Abstract:
An electromagnetic needle tracking system includes a needle assembly, a calibration system and a computing system. The needle assembly includes a needle stylet and a sensor assembly. The sensor assembly includes a sensor that measures position and angular orientation data when placed within an electromagnetic field. The calibration system measures the sensor&#39;s position and angular orientation for a known needle tip position and angular orientation within a calibration fixture and calculates a position offset and an angular orientation offset of the sensor relative to the needle tip position and angular orientation. The computing system computes position and angular orientation data of the needle tip by adding the sensor position offset and angular orientation offset to the measured position and angular orientation data, respectively.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a magnetically tracked needle assembly, and more particularly to a disposable, magnetically tracked needle assembly, that includes a sensor coaxially inserted into a disposable stylet. 
       BACKGROUND OF THE INVENTION 
       [0002]    Magnetic tracking of instruments with respect to imaged anatomy is widely employed in medical practice. Imaging systems that are enhanced with magnetic tracking may be used to track and display position and orientation of a biopsy needle assembly relative to the imaging plane. They help the clinician guide the instrument to a chosen target with reduced error compared to an unguided biopsy needle. Furthermore, the visual representation of the tracked biopsy needle is not constrained to the ultrasound imaging plane, thus enabling the clinician with more freedom of motion. 
         [0003]    For magnetic tracking of an instrument, an electromagnetic sensor is included in a location of the instrument. Electromagnetic sensors are usually electromagnetic coils that surround or are close to the objects whose location is being tracked. When the instruments with the included sensors are placed within a varying electromagnetic field a voltage is generated in the electromagnetic sensor. This generated voltage is used to determine and track the locations and relative positioning of the instrument, within the electromagnetic field. Ultrasound system enhanced with magnetic tracking of sensors, displays 3-dimensional merger of ultrasound generated anatomical features and the visual representation of the instrument position and orientation. 
         [0004]    Prior art magnetic tracking systems require that the sensor be located in a predetermined location inside the instrument. This is accomplished by precisely controlling the physical location of the sensor within a tube containing the instrument. Such precision increases the complexity and the time required to manufacture the sensor assembly. These prior art tracking system usually do not provide a way to correct small manufacturing errors which are outside of the allowed positioning tolerances. Prior art tracking systems also do not provide a way for correcting the angular misalignment caused by tolerances in the sliding fit between the sensor assembly and the elongated instrument. This orientation error is generally greater than that inherent in the magnetic tracking system and results in the system displaying an incorrect trajectory for the instrument. 
         [0005]    Medical procedures are increasingly cost driven. Magnetic tracking, while improving procedure outcomes, adds a cost element to the equipment. Labor cost associated with assembly and test of needle increases with design and process complexity. The needle sensor position and orientation relative to the stylet tip position and orientation is unique, with calibration data associated with each needle assembly produced. Overhead costs associated with serial number traceability maintenance of this needle assembly sensor calibration data are hidden, but significant. 
         [0006]    It is desirable to have an electromagnetically tracked needle that is cost effective, provides accurate needle position and angular orientation and does not require repeated sterilization. 
       SUMMARY OF THE INVENTION 
       [0007]    This invention relates to electromagnetic tracking of medical instruments, specifically tracking of minimally invasive instruments such as biopsy needles, ablation instruments, and various other instruments for which the end of a cannula must be precisely placed to enable diagnostic or interventional procedures at the distal end of the cannula. Such procedures are often performed using an imaging modality such as ultrasound, where electromagnetic tracking of the instrument allows spatial location and visualization of anatomy and instrument location. 
         [0008]    In general, in one aspect, the invention features an electromagnetic needle tracking system including a needle assembly, a calibration system and a computing system. The needle assembly includes a needle stylet and a sensor assembly. The needle stylet includes an elongated hollow tube having an open proximal end and a distal end comprising a needle tip. The sensor assembly includes an elongated body and a sensor attached to the elongated body and the elongated body is shaped and dimensioned to be inserted into the elongated hollow tube. The sensor is configured to measure position and angular orientation data when placed within an electromagnetic field. The calibration system includes a calibration fixture and is configured to measure the sensor&#39;s position and angular orientation for a known needle tip position and angular orientation within the calibration fixture and to calculate a position offset and an angular orientation offset of the sensor relative to the needle tip position and angular orientation. The computing system computes position and angular orientation data of the needle tip by adding the sensor position offset and angular orientation offset to the measured position and angular orientation data, respectively. 
