KEYWORDS: Calibration, Image segmentation, Ultrasonography, Position sensors, Sensors, Distance measurement, 3D image processing, Ultrasonics, 3D metrology, Image processing algorithms and systems
Z-fiducial phantoms allow 3D ultrasound probe calibration with a single B-scan. One of the main difficulties in using this phantom is the need for reliable segmentation of the wires in the ultrasound images, which necessitates manual intervention. In this paper, we have shown how we can solve this problem by mounting a thin rubber membrane on top of the phantom. The membrane is segmented automatically and the wires can be easily located as they are at known positions relative to the membrane. This enables us to segment the wires automatically at the full PAL frame rate of 25Hz, to produce calibrations in real-time, while achieving accuracies similar to those reported in the literature. It takes
approximately two minutes to set up the experiment-submerge the phantom in the water bath and locate the phantom in space with a pointer. After this, spatial calibration can be performed in real-time at 25 calibrations per second.
The most intuitive way to perform freehand 3D ultrasound calibration is to locate fiducials in the scan plane directly with a pointer. The main problem of using this approach is the difficulty of aligning the tip of the pointer with the scan plane. The thick beam width makes the tip of the pointer visible in the B-scan even if the tip is not exactly in the elevational centre of the scan plane. Furthermore, reliable automatic segmentation of isolated points in ultrasonic imaging systems is difficult. As a result, it is usual to rely on manual segmentation, making calibration a lengthy process. In this paper, we present a novel phantom that solves the alignment problem and simplifies the segmentation process, while maintaining the other advantages of using a pointer.
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