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Stereotactic Frames: Technical

Considerations

Ashwini D. Sharan and David W. AndrewsThomas Jefferson University, Philadelphia, Pennsylvania,

U.S.A.

1 INTRODUCTION

The term stereotaxis, derived from the Greek stereo- for three-dimensional

and -taxic for an arrangement, was coined by Horsley and Clarke in 1908

[1]. It was their use of a three-dimensional Cartesian coordinate system that

provided the basis for all stereotactic systems used in modern day neuro-

surgery. Human stereotaxy was initially developed for the placement of deep

lesions in patients with Parkinsons disease but lost favor with the devel-

opment of dopamine agonist medications. The introduction of computedtomography (CT) renewed interest in stereotaxy and, together with the sub-

sequent introduction of magnetic resonance imaging (MRI), broadened in-

dications for stereotactic approaches dramatically as deeper areas of the brain

could now be targeted with great accuracy. As radiosurgery developed, in-

dications for the use of stereotactic frames broadened further. A thorough

review of the history of stereotaxy and the development of frame-based

systems can be found in Gildenberg and Taskers definitive textbook [2].

This chapter will be devoted to three current frames systems, including tech-

nical aspects of frame application and target localization. Other frames will

Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved.

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be described elsewhere in the book; our goal is to describe some of the

theoretical underpinning for the use of stereotactic frames in the era of digital

imaging.

2 GENERAL PRINCIPLES

The stereotactic approach to intracranial targets involves the rigid application

of a stereotactic frame, a localizer, and an image data set derived from either

CT, MRI, or angiography. With a fixed relationship between the patients

head and the fiducial localizers [3], any intracranial target can be reached

with an optimal trajectory and great accuracy. The standard performance

specifications for cerebral stereotactic systems, specified by the American

Society for Testing and Materials, stipulate a mechanical accuracy below 1

mm [4]. Within a Cartesian coordinate system, the x- and y-axes refer to amediallateral and anteriorposterior location, respectively, whereas the z-

axis refers to a basevertex location. Many methods have been outlined to

determine the z-axis, but the most popular method uses posts with an N

shape configuration where the position of the oblique rod relative to the

vertical rods defines the z plane of the slice [3]. Once the target is localized,

the arc method is used to direct a probe to the selected target and carry out

the remainder of the procedure. These features are discussed in more detail

below.

2.1 Frame Application

With experience and assistance, a stereotactic frame application should take

minimal time. Before applying the frame, the neurosurgeon must have a

clear idea of the anatomical localization of the lesion and should bear in

mind a suitable entrance point for the probe. When applying a Leksell frame

for radiosurgery, the frame must be shifted as much as possible to center

the lesion in Leksell space (Fig. 1A). Failure to do so may result in a col-

lision with the collimation helmet. When using CT data, the surgeon must

remember that the headpins may cause significant artifact, which may ob-scure the target if small, as might the beam-hardening artifacts of the tem-

poral bone if the lesion is located in a low temporal or posterior fossa

location. Frame application may be performed at the bedside or in the op-

erating room and is most easily accomplished with the patient in the sitting

position. Our preference is to sterilize the scalp with an alcohol or betadine

prep without shaving hair. The assistant stands either behind or on the side

of the patient and stabilizes the ring. The ring should be applied parallel to

the cranial floor through the use of ear bars, but some frame parallax is

acceptable. As one exception, Leksell frame application must be within 3


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FI UR A Axial gadolinium enhanced T 1 weighted image with field of view inccoordinates. The frame was shifted to the patients left to center an acoustic tumorradiosurgery. Note the annotations that depict x y and z measurements. B Axialweighted image with field of view including Leksell stereotactic coordinates for trbrain metastasis.