         [0009]    Implementations of this aspect of the invention may include one or more of the following features. The system may further include a non-volatile storage circuitry configured to store the calculated sensor position and angular orientation offsets. The needle stylet further includes a stylet receiver attached to the proximal end of the elongated hollow tube and the needle assembly further includes means for attaching the elongated body&#39;s proximal end to the stylet receiver. The elongated body&#39;s proximal end is attached to the stylet receiver with an adhesive. The adhesive may be cyanoacrylate, epoxy, hot melt or solvent bonding. The stylet receiver includes a cavity and the cavity is tapered. The sensor assembly further includes an insulated cable and a pair of twisted insulated wires connected to the distal end of the insulated cable. The distal end of the insulated cable is inserted in the receiver cavity and the proximal end is connected to the computing system. The tapered cavity provides a hard stop for the inserted distal end of the insulated cable. The stylet receiver includes a cavity extending coaxially with the elongated hollow tube. The stylet receiver includes a cavity extending parallel to but offset from the elongated hollow tube. The assembly may further include an outer cannula and the needle stylet is configured to be inserted into the outer cannula. 
         [0010]    In general, in another aspect, the invention features a needle assembly including a needle stylet, a sensor assembly and a computing system. The needle stylet includes an elongated hollow tube and a needle. The elongated hollow tube extends along a first axis and has an open proximal end and a closed distal end. The needle is attached to the closed distal end of the elongated hollow tube and has a tip end that extends a first distance from the closed distal end of the elongated hollow tube along the first axis. The sensor assembly includes an elongated body extending along a second axis, and a sensor placed at a second distance from the distal end of the elongated body. The elongated body is shaped and dimensioned to be inserted into the elongated hollow tube so that the distal end of the elongated body is placed in contact with the closed distal end of the elongated hollow tube. The sensor is configured to measure position and angular orientation data when placed within an electromagnetic field. The computing system computes the position and angular orientation of the tip end of the needle by adding the sum of the first and the second distances to the measured position data and by adding the angular difference between the first and second axes to the measured angular orientation data, respectively. The assembly may further include a non-volatile storage circuitry configured to store calibration data comprising the first and second distances, the sum of the first and second distances, and the angular difference between the first and second axes. 
         [0011]    In general, in another aspect, the invention features a needle assembly including a needle stylet, a sensor assembly and a computing system. The needle stylet includes an elongated hollow tube and a needle. The elongated hollow tube extends along a first axis and has an open proximal end and a closed distal end. The needle is attached to the closed distal end of the elongated hollow tube and has a tip end that extends a first distance from the closed distal end of the elongated hollow tube along the first axis. The sensor assembly includes an elongated body extending along a second axis, and a sensor located at a second distance from the distal end of the elongated body. The elongated body is shaped and dimensioned to be inserted into the elongated hollow tube so that the distal end of the elongated body is placed in contact with the closed distal end of the elongated hollow tube. The sensor is configured to measure position and angular orientation data when placed within an electromagnetic field. The system also includes means for fixing the sensor&#39;s angular orientation within the elongated hollow tube to be coaxial with the first axis. The computing system computes the position of the tip end of the needle by adding the sum of the first and the second distances to the measured position data. 
