Abstract:
An improved system for magnetic position tracking of a device includes a magnetic transmitter, a magnetic sensor, a computing system and a polarity inverter. The magnetic transmitter includes at least one transmitter coil that outputs a transmitted magnetic field having a time derivative component. The magnetic sensor includes at least one sensor coil that has coil terminals having a polarity, and the sensor coil is responsive to the time derivative component of the transmitted magnetic field and outputs a sensor signal. The computing system computes position and angular orientation data of a device based on the sensor signal and the polarity inverter is configured to connect to the coil terminals and to cause the polarity of the coil terminals to be reversed according to a switching signal.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to an improved system and method for magnetic position tracking, and more particularly to a system and a method that reduces the magnetic field induced noise signal in the sensor interconnect system by periodically switching the polarity of the noise signal. 
       BACKGROUND OF THE INVENTION 
       [0002]    Magnetic position tracking systems are becoming more widely used in the medical field, particularly when paired with an ultrasound imaging system. Due to the problems introduced into magnetic systems by conductive metals, medical magnetic tracking systems may operate in a low frequency band, in the sub 2 KHz range down to near DC levels. Distortion of the transmitted fields due to nearby conductive metals is minimized when operating in this low frequency range. A problem which arises due to these low frequencies is that the magnetic signals tend to be less affected by signal shielding materials such as aluminum or copper which are effective at higher frequencies. The shields for low frequency must employ high permeability materials and the design must be optimized such that leakage fields are well controlled. This makes the design of low frequency shielding much more difficult than for higher frequencies where thin conductive foils and loosely fitting shells can be employed. Due to the sensitive nature of the signals from the magnetic sensors, the signal path interconnect must be carefully designed to minimize sensitivity to the transmitted field. Electromotive force (EMF) errors are induced into the interconnect system if there is an unbalanced loop area within the interconnect system that is exposed to the transmitted field. In the case of an ultrasound probe, the probe interconnect system is designed to accommodate hundreds of co-axial cable elements and their associated terminations. This type of interconnect presents a relatively large unbalanced loop area into the signal path of the magnetic sensor. 
         [0003]    Prior art systems have avoided this problem by running the optimized magnetic interconnect cable assembly adjacent to the probe interconnect cable assembly. The external mounting of the magnetic sensor and the bulk of a second independent cable running alongside the probe cable is objectionable to many end users. In order to disconnect a probe from the ultrasound chassis, both the probe interconnect and magnetic sensor interconnect must be disconnected. The mass of the probe interconnect, which is attached to the magnetic sensor cable and connector, stresses the smaller interconnect causing reliability concerns. Another limitation of prior art systems is seen when the sensor signals must be passed through a connector which shares the same physical structure as a therapeutic device, such as is found on an endoscope. In this case, the magnetic signal must be contained within the instrument due to size constraints. Currently, prior art systems employ magnetic shielding around the magnetic portion of the instrument connector. This shielding can become bulky, complex, and expensive. Sterilization and reprocessing are needed in order to safely re-use such an instrument, and these costs are moving the industry towards inexpensive disposable devices. The ability to pass the magnetic sensor signals through a single, uncomplicated, low cost interconnect, without adding large cost elements to the magnetic sensor, is thus very desirable. 
       SUMMARY OF THE INVENTION 
       [0004]    In general, in one aspect, the invention features a system for magnetic position tracking of a device including a magnetic transmitter, a magnetic sensor, a computing system and a polarity inverter. The magnetic transmitter includes at least one transmitter coil that outputs a transmitted magnetic field having a time derivative component. The magnetic sensor includes at least one sensor coil that has coil terminals having a polarity, and the sensor coil is responsive to the time derivative component of the transmitted magnetic field and outputs a sensor signal. The computing system computes position and angular orientation data of a device based on the sensor signal and the polarity inverter is configured to connect to the coil terminals and to cause the polarity of the coil terminals to be reversed according to a switching signal. 
         [0005]    Implementations of this aspect of the invention may include one or more of the following features. The system may further include a sign inverter configured to invert a digitized output from an analog to digital (A/D) converter. The sign inverter is operated concurrently with the switching signal, so that the polarity of the coil terminals is maintained at the computing system&#39;s input. The system may further include a synchronizer configured to operate concurrently with the magnetic transmitter. The system may further include averaging means. The sign inverter is also configured to invert the sensor signal at the A/D converter&#39;s input. The transmitted magnetic field may be a sinusoid. The sinusoid may include a plurality of sine waves. The sinusoid may be continuous with respect to time. The sinusoid may be time division multiplexed. The transmitted magnetic field may have one of trapezoidal, triangular, half sinusoid, exponential, or square amplitude versus time characteristics shape. The polarity inverter is located adjacent to the magnetic sensor. The polarity inverter is connected to the magnetic sensor via a twisted pair cable. The polarity inverter may be an analog switch. The switching signal is transmitted wirelessly or via a wired connection. The averaging means is configured to sum signals received with opposite polarity from the sign inverter. The averaging means may be a lowpass filter. 
