Abstract Ultrasound probe calibration is a necessary step in the process of 3D ultrasound imaging by a freehand 2D ultrasound probe. In order to calibrate ultrasound probe quickly and effectively with small number of calibration images, a flexible calibration method based on optical tracker was presented for determining the unknown transformation matrix. First, a calibration pattern containing two mutually perpendicular planes was developed. In the process of calibration, the calibration pattern was scanned by a freehand 2D ultrasound probe. Transformation matrice were calculated from camera to calibration pattern and from ultrasound probe to ultrasound image. There were 3 crosspoints between ultrasound beam and three orthogonal axes of the pattern in each trial, which setup 6 calibration equations. The unknown transformation matrix and 2 scaling factors were solved by nonlinear fitting, when at least 2 calibration images were captured. Experiments with synthetic data show that calibration error decreases with increased number of images. The error was 1 mm when the number of images was 30. Experiments with real data show that our method has high accuracy with only 20 calibration images, meeting the demand of ultrasound surgical navigation and lays the groundwork for 3D ultrasound imaging reconstruction.
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