Figure 1.

Coordinate systems used in the image re-registration. Solid arrows indicate transformations determined from calibration, and are fixed. Dashed arrows indicate transformations determined from registration, and are subject to adjustment through the re-registration process. A transformation reversing the arrow direction is obtained by matrix inversion. Intraoperative US images obtained from single and sequence acquisitions before dural opening for a typical patient case are overlaid on the patient head, demonstrating the difference in the two types of acquisitions as well as the incomplete scanning and∕or nonuniform sampling of the tumor volume resulting from free-hand scanning.