Surface registration of a ct image with a magnetic tracking system

Abstract

Apparatus, including a magnetic tracking system, which generates a magnetic field near a living subject's body, and a probe, having a distal end that can contact registration points on a surface of the body. The probe has a contact sensor, located within the distal end, that outputs first signals indicative of a quality of the contact between the distal end with the registration points. Located within the distal end is a magnetic detector that outputs second signals that are indicative of respective positions of the registration points in a coordinate frame of the magnetic tracking system. A processor receives a tomographic image of the subject, and verifies, based on the quality of the contact indicated by the first signals, that the registration points are valid, and registers the tomographic image in the coordinate frame of the magnetic tracking system using the positions of the valid registration points.

We claim: 
     
         1 . Apparatus, comprising:
 a magnetic tracking system, which is configured to generate a magnetic field in a vicinity of a body of a living subject;   a probe, having a distal end configured to be brought into contact with one or more registration points on a surface of the body, and comprising:
 a contact sensor, located within the distal end and configured to output first signals indicative of a quality of the contact between the distal end with the one or more registration points; and 
 a magnetic detector, located within the distal end and configured to output second signals in response to the magnetic field that are indicative of respective positions of the one or more respective registration points in a coordinate frame of the magnetic tracking system; and 
   a processor configured to receive a tomographic image of the subject, and to verify, based on the quality of the contact indicated by the first signals, that the one or more registration points are valid, and to register the tomographic image in the coordinate frame of the magnetic tracking system using the positions of the one or more registration points that were verified to be valid.   
     
     
         2 . The apparatus according to  claim 1 , wherein the contact sensor comprises an electrode, and wherein verifying that the one or more registration points are valid comprises verifying that an impedance measured by the electrode on contact with each of the registration points lies within a predetermined range. 
     
     
         3 . The apparatus according to  claim 2 , wherein the predetermined range is 20 kΩ-40 kΩ when the impedance is measured at 20 kHz. 
     
     
         4 . The apparatus according to  claim 2 , wherein the predetermined range is evaluated in response to a position of a grounding element on the surface of the body, and in response to a degree of contact of the grounding element with the surface. 
     
     
         5 . The apparatus according to  claim 1 , and comprising a handle connected to the probe so as to form a rigid probe assembly. 
     
     
         6 . The apparatus according to  claim 1 , wherein the one or more registration points are selected to be visible to a naked eye of a user of the probe. 
     
     
         7 . The apparatus according to  claim 1 , wherein the one or more registration points are selected to be immobile on the surface of the body. 
     
     
         8 . The apparatus according to  claim 1 , wherein the contact sensor comprises a force sensor, and wherein verifying that one or more registration points are valid comprises verifying that a force measured by the force sensor on contact with each registration point lies within a predetermined range. 
     
     
         9 . The apparatus according to  claim 8 , wherein the predetermined range is 2 gm-8 gm. 
     
     
         10 . The apparatus according to  claim 8 , wherein the predetermined range is evaluated in response to a user of the probe contacting preselected points on the surface of the body. 
     
     
         11 . A method, comprising:
 generating, with a magnetic tracking system, a magnetic field in a vicinity of a body of a living subject;   bringing a distal end of a probe into contact with one or more registration points on a surface of the body, the probe comprising:
 a contact sensor, located within the distal end and configured to output first signals indicative of a quality of the contact between the distal end with the one or more registration points, and 
 a magnetic detector, located within the distal end and configured to output second signals in response to the magnetic field that are indicative of respective positions of the one or more respective registration points in a coordinate frame of the magnetic tracking system; 
   receiving a tomographic image of the subject;   verifying, based on the quality of the contact indicated by the first signals, that the one or more registration points are valid; and   registering the tomographic image in the coordinate frame of the magnetic tracking system using the positions of the one or more registration points that were verified to be valid.   
     
     
         12 . The method according to  claim 11 , wherein the contact sensor comprises an electrode, and wherein verifying that the one or more registration points are valid comprises verifying that an impedance measured by the electrode on contact with each of the registration points lies within a predetermined range. 
     
     
         13 . The method according to  claim 12 , wherein the predetermined range is 20 kΩ-40 kΩ when the impedance is measured at 20 kHz. 
     
     
         14 . The method according to  claim 12 , and comprising evaluating the predetermined range in response to a position of a grounding element on the surface of the body, and in response to a degree of contact of the grounding element with the surface. 
     
     
         15 . The method according to  claim 11 , and comprising connecting a handle to the probe so as to form a rigid probe assembly. 
     
     
         16 . The method according to  claim 11 , and comprising selecting the one or more registration points to be visible to a naked eye of a user of the probe. 
     
     
         17 . The method according to  claim 11 , and comprising selecting the one or more registration points to be immobile on the surface of the body. 
     
     
         18 . The method according to  claim 11 , wherein the contact sensor comprises a force sensor, and wherein verifying that one or more registration points are valid comprises verifying that a force measured by the force sensor on contact with each registration point lies within a predetermined range. 
     
     
         19 . The method according to  claim 18 , wherein the predetermined range is 2 gm-8 gm. 
     
     
         20 . The method according to  claim 18 , and comprising evaluating the predetermined range in response to a user of the probe contacting preselected points on the surface of the body.