C-arm angle measurement with accelerometer for brachytherapy: an accuracy study

Abstract

Purpose

 X-ray fluoroscopy guidance is frequently used in medical interventions. Image-guided interventional procedures that employ localization for registration require accurate information about the C-arm’s rotation angle that provides the data externally in real time. Optical, electromagnetic, and image-based pose tracking systems have limited convenience and accuracy. An alternative method to recover C-arm orientation was developed using an accelerometer as tilt sensor.

Methods

    The fluoroscopic C-arm’s orientation was estimated using a tri-axial acceleration sensor mounted on the X-ray detector as a tilt sensor. When the C-arm is stationary, the measured acceleration direction corresponds to the gravitational force direction. The accelerometer was calibrated with respect to the C-arm’s rotation along its two axes, using a high-accuracy optical tracker as a reference. The scaling and offset error of the sensor was compensated using polynomial fitting. The system was evaluated on a GE OEC 9800 C-arm. Results obtained by accelerometer, built-in sensor, and image-based tracking were compared, using optical tracking as ground truth data.

Results

The accelerometer-based orientation measurement error for primary angle rotation was $-0.1\pm 0.0^{\circ }$ and for secondary angle rotation it was $0.1\pm 0.0^{\circ }$. The built-in sensor orientation measurement error for primary angle rotation was $-0.1\pm 0.2^{\circ }$, and for secondary angle rotation it was $0.1\pm 0.2^{\circ }$. The image-based orientation measurement error for primary angle rotation was $-0.1\pm 1.3^{\circ }$, and for secondary angle rotation it was $-1.3\pm 0.3^{\circ }$.

Conclusion

The accelerometer provided better results than the built-in sensor and image-based tracking. The accelerometer sensor is small, inexpensive, covers the full rotation range of the C-arm, does not require line of sight, and can be easily installed to any mobile X-ray machine. Therefore, accelerometer tilt sensing is a very promising applicant for orientation angle tracking of C-arm fluoroscopes.