Ultrasound Probe Calibration Using a Phantom Embedded with Randomly Positioned Spherical Objects

Abstract

We present a rapid and accurate freehand ultrasound probe spatial calibration technique that uses a simple phantom easily fabricated in laboratory settings. The phantom incorporates randomly distributed spherical inclusions made from gelatin mixtures with different ultrasonic velocities and is CT-visible, allowing precise geometric reconstruction without requiring high-precision manufacturing. An optical tracking system mapped the phantom coordinates to the probe coordinates, and a synchronization algorithm corrected approximately 150 ms of system latency. Elliptical projections in ultrasound images were analyzed by extracting ellipse centers and axes to estimate calibration parameters using our new alignment algorithm. Calibration performed at 6, 9, and 12 cm depth demonstrated consistent accuracy, with three operators freely sweeping the probe across the phantom and achieving a mean error of 0.6656 mm. This method addresses common limitations in existing approaches—such as complex phantom fabrication, limited generalizability, and high sensitivity to deviations—and enables reliable real-time calibration across multiple depths with minimal error, supporting its potential for broad clinical and research adoption.