A novel ultrasound probe calibration method for multimodal image guidance of needle placement in cervical cancer brachytherapy

Purpose: Interstitial needles placement is a critical component of combined intracavitary/interstitial (IC/IS) brachytherapy (BT). To ensure precise placement of interstitial needles, we proposed a novel ultrasonic (US) probe calibration method to accurately register the US image in the magnetic resonance imaging (MRI) image and provide multimodal image guidance for needle placement. Methods: A wire-based calibration phantom combined with the stylus was developed for the calibration of US probe. The calibration phantom helps to quickly align the imaging plane of the US probe with the fiducial points to obtain US images of these points. The coordinates of fiducial points in US images were located automatically by feature extraction algorithms and were further corrected by the proposed correction method. Ingenious structures were designed on both sides of the calibration phantom to accurately obtain the coordinates of the fiducial points relative to the tracking device. Marker validation and pelvic phantom study were performed to evaluate the accuracy of the proposed calibration method. Results: In the marker validation, the US probe calibration method with corrected transformation achieves a registration accuracy of 0.694 +/- 0.014 mm, and the uncorrected one is 0.746 +/- 0.018 mm. In the pelvic phantom study, the needle tip difference was 1.096 +/- 0.225 mm and trajectory difference was 1.416 +/- 0.284 degrees. Conclusion: The proposed US probe calibration method is helpful to achieve more accurate multimodality image guidance for needle placement.