         [0006]    In general, in another aspect, the invention features a method for magnetic position tracking of a device including the following steps. Providing a magnetic transmitter having at least one transmitter coil. The transmitter coil outputs a transmitted magnetic field having a time derivative component. Providing a magnetic sensor having at least one sensor coil. The sensor coil has coil terminals having a polarity, and the sensor coil is responsive to the time derivative component of the transmitted magnetic field and outputs a sensor signal. Providing a computing system for computing position and angular orientation data of a device based on the sensor signal and providing a polarity inverter configured to connect to the coil terminals and to cause the polarity of the coil terminals to be reversed according to a switching signal. 
         [0007]    This invention is applicable to electromagnetic tracking of medical instruments. Applications include tracking of instruments such as ultrasound probes, biopsy needles, ablation instruments, and so on. 
         [0008]    The details of one or more embodiments of the invention are set forth in the accompanying drawings and description below. Other features, objects, and advantages of the invention will be apparent from the following description of the preferred embodiments, the drawings, and the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    Referring to the figures, wherein like numerals represent like parts throughout the several views: 
           [0010]      FIG. 1  illustrates a schematic of a magnetic transmitter and sensor integrated with an ultrasound imaging system; 
           [0011]      FIG. 2  illustrates a schematic of prior art magnetic sensor cable and signal conditioning elements; 
           [0012]      FIG. 3  illustrates a schematic of the magnetic sensor cable with improved signal conditioning elements; 
           [0013]      FIG. 4  illustrates states and transitions of a single-channel DC pulsed magnetic sensor signal with and without the improved signal conditioning elements; 
           [0014]      FIG. 5  illustrates extension of the single-channel DC pulsed magnetic sensor signal to a three channel magnetic sensor signal stream with polarity switch timing; 
           [0015]      FIG. 6  illustrates application of the improved signal conditioning schema to a single-channel AC driven magnetic sensor signal with polarity switch timing. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0016]    The ideal magnetic tracking system receives 100% of its signal input exclusively from the sensor coil, where the sensor signal is a response to a transmitted time varying magnetic field. The sensor coil signal traverses the sensor assembly interconnect system travelling from the sensor coil through cable wires, to and through the connector, and through signal conditioning such as an amplifier and analog-to-digital converter mounted on a printed circuit board. The interconnect system components generate spurious signals in response to the transmitted time varying magnetic field. These spurious signals sum to corrupt the otherwise ideal sensor coil signal, and thus induce position and orientation error of the tracked instrument. 
         [0017]    The invention described herein electronically periodically switches polarity of the summed spurious signal, enabling its self-cancellation. The invented polarity switch method and apparatus is applied to remove the spurious error-inducing signals generated within the interconnect, leaving the desired sensor coil signal uncorrupted. 
         [0018]    Referring to  FIG. 1 , a magnetic position tracking system  100  includes a magnetic sensor  1 , a magnetic transmitter  4 , a computer  7  and an instrument  2  whose position is being tracked. Magnetic sensor  1  is connected to the computer  7  via cable  5  and connector  6 . Magnetic transmitter  4  is connected to computer  7  via cable  50 . Magnetic sensor  1  outputs signals in response to the time derivative of magnetic fields, dB/dt, generated by the magnetic transmitter  4 . Computer  7  receives the output signals from the magnetic sensor  1  by way of cable  5  and connector  6  and computes the position of magnetic sensor  1  relative to the magnetic transmitter  4 . 
         [0019]    Magnetic sensor  1  may contain one or more signal channels. In one example, a typical 6 degree of freedom magnetic position tracking system may be constructed using 3 signal channels within magnetic sensor  1  combined with 3 orthogonal magnetic transmitting coils housed within transmitter  4 . For better clarity in this description, a single signal channel is described, because the operation of any additional signal channel is identical. 
         [0020]    Referring to  FIG. 2 , a single signal channel magnetic position tracking system  110 , includes a magnetic sensor coil  13 , a connector  6 , an amplifier  8 , an analog to digital (A/D) converter  9  and a processor  10 . Coil  13  is connected to the amplifier  8  via a pair of twisted wires  5  and via connector  6 . The sensing signal passes through the amplifier  8 , then through the A/D converter  9  and then goes to processor  10 . 
         [0021]    Coil  13  detects the time derivative of the magnetic field, dB/dt, generated by the transmitter  4  according to the formula 
         [0000]    
       
