The stereotactic method was first developed by two British scientists in 1908, working at University College London Hospital, Sir Victor Horsley, a physician and neurosurgeon, and Robert H. Clarke, a physiologist. The Horsley-Clarke apparatus they developed was used for animal experimentation and implemented a Cartesian (three-orthogonal axis) system. Improved designs of their original device came into use in the 1930s for animal experimentation and are still in wide use today in all animal neuroscience laboratories.
Using the Horsley-Clarke apparatus for human brains was difficult because of the inability to visualize intracranial anatomic detail via radiography. However, contrasted brain radiography permitted the visualization of intracranial anatomic reference points or landmarks.
The first stereotactic devices for humans used the pineal gland and the foramen of Monro as landmarks. Later, other anatomic reference points such as the anterior and posterior commissures were used as intracranial landmarks. These landmarks were used with a brain atlas to estimate the location of intracranial anatomic structures that were not visible in radiographs. Using this approach between 1947 and 1949, two American neurosurgeons, Ernest A. Spiegel and Henry T. Wycis, and a Swedish neurosurgeon, Lars Leksell, developed the first stereotactic devices that were used for brain surgery in humans. Spiegel and Wycis used the Cartesian coordinate system (also called the translational system) for their device. Leksell's device used the polar coordinate system (also called spherical) that was far easier to use and calibrate in the operating room. The stereotactic localization system was also used by Leksell in his next invention, a device for radiosurgery of the brain.
In 1978, Russell A. Brown, an American physician and computer scientist, invented a simple technique to guide stereotactic surgery using computed tomography. This technique significantly improves surgical precision because computed tomography permits direct visualization of intracranial anatomic detail. The technique uses fiducials to create extracranial landmarks in each tomographic image or section. These landmarks specify the spatial orientation of that section with respect to the stereotactic device. Brown's invention stimulated intense interest in stereotaxy and radiosurgery. It is widely used today in the Brown-Roberts-Wells (BRW) stereotactic system as well as other stereotactic and radiosurgical devices.
Stereotactic surgery is a minimally invasive form of surgical intervention which makes use of a three-dimensional coordinates system to locate small targets inside the body and to perform on them some action such as ablation (removal), biopsy, lesion, injection, stimulation, implantation, radiosurgery (SRS) etc. In theory, any organ system inside the body can be subjected to stereotactic surgery. Difficulties in setting up a reliable frame of reference (such as bone landmarks which bear a constant spatial relation to soft tissues), however, mean that its applications have been limited to brain surgery. Besides the brain, biopsy and surgery of the breast are done routinely to locate, sample (biopsy) and remove tissue. Plain X-ray images (radiographic mammography) and computed tomography can be used to guide the procedure.
This technique uses images of the brain to guide the surgeon to a target within the brain. A colourful term for this surgery is neuro-navigation. This technique may utilise an external frame attached to the head (frame-based) or imaging markers attached to the scalp (frameless or image-guided surgery) to orient the surgical in his approach. The term "stereotactic" was coined from Greek and Latin roots meaning "touch in space." With frame-based stereotactic surgery, a light-weight frame is attached to the head using local anaesthesia. The head is imaged by CT, MR or angiography to identify the target in relationship to the external frame. Since both the frame and the target are "seen" in the images, the distance of the target from reference points on the frame can be measure in three dimensions. Surgical apparatus attached to the head frame can be adjusted to the three dimensional coordinates of the target and the target can be accurately approached by the surgeon. A common example is stereotactic brain biopsy. Deep tumours within the brain may be difficult and dangerous to approach by an open operation. Using a stereotactic biopsy apparatus fixed to the head frame and adjusted to the target coordinates, a biopsy probe is passed through a small hole in the skull to sample tissue for diagnosis. This technique is used to place electrodes in the deep brain to treat movement disorders, such as Parkinson's disease.
This system is also used by the Gamma Knife device, and by other neurosurgeons, using linear accelerators, proton beam therapy and neutron capture therapy.
Cross section of a Gamma Knife
Frameless stereotactic surgery relies on fiducial markers which are taped to the scalp before the brain is imaged. In the operating room the orientation of these markers is used to register the computer containing the brain images. Once registration is completed, the computer can show the relationship of our surgical instruments to the imaged brain. Frameless or image guided surgery is very helpful for the accurate approach and removal of large brain tumours.
The stereotactic method has continued to evolve, and at present uses an elaborate mixture of image-guided surgery using computed tomography, magnetic resonance imaging and stereotactic localization.
Stereotactic surgery works on the basis of three main components:
- A stereotactic planning system, including atlas, multimodality image matching tools, coordinates calculator, etc.
- A stereotactic device or apparatus
- A stereotactic localisation and placement procedure
Modern stereotactic planning systems are computer based. The stereotactic atlas is a series of cross sections of anatomical structure (for example, a human brain), depicted in reference to a two-coordinate frame. Thus, each brain structure can be easily assigned a range of three coordinate numbers, which will be used for positioning the stereotactic device. In most atlases, the three dimensions are: latero-lateral (x), dorso-ventral (y) and rostro-caudal (z).
The stereotactic apparatus uses a set of three coordinates (x, y and z) in an orthogonal frame of reference (cartesian coordinates), or, alternatively, a polar coordinates system, also with three coordinates: angle, depth and antero-posterior location. The mechanical device has head-holding clamps and bars which puts the head in a fixed position in reference to the coordinate system (the so-called zero or origin). In small laboratory animals, these are usually bone landmarks which are known to bear a constant spatial relation to soft tissue.
For example, brain atlases often use the external auditory meatus, the inferior orbital ridges, the median point of the maxilla between the incisive teeth. or the bregma (confluence of sutures of frontal and parietal bones), as such landmarks. In humans, the reference points, as described above, are intracerebral structures which are clearly discernible in a radiograph or tomogram.
Guide bars in the x, y and z directions (or alternatively, in the polar coordinate holder), fitted with high precision vernier scales allow the neurosurgeon to position the point of a probe (an electrode, a cannula, etc.) inside the brain, at the calculated coordinates for the desired structure, through a small trephined hole in the skull.
Currently, a number of manufacturers produce stereotactic devices fitted for neurosurgery in humans, as well as for animal experimentation.
Edited by John Sandham