied field
can assist in distinguishing the responsive field from the
environmental background fields, by synchronization of the sensing
circuitry with the power supply. In a region surrounding the
centers of the two PC boards 14, 16, the magnetic field produced by
this arrangement approximates that produced by a pair of parallel,
uniform sheets of current flowing in the y-direction. In the space
between the centers of the two PC boards, the net magnetic field is
nearly zero since the contributions from the two current sheets
approximately cancel each other. There is a small residual magnetic
field, since perfect field cancellation is prevented by the finite
size of the regions of the current sheets, and the presence of the
return paths 20 along the outer edges of the PC boards 14, 16.
However, due to the symmetry of the current paths in the two PC
boards, the magnetic field is substantially zero in the plane
midway between two PC boards. The magnetic sensor(s) 24 are placed
in a plane parallel to the PC boards 14, 16, with the plane of the
sensors being located at the midpoint MP between the two PC boards
14, 16, so that the sensors are nearly in a zero field state with
respect to magnetic fields generated by the applied field coils
18.
[0041] FIG. 2 shows another view of the sandwiched field coil with
a magnetic sensor 24, preferably an MR sensor, placed in a
low-field region between parallel circuit boards 14 and 16 as shown
in FIG. 1. The current paths are shown with lines and arrows. The
magnetic sensor 24 is sandwiched between two printed circuit boards
14, 16. The central region of each circuit board 14, 16 contains a
number of parallel, evenly spaced traces 19 which are connected in
series and which carry identical applied field currents.
[0042] FIG. 3 shows where the sensor 24 is placed with respect to
the applied field coil 18. The top coil has been removed to show
sensor positioning. The arrow on the sensor 24 indicates the
direction of its field sensitivity. Two methods are used to null
out the field at the sensor location. First, a set of shims is used
to adjust the position of the sensor between the two current
sheets. This adjustment is needed because the applied field, given
the finite size of the circuit boards 14, 16 used, is zero only on
the plane of symmetry midway between the two current sheets. With
this coarse adjustment, a reduced residual field occurs at the
sensor to a value roughly 300 times smaller than the field at the
outer surface of the coil set. A fine balance adjustment is made by
placing small tabs of metal near the sensor. By using balance tabs
of both steel and aluminum foil, the in-phase and the out-of-phase
components are canceled out of the magnetic field with respect to
the ac current supplied to the applied field coil. A reduced
residual field to a level roughly 30,000 times smaller than the
field at the outer surface of the coil set occurs when current is
applied. Any noise due to the variations in the ac supply current
is less than 10.sup.-8 of the field applied to an examined sample,
that is the tissue.
[0043] In the detector 10, geometrical variation of the applied
field coils 18 and sensor(s) 24 is an important effect that this
field-nulling system cannot remove. Temperature variations may
cause subtle distortions in the geometry of the applied-field
coils, or in the position of the magnetic sensor within the coils.
Such distortions can perturb the balance of the field-canceling
system, producing noise in the magnetic measurements.
[0044] The detector assembly provided herein minimizes effects
caused by geometric distortion of the detector assembly, by
modulating a distance between an area of interest and the
instrument's detector assembly at up to several hertz, with
displacement of the detector assembly up to six inches. The change
in the magnetic signal at the modulation frequency is then
measured. The invention departs from methods used with conventional
SQUID devices by moving the detector assembly 10 while the patient
remains stationary. The instrument herein performs this function by
mounting the detector assembly, which includes the applied field
coils 18 and the sensor 24, on a nonmagnetic platform, and
oscillating the detector assembly 10 back and forth at several
hertz using a motor. The motor can drive a mechanism for producing
oscillatory movement of the detector assembly. This mechanism can
be a cam driven, spring biased plate, where the cam member is belt
driven by the motor, or a reciprocating rod where the detector
assembly is mounted to a plate that oscillates by a linear drive
member. Other reciprocating motion-type devices can be used as well
to provide proper oscillatory motion with displacements of up to
and around six inches, at motion frequencies up to and around 10
hertz. The detector assembly is mounted in a housing that provides
support and positioning for the instrument. The housing and the
components of the oscillatory motion mechanism are made of
nonmetallic, nonmagnetic materials. Signal analysis described below
extracts information from the detector assembly's 10 signal output
from the magnetic sensor 24.
