A motion constrained cross-wire phantom for tracked 2D ultrasound calibration

PurposeUltrasound-guided 3D interventions require calibration to relate real-time 2D images with the position and orientation of the ultrasound probe. Capturing several images of a single fixed point from different viewpoints is a simple and commonly used approach, but it is cumbersome and tedious. A new phantom for calibration was designed, built and tested to simplify this process.MethodsA mechanical phantom that restricts the motion of the ultrasound probe was designed such that the ultrasound image always captures a designated fixed point. Software was implemented which computes calibration parameters. Although the software provides no scientific novelty, it is required to demonstrate the proof of concept and to assess the accuracy and precision of the calibration phantom. The software also illustrates how the phantom enables the fixed point to be located automatically, both in tracker device coordinates and in image pixel coordinates.ResultsThe phantom was used to capture several hundred images of a single fixed point in less than 1 min, with different probe positions and orientations around the fixed point and with the single fixed point located in different parts of the ultrasound image. It would not be feasible to capture the same number of images by manual alignment of the probe with the fixed point.ConclusionImages for single fixed point calibration can be captured easily and quickly with a new calibration phantom. Since a larger number of images can be used to compute the required parameters, the calibration robustness is increased.