A fully automatic calibration procedure for freehand 3D ultrasound

Describes a novel method for calibration of freehand three-dimensional (3D) ultrasound. A position sensor is mounted on a conventional ultrasound probe, thus the set of B-scans can be localized in 3D, and can be compounded into a volume. The calibration process aims at determining the transformation (translations, rotations, scaling) between the coordinates system of images and the coordinate system of the localization system. In our study, the phantom used to calibrate the 3D ultrasound system is a plane. It provides in each image a strong, straight line. The calibration process is based on the set of lines in 2D images forming a plane in 3D. Points of interest are extracted from the ultrasound sequence. The eight parameters of the transformation are determined with an iterative algorithm which is based on the principle that correct registration between the plane and the points of interest provides correct calibration. Validation of this method has been performed on synthetic sequences. This calibration method is shown to be easy to perform, completely automatic and fast enough for clinical use.