3D volume registration: experiments on tissue phantom and liver

The fusion of multiple ultrasound scans, taken from different orientations and different times, can enhance interventional procedures and can quantify response to therapy. However, fusion of ultrasound scans requires precision alignment of recognizable fiducial markers. We present the results of volume registration of liver tissue and phantom material to test the accuracy of a semi-automated registration algorithm. Both linear tracking and magnetic "Flock of Birds" (FOE) position sensing are employed to register sequential frames of 2D ultrasound into 3D space. Major vessels and/or interventional probes are utilized as internal fiducial markers for a correlation registration algorithm. US scans were obtained from a volunteer with a known hemangioma, a benign, stable, echogenic liver tumor. Complementary, non-parallel scans were then fused to determine hemangioma co-registration displacement error. The average central displacement error was 4.1 mm. To further test our algorithm, we obtained perpendicular scans of a radio-frequency (RF) ablation probe within a tissue phantom. The volumes were registered and fused successfully, with an average displacement error of 1.1 mm. In conclusion, internal structures such as vessels and interventional probes provide adequate markers for volume fusion, producing misregistration errors less than 7.4 mm.