Locatable catheter

1. A locatable medical instrument insertable through a working channel of a bronchoscope comprising:

an elongate flexible sheath defining a longitudinal lumen extending therethrough and having a distal end, the sheath configured to receive a probe therethrough for positioning the sheath adjacent target tissue;

at least three coils embedded within the flexible sheath proximate said distal end, the at least three coils overlying one another and having a common center point;

wherein each of said at least three coils is configured to generate a signal when placed in an electromagnetic field, said signal indicative of a strength of said electromagnetic field and the position of the flexible sheath when the probe is removed therefrom.

2. The locatable medical instrument of claim 1 wherein at least three coils is approximately perpendicular to a longitudinal axis of said sheath.

3. The locatable medical instrument of claim 1 wherein two of said at least three coils are approximately perpendicular to each other.

4. The locatable medical instrument of claim 1 wherein two of said at least three coils are approximately perpendicular to each other.

5. A system for navigating through a branched structure comprising:

a sheath having a distal end and defining a lumen extending longitudinally therethrough;

a first electromagnetic field sensor configuration embedded within the sheath and located near said distal end of said sheath, the first electromagnetic field sensor configuration including at least three coils overlying one another and sharing a common center point;

a probe extendable through said sheath and having a second electromagnetic field sensor embedded within a distal tip thereof;

an electromagnetic field generation device configured to create electromagnetic fields having a plurality of frequencies and configured to induce signals in said first and second sensors,

wherein said first electromagnetic field sensor is configured to generate signals indicating its position relative to the branched structure when said probe is removed from said sheath.

6. The system of claim 5 wherein said first electromagnetic field sensor comprises a six degree-of-freedom sensor.

7. The system of claim 5 wherein said second electromagnetic field sensor comprises a six degree-of-freedom sensor.

8. The system of claim 5 wherein said electromagnetic field generation device creates electromagnetic fields of at least four frequencies, three of said frequencies detected by said second electromagnetic field sensor, thereby allowing said second electromagnetic field sensor to generate six degree-of-freedom location information.

9. The system of claim 8 wherein one of said at least four frequencies is detected by said first electromagnetic field sensor, thereby allowing said first electromagnetic field sensor to generate a signal including location information.

10. The system of claim 5 wherein at least one coil of the at least three coils is approximately perpendicular to the lumen.

11. The system of claim 5 wherein at least two coils of the at least three coils are approximately perpendicular to each other.

12. A method for accessing a target in a branched structure comprising:

providing an electromagnetic field generator defining a sensing volume containing said branched structure;

advancing a probe through said branched structure, said probe including a probe electromagnetic field sensor that is configured to sense electromagnetic fields generated by said generator and creating probe location signals as a result;

using said probe location signals to locate said target;

advancing a sheath over said probe to said target, said sheath having at least three coils at a distal end thereof that are configured to sense electromagnetic fields generated by said generator and creating sheath location signals as a result, the at least three coils embedded within the sheath and overlying one another so as to share a common center point;

removing said probe proximally from said sheath;

monitoring a location of said sheath using said sheath location signals, to ensure the distal end of said sheath remains at said target; and

accessing said target with one or more instruments by passing said one or more instruments through said sheath.

13. The method of claim 12 wherein said step of monitoring a location of said sheath comprises reading signals from said sensor on said sheath at a frequency that is independent of frequencies of signals from said sensor on said probe.

14. The method of claim 12 wherein said step of monitoring a location of said sheath comprises reading signals from said sensor on said sheath at a frequency that is the same as frequencies of signals from said sensor on said probe.