Ferromagnetic foreign body detection with background canceling

• US 20020077537A1
• Filed: 09/21/2001
• Published: 06/20/2002
• Est. Priority Date: 06/25/1996
• Status: Active Grant

• Current Assignee: Quantum Magnetics, Inc. (GE Aerospace), Mednovus Incorporated (Kopp Development Incorporated)
• Original Assignee: Quantum Magnetics, Inc. (GE Aerospace)
• Inventors: Massengill, R. Kemp, Avrin, William F., Czipott, Peter V.
• Granted Patent: US 6,496,713 B2
• Time in Patent Office: Days
• US Class Current: 600/409
• CPC Class Codes: A61B 5/05 Detecting, measuring or recording for diagnosis by means of electric currents or magnetic fields; Measuring using microwaves or radio waves impedance plethysmography A61B5/0295; measuring movement of the entire body or parts thereof A61B5/11; detecting, measuring or recording bioelectric or biomagnetic signals of the body or parts thereof A61B5/24
• CPC Class Codes: A61B 5/242 Detecting biomagnetic fields, e.g. magnetic fields produced by bioelectric currents
• CPC Class Codes: A61B 5/7264 Classification of physiological signals or data, e.g. using neural networks, statistical classifiers, expert systems or fuzzy systems neural networks per se G06N3/00; expert systems per se G06N5/00
• CPC Class Codes: G01R 33/16 Measuring susceptibility G01R33/1238 takes precedence
• CPC Class Codes: G01V 3/08 operating with magnetic or electric fields produced or modified by objects or geological structures or by detecting devices with electromagnetic waves G01V3/12

Abstract

A method for rejecting the magnetic response of surrounding biological tissues, in measuring the magnetic susceptibility response of a region of interest in a patient, to detect the presence of ferromagnetic foreign bodies. The magnetic response of the surrounding tissues is rejected by defining a statistically determined normal range of magnetic susceptibility responses, or by rejecting magnetic field responses characteristic of materials with isotropic magnetization properties.

Claims

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 means for processing sensed signals from said at least one 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 magnetic response from said region of interest with said magnetic sensor;

rejecting the magnetic response from biological tissues in said region of interest; and

outputting data corresponding to the magnetic response of a ferromagnetic foreign body within said region of interest.

2. The method recited in claim 1, further comprising:

providing said applied field coil wound on a cylindrical coilform; and

providing sensor coils axially spaced from said applied field coil on either side thereof;

wherein said sensor coils are oppositely wound in a gradiometer configuration.

3. The method recited in claim 2, further comprising positioning the instrument in proximity to the eye of the patient.

4. The method recited in claim 2, further comprising positioning the instrument in proximity to the brain of the patient.

5. The method recited in claim 1, further comprising positioning next to the patient a flexible bag substantially filled with deformable material having a magnetic susceptibility substantially similar to that of body tissue, said bag being attached to a substantially rigid barrier, said barrier being spaced from the patient by the flexible bag.

6. The method recited in claim 5, further comprising positioning the instrument in proximity to the eye of the patient.

7. The method recited in claim 5, further comprising positioning the instrument in proximity to the brain of the patient.

8. The method recited in claim 1, further comprising using a neural network for said rejecting of all magnetic responses from biological tissues in said region of interest.

9. The method recited in claim 1, further comprising using a Bayesian filter for said rejecting of all magnetic responses from biological tissues in said region of interest.

10. The method recited in claim 1, wherein:

said rejecting of said magnetic response from said biological tissues comprises rejecting, with said instrument, all magnetic responses within a statistically determined normal range of magnetic responses from said biological tissues in said region of interest; and

said output data comprises indication of a magnetic response having a magnitude outside said normal range of magnetic responses from said biological tissues in said region of interest.

11. The method recited in claim 10, further comprising:

measuring magnetic responses of said region of interest in a statistically significant number of subjects to create a baseline of magnetic responses;

computing the mean and standard deviation of said baseline magnetic responses;

rejecting, with said instrument, all magnetic responses from said region of interest of the patient that fall within a selected number of standard deviations of said mean of said baseline magnetic responses; and

indicating the presence of a ferromagnetic foreign body for any magnetic response that differs from said mean by said selected number of standard deviations.

