4.1 Introduction

Advances in modern neurosurgery have been synchronous with advances in neuroimaging. Roentgen’s X-rays in 1895, Dandy’s pneumoencephalography in 1918 and Moniz’s angiography in 1927 each provided a stepping-stone for neurosurgical progress. CT and MRI imaging techniques and subsequent refinements in its diagnostic equipment have since led to dramatic improvements in neurosurgery, and to the patients served by it. Such benefits include improved diagnosis, more appropriate patient selection, enhanced target resolution, and optimized trajectory planning—all crucial to the field of stereotactic neurosurgery. This chapter will focus on current techniques for CT and MRI imaging in stereotactic neurosurgery, with an emphasis on imaging of the basal ganglia for the surgical treatment of movement disorders.

4.2 Imaging in Frames

In 1908 Horsley and Clarke established the principles of stereotactic technique for vertebrates. Their guiding device provided accurate localization of feline intracranial structures by ascertaining three-dimensional target coordinates with reference to skull landmarks and an external coordinate system. ¹ In the 1940s, Spiegel and Wycis developed a skull-mounted rigid stereotactic frame and used contrast ventriculography with it to identify cerebral landmarks. The anterior and posterior commissural coordinates (AC-PC) are acquired by imaging the brain while the patient wears an applied frame. These points are correlated with stereotactic atlases to define spatial relationships (▶ Fig. 4.1). Important to movement disorders are the identifiable targets: ventrointermediate nucleus of the thalamus (Vim), subthalamic nucleus (STN), and the globus pallidum pars interna (GPi). Indirect targeting is a technique that makes use of consensus coordinates for the location of each target structure. Stereotactic atlases based on neural landmarks do not eliminate the problem of anatomic variability in patients, and either microelectrode recording or macrostimulation must be used to confirm the functional target. Direct targeting uses MRI and/or CT-MRI fusion to identify brain targets based on direct visualization of the target on MRI.

Visualization of the Vim, the target for the treatment of tremor, is not possible using available MRI sequences at 3-Tesla, requiring indirect targeting with physiological confirmation techniques. 7-Tesla MRIs can visualize internal thalamic nuclei, and hold promise for improving anatomic-based targeting in DBS surgery. ²

Modern stereotactic frames are MRI compatible and safe within MRI systems, with minimal distortion of the magnetic field. Still, even slight geometric distortion can be an issue, particularly close to the base of the frame, which therefore is positioned as far away from the target region as possible. Frames with smaller fiducial indicator boxes, such as the Laitinen and Leksell systems, produce less geometric distortion than their larger counterparts. ¹

4.3 Frameless Imaging

While rigid head frames have been essential to the origin of stereotactic technique, they have disadvantages. Frames simplify targeting of a single point in space with a probe. They are not designed to localize an instrument on an image. ³ Also, the physical frame itself is a constraint during surgery. It is uncomfortable for patients to wear them. Large head circumferences may make them difficult to apply. In awake patients frameless stereotaxy can be combined with microelectrode recording and with macrostimulation to verify correct placement. In asleep patients intraoperative MRI imaging alone can be used with a frameless system.