Freehand ultrasound calibration: Phantom versus tracked pointer

PURPOSE: Ultrasound-guided tracked navigation requires spatial calibration between the ultrasound beam and the tracker. We examined the reproducibility and accuracy of two popular open source calibration methods¹ with a handheld linear ultrasound transducer. METHODS : A total of 10 calibrations were performed using (1) a double N-wire phantom with automatic image segmentation and registration; (2) and registration of landmark points collected with a tracked pointer. Reproducibility and accuracy were characterized by comparing the resulting transformation matrices, and by comparing ground truth landmark points. RESULTS: Transformation matrices calculated with an N-wire phantom showed a variance of X: 0.02 mm (in the direction of sound propagation), Y: 0.03 mm (in the direction of transducer elements) and Z: 0.21 mm (in the elevation direction). Transformation matrices obtained with tracked pointer showed a variance of X: 0.1 mm, Y: 0.10 mm and Z: 0.43 mm. Calibration accuracy was tested with ground truth cross wire points. The N-wire phantom provided a calibration with a distance from ground truth of X: 2.44 ± 1.44 mm, Y: 1.21 ±0.88 mm, and Z: 1.12 ± 0.82 mm. Tracked pointer calibration had a distance from the ground truth of X: 0.23 ± 0.16 mm, Y: 0.62 ± 0.31 mm, and Z: 0.45 ± 0.33 mm. Distance from ground truth was significantly less (p