         
           
             
               EMF 
               coil 
             
             = 
             
               A 
               * 
               N 
               * 
               U 
               * 
               
                 
                    
                   B 
                 
                 
                    
                   t 
                 
               
             
           
         
       
     
         [0022]    A=area of coil  13  in square meters 
         [0023]    N=number of turns in coil  13   
         [0024]    U=free space permeability 
         [0025]    dB/dt=time rate of change of the magnetic flux density, B, from transmitter  4 , in Tesla per second. 
         [0026]    It is important to ensure that coil  13  is the only element of magnetic sensor  1  that is responsive to the magnetic signal from transmitter  4 . Any additional signal sources between coil  13  and A/D converter  9  will result in an incorrect position computation for sensor  1 . Prior art systems depend upon a high quality twisted pair cable  5  to conduct the EMF from coil  13  to connector  6 . The twisted pair cable  5  provides cancellation of magnetic signals by way of forming small opposing loops along its length, causing the EMF of each successive loop to change polarity with respect to its neighbors and thereby to cancel the effects of any external magnetic fields. This cancellation works well in a uniform magnetic field. However, in a gradient magnetic field, the dB/dt magnitude is not uniform along cable  5  and therefore the EMF for successive loops is not uniform. In this case cable  5  introduces a cable error, EMF cable . EMF cable  has the highest magnitude when cable  5  is placed on or near the transmitter  4 , due to the high gradient field near the transmitter  4 . An example of this occurrence is when instrument  2  is an ultrasound transducer and the operator inadvertently pulls cable  5  across the transmitter  4 . 
         [0027]    An additional source of error occurs where the signals from coil  13  pass through connector  6 . In most high density pin type connectors, the pins form a parallel path over their mating length. This path has a net area described by the product of pin length and pin separation. This net area is shown as a connector pin loop  14  in  FIG. 2 . The EMF from connector pin loop  14  is then described as: 
         [0000]    
       
         
           
             
               EMF 
               connector 
             
             = 
             
               
                 L 
                 
                   p 
                    
                   
                       
                   
                    
                   i 
                    
                   
                       
                   
                    
                   n 
                 
               
               * 
               
                 W 
                 
                   p 
                    
                   
                       
                   
                    
                   i 
                    
                   
                       
                   
                    
                   n 
                 
               
               * 
               U 
               * 
               
                 
                    
                   B 
                 
                 
                    
                   t 
                 
               
             
           
         
       
     