         [0012]    Implementations of this aspect of the invention may include one or more of the following features. The means for fixing the sensor&#39;s angular orientation includes a sleeve of hot-melt plastic and the sleeve is configured to be positioned around the elongated body and to be tacked to the elongated body by heating. The sensor&#39;s angular orientation is fixed to be coaxial with the first axis by iteratively heating and melting the sleeve of hot-melt plastic, orienting the elongated body, cooling and solidifying the sleeve of hot-melt plastic and measuring the resulting angular difference between the first and second axes until the elongated body is coaxial with the elongated hollow tube. The sensor may be a magnetic sensor and the elongated body is oriented within the elongated hollow tube by applying a magnetic force. The means for fixing the sensor&#39;s angular orientation include first and second heat-shrink rings. The first and second heat-shrink rings are positioned coaxially and around the sensor&#39;s first and second ends, respectively, and subsequently the sensor assembly is inserted into the elongated hollow tube and the heat-shrink rings are heated at a controlled temperature and for a controlled time period until the outer diameter of the heat-shrink rings expands to be slightly smaller than the inner diameter of the elongated hollow tube, and thereby orienting and fixing the elongated body coaxially with the elongate hollow tube. The outer surface of each of the first and second heat-shrink rings has a groove and the groove is oriented parallel to the ring&#39;s axis. 
         [0013]    In general, in another aspect, the invention features a needle assembly including a needle stylet, a sensor assembly and a computing system. The needle stylet includes an elongated hollow tube and a needle. The elongated hollow tube extends along a first axis and has an open proximal end and a closed distal end. The needle is attached to the closed distal end of the elongated hollow tube and has a tip end that extends a first distance from the closed distal end of the elongated hollow tube along the first axis. The sensor assembly includes an elongated body, a sensor located at the distal end of the elongated body and a stop-plug configured to be placed over the elongated body&#39;s distal end. The elongated body is shaped and dimensioned to be inserted into the elongated hollow tube so that the distal end of the stop-plug is in contact with the closed distal end of the elongated hollow tube. The stop-plug has an outer diameter slightly smaller than the inner diameter of the elongated hollow tube and is configured to orient the elongated body coaxially with the elongated hollow tube. The stop-plug has an inner diameter slightly larger than the outer diameter of the elongated body and is configured to receive and place the elongated body&#39;s distal end at a second distance from the stop-plug&#39;s distal end. The computing system computes the position of the tip end of the needle by adding the sum of the first and the second distances to the measured position data. 
         [0014]    Implementations of this aspect of the invention may include one or more of the following features. The outer surface of the stop-plug comprises a groove and the groove is oriented parallel to the stop-plug&#39;s axis. The needle stylet further includes a stylet receiver attached to the proximal end of the elongated hollow tube and the needle assembly further includes means for attaching the elongated body&#39;s proximal end to the stylet receiver. 
         [0015]    In general, in another aspect, the invention features method for electromagnetic tracking of a needle including providing a needle assembly, providing a calibration system and providing a computing system. The needle assembly includes a needle stylet and a sensor assembly. The needle stylet includes an elongated hollow tube having an open proximal end and a distal end comprising a needle tip. The sensor assembly includes an elongated body, and a sensor attached to the elongated body. The elongated body is shaped and dimensioned to be inserted into the elongated hollow tube, and the sensor is configured to measure position and angular orientation data when placed within an electromagnetic field. The calibration system includes a calibration fixture and is configured to measure the sensor&#39;s position and angular orientation for a known needle tip position and angular orientation within the calibration fixture and to calculate a position offset and an angular orientation offset of the sensor relative to the needle tip position and angular orientation. The computing system computes position and angular orientation data of the needle tip by adding the sensor position offset and angular orientation offset to the measured position and angular orientation data, respectively. 
         [0016]    In general, in another aspect, the invention features a method for electromagnetic tracking of a needle including providing a needle stylet, providing a sensor assembly and providing a computing system. The needle stylet includes an elongated hollow tube and a needle and the elongated hollow tube extends along a first axis and has an open proximal end and a closed distal end. The needle is attached to the closed distal end of the elongated hollow tube and has a tip end that extends a first distance from the closed distal end of the elongated hollow tube along the first axis. The sensor assembly includes an elongated body extending along a second axis, and a sensor placed at a second distance from the distal end of the elongated body. The elongated body is shaped and dimensioned to be inserted into the elongated hollow tube so that the distal end of the elongated body is placed in contact with the closed distal end of the elongated hollow tube. The sensor is configured to measure position and angular orientation data when placed within an electromagnetic field. The computing system computes the position and angular orientation of the tip end of the needle by adding the sum of the first and the second distances to the measured position data and by adding the angular difference between the first and second axes to the measured angular orientation data, respectively. 
