It is important to check that the instrument in the surgical field and the CT images correlate well. The correlation of the x, y, and z coordinates in the three orthogonal image planes to the actual position of the probe defines the accuracy of the system (Fig. 10.33).
A number of techniques for registration have been described. One method is to register artificial reference points that are stuck onto the skin or are included in the headgear during both the preoperative CT scan and the operative procedure. A malleable registration mask with nickel fiducial markers has recently been described (Albritton et al., 2001) and it is likely that processes such as these will be simplified as well as being made more accurate.
Natural landmarks can be used to perform "surface registration" that allows preoperative scanning without any headset or patient-mounted fiducials. This involves the surgeon touching a finite number of a
discrete points on the patient with a probe and registering the corresponding points on the three-dimensional model depicted on the computer display (Anon et al., 1994; Fried and Morrison, 1998). Four reference points are usually chosen and may include the following: the tragi, lateral orbital rims and lateral canthi, medial canthi, the deepest part of the glabella, the columellar-labial angle, and the nasal alar rims. Once registration has been completed, the surgeon should perform an anatomical check with known structures to evaluate positional accuracy and estimate target error. With the electromagnetic system, once the headframe has been applied, registration is quickly checked using the spine, origin of the middle turbi-nate, and anterior lacrimal crest, and posteriorly the base of the middle turbinate and root of the vomer. This gives excellent accuracy (Fig. 10.34a, b).
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