[0045] The ability to move the detector assembly 10 instead of the
patient is significant since the overall instrument is much simpler
and less expensive. Moving a SQUID type magnetic sensor is not
permitted since any magnetic gradients in the environment produce
signals that interfere with the direct current magnetic response
measurements. These ambient magnetic gradients do not present
problems in the proposed invention's measurements. For example, the
room temperature sensor(s) 24 have much more tolerance compared to
SQUIDs when being moved in the presence of the earth's magnetic
field.
[0046] Another feature of the invention is the ability to measure
weak variations of the magnetic field response of the item of
interest. It is sometimes desirable that the applied field
penetrate more deeply into the body than is possible with the
applied field coils 18 in detector 10. Also it is desirable to
maximize the magnetic response from the item of interest with
respect to the magnetic response from the overlying tissue and from
any nearby organ. The applied field coil of the present application
optimizes the response of the item of interest with respect to the
sensor noise and with respect to the interfering signals from the
overlying tissues and adjacent organs.
[0047] FIGS. 4 and 5 show the instant invention's design of an
applied field coil arrangement 35 whose geometrical design
optimizes a response signal from the item of interest. Such a
design adjusts the diameter of the applied field coils 26, 30 to
control how deeply the applied magnetic field penetrates into the
patient's body. A circular coil of radius "a" produces a field that
falls off rather slowly out to distances comparable to "a," and
then decays as 1/r.sup.3 at longer distances. The two main field
coils 26, 30 allow for measurement of the response of the item of
interest, and evaluation of the response due to the susceptibility
of the overlying tissues.
[0048] FIG. 5 shows the detector assembly's applied field coil
arrangement. The detector assembly comprises three concentric
circular spiral coils, but can include additional coils for designs
that encompass the present invention's design. FIG. 5 shows the
first coil 26, with a relatively large diameter, which produces a
field that reaches deep into a patient's body. The resulting
magnetic susceptibility response contains contributions from both
the item of interest and the overlying tissues. The diameter of
this coil 26 maximizes the contribution of the item of interest and
minimizes the overlying tissue contribution, so that variations in
the susceptibility of the overlying tissue have as little effect as
possible on the measurement of susceptibility of the item of
interest. A mean diameter in a range of around 15-50 cm for the
outer coil 26 is preferred.
[0049] FIG. 5 shows the small, innermost applied field coil 28.
During magnetic susceptibility measurements, this smaller coil is
connected in series with the outer coil 26, in such a way that the
current in the inner coil 28 is in the opposite direction from that
in the outer coil 26. The diameters and numbers of turns in the two
coils are adjusted so that the magnetic field due to the inner coil
cancels the magnetic field due to the outer coil, in a region near
the common center of the two coils, producing a small zone of
substantially zero magnetic field. The magnetic sensor (24 in FIG.
4) is then placed in this zone of substantially zero magnetic field
so that, as discussed above, fluctuations of the current in the
applied field coils produce very little noise in the magnetic
susceptibility measurements. The inner coil can have a mean
diameter of about 1.5 to 8 cm. Since the magnetic field due to the
small, innermost coil 28 dies away rapidly with distance, the
magnetic field in the patient's body tissues is produced almost
entirely by the outer coil 26.
[0050] FIG. 5 also shows the intermediate-diameter coil 30 which
can optionally be used, in place of the outer coil 26, to produce a
magnetic field that reaches a relatively short distance into the
patient's body. Magnetic susceptibility measurements made using
this intermediate-diameter coil 30 will produce a magnetic
susceptibility response whose main contribution comes from the
patient's overlying tissues. The results of these measurements can
be used to evaluate the magnetic susceptibility of the overlying
tissues. This information can then be combined with the results of
magnetic susceptibility measurements made using the outer coil 26,
to evaluate the magnetic susceptibility of the item of interest,
while removing errors due to the susceptibility of the overlying
tissues. In magnetic susceptibility measurements made using the
intermediate-diameter coil 30, the intermediate-diameter coil 30 is
connected in series with the small, inner coil 28, in such a way
that the magnetic field is canceled at the location of the magnetic
sensor.
[0051] FIG. 5 shows exemplary dimensions of the three concentric
coils that make up the applied field coil. Each coil consists of
one or more concentric loops. The number of loops in each coil is
proportional to its diameter. This ensures that if any two coils
are energized with equal but opposite current, the field at the
center will be zero. This equal and opposite current is realized by
making the appropriate electrical interconnections between the
inner and outer coils and applying current to the two coils using
the same current source. In this example, the outermost coil 26 has
exactly four times the diameter of the innermost coil 28, and has
exactly four times as many turns. When magnetic susceptibility
measurements are made using the outer coil 26, this coil is
connected in series with the innermost coil 28, but with opposite
polarity, as shown in FIG. 4 by the two oppositely directed arrows.