         [0017]    In general, in another aspect, the invention features a method for electromagnetic tracking of a needle including providing a needle stylet, providing a sensor assembly and providing a computing system. The needle stylet includes an elongated hollow tube and a needle. The elongated hollow tube extends along a first axis and has an open proximal end and a closed distal end. The needle is attached to the closed distal end of the elongated hollow tube and has a tip end that extends a first distance from the closed distal end of the elongated hollow tube along the first axis. The sensor assembly includes an elongated body extending along a second axis, and a sensor located at a second distance from the distal end of the elongated body. The elongated body is shaped and dimensioned to be inserted into the elongated hollow tube so that the distal end of the elongated body is placed in contact with the closed distal end of the elongated hollow tube. The sensor is configured to measure position and angular orientation data when placed within an electromagnetic field. The computing system computes the position of the tip end of the needle by adding the sum of the first and the second distances to the measured position data. The method also includes fixing the sensor&#39;s angular orientation within the elongated hollow tube to be coaxial with the first axis. The sensor&#39;s angular orientation may be fixed via a sleeve of hot-melt plastic and the sleeve is configured to be positioned around the elongated body and to be tacked to the elongated body by heating. The sensor&#39;s angular orientation may be fixed via first and second heat-shrink rings. The first and second heat-shrink rings are positioned coaxially and around the sensor&#39;s first and second ends, respectively, and subsequently the sensor assembly is inserted into the elongated hollow tube and the heat-shrink rings are heated at a controlled temperature and for a controlled time period until the outer diameter of the heat-shrink rings expands to be slightly smaller than the inner diameter of the elongated hollow tube, and thereby orienting and fixing the elongated body coaxially with the elongate hollow tube. 
         [0018]    In general, in another aspect, the invention features a method for electromagnetic tracking of a needle including providing a needle stylet, providing a sensor assembly and providing a computing system. The needle stylet includes an elongated hollow tube and a needle. The elongated hollow tube extends along a first axis and has an open proximal end and a closed distal end. The needle is attached to the closed distal end of the elongated hollow tube and has a tip end that extends a first distance from the closed distal end of the elongated hollow tube along the first axis. The sensor assembly includes an elongated body, a sensor located at the distal end of the elongated body and a stop-plug configured to be placed over the elongated body&#39;s distal end. The elongated body is shaped and dimensioned to be inserted into the elongated hollow tube so that distal end of the stop-plug is in contact with the closed distal end of the elongated hollow tube. The stop-plug has an outer diameter slightly smaller than the inner diameter of the elongated hollow tube and is configured to orient the elongated body coaxially with the elongated hollow tube. The stop-plug has an inner diameter slightly larger than the outer diameter of the elongated body and is configured to receive and place the elongated body&#39;s distal end at a second distance from the stop-plug&#39;s distal end. The computing system computes the position of the tip end of the needle by adding the sum of the first and the second distances to the measured position data. 
         [0019]    Among the advantages of this invention may be one or more of the following. The invention provides a cost-reduced sensor assembly, where costs associated with assembly labor, traceability overhead, high inventory mix, and component materials are minimized. The low cost of the sensor assembly enables a single-use protocol, thus eliminating the procedural cost, borne by the end-user, of repeated sterilization for re-uses. The invention also provides a single-use disposable needle configuration, that eliminates the substantial cost of repeatedly sterilizing the sensor assembly. The invention also provides means for correcting the angular misalignment caused by tolerances in the sliding fit between the sensor assembly and the elongated instrument, or stylet axis. 
         [0020]    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 
         [0021]    Referring to the figures, wherein like numerals represent like parts throughout the several views: 
           [0022]      FIG. 1  illustrates an ultrasound imaging system enhanced with magnetic instrument tracking; 
           [0023]      FIG. 2A  is an exploded perspective diagram of a magnetically tracked, disposable needle assembly, according to this invention; 
           [0024]      FIG. 2B  and  FIG. 2C  are magnified views of two different embodiments of the stylet receiver (area A of  FIG. 2A ), according to this invention; 
           [0025]      FIG. 2D-FIG .  2 G are magnified views of four different sensor configuration embodiments (area B of  FIG. 2A ), according to this invention; 
           [0026]      FIG. 3A-FIG .  3 C are schematic views of the components and methods for measuring, positioning and orientating the electromagnetic sensor of  FIG. 2D  relative to the stylet tip of  FIG. 2A ; 
