Abstract:
A system and method is provided for automatic navigation of medical tools. The system can include a tracking system operable to provide information relating to the position of at least a portion of a tool which is inside a body, and an automated navigation system operable to apply forces upon a portion of the tool. The automated navigation system can include a computer operable to store a 3-dimensional route. Furthermore, the automated navigation system can be operable to use the information relating to the position of at least the portion of the tool from the tracking system to apply calculated forces upon the tool, the calculated forces being associated with movement of the tool along the 3-dimensional route.

Description:
RELATED APPLICATIONS 
       [0001]    This is a continuation of U.S. patent application Ser. No. 12/917,052, filed Nov. 1, 2010, which is a continuation of U.S. patent application Ser. No. 10/599,963, filed Apr. 24, 2007, which is a U.S. National Phase Application of PCT Application No. PCT/IL2005/000871, filed Aug. 11, 2005, and claims priority to and the benefit of the filing date of U.S. Provisional Application Nos. 60/600,725 filed on Aug. 12, 2004, entitled “Medical Navigation System Base on Differential Sensor,” 60/619,792 filed on Oct. 19, 2004, entitled “Using a Catheter or Guidewire Tracking System to provide positional feedback for an automated catheter or guidewire navigation system,” and 60/619,897 filed on Oct. 19, 2004, entitled “Using a Radioactive Source as the Tracked element for a tracking system,” the disclosures of which are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to location and tracking of a source of ionizing radiation, for example within a body of a subject. 
       BACKGROUND OF THE INVENTION 
       [0003]    Existing techniques for intrabody tracking include direct video imaging using a laparoscope; fluoroscopy (performance of the procedure under continuous or periodic X-Ray imaging); electromagnetic tracking, optical tracking, computerized tomography (CT) tracking and ultrasonic image assisted tracking. Some of these techniques explicitly avoid ionizing radiation. Those techniques which employ ionizing radiation, such as fluoroscopy and CT, require sufficient amounts of ionizing radiation that radiation exposure for subjects and medical staff is a subject of concern. 
         [0004]    Some applications which require intrabody tracking, such as cardiac catheterization, require concurrently acquired images because the tissue through which the tracked medical device is being navigated moves frequently. Other applications which require intrabody tracking, such as intracranial procedures, are more amenable to the use of pre-acquired images because the relevant tissue is relatively static. 
         [0005]    Although catheter tracking systems have been in existence for a number of years using a number of different technologies (RF tracking, magnetic tracking, MR Tracking, MRI gradient field tracking, etc. . . . ) they have generally been used for special purpose tracking (i.e. cardiac mapping), and not for aiding in navigation during catheterization. This is most likely because the navigation during catheterization is performed primarily with a guidewire and the existing catheter tracking technologies require a tracked element which is too large to be placed on a standard guidewire. 
         [0006]    Guidewire and catheter navigation is generally guided by fluoroscopy. This is true for manual catheterization as well as for automated navigation systems such as the magnet-driven Niobe navigation system produced by Stereotaxis Inc. 
       SUMMARY OF THE INVENTION 
       [0007]    An aspect of some embodiments of the present invention relates to using ionizing radiation from a source in order to detect its position, optionally in or near the body of a subject, without production of an image. Optionally, the source is integrally formed with or attached to a medical device. Medical devices include, but are not limited to, tools, implants, navigational instruments and ducts. 
         [0008]    In an exemplary embodiment of the invention, position of the source is determined by non-imaging data acquisition. For purposes of this specification and the accompanying claims, the phrase “non-imaging” indicates data acquired independent of an image acquisition process that includes the source and anatomical or other non-source features in a same image. 
         [0009]    Optionally, position is determined using a sensor which has angular sensitivity resulting in a detectable change in output resulting from radiation detection according to an effective angle of incidence of radiation from the source. Greater sensitivity in effective angle of incidence provides greater efficiency of the position determination in terms of speed and accuracy. Embodiments with an angular range of less than ±100 milliradians, optionally less than ±50 milliradians are disclosed. In an exemplary embodiment of the invention, greater sensitivity to effective angle of incidence can be achieved by moving a radiation detector and/or a shield. 
         [0010]    Optionally, the source of ionizing radiation has an activity in the range of 0.01 mCi to 0.5 mCi. Optionally, the source of ionizing radiation has an activity less than 0.1 mCi. Optionally, the source of ionizing radiation has an activity of about 0.05 mCi. In an exemplary embodiment of the invention, a radiation source which poses no significant health risk to a patient (i.e short term exposure) and/or medical personnel (i.e. long term exposure) may be employed. 
         [0011]    Optionally, the refresh rate for the location data insures that the locational information is temporally well correlated to the actual location of a tracked object (e.g. medical device). Recommended refresh rates vary according to the speed at which the tracked object moves and according to the environment in which the tracked object moves. In an exemplary embodiment of the invention, for tracking of medical devices through body parts which are more static, such as brain or digestive tract, lower refresh rates, for example 10 times/second may be adequate. In embodiments for tracking of medical devices through body parts which move frequently, such as the heart, higher refresh rates, for example 20 times/second may be desirable. Optionally, gating to an ECG output may be implemented so that positions from selected cardiac cycle phases are plotted. 
         [0012]    Optionally, the RMS error of a calculated position of the source of ionizing radiation is less than 10 rnm, optionally less than 5 mm, optionally less than 2 mm, optionally less than 1 mm, optionally 0.5 to 0.8 mm or better. 
         [0013]    Variables which may influence the accuracy of determined position(s) include activity of the source in DPM, the accuracy and/or response time of radiation sensors employed for detection, and the speed of the implanted medical device. Improvement in one or more of these variables may compensate for one or more other variables. Optionally, reducing the speed of a tracked medical device may be employed to compensate for other variables. Optionally, location information is displayed in the context of anatomical imaging data. Optionally, relevant anatomical features are highlighted to facilitate navigation of the medical device by medical personnel. Optionally, determined positions may be displayed in the context of a separately acquired image. 
         [0014]    Optionally, two or more sources may be tracked concurrently. Optionally, multi-source tracking is used in determining orientation of an asymmetric medical device. Optionally, multi-source tracking is used in coordinating activity of two or more medical devices for a medical procedure. 
         [0015]    An aspect of some embodiments of the present invention relates to using a sensor with angular sensitivity to detect a direction towards a source of ionizing radiation. Optionally, two or three or more directions are determined, either concurrently or successively, so that a position may be determined by calculating an intersection of the directions. If three or more directions are employed, the location may be expressed as a three dimensional position. Optionally, a direction is used to determine a plane in which the source resides. 
         [0016]    Optionally, sensors for detection of radiation from the source achieve the desired angular sensitivity by rotation of at least a portion of the sensor about an axis through a rotation angle. For example, detectors or radiation shields may be rotated. Alternately or additionally, sensors may achieve the desired angular sensitivity by translational motion. 
         [0017]    An aspect of some embodiments of the present invention relates to a sensor with an angular sensitivity which causes changes in an output signal from at least one radiation detector in response to an effective angle of incidence between the detector and a source. A target value of the output signal is achieved at an angle indicating the direction towards the source. The direction is optionally used to determine a plane in which the source resides. 
         [0018]    Optionally the sensor may include more than one radiation detector, each radiation detector having a separate output signal. Optionally, one or more radiation shields may be employed to shield or shadow at least a portion of at least one of the radiation detectors from incident radiation. The degree of shielding changes as deviation from the angle indicating a direction towards the source occurs and the output signal varies according to the degree of shielding. 
         [0019]    Optionally, multiple radiation shields are employed in concert to form a collimator. The radiation shields may be either parallel to one another or skewed inwards. Optionally, the multiple radiation shield, whether parallel or skewed, may be rotated. 
         [0020]    Optionally, the deviation from target output is 1% of the output range per milliradian of angular displacement away from an angle indicating a direction towards the source. Optionally deviation in output indicates direction of deviation as well as magnitude of deviation. According to various embodiments of the invention, radiation detectors and/or radiation shields may be displaced to impart angular sensitivity. This displacement may be rotational and/or translational. 
         [0021]    An aspect of some embodiments of the present invention relates to a computerized system for locating a medical device, optionally within a body of a subject by using angular sensitivity of a sensor module to determine a direction. The sensor module measures incident radiation on one or more radiation detectors. Incident radiation produces an output signal which is translated to directional information by the system. 