12. The method recited in claim 11, wherein said selected number of standard deviations is empirically determined for each said region of interest.

13. The method recited in claim 12, wherein said selected number of standard deviations is between two and four.

14. The method recited in claim 10, further comprising:

measuring magnetic responses of two substantially symmetrical sides of said region of interest of the patient;

computing the difference between said magnetic responses of said two substantially symmetrical sides of said region of interest in the patient;

rejecting, with said instrument, all magnetic responses from said region of interest of the patient for which said difference between said magnetic responses of said two substantially symmetrical sides of said region of interest of the patient falls within a statistically determined normal range; and

indicating the presence of a ferromagnetic foreign body for any magnetic response for which said difference between said magnetic responses of said two substantially symmetrical sides of said region of interest of the patient is outside said statistically determined normal range.

15. The method recited in claim 10, further comprising:

measuring magnetic responses of two substantially symmetrical sides of said region of interest in a statistically significant number of subjects;

computing the difference between said magnetic responses of said two substantially symmetrical sides of said region of interest in each said subject to create a baseline of magnetic response differences;

computing the mean and standard deviation of said baseline differences between said magnetic responses;

measuring magnetic responses of two substantially symmetrical sides of said region of interest of the patient;

computing the difference between said magnetic responses of said two substantially symmetrical sides of said region of interest in the patient;

rejecting, with said instrument, all magnetic responses from said region of interest of the patient for which said difference between said magnetic responses of said two substantially symmetrical sides of said region of interest of the patient falls within a selected number of standard deviations of said mean of said baseline differences; and

indicating the presence of a ferromagnetic foreign body for any magnetic response for which said difference between said magnetic responses of said two substantially symmetrical sides of said region of interest of the patient differs from said mean of said baseline differences by said selected number of standard deviations.

16. The method recited in claim 15, wherein said selected number of standard deviations is empirically determined for each said region of interest.

17. The method recited in claim 16, wherein said selected number of standard deviations is between two and four.

18. The method recited in claim 10, further comprising:

measuring at least one dimension of said region of interest in a statistically significant number of subjects;

measuring the magnetic response of said region of interest in each of said subjects;

correlating each said magnetic response to each said at least one dimension of said region of interest;

measuring said at least one dimension of said region of interest of the patient;

measuring the magnetic response of said region of interest of the patient;

rejecting all magnetic responses from said region of interest of the patient which fall within a normal range of magnetic responses for said measured dimension of the patient; and

indicating the presence of a ferromagnetic foreign body for any magnetic response having a magnitude above said normal range.

19. The method recited in claim 18, further comprising using a neural network for said correlating of each said magnetic response to each said at least one dimension of said region of interest, and for said rejecting of all magnetic responses from said region of interest of the patient which fall within a normal range.

20. The method recited in claim 18, further comprising using a Bayesian filter for said correlating of each said magnetic response to each said at least one dimension of said region of interest, and for said rejecting of all magnetic responses from said region of interest of the patient which fall within a normal range.

21. The method recited in claim 10, further comprising:

measuring all nine components of the magnetic-field response on both sides of said region of interest, for a total of 18 measurements, on each subject of a population of subjects known not to have FFBs;

calculating the covariance matrix of the 18 quantities over said population of normal subjects;

finding the eigenvectors and eigenvalues of said covariance matrix;

defining an 18-dimensional probability distribution for all the measured quantities in said population of normal subjects, wherein the surfaces of constant probability are ellipsoids, and where the principal axes of said ellipsoids correspond to the eigenvectors of said covariance matrix; and

defining a threshold value of how far a given patient'"'"'s measurements fall from the mean for the normal population, such that values below that threshold would be considered within the range of normal variation and values above that threshold would indicate the possible presence of an FFB.

22. The method recited in claim 1, wherein:

said rejecting of said magnetic response from said biological tissues comprises rejecting any magnetic response not indicating an item substantially subject to rotation by an applied magnetic field;

wherein said output data comprises indication of a magnetic response characteristic of a ferromagnetic foreign body subject to rotation by an applied magnetic field.

