The relationship of imaging techniques to the accuracy of frameless stereotaxy.

Objective: We analyzed the accuracy of a frameless stereotactic system using computed tomographic (CT) and magnetic resonance imaging (MRI) scans of different slice thickness and T(1) versus T(2) weighting of MRI.

Methods: An open skull with graphite pegs fixed to its base was used for all scans. CT scans were done with slice thicknesses of 1, 2 and 3 mm. MRI-visible markers were placed on top of pegs for T(1)-weighted and T(2)-weighted MRI scans, which were acquired at thicknesses of 1.5, 3 and 5 mm. For each scan, 3 separate registrations of a probe were performed; the distance between the actual probe location and that displayed on the registered image was noted. Each measurements was repeated 3 times for each registration.

Results: Greatest accuracy was achieved with 3-mm-slice CT scans; this was not improved by using thinner slices. T(1)-weighted 1.5-mm MRI scans were 23% less accurate and T(2)-weighted 3-mm scans 37% less accurate.

Conclusions: Frameless stereotaxy should be done using CT scans when the greatest possible accuracy is desired. There appears to be no advantage to using slice thicknesses less than 3 mm. For most craniotomy applications, T(1)-weighted MRI using 3-mm slices provides sufficient accuracy. Lesions imaged only on T(2)-weighted MRI also can be approached with adequate precision using 3-mm scans.