Abstract:
An elongate medical probe, having proximal and distal ends, whose position is tracked within the body of a subject includes a magnetic-field responsive optical element adjacent to the distal end, which modulates light passing therethrough responsive to an externally-applied magnetic field. The probe also includes a fiberoptic coupled to receive modulated light from the optical element and convey it to the proximal end of the probe for analysis of the modulation.

Description:
This application is a nonprovisional application of U.S. provisional application Ser. No. 60/072,148 filed Jan. 21, 1998. 
    
    
     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&#39;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&#39;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: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a schematic illustration showing a catheter including a fiberoptic position sensor, in accordance with a preferred embodiment of the present invention; 
     FIGS. 1B and 1C are schematic illustrations showing fiberoptic position sensors for use in the catheter of FIG. 1A, in accordance with alternative preferred embodiments of the present invention; 
     FIG. 2 is a graph showing a hysteresis curve associated with the sensor of FIG. 1A or FIG. 1B or FIG. 1C; 
     FIG. 3 is a schematic illustration showing a catheter including a fiberoptic position sensor having a magnetostrictive cladding, in accordance with a preferred embodiment of the present invention; and 
     FIG. 4 is a schematic illustration showing a catheter including a single fiberoptic comprising a plurality of gratings for position sensing, in accordance with a preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Reference is now made to FIG. 1A, which shows a probe  24 , preferably a catheter, including a magnetic-field responsive position sensor  20 , in accordance with a preferred embodiment of the present invention. Sensor  20  comprises a magneto-optic crystal  22 , preferably yttrium iron garnet (YIG), and an optical fiber  30 , in this case a single-mode, polarization-preserving fiber. In the preferred embodiment shown in FIG. 1A, crystal  22  has two opposing parallel faces, proximal face  26  and distal face  28 , orthogonal to a symmetry axis  50  of the distal end of fiberoptic  30 . Fiber  30  is connected to face  26 , preferably by optical cement. Face  28  is preferably coated with a reflecting material, for example, an aluminum or dielectric coating, as is known in the art, so that light incident from the fiberoptic onto the magneto-optic material and passing through face  26  is largely reflected from face  28  back to the fiberoptic. In the presence of a magnetic field, the plane of polarization of the reflected light will be rotated by an angle proportional to the component of the magnetic field parallel to axis  50 . 
     As shown in FIG. 1A, probe  24  is placed in the magnetic field of one, two, or more magnetic radiator coils  48 , the fields of said coils preferably having been previously mapped and/or calibrated using methods known in the art. Generally the magnetic field of the radiator coils is a DC field, or an AC field, or a combination of a DC and an AC field. 
     In the preferred embodiment shown in FIG. 1A, polarized light having a wavelength λ 1 , where λ 1  is preferably of the order of 1 μm, is injected from a source  46  into the proximal end of fiberoptic  30 , preferably via a beamsplitter  32 . The light traverses the fiberoptic to the distal end thereof and enters into magneto-optic crystal  22 . It is reflected from face  28  through the magneto-optic crystal and the fiberoptic, back to beamsplitter  32 . Beamsplitter  32  is constructed so as to direct the reflected light onto a detector  34 , which measures the intensity of the reflected light. In the preferred embodiment shown in FIG. 1A, detector  34  generally comprises a polarizing element  52 . It will be appreciated that the intensity of the reflected light measured by the detector is dependent on the degree of rotation of the plane of polarization caused by the magneto-optic crystal  22 . 
     Although the intensity of the reflected light is a measure of the component of the magnetic field along axis  50  at magneto-optic crystal  22 , the intensity may also be affected by temperature changes or mechanical deformation in the fiberoptic. Therefore, a second compensating fiberoptic  40 , not coupled to the magneto-optical material, is fixed in the probe in proximity to fiberoptic  30 , and light is injected into fiberoptic  40  as described above regarding fiberoptic  30 . Light reflected from the distal end of fiberoptic  40  passes through beam splitter  42  and a polarizing element  54  to detector  44 . It will be appreciated that the intensity of the reflected light measured by detector  44  is not dependent on the magnetic field acting on the magneto-optical material  22 . Electrical signals from detectors  34  and  44  are fed by wires  38  to signal processing circuitry  36 , which processes the signals by difference or other signal processing techniques known in the art to determine the amplitude of the magnetic field at crystal  22 . As described in U.S. Pat. No. 5,391,199, the location of the sensor is derived from the amplitude of the magnetic field. 
