Headshape Digitization using an Ultrasound 3D Acquisition System

Abstract

Localization of neuromagnetic sources requires co-registration in order to map data registered by magnetoencephalography (MEG) onto the corresponding anatomic structure. Co-registration on the other hand often involves head shape digitization, which is usually carried out using a 3D digitization system that has a reference point which is not fixed at the subjects head. Therefore, the major drawback of such systems is that head motion artefacts during the digitization process cannot be compensated. We instead use the Zebris 3D Motion Analyzer for headshape digitization outside the magnetically shielded room providing a much more comfortable measurement, especially for patients. The foremost advantage, however, is that this system makes use of a 3D ultrasound acquisition device providing three or more reference markers that can be fixed on the surface of the subject's head. Using these reference markers we are able to compensate for any head movements during head shape acquisition. Furthermore, we developed a new 3D localization software to control the Zebris system that is now integrated in our MEG measurement environment (4D-Neuroimaging). With our software we automatically can check the quality of the digitization process, and it may also be used to localize attached head location coils or surface electrodes that are used for the MEG acquisition. Our tests show that the headshape digitization system works reliably and provides an acquisition accuracy of about 1mm, while it automatically compensates for any head motion artefacts.