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The influence of non‐rigid anatomy and patient positioning on endoscopy‐ CT image registration in the head and neck
Author(s) -
Ingram W. Scott,
Yang Jinzhong,
Wendt Richard,
Beadle Beth M.,
Rao Arvind,
Wang Xin A.,
Court Laurence E.
Publication year - 2017
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1002/mp.12352
Subject(s) - image registration , head and neck , medical imaging , endoscopy , medicine , head (geology) , computed tomography , radiology , anatomy , computer vision , computer science , image (mathematics) , surgery , biology , paleontology
Purpose To assess the influence of non‐rigid anatomy and differences in patient positioning between CT acquisition and endoscopic examination on endoscopy‐ CT image registration in the head and neck. Methods Radiotherapy planning CT s and 31–35 daily treatment‐room CT s were acquired for nineteen patients. Diagnostic CT s were acquired for thirteen of the patients. The surfaces of the airways were segmented on all scans and triangular meshes were created to render virtual endoscopic images with a calibrated pinhole model of an endoscope. The virtual images were used to take projective measurements throughout the meshes, with reference measurements defined as those taken on the planning CT s and test measurements defined as those taken on the daily or diagnostic CT s. The influence of non‐rigid anatomy was quantified by 3D distance errors between reference and test measurements on the daily CT s, and the influence of patient positioning was quantified by 3D distance errors between reference and test measurements on the diagnostic CT s. The daily CT measurements were also used to investigate the influences of camera‐to‐surface distance, surface angle, and the interval of time between scans. Results Average errors in the daily CT s were 0.36 ± 0.61 cm in the nasal cavity, 0.58 ± 0.83 cm in the naso‐ and oropharynx, and 0.47 ± 0.73 cm in the hypopharynx and larynx. Average errors in the diagnostic CT s in those regions were 0.52 ± 0.69 cm, 0.65 ± 0.84 cm, and 0.69 ± 0.90 cm, respectively. All CT s had errors heavily skewed towards 0, albeit with large outliers. Large camera‐to‐surface distances were found to increase the errors, but the angle at which the camera viewed the surface had no effect. The errors in the Day 1 and Day 15 CT s were found to be significantly smaller than those in the Day 30 CT s ( P < 0.05). Conclusions Inconsistencies of patient positioning have a larger influence than non‐rigid anatomy on projective measurement errors. In general, these errors are largest when the camera is in the superior pharynx, where it sees large distances and a lot of muscle motion. The errors are larger when the interval of time between CT acquisitions is longer, which suggests that the interval of time between the CT acquisition and the endoscopic examination should be kept short. The median errors found in this study are comparable to acceptable levels of uncertainty in deformable CT registration. Large errors are possible even when image alignment is very good, indicating that projective measurements must be made carefully to avoid these outliers.