Fig. 4.53. (a) Brain tissue after exposure to a picosecond Nd:YLF laser (pulse duration: 30 ps, pulse energy: 0.5mJ). (b) Histologic section of brain tissue after exposure to the same laser (bar: 50 ^m). Reproduced from Fischer et al. (1994) by permission. © 1994 Springer-Verlag
A very precise technique is called stereotactic neurosurgery which was described in detail by Kelly et al. (1982). It requires a so-called stereotactic head ring made of steel or carbon fibers which is tightly fastened to the patient's skull by several screws. This ring defines a coordinate system which serves as a valuable means of orientation during surgery. The head ring appears on CT or magnetic resonance images (MRI) and thus determines the coordinates of the tumor. Various kinds of aiming devices can be mounted to the ring allowing for precise operation in all three dimensions. The main goal of stereotactic neurosurgery is to plan a suitable penetration channel in advance of surgery, set its coordinates with respect to the head ring, and then keep this channel during surgery. By this procedure, the risk of hitting a vital center within the brain can be significantly reduced, and the success of a treatment becomes more predictable.
The concept of stereotactic laser-neurosurgery according to Bille et al. (1993) is illustrated in Fig. 4.54. By means of a stereotactic head ring, a laser probe is inserted into the brain. CT and NMR data are used to correctly position the distal end of the probe inside the tumor. A schematic drawing of the laser probe is given in Fig. 4.55. It basically consists of a conical tube which contains a rotating mirror at its distal end, a movable focusing lens, and additional channels for aspiration and rinsing. Aspiration is necessary to maintain a constant pressure at the site of operation. The laser probe is rinsed to remove debris from the rotating mirror and to increase the efficiency of the ablation process. The rotating mirror deflects the laser beam perpendicularly to the axis of rotation. Tissue is thus ablated in cylindrical layers as shown in Fig. 4.55. Furthermore, it is planned to integrate a confocal laser scanning microscope into this system for the automatic detection of blood vessels as illustrated in Fig. 4.56.
Fig. 4.54. Concept of stereotactic laser-neurosurgery
Stereotactic CT & NMR data head ring
Fig. 4.54. Concept of stereotactic laser-neurosurgery
Fig. 4.56. Topology of tumor ablation
It should be mentioned that stereotactic neurosurgery is already a well-established clinical discipline. Stereotactic techniques are not only applied in combination with lasers but with alternative therapies, e.g. insertion of radioactive seeds (60Co or 125J) and high-frequency coagulation, as well. In general, any of these procedures alone might not lead to a complete necrosis of all tumor cells. In these cases, however, the stereotactic concept provides a useful combination of several treatment techniques by simply exchanging the surgical equipment mounted to the head ring of the patient. The principal advantages of stereotactic surgery, of course, are its high precision - within tenths of a millimeter - in aiming at the tumor and the ability to manage surgery with a tiny hole in the skull of less than 1cm in diameter8. Stereo-tactic surgery thus certainly belongs to the favored treatments of minimally invasive surgery (MIS).
An exciting technique of sutureless microvascular anastomosis using the Nd:YAG laser was developed by Jain (1980). It is performed in some cases of cerebrovascular occlusive disease. During anastomosis, a branch of the superficial temporal artery is connected to a cortical branch of the middle cerebral artery. Typical laser parameters are powers of 18 W, focal spot sizes of 0.3 mm, and single exposure durations of 0.1 s. This method is considerably faster than conventional suture techniques, it does not induce damage to the endothelium of the vessel, and it can be performed on relatively small and/or deeply located blood vessels, as well. The mechanism of vessel welding is not completely understood but is believed to rely on heat-induced alterations in collagen of the vessel. First clinical results had already been reported by Jain (1984a), but a high rate of associated complications soon slowed down the initial euphoria. Later, Neblett et al. (1986) and Ulrich et al. (1988) combined the application of a Nd:YAG laser with conventional techniques of anastomosis, and they achieved more promising results. In blood vessels with diameters of 0.8-1.2 mm, neither short-term nor long-term complications occurred.
Spinal surgery is the other principal field of neurosurgical treatments. According to Jain (1984b), the CO2 laser has proven to be useful in treating tumors of the spinal cord. Such tumors can be coagulated without severe complications. Ascher and Heppner (1984) have reported on the successful dissection of intramedullary gliomas of the spinal cord with a pulsed CO2 laser. Moreover, some basic procedures concerning pain relief of the spinal cord can be performed with this laser. Spinal laser surgery is still in its infancy, and considerable progress is expected within the next few years when miniaturized surgical instruments become available, e.g. in the technique of laser-assisted nerve repair as already proposed by Bailes et al. (1989). The combination of highly sophisticated endoscopes and appropriate laser systems might then turn into a powerful joint venture.
8 Conventional craniotomies usually require openings in the skull of at least 5 cm in diameter.
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