An image-based method to synchronize cone-beam CT and optical surface tracking.

Aurora Fassi,Joël Schaerer,Guido Baroni,Marco Riboldi,David Sarrut

doi:10.1120/jacmp.v16i2.5152

Abstract

The integration of in‐room X‐ray imaging and optical surface tracking has gained increasing importance in the field of image guided radiotherapy (IGRT). An essential step for this integration consists of temporally synchronizing the acquisition of X‐ray projections and surface data. We present an image‐based method for the synchronization of cone‐beam computed tomography (CBCT) and optical surface systems, which does not require the use of additional hardware. The method is based on optically tracking the motion of a component of the CBCT/gantry unit, which rotates during the acquisition of the CBCT scan. A calibration procedure was implemented to relate the position of the rotating component identified by the optical system with the time elapsed since the beginning of the CBCT scan, thus obtaining the temporal correspondence between the acquisition of X‐ray projections and surface data. The accuracy of the proposed synchronization method was evaluated on a motorized moving phantom, performing eight simultaneous acquisitions with an Elekta Synergy CBCT machine and the AlignRT optical device. The median time difference between the sinusoidal peaks of phantom motion signals extracted from the synchronized CBCT and AlignRT systems ranged between ‐3.1 and 12.9 msec, with a maximum interquartile range of 14.4 msec. The method was also applied to clinical data acquired from seven lung cancer patients, demonstrating the potential of the proposed approach in estimating the individual and daily variations in respiratory parameters and motion correlation of internal and external structures. The presented synchronization method can be particularly useful for tumor tracking applications in extracranial radiation treatments, especially in the field of patient‐specific breathing models, based on the correlation between internal tumor motion and external surface surrogates.PACS number: 87

Highlights

118 Fassi et al.: cone-beam computed tomography (CBCT) and surface tracking synchronization extracranial tumors for the assessment of target position and for the compensation of inter- and intrafraction motion.[2]
The insertion of fiducials within the lung can be associated to the risk of pneumothorax or fiducial migration.[5] optical surface tracking provides noninvasive patient motion monitoring and can be applied for deriving a breathing surrogate from which tumor motion is indirectly estimated by means of external–internal correlation models.[6]. Optical localization devices are used to capture the displacement of the patient surface, by reconstructing the position of active or passive markers placed on the patient skin[7] or by scanning the entire markerless surface.[8,9] A multidimensional respiratory motion signal can be extracted both from surface marker trajectories and from markerless optical surfaces by applying deformable mesh registration.[10]
Internal correlation model is initialized before treatment and periodically updated throughout the whole fraction by simultaneously acquiring the external surface surrogate and the internal tumor motion.[14]. The integrated approach overcomes the drawbacks of the single techniques: the noninvasivity of optical tracking allows the continuous monitoring of intrafraction organ motion during the entire treatment course, whereas anatomical data provided by X-ray imaging enables the update of the external–internal correlation to reduce tumor position estimation uncertainties.[15,16] The accuracy of tumor tracking systems based on correlation models was quantified within 2.5 mm in the anteroposterior direction and 1.9 mm in the mediolateral and superior–inferior (SI) directions.[17]

Summary

Introduction

118 Fassi et al.: CBCT and surface tracking synchronization extracranial tumors for the assessment of target position and for the compensation of inter- and intrafraction motion.[2]. Internal correlation model is initialized before treatment and periodically updated throughout the whole fraction by simultaneously acquiring the external surface surrogate and the internal tumor motion.[14] The integrated approach overcomes the drawbacks of the single techniques: the noninvasivity of optical tracking allows the continuous monitoring of intrafraction organ motion during the entire treatment course, whereas anatomical data provided by X-ray imaging enables the update of the external–internal correlation to reduce tumor position estimation uncertainties.[15,16] The accuracy of tumor tracking systems based on correlation models was quantified within 2.5 mm in the anteroposterior direction and 1.9 mm in the mediolateral and superior–inferior (SI) directions.[17]. Breathing motion models can be extracted from planning cine-CT[26] or four-dimensional (4D) CT[27] and adapted to interfraction baseline variations based on daily CBCT imaging.[28]

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Conclusion