     FIG. 1B schematically illustrates another magneto-optic position sensor  60 , similarly suitable for use in probe  24 , in accordance with an alternative preferred embodiment of the present invention. Apart from the differences described below, the operation of position sensor  60  is generally similar to that of position sensor  20 , whereby components with the same reference numerals are generally identical in construction and operation. In the preferred embodiment shown in FIG. 1B, polarized light having a wavelength λ 1  and a reference wavelength λ 2 , where λ 2  is substantially different from λ 1 , is injected from a source  47  into the proximal end of fiberoptic  30 , preferably via first and second dichroic beamsplitters  33  and  35 . Beamsplitter  33  is designed to substantially fully transmit λ 1  and to deflect light at λ 2 . Beamsplitter  35  is designed to substantially fully transmit λ 2  and to deflect light at λ 1 . Beamsplitter  33  directs reflected light of wavelength λ 1  onto detector  34  via polarizer  52 . Beamsplitter  35  directs reflected light of wavelength λ 2  onto detector  44  via polarizer  54 . A dichroic mirror  27 , which substantially transmits λ 1  and reflects λ 2 , is placed between the distal end of fiberoptic  30  and proximal end  26  of crystal  22 . 
     Thus, the intensity of the reflected light measured by detector  34  is dependent on the degree of rotation of the plane of polarization caused by the magneto-optic crystal  22 , while the intensity of the reflected light measured by detector  44  is substantially independent of the magnetic field acting on magneto-optical material  22 . Both intensities are substantially equally affected by temperature changes or mechanical deformation in the fiberoptic, so that the signal from detector  44  may be used as a compensating reference signal. As described above regarding sensor  20 , electrical signals from detectors  34  and  44  are used to determine the amplitude of the magnetic field at crystal  22 , and the location of the sensor is derived from the amplitude of the field. 
     FIG. 1C schematically illustrates yet another magneto-optic position sensor  120 , similarly suitable for use in probe  24 , in accordance with an alternative preferred embodiment of the present invention. In sensor  120 , a polarizer  56  is placed between the distal end of fiberoptic  30  and proximal face  26  of magneto-optical crystal  22 . In this preferred embodiment, the light injected into the fiberoptic by source  46  is generally unpolarized, and fiberoptic  30  is not necessarily a single mode or a polarization preserving fiberoptic. Polarizer  56  thus acts as an analyzer of light reflected from face  28 . The light reflected passes to beam splitter  32  and to detector  34 , which generates electrical signals used to determine the amplitude of the magnetic field, as described above. 
     Although the preferred embodiments described above measure only a single directional component of the magnetic field, those skilled in the art will appreciate that similar sensors may be produced for measuring two or three components of the field, preferably by using a plurality of magneto-optic crystals, each with a respective fiberoptic and detector. The crystals are oriented so that each respective crystal axis is aligned along a different field axis. In a preferred embodiment of the present invention, not shown in the figures, three such crystals, in mutually substantially orthogonal orientations, may be used to measure six-dimensional position and orientation coordinates of a probe, using methods described in the above-mentioned PCT publication WO96/05768. In another preferred embodiment, three separate fiberoptics are connected to one magneto-optic crystal so as to inject into the crystal and receive therefrom three mutually substantially orthogonal beams of light, whereby the six-dimensional position and orientation coordinates are found. 
     In the preferred embodiments described above, the magneto-optic polarization effect of crystal  22  may be characterized by a hysteresis curve, as is shown schematically in FIG.  2 . Therefore, a given polarization rotation may correspond to two different values of magnetic field strength, depending on where on the hysteresis curve the crystal is operating. Preferably, signal processing circuitry  36  tracks changes of polarization rotation in the light received by detectors, so as to compensate and correct for ambiguities due to hysteresis. Alternatively, a combination of DC and AC fields may be applied to magnetic radiator coils  48  in order to compensate for hysteresis effects, by methods known in the art, in crystal  22 . 
