stereotaxy

When Horsley and Clarke invented the technique of controlled insertion of an electrode into the brain of an experimental animal in 1906, they also invented the term stereotaxic to define it. The name they chose, stereotaxic, is derived from the Greek stereos meaning solid or three-dimensional and taxis meaning an arrangement (as in taxonomy). It was argued by some that stereotaxic should be the proper spelling because Horsley and Clarke had used it. It was thought, however, that "a three dimensional arrangement" did not adequately describe the field. Stereotactic surgery was proposed, being derived from the Greek stereos, for "three-dimensional," and tactus, from the Latin, meaning, "to touch". The field involved not only identifying a target in three-dimensional coordinates, but also actually touching the target with a probe, electrode, or surgical instrument. The technique they described involves localizing a target in space, and, in their proposal, the anatomical structure that lay at that point in space.

It is a three-dimensional concept based on the Cartesian coordinate system, which states that one point and only one point in space can be defined by its relationship to three planes intersecting each other at right angles. The point can be defined by three numbers, indicating distances from those three planes (anteroposterior, lateral, and vertical). For their experiments with monkeys (and later other animals), they proposed the following: the basal plane would pass through the external auditory canals and the inferior orbital rims (it takes at least three points to define the location of a plane) or a plane parallel to that; the midsagittal plane would pass through the midline at right angles to the basal plane; and the coronal plane would pass through the external auditory canals at right angles to the first two planes.

Thus, a target point could be defined as follows:

Anteroposterior - mms anterior or posterior to the coronal plane;

Lateral - mms lateral to the midsagittal plane; and

Vertical - mms above or below the basal plane

Stereotactic biopsy:.

The primary purpose of any brain tumor surgery is to find out what kind of tumor there is in the brain. One of the simplest and easiest ways is to biopsy the tumor by passing a special needle into the tumor, which can take a piece of it. In the brain, however, needles must be guided to the tumor by using the neuro-image from a CT or MRI scan. These scans provide computer data, which can be used in the operating room to guide a needle into the mass. Before this can be done, a reference frame needs to be applied to the head so that the three-dimensional coordinate system used in the scanner will be the same in the operating room.

This is the stereotactic device.

First, a base ring is fitted to the skull using sterile technique and local anesthesia. Usually the patient is given some sedation as well. A localizing ring is then attached to the base ring and a scan is obtained. The localizing ring has a series of rods, which are arranged in such a way that they can be seen with the CT, or MRI scans for each slice. The computer uses the location of these rods to place each individual scan slice precisely in three-dimensional space.

In the operating room, the localizing ring is removed and the arc.ring is attached to the base ring. The arc ring has a moveable guide tube for

the biopsy needle, which can be adjusted to the exact trajectory calculated by the operating room computer to approach the tumor.

A small incision is made in the scalp under local anesthesia and a small hole is made in the skull with a drill. The needle is passed through the guide tube to a pre-measured depth, and biopsy samples are obtained. Once the biopsies are obtained, the scalp incision is closed and the patient is returned to their room. Most patients will go home the following day after a period of observation to ensure that there is no significant post-operative bleeding.

The benefits of a stereotactic biopsy are that a diagnosis can be made with a relatively small operation.

The main limitation of the procedure is that the tumor remains.

The risks of the surgery include the general risks that exist with any operation which include the risks of infection, bleeding, anesthesia complications and medical complications.

Risks that are specific to the operation are primarily related to the risk of bleeding. If there is significant bleeding, there is a risk of a stroke and a major operation might be required to remove the blood clot. The risk of significant bleeding is about 1 to 2%.

Stereotactic craniotomy:

Stereotactic craniotomy is performed where excision rather than biopsy of a lesion is planned. Stereotactic localization for craniotomy is important in small superficial cortical or subcortical lesions or deep lesions that can be easily missed by conventional means; and also when accurate localization is crucial to excise tumors in highly eloquent areas. This procedure is routinely performed under general anesthesia.

Although MRI may be used, CT is good enough for most lesions. After applying the head frame and localiser frame. CT scan is done and image acquisition is completed for choosing appropriate slices for target selection. Normally for this procedure seven targets are chosen – lesion center, lateral edge, medial edge, posterior edge, anterior edge – these five are calculated from the same axial slice, and the superior edge and inferior edge – these are calculated from slices showing the upper and lower limits of the lesion.

The calculation of multiple target coordinates enables a more accurate planning of the craniotomy as well as aiding in volumetric excision.

The base ring is attached to the Mayfield adaptor and head is positioned as to be approximately horizontal with care taken to prevent compression of neck veins. The posterior, anterior, superior and inferior edges of the lesion are marked out on the skin using the sterile pointer – this outlines the lesion. Generally the center target is used to plan the trajectory. Once the outlining is over, the arc is swung away and craniotomy performed.Before opening the dura, the arc is swung back and trajectory confirmed.

