Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
2001-09-21
2002-12-17
Lateef, Marvin M. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
C324S207210, C324S260000
Reexamination Certificate
active
06496713
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
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.
2. Background Art
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.
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.
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.
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.
The present instrument obviates the need for cryogenically cooled SQUIDs by providing operational use at room temperature, making for much less expensive fabrication and use. The instrument 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.
A key problem with the magnetic susceptibility method is that the patient's body tissues have their own magnetic susceptibility response, which is superimposed upon the response due to the FFB. To detect the smallest possible FFBs, it would be advantageous to distinguish the signature of the FFB, in the presence of this background response due to body tissues.
U.S. Pat. No. 5,408,178 to Wikswo et. al. describes an apparatus and method for imaging the structure of diamagnetic and paramagnetic objects. The Wikswo et al. method involves applying a magnetic field to a specimen, and measuring the resulting magnetic susceptibility response. Specifically, the Wikswo method attempts to image, or map, the magnetic susceptibility variations within a specimen composed of paramagnetic or diamagnetic material. Wikswo inverts a set of equations, using the measured response as a function of sensor position as well as applied-field direction and measured-field direction, to map out the magnetic susceptibility distribution of the paramagnetic or diamagnetic material within the specimen itself.
The method of the present invention and that of Wikswo et al. make different kinds of measurements and process the data in different ways, to achieve very different results. Specifically, the Wikswo et al. method attempts to image, or map, the magnetic susceptibility variations within a specimen composed of paramagnetic, or diamagnetic material. The present method cancels out the response of paramagnetic or diamagnetic material, to more readily detect the presence of a ferromagnetic foreign body within the specimen. The present method exploits the symmetry properties of the response, as a function of the applied-field and measured-field directions, in order to cancel the response of paramagnetic and diamagnetic material within the specimen, and thus detect the presence of a ferromagnetic foreign body. Wikswo et al. invert a set of equations, using the measured response as a function of sensor position as well as applied-field direction and measured-field direction, in an attempt to map out the magnetic susceptibility distribution of the paramagnetic or diamagnetic material within the specimen itself.
BRIEF SUMMARY OF THE INVENTION
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, or a permanent magnet, 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.
The applied field may be produced using an applied field coil or a permanent magnet. The use of an applied field coil is preferred for a number of reasons. First, it lends itself to the application of an alternating magnetic field. The use of an alternating magnetic field reduces sensor noise, reduces noise due to ambient magnetic fields, and facilitates the modulation of the sensor-sample distance in order to reduce the effects of temperature drift in the sensing apparatus. Also, the use of an applied-field coil, or coils, lends itself to the cancellation of the signal due to the applied magnetic field.
An alternative embodiment is to use a permanent magnet to produce the applied magnetic field. A permanent magnet produces a constant DC field, which lacks many of the advantages of an alternating field. In principle, one could produce an alternating magnetic field by appropriate movement, such as reciprocating motion, of one or more permanent magnets.
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. In some cases, the noise in the magnetic field measurements can be reduced by using an induction coil sensor, which detects a changing magnetic field by measuring the voltage induced in a coil of electrically conductive wire.
The applied field coil dimensions are such that an applied field is optimized for maximum respon
Avrin William F.
Czipott Peter V.
Massengill R. Kemp
Lateef Marvin M.
Mantis Mercader Eleni
MedNovus, Inc.
Spinks Gerald W.
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