Rapid and Robust Ultrasonic Probe Calibration Employing a CT-Visible Randomized Spherical Phantom

Abstract

Our study presents a robust and rapid calibration technique for ultrasonic probes, utilizing a straightforward phantom that can be easily fabricated in a laboratory environment, alongside a newly developed algorithm. We designed a phantom with randomly placed spherical features of various gelatins. The phantom is CT-visible so that the phantom features are accurately measured via CT, obviating the need for high-precision phantom fabrication. The ultrasonic probe and phantom were tracked by an optical tracking system to transform the phantom's coordinate system into the probe's coordinate system. Furthermore, an algorithm for synchronization was developed to compensate for the 150ms delay in the system. We developed an easily-fabricated phantom and conducted a CT scan to accurately capture its geometry for ultrasonic spatial calibration. The projected images of the phantom, captured using an ultrasonic probe, showed ellipses. By measuring the centers, major, and minor axes of these ellipses, we estimated calibration parameters with the newly proposed algorithm that aligns the images with the transformation between the probe and the phantom. We conducted the calibration at three ultrasonic imaging depths: 6 cm, 9 cm, and 12 cm. Three operators continuously swept the probe over the phantom without stopping or controlling speed, achieving an average error of 1.2106mm. While numerous studies have proposed various methodologies, they face challenges in phantom fabrication, non-universality of the approach, and susceptibility to significant errors when deviating slightly from predefined constraints. Our approach, featuring real-time spatial calibration at various depths, consistently achieves precise alignments with minimal errors, proving its potential for widespread applications.