Development of an optical-based image guidance system: Technique detecting external markers behind a full facemask

Hwiyoung Kim, Yang Kyun Park, Il Han Kim, Kunwoo Lee, Sung Joon Ye

Research output: Contribution to journalArticlepeer-review

7 Scopus citations


Purpose: Optical image-guided systems (e.g., AlignRT®, frameless SonArray, ExacTrac®) have been used with advantages of avoiding excessive radiation exposure and real-time patient monitoring. Although these systems showed proven accuracy, they need to modify a full facemask for patients with HN cancer and brain tumor. We developed an optical-based guidance system to manage interfractional and intrafractional setup errors by tracking external markers behind a full facemask. Methods: Infra-red (IR) reflecting markers were attached on the face of a head phantom and then the phantom was immobilized by a full face thermoplastic mask. A stereo camera system consisting of two CCD cameras was mounted on the inferior wall of treatment room. The stereo camera system was calibrated to reconstruct 3D coordinates of multiple markers with respect to the isocenter using the direct linear transform (DLT) algorithm. The real-time position of the phantom was acquired, through the stereo camera system, by detecting the IR markers behind the full facemask. The detection errors with respect to the reference positions of planning CT images were calculated in six degrees of freedom (6-DOF) by a rigid-body registration technique. Results: The calibration accuracy of the system was in submillimeter (0.33 mm 0.27 mm), which was comparable to others. The mean distance between each of marker positions of optical images and planning CT images was 0.50 mm 0.67 mm. The maximum deviations of 6-DOF registration were less than 1 mm and 1 for the couch translation and rotation, respectively. Conclusions: The developed system showed the accuracy and consistency comparable to the commercial optical guided systems, while allowing us to simultaneously immobilize patients with a full face thermoplastic mask.

Original languageEnglish (US)
Pages (from-to)3006-3012
Number of pages7
JournalMedical physics
Issue number6
StatePublished - Jun 2011


  • IR marker
  • full facemask
  • localization
  • optical-based image guidance

ASJC Scopus subject areas

  • Biophysics
  • Radiology Nuclear Medicine and imaging


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