The magnetic field cancels out almost completely at the location of
the detector 10. The intermediate-diameter coil 30 has exactly
twice the diameter, and twice the number of turns, as the innermost
coil 28. When measurements are made using this intermediate coil
30, it is connected in series with the innermost coil to cancel out
the magnetic field at the sensor 24 location.
[0052] The applied field coils 26, 28, 30 can comprise traces on a
printed circuit board. To generate the maximum field for a given
current magnitude, similar coil sets can be positioned on both
sides of the circuit board 14, thus doubling the number of turns of
each coil. In addition, stacks of circuit boards 14 can provide
sufficiently strong field to the examined tissue sample, without
the excessive ohmic heating (and the resulting undesirable thermal
drifts) that can occur if too large a current is passed through a
single circuit board. Alternatively, the printed circuit board can
be replaced by wires, metal rods, or other electrical conductors
supported by a rigid support structure that maintains the
appropriate spatial relationship of the current carrying
elements.
[0053] FIG. 5 shows a PC board 14 which has a number of circular
holes for bolting individual boards together rigidly to a solid
G-10 fiberglass plate for structural stability. The larger
noncircular holes facilitate electrical connections between the
coils 26, 28, 30 on the stacked circuit boards. A hole at the
center of the coil set allows for placement of a sensor 24 in a low
field region close to the sample. The magnetic sensor 24 is placed
in the appropriate orientation so as to sense magnetic fields
normal to the plane of the applied field coils (as indicated by
FIG. 4). In this zero-field region, the sensor is practically
immune to the applied field directly and only senses the body's
response to the applied field.
[0054] In an example of the design shown in FIG. 5, the outer coil
26 consists of 16 equally spaced concentric loops with a mean
diameter of 20 cm. The inner coil 28 consists of 4 equally spaced
concentric loops with a mean diameter of 5 cm. The middle coil 30
has 8 equally spaced concentric loops with a mean diameter of 10
cm. The applied field coil design ensures that when any pair of
coils is energized with equal and opposite current the applied
field at the center of the coils is zero.
[0055] Measurements of liver iron concentration involving the
cryogenically cooled SQUID systems usually use a "water bag" to
help discriminate the signal from the liver from that of the
overlying abdominal tissue. The magnetic susceptibility of the
liver (1.6.times. 10.sup.-6 SI units) is only slightly different
from that of the abdominal tissue (value close to that of water;
-9.times.10.sup.-6 SI units). The susceptibility contrast between
the liver and the abdominal tissue is typically smaller than that
between the air and the abdominal tissue. Hence the liver will
appear as an anomaly in the body which is itself an anomaly in the
surrounding air space. In biomagnetic susceptibility measurements,
the susceptibility contrast between the abdominal tissue and the
air produces a magnetic response which interferes with the
measurement of the response due to the liver iron itself. In order
to minimize this interfering signal, a bag filled with water is
positioned to fill the space between the sensor and the surface of
the patient's abdomen. The water, whose magnetic susceptibility is
nearly the same as that of the abdominal tissue, essentially
removes any magnetic susceptibility contrast at the outer surface
of the abdomen, as if the entire magnetic measurement were being
made in an environment filled with material of a constant magnetic
susceptibility approximately equal to that of the abdominal tissue.
The magnetic susceptibility measurement then responds primarily to
the magnetic susceptibility contrast between the liver and the
surrounding abdominal tissue. This magnetic susceptibility anomaly
is due almost entirely to the iron in the liver.
[0056] The room temperature instrument of the present invention can
also be used with a water bag, if necessary, to remove the
interfering signal from the overlying tissue. Alternatively, the
contribution of the overlying tissue to the signal can be measured
and subtracted out by using the middle coil 30. Since the middle
coil is smaller than the outer coil, the magnetic field generated
by the middle coil will not penetrate as deeply into the body as
will the field generated by the outer coil. Therefore, with the
proper choice of coil dimensions, the response signal due to the
applied field of the middle coil will be mostly due to the
overlying tissue closer to the surface of the body, whereas the
response signal due to the applied field of the outer coil will be
due to both the item of interest and the overlying tissue.