         [0028]    L pin =length of a connector pin 
         [0029]    W pin =pin separation distance 
         [0030]    U=free space permeability 
         [0031]    dB/dt=time rate of change of the magnetic flux density, B, from transmitter  4   
         [0032]    An important factor with the EMF error from loop  14  is that loop  14  may be located near transmitter  4  while sensor  1  may be near the outside limits of its range. Thus dB/dt at loop  14  may be orders of magnitude larger than the dB/dt at coil  13 . This could occur, for example, if an ultrasound operator positions computer  7  and connector  6  near the transmitter  4  due to space constraints in a procedure room. Prior art systems commonly place a restriction on the position of the connector  6  relative to the transmitter  4 , a common restriction being 0.6 meters of minimum separation. Prior art systems also commonly employ a magnetic shield around connector  6 , to decrease the dB/dt magnitude at loop  14 . Such a shield adds cost and bulk to connector  6 , and can cause distortion of the magnetic field transmitted by transmitter  4  if placed too closely. 
         [0033]    An additional source of EMF error is the net loop area of the printed circuit board traces, as the physical paths of the signal lines through amplifier  8  and into A/D  9  are separate. The loop formed by these printed circuit board traces is shown by trace area  15  in  FIG. 2 . Trace area  15  error is important because circuitry used to energize transmitter  4  is contained within computer  7  and there is commonly some leakage dB/dt from this circuitry. Since it is desirable to fit computer  7  into a small form factor, the spacing between this energizing circuitry and trace area  15  may be only a few tens of millimeters. This can result in a significant leakage dB/dt component being present at trace area  15 , giving: 
         [0000]    
       
         
           
             
               EMF 
               trace 
             
             = 
             
               
                 A 
                 trace 
               
               * 
               U 
               * 
               
                 
                    
                   B 
                 
                 
                    
                   t 
                 
               
             
           
         
       
     
         [0034]    A trace =trace loop area 
         [0035]    U=free space permeability 
         [0036]    dB/dt=time rate of change of the magnetic flux density, B, from transmitter  4   
         [0037]    Prior art systems protect area  15  using magnetic shielding and also attempt to locate the transmitter drive circuitry as far from area  15  as is practical. 
         [0038]    Once the signal from coil  13  is digitized by the A/D converter  9  it is no longer susceptible to dB/dt effects from transmitter  4  and is processed by processor  10 . 
         [0039]    The total signal at the input of the A/D converter  9  is thus: 
         [0000]      EMF total =EMF coil +EMF cable +EMF connector +EMF trace    
         [0040]    The last three terms of this equation are significant errors that need to be minimized. 
         [0041]    The above mentioned cable, connector and trace errors (EMF coil , EMF connector , EMF trace ) are minimized in the present invention by periodically switching the polarity of the noise signal. Referring to  FIG. 3 , in one embodiment of the present invention, a single signal channel magnetic position tracking system  120 , includes a magnetic sensor coil  13 , a connector  6 , a dual single-pole-double-throw (SPDT) analog switch  18 , an amplifier  8 , an analog to digital (A/D) converter  9 , a polarity control  11 , a multiplier  12  and a processor  10 . Coil  13  is connected to the amplifier  8  via a pair of twisted wires  5  and via connector  6 . The sensing signal passes through the SPDT analog switch  18 , the amplifier  8 , then through the A/D converter  9 , then through the multiplier  12  and then goes to processor  10 . Multiplier  12  also receives information from the polarity control  11 . Polarity control  11  controls the polarity of the sensor signal at the end of the coil terminals. Polarity control  11  is set to output a logic 0 or a logic 1. Logic 0 is interpreted by multiplier  12  and switch  14  as normal or non-inverting polarity (value=1) and logic 1 is interpreted as inverted polarity (value=−1). The effect of switch  18  and multiplier  12  is to negate the polarity of coil  13  as seen by the A/D converter  9 , and to simultaneously negate the data from the A/D converter  9  as seen by processor  10 . The net effect is that the signal from coil  13  as seen by processor  10  does not change sign regardless of the state of polarity control  11 . The error inputs, EMF cable , EMF connector , and EMF trace , however, change polarity at processor  10  in accordance with the state of polarity control  11 . 
         [0042]    Referring to  FIG. 4 , the state of polarity control  11  is synchronized with the operation of transmitter  4  so that it is logic 0 (non-inverting) during the first pulse A  19  and logic 1 (inverting) during the second pulse B  20 . EMF total  for the rising and falling edges of pulse  19  are integrated within processor  10  to produce an output proportional to 
         [0000]      EMF coil +EMF cable +EMF connector +EMF trace    
         [0043]    This equation is described in U.S. Pat. No. 6,172,499, the contents of which are expressly incorporated herein by reference. At the boundary between pulse  19  and pulse  20 , polarity control  11  is switched to logic 1 and multiplier  12  is set to negate data from A/D  9 . The EMF total  for the rising and falling edges of pulse B is integrated within processor  10  to produce an output proportional to 
         [0000]      EMF coil −EMF cable −EMF connector −EMF trace  
 