         [0022]    An aspect of some embodiments of the invention relates to association of a source of ionizing radiation with a medical device to facilitate determination of a location of the device, optionally as the device is navigated within or near a subject&#39;s body during a medical procedure. Optionally, the source of ionizing radiation has an activity in the range of 0.01 mCi to 0.5 mCi. Optionally, the source of ionizing radiation has an activity less than 0.1 mCi. Optionally, the source of ionizing radiation has an activity of about 0.05 mCi. Association includes integrally forming the source and the device as a single unit. Association also includes attaching the source to the device. Optionally, the source is concentrated in an area having a largest dimension less than 10 mm, optionally less than 5 mm, optionally less than 2.5 mm, optionally less than 1 mm. 
         [0023]    An aspect of some embodiments of the invention relates to use of an ionizing radiation source with an activity of 0.1 mCi or less as a target for non imaging localization or tracking, optionally in a medical context. The source of ionizing radiation is selected to reduce a biological effect on the patient and/or medical personnel. This selection involves consideration of radiation strength, radiation type and/or amount of exposure time (e.g. time in the body for a patient undergoing a procedure). Alternatively or additionally, radiation sources which are constructed of biocompatible material and/or coated with biocompatible coatings may be employed. 
         [0024]    Some embodiments of the invention include a medical catheterization method based on utilizing a guidewire and/or catheter tracking system to provide position feedback to an automated guidewire or catheter navigation system. 
         [0025]    Some embodiments of the invention include the use of a guidewire or catheter tracking system as the primary guiding system for an automated catheter or guidewire navigation system. The automated navigation system is capable of applying a force in any direction upon the tip of the guidewire. 
         [0026]    In an exemplary embodiment of the invention, a computerized system for tracking and locating a source of ionizing radiation is provided. The system comprising: 
         [0027]    (a) at least one non-imaging sensor module comprising at least one radiation detector, the at least one radiation detector capable of receiving ionizing radiation from the radiation source and producing an output signal; and 
         [0028]    (b) the CPU designed and configured to receive the output signal and translate the output signal to directional information. 
         [0029]    Optionally, the source of radiation is integrally formed with or attached to a medical device. 
         [0030]    Optionally, the at least one sensor module includes at least two sensor modules. 
         [0031]    Optionally, the at least two sensor modules includes at least three sensor modules. 
         [0032]    Optionally, the at least one of the at least one sensor module further comprises a locomotion device capable of imparting translational motion to the sensor module so that the sensor module is moved to a new location. 
         [0033]    Optionally, the locomotion device is operable by a translational motion signal from the CPU. 
         [0034]    Optionally, the system additionally comprises an imaging module, the imaging module capable of providing an image signal to the CPU, the CPU capable of translating the image signal to an image of a portion of the body of the subject. 
         [0035]    Optionally, the system further comprises a display device. 
         [0036]    Optionally, the display device is capable of displaying the image of the portion of the body of the subject with a determined position of the medical device superimposed on the image of the portion of the body of the subject. 
         [0037]    Optionally, the CPU receives at least two of the output signals and computes a position of the radiation source based on the output signals, 
         [0038]    Optionally, the CPU receives at least three of the output signals and computes a position of the radiation source based on the at least three output signals. 
         [0039]    Optionally, wherein the CPU computes the position repeatedly at intervals so that a position of the radiation source as a function of time may be plotted. 
         [0040]    Optionally, wherein the radiation source employs an isotope with a half life in the range of 6 to 18 months. 
         [0041]    Optionally, the system further comprises additionally comprising the radiation source capable of providing the radiation. 
         [0042]    Optionally, the directional information is produced when the source has an activity in the range of 0.01 mCi to 0.5 mCi. 
         [0043]    In an exemplary embodiment of the invention, a sensor for directionally locating an ionizing radiation source is provided. The sensor comprises: 
         [0044]    (a) at least one functional component; and 
         [0045]    (b) a displacement mechanism which imparts angular sensitivity to the sensor by moving the at least one functional component. 
         [0046]    Optionally, the at least one functional component comprising at least one radiation detector, the at least one radiation detector capable of receiving radiation from the radiation source and producing an output signal; 
         [0047]    wherein the displacement mechanism is capable of rotating the at least one radiation detector through a rotation angle so that the output signal varies with the rotation angle. 
         [0048]    Optionally, the at least one radiation detector comprises at least one first radiation detector and at least one second radiation detector and the output signal comprises at least one first output signal from the at least one first radiation detector and at least one second output signal from the at least one second radiation detector. 
         [0049]    Optionally, the sensor comprises at least one radiation shield installed at a fixed angle with respect to the at least one first radiation detector and the at least one second radiation detector so that a magnitude of the first output signal from the at least one first radiation detector and a magnitude of the second output signal from the second radiation detector vary with the rotation angle. 
         [0050]    Optionally, the sensor comprises: 
         [0051]    (a) at least one first radiation detector and at least one second radiation detector, each of the at least one first radiation detector and at least one second radiation detector capable of receiving radiation from the radiation source and producing at least one first output signal from the at least one first radiation detector and at least one second output signal from the at least one second radiation detector; 
         [0052]    (b) at least one radiation shield, the radiation shield rotatable about an axis of shield rotation through an angle of shield rotation, so that a magnitude of the first output signal from the at least one first radiation detector and a magnitude of the second output signal from the second radiation detector each vary with the angle of shield rotation. 
         [0053]    Optionally, the at least one radiation shield comprises: 
         [0054]    (i) a primary radiation shield located between the at least one first radiation detector and the at least one second radiation detector; 
         [0055]    (ii) at least one first additional radiation shield deployed to interfere with incident radiation directed towards the at least one first radiation detector; and 
         [0056]    (iii) at least one second additional radiation shield deployed to interfere with incident radiation directed towards the at least one second radiation detector. 
         [0057]    Optionally, wherein the at least one first additional radiation shield and the at least one second additional radiation shield are each inclined towards the primary radiation shield. 
         [0058]    Optionally, wherein the at least one first radiation detector and the at least one second radiation detector are organized in pairs, each pair having a first member and a second member and each radiation shield of the primary and additional radiation shields is located between one of the first member and one of the second member of one of the pairs so that the output signal varies with the rotation angle. 
         [0059]    Optionally, the sensor is additionally capable of revolving the at least a functional component about an axis of revolution through an angle of revolution. 
         [0060]    In an exemplary embodiment of the invention, a method of determining a location of a device is provided. The method comprises: 
         [0061]    (a) providing a device having a radiation source associated therewith; 
         [0062]    (b) determining a direction towards the radiation source; 
         [0063]    (c) further determining at least a second direction towards the radiation source; 
         [0064]    (d) locate the device by calculating an intersection of the first direction and the at least a second direction. 
         [0065]    Optionally, the further determining at least a second direction towards the radiation source includes determining at least a third direction towards the radiation source and additionally comprising: 
         [0066]    (e) calculating a point of intersection of the first direction, the second direction and the at least a third direction. 
         [0067]    In an exemplary embodiment of the invention, a method of manufacturing a trackable medical device is provided. The method comprises incorporating into or fixedly attaching a detectable amount of a radioactive isotope to the medical device. 
         [0068]    Optionally, the detectable amount is in the range of 0.01 mCi to 0.5 mCi. 
         [0069]    Optionally, the detectable amount is 0.1 mCi or less. 
         [0070]    Optionally, the detectable amount is 0.05 mCi or less. 
         [0071]    Optionally, the isotope is Iridium-192. 
         [0072]    An aspect of some embodiments of the invention relates to use of an ionizing radiation source with an activity of 0.1 mCi or less as a target for non imaging localization or tracking. 
         [0073]    In an exemplary embodiment of the invention, a system is provided consisting of an automated catheter/guidewire navigation system, a C-arm fluoroscopy system (for visualization of the intravascular procedure once navigation to the target location has been completed, blood flow diagnostics, initial 3-D angiography, etc. . . . ), and a 3-dimensional guidewire and/or catheter tracking system. 
         [0074]    Optionally, a 3-dimensional angiography data set will either be provided from previous MRI or CT angiography, or will be produced at the outset of the procedure using the fluoroscopy system. 
         [0075]    Optionally, the 3-dimensional angiography data set will be used to produce a 3-dimensional model of the vasculature within which the system will navigate. The target will then be indicated by the doctor on the 3-dimensional model and the route to the target will be either automatically or manually determined. Once the guidewire is brought into the region of the 3-dimensional vasculature model, the automated navigation system will pull the tip of the guidewire along the predetermined route based on real-time position feedback from the tracking system. 