Consequently, two successive magnetic susceptibility measurements,
using the outer coil and the middle coil, provide two independent
pieces of information, which can be used to solve mathematically
for two unknown quantities, the magnetic susceptibilities of the
item of interest and the overlying tissue. This method determines
the magnetic susceptibility of the item of interest, while removing
errors due to variability in the magnetic susceptibility of the
overlying tissue.
Ancillary Hardware and Method of Use
[0057] FIG. 6 shows the detector assembly 10 and the interface
assembly components attached thereto. Preferably, an MR sensor can
be used along with a feedback coil mounted on the sensor, which
allows the sensor to be "locked" at its optimum operating point by
applying a compensating field to cancel changes in the ambient
field. This technique maintains constant sensitivity of the sensor.
The detection assembly's MR sensor is part of a Wheatstone bridge
wherein transduced resistance measurements are related to
transduced sensed magnetic measurements. An electronic feedback
circuit amplifies the voltage of the Wheatstone bridge, and
supplies a feedback current to a small feedback coil wrapped around
the MR sensor, compensating for changes in the ambient magnetic
field and maintaining the sensing element at a constant magnetic
field. In this "field-lock" scheme, the ambient magnetic field is
actually measured by monitoring the current applied to the
compensating coil by the feedback loop. This approach eliminates
potential errors due to either the sensor's nonlinear response
function or its temperature-dependent responsivity.
[0058] A phase sensitive detector measures the component of the
output of the magnetic sensor that oscillates in phase with an AC
applied field. A Fourier transform analyzer calculates the
component of the output of the phase-sensitive detector that
oscillates in phase with the modulation of the sample-sensor
distance. This provides a way to distinguish the signal of interest
from the low-frequency noise caused by thermal drifts. The function
of the phase sensitive detector can be performed by a lockin
amplifier, and the function of the Fourier transform analyzer can
be performed by a spectrum analyzer. Preferably, either or both
functions can be performed on a computer.
[0059] A signal source is used to generate an AC signal between 25
Hz and 2 kHz. This signal, amplified by an audio frequency
amplifier, provides a constant amplitude oscillating current
through the applied field coils on the detection head assembly.
[0060] FIG. 7 shows the computer analyzer and control functions
which process response signals from the sensor 24, and output
information regarding the magnetic susceptibility of materials in
the patient's body. In FIG. 7, the computer integrates and controls
all instrument functions, including the modulation of the
sensor-sample distance, the generation of the AC field coil
current, and the processing of the magnetic sensor outputs to
determine the magnetic susceptibility of the sample. The computer
can be a personal computer with the required functioning signal
cards and processors included. The motor indicated in FIG. 7 is
preferably used to move the detector assembly toward and away from
a patient's area of interest. The fast Fourier transformer is used
to resolve the variation of the received signal in synchrony with
this motion. The waveform synthesizer is used to generate an AC
signal, which is then amplified by the power amplifier to generate
an AC current for the applied field coil. The waveform synthesizer
function can be incorporated by the computer. The AC signal can
have frequencies up to around 2,000 hertz, preferably avoiding
harmonics of the power line frequency. The AC signal can be
synchronized with the power lines, at a frequency commensurate with
the power line frequency, in order to minimize noise due to the
power lines.
[0061] Actual output from the computer can be a data storage
device, a video display of useful medical information, or a
connection to a computer system network.
[0062] The magnetic sensor control electronics, a motor/crank rod
arrangement for oscillatory movement of the instrument's distal end
detector assembly, a waveform synthesizer and power amplifier, a
lock-in amplifier, and a spectrum analyzer or equivalent computer
device for signal analysis can be incorporated in a single medical
instrument unit as shown in exemplary form in FIG. 8 as unit 100.
FIG. 8 shows the probe instrument 100 with an elongated positioning
arm 130 wherein the detector assembly 10 is mounted at the distal
end of the arm 110 which has a motor 125 within, with the required
oscillatory drive members 120 that move the detector assembly 10
toward and away from a patient.
[0063] FIG. 8 shows the patient on a non-metallic table. The
detector assembly 10 is positioned over a tissue area of interest.
The detector assembly 10 has the sensor mounted to a reciprocating
member 120 located within the arm 110 that moves the detector
assembly 10 translationally toward and away from the distal end of
the head member, preferably between one and six inches. The
reciprocating action typically is in a range between around 0.5 to
10 hertz such that modulation of the detector assembly 10 filters
out signal noise caused by temperature drifts in the applied field
coils.
