A Novel Approach for Rapid and Precise Freehand Ultrasound Probe Spatial Calibration Utilizing an Easily Fabricated Phantom

Abstract

Objectives: Our study demonstrates an efficient, rapid calibration technique for ultrasound probes using a simple phantom, which can be readily constructed in a laboratory setting.

Methods: 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. Also, an algorithm for synchronization was developed to compensate for the 150ms delay in the system.

Results: We developed a robust phantom and conducted a CT scan to accurately capture its geometry for ultrasound spatial calibration. The projected images of the phantom, captured using an ultrasound 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 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 0.6107mm.

Conclusions: Numerous studies have proposed various methodologies, albeit confronted with challenges in phantom fabrication, non-universality of the approach, and susceptibility to significant errors when deviating slightly from predefined constraints. This approach, featuring real-time spatial calibration at various depths, consistently achieves precise alignments with minimal errors, proving its potential for widespread uses.