An image registration approach to automated calibration for freehand 3D ultrasound

This paper describes an image registration approach to calibration for freehand three-dimensional (3D) ultrasound, If a conventional ultrasound probe is tracked using a position sensor, and the relation between this sensor and the two-dimensional (2D) image plane is known, the resulting set of B-scans may be correctly compounded into an image volume. Calibration is the process of determining the transformation (rotation, translation, and optionally image scaling) that maps B-mode image slice coordinates to points in the coordinate system of the tracking sensor mounted on the ultrasound probe. A set of 2D ultrasound images of a calibration phantom is obtained using a tracked ultrasound probe. Calibration is performed by searching for the calibration parameters that maximise the similarity between a model of the calibration phantom, which can be an image volume or a geometrical model, and the ultrasound images transformed into the coordinate space of the phantom. Validation of tl-Lis calibration method is performed using a gelatin phantom. Measures of the calibration reproducibility, reconstruction precision and reconstruction accuracy are presented for this technique, and compared to those obtained using a conventional cross-wire phantom. Registration-based calibration is shown to be a rapid and accurate method of automatic calibration for freehand 3D ultrasound.