Noninvasive room temperature instrument to measure magnetic susceptibility variations in body tissue

• US 6,208,884 B1
• Filed: 08/18/1998
• Issued: 03/27/2001
• Est. Priority Date: 06/25/1996
• Status: Expired due to Term

• Current Assignee: Quantum Magnetics, Inc. (GE Aerospace)
• Original Assignee: Quantum Magnetics, Inc. (GE Aerospace)
• Inventors: Avrin, William F., Kumar, Sankaran
• Primary Examiner(s): Lateef, Marvin M.
• Assistant Examiner(s): MANTIS MERCADER, ELENI M
• Application Number:
  US09/135,890
• Time in Patent Office: 952 Days
• Field of Search: 600/407, 600/409, 600/410, 324/260, 324/261, 324/207.11, 324/207.12, 324/207.13, 324/207.14, 324/207.15, 324/207.16, 324/207.17, 324/207.18, 324/207.19, 324/207.21
• 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 5/055 involving electronic [EMR] or nuclear [NMR] magnetic resonance, e.g. magnetic resonance imaging
• CPC Class Codes: A61B 5/242 Detecting biomagnetic fields, e.g. magnetic fields produced by bioelectric currents
• CPC Class Codes: A61B 5/416 the spleen
• CPC Class Codes: A61B 5/4244 liver
• 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 probe instrument using room-temperature sensor(s) that can measure magnetic susceptibilities variations. The instrument has sufficient resolution to monitor paramagnetic materials in a human body, and preferably iron in a human liver by noninvasively examining patients with iron-overload diseases. The instrument includes room temperature magnetic sensors, and detects the sample, that is the tissue response to an alternating current field applied by 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 liver while minimizing the effects due to the overlying abdominal tissue and at the same time not unduly increasing the sensitivity of the instrument to the lung. 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, i.e. the real and imaginary parts of the applied field are canceled. 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 sensor-sample distance is modulated. The detector assembly is oscillated while the examined patient remains stationary. The detector'"'"'s applied field coil is fabricated on a printed circuit board and is attached firmly to a solid nonmetallic support structure forming part of a detector. This improves mechanical and thermal stability, and makes for a precise and inexpensive fabrication of the applied field coil. The detector assembly forms part of a probe instrument for performing noninvasively the paramagnetic concentration of a patient.

Claims

1. A method for noninvasive determination of magnetic susceptibility variation in a human body by measuring magnetic susceptibilities of tissue, the method comprising:

providing an instrument which includes at least one magnetic sensor, an applied field coil and a current source connected to the applied field coil and means for processing sensed signals from the at least one magnetic sensor, wherein the instrument functionally operates at room temperature and minimizes noise due to temperature fluctuations at the magnetic sensor;

positioning the instrument external to the patient in proximity to the tissue of interest;

supplying the applied field coil with current thereby applying a magnetic field to the tissue of interest;

sensing a response from the tissue of interest with the instrument; and

outputting data corresponding to the magnetic susceptibility variation in the tissue.

2. The method recited in claim 1, wherein the supplying of current to the applied field coil comprises alternating current operating between about 25 to 2,000 hertz.

3. The method recited in claim 1, wherein the method further comprises providing the instrument with displacement means for displacing the magnetic sensor and the applied field coil simultaneously thereby compensating for noise introduced to the sensed signals.

4. The method recited in claim 3, wherein the displacement means operates between about one to six inches.

5. The method recited in claim 3, wherein the displacement means operates between about 0.5 to 10.0 hertz.

6. The method recited in claim 1, wherein the outputting of data corresponding to the magnetic susceptibility variation in the human body comprises concentrations of paramagnetic material in the tissue of interest.

7. The method recited in claim 6, wherein the paramagnetic material is iron and the tissue of interest is a liver.

8. The method recited in claim 7, wherein the outputting of data corresponds to concentrations of iron in the liver and the resolution of the measurements corresponds to 30 micrograms per milliliter.

9. An apparatus for noninvasively measuring magnetic susceptibility variations in body tissue to determine a compositional state in the body, the apparatus comprising:

a detector assembly that includes;

at least one magnetic sensor and an applied field coil for generating a magnetic field;

a current signal generating source which connects to the applied field coil;

a means for processing signals from the at least one magnetic sensor of observed magnetic susceptibility variations in body tissue; and

a means for outputting data corresponding to a compositional state in the body;

wherein the detector assembly functionally operates at room temperature and minimizes noise due to temperature fluctuations at the magnetic sensor.

10. The apparatus recited in claim 9, wherein the current signal generating source provides alternating current (AC) and operates between about 25 to 2,000 hertz and avoiding power line frequency and harmonics thereof.

11. The apparatus recited in claim 9, wherein the means for processing signals from the at least one magnetic sensor comprises a processor for analyzing the signals obtained from the magnetic sensor.

12. The apparatus recited in claim 9, wherein the means for processing signals further comprises outputting means for displaying paramagnetic material concentration.

13. The apparatus recited in claim 12, wherein the paramagnetic material is iron.

14. The apparatus recited in claim 9, wherein the applied field coil is designed to create a first zone of a finite magnetic field, within a selected region of the body, and a second zone of substantially zero magnetic field, outside the selected region;

and

the at least one magnetic sensor is positioned within said second zone of substantially zero magnetic field.