         [0076]    Optionally, the navigation system computer will control the direction in which the navigation system pulls the guidewire tip at each point in time based on the current tip position provided by the tracking system and the predetermined route on the 3-dimensional vasculature model. 
     
    
     
       BRIEF DESCRIPTION OF FIGURES 
         [0077]    In the Figures, identical structures, elements or parts that appear in more than one Figure are generally labeled with the same numeral in all the Figures in which they appear. Dimensions of components and features shown in the Figures are chosen for convenience and clarity of presentation and are not necessarily shown to scale. The Figures are listed below. 
           [0078]      FIG. 1  is a side view of one embodiment of a sensor module according to an exemplary embodiment of the present invention; 
           [0079]      FIG. 2  is a schematic representation of a computerized tracking system according to an exemplary embodiment of the present invention; 
           [0080]      FIG. 3  is a side view of an additional embodiment of a sensor module according to an exemplary embodiment of the present invention illustrating receipt of a signal by the module; 
           [0081]      FIG. 4  is a perspective view of a computerized tracking system according to an exemplary embodiment of the present invention illustrating one possible arrangement of sensor modules with respect to a patient; 
           [0082]      FIG. 5  is a side view of another additional embodiment of a sensor module according to an exemplary embodiment of the present invention; 
           [0083]      FIGS. 6A and 6B  are side views of further additional embodiments of a sensor module according to exemplary embodiments of the present invention; 
           [0084]      FIGS. 7A and 7B  are graphs of simulated response time and simulated rms position error respectively plotted as a function of sensor rotation per photon impact using a system according to an exemplary embodiment of the present invention; 
           [0085]      FIGS. 8A and 8B  are graphs of simulated response time and simulated rms position error respectively plotted as a function of sampling time using a system according to an exemplary embodiment of the present invention; 
           [0086]      FIGS. 9A and 9B  are graphs of simulated response time and simulated rms position error respectively plotted as a function of specific activity of a radioactive signal source using a system according to an exemplary embodiment of the present invention; 
           [0087]      FIG. 10A  is a graph of position as a function of time. Simulated position output from a system according to an exemplary embodiment of the present invention is overlaid on a plot of actual input position for the simulation; 
           [0088]      FIG. 10B  is a graph of rms position error plotted as a function of time based upon the two plots of  FIG. 10A ; 
           [0089]      FIG. 11  is a simplified flow diagram of a method according to exemplary embodiments of the present invention; and 
           [0090]      FIG. 12  is a graph of sensor output as a function of rotation angle. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0091]    According to one embodiment of the invention ( FIGS. 2 and 4 ), a computerized system  40  locates and/or tracks a device. In the embodiment depicted in  FIG. 4 , the device is a medical device. Medical devices include, but are not limited to, tools, implants, navigational instruments and ducts. Tools include, but are not limited to, catheters, canulae, trocar, cutting implements, grasping implements and positioning implements. Implants include, but are not limited to, brachytherapy seeds, stents and sustained release medication packets. Navigational instruments include, but are not limited to, guidewires. Ducts include, but are not limited to, tubing (e.g. esophageal tubes and tracheal tubes). In exemplary embodiments of the invention, one or more moving tools are tracked. 
         [0092]    In an exemplary embodiment of the invention, position of the source is determined by non-imaging data acquisition. For purposes of this specification and the accompanying claims, the phrase “non-imaging” indicates data not acquired as part of an image acquisition process that includes the source and anatomical or other non-source features in a same image. Optionally, a sensor which is not suitable for and not connected to imaging circuitry is employed. Imaging relies upon information about many points, including at least one point of interest, and image analysis of the information determines characteristics of the point(s) of interest, for example position relative to an object. In an exemplary embodiment of the invention, position sensing provides information only about the source. This can improve detectability and/or accuracy. 
         [0093]    Optionally, the medical device is at least partially within a body of a subject  54  during at least part of the path upon which its location is determined. In  FIG. 4 , an exemplary embodiment in which system  40  is configured to track a device through the head of subject  54  during an intracranial medical procedure is depicted. This drawing is purely illustrative and should not be construed as a limitation of the scope of the invention. 
         [0094]      FIG. 2  shows an embodiment of system  40  including three sensor modules  20  which rely on angular detection acting in concert to determine a location of radioactive source  38 . In the pictured embodiment, each of sensors  20  determines an angle of rotation  32  indicating a direction towards source  38 . This angle of rotation  32  ( FIG. 1 ) defines a plane in which source  38  resides and which crosses radiation detector  22 . Angle of rotation  32  is provided as an output signal  34  which is relayed to computerized processing unit (CPU)  42 . CPU  42  determines an intersection of the three directions (planes) which is expressed as a point. 
         [0095]    According to some embodiments of the invention, a source  38  located within the boundaries  24  of detection ( FIG. 1 ) of sensor  20  may be accurately located by system  40  as radiation detector  22  of sensor module  20  is rotated through a series of rotation angles  32 . A source  38  located outside of boundaries  24  will not be accurately located. For this reason, it is desirable, in some embodiments, that each of sensors  20  is deployed so that the predicted path of source  38  lies within boundaries  24 . According to some embodiments of the invention, sensor  20  may move to keep source  38  within boundaries  24 . The size and shape of boundaries  24  vary according to the configuration of sensor  20 . 
         [0096]    Accuracy of determination of target rotation angle  32  contributes to accuracy of the location of source  38  as determined by system  40 . Various modifications to sensor module  20  which can increase the sensitivity to small differences in rotation angle  32  are depicted as exemplary embodiments in  FIGS. 3 ,  5 ,  6 A and  6 B and explained in greater detail hereinbelow. 
         [0097]      FIG. 4  provides a perspective view of an exemplary system  40  which employs angular detection and includes three sensor modules  20  dispersed upon the circumference of a circle  58 . In the pictured embodiment, modules  20  feature radiation shields  36 . In the pictured embodiment, each module  20  rotates about an axis tangent to circle  58 . This rotation allows tracking of the medical device as explained in greater detail hereinbelow. According to various embodiments of the invention, rotational motion or translational motion may be employed to facilitate the desired angular detection. According to the embodiment depicted in  FIG. 4 , sensor modules  20  are situated below the head of subject  54  such that the vertical distance between the plane of sensor modules  20  and the region of interest within the head is approximately equal to the radius of circle  58 . This arrangement assures that each of sensors  20  are deployed so that the predicted path of source  38  lies within boundaries  24 . This arrangement may be repeatably and easily achieved by providing three of sensors  20  mounted on a board equipped with a raised headrest in the center of circle  58 . This optionally permits a reclining chair or adjustable examination table to be easily positioned so that subject  54  is correctly placed relative to sensors  20  without an extensive measuring procedure. 
         [0098]    Positioning volume of system  40  is the set of spatial coordinates in which a location of source  38  may be determined. Positioning volume of system  40  has a size and/or shape dependent upon positions of sensor(s)  20 , their design and/or their performance characteristics. Optionally, positioning volume of system  40  can be expressed as the intersection of boundaries of detection  24  of sensors  20 . Optionally, two or more positioning volumes may be created, by using multiple sets of sensors  20 . Optionally, these positioning volumes may overlap. 
         [0099]    The 3-dimensional position of the center of mass of a radiation source  38  is calculated by CPU  42  from the angle  32  measured by each of sensor modules  20 , given the known location and rotation axis of each of modules  20 . According to some embodiments of the invention, source  38  will be a piece of wire with a length of 1 to 10 mm. This range of lengths reflects currently available solid isotope sources  38  supplied as wires with useful diameters and capable of providing a sufficient number of DPM to allow efficient operation of system  40 . System  40  determines the position of the middle of this piece of wire  38  and resolves the determined position to a single point, optionally indicating margins of error. 
         [0100]    Sensor module  20  includes at least one radiation detector  22 . Radiation detector  22  is capable of receiving radiation from radiation source  38  attached to the medical device and producing an output signal  34 . Radiation detector  22  may employ any technology which transforms incident radiation into a signal which can be relayed to CPU  42 . If source  38  is a gamma radiation source, radiation detector  22  may be, for example, an ionization chamber, a Geiger-Mueller Counter, a scintillation detector, a semiconductor diode detector, a proportion counter or a micro channel plate based detector. Radiation detectors  22  of various types are commercially available from, for example, EVproducts (Saxonburg Pa., USA); Hammatsu Photonics (Hamamatsu City, Shizuoaka, Japan); Constellation Technology, (Largo, Fla., USA); Soltec Corporation (San Fernando Calif., USA); Thermo Electron Corporation, (Waltham Mass., USA): Bruker-biosciences (Billerica Mass., USA); Saint Gobain crystals (Newbury Ohio, USA) and Silicon Sensor GMBH (Germany). A suitable commercially available radiation detector  22  can be incorporated into the context of system  40  as part of sensor  20 . Embodiments of the invention which rely upon a source  38  producing a small number of DPM and S types of detectors  22  which offer good sensitivity (i.e. high ratio between CPM and DPM) will improve the performance of sensor modules  20 . As the distance between sensor  20  and source  38  increases, this consideration becomes more relevant. Embodiments of the invention which rely upon source  38  with a greater DPM output may permit use of less sensitive radiation detectors  22 . 
