Comparison of freehand 3-D ultrasound calibration techniques using a stylus.

In a freehand 3-D ultrasound system, a probe calibration is required to find the rigid body transformation from the corner of the B-scan to the electrical center of the position sensor. The most intuitive way to perform such a calibration is by locating fiducial points in the scan plane directly with a stylus. The main problem of this approach is the difficulty in aligning the tip of the stylus with the scan plane. The thick beamwidth makes the tip of the stylus visible in the B-scan, even if the tip is not exactly at the elevational center of the scan plane. We present a novel stylus and phantom that simplify the alignment process for more accurate probe calibration. We also compare our calibration techniques with a range of styli. We show that our stylus and cone phantom are both simple in design and can achieve a point reconstruction accuracy of 2.2 mm and 1.8 mm, respectively, an improvement from 3.2 mm and 3.6 mm with the sharp and spherical stylus. The performance of our cone stylus and phantom lie between the state-of-the-art Z-phantom and Cambridge phantom, where accuracies of 2.5 mm and 1.7 mm are achieved.

A mechanical instrument for 3D ultrasound probe calibration.

We present a novel technique for 3D ultrasound probe calibration. The principle of operation is that the beam is aligned with a set of coplanar wires strung across a rigid frame. The probe and frame are mounted on a precision-manufactured mechanical instrument which allows adjustment and measurement of their relative pose. Semi-automatic image processing facilitates alignment of the beam and wires to within a tolerance of around 0.2 mm, despite the considerable beam thickness. The calibration process requires just a single view and relatively little user expertise. In a series of experiments with different ultrasound probes, we demonstrate the technique's high accuracy and precision. The latter is partly due to the elimination of the position sensor, a significant source of measurement noise, from the end-user calibration process.