Improving N-wire phantom-based freehand ultrasound calibration

Area of Science:

• Medical Imaging
• Biomedical Engineering
• Surgical Navigation

Background:

• Freehand tracked ultrasound imaging is a cost-effective, non-invasive technique vital for guided interventions.
• Spatial calibration between the tracker and ultrasound image plane is essential for this technique.
• N-wire phantoms are commonly used for automatic calibration due to straightforward fiducial segmentation and real-time localization.

Purpose of the Study:

• To investigate if incorporating information from all wires in N-wire phantoms improves spatial calibration accuracy compared to using only middle wires.
• To evaluate the precision and accuracy of implemented calibration methods for comparison with existing techniques.

Main Methods:

• A novel cost function was developed, utilizing in-plane errors between segmented wire points and their intersections with the image plane.
• The cost function was minimized iteratively, with an initial seed derived from a closed-form solution based on middle wires.
• Calibration precision and accuracy were assessed using both shallow and deep probes with an N-wire phantom.

Main Results:

• Mean calibration precision of approximately 0.5 mm was achieved with a shallow probe.
• Mean accuracy was around 1.4 mm across all implemented methods.
• Precision using a deep probe was approximately 2.0 mm.

Conclusions:

• The N-wire phantom calibration, particularly with a shallow probe, demonstrates precision and accuracy comparable to more established methods.
• Optimizing calibration parameters with the proposed cost function enhances the consistency of N-wire based calibration.