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

To quantify changes in electromagnetic (EM) tracking accuracy due to EM field distortion produced by metal components in conventional computed tomography (CT) tables. Correction methods are presented for restoring accuracy to clinically acceptable levels.

EM tracking accuracy was evaluated as a function of sensor location and orientation for an NDI Aurora EM tracking system with conventional GE and Siemens CT scanners. Distortion effects were quantified as a function of sensor distance from the EM field generator (EFG) and height above and longitudinal distance along the CT table. A map-based distortion correction method was developed and evaluated using interstitial tandem and ovoid/ring applicator phantoms.

Conventional CT tabletops produced EM distortion that resulted in clinically significant localization errors. The error magnitude increased when the sensor was located closer to the CT table surface and further from the EFG. The greatest errors occurred when the sensor position was fixed, but the orientation was varied. Under typical brachytherapy conditions, errors due to sensor translation were <1 mm for translations of ±50 mm, while errors produced by orientation averaged (maximum) 3.5 (5.3) mm for DICOM YZ plane rotations and 2.0 (2.9) mm for DICOM XZ plane rotations. A clinical interstitial tandem and ovoid applicator set had median (min, max) distortion errors of 1.5 (1.3, 4.7) mm for applicator tips and 0.6 (0.0, 4.0) mm for shafts. Map-based distortion corrections reduced these errors to 0.6 (0.2, 1.7) mm and 0.4 (0.0, 1.8) mm.

The metal components in conventional CT scanner tables produce EM distortion that can result in EM tracking localization errors >5 mm for nonparallel brachytherapy applicator geometries such as the tandem and ovoid or ring. The effects of EM distortion may be reduced by placing the EM field generator closer to the patient or elevating the patient above the table with pads. Distortion corrections can be used to reduce position errors to clinically acceptable levels.