User-friendly freehand ultrasound calibration using Lego bricks and automatic registration

Purpose As an inexpensive, noninvasive, and portable clinical imaging modality, ultrasound (US) has been widely employed in many interventional procedures for monitoring potential tissue deformation, surgical tool placement, and locating surgical targets. The application requires the spatial mapping between 2D US images and 3D coordinates of the patient. Although positions of the devices (i.e., ultrasound transducer) and the patient can be easily recorded by a motion tracking system, the spatial relationship between the US image and the tracker attached to the US transducer needs to be estimated through an US calibration procedure. Previously, various calibration techniques have been proposed, where a spatial transformation is computed to match the coordinates of corresponding features in a physical phantom and those seen in the US scans. However, most of these methods are difficult to use for novel users. Methods We proposed an ultrasound calibration method by constructing a phantom from simple Lego bricks and applying an automated multi-slice 2D–3D registration scheme without volumetric reconstruction. The method was validated for its calibration accuracy and reproducibility. Results Our method yields a calibration accuracy of 1.23 ± 0.26 mm and a calibration reproducibility of 1.29 mm. Conclusion We have proposed a robust, inexpensive, and easy-to-use ultrasound calibration method.