Breakthrough technology has emerged which allows the navigation of a catheter tip through a tortuous channel, such as those found in the pulmonary system, to a predetermined target.  This technology compares the real-time movement of a locatable guide (LG) against a three-dimensional digital map of the targeted area of the body (for purposes of explanation, the pulmonary airways of the lungs will be used hereinafter, though one skilled in the art will realize the present invention could be used in any body cavity or system: circulatory, digestive, pulmonary, to name a few). 
Such technology is described in U.S. Pat.  Nos.  6,188,355; 6,226,543; 6,558,333; 6,574,498; 6,593,884; 6,615,155; 6,702,780; 6,711,429; 6,833,814; 6,974,788; and 6,996,430, all to Gilboa or Gilboa et al.;  and U.S. Published Applications Pub.  Nos.  2002/0193686; 2003/0074011; 2003/0216639; 2004/0249267 to either Gilboa or Gilboa et al.  All of these references are incorporated herein in their entireties. 
One aspect of this background technology pertains to the registration of the CT images that were used, collectively, as a three-dimensional digital map against the actual movement of the LG through the pulmonary system.  The user interface shows three separate CT-based images reconstructed by software from x, y, and z directions, simultaneously with the LG location superimposed onto the intersection point of the reconstructed images.  If the CT images do not accurately reflect the actual location of the airways, the LG will quickly appear to drift out of the airways as the LG is advanced, thereby diminishing the utility of the navigation system. 
Presently, registration points at chosen known landmarks in the central area of lungs are used to register or align the CT based digital map with the patient's chest cavity.  These registrations points are first chosen during a planning stage and marked on the internal lung surface.  At the beginning of the procedure, the corresponding points are touched and recorded using the LG aided by a bronchoscope in the patient's airways.  Doing so allows a computer to align the digital map with the data received from the LG such that an accurate representation of the LG's location is displayed on a monitor. 
However, due to various factors, the accuracy of the registration diminishes as the distance between LG and the registration points increases.  In other words, the navigation system is less accurate at the periphery of the lungs, where it is most needed.  This is due to various factors, two of which are the focus of the present invention.  The first factor involves the rigidity of the CT digital image utilized as a digital map by current system while the lung structure is flexible.  Second, as the distance increases from the last registration point, errors compound.  Compounded errors, coupled with the flexible airways, result in LG that appear to be outside of the airways on the CT images. 
As a result of the accumulative inaccuracies, the performance of the existing system is limited.  For example, once the bronchoscope is too big to advance, the existing system provides guidance to the user as to whether the LG is being advanced in the direction of the target ignoring the inaccuracies created by the flexibility and internal movement of the living airways.  In addition, the guidance instructions to the target are given without regard to the geometries of the airways leading to the target.  As a result, user gently advances the LG and watches whether the LG is moving in the direction of the target.  If it is not, the LG is retracted and the user “feels” for another airway that may lead to the target rather than see it directly on the CT cross-sections.  Hence, two problems arise.  First, the LG no longer appears to be located within the airways.  Second, the guidance provided does not guide the user along a logical path, it merely provides a general direction to the lesion. 
The present invention addresses these two issues by using a unique algorithm to create a BT skeleton, which is a three-dimensional virtual map of the bronchial airways, and by continuously and adaptively matching the LG path to the BT skeleton.  Due to the increased accuracy of the BT skeleton and the registration, three-dimensional guidance is extended past the limits of the bronchoscope.