Optical position sensors

Details Optical position sensors Optical position sensors

1999-01-15

2001-04-24

Kamm, William E. (Department: 3762)

Surgery

Diagnostic testing

Detecting nuclear, electromagnetic, or ultrasonic radiation

Reexamination Certificate

active

06223066

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to object tracking systems, and specifically to non-contact, electromagnetic methods and devices for tracking the position and orientation of an object.
BACKGROUND OF THE INVENTION
Non-contact methods of determining the position of an object based on generating a magnetic field and measuring its strength at the object are well known in the art. For example, U.S. Pat. No. 5,391,199, and PCT patent publication WO96/05768, which are incorporated herein by reference, describe such systems for determining the coordinates of a medical probe or catheter inside the body. These systems typically include one or more coils within the probe, generally adjacent to the distal end thereof, connected by wires to signal processing circuitry coupled to the proximal end of the probe.
U.S. Pat. No. 4,849,692, to Blood, describes a position tracking system based on detection of a DC magnetic field. Preferred embodiments described in this patent are based on detecting electrical currents generated in response to the field. Mention is made of the possibility of using a fiberoptic magnetic field sensor, but the patent gives no further information on possible implementations of such a sensor in position measurement.
The use of magneto-optic materials to measure magnetic field strength is known in the art, as described, for example, by M. N. Deeter et al., in “Novel Bulk Iron Garnets for Magneto-Optic Magnetic Field Sensing, Proceedings of SPIE, Vol. 2922, which is incorporated herein by reference. Magneto-optic materials rotate the polarization of polarized light passing through them, by an amount proportional to the strength of the magnetic field. The polarization rotation is characterized by a parameter known as Verdet's constant, expressed in units of deg/cm/Tesla. For strongly magneto-optic materials, such as yttrium iron garnet (YIG), the Verdet constant is about 10
8
. However, magneto-optic materials exhibit hysteresis, causing difficulties in field measurement when time-varying non-constant fields are involved.
Magnetostrictive fiberoptic strain gauges are also known in the art. For example, the article “Optical Fibre Magnetic Field Sensors,” by K. P. Koo, Optics Letters, which is incorporated herein by reference, describes a method for measuring magnetic fields using magnetostrictive perturbation of a fiberoptic. A grating is produced within the fiber, for example by irradiating the fiber with an excimer laser. The grating generally comprises a periodically varying refractive index within the fiber. When light having a wavelength equal to twice the grating spacing is injected into the proximal end of the fiber, constructive interference of the reflected waves will give a strong reflection back to the proximal end. When a mechanical strain is applied to stretch the fiber, the grating spacing changes, so that the wavelength response of the reflected light is proportional to the mechanical strain and hence to the magnetic field.
SUMMARY OF THE INVENTION
It is an object of some aspects of the present invention to provide improved position sensing apparatus based on optical sensing of a magnetic field.
In one aspect of the present invention, the apparatus is used to determine the position of an invasive probe within the body of a patient.
In preferred embodiments of the present invention, apparatus for sensing the position of a catheter comprises an optical fiber embedded in the catheter, which senses an external magnetic field that is applied to the catheter. Light is injected into the fiber at the proximal end of the catheter and propagates down to the distal end thereof, where it is modulated by the effect of the magnetic field, as described below. The modulated light is reflected back to the proximal end, where it is monitored to provide a measure of the magnetic field at the distal end. The magnetic field measurement is used to determine coordinates of the distal end of the catheter, by methods of signal analysis similar to those described in the above-mentioned U.S. Pat. No. 5,391,199 and PCT publication WO96/05768.
In some preferred embodiments of the present invention, the fiber is coupled at its distal end to one face of a magneto-optic crystal, preferably yttrium iron garnet (YIG), suitably oriented, adjacent to the distal end of the catheter. An opposing face of the crystal is coated for reflection. Preferably, the fiber is a single-mode, polarization preserving fiber, as is known in the art. Polarized light is injected into the fiber's proximal end, and is rotated by the YIG crystal by an angle proportional to the magnetic field strength. The polarization of the reflected light returning to the proximal end is analyzed to determine the field strength, and hence, the position of the distal end of the catheter.
Alternatively, there is a polarizer placed between the distal end of the fiber and the crystal, and the intensity of the reflected light is detected to determine the polarization rotation angle. In this case, it is not necessary that the fiber be of the polarization-preserving type.
In these preferred embodiments, there is preferably an additional fiber in the catheter, not coupled to the crystal, to serve as a temperature reference. Reflection signals received from the additional fiber are used to compensate for changes in signals in the sensor fiber due to temperature changes.
Furthermore, in order to account for hysteresis in the polarization rotation effect, in preferred embodiments of the present invention, signal processing circuitry associated with the catheter preferably tracks changes of polarization of the light reflected back from the crystal, to determine where on the hysteresis curve the sensor is operating.
In other preferred embodiments of the present invention, the fiber contains a grating structure, as described in the above-mentioned article by Koo, and is clad with a magnetostrictive material. The magnetostrictive material expands or contracts in direct proportion to the external magnetic field. Such expansion or contraction changes the spacing of the grating in the fiber, so that the reflected light intensity may be used to measure the field strength and thus to determine the position of the catheter, as described above.
Preferably the magnetic field has an AC field component, at a frequency that is low enough so that the magnetostrictive material will contract and expand synchronously with the field variation. Detection of the reflected light is locked to the magnetic field AC frequency, so as to cancel out spurious changes in reflection due to other strains on the catheter, such as bending.
In some of these preferred embodiments, the fiber includes several gratings at different points along its length, each grating having a different, respective grating spacing. Polychromatic light having wavelengths corresponding respectively to the different spacings of the gratings is injected into the fiber, and changes of intensity at each wavelength are monitored to detect the magnetic field at (and hence the positions of) the different grating points along the length of the fiber. In this manner, a single fiber is used to make multiple position measurements simultaneously.
The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawings in which:


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patent: 5

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