         [0101]    Various types of sensor modules  20  are described in greater detail hereinbelow. 
         [0102]    System  40  further includes radiation source  38  capable of providing a sufficient amount of radiation for location and/or tracking at a rate which will not adversely affect a procedure being carried out by the medical device. For most medical procedures, 10 locations/second is sufficient to allow an operator of system  40  to comfortably navigate the medical device to a desired location. Based upon results from a computerized simulation described in greater detail hereinbelow, the amount of radiation to meet these criteria can be made low enough that it does not pose any significant risk to a patient undergoing a procedure of several hours duration with source  38  inside their body. Alternatively or additionally, the amount may be made low enough so that an operator of system  40  is not exposed to any significant risk from radiation exposure over time as explained hereinbelow. 
         [0103]    For example, using Iridium-192 increasing the activity of radiation source  38  from 0.01 mCi to 0.5 mCi improves accuracy only by a factor of 2 ( FIG. 9B ). However, activity levels below 0.1 mCi adversely affect response time ( FIG. 9A ). Activities greater than 0.1 mCi do not significantly improve response time. An activity of 0.05 mCi offers an acceptable trade-off between latency and accuracy as described in greater detail hereinbelow and provides a good compromise between performance and radiation dose. 
         [0104]    A 0.05 mCi source  38  meets permits system  40  to achieve adequate speed and accuracy with an amount of radiation produced so low that it may be safely handled without gloves. Radiation exposure for the patient from a 0.05 mCi source  38  is only eight times greater than average absorbed background radiation in the United States. For purposes of comparison to previously available alternatives, a 0.05 mCi source  38  exposes the patient to an Effective Dose Equivalent (EDE) of 0.0022 mSv/hr. A typical fluoroscopy guided procedure has an EDE of 1-35 mSV per procedure and a typical Nuclear Medicine procedure has an EDE of 5 mSv. Thus, some embodiments of the invention may be employed to significantly reduce patient radiation exposure. 
         [0105]    Medical personnel are optionally exposed to even less radiation, with the level of exposure decreasing in proportion to the square of the intervening distance. For example, a doctor located one meter from a 0.05 mCi source  38  and performing procedures for 6 hours per day, 5 days a week, 52 weeks a year would accumulate a total annual EDE of 0.22 mSv. This is approximately 5% of the radiation exposure level at which exposure monitoring is generally implemented. This level of exposure corresponds to 1.4e −4  mSV/hr which is orders of magnitude less than the 1-12 mSv/hr associated with a typical dose from fluoroscopic procedures. 
         [0106]    Iridium-192 has been used as an example because it is already approved for use in medical applications and is generally considered safe to introduce into the body of a subject. However this isotope is only an illustrative example of a suitable source  38 , and should not be construed as a limitation of system  40 . When choosing an isotope for use in the context of system  40 , activity (DPM), type of radiation and/or half life may be considered. Activity has been discussed above. In addition, it is generally desired that disintegration events be detectable with reasonable efficiency at the relevant distance, for example 20-50 cm. Long half lives may be preferred because they make inventory control easier and reduce total costs in the long run by reducing waste. However, short half lives may reduce concerns over radioactive materials and/or may allow smaller sources to be used. 
         [0107]    According to some embodiments of the invention, source  38  is a source of positron emissions. According to these embodiments, sensors  20  determine a direction from which photons released as a result of positron/electron collisions originate. This difference optionally does not affect accuracy of a determined location to any significant degree because the distance traveled by a positron away from source  38  before it meets an electron is generally very small. Use of positrons in source  38  can effectively amplify total ionizing radiation emissions available for detection. Optionally, the use of multiple detector may allow the detection of pairs of positron annihilation events to be detected. Other examples of source types include gamma sources, alpha sources, electron sources and neutron sources. 
         [0108]    Regardless of the isotope, source  38  may be incorporated into a medical device (e.g. guidewire or catheter) which is to be tracked. Incorporation may be, for example, at or near the guidewire tip and/or at a different location in a catheter or in an implant. The source of ionizing radiation may be integrally formed with, or attached to, a portion of the guidewire or to a portion of the medical device. Attachment may be achieved, for example by gluing, welding or insertion of the source into a dedicated receptacle on the device. Attachment may also be achieved by supplying the source as an adhesive tag (e.g. a crack and peel sticker), paint or glue applicable to the medical device. Optionally, the source of ionizing radiation is supplied as a solid, for example a length of wire including a radioactive isotope. A short piece of wire containing the desired isotope may be affixed to the guidewire or medical device. This results in co-localization of the medical device and the source of radiation. Affixation may be accomplished, for example, by co-extruding the solid source with the guidewire during the manufacture of the guide wire. Alternately, or additionally, the source of ionizing radiation may be supplied as a radioactive paint which can be applied to the medical device and/or the guidewire. Regardless of the exact form in which the ionizing radiation source is supplied, or affixed to the guidewire or medical device, it should not leave any significant radioactive residue in the body of the subject after removal from the body at the end of a medical procedure. 
         [0109]    While source  38  is illustrated as a single item for clarity, two or more sources  38  may be tracked concurrently by system  40 . System  40  may identify multiple sources  38  by a variety of means including, but not limited to, discrete position or path, frequency of radiation, energy of radiation or type of radiation. According to some embodiments of the invention, use of two or more resolvable sources  38  provides orientation information about the item being tracked. In other words, these embodiments permit determination of not only a 3-dimensional position defined by co-ordinates X, Y and Z, but also information about the orientation of the tracked object at the defined location. This feature is relevant in a medical context when a non-symmetrical tool is employed. 
         [0110]    System  40  may include a channel of communication  48  capable of conveying a data signal between the one or more sensor modules  20  and a computerized processing unit (CPU)  42 . Channel of communication may be wired or wireless or a combination thereof. Wired channels of communication include, but are not limited to direct cable connection, telephone connection via public switched telephone network (PSTN), fiber optic connection and construction of system  40  as an integrated physical unit with no externally apparent wires. Wireless channels of communication include, but are not limited to infrared transmission, radio frequency transmission, cellular telephone transmission and satellite mediated communication. The exact nature of channel of communication  48  is not central to operation of system  40  so long as signal transmission permits the desired refresh rate. Channels of communication  48  may optionally permit system  40  to be operated in the context of telemedicine. Alternately, or additionally, channels of communication  48  may serve to increase the distance between source  38  and medical personnel as a means of reducing radiation exposure to the medical personnel to a desired degree. 
         [0111]    CPU  42  is designed and configured to receive output signal  34  via channel of communication  48  and translate output signal  34  to directional information concerning radiation source  38 . This directional information may be expressed as, for example, a plane in which radiation source  38  resides. Output signal  34  includes at least rotation angle  32 . Optionally, output signal  34  may also include a signal strength indicating component indicating receipt of a signal from source  38 . Receipt of a signal from source  38  may be indicated as either a binary signal (yes/no) or a signal magnitude (e.g. counts per minute). According to various embodiments of the invention, output signal  34  may be either digital or analog. Translation of an analog signal to a digital signal may be performed either by sensor module  20  or CPU  42 . In some cases, locating radiation source  38  in a single plane is sufficient. However, in most embodiments of the invention, it is desirable that CPU  42  receives two of output signals  34  and computes an intersection. If output signals  34  are expressed as planes, this produces a linear intersection  44  of two of the planes. This locates radiation source  38  upon the linear intersection  44 . Optionally, results  44  of this calculation are displayed on a display device  43  as described in greater detail hereinbelow. In additional embodiments of the invention, CPU  42  receives at least three of output signals  34  and computes their intersection. If output signals  34  are expressed as planes and sensors  20  are positioned on the circumference of circle  58 , this produces a point of intersection  44  of at least three planes, thereby locating radiation source  38  at the calculated point of intersection  44 . 