     FIG. 3 schematically illustrates another position sensor  78  within a probe  68 , in accordance with an alternative preferred embodiment of the present invention. Sensor  78  comprises a fiberoptic  62  including a grating structure  64 , preferably etched into its outer surface or alternatively formed within the core of the fiberoptic, using methods known in the art. The fiberoptic is clad in the area of the grating with a magnetostrictive cladding  66 , preferably nickel. The scale of the elements of sensor  78  is exaggerated in the figure for clarity of illustration. 
     As described above, light of wavelength λ 1  is injected into the proximal end of fiberoptic  62 , generally via a beam-splitter  72  from a light source  76 , which emits generally coherent light. The period of grating  64  is preferably of the order of half the wavelength λ 1  of the light injected. Magnetostrictive cladding  66  expands or contracts as a function of the external magnetic field component, parallel to grating  64 , generated by magnetic radiator coils  63 , thus altering the grating period. Consequently the intensity of the light at wavelength λ 1  reflected from grating  64  back to beam-splitter  72  and measured at detector  74  is a function of the magnetic field component applied along probe  68 . Electrical signals from detector  44  are fed to signal processing circuitry, as shown in FIG. 1A, and the signals are processed to determine the amplitude of the magnetic field at grating  64 , and thus to determine the position of probe  68 . 
     Preferably, the magnetic field produced by coils  63  comprises an AC field, such that magnetostrictive cladding  66  contracts and expands synchronously with the field. The detection of signals from detector  74  is most preferably locked to the frequency of the AC field, so as to minimize interference due to spurious changes in reflected light intensity caused by non-magnetostrictive changes in fiberoptic parameters. 
     FIG. 4 schematically illustrates a set of position sensors  80  within a probe  90 , in accordance with a further preferred embodiment of the present invention. Sensors  80  operate in conjunction with a polychromatic light source  94 , preferably a laser, emitting a plurality of substantially different coherent wavelengths λ 1 , λ 2 , λ 3 , and λ 4 . The light is injected via a broadband beamsplitter  88 , or by other methods known in the art, into the proximal end of a fiberoptic  82 . The fiberoptic comprises a plurality of gratings  84 , formed as described above, corresponding to the plurality of injected wavelengths. Each of the gratings  84  has a substantially different grating period  92 , preferably equal to half of a respective one of the plurality of wavelengths of the light injected. The wavelengths λ 1 , λ 2 , λ 3 , and λ 4  are selected so that each grating generally reflects one of the wavelengths and largely transmits the others. 
     Each of gratings  84  is separately clad by a magnetostrictive cladding  86 , which in the presence of a magnetic field, applied by magnetic radiator coils  100 , alters the grating period as described above, and consequently changes the intensity of the light reflected from each grating. The reflected light is transferred via beamsplitter  88  to a diffraction grating  96 , or other suitable wavelength-dispersive element. Grating  96  disperses the light according to wavelength onto a detector  98 , most preferably a linear array detector, giving separate outputs for each of the plurality of wavelengths. As described above, the intensity of the light reflected from each of the gratings  84  and measured at detector  98  is a function of the magnetic field generated by the magnetic radiator coils at the respective grating. Electrical signals from detector  98  are fed to signal processing circuitry, as shown in FIG. 1A, and the signals are processed to determine the amplitude of the magnetic field at each grating. Thus, the respective positions of multiple points along probe  90 , corresponding to multiple gratings  84 , are determined. 
     Preferably the magnetic field in the preferred embodiment comprises an AC field, such that the magnetostrictive material contracts and expands synchronously with the field. The detection of signals from detector  98  is most preferably locked to the frequency of the AC field, as described above. 
     Although the preferred embodiments described above use reflection, from crystal  22  or gratings  64  or  84  back through the fiberoptic, to transfer the modulated light from the sensors to the detectors, it will be appreciated by those skilled in the art that other optical configurations can also be used to accomplish the transfer. Specifically, the modulated light from the sensors can be transferred to the detectors using transmission through the crystal or gratings. 
     It will be appreciated that the preferred embodiments described above are cited by way of example, and the full scope of the invention is limited only by the claims.