Effect of Metal Hip Prostheses On the Accuracy of Electromagnetic Localization and Tracking
B Kurko*, W Butler, G Merrick, Wheeling Hospital, Wheeling, WVSU-E-J-154 Sunday 3:00PM - 6:00PM Room: Exhibit Hall
Purpose: To determine the effect of metal hip prostheses on the accuracy of Calypso electromagnetic transponder tracking. Calypso warns that prosthetic pelvic implants are contraindicated.
Methods: Beacon transponders were implanted into prostate phantoms - one as a triangular array and the other with the transponders side-by-side. The electromagnetic tracking array was placed 14 cm above the isocenter set by the transponders in the phantom, and two different metal hips were placed either singly or on both sides of the phantom, resting on Styrofoam to be level with the isocenter plane. For each phantom and hip setup, positional data was acquired by the Calypso at 10 Hz for 30 s.
Results: With the Beacons forming an isosceles triangle with 1.5 cm long legs, there was no configuration of metal hips, either singly or doubly, that caused an isocenter offset greater than 0.1 mm in any Cartesian direction or in the calculated net vector displacement. However, in the model of a poorly executed Beacon placement with transponders nearly side-by-side, almost all the measured mean displacements were greater than 0.1 mm, but only when the proximal part of the metal hip was 5 cm from isocenter. Maximum displacements for single hips were in the lateral direction (~ 0.5 mm), while displacements in the orthogonal plane were less than half as great. The presence of bi-lateral hips 5 cm from isocenter had countervailing but not negating effects because the composition of each hip was different. The mean vector displacement was 0.3 mm with close bilateral hip placement and was 0.5 mm with either single hip.
Conclusion: Calypso localization and tracking had an error greater than 0.1 mm only when the metal hips were within 5 cm of the transponder isocenter. At clinically realistic distances, all isocenter displacements for mono- and bi-lateral hips were negligible.