Neurosurgery deals with diseases of the central nervous system (CNS), i.e. the brain and the spine. Surgery of brain tumors is very difficult, since extremely localized operations are necessary due to the complicated structure and fragility of the brain. Moreover, the tumor itself is often not easily accessible, and very important vital centers are situated beside it. Therefore, it is not surprising that a considerable amount of research funds is currently being spent in this field, especially since any kind of brain tumor - even benign tumors - are extremely life-threatening. This is because space inside the skull is very limited. Hence, growth of new tissue increases the pressure inside the brain which leads to mechanical damage of other neurons. A schematic cross-section of the brain is shown in Fig. 4.50.
The major parts of the brain are the cerebrum, diencephalon, cerebellum, and brainstem. The diencephalon can be further divided into the hypothalamus, hypophysis, thalamus, and epiphysis, whereas the brainstem consists of the mesencephalon, pons, and medulla oblongata. Usually, tumors of the brainstem are highly malignant and, unfortunately, they reside in an inaccessible location. In general, brain tissue can be divided into gray matter and white matter which are made up of cell nuclei and axons, respectively. Blood perfusion of gray and white matter differs remarkably. The corresponding ratio is about five to one.
Fig. 4.50. Scheme of a human brain
Hypophysis Pons Medulla oblongata
The application of lasers in neurosurgery has been extremely slow compared with other medical fields, e.g. ophthalmology. This was mainly due to two reasons. First, studies by Rosomoff and Caroll (1966) revealed that the ruby laser was not of great help in neurosurgery. Second, initial experiments with the CO2 laser were performed at too high energy levels, e.g. by Stellar et al. (1970), which was dangerous and completely unnecessary. It then took some time until Ascher (1979), Beck (1980), and Jain (1983) reawakened interest in neurosurgical lasers, especially moderate CO2 lasers and Nd:YAG lasers. The principal advantages of lasers in neurosurgery are evident. Lasers are able to cut, vaporize, and coagulate tissue without mechanical contact. This is of great significance when dealing with very sensitive tissues. Simultaneous coagulation of blood vessels eliminates dangerous hemorrhages which are extremely life-threatening when occurring inside the brain. Moreover, the area of operation is sterilized as lasing takes place, thereby reducing the probability of potential infections.
According to Ascher and Heppner (1984) and Stellar (1984), the main advantage of the CO2 laser is that its radiation at a wavelength of 10.6 |im is strongly absorbed by brain tissue. By this means, very precise cuts can be performed. However, CO2 lasers are not appropriate for the coagulation of all blood vessels. In particular, arteries and veins with diameters > 0.5 mm tend to bleed after being hit by the laser beam. Nd:YAG lasers, on the other hand, are effective in coagulating blood vessels as stated by Fasano et al. (1982) and Wharen and Anderson (1984b). Ulrich et al. (1986) even observed very good results on both ablation and coagulation when combining a Nd:YAG laser emitting at 1.319 | m and a 200 | m fiber. The biological response of brain tissue to radiation from Nd:YAG lasers was extensively studied by Wharen and Anderson (1984a). A preliminary report on the clinical use of a Nd:YAG laser was given by Ascher et al. (1991). Moreover, neurosurgical applications of argon ion lasers had been investigated by Fasano (1981) and Boggan et al. (1982), but they seem to be rather limited, since radiation from these lasers is strongly scattered inside brain tissue.
The main problem with CW lasers is that they do not remove brain tumors but only coagulate them. Necrotic tissue remains inside the brain and can thus lead to the occurrence of severe edema. Moreover, adjacent healthy tissue might be damaged due to heat diffusion, as well. Recently, two alternative lasers have been investigated concerning their applicability to neurosurgery: Er:YAG and Nd:YLF lasers. Cubeddu et al. (1994) and Fischer et al. (1994) have studied the ablation of brain tissue using free-running and Q-switched Er:YAG lasers. They observed limited thermal alterations of adjacent tissue. However, mechanical damage was very pronounced. Since the Er:YAG laser emits at a wavelength of 2.94 |im, its radiation is strongly absorbed in water as already discussed in Sect. 3.2. Thus, soft brain tissue - having a high water content - is suddenly vaporized which leads to vacuoles inside the tissue with diameters ranging up to a few millimeters. In Fig. 4.51a, mechanical damage
up to a depth of at least 1.5 mm is clearly visible. Therefore, it is not very helpful that even large volumes of brain tissue can be ablated with Er:YAG lasers as shown in Fig. 4.51b.
The ablation of brain tissue with a picosecond Nd:YLF laser system was investigated by Fischer et al. (1994). In Fig. 4.52, the ablation depths of white and gray brain matter are given, respectively. Obviously, there is no significant difference in ablating either substance. It is interesting to observe, though, that there is no saturation in ablation depth even at energy densities as high as 125 J/cm2. Thus, higher laser powers will probably enable ablation depths > 200 |im. Fischer et al. (1994) state that the corresponding ablation threshold is at approximately 20 J/cm2.
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