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Point-based registration for image-guided neurosurgery has become the industry standard. While the use of intrinsic
points is appealing because of its retrospective nature, affixing extrinsic objects to the head prior to scanning has
been demonstrated to provide much more accurate registrations. Points of reference between image space and
physical space are called fiducials. The extrinsic objects which generate those points are fiducial markers. The
markers can be broken down into two classifications: skin-mounted and bone-implanted. Each has distinct
advantages and disadvantages. Skin-mounted fiducials require simply sticking them on the patient in locations
suggested by the manufacturer, however, they can move with tractions placed on the skin, fall off and perhaps the
most dangerous problem, they can be replaced by the patient. Bone implanted markers being rigidly affixed to the
skull do not present such problems. However, a minor surgical intervention (analogous to dental work) must be
performed to implant the markers prior to surgery. Therefore marker type and use has become a decision point for
image-guided surgery. We have performed a series of experiments in an attempt to better quantify aspects of the two
types of markers so that better informed decisions can be made. We have created a phantom composed of a full-size
plastic skull [Wards Scientific Supply] with a 500 ml bag of saline placed in the brain cavity. The skull was then
sealed. A skin mimicking material, DragonSkinTM [SmoothOn Company] was painted onto the surface and allowed
to dry. Skin mounted fiducials [Medtronic-SNT] and bone-implanted markers [Z-Kat]were placed on the phantom.
In addition, three additional bone-implanted markers were placed (two on the base of the skull and one in the eye
socket for use as targets). The markers were imaged in CT and 4 MRI sequences (T1-weighted, T2 weighted, SPGR,
and a functional series.) The markers were also located in physical space using an Optotrak 3020 [Northern Digital
Inc]. Registrations between image space and physical space were performed and fiducial and target registration
errors were determined. Finally the 5 bone-implanted makers which penetrated the skin were removed and a traction
equivalent to 25% of the weight of the average human head was applied to the “skin” surface. Target and fiducial
registrations were again performed.
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