         [0112]    Because system  40  is most often employed to track a medical instrument during a medical procedure, CPU  42  is often employed to compute the point of intersection repeatedly at predetermined intervals so that a position of radiation source  38  as a function of time may be plotted (see  FIG. 10  a). The accuracy of each plotted position and of the plot as a whole may be influenced by the activity of source  38 , the accuracy and response time of sensors  20  and the speed at which the implanted medical device is moving through subject  54 . Because medical procedures generally favor precision over speed, an operator of system  40  may compensate for deficiencies in source  38 , or accuracy or response time of sensors  20 , by reducing the rate of travel of the medical device being employed for the procedure.  FIG. 10B  illustrates output of a simulated system  40  with tracking accuracy in the range of ±2 mm. CPU  42  may also optionally employ channel of communication  48  to send various signals to sensor module(s)  20  as detailed hereinbelow. Alternately, or additionally, CPU  42  may also optionally employ channel of communication  48  to send various signals to the medical device. According to various embodiments of the invention, system  40  may be employed in the context of procedures including, but not limited to, angioplasty (e.g. balloon angioplasty), deployment procedures (e.g. stent placement or implantation of radioactive seeds for brachytherapy), biopsy procedures, excision procedures and ablation procedures. 
         [0113]    While CPU  42  is depicted as a single physical unit, a greater number of physically distinct CPUs might actually be employed in some embodiments of the invention. For example, some functions, or portions of functions, ascribed to CPU  42  might be performed by processors installed in sensor modules  20 . For purposes of this specification and the accompanying claims, a plurality of processors acting in concert to locate source  38  as described herein should be viewed collectively as CPU  42 . 
         [0114]    According to some embodiments of the invention, system  40  concurrently employs three or more sensor modules  20  in order to concurrently receive three or more output signals  34  and compute three or more directions indicating signal source  38 . If the directions are expressed as planes, the three or more planes intersect in a single point. However, system  40  includes alternate embodiments which employ two, or even one, sensor module  20  to localize source  38  to a single point. This may be achieved in several different ways as described hereinbelow. 
         [0115]    According to some embodiments of system  40  at least one of sensor module  20  is capable of rotating the at least one radiation detector  22  through a series of positions. Each position is defined by a rotation angle  32  so that receiving the radiation from source  38  upon detector  22  varies with rotation angle  32 . This rotation may be accomplished in a variety of ways. For example, rotation mechanism  26  may be operated by feedback from  28  from radiation detector  22  according to a rule with amount of received radiation as a variable. Alternately, rotation mechanism  26  may be operated by a signal from CPU  42  according to a rule including amount of received radiation and/or time as variables. Alternately, rotation mechanism  26  may rotate radiation detector  22  according to a fixed schedule, with no regard to how much radiation impinges upon radiation detector  22  at any particular rotation angle  32 . Rotation mechanism  26  may employ a wide variety of different mechanisms for achieving rotation angle  32 . These mechanisms include, but are not limited to, mechanical mechanisms, hydraulic mechanisms, pneumatic mechanisms, electric mechanisms, electronic mechanisms and piezoelectric mechanisms. Optionally, an independent angle measuring element  30  may be employed to more accurately ascertain the actual rotation angle  32 . Although angle measuring element  30  is depicted as a physically distinct component in  FIGS. 1 ,  2  and  3 , it could be physically integrated into rotation mechanism  26  without affecting performance of system  40  to any significant degree. Regardless of the exact operational details, the objective is to detect the rotation angle  32  at which sensor module  20  is pointing directly towards source  38 . This angle will be referred to as the target rotation angle  32 . 
         [0116]    According to some embodiments of system  40 , radiation detector  22  ( FIGS. 3 ,  5 ,  6 A and  6 B) includes at least one first radiation detector  22 A and at least one second radiation detector  22 B. These embodiments of system  40  rely upon comparison of output signals  34  from radiation detectors  22 A and  22 B for each rotation angle  32 . A target angle of rotation  32  which produces output signals  34  from radiation detectors  22 A and  22 B with a known relationship indicates that radiation detectors  22 A and  22 B are both facing source  38  to the same degree. When radiation detectors  22 A and  22 B have identical receiving areas, the known relationship is equality. This target angle of rotation  32  is employed to determine a plane in which source  38  resides. 
         [0117]    In order to increase the sensitivity of system  40  to small differences between output signals  34  from radiation detectors  22 A and  22 B it is possible to introduce one or more radiation shields  36  at a fixed angle with respect to radiation detectors  22 A and  22 B. Radiation Shield  36  causes a magnitude of the component of output signal  34  from first radiation detector  22 A and a magnitude of the component of output signal  34  from second radiation detector  22 B to each vary with rotation angle  32  (see  FIG. 3 ). Radiation shield  36  differentially shadows either radiation detectors  22 A or  22 B depending upon the relationship between angles of incidence  39  and  41 . At some angle of rotation  32 , neither radiation detector  22 A nor  22 B will be shadowed by radiation shield  36 . This angle of rotation  32  is employed to determine a plane in which source  38  resides. This configuration insures that small variations from this target angle of rotation  32  cause relatively large differences in the output signals  34  from radiation detectors  22 A and  22 B because of the shadow effect. Therefore, use of radiation shield  36  in sensor module  20  increases the sensitivity of system  40 . This increased sensitivity permits sensor module  20  to function effectively even with a low number of detectable radioactive counts. 
         [0118]      FIG. 6A  illustrates an additional embodiment of sensor module  20  in which the radiation shield includes a primary radiation shield  36  located between first radiation detector  22 A and second radiation detector  22 B. The picture embodiment also includes a series of first additional radiation shields ( 36 A 1 ,  36 A 2 , and  36 A 3 ) which divide first radiation detector  22 A into a series of first radiation detectors and interfere with incident radiation directed towards first radiation detector  22 A. The pictured embodiment also includes a series of second additional radiation shields ( 36 B 1 ,  36 B 2 , and  36 B 3 ) which divide second radiation detector  22 B into a series of second radiation detectors and interfere with incident radiation directed towards second radiation detector  22 B. This configuration can insure that even smaller variations from target rotation angle  32  cause relatively large differences in the output signals  34  from radiation detectors  22 A and  22 B by increasing the shadow effect in proportion to the number of additional radiation shields ( 36 A 1 ,  36 A 2 ,  36 A 3 ,  36 B 1 ,  36 B 2 , and  36 B 3  in the pictured embodiment). Therefore, use of additional radiation shields (e.g.  36 A 1 ,  36 A 2 ,  36 A 3 ,  36 B 1 ,  36 B 2 , and  36 B 3 ) in sensor module  20  may serve to achieve an additional increase in sensitivity of system  40 . Optionally, secondary radiation shields ( 36 A 1 ,  36 A 2 ,  36 A 3 ,  36 B 1 ,  36 B 2 , and  36 B 3  in the pictured embodiment) are inclined towards primary radiation shield  36 . The angle of secondary radiation shields  36 A 1 ,  36 A 2 ,  36 A 3 ,  36 B 1 ,  36 B 2 , and  36 B 3  towards primary shield  36  can be changed, for example, using a motor to improve focus and/or define imaging volume. 
         [0119]    A similar effect may be achieved by holding radiation detectors  22 A and  22 B at a fixed angle and subjecting radiation shield(s)  36  ( FIG. 6B ) to angular displacement. Therefore, system  40  also includes embodiments in which radiation detector  22  includes at least one first radiation detector  22 A and at least one second radiation detector  22 B and output signal  34  includes discrete components from detectors  22 A and  22 B with at least one radiation shield  36  rotatable about an axis of shield rotation through an angle of shield rotation  32  so that a magnitude of discrete components of output signal  34  from detectors  22 A and  22 B each vary as a function of the angle of shield rotation  32 . 
