The feasibility of endoscopy-CT image registration in the head and neck without prospective endoscope tracking.

W Scott Ingram,Richard Wendt,Jinzhong Yang,Xin A Wang,Laurence E Court,Beth M Beadle,Arvind Rao,Qinghui Zhang

doi:10.1371/journal.pone.0177886

W Scott Ingram, Richard Wendt + Show 6 more

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https://doi.org/10.1371/journal.pone.0177886

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Abstract

PurposeEndoscopic examinations are frequently-used procedures for patients with head and neck cancer undergoing radiotherapy, but radiation treatment plans are created on computed tomography (CT) scans. Image registration between endoscopic video and CT could be used to improve treatment planning and analysis of radiation-related normal tissue toxicity. The purpose of this study was to explore the feasibility of endoscopy-CT image registration without prospective physical tracking of the endoscope during the examination.MethodsA novel registration technique called Location Search was developed. This technique uses physical constraints on the endoscope’s view direction to search for the virtual endoscope coordinates that maximize the similarity between the endoscopic video frame and the virtual endoscopic image. Its performance was tested on phantom and patient images and compared to an established registration technique, Frame-To-Frame Tracking.ResultsIn phantoms, Location Search had average registration errors of 0.55 ± 0.60 cm for point measurements and 0.29 ± 0.15 cm for object surface measurements. Frame-To-Frame Tracking achieved similar results on some frames, but it failed on others due to the virtual endoscope becoming lost. This weakness was more pronounced in patients, where Frame-To-Frame tracking could not make it through the nasal cavity. On successful patient video frames, Location Search was able to find endoscope positions with an average distance of 0.98 ± 0.53 cm away from the ground truth positions. However, it failed on many frames due to false similarity matches caused by anatomical structural differences between the endoscopic video and the virtual endoscopic images.ConclusionsEndoscopy-CT image registration without prospective physical tracking of the endoscope is possible, but more development is required to achieve an accuracy suitable for clinical translation.

Highlights

Many head and neck cancers are visible by direct inspection, or more frequently, by the use of nasopharyngolaryngoscopy
The endoscopic examination is crucial for understanding the location and extent of disease prior to radiation treatment planning, since mucosal changes can indicate tumor involvement that may or may not correlate with the findings using crosssectional imaging such as computed tomography (CT) or magnetic resonance imaging (MRI)
The difference is due to the fact that the virtual endoscope became lost about two thirds of the way through video sequence 1, resulting in very large errors for the last seven registration frames

Summary

Introduction

Many head and neck cancers are visible by direct inspection, or more frequently, by the use of nasopharyngolaryngoscopy. In order to use the endoscopic video quantitatively and objectively, the two modalities must be registered so that the information contained in the video can be mapped into CT space, i.e. the coordinate system of the CT scan. This additional information could be used to enhance tumor delineation during the treatment planning process and to provide new methods to analyze radiation-related normal tissue toxicity, which may not be appreciable using other imaging techniques

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