Electromagnetic tracking in ultrasound-guided high dose rate prostate brachytherapy
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Electromagnetic (EM) tracking assistance for ultrasound-guided high-dose-rate (HDR) prostate brachytherapy has recently been introduced in order to enable prompt and uncomplicated reconstruction of catheter paths with respect to the prostate gland. However, EM tracking within a clinical setting is notorious for fluctuating measurement performance. In fact, measurements are prone to errors due to field distortions caused by magnetic and conductive objects and can compromise the outcome of the procedure. Enhancing these measurements is therefore paramount. The objective of this thesis is to enable robust and accurate reconstructions of HDR catheter paths on the ultrasound images of the prostate gland using EM tracking. To achieve this objective, the measurement uncertainty of an electromagnetic system was first characterized in various environments; this characterization enabled us to identify optimum setup configurations and model the measurement uncertainty. Second, we designed and implemented a specialized filtering method for catheter path reconstructions, combining the nonholonomic motion constraints which apply to the EM sensor, with both the position and orientation measurements of the sensor. Finally, ultrasound scan planes were robustly tracked with the help of a simultaneous localization and calibration algorithm; this method allows for dynamic tracking of the ultrasound probe while simultaneously mapping and compensating for the EM field distortions. We experimentally validated the performance of our advanced filter for catheter path reconstructions in a HDR brachytherapy suite; the EM sensor was threaded within paths of various curvatures at several insertion speeds. The simultaneous ultrasound probe tracking and EM field distortion compensation approach was also assessed in the brachytherapy suite. The performances of our approaches were compared to conventional error minimization methods. The advanced methods effectively increased the EM tracking accuracy of catheter paths and ultrasound probes. With the help of our proposed approaches, EM tracking can provide effective assistance for a plurality of clinical applications.