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Ultrasound Transducer Localization Using the CyberKnife's X-Ray System


R Bruder

R Bruder1*, S Ipsen1, P Jauer1, F Ernst1, O Blanck2, A Schweikard1, (1) University of Luebeck, Luebeck, SH, (2) CyberKnife Center Northern Germany, Guestrow, Mecklenburg / Vorpommern

MO-D-144-2 Monday 2:00PM - 3:50PM Room: 144

Purpose: 4D ultrasound has become an alternative for image guidance and motion compensation in radiosurgery. Nevertheless, a two-step localization has to be performed when using ultrasound. In addition to target localization inside the ultrasound volume, the transducer itself has to be localized and the target position has to be transformed into treatment coordinates.
Methods: The CyberKnife (Accuray Inc.) features a stereo X-ray system which is used for patient and fiducial marker localization. Accessing only the raw X-ray images, we designed a software package for additional marker detection. To measure the physical camera parameters, X-ray phantoms were designed and a non-orthogonal stereo camera calibration was performed on 50 calibration X-ray pairs. Algorithms were developed for three-dimensional single marker localization and six-dimensional localization of marker geometries. Different X-ray marker geometries were designed and attached to the ultrasound transducer. The final system was evaluated and compared to the on-board localization system using a 6-axis industrial robot to position different marker geometries at 150 randomly distributed positions over the acquisition volume.
Results: Two marker geometries with 20 and 50mm base lengths were localized using the on-board and newly developed software. The mean translational error for the on-board localization is 0.202mm, for the new system 0.218mm. The rotational error of the on-board system could be reduced from 0.212 to 0.053 degrees for the 50mm marker and from 0.272 to 0.076 degrees for the 20mm marker geometry.
Conclusion: We have shown that ultrasound transducers can be localized using the CyberKnife x-ray system. The rotational accuracy of the localization could be increased by a factor of four, which is important for high marker-to-ultrasound-target distances. Furthermore, using the full area of the flat-panel detectors without pre-processing steps the tracking volume was increased by 70 percent, which helps detecting patient/fiducials and transducer at the same time.

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