[0064] The reciprocating member 120 within the arm of the probe
instrument 100 allows modulation of the distance between the
examined tissue and the detector assembly 10, as explained above.
The reciprocating member is made of nonmagnetic materials. In use,
a water bag (not shown) may be placed between the detector assembly
10 and the patient.
[0065] Analysis is performed on the signal detected by the sensor
to provide output information corresponding to the magnetic
susceptibility of items detected in the area of interest.
[0066] Variations to the invention include the following:
[0067] a) Modulation of the distance between the sample and the
detector assembly can improve the signal-to-noise ratio of magnetic
susceptibility measurements on any type of sample (i.e., including
samples other than the human body).
[0068] b) The methods and apparatus described in the parent U.S.
patent application Ser. No. 08/670,393 can be modified by the
presently described modulation of the sample-sensor distance to
improve the signal-to-noise ratio of magnetic susceptibility
measurements for the detection of ferromagnetic foreign bodies
(FFBs) within the eye, brain, or body of a patient.
[0069] The instant invention describes an applied-field coil
configuration, as shown in FIG. 5, consisting of two concentric
circular loops carrying currents in opposite directions, in which
the diameters and number of turns in the two loops are adjusted so
as to cancel the magnetic field at the common center of the two
coils. This applied-field coil design may be used in other types of
magnetic susceptibility measurements.
[0070] In particular, the concentric-loop coil design (FIG. 5) may
be used with the apparatus and methods described in the parent U.S.
patent application Ser. No. 08/670,393, for the detection of
ferromagnetic foreign bodies (FFBs) within the eye, brain, or body
of a patient. The use of the concentric-loop coil would increase
the magnetic susceptibility response of FFBs located deep below the
surface of the patient's face, head, or body.
[0071] For the detection of FFBs in the eye, brain, or body, the
parent U.S. patent application Ser. No. 08/670,393 teaches the
measurement of appropriate magnetic-field gradients, or
alternatively, the mapping of the magnetic-susceptibility response
as a function of position, in order to compute the location of the
FFB within the host. This spatial mapping or magnetic gradient
measurement may be achieved either by using an array of more than
one magnetic sensor, or by using a single magnetic sensor and
moving the detection unit (applied field coils and magnetic
sensor). Either approach may be used in conjunction with the
concentric-loop applied field coil design shown in FIG. 5.
[0072] The applied-field coil design of FIG. 5 may be modified to
accommodate an array of more than one magnetic sensor. The parent
U.S. patent application Ser. No. 08/670,393 discloses that to
reduce the noise produced by variations in the applied magnetic
field, it is desirable to ensure that the applied magnetic field is
as small as possible at the location of each magnetic sensor. The
concentric-loop coil described above cancels the magnetic field at
a single point, the common center of the at least two concentric
loops. If the radius of the inner coil is decreased slightly in
relation to that of the outer coil, or if the current in the inner
coil is increased slightly in relation to that of the outer coil,
the magnetic field will be canceled not at a single point, but
along a circle concentric with the two loops. Multiple sensors may
then be placed at different locations on this circle, and the
applied magnetic field will be canceled out at the location of each
sensor. This arrangement makes possible the simultaneous
measurement of the magnetic field response at multiple points in
space.
[0073] As an alternative, the noise produced by applied-field
variations may be minimized by measuring differences in magnetic
field between two or more magnetic sensors, as long as the magnetic
sensors are positioned within the applied-field coils in such a way
that the applied magnetic field is the same for each of the
sensors. Such a result may be achieved with an applied field coil
consisting of a circular loop, or multiple concentric loops, by
placing each of the magnetic sensors at the same distance from the
center of the loop(s).
[0074] Moreover, the applied field coils of the concentric coil
design shown in FIGS. 4 and 5 can have differing dimensions and
configurations to measure at other tissue regions in the body.
Also, switchable configurations of the applied field coil
connections can be controlled by the instrument's computer allowing
for adaptive control of the instrument for multiple examining
capabilities.
[0075] While the particular invention as herein shown and disclosed
in detail is fully capable of obtaining the objects and providing
the advantages hereinbefore stated, it is to be understood that
this disclosure is merely illustrative of the presently preferred
embodiments of the invention and that no limitations are intended
other than as described in the appended claims.
* * * * *