           [0027]      FIG. 4A-FIG .  4 D are schematic views of the components and methods for measuring, positioning and orientating the electromagnetic sensor of  FIG. 2E  in a stylet tip of  FIG. 2A ; 
           [0028]      FIG. 5A-FIG .  5 D are schematic views of the components and methods for measuring, positioning and orientating the electromagnetic sensor of  FIG. 2F  in a stylet tip of  FIG. 2A ; 
           [0029]      FIG. 6A-6D  are schematic views of the components and methods for measuring, positioning and orientating the electromagnetic sensor of  FIG. 2G  in a stylet tip of  FIG. 2A ; 
           [0030]      FIG. 7A  is a schematic perspective view of the press-fit capture of the sensor cable in the coaxial stylet receiver cavity of  FIG. 2B ; 
           [0031]      FIG. 7B  is a magnified view of area C in  FIG. 7A ; 
           [0032]      FIG. 7C  is a magnified view of area D in  FIG. 7A ; 
           [0033]      FIG. 8A  is a schematic perspective view of the press-fit capture of the sensor cable and fixing of the sensor tube in an axially offset receiver cavity of  FIG. 2C ; 
           [0034]      FIG. 8B  is a magnified view of area E in  FIG. 8B ; 
           [0035]      FIG. 9  is a schematic perspective view of the fully assembled disposable needle, according to this invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0036]    In a biopsy application, precise knowledge of needle tip position and orientation is critical. In such applications, it is optimal to locate the magnetic sensor as close to the distal end of the needle as is practical, because the needle flexes while penetrating anatomy. Furthermore, care must be taken to ensure that pathogens are not passed from patient to patient using this assembly, thus component sterilization and reprocessing are required if the sensor assembly is to be re-used. A single-use disposable configuration, eliminates the substantial cost to repeatedly sterilize the sensor-assembly. The invention described herein is motivated by the need for a cost-reduced sensor assembly, where costs associated with assembly labor, traceability overhead, high inventory mix, and component materials are minimized. The low cost of the sensor assembly enables a single-use protocol, thus eliminating the procedural cost, borne by the end-user, of repeated sterilization for re-uses. 
         [0037]    The present invention addresses the problem of positioning and orientating the magnetic sensor relative to a stylet tip. Furthermore, the present invention addresses the problem of fastening the sensor cable and sensor tube proximal end into a stylet receiver. 
         [0038]    Referring to  FIG. 1 , imaging tools, such as ultrasound system  30 , are used to image detailed anatomical features in a spatial slice (or imaging plane)  36 . Ultrasound system  30  includes a hand-held probe  34 , a display  32   a  and electronics  32   b . For magnetic tracking of an instrument  190  with ultrasound system  30 , electromagnetic sensors  34   a  and  52   f  are included in the hand-held ultrasound probe  34  and in a location of instrument  190 , respectively. Sensors  34   a  and  52   f  are usually electromagnetic coils that surround or are close to the objects whose location is being tracked. In the example of  FIG. 1 , instrument  190  is a needle assembly and sensor  52   f  is close to the needle tip  190   a . When sensor  52   f  is placed within a varying electromagnetic field a voltage is generated in the electromagnetic sensor  52   f . Similarly, when hand-held ultrasound probe  34  with the embedded sensor  34   a  is placed within the varying electromagnetic field a voltage is generated in the electromagnetic sensor  34   a . These generated voltages in sensors  34   a ,  52   f  are used to determine and track the locations and relative positioning of ultrasound probe  34  and needle tip  190   a , respectively, within the electromagnetic field. Ultrasound system  30  enhanced with magnetic tracking of sensors  34   a    52   f , displays the 3-dimensional merger of ultrasound generated anatomical features  36   a  and the visual representation of the instrument position and orientation  190   a.    
         [0039]    Referring to  FIG. 2A , needle assembly  190  includes an outer cannula  58 , a stylet  54 , and an electromagnetic sensor assembly  52 . Outer cannula  58  includes an elongated tube  58   c  having a plastic receiver  58   b  at the proximal end  58   e  and a cannula tip  58   a  at the distal end  58   f . Outer cannula  58  also includes an axial though-opening  58   d  extending the entire length of the elongated tube  58   c . Through-opening  58   d  is shaped and dimensioned to match and receive the stylet  54 . Cannula tip  58   a  is often a spade or a truncated cone, as shown in  FIG. 2A . The truncated cone tip form  58   a  is common for soft-tissue biopsy applications and is dimensioned to follow the stylet tip  54   a  during tissue insertion. 
         [0040]    Stylet  54  includes an elongated hollow tube  54   c  having a stylet receiver/handle  54   g  at the proximal end  54   e  and a sharp tip  54   a  at the distal end  54   f . Stylet  54  also includes an axial opening  54   d . Opening  54   d  is shaped and dimensioned to receive the sensor assembly  52 . In one example, hollow tube  54   c  is made of stainless steel and sharp tip  54   a  is hardened, sharpened and welded to the distal end of the hollow tube  54   c . The fabrication process of sharp tip  54   a  slightly magnetizes the tip. Sharp tip  54   f  is available in many forms and shapes, for instance a trocar tip (as shown in area B in  FIG. 2A ), a spade tip, or a conical tip. 