         [0044]    If we add the integral results from first pulse  19  and second pulse  20  and divide by two, the resulting average is an integral proportional only to EMF coil . Since the positions of computer  7 , connector  6 , and portions of cable  5  are relatively stable with respect to transmitter  4 , the magnitudes of EMF cable , EMF connector , and EMF trace  remain essentially constant during the pulse AB sequence. The present invention thus eliminates the need to shield loop  14 , area  15 , and eliminates gradient error from cable  5 . 
         [0045]    Placing a lowpass filter at the output of multiplier  12  can also accomplish the averaging function of the first pulse  19  and second pulse  20  sequence. The lowpass filter should be chosen such that the ripple at the output of multiplier  12  as an amplitude function of 
         [0000]      EMF cable +EMF connector +EMF trace    
         [0046]    is within acceptable limits and the system response bandwidth is adequately fast. For example, in a system employing the present invention, a 4th order infinite impulse response (IIR) filter, implemented in a digital signal processor (DSP), with a cutoff frequency of 2 Hz is adequate for a system employing a three axis transmitter  4  and a three axis sensor  1  operating at 240 transmitter pulses per second. 
         [0047]    In addition to magnetic EMF error cancellation, the present invention may also be employed to remove EMF errors from sources such as ground coupling. Current from computer  7  flowing into transmitter  4  may induce some resistive voltage drops within the conductors of computer  7 . One important conductor is the grounding system. Generally the circuitry will employ a ground plane on a printed circuit board. This ground plane generally has a small but measurable resistance, on the order of a milliohm for points a few centimeters apart. Imperfections in amplifier  8 , ground feedthrough from biasing circuitry, and numerous other parasitic sources can cause error signals to appear at the output of amplifier  8 . Collectively these EMF error sources are shown as circuit error source  17 . Source  17  will exhibit a reasonably constant response to each of pulse  19  and pulse  20  in  FIG. 4 . Due to the constant nature of this response, the multiplier  12  and polarity control  11  will cause the error from source  17  to be periodically inverted. The error source  17  is thus removable by averaging or lowpass filtering as previously described. 
         [0048]    Referring to  FIG. 5 , in another embodiment of the present invention, pulse  19  and pulse  20  are each comprised of multiple pulses. In this example, transmitter  4  is comprised of 3 orthogonal coils, referred to as X, Y, and Z respectively, energized sequentially. X axis pulse  21  represents the X coil excitation, Y axis pulse  22  represents the Y coil excitation, and Z axis pulse  23  represents the Z coil excitation. The combination of pulses  21 ,  22 , and  23  herein referred to first transmitter sequence  24  and second transmitter sequence  25 . Using the device described in U.S. Pat. No. 6,172,499, as an example, the response of sensor  1  to each of the pulses  21 ,  22 ,  23  in first sequence  24  is processed in the same manner as previously disclosed for first pulse  19  and stored. Next, polarity control  11  is switched and the response of sensor  1  to each of the pulses  21 ,  22 ,  23  in second sequence  25  is computed and averaged with the corresponding response values from first sequence  24 . The sequence of  FIG. 5  is useful because analog switch  18  may have some undesirable parasitic error effects on the output of coil  13 . One such effect is commonly known as charge injection. The injection components change amplitude and polarity synchronously with polarity control  11  and thus appear as a transient offset at the output of multiplier  12 . Introducing a short amount of dead time  26  between the first sequence  24  and the second sequence  25  will allow this transient offset to decay to zero before being sampled by processor  10 . 
         [0049]    The system of  FIG. 3 , may also be use for error reduction in an AC magnetic tracking system.  FIG. 6  shows a pictorial description of key waveforms present at the input of processor  10  when the system  120  of  FIG. 3  is operated to cancel transmitter induced offset signals in an AC magnetic tracking system. Transmitter  4  emits an AC magnetic field  27 . Sensor coil  13  outputs an EMF proportional to the time derivative of magnetic field  27  according to the formula 
         [0000]      EMF coil   =A*N*U*B *sin ω t  
 