Further surgery is the standard procedure of tumor excision.

In deep-seated lesions the sterile pointer may be passed directly into the target and locked in position. This will act as a guide to the target, with dissection being carried around the pointer. There is inevitably some movement of the brain on performing a craniotomy, even under stereotactic conditions, and this will affect the accuracy. This can be overcome by passing a fine silicon catheter into the target through a burr hole, before performing the craniotomy. Once the excision is complete the rest of the closure is routine.

Fixing the base-ring under local Anaesthesia.

Localiser frame being fitted to the base-ring on the CT scanner table.

Target localisation and obtaining the Localiser Co-ordinates from the CT scanner.

Verifying the Target Co-ordinates on the Phantom base.

Arc-localiser fitted to the base-ring and trajectory chosen, preparing for biopsy

Biopsy in Progress.

Stereotactic aspiration:

Aspiration of deep seated abscesses, acute hematomas, and colloid cysts of the 3rd ventricle are facilitated by stereotactic applications.

The procedure is similar to stereotactic biopsy.

In case of the colloid cyst, the recurrence is high and many neurosurgeons prefer microsurgery. However, in selected cases, this is an effective alternative.

colloid cyst-pre & post aspiration CT

Stereotactic functional surgery:

When radiosurgery was born, stereotactic neurosurgery was more or less synonymous with functional neurosurgery. Movement disorders such as Parkinson's disease, intractable pain due to cancer, trigeminal neuralgia, and psychological disorders such as obsessive-compulsive neurosis are the disorders treated.

Some of the commoner procedures, although not yet well established, are:

Thalamotomies -to arrest the tremors in Parkinson's disease.

Pallidotomy -to ameliorate dyskinesia and rigidity of Parkinson's disease.

Thalamotomy or hypophysectomy -to relieve intractable cancer pain.

Lesiong of trigeminal root zone at its exit from brainstem -in primary trigeminal neuralgia.

Bilateral lesioning of the anterior internal capsule -in obsessive compulsive neurosis.

It is claimed that the Gamma-knife is more suited for these procedures.

Test lesioning is not possible and there is a latent period before the onset of relief.

Recently radiosurgery is being tried for intractable epilepsy as well.

Stereotactic Radiosurgery (SRS):

SRS is a technique that delivers a dose of high-energy radiation to a targeted cranial abnormality. Unlike whole brain radiation, X-Knife Stereotactic Radiosurgery enables precise lesion location and treatment planning with computer imaging equipment, and then uses precisely guided beams of focused radiation from a LINAC to treat it. An X-Knife SRS procedure is completed in one day and the actual treatment time typically takes less than 30 minutes. X-Knife produces a radiation dose that results in an effective treatment of the lesion target,while greatly reducing the dose of radiation to the surrounding healthy tissue.This non-invasive treatment avoids the complicationsand inconveniences of open surgery.
Stereotactic Radiotherapy (SRT):	Treatment planning using computer for a Para-sellar mass lesion.
SRT accurately delivers lower levels of focused radiation over a series of treatment sessions called "multiple fractions." This technique is particularly important in cases where tumors are adjacent to radiosensitive tissues such as the brain stem, eyes, or optic nerves, or in cases of pediatric tumors. By treating the lesion with lower dose fractionated therapy, spaced overmultiple sessions, the SRT method enhances the desired effect on the tumor while reducing the amount of radiation to nearby critical structures. An X-Knife procedure is a team effort. It requires a neurosurgeon, radiation oncologist, physicist, dosimetrist, and radiation technician. The neurosurgeon fixes the head frame to the patient. The head frame will remain on the patient for the entire procedure; it provides a reference for the location of the patient's anatomy and
tumor during imaging. It will also serve to immobilize the patient during treatment. After the frame is in place, a series of CT and/or MR scans are taken.	LINAC based Radiosurgery in Progress.

Depending on the type of lesion, angiographic films may also be ordered.

After the scan is taken, the imaging data is transferred to the X-Knife computer system. The treatment planning begins by using the imaging data to produce a 3-dimensional model of the tumor and nearby critical anatomical structures, such as the optic nerves or brain stem. The computer system determines the precise target position, dosage and configuration of radiation beams. This positioning optimizes the dose to the tumor, while minimizing the exposure of healthy tissue. Once the physician approves the treatment plan, the LINAC undergoes a series of quality assurance checks, and then treatment can begin. The patient is moved into the X-Knife LINAC Suite and is positioned. The actual dose administration can take as little as 30 minutes. This involves the movement of the LINAC around the patient as the focused radiation beams converge on the target. After the treatment, the head frame is removed. If no complications are observed, patients are free to leave the hospital the same afternoon.

