Introduction: Surgical navigation requires registration of the patient’s anatomy to preoperative images. Traditionally this is accomplished with fiducials or skin surface tracings, but can be difficult with lateral or prone positioning. Here we show how an A-mode ultrasound can be used to register skull surface points through skin and hair on a cadaver model as accurately as traditional methods.

Methods: Three cadaver heads were fitted with bone screw fiducials for registration. A 7.5 megahertz ultrasound transducer was calibrated and tracked with a commercially available surgical navigation system. The distance from the tip of the ultrasound transducer on the cadaver skin surface to the skull surface was calculated using a digital signal processing algorithm developed at the Southern Illinois University department of biomedical engineering. The measured skull surface points were loaded into the standard registration software of the surgical navigation system. The individual skull surface points measured by ultrasound and their distance to the skull surface from the computed tomography scan of the cadaver head was calculated. Computed error metrics were then used to validate that ultrasound measured skull surface points could register the skull with increased accuracy for lateral and prone surgical positions.

Results: The error of the skull surface points as measured by the ultrasound probe was less than 1mm for most regions of the skull surface. Artificial hair saturated with ultrasound gel did not significantly affect measurements indicating that hair clipping would not be necessary for most patients.

Conclusions: The computed error metrics after registration validated the hypothesis that ultrasound measurements of the skull surface can accurately register preoperative images to the patient on the operating table with cranial fixation. Surgical registration technology can use ultrasound measurements with the digital signal processing method we developed. This technological innovation can increase the accuracy of surgical navigation by replacing fiducials or tracing methods.

Patient Care: This shows a new method of registering patients to pre-operative exams for intraoperative navigation. This can be used accurately to register patients, especially in situations where skin tracing may not be accurate or may not be feasible and when skin fiducials could not be applied. This can increase patient safety and navigation reliability in cranial surgery, most specifically skull base and posterior fossa surgery.

Learning Objectives: Understand the basics of surgical navigation and registration. Understand how A-mode ultrasound can be used to register patient's to preoperative CT and MRI scans.

References: 1. Raabe et al, "Face scanning for patient registration in intracranial image-guided surgery." Neurosurgery. 2002;50:797-803 2. P. Besl al et. “A method for registration of 3-D shapes”, IEEE transactions on Pattern Analysis and Machine Intelligence, 14(2), 239-254, 1992. 3. Caversaccio M et al, "Practical aspects for optimal registration (matching) on the lateral skull base with an optical frame- less computer-aided pointer system." Am J Otol. 2000;21:863-870.