         [0120]    Referring now to  FIG. 5 , alternate embodiments of sensor module  20  of system  40  are configured so that radiation detector  22  includes a plurality of radiation detectors  22  and a plurality of protruding radiation shields  36  interspersed between the plurality of radiation detectors  22 . According to these embodiments, plurality of radiation detectors  22  is organized in pairs, each pair having a first member  21  and a second member  23  and each protruding radiation shield  36  of the plurality of protruding radiation shields is located between first member  21  and second member  23  of the pair of radiation detectors  22 . According to this embodiment, sensor module  20  is capable of rotating the radiation detectors  22  through a series of rotation angles  32  so that the receiving the radiation from radiation source  38  upon radiation detectors  22  varies with rotation angle  32 . Each radiation detector produces an output signal  34 . CPU  42  sums output signals  34  from all first members  21  to produce a first sum and all second members  23  to produce a second sum. Assuming that all of radiation detectors  22  are identical, when the sensor is aimed directly at the center of mass of source  38  (target rotation angle  32 ), the first sum and the second sum are equivalent. This embodiment insures that the total output for the entire module  20  increases rapidly with even a very slight change in rotation angle  32  in either direction. Alternately, or additionally, the sign of the total output for the entire module  20  indicates the direction of rotation required to reach the desired rotation angle  32  at which total output for the entire module  20 . Thus, this configuration serves to increase both speed of operation and overall accuracy of system  40 . This type of sensor module  20  may be operated (for example) by implementation of a first algorithm summing gamma ray impacts from source  38  for a period of time and allowing CPU  42  to decide, based on the sign of total output for the entire module  20 , in which direction and to what degree to rotate radiation detectors  22  in an effort to reach a desired rotation angle  32 . Alternately, CPU  42  may (for example) implement a second algorithm rotates radiation detectors  22  a very small amount in response to every detected count. Performance data presented herein is based upon a simulation of the second algorithm, but the first algorithm is believed to be equally useful. 
         [0121]    According to additional embodiments of system  40 , a single sensor module  20  may be employed to determine two intersecting planes in which source  38  resides. This may be achieved, for example, by revolution of sensor module  20  or by moving sensor module  20  to a new location. 
         [0122]    According to some embodiments of the invention, sensor module  20  may be additionally capable of revolving radiation detector  22  about an axis of revolution  25  through an angle of revolution  29 . Revolution is produced by a revolution mechanism  27  which may function in a variety of ways as described hereinabove for rotation mechanism  26 . According to these embodiments of the invention angle of revolution  29  is included as a component of the orientation of sensor module  20  and is included in output signal  34 . Revolution may be employed in the context of any or all of the sensor module  20  configurations described hereinabove and hereinbelow. Revolution may occur, for example, in response to a revolution signal  46  transmitted to sensor module  20  from CPU  42  via channel of communication  48 . 
         [0123]    According to additional embodiments of the invention, sensor module  20  includes a locomotion device  31  capable of imparting translational motion  33  to module  20  so that the location of module  20  is changed. Locomotion may be initiated, for example, in response to a translational motion signal  46  transmitted to sensor module  20  from CPU  42  via channel of communication  48 . According to various embodiments of the invention, locomotion may be used to either permit a single sensor module  20  to operate from multiple locations or to provide angular sensitivity to sensor module  20 . In other words, translational motion may be used as a substitute for angular displacement, especially in embodiments which employ at least one radiation shied  36 . In embodiments which employs translational motional, translation of a single sensor  20  in a first dimension permits acquisition of a first set of directional information. For example, in the embodiment of system  40  depicted in  FIG. 4 , successive vertical displacement of sensor  20 A could be used to determine a first plane in which source  38  resides. Successive horizontal displacement of sensor  20 B could be used to determine a second plane in which source  38  resides. Alternately, or additionally, a single sensor  20  may be subject to both vertical and horizontal displacement. Successive vertical and horizontal displacement permits a single sensor  20  to determine two non-parallel planes in which source  38  resides. Concurrent vertical and horizontal displacement along a single line permits a single sensor  20  to determine a single plane in which source  38  resides. Determination of intersection of 2 or 3 or more planes is as determined above. Optionally locomotion and revolution may be employed in the same embodiment of the invention. 
         [0124]    Optionally, system  40  further includes an imaging module  50  including an image capture device  56  capable of providing an image signal  52  to CPU  42 . Imaging module  50  optionally includes an interface to facilitate communication with CPU  42 . CPU  42  is capable of translating image signal  52  to an image of a portion of the body of subject  54 . According to various embodiments of the invention, imaging module  50  may rely upon fluoroscopy, MRI, CT or 2D or multi-plane or 3D angiography. For intracranial procedures, imaging generally need not be conducted concurrently with the procedure. This is because the brain does not shift much within the skull. Images captured a day or more before a procedure, or a few hours before a procedure, or just prior to a procedure, may be employed. According to alternate embodiments of the invention, image data is acquired separately (i.e. outside of system  40 ) and provided to CPU  42  for alignment. 
         [0125]    Alignment methods and the algorithms for anatomical image display and tracking information overlay are reviewed in Jolesz (1997) Radiology. 204(3):601-12. The Jolesz article, together with references cited therein, provides enablement for a skilled artisan to accomplish concurrent display and alignment of image data and tracking data. The Jolesz reference, together with references cited therein, are fully incorporated herein by reference to the same extent as if each individual reference had been individually cited and incorporated by reference. 
         [0126]    In an exemplary embodiment of the invention, the location(s) determined by system  40  are registered with respect to the image. This may be accomplished, for example by registering system  40  and/or sensors  20  to image capture device  56 . 
         [0127]    Regardless of which type of sensor module  20  is employed, system  40  may include display device  43  in communication with CPU  42 . Display device  43  may display the image of the portion of the body of the subject with a determined position of the medical device (corresponding to a position of source  38 ) superimposed on the image of the portion of the body of the subject. The superimposed determined position is optionally represented as a point on display screen  43 . Optionally the point is surrounded by an indicator of a desired confidence interval determined by CPU  42 . The confidence interval may be displayed, for example, as a circle, as two or more intersecting lines or as one or more pairs of brackets. Alternately, or additionally, display device  43  may display position coordinates of a determined position of the medical device (e.g., corresponding to a position of source  38  at a tip of guidewire). 
         [0128]    Display device  43  may be provided with a 3-dimensional angiography dataset from CT, MRI, or 3-D angiography, imaged either during the procedure or prior to the procedure. Appropriate software can be employed to extract a 3-D model of the vasculature from the angiography dataset, and display this model using standard modes of 3-D model visualization. A 3-dimensional graphical representation of the guidewire or catheter can be integrated into the 3-D model of the vasculature and updated with minimal temporal delay based on the position information provided by system  40  to indicate the position of the guidewire or catheter within the vasculature. The entire 3-D model including the vasculature and the catheter can be zoomed, rotated, and otherwise interactively manipulated by the user during performance of the procedure in order to provide the best possible visualization. 
         [0129]    Optionally, system  40  may further include one or more user input devices  45  (e.g. keyboard, mouse, touch screen, track pad, trackball, microphone, joystick or stylus). Input device  45  may be used to adjust an image as described hereinabove on display device  43  and/or to issue command signals to various components of system  40  such as rotation mechanism  26 , revolution mechanism  27 , locomotion device  31  or image capture device  56 . 
         [0130]    The invention optionally includes a sensor  20  for determining a plane in which a radiation source resides as depicted in  FIG. 3  and described hereinabove. Briefly, the sensor  20  includes at least one radiation detector  22 , the at least one radiation detector capable of receiving radiation from radiation source  38  and producing an output signal  34 . Sensor  20  is capable of rotating radiation detector  22  through a series of positions, each position defined by a rotation angle  32  so that the receiving the radiation from radiation source  38  upon radiation detector  22  varies with rotation angle  32 . Rotation is optionally achieved as described hereinabove. A rotation angle  32  which produces a maximum output signal indicates the plane in which radiation source  38  resides. 
         [0131]    According to some embodiments of sensor  20 , radiation detector  22  includes at least one first radiation detector  22 A and at least one second radiation detector  22 B and output signal  34  includes a first output signal from first radiation detector  22 A and a second output signal from radiation detector  22 B. 
         [0132]    According to some embodiments of sensor  20 , at least one radiation shield  36  is further installed at a fixed angle with respect to detectors  22 A and  22 B. As a result, a magnitude of the first output signal  34  from the at least one first radiation detector and a magnitude of the second output signal  34  from radiation detector  22 B each vary with rotation angle  32  as detailed hereinabove. 
         [0133]    A sensor  20  for determining a plane in which a radiation source resides and characterized by at least one radiation shield  36  rotatable about an axis of shield rotation through an angle of shield rotation  32  as described hereinabove in detail ( FIG. 6B ) is an additional embodiment of the invention 
         [0134]    Sensor  20  for determining a plane in which a radiation source resides as depicted in  FIG. 5  and described hereinabove is an additional embodiment of the invention. 
         [0135]    According to alternate embodiments of the invention, a method  400  ( FIG. 11 ) of determining a location of a medical device within a body of a subject is provided. Method  400  includes co-localizing  401  a radioactive signal source  38  with a medical device. Co-localization may be achieved, for example, by providing a device having a radiation source associated therewith or by associating a radiation source with a device. 
