Three-dimensional CT-guided bronchoscopy with a real-time electromagnetic position sensor: A comparison of two image registration methods

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 I 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 image. 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 I 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 bronchoscopy.