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Abbreviations: MR, Magnetic resonance; STN, Subthalamic nucleus; GPi, Globus pallidus internal.

Abbreviations: MR, Magnetic resonance; STN, Subthalamic nucleus; GPi, Globus pallidus internal.

Magnetic resonance imaging has the disadvantage, compared with CT and ventriculography, of several types of image distortion effects [11]. Image distortion may result in apparent positions of the target or fiducial markers on the image that are different from their actual positions in real space. Distortion effects can vary widely between different scanners, sequence protocols, and frame systems. Before using MRI as the sole modality for anatomical localization, the degree of distortion should be estimated by performing a phantom study [12]. Distortion effects may be partially corrected using computational algorithms [11], or CT-MR image fusion techniques [13,14]. Alternatively, some groups use a nonstereotactic MRI to plan "individualized" AC-PC based coordinates, then perform the actual stereotactic localization with ventriculography or with CT [4,15].

2.3 Frame Placement

Stereotactic procedures begin with fixation of the frame to the patient's head. ''Scanner-dependent'' frame systems, such as the Leksell (Atlanta, GA) series G frame or the Radionics (Burlington, MA) CRW-fn frame are frequently used for functional stereotaxy. These frames are designed to be fixed with the vertical axis parallel to the MR or CT gantry. Unless special surgical planning software is used to calculate the target coordinates, scanner-dependent stereotactic systems also require that images be obtained orthogonal to the frame axes.

It is important to place the frame with its axes orthogonal to standard anatomical planes of the brain. Frame misalignments are often described using the terms "pitch," ''roll,'' and "yaw," following nautical terminology [16]. Good frame placement is facilitated by the use of the earplugs provided with most scanner-dependent systems. These align the frame with the external auditory canals. The use of the earplugs ensures that the mediolateral (X) axis of the frame is perpendicular to the midsagittal plane of the brain, thus avoiding any roll (lateral tilt) or yaw (rotation) of the frame with respect to the brain. To adjust the pitch, the anteroposterior axis of the frame may then be angled according to superfical landmarks so as to parallel the AC-PC line. A line between the inferior orbital rim and the external auditory canal is approximately parallel to the AC-PC [1], as is a line from the glabella to the inion [17]. The eyes and mouth should remain unencumbered by the frame, so as to allow visual field examination and airway access. Once the frame is aligned in all dimensions, the skull pins are advanced symmetrically. It is important to begin withdrawing the earplugs before the pins are fully tightened, to avoid severe pain in the external auditory canals. Intravenous sedation (such as 1-2 mg of Versed) is desirable when the earplugs are used.

Straight frame placement ensures that the preoperative images, as well as maps made from intraoperative physiological exploration, are interpretable in terms of familiar anatomy corresponding to standard brain atlases. In addition, the use of indirect targeting from the AC-PC line assumes that there is no pitch, yaw, or roll of the frame with respect to the brain; such deviations will reduce accuracy. When the frame is imperfectly aligned, computational algorithms [16] or surgical planning software [18] may be used to correct for this.

2.4 Surgical Planning Software

Surgical planning software is a useful, although not essential, adjunct to frame-based stereotactic targeting and is available from a variety of commercial sources including Elekta (Atlanta, GA), Radionics (Burlington, MA), and Sofamor-Danek (Memphis, TN) [18]. Such software usually provides a variety of functions. Among the most useful is multiplanar visualization of the probe trajectory based on a given arc and ring angle, which assists greatly in planning a trajectory that avoids large cortical vessels, sulci, or ventricles. Another helpful feature is semiautomated registration of the image set in stereotactic space as defined by the fiducial markers. This may improve targeting speed and reduce human error. Other functions are useful only in specific situations. Many software packages allow the user to reformat MR images along standard anatomical planes orthogonal to the AC-PC line and midsagittal plane. However, to preserve a high degree of resolution in the reformatted images, the original image set should be acquired volu-metrically. This limits the choice of pulse sequences and is generally not necessary if the headframe is carefully placed orthogonal to standard anatomical planes. Computational fusion of nonstereotactically acquired images (usually MR) with stereotactic images (usually CT) is a useful function for operators whose MR units suffer from spatial distortion. Superposition of a "deformable" human brain atlas onto MR or CT images is a function used by many surgeons. However, accurately matching an individual brain to a standard atlas may not be possible, even if the atlas is "deformable.'' A reasonable alternative is to use MR pulse sequences, which provides maximal visualization of the target nucleus.

2.5 Limitations of Anatomical Targeting

A number of factors limit the accuracy of anatomical targeting that is based on historical data. These are summarized in Table 3. The accuracy of any stereotactic system, regardless of imaging modality, is limited by mechanical properties of the frame, and in CT- or MR-based stereotaxy by slice thick-

Table 3 Factors Limiting the Accuracy of Anatomical Targeting

Method

Any method that uses historical images in conjunction with a frame-based coordinate system

MR-based targeting

"Indirect" targeting by measuring fixed distances

"Direct" targeting by visualization of the target structure's boundaries

Any purely anatomical method

Abbreviation : MR, Magnetic resonance.

Factors limiting accuracy

1. Application accuracy of stereo-tactic system

2. Brain shifts that may occur after imaging

Image distortion

1. Anatomic variability between individuals

2. Requires perfectly straight frame placement

Imperfect visualization of the target structure

Anatomy ^ physiology. A given physiological function may not always occur in the same anatomical structure ness. The "application accuracy'' of a stereotactic system is a term that describes the accuracy of the system as it is used clinically. The application accuracy of standard frame-based stereotactic systems, with 1-mm thick CT slices, has been measured to be approximately 1.5 mm at the 95% confidence limit [19]. The application accuracy of frameless systems is probably less than this [18,20], explaining the continued widespread use of frame-based systems for functional work, whereas tumor stereotaxis is now largely performed with frameless systems.

In practice, the theoretical maximum accuracy of a stereotactic system is rarely achieved, as many other factors in a given case can further decrease the accuracy of anatomical targeting (Table 3). Thus, image-guided stereotaxis alone is usually adequate for placing a stimulator or lesion probe within several millimeters of the target, but physiologic studies are important to adjust or confirm final placement.

The use of intraoperative MR, currently under investigation for tumor surgery [21], could improve stereotactic targeting for functional neurosur-gery by providing a real time update of the actual probe position. Application of this technique to basal ganglia surgery, however, awaits improvements in the image quality for these units, as well as the development of MR-compatible, low-artifact instrumentation for guiding probes into place.

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