         [0136]    Method  400  further includes determining  402  a first plane in which the omni directional signal generator resides, further determining  403  a second plane in which the omni directional signal generator resides, calculating  404  a linear intersection of the first plane and the second plane as a means of determining a line upon which the medical device resides. 
         [0137]    Method  400  optionally includes further determining  405  at least one additional plane in source  38  resides. 
         [0138]    Method  400  optionally includes calculating  406  a point of intersection of the first plane, the second plane and the at least one additional plane as a means of determining a location of the medical device. 
         [0139]    Optionally, method  400  is successively iterated  408  so that a series of location are generated to track an implanted medical device in motion. Calculated locations may be displayed  410  in conjunction with anatomical imaging data if desired. 
         [0140]    The various aspects and features of system  40  and/or sensors  20  described in detail hereinabove may be employed to enable or enhance performance of method  400 . 
         [0141]    System  40  and method  400  may employ various mathematical algorithms to compute the location of source  38 . One example of an algorithm suited for use in the context of some embodiments of the invention calculates the position of source  38  from sensor output signal  34 , sensor position, and sensor orientation (i.e. rotation angle  32 ) of three sensors as follows: 
         [0142]    1) the plane defined by each sensor module  20  is calculated using an equation of the form 
         [0000]    
       
      
       Ax+By +Cz=D  
      
     
         [0143]    2) the coefficients A, B, C, and D are calculated as follows:
       a. Three non-collinear points are defined within sensor  20 &#39;s internal reference frame.   b. These three points are then shifted by the position of sensor  20  and rotated by the sensor orientation. This defines the plane in which source  38  would lie if output signal  34  was zero.   c. These three points are then rotated about the axis of rotation angle  32  of sensor  20  by rotation angle  32  indicated by output signal  34 . This defines the plane in which source  38  lies as measured by a particular sensor  20 .   d. Using the x,y,z coordinates of the three points, x 1 ,y 1 ,z 1 ,x 2 ,y 2 ,z 2 ,x 3 ,y 3 ,z 3  in the following equations, A, B, C, and Dare calculated as follows:
           i. A=y 1 (z 2 −z 3 )+y 2 (z 3 −z 1 )+y 3 (z 1 −z 2 )   ii. B=z 1 (x 2 −x 3 )+z 2 (x 3 −x 1 )+z 3 (x 1 −x 2 )   iii. C=x 1 (y 2 −y 3 )+x 2 (y 3 −y 1 )+x 3 (y 1 −y 2 )   
               
 
         [0151]    iv. D=x 1 (y 2 *z 3 −y 3 *z 2 )+x 2 (y 3 *z 1 −y 1 *z 3 )+x 3 (y 1 *z 2 −y 2 *z 1 ) 
         [0152]    3) Calculation of A, B, C, and D for each of three sensors  20  produces a system of three equations in three unknowns: 
         [0000]    
       
         
           
             
               
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         [0153]    This system of equations can be solved to provide an exact solution for (x,y,z) (or part of the vector), the point of intersection of the three planes, which is the position of the source  38 . 
         [0154]    Use of additional sensors  20  improves the accuracy by averaging the errors in the individual sensors, and may also provide a means of estimating the accuracy of the position measurement by indicating the extent to which the sensors agree with each other. 
         [0155]    When 4 or more sensors are used, the algorithm is as follows: 
         [0156]    Steps 1 and 2 above remain the same—the equation of the plane indicated by each sensor is calculated. Step 3 is modified as follows:
       3) Once A, B, C, and D have been calculated for each of the sensors an over-determined system of more than three equations in three unknowns results:       
 
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             This over-determined system can be solved in a least square sense using methods familiar to those skilled in the art in order to obtain the best solution for (x,y,z), which is the most likely position of the tracked element. There is generally no exact solution due to the error in the sensor outputs, there may be no single point through which all of the planes pass. 
             In order that the least square solution may be based on the error defined by the Euclidian distance between each plane and the solution for (x,y,z), it is necessary to scale all of the coefficients defining each plane by the lengths of their respective Normal vectors (the Normal vector is the vector defined by (A,B,C)). This is done by dividing A, B, C, and D by sqrt (AA 2 +BA 2 +CA 2 ) before performing the least square solution. 
             4) The Euclidian distance between each of the planes and the calculated position can be used as a measure of the accuracy of the position measurement. Once the coefficients have been scaled by the length of the Normal vector, this distance can be calculated for each sensor as Ax+By+Cz−D. The mean value of the distances from each plane to the calculated position gives a measure of the extent to which all of the sensors agree on the position that was calculated. 
           
         
       
     
         [0161]    Overdeteimined systems of equations may be solved using least square solution algorithms. Suitable least square algorithms are available as components of commercially available mathematics software packages. 
         [0162]    Optionally, other methods of solving equation sets as known in the art are used. Optionally, instead of a set of equations, other calculation methods are used, for example, neural networks, rule based methods and table look up methods in which the signals from the sensors are used to look-up or estimate a resulting position. In systems where the sensors move linearly, other solution methods may be used, for example, translating linear positions of the sensors into spatial coordinates of the source. 
         [0163]    In order to increase the accuracy and performance of system  40  and method  400 , advance calibration may optionally be performed. The position and orientation of each of the sensor modules  20  can be calibrated instead of relying upon values based on the mechanical manufacturing of the system. The calibration procedure involves using system  40  to measure the 3-dimensional position of a source  38  at a number of known positions defined to a high degree of accuracy. Since the position of source  38  is known, the equations normally used to calculate the positions (described above) can now be used with the sensor positions and orientations as unknowns in order to solve for these values. Various minimization procedures are known in the art. The number of measurements needed to perform such a calibration may depends on the number of sensor modules  20  in system  40 , since it is useful to make enough measurements to provide more equations than unknowns. This calibration procedure also defines the origin and frame of reference relative to which system  40  measures the position of the source, and can therefore provide alignment between the tracking system and another system to which it is permanently attached, such as a fluoroscopy system or other imaging system. 
         [0164]    In an exemplary embodiment of the invention, once a position of the source is known, the sensors can remain aimed at the source and not change their orientation. Optionally, if the source moves, determined for example, by a significant change in detected radiation (e.g., a drop of 30%, 50%, 70%, 90% or a greater or intermediate drop), the sensor is moved to scan a range of angles where the source is expected to be in. Optionally, the sensor generates a signal indicating on which side of the sensor the source is located, for example as described below. Optionally, the range of scanning depends on an expected angular velocity of the source, for example, based on the procedure, based on the history and/or based on a user threshold. If scanning within the range fails, the range is optionally increased. 
         [0165]    Optionally, if multiple target sources are provided (e.g., ones with different count rates and/or different energy of emission), the sensors jump between target angles. Optionally, a steady sweep between a range of angles encompassing the two (or more) sources is provided. Optionally, sweeping is provided by ultrasonic or sonic vibrations of the sensor or part thereof, for example, comprising a range of angles 1, 5, 10, 20, 50 or more times a second. Optionally, the amplitude of the vibration determines the range of angles. Optionally, the sensors or sensor motion is in resonance with one or more vibration frequencies. 
         [0166]    Optionally, scanning of the sensors, at least in a small range of angles, such as less than 10 or less than 5 or less than 1 degree, is provided even when the sensor is locked on a target source. 
         [0167]    The tracking accuracy of system  40  using Iridium-192 as source  38  as described hereinabove has been evaluated only by computer simulation. The simulation is a model of the random distribution of gamma photons emitted by a source  38  within a model head and absorbed by the photon-sensitive elements  22  in a compound differential sensor unit  20  of the type illustrated in  FIG. 5 . According to the simulation, radiation detector  22  of sensor module  20  rotates so that a new rotation angle  32  is defined every time a photon is absorbed by detector  22 . If the photon is absorbed by a positive radiation detector  21  then radiation detector  22  of sensor module  20  rotates in the positive direction, and if it is absorbed by a negative sensor  23  then radiation detector  22  of sensor module  20  rotates in the negative direction. Total output signal  34  of sensor module  20  is its average orientation during the sample time. 
         [0168]    According to the simulation, performance is defined by two parameters, however other parameters may be used in a practical system:
       1) The Root Mean Square (RMS) error when the target is stationary   2) The time to indicate a 9 mm change in calculated location after a 10 mm change in actual location of source  38 .       
