金属球マーカーを用いた自動かつ高精度なフリーハンド超音波キャリブレーション

Abstract

Since ultrasonic diagnostic equipment is small and inexpensive compared to CT or MRI, 3D-information acquiring methods by ultrasound (US) are widely studied. For that purpose, calibration for identifying of the spatial relationship between an US probe and a position tracking device is required. To apply the calibration in a clinical setting, not only the accuracy but the simplicity of a calibration phantom and procedures are important. In this study, we propose an automatic and accurate US calibration method using a metal ball marker. First, US images were captured freehand keeping the depth position of the marker. Next, an image passing through the center point of the marker was selected by comparison with mean brightness of comet-tail artifacts. Then, the center position was calculated by least-square circular fitting. When several center positions of the metal ball were obtained, the transformation matrix was determined by point-based registration. From the results, average of calibration errors was 0.26mm by a convex probe. Moreover, the calibration time including freehand image acquisition and image processing was less than 10 minutes. The results demonstrate that the proposed method is accurate and convenient for US calibration.