         [0050]    A=area of coil  13  in square meters 
         [0051]    N=number of turns in coil  13   
         [0052]    U=free space permeability 
         [0053]    B=peak to peak magnitude of field, in Tesla 
         [0054]    ω=angular frequency of magnetic field, in radians per second 
         [0055]    t=time, in seconds 
         [0056]    Parasitic, unbalanced loops exposed to the magnetic field from transmitter  4  are added to the signal from coil  13  and the digitized signal at processor  10  is described as 
         [0000]      EMF total =(EMF coil +EMF cable +EMF connector +EMF trace )*sin ω t  
 
         [0057]    EMF coil  sin ωt=signal from coil  13  due to magnetic field from transmitter  4   
         [0058]    EMF cable  sin ωt=induced EMF due to gradient field of transmitter  4  acting on cable  5 . 
         [0059]    EMF connector  sin ωt=induced EMF in connector pin loop  14  due to magnetic field from transmitter  4 . 
         [0060]    EMF trace  sin ωt=induced EMF from printed circuit board trace loops 
         [0061]    EMF trace  sin ωt=induced EMF in trace area  15  due to magnetic field from transmitter  4   
         [0062]    Ideally, EMF coil  sin ωt would be the only signal digitized by the A/D converter  9  and processed by processor  10  and by a demodulator. EMF cable  sin ωt, EMF connector  sin ωt, and EMF trace  sin ωt are undesireable signals. 
         [0063]    The total signal at the A/D converter  9  due to transmitter  4  is described as 
         [0000]      EMF total =(EMF coil +EMF cable +EMF connector +EMF trace )*sin ω t  
 
         [0064]    After demodulation and detection in processor  10 , the value corresponding to EMF total  is stored and the polarity control  11  is switched. The output of the A/D converter  9  is then equal 
         [0000]      EMF total =(EMF coil −EMF cable −EMF connector −EMF trace )*sin ω t  
 
         [0065]    Demodulating and detecting this second sequence and averaging with the stored result from the first results in an output value proportional only to EMF coil . It should be noted that it is not required that the AC magnetic field  27  be continuous, nor fixed in frequency. The technique shown will work with time division multiplexed AC magnetic fields, and with fixed, variable, or multiple frequencies. 
         [0066]    In the embodiment of  FIG. 6 , the gain of amplifier  8  was set to unity to simplify the expressions. The waveforms are shown in continuous time format for clarity purposes, although in actuality the waveforms shown in  FIG. 6  are discrete digital values output by the A/D converter  9 .  FIG. 6 , assumes that the sampling rate of the A/D converter  9  is high enough to accurately capture the details shown. 
         [0067]    The embodiment of  FIG. 3  may be employed on numerous other signal transmission methods used in magnetic tracker art by employing the following principals: 
         [0068]    1) Define a measurement sequence, including magnetic transmitter excitations and receipt of magnetic signals from sensor coils. 
         [0069]    2) Feeding coil signals into a switching array capable of reversing the coil polarity relative to subsequent interconnect and processing elements. The switching array should be located such that parasitic loops are located between the switching array and the A/D converter. 
         [0070]    3) Controlling the switching array such that the processor receiving A/D data inverts the data synchronously with coil polarity changes at the output of the switching array. 
         [0071]    4) Alternating the polarity of the switching array and A/D sign inversion such that these operations are synchronous with the defined magnetic transmitter excitation sequences. 
         [0072]    5) Averaging alternate sign inverted processed data sequences such that the offset components cancel, or alternatively low pass filtering the processed data sequence, or alternatively storing a sequence of a first polarity, subtracting a sequence of opposing polarity, and utilizing the remainder offset value to correct future readings. 
         [0073]    Several embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.