 
         [0171]    The following parameter values are fixed in the simulation:
       1) Distance from the source to the sensor=25 cm (worst case distance)   2) Source distance for which sensor is geometrically optimized=25 cm   3) Width of photon-sensitive surface in each sub sensor=2 mm ( 18  in  FIG. 5 )   4) Sensor length=10 cm ( 14  in  FIG. 5 )   5) Height of dividing walls between sensors=5 cm ( 35  in  FIG. 5 )   6) Width of dividing walls at their base=4 mm ( 37  in  FIG. 5 )   7) Number of subsensors defined by walls in the compound sensor=7 ( 36  in  FIG. 5 )   8) Sensor sensitivity (fraction of incoming gamma rays which are detected)=0.3       
 
         [0180]    The simulation evaluated and optimized the following parameters with respect to influence on performance: 
         [0181]    1) Rotation magnitude per absorbed photon ( FIGS. 7   a  and  7   b ) 
         [0182]    2) Sample time ( FIGS. 8A and 8B ) 
         [0183]    3) Photons per second (source activity level) ( FIGS. 9A and 9B ) 
         [0184]    4) Overall tracking accuracy ( FIGS. 10   a  and  10   b ) 
         [0185]    The simulation determined that as rotation per photon impact increases, response time is improved ( FIG. 7   a ). However, as rotation per photon impact increases, RMS position error also increases ( FIG. 7   b ). There is clearly a trade-off between latency and accuracy. This parameter can be modified in real-time in order to optimize the trade-off using a motion detection algorithm as described hereinbelow. 
         [0186]    The simulation determined that sample time no significant impact on latency or accuracy ( FIGS. 8A and 8B ). This is because for small values of rotation per impact, the number of impacts per sample has minimal effect on accuracy and only determines the latency (the total amount of rotation per sample). However, if the number of impacts per sample is reduced as a result of a reduction in the sample time, then the reduction in sample time exactly compensates for the reduced response per sample leaving the latency unchanged. 
         [0187]    Radioactivity (number of photons emitted per second) has a very slight effect on accuracy, improving accuracy only by a factor of 2 as the activity increases from 0.01 mCi up to 0.5 mCi ( FIG. 9B ). It has a drastic effect on response time at low activity levels ( FIG. 9A ) where there simply are not enough photons to induce rapid rotation, however at activity levels above 0.1 mCi there is minimal improvement with increased activity level. Optimization of this trade-off between latency and accuracy (see below) is achieved with 0.05 mCi. This specific activity provides a good compromise between performance and radiation dose, providing a performance suitable for a typical medical application without imposing a safety risk to the patient or doctor. 
         [0188]    In order to optimize the tradeoff between accuracy and latency a motion detection algorithm was employed to increase the rotation per photon during motion of tracked source  38 . This decreased latency time and increased accuracy. In the simulation, the percentage of photons hitting receiving elements  22  classed as positive  21  versus those classed as negative  23  was used as an indication of motion of tracked source  38 . As the percentage moved farther away from 50% the rotation per photon was increased, reducing latency at the expense of accuracy during motion. In other words, system  40  begins by moving towards an estimated target rotation angle  32  in large steps. As estimated target rotation angle  32  is approached, the size of the steps is decreased. If target rotation angle  32  is passed, a small compensatory step in the opposite direction is employed. Results are summarized graphically in  FIGS. 10   a  and  10   b . Briefly, the RMS error of system  40  tracking a moving source  38  is 0.71 mm on average. Location of a stationary source  38  by system  40  produces an rms error of 0.62 mm. 
         [0189]    In summary, the simulation results indicate that with an activity of 0.05 mCi of 192Ir, compound differential sensors of the type illustrated in  FIG. 5 , and a motion detection algorithm which trades-off latency against accuracy, system  40  can achieve overall accuracy of approximately 1 mm RMS. 
         [0190]    Simulated sensitivity of sensor module  20  to changes in rotation angle  32  is illustrated in  FIG. 12  which is a plot of output signal  34  as a function of rotation relative to target rotation angle  32  for a sensor of the type indicated in  FIG. 5 . The graph was produced using the formula: 
         [0000]      Total Output 34= A /( A+B ) 
         [0191]    Where A is the sum of all right side sensors  21 ; and 
         [0192]    B is the sum of all left side sensors  23  and B 
         [0193]    The total range of output  34  (Y axis) from sensor  20  was arbitrarily defined as being in a range from 0 to 1. On the X axis, 0 indicates the angle of rotation  32  which indicates the direction of source  38 . The total rotational range of sensor  20  was ±32 milliradians from this target rotation angle  32 . Deviation of more than 32 milliradians away from target rotation angle  32  produced an output  34  of either 0 or 1, indicating the direction of rotation for a return to target rotation angle  32 , but not the amount of rotation to reach target rotation angle  32 . When output  34  is 0 or 1, the only conclusion that can be drawn about deviation from target rotation angle  32  is that it is greater than 32 milliradians in the indicated direction. 
         [0194]    The graph of  FIG. 12  depicts output  34  for target rotation angle  32  as the middle of the dynamic range (0.5). Therefore, if output  34  is 0.6, a correctional rotation of 10 milliradians in the plus direction is indicated to achieve target rotation angle  32 . An output  34  of 0.6 indicates a correctional rotation with the same magnitude (10 milliradians), but in the minus direction. Another way of depicting the same information would be to indicate a total dynamic range of +0.5 to −0.5 on the Y axis. This middle of the range could be zero, with one direction being positive and the other negative, or it can be any arbitrary number, with one direction being higher and the other lower. 
         [0195]    As illustrated in  FIG. 12 , at target angle  32  simulated sensitivity of sensor  20  to rotation is approximately 1% of the dynamic range per milliradian of rotation. 
         [0196]    This 1% sensitivity per milliradian is sufficient to provide the desired accuracy (1 mm rns), using a 5 cm×10 cm sensor module  20  with shields  36  having a height  35  of 5 cm interspersed between radiation detectors  22  and located 25 cm from source  38  with an activity of 0.05 mCi. Adjusting accuracy parameters, increasing the size of detectors  22 , reducing the distance between sensor  20  and source  38  and increasing the activity of source  38  could each serve to reduce the level of directional sensitivity desired of sensor  20 . 
         [0197]    Simulation results (not shown) using a sensor  20  of the type shown in  FIG. 6A  were similar to those described hereinabove. 
         [0198]    System  40  and/or sensors  20  rely upon execution of various commands and analysis and translation of various data inputs. Any of these commands, analyses or translations may be accomplished by software, hardware or firmware according to various alternative embodiments. In an exemplary embodiment of the invention, machine readable media contain instructions for transforming output signal  34  from one or more sensor modules  20  into position co-ordinates of source  38 , optionally according to method  400 . In an exemplary embodiment of the invention, CPU  42  executes instructions for transforming output signal  34  from one or more sensor modules  20  into position co-ordinates of source  38 , optionally according to method  400 . 
         [0199]    According to an exemplary embodiment of the invention a trackable medical device is manufactured by incorporating into or fixedly attaching a detectable amount of a radioactive isotope to the medical device. The radioactive isotope may or may not have a medical function according to various embodiments. Optionally, the radioactivity of the isotope has no medical function. Optionally, the radioactive isotope may be selected so that it can be used in the body without a protective coating without adverse reaction with tissue. In an exemplary embodiment of the invention, the detectable amount of isotope is in the range of 0.5 mCi to 0.001 mCi. Use of isotope source  38  with an activity in the lower portion of this range may depend on lower speeds of the device, sensitivity of detector(s)  22 , distance from sensor  20 . Optionally, at least 1, optionally at least 5, optionally at least 10, optionally at least 100 detectable counts per second are produced by the incorporated radioactive isotope. 
         [0200]    In the description and claims of the present application, each of the verbs “comprise”, “include” and “have” as well as any conjugates thereof, are used to indicate that the object or objects of the verb are not necessarily a complete listing of members, components, elements or parts of the subject or subjects of the verb. 
         [0201]    The present invention has been described using detailed descriptions of embodiments thereof that are provided by way of example and are not intended to necessarily limit the scope of the invention. The described embodiments comprise different features, not all of which are required in all embodiments of the invention. Some embodiments of the invention utilize only some of the features or possible combinations of the features. Variations of embodiments of the present invention that are described and embodiments of the present invention comprising different combinations of features noted in the described embodiments can be combined in all possible combinations including, but not limited to use of features described in the context of one embodiment in the context of any other embodiment. The scope of the invention is limited only by the following claims.