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of the coaxial imaging plane of Leksell Gamma Plan rejects further attempts

at treatment planning for radiosurgery. We anesthetize the scalp and peri-

osteum with a mixture of 9 parts 0.5% Marcaine (bupivicaine) and 1 part

sodium bicarbonate, which reduces the burning sensation of the local an-

esthetic. With adequate local anesthesia, we have obviated the need for se-dation. We prefer to place the two posterior pins first and then we place the

anterior pins and hand tighten all four pins, before using the wrench, with

a two-fingers method. During the frame application, the patient may be in-

jected with intravenous contrast if the localization scan is to be performed

immediately after the procedure. Otherwise, the patient can return to the bed

with a pillow under the neck for comfort.

2.2 Target Localization for Stereotactic Biopsy

Imaging modality should depend on the modality that best demonstrates the

lesion: either CT, MRI, or angiography. Basic principles that should be ap-

plied when planning a trajectory to target. The instruments trajectory should

avoid eloquent brain and breach only one pial surface to minimize the

change of hemorrhage. This is particularly true for lesions near the sylvian

fissure or pineal region. When possible, the instrument should penetrate the

brain parallel to white matter tracts, especially when interested in brainstem

lesions. Generally, the majority of the cerebrum, basal ganglia, thalamus,

and brainstem can be approached with entry points anterior to the coronalsuture. For lesions in the occipital, parietal, temporal lobe or the pineal

region, a superior parieto-occipital approach is better. Temporal lesions may,

additionally, be approached laterally and cerebellar lesions approached

posteriorly.

With the patient still in the scanner, it is important to ensure that all

fiducial markers are visible on all images. With the advent of MRI-compat-

ible localizers, MRI has provided superior target identification. Typically an

axial T-1-weighted gadolinium-enhanced image will provide enough spatial

information for target localization. For deep grey matter lesions, coronal and

sagittal images provide ring and slide angles for isocentric frames (Fig.

2A and B). For brainstem lesions near midline, we recommend frontal lobe

entry points with long-axis trajectories to avoid additional pial planes. For

such cases, we obtain fiducial and target coordinates in all three orthogonal

planes and average the three paired coordinates with the greatest spatial

accuracy, eliminating the coordinate in each orthogonal plane which is, by

definition, less accurate because of volume averaging. We always select a

contrast-enhancing target if present, or abnormal signal visualized in a

FLAIR sequence, if not.


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FI UR 2 Patient with a small punctate contrast enhancing lesion near speech that wuation for a new onset seizure disorder. This lesion was biopsied and diagnosed asCoronal gadolinium enhanced T 1 weighted image with annotation reflecting coronawill establish the slide angle on the CRW stereotactic frame. B Sagittal gadolinium enwith annotation reflecting sagittal angle of trajectory which will establish the ring orstereotactic frame.


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2.3 The Leksell Frame

After an inspiring visit with Spiegel and Wycis in Philadelphia, Leksell

developed his first stereotactic frame [5]. His design included an arc system

that was attached to a patients head with pins such that the center of the

arc corresponded to the selected target. The radius of the arc in the Leksell

system is 190 mm, and stereotactic space is designated in a Cartesian co-

ordinate system with center established, in millimeters, at x = 100, y = 100,

and z = 100 and zero, by convention, is right, posterior, and superior. This

elegant system allows the surgeon the opportunity to quickly and easily

establish the targets coordinates on the MRI or CT monitor. The frame

center at x and y = 100 is determined at the center of the intersecting lines

drawn from the fiducial points at the corners of the localizing frame, as long

as the frame is maintained in an orthogonal relationship with the scanner

table. This may be confirmed with a carpenters level; periodic adjustmentsof the frame attachment to the table may be needed. The Leksell z coordinate

is established by measuring the distance from the ipsilateral superior fiducial

coordinate to the diagonal coordinate and adding 40 mm (Fig. 1A and B).

Alternatively, the images can be transferred by tape or ethernet to a surgical

planning system. For Gamma Knife radiosurgery, we cross-check the treat-

ment planning software determination (Leksell Gamma Plan) with the man-

ually derived coordinates. If a lesion is left, posterior, and superior, for

example, its location should be associated with x > 100, y < 100, and z

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