         [0041]    Sensor assembly  52  includes a sensor tube  52   e  having an electromagnetic sensor  52   f  at its distal end  52   k  and an insulated sensor cable  52   a  connected to its proximal end  521 . The delicate sensor assembly  52  is inserted inside opening  54   d  of the hollow stylet tube  54   c . Electromagnetic sensor  52   f  must be positioned at a minimum distance form the magnetized tip  54   a  in order to avoid distortions in the position and orientation readings. Sensor cable  52   a  includes a twisted pair of insulated wires  52   c . The length of cable  52   a  depends upon its intended use. In one example, cable  52   a  has a nominal length of 3 meters. The twisted pair of wires  52   c  is fragile, yet necessarily unconstrained between their exit point at the end of cable  52   a  and the entrance to the sensor tube  52   e . The sensor cable  52   a  also includes a strength member  52   b  that extends a short distance from the cable distal end. In one example, strength member  52   b  is made of Kevlar filament. The cable cross section also features an insulating layer  52   g . The sensor assembly  52  also includes a connector  52   d  at the proximal end of cable  52   a . Connector  52   d  connects the cable  52   a  to the tracking electronics  32   b , as shown in  FIG. 1 . Connector  52   d  also contains non-volatile storage circuitry  52   h  that is used for storing sensor  52   f  to stylet tip  54   a  position and orientation offset calibration data. Sensor tube  52   e  is rigid to compressive force along its axis, but bends like a spline when a point force is applied normal to its axis. In one example, sensor tube  52   e  is made of Polyamide. As was mentioned above, sensor tube  52   e  contains the magnetic sensor  52   f  at its distal end and the twisted pair of wires  52   c  that electrically connect to the sensor  52   f  wires. The sensor tube  52   e  is cut to a precise length per the known length of the stylet cavity  54   d  and the outer diameter of the sensor tube  52   e  is less than the inner diameter of the stylet cavity  54   d.    
         [0042]    Referring to  FIG. 2B  and  FIG. 2C , receiver  54   g  is made of clear or opaque plastic and has a press-fit design with internal surfaces and form that receive and fit snugly around the distal end of the inserted cable  52   a . In some embodiments, receiver  54   g  includes gripping features and features for connecting to a press-handle (not shown) for high-force insertion medical procedures. Receiver  54   g  also includes a male friction lock feature  56   e ,  59   e  for fixing and stopping the stylet  54  when fully inserted into the cannula  58 . The plastic receiver  58   b  of the outer cannula  58 , functions as the female friction lock to the stylet receiver male friction lock features  56   e ,  59   e . The outer diameter of the stylet  54  is less than the inner diameter of the cannula  58  opening  54   d . In the embodiment  56  of FIG.  2 B, receiver  54   g  has a central axial opening  54   d  and the in the embodiment of  59  of  FIG. 2C , receiver  54   g  has an off-axis opening  54   d.    
         [0043]      FIG. 2D-FIG .  2 G, depict four different sensor configuration embodiments including compensated sensor configuration  70 , manipulated sensor configuration  90 , blind-set shim-rings sensor configuration  110 , and blind-set stop-plug sensor configuration  130 , respectively. In the compensate sensor configuration  70 , the end  52   m  of sensor  52   f  is positioned at a distance t from the end  52   k  of sensor tube  52   e . In the manipulated sensor configuration  90 , the end  52   m  of sensor  52   f  is positioned at a distance t from the end  52   k  of sensor tube  52   e  and a sleeve of hot-melt plastic  92  is positioned around the sensor  52   f , as shown in  FIG. 2E . In the blind-set shim-rings sensor configuration  110 , the end  52   m  of sensor  52   f  is positioned at a distance t from the end  52   k  of sensor tube  52   e  and two heat shrink rings  112   a ,  112   b  are positioned separately and coaxially around the two ends  52   m ,  52   n  of the sensor  52   f , as shown in  FIG. 2F . In the blind-set stop-plug sensor configuration  130 , the end  52   m  of sensor  52   f  is positioned to coincide with the end  52   k  of sensor tube  52   e  and a stop-plug  132  is positioned over the ends  52   k  and  52   m , as shown in  FIG. 2G . All of these sensor configuration embodiments provide cost reduced apparatuses and methods for positioning and orienting the sensor  52   f  with respect to the stylet tip  54   a  and stylet axis  56   a , respectively, where the diameter of the sensor tube  52   e  is significantly smaller than the diameter of the stylet cavity  54   b . Cost reduction is realized because one sensor tube  52   e  outer diameter and length is applicable to a wide range of larger stylet cavity  54   b  dimensions. This reduction in the sensor assembly product mix enables economy of scale effects in the cost of the sensor assembly models. TABLE 1 shows the attributes associated with each of the configuration options. 
         [0000]    
       
