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Neuroimaging in Stereotactic Functional Neurosurgery
This chapter includes an accompanying lecture presentation that has been prepared by the authors: 
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Key Concepts
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Neuroimaging is a key component of safe and effective stereotactic intervention.
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Dedicated structural MRI sequences enable visualization of commonly used anatomic targets.
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MRI connectivity may compliment structural imaging.
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Stereotactic imaging allows confirmation of accurate anatomic targeting during the intervention.
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Neurosurgeons must understand potential neuroimaging pitfalls if they are to avoid them.
Stereotactic functional neurosurgery involves precise surgical targeting of anatomic structures to modulate neurological function. The ultimate aim is to improve the symptoms and quality of life of patients suffering from chronic neurologic or neuropsychiatric disorders; this demands minimal risk for inflicting morbidity and mortality.
Stereotactic imaging requires the application of fiducials, which are fixed external reference points, to the skull. The location of visible intracranial structures on imaging can then be determined in relation to the external fiducial markers. This process is analogous to laying a reference grid over a map to assist in navigation. Calculating the triplanar coordinates of an intracranial structure within the fiducial reference frame provides surgical access to targets deep within the brain in a minimally invasive fashion.
Historical inability to visualize detailed intracranial anatomy required the development of stereotactic atlases depicting the spatial relations of neuroanatomy with respect to internal landmarks visible on available imaging techniques. Such “indirect targeting” was an informed “best guess,” and several brain passes were often required before the intended anatomic and functional target was acquired. Surgeons relied on surrogate markers, such as physiologic and clinical observations, to “map” the region of interest.
Advances in imaging have had an immense impact on every branch of neurosurgery, and functional neurosurgery is no exception. Leksell and colleagues quickly recognized the fundamental importance of magnetic resonance imaging (MRI) in the practice of functional neurosurgery:
In clinical practice, brain imaging can now be divided in two parts: the diagnostic neuroradiology and the preoperative stereotactic localization procedure. The latter is part of the therapeutic procedure. It is the surgeon’s responsibility and should be closely integrated with the operation.
Image-Guided Surgery
The accuracy, speed, and noninvasive nature of MRI make it the primary tool for anatomic targeting by functional neurosurgeons. MRI provides a tailored map of the brain for each patient undergoing surgery. The ability to visualize most of the relevant anatomic structures in functional neurosurgery raises the possibility of acquiring the target with a single brain pass in the majority of cases, further reducing morbidity and mortality. Using dedicated software packages, reconstruction of MR images allows accurate planning of a surgical trajectory that avoids eloquent cortex, sulci, and ventricles with their enclosed vessels, and maximizes the number of deep brain stimulation (DBS) contacts within the target structure.
Stereotactic MRI is considered the “gold standard” in stereotactic targeting. Dedicated stereotactic MRI sequences can reliably demonstrate the intended target. On T2-weighted images, the subthalamic nucleus (STN) presents a characteristic hypointense signal attributed to the presence of iron, allowing direct targeting of this structure. Advances in MR imaging sequences and techniques make it ever easier to identify the STN and its borders in clinical practice ( Fig. 108.1A ).
A number of published MRI protocols allow clear visualization of pallidal architecture. , A proton-density sequence for targeting the posteroventral pallidum is shown in Fig. 108.1B . The putamen, internal and external pallidum, and pallidocapsular border can easily be seen at the level of the anterior commissure. This guides the surgical trajectory to the posteroventral pallidum that lies around 5 mm deeper, immediately superior and lateral to the optic tract.
Visualization of individual thalamic nuclei is unreliable at 1.5T, and attempts have been made to visualize these at higher field strengths. , Indirect targeting currently retains an important role when using this target; however, the lateral coordinates can be guided by visualization of the thalamocapsular border.
Unlike diagnostic radiology, stereotactic imaging requires more than just visualization of structures. Geometric distortion of the acquired images can represent a significant obstacle to accurate spatial representation and can render such MR images useless for the purpose of accurate anatomic targeting. However, MRI scanners have incorporated software solutions that minimize distortion, such that geometric errors can be reduced to the submillimeter range (see Fig. 108.1C ).
Coregistration or “fusion” of nonstereotactic MRI with stereotactic computed tomography (CT) to guide the initial trajectory introduces another step to the stereotactic procedure. “Fusion” errors are analogous to malalignment of a reference grid over a coordinate map and can be minimized by obtaining high-resolution postcontrast images to visualize the intracranial vasculature and increase the number of available data points. Nevertheless, using commercially available planning software, the mean coregistration error is approximately 1 mm, rising above 2 mm in one-fourth to one-third of patients and >4 mm in individual patients. , Introducing such errors when planning the initial trajectory will require intraoperative correction and the use of multiple brain tracks.
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