23. The method recited in claim 22, further comprising:

making said magnetic field substantially uniform and substantially unidirectional, in the region of interest;

applying said magnetic field to said region of interest in each of three mutually orthogonal directions;

sensing said magnetic response from said region of interest in the direction of said application of the applied magnetic field, for each said direction of application of said magnetic field; and

rejecting any magnetic response of materials for which the magnitude of magnetization response does not substantially vary with the direction of said applied field.

24. The method recited in claim 22, further comprising:

making said magnetic field substantially uniform and substantially unidirectional, in the region of interest;

applying said magnetic field to said region of interest in each of three mutually orthogonal directions;

sensing said magnetic response from said region of interest in each of said three mutually orthogonal directions, for each said direction of application of said magnetic field; and

rejecting any magnetic response of materials for which the magnitude of magnetization response does not substantially vary with the direction of said applied field.

25. The method recited in claim 24, wherein:

said rejecting of said magnetic responses comprises;

rejecting all magnetic responses in which the direction of said magnetization response is always in said direction of said applied field and in which the magnitude of said magnetic response is independent of said direction of said applied field; and

said output data comprises;

indication of a magnetization response having a direction different from said direction of said applied field; and

indication of a magnetization response having a magnitude substantially dependent upon said direction of said applied field.

26. The method recited in claim 24, wherein:

said three magnetic fields are applied simultaneously in said three mutually orthogonal directions;

each of said three magnetic fields is at a frequency different from the frequencies of the other two of said three magnetic fields;

said three magnetic responses in said three mutually orthogonal directions are sensed simultaneously, for each of said three magnetic fields; and

said three magnetic responses resulting from each said applied field are sensed at the frequency of said applied field.

27. The method recited in claim 24, wherein:

said three magnetic fields are applied sequentially in said three mutually orthogonal directions; and

said three magnetic responses in said three mutually orthogonal directions are sensed simultaneously, for each of said three magnetic fields.

28. The method recited in claim 1, further comprising outputting data corresponding to a magnetization response which is not parallel to said applied magnetic field.

29. The method recited in claim 1, further comprising outputting data corresponding to a magnetization response which has a magnitude that varies with the direction of said applied magnetic field.

30. A method for noninvasive screening of a human body for the presence of a ferromagnetic foreign body, said method comprising:

providing a test instrument at a remote location, including at least one magnetic sensor and an applied field coil;

providing a central computer system;

supplying said applied field coil with current, thereby applying a magnetic field to a region of interest of a patient;

sensing a magnetic response from said region of interest with said magnetic sensor;

transmitting said magnetic response to said central computer system;

rejecting said magnetic response from biological tissues, with said central computer system;

outputting data corresponding to the magnetic response of a ferromagnetic foreign body, with said central computer system; and

transmitting said output data to said remote location.

31. The method recited in claim 30, wherein:

said rejecting of said magnetic response from said biological tissues comprises rejecting, with said central computer system, all magnetic responses within a statistically determined normal range of magnetic responses from said biological tissues in said region of interest; and

said output data comprises indication of a magnetic response having a magnitude outside said normal range of magnetic responses from said biological tissues in said region of interest.

32. The method recited in claim 30, wherein:

said rejecting of said magnetic response from said biological tissues comprises rejecting, with said central computer system, any magnetic response not indicating an item substantially subject to rotation by an applied magnetic field;

said output data comprises indication of a magnetic response characteristic of a ferromagnetic foreign body subject to rotation by an applied magnetic field.

33. The method recited in claim 30, further comprising:

transmitting said magnetic response from said remote location to said central computer system via the Internet; and

transmitting said output data from said central computer system to said remote location via the Internet.

34. 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, a permanent magnet, and means for processing sensed signals from said at least one magnetic sensor;

positioning said instrument external to a patient in proximity to a region of interest;

applying a magnetic field to said region of interest;

sensing a magnetic response from said region of interest with said magnetic sensor;

rejecting the magnetic response from biological tissues in said region of interest; and

outputting data corresponding to the magnetic response of a ferromagnetic foreign body within said region of interest.