         
               
             
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Sensor Configuration Application Matrix 
               
             
          
           
               
                   
                 Apparatus/Method 
               
             
          
           
               
                   
                   
                   
                 Blind-set 
                 Blind-set 
               
               
                   
                 Compensated 
                 Manipulated 
                 Shim-Rings 
                 Stop-Plug 
               
               
                 Attribute 
                 70 
                 90 
                 110 
                 130 
               
               
                   
               
               
                 Small Batch 
                 X 
                 X 
                   
                   
               
               
                 Large Batch 
                   
                   
                 X 
                 X 
               
               
                 Compensated 
                 X 
               
               
                 offsets 
               
               
                 Corrected 
                   
                 X 
                 X 
                 X 
               
               
                 offsets 
               
               
                 Offset 
                 100% 
                 100% 
                 Sampled 
                 Sampled 
               
               
                 calibration 
               
               
                 Non-volatile 
                 X 
                 Optional 
               
               
                 offset storage 
               
               
                   
               
             
          
         
       
     
         [0044]    Referring to TABLE 1, small batch, low volume, fabrication of the disposable sensor sub-assembly does not apply to precision blind-set components such as shim-rings  112  and stop-plugs  132 . Small batches are semi-custom and require the manual means of compensating  70  and/or manipulating  90  the sensor  52   f  position and orientation with respect to the stylet tip  54   a . Conversely, large batch, high volume, fabrication cost-justifies use of the blind-set components. For large batch (high unit count) fabrication, the blind-set components are available, affordable, and stocked in quantity. 
         [0045]      FIG. 3A-FIG .  3 C illustrate the offset compensation method for small batch disposable needle fabrication  70 . Referring to  FIG. 3C , the sensor tube  52   e  distal end  52   k  is trimmed to position the sensor end  52   m  at distance, t, from the sensor tube distal end  52   k . The distance, t, is large enough to separate the sensor  52   f  from the slightly magnetized stylet tip  54   a . Referring to  FIG. 3A , the sensor tube  52   e  is inserted fully in the stylet cavity  54   b  and fastened at the proximal end  54   e  of the stylet  54 . Referring to  FIG. 3B , the resulting assembly of the sensor  52   f  and the stylet  54  is placed in a calibration fixture (not shown) where the sensor position offset, m, and sensor orientation offset, μ are measured and recorded. The sensor position offset m is the distance of the end  52   m  of sensor  52   f  from the stylet tip end  54   m . The sensor orientation offset μ is the angle between the stylet axis  56   a  and the sensor tube axis  52   p . The sensor position offset m and the sensor orientation offset μ are used as calibration values and are stored in the sensor sub-assembly nonvolatile storage circuitry  52   h . The stored calibration values are read by the tracking electronics  32   b  and applied as compensation during disposable needle tracking. 
         [0046]      FIG. 4A-FIG .  4 D illustrate the sensor manipulation  90  method for small batch disposable needle fabrication. Referring to  FIG. 4D , the sensor tube  52   e  distal end  52   k  is trimmed to position the sensor end  52   m  at distance t, from the sensor tube distal end  52   k . The distance, t, is large enough to separate the sensor from the slightly magnetized stylet tip  54   a . A preformed sleeve of hot-melt plastic  92  is positioned around and tacked to the sensor tube  52   e  with a heat-gun  94 , shown in  FIG. 4B . Referring to  FIG. 4A , the sensor tube  52   e  is inserted fully in the stylet cavity  54   b  and fastened at the proximal end  54   e  of the stylet  54 . Referring to  FIG. 4B  the resulting assembly of the sensor  52   f  and the stylet  54  is placed in a calibration fixture (not shown) where the sensor position offset m, and sensor orientation offset μ, calibration values are measured while the stylet tip  54   a  is heated with a heat-gun  94  to melt and flow the hot-melt sleeve  92 . Concurrently, a magnet  96  is introduced to induce a manipulating force on the highly magnetic sensor  52   f . The sensor  52   f  is iteratively manipulated with the magnet  96  until the sensor orientation offset μ is measured to be zero degrees. Referring to  FIG. 4C , when the sensor orientation offset μ is set to zero degrees the heat-gun is removed and the hot-melt solidifies to fix the sensor coaxially to the stylet  54 . The last iteration calibration position and orientation offset data, although very close to m=t+d and μ=zero degrees, respectively, can optionally be written to the sensor nonvolatile storage circuitry  52   h . This small batch manipulation method  90 , corrects the sensor offsets, and is preferable to the small batch compensation method  70  if the end-user requires the sensor to be physically coaxial with the stylet. 
         [0047]      FIG. 5A-FIG .  5 D illustrate the sensor blind-set method  110  used for large batch disposable needle fabrication. Referring to  FIG. 5C , the sensor tube  52   e  distal end  52   k  is trimmed to position the sensor end  52   m  at a distance, t, from the sensor tube distal end  52   k . The distance, t, is large enough to separate the sensor  52   f  from the slightly magnetized stylet tip  54   a . At least two preformed heat shrink rings  112   a ,  112   b  are positioned separately and coaxially on the distal end of the sensor tube  52   e  so that they around the ends  52   m ,  52   n  of the sensor  52   f . The rings  112   a ,  112   b  are heated using conventional means, such as a hole-in-hot-block fixture, and collapsed to a tight fit around the sensor tube  52   e . The heating time and temperature are precisely controlled, to achieve a predetermined outer diameter, φ 1 , of the heat shrink ring  112   a ,  112   b . The outer diameter, φ 1 , is slightly smaller than the diameter of the intended stylet cavity  54   b . Each heat shrink ring  112   a ,  112   b  is