Effect of metal hip prosthesis on the accuracy of electromagnetic localization tracking

To quantify the effect of metal hip prosthesis on the ability to track and localize electromagnetic transponders.

Three Calypso Beacon (Varian Medical Systems, Palo Alto, CA) transponders were implanted into 2 prostate phantoms. The geometric center of the transponders were identified on computed tomography and set as the isocenter. With the phantom stationary on the treatment table and the tracking array 14-cm above the isocenter, data were acquired by the Calypso system at 10 Hz to establish the uncertainty in measurements. Transponder positional data were acquired with unilateral hip prostheses of different metallic compositions and then with bilateral hips placed at variable separation from the phantom.

Regardless of hip prosthesis composition, the average vector displacement in the presence of a unilateral prosthesis was <0.5 mm. The greatest contribution to overall vector displacement occurred in the lateral dimension. With bilateral hip prosthesis, the average vector displacement was 0.3 mm. The displacement in the lateral dimension was markedly reduced compared with a unilateral hip, suggesting that there was a countervailing effect with bilateral hip prosthesis. The greatest average vector displacement was 0.6 mm and occurred when bilateral hip prostheses were placed within 4 cm of the detector array.

Unilateral and bilateral hip prostheses did not have any meaningful effect on the ability to accurately track electromagnetic transponders implanted in a prostate phantom. At clinically realistic distances between the hip and detection array, the average tracking error is negligible.