**2.4. Biological effect of LITT**

The laser probe tip is made of either sapphire or quartz to avoid altering the optical properties of the laser light. This design results in a spherical light distribution at the tip of the probe, and as a result, thermal energy is delivered in a symmetrical ellipsoid shape that is centered along the probe axis. The NeuroBlate System, in addition to the spherical probe design, also offers a side-firing probe which allows the surgeon to robotically control the direction of maximal heat distribution and may have an advantage in treating irregularly shaped lesions, or lesions

The Visualase system uses a 15 W 980 nm diode laser that is cooled with circulating sterile saline solution [25]. The diameter of the catheter is 1.65 mm. The laser probe tip comes with a light diffusing tip that results in spherical light distribution producing an ellipsoid area of tissue damage. This non-pulsed system produces faster lesions but the application of heat is limited to several minutes. The system is connected to a workstation that displays real-time thermography data as "thermal" and "damage" images [26, 27]. A number of safe points can be set on the pre-treatment MRI, and when the set temperature is reached at that point, the

The NeuroBlate System uses a 12 W solid-state Dornier diode laser that operates at Nd:YAG

in two diameters: 3.2 and 2.2 mm. The light diffusing tip comes in two configurations: spherical, used to produce elliptical lesions along the probe axis, and side-firing probes, that enable treatment of complex and irregularly shaped lesions. The computer interface displays thermal damage as thermal-damage-threshold (TDT) lines. The yellow line represents tissue volume that is exposed to the equivalent of 43°C for 2 min, the blue line is equivalent to exposure to 43°C for 10 min, and the white line surround the volume that received the equivalent of thermal energy of 43°C for 60 min. Based on the Arrhenius equation, the higher the tempera-

The use of laser ablation for treatment of tumors was first described by Bown in 1983 [20]. The first report of intracranial use for brain lesion laser ablation came out in 1990 [22]. Despite that, laser interstitial thermal therapy did not gain wide-spread use due to lack of the ability to monitor the extent of ablation and tissue damage. A variety of methods were attempted to measure thermal energy delivered to tissues and included skin thermometers, subcutaneous and interstitial probes, infrared detectors, and thermographic cameras, none of which were accurate enough to predict the size of the resulting thermal lesion [30–32]. Introduction of MR thermography revolutionized the application of laser thermal therapy since for the first time it allowed monitoring of the extent of tissue damage in real time [27]. The principle of MR thermography relies on detecting differential temperature-specific proton resonance frequency in the water molecules. At a given temperature, a proportion of water molecules are interconnected in space via hydrogen bonds between molecules. As the temperature of tissues increase during laser ablation, more water molecules are freed up from the hydrogen

free water molecules are mobilized more effectively resulting in a different proton resonance

O molecules. During application of the magnetic field, proton nuclei within

gas [29]. The probes come

wavelength of 1064 nm [28]. The laser catheter is cooled with CO2

ture, the less time it takes to generate each TDT-line.

near eloquent areas.

190 Glioma - Contemporary Diagnostic and Therapeutic Approaches

laser is deactivated.

**2.3. MR thermography**

bonds between H2

Heating tissue results in different types of tissue damage. Several different zones of tissue damage have been described. Heating tissues to up to 40°C typically does not disrupt cellular homeostasis. Once the temperature increases in the range of 42–45°C, the cells display marked susceptibility to cellular damage [34]. This range is typically explored in hyperthermia experiments. Further increase in temperature from 46 to 60°C results in significant cytotoxicity and consequent rapid cell death [35]. At temperatures exceeding 60°C, the damage sustained by mitochondrial enzymes, as well as cellular nucleic acids and proteins is so severe that coagulative necrosis takes place [36]. Finally, heating tissues to near boiling temperatures results in charring, tissue evaporation and carbonization, that may result in life-threatening intracranial pressure increases if not immediately relieved. In addition to temperature thresholds, the length of time that the tissue exposed to a particular temperature determines the extent of tissue damage with longer exposures resulting in equivalent damage that is observed at higher temperatures [18]. For instance, heating tissues to 43°C for 2 min will result in reversible tissue damage. Whereas heating tissues to this temperature for 10 min will result in permanent injury, and for 60 min will result in coagulative necrosis.

As tissue heating occurs, concentric zones of damage can be identified [18, 37–39]. In the area around the fiber, the temperatures can reach high numbers in excess of 60°C resulting in central core area of coagulative necrosis. If the temperature in the area adjacent to the fiber inadvertently reaches 100°C, tissue vaporization occurs and a pseudocavity is formed. Immediately outside the core area lies the intermediate zone of permanently damaged tissue with increased interstitial fluid content. The outermost zone of damage that represents marginal zone consists of edematous but viable brain tissue. Histologically, the marginal zone is defined by lack of evidence of apoptosis and vessel thrombosis, and containing axonal swelling, shrinking neurons, and hypertrophied endothelial cells—markers of reversible tissue injury. Following a laser ablation procedure, tissues typically exhibit an increase in size due to the presence of necrotic tissue and perilesional edema. Over time, however, the necrotic core of the lesion is replaced by granulation tissue resulting in lesion shrinkage and scar formation.