scored  112   c  parallel to its axis, to enable pass-by of trapped air as the sensor tube  52   e  is inserted into the stylet cavity. Referring to  FIG. 5A , the sensor tube  52   e  is inserted into the stylet cavity  54   a  to maximum depth. The maximum depth is confirmed by visual inspection of the position of the proximal end  521  of the sensor tube relative to the proximal end  54   e  of the stylet cavity. The sensor tube  52   e  is fastened by means described herein at the proximal end of stylet  54 . Referring to  FIG. 5B , the sensor  54   f  is now fixed at a known distance, m=t+d, from the stylet tip end  54   m  and is coaxially oriented to the stylet axis. A random sample subset of each batch of disposable needle assemblies fabricated with this method  110  may be measured to ensure the expected position and orientation offsets are in fact m=t+d and μ=zero degrees, respectively. With this blind-set method  110 , there is no need to store calibration offset data in the nonvolatile storage circuitry  52   h  because the resulting offset data are known and invariable for each stylet model. 
         [0048]      FIG. 6A-FIG .  6 D, illustrates the sensor blind-set method  130  used for large batch disposable needle fabrication. Referring to  FIG. 6C , the sensor tube  52   e  distal end  52   k  is trimmed to coincide with the distal end  52   m  of the sensor  52   f  and then a stop-plug  132  is placed over it. Stop-plug  132  is selected so that the outer diameter,  12 , is slightly smaller than the inner diameter of the stylet cavity  54   b , and the inner diameter, θ, is slightly larger than the outer diameter of sensor tube  52   e . The stop-plug  132  receives the distal end  52   k  of the sensor tube  52   e  to a maximum stop depth and thereby the sensor  52   f  tip  52   m  is placed at a known distance, p, from the distal end  132   b  of the stop-plug  132 . The distance, p, is large enough to separate the sensor from the slightly magnetized stylet tip  54   a . Optionally, a small amount of quick set-adhesive is applied to fix the sensor tip  52   m  in the stop-plug  132 . The stop-plug  132  features a groove  132   a  parallel to its axis, to enable pass-by of trapped air as the sensor tube  52   e  is inserted into the stylet cavity. Referring to  FIG. 6A  the sensor tube  52   e  is inserted into the stylet cavity  54   b  to maximum depth. The maximum depth is confirmed by visual inspection of the position of the proximal end  521  of the sensor tube  52   e  relative to the proximal end  54   e  of the stylet cavity. The sensor tube is fastened by means described herein at the proximal end of stylet  54 . Referring to  FIG. 6B , the sensor  54   f  is now fixed at a known distance, n=p+d, from the stylet tip  54   a  and is coaxially oriented to the stylet axis  56   a . A random sample subset of each batch of disposable needle assemblies fabricated with this method  130  may be measured to ensure the expected position and orientation offsets are in fact n=p+d and μ=zero degrees, respectively. With this blind-set method  130 , there is no need to store calibration offset data in the nonvolatile storage circuitry  52   h  because the resulting offset data are known and invariable for each stylet model. 
         [0049]      FIG. 7A  and  FIG. 8A  depict two stylet receiver configuration apparatuses and methods  150 ,  170 , respectively, for securing and positioning the sensor assembly cable  52   a  in the receiver  56 . The receiver cavity axis  56   d  in configuration  150  is coaxial to the stylet cavity  54   b , as shown in  FIG. 7A . The receiver cavity axis  59   d  in configuration  170  is parallel to but offset from the stylet cavity axis, as shown in  FIG. 8A . 
         [0050]    Referring to  FIG. 7C , the sensor tube  52   e  proximal end  521  is fixed in the proximal end  54   e  of stylet cavity  54   b  with a droplet of quick-set adhesive  152 . Referring to  FIG. 7B , the receiver cavity  56   b  features a slight taper to act as a press-fit to the sensor cable  52   a . The receiver cavity  56   b  features a hard stop for the inserted cable end, and the hard stop allows a remaining cavity volume  56   a  wherein the unconstrained twisted pair  52   c  is enclosed. Adhesive  56   c  is applied to the press-fit surface of the receiver cavity  56   b . The sensor cable strength member  52   b  is folded back along the sensor cable  52   a . The sensor cable is inserted into the press-fit receiver cavity to maximum depth, with care given to ensure the sensor twisted pair  52   c  is loosely housed in the remaining cavity volume  56   a . Examples of quick-set adhesive material applied in this configuration option  150  are hot-melt preform and/or Cyanoacrylate. 
         [0051]    Referring to  FIG. 8A , configuration  170  features the press-fit receiver cavity axis  59   d  being offset from the stylet  54  axis. The axis offset is such that the sensor cable insulating material  52   g  is aligned to contact and lightly compress against the extended proximal end of the sensor tube  52   e . The receiver cavity  59   b  features a slight taper to act as a press-fit to the sensor cable  52   a , as shown in  FIG. 8B . The receiver cavity  59   b  features a hard stop for the inserted cable end, and the hard stop allows a remaining cavity volume  59   a  wherein the unconstrained twisted pair  52   c  is enclosed. Adhesive  59   c  is applied to the press-fit surface of the receiver cavity  59   b . The sensor cable strength member  52   b  is folded back along the sensor cable  52   a . The sensor cable is inserted into the press-fit receiver cavity to maximum depth. Sensor tube  52   e  has been pre-cut to a specified length such that the proximal end  521  of the sensor tube contacts the cable insulating material  52   g , thus lightly compressing the proximal end of the sensor tube  52   e . Attention is given to ensure the sensor twisted pair  52   c  is loosely housed in the remaining cavity volume  56   a . The compression means eliminates the need to glue the proximal end of the sensor tube to the proximal end of the stylet cavity  54   b , thereby further reducing the labor component of the disposable needle  190  cost. 
         [0052]    Referring to  FIG. 9 , the sensor-loaded stylet  54  is inserted into the outer cannula  58  with sufficient force to mate the stylet receiver male feature  56   e  with the female cannula receiver  58   b . The magnified views  150 ,  170 ,  70 ,  90 ,  110  and  130  illustrate the permutations of cost reducing apparatuses and methods described herein. The stylet tip  54   a  is oriented as designed relative to the cannula tip  58   a , and the sensor  52   f  is precisely positioned at a known distance and orientation relative to the stylet tip and thus the cannula tip. 
         [0053]    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.