The indications have increased dramatically. The following therapies have been reported:

A. Benign Non-Invasive Tumors: When small and radiographically distinct, radiosurgery can be curative: pituitary adenomas, acoustic neuromas, meningiomas, etc.

B. Small, Solitary Metastases.

C. Arterovenous Malformations (AVMs): The technique is very effective for small and medium sized AVMs in eloquent areas or when age dictates against conventional craniotomy.

D. Adjunct or Boost Therapy: To treat identifiable residual tumor not removed during surgery, or to augment conventional radiotherapy.

E. Salvage Therapy: To treat inoperable benign or malignant tumors in patients who have previously been irradiated?

Frameless stereotaxy:

The application of stereotactic techniques to the surgical resection of brain tumors provides information that allows the use of minimal craniotomies, accurately localizes subcortical lesions, and may assist in determining lesion boundaries. As such, stereotaxy-assisted craniotomy may reduce wound and neurological morbidity and increase the extent of tumor resection over conventional methods. However, craniotomies using commercially available stereotactic head frames can be logistically cumbersome, and techniques that provide information about tumor boundary demarcation may be either tedious or costly. The development of frameless stereotactic techniques depended on the development of the improved spatial fidelity of neuroimagers and the availability of graphics computers at reasonable costs.

Frameless stereotactic techniques promise to overcome many of these shortcomings while providing real-time localizing information throughout the craniotomy. The stereotactic microscope, as developed by Friets et al. and Roberts et al., and Watanabe et al.’s neuronavigator arm were pioneering efforts in this area.

The wand tip and trajectory are determined by proprietary computer software. Real-time display of this information is presented in multiple, two-dimensional or three-dimensional displays. When possible, patients were positioned with the anticipated surgical trajectory nearly vertical. Although not strictly necessary, this orientation optimizes the accuracy of the wand, prevents so-called "line-of-site" error, provides a comfortable working position, and minimizes the effects of "brain shift" after opening the dura. The present location of the table-mounted detector array precludes the use of an overhead sterile instrument table, but draping the patient is otherwise routine .The localizing wand is used to assist the determination of the lesion boundary. When visuotactile information suggested tumor edge, the wand proved confirmatory.

Commonly, in low-grade or deep portions of malignant astrocytomas (newly diagnosed or recurrent), the boundary was not apparent and the wand provided guidance that was equal to and more intuitive than cross- sectional information provided by the stereotactic frame system.

As in the performance of volumetric resections with frame systems, it was confirmed that the tumor should be removed in a near en bloc fashion to minimize otherwise unpredictable distortions in the spatial fidelity of the tumor/brain interface.

Although brain shift occurs, careful patient positioning limited this largely to a vertical axis that was readily detectable on the triplanar display and easily compensated once it was recognized.

An Optical Tracking System(OTS) procedure begins with the placement of a number of small, donut-shaped stickers, called fiducial markers; on the patient's head prior to taking a CT or MR scan. These markers appear in the scan images and will be used later to match the patient's anatomy to the CT/MR images in the operating room.

The scan is transferred to the OTS computer, and the OTS then reconstructs the scan images onto the computer screen as both two-dimensional and three-dimensional views. The surgeon can now conduct a review of all scan images from many angles, and determine the optimal point of entry and trajectory to remove the tumor.

Frameless Stereotaxy – the viewing wand and the computer work-station – for placement of scalp incision.

Frameless Stereotaxy – the wand, the fiducial, the optical tracking device and the computer work-station are seen

The author with Prof Connolly and Dr. M.F.Pell, in a frameless stereotaxy procedure, at the St. Vincent’s Hospital, Sydney.

The next step is to "register" the patient to his or her scan images so they can be used interactively during surgery. This important step is accomplished by just touching each of the markers on the patient's head with a special probe. This probe can be "seen" by the OTS camera, and the camera relays the position of the probe back to the computer.

This relationship allows the computer to know the position of the instrument in and around the patient anatomy at all times. When the instrument is placed on the patient, the exact location of the instrument tip is displayed on the same location on the scan images on the computer. This enables the surgeon to see the exact location of the anatomy, which may be obscured by blood and other obstructive tissue.

The OTS offers numerous unique features the Pointer-as-a-Mouse feature, which allows the surgeon full control of the software from the sterile field; the Depth Probe, or "virtual probe", which provides the surgeon the ability to simulate passage through patient anatomy and visualize critical anatomical structures before making an incision; and the Universal Instrument Registration feature, which allows the surgeon to quickly register and then track virtually any tool during surgery.

With the Optical Tracking System, the neurosurgeon can plan the most optimal approach to remove the tumor, as well as perform a smaller craniotomy. This means shorter surgery, reduced recovery time, and shorter hospital stay.