15. The apparatus recited in claim 14, wherein the applied field coil comprises two parallel flat coils and a connecting circuit between the two parallel coils causing current to flow in identical directions at corresponding locations in the two flat coils, thereby simulating parallel uniform sheets of current.

16. The apparatus recited in claim 14, wherein the excitation field coil comprises at least two concentric coils and a connecting circuit between the at least two concentric coils causing current to flow in opposite directions, thereby canceling the effects of a magnetic field caused by current flowing through the applied field coil at a central region of the at least two concentric coils.

17. The apparatus recited in claim 16, wherein the largest concentric coil has a diameter between around 15 to 50 centimeters.

18. The apparatus recited in claim 16, wherein there are at least three concentric coils, the outermost coils include at least two coils which are alternatively switched with the current source, whereby sufficient information can be derived independently as to the susceptibility of a deep lying tissue area in the body compared to a corresponding surface tissue area.

19. The apparatus recited in claim 18, wherein the deep lying tissue area is the liver and the overlying surface tissue area is abdominal tissue.

20. The apparatus recited in claim 16, wherein the current in each concentric coil is the same.

21. The apparatus recited in claim 14, wherein the detector assembly is multiple stacked applied field coils.

22. The apparatus recited in claim 9, wherein the at least one magnetic sensor attaches to a central region relative to the applied field coil.

23. The apparatus recited in claim 22, wherein the applied field coil is attached to a planar substrate.

24. The apparatus recited in claim 9, wherein the at least one magnetic sensor is a magnetoresistive sensor.

25. The apparatus recited in claim 24, wherein the at least one magnetoresistive sensor is part of a Wheatstone bridge sensing circuit.

26. The apparatus recited in claim 25, further including magnetic sensor compensating electronics and a feedback coil disposed about the magnetoresistive sensor for locking an optimum operating point by applying a compensating electrical current from compensating electronics to the feedback coil thereby maintaining constant measurement sensitivity of the apparatus.

27. The apparatus recited in claim 9, wherein the at least one magnetic sensor is a fluxgate sensor.

28. The apparatus recited in claim 9, wherein the at least one magnetic sensor is a magnetoinductive sensor.

29. The apparatus recited in claim 9, wherein the detector assembly further comprises a means for oscillating the detector assembly.

30. The apparatus recited in claim 29, wherein the detector assembly is housed in a housing structure for positioning the detector assembly in proximity to a surface of the human body, and the means for oscillating the detector assembly comprises a motor with attached drive members that move the detector assembly.

31. A magnetic susceptibility detector device comprising:

an applied field coil configured to connect to a current source, the field coil being designed to create a first zone of a finite magnetic field, within a selected region of an observed specimen, and a second zone of substantially zero magnetic field, outside said selected region, the applied field coil having at least two concentric electric current carrying coils of conductor material; and

a sensing device which is at least one magnetic sensor, the at least one sensor being positioned within said second zone of substantially zero magnetic field.

32. The device recited in claim 31, wherein the at least one magnetic sensor is a magnetoresistive sensor.

33. The device recited in claim 31, wherein the at least one magnetic sensor is a fluxgate sensor.

34. The device recited in claim 31, wherein the at least one magnetic sensor is a magnetoinductive sensor.

35. An apparatus for noninvasively determining magnetic susceptibility variation in body tissue, the apparatus comprising:

a detector assembly that includes;

at least one magnetic sensor and an applied field coil for generating a magnetic field, wherein the applied field coil is designed to create a first zone of a finite magnetic field, within a selected region of the body, and a second zone of substantially zero magnetic field, outside the selected region;

the at least one magnetic sensor is positioned within said second zone of substantially zero magnetic field; and

a means for oscillating the detector assembly;

the detector assembly attaches to said means for oscillating the detector assembly;

a current signal generating source which connects to the applied field coil; and

a means for processing signals from the at least one magnetic sensor of observed magnetic susceptibility variation in the body;

wherein the detector assembly functionally operates at room temperature and minimizes noise due to temperature fluctuations at the magnetic sensor.

36. The apparatus recited in claim 35, wherein the magnetic sensor attaches to a central region relative to the applied field coil.

37. The apparatus recited in claim 35, wherein the applied field coil includes two parallel flat coils and a connecting circuit between the two parallel coils causing current to flow in identical directions at corresponding locations in the two flat coils, thereby simulating parallel uniform sheets of current.

38. The apparatus recited in claim 35, wherein the excitation field coil comprises at least two concentric coils and a connecting circuit between the at least two concentric coils causing current to flow in opposite directions, thereby canceling the effects of a magnetic field caused by current flowing through the applied field coil at a central region of the at least two concentric coils.

39. The apparatus recited in claim 35, wherein the detector assembly is multiple stacked applied field coils.

40. The apparatus recited in claim 35, wherein the at least one magnetic sensor is a magnetoresistive sensor;

the sensor forms part of a Wheatstone bridge circuit and further includes a means for compensating the magnetic sensor for locking an optimal operational state by applying a compensating electrical current from the compensating means thereby maintaining sensitivity of the apparatus.

41. The apparatus recited in claim 35, wherein the at least one magnetic sensor is a fluxgate sensor.

42. The apparatus recited in claim 35, wherein the at least one magnetic sensor is a magnetoinductive sensor.