EM tracking with conventional CT tables: Quantifying and correcting for electromagnetic field distortion

Area of Science:

• Medical Physics
• Imaging Technology
• Radiation Oncology

Background:

• Electromagnetic (EM) tracking systems are crucial for precise instrument localization in image-guided procedures.
• Conventional computed tomography (CT) tables contain metal components that can interfere with EM fields, potentially compromising tracking accuracy.

Purpose of the Study:

• To quantify the impact of EM field distortion from CT table metal components on EM tracking accuracy.
• To develop and evaluate correction methods for restoring EM tracking accuracy to clinically acceptable levels.

Main Methods:

• Evaluated NDI Aurora EM tracking system accuracy with GE and Siemens CT scanners.
• Quantified distortion effects based on sensor proximity to the EM field generator (EFG) and CT table.
• Developed and tested a map-based distortion correction method using brachytherapy applicator phantoms.

Main Results:

• CT tables introduced clinically significant EM tracking localization errors, particularly with varying sensor orientation.
• Errors averaged up to 3.5 mm for rotations in the YZ plane and 2.0 mm for XZ plane rotations.
• Map-based corrections reduced applicator tip and shaft localization errors to sub-millimeter levels (median 0.6 mm and 0.4 mm, respectively).

Conclusions:

• Metal components in CT tables cause substantial EM tracking errors, exceeding 5 mm for complex brachytherapy geometries.
• Mitigation strategies include positioning the EFG closer to the patient or elevating the patient.
• Developed distortion correction methods successfully restore EM tracking accuracy to clinically acceptable levels.