Study objectives: To compare two different image
registration methods for accurately displaying the position of a
flexible bronchoscope on a previously acquired three-dimensional CT
scan during bronchoscopy.
Setting: Bronchoscopy suite
of a university hospital.
Patients: Fifteen adult
patients scheduled for nonemergent bronchoscopy.
Methods: A miniature electromagnetic position
sensor was placed at the tip of a flexible bronchoscope. Previously
acquired three-dimensional CT scans were registered with the patient in
the bronchoscopy suite. Registration method 1 used multiple skin
fiducial markers. Registration method 2 used the inner surface of the
trachea itself for registration. Method 1 was objectively assessed by
measuring the error distance between the real skin marker position and
the computer display position. Methods 1 and 2 were subjectively
assessed by the bronchoscopist correlating visual bronchoscopic
anatomic location with the computer display position on the CT
Results: The error distance (± SD) from known
points for registration method 1 was 5.6 ± 2.7 mm. Objective error
distances were not measured for method 2 because no accurate placement
of the bronchoscope sensor could be correlated with CT position.
Subjectively, method 2 was judged more accurate than method 1 when
compared with the fiberoptic view of the airways through the
bronchoscope. Additionally, method 2 had the advantage of not requiring
placement of fiducial markers before the CT scan. Respiratory motion
contributed an error of 3.6 ± 2.6 mm, which was partially compensated
for by a second tracking sensor placed on the patient’s chest.
Conclusion: Image registration method 2 of surface fitting
the trachea rather than method 1 of fiducial markers was subjectively
judged to be superior for registering the position of a flexible
bronchoscope during bronchoscopy. Method 2 was also more practical
inasmuch as no special CT scanning technique was required before