**4.2. Lasers and probes used for LITT**

boiling, vaporization, and carbonization which if not released immediately might culminate in increased intracranial pressure. Apart from the true values of the temperature used, the time of exposure to such temperatures is also important. For example, 43°C for 2 min causes reversibledamage to the tissue, while the same temperature for 10 min causes permanenttissue damage and for 60 min causes coagulative necrosis [22, 45]. Based upon the Arrhenius equation, only shorter intervals are needed when using high temperatures to get the same

The target lesion usually undergoes central coagulation necrosis following LITT therapy, surrounded by a zone of edema next to the undamaged tissue [36]. By the end of 1st week, granulation tissue gradually replaces the zone of necrosis. The targeted lesion then develops into a cystic lesion with remnant necrotic debris surrounded by reactive gliosis with mesen‐

Three distinct zones can be identified on MRI following LITT. The first central zone repre‐ sents the zone of coagulation necrosis and ifthe temperature inadvertently exceeds 100°C, then there is a chance of charring and vaporization followed by a pseudo cavity formation. Just outside the core area lays a non-viable part with increased interstitial fluid called the inter‐ mediate zone. The outermost marginal zone is viable consisting of edematous viable sur‐ rounding brain parenchyma following thermal exposure and sharply delineates itself from the inner two zones. The ultra-structure of the inner two zones of thermal injury show disrupt‐ ed organelles and evidence of apoptosis, whereas the outer zone shows only axonal swelling, neuronal shrinkage and hypertrophied endothelial cells with no evidence of vessel thrombo‐ sis [4, 49–51]. Following LITT therapy, the target lesions might exhibit an increase in size due to necrosis and perilesional edema, but eventually will shrink and form a rim of granulation

**4. Technical aspects and commercially available components of LITT**

After numerous attempts of measuring the thermal energy delivery to the target tissue during LITT, including the use of skin thermometers, subcutaneous and interstitial probes, infrared detectors and thermographic cameras [28, 29, 52–56], it was the addition of MR thermometer that played the most significant role in allowing real-time monitoring and quantification of thermal energy delivery leading to thermal ablation [27]. MR thermography based on the temperature-dependent water proton resonance frequency (PRF) is capable of providing visual imaging together with a quantification model of thermal deposition with accurate temporal and spatial resolution. The theory behind PRF is based on the fact that as tempera‐ ture increases during LITT, the number of free H2O molecules also increases due to break‐ age of hydrogen bonds between H2O molecules. The hydrogen nuclei (proton) are mobilized more efficiently within the gradient field when in the free H2O molecule state, producing realtime imaging that can be interpreted and visualized using the proper computer software in

results [46].

tissue.

**4.1. MR thermometry**

the treatment workstation [57, 58].

chymal deposits [47, 48].

286 Neurooncology - Newer Developments

The two most common types of lasers used for LITT are the continuous-wave neodymiumdoped yttrium aluminum garnet (Nd:YAG), with a wavelength of 1064 nm, and diode lasers with wavelengths between 800–980 nm, which operate at a wide range of powers [1, 59, 60]. Nd:YAG lasers are capable to achieve deeper tissue penetration compared to diode lasers, especially in soft tissues with high blood perfusion at wavelengths between 1000–1100 nm [59, 61]. Diode lasers have the advantage of producing lesions faster, but typically with less penetration [2].

LITT probes have three main components: an optical fiber with a 600 μm diameter, a heatresistant terminal tip made of sapphire or quartz, measuring around 10 mm [59] and a cooling system, which is required to avoid overheating, tissue carbonization and optical fiber damage [61]. The current cooling mechanisms use either a cooled gas system (CO2) or a constant stream of fluid (water or saline) delivered to the tip of the probe through a sheath associated to the optic fiber [60, 62]. The thermal energy delivery at the probe tip has been classically described as a symmetrical ellipsoid shape centered along the axis of the probe. Recent advances in probe design, most specifically by the NeuroBlate® System (Monteris Medical Corporation, Plymouth, MN, USA), also led to the development of side firing laser probes, which allows the surgeon to control the laser ablation of complex shaped tumors in a real-time fashion.

## **4.3. Commercially available LITT systems used in neurosurgery**

Currently, there are two commercially available FDA-cleared LITT systems for neurosur‐ gery in the United States: the NeuroBlate® System (Monteris Medical Corporation, Plymouth, MN, USA) and the Visualase Thermal Therapy System (Medtronic Inc., Minneapolis, MN, USA).

*The Visualase Thermal Therapy System* uses a 15 W 980 nm diode laser generator that supplies energy to a disposable 1.65-mm diameter outer cooling catheter, which contains a 1cm-long fiber optic with a light diffusing, tip [2, 63]. The cooling mechanism is provided by circulat‐ ing sterile saline [2] and limits the duration of laser application to several minutes. Thermal energy is delivered in an ellipsoid-cylindrical fashion. The system is a MRI-guided laser ablation system, which is connected to a computer workstation capable of displaying real-time thermography data at the target location. Thermal information produces color-coded "thermal" and "damage" images [3, 27]. Limit temperatures can be designated as safety points on the pre-treatment MRI such that if during treatment an increase in temperature beyond the designated limit is detected at those points, the laser is automatically deactivated.

*The NeuroBlate® system* consists of a solid-state Dornier diode laser operating at the Nd:YAG wavelength (1064 nm) with a laser output of 30 W. The probes are available at diameters of 3.2 and 2.2 mm. The cooling mechanism is provided by a CO2 gas-cooled system [22, 45]. One unique feature of this system is that both side-firing and diffuse-tip probes are available. The NeuroBlate® directional side-firing laser probe is aimed for contoured ablation of complex shaped targets while the diffusing-tip laser probe is designed to provide fast volumetric ablation in a concentric fashion. The probes are inserted using frameless stereotactic guid‐ ance. The Monteris® Mini-Bolt provides rigid skull fixation and allows a direct interface to the NeuroBlate laser probe. The system is a MRI-guided laser ablation system, which is connect‐ ed to a computer workstation capable of displaying real-time thermography data at the target location. The NeuroBlate software displays the extent of thermal energy delivered as ther‐ mal-damage-threshold (TDT) lines. The yellow line surrounds the target volume that has received the thermal energy equivalent of 43*°C* for at least 2 min; the blue line surrounds the target volume that has been exposed to 43*°C* for at least 10 min; finally, the white line corresponds to tissue exposed to 43*°C* for 60 min. Tissues located outside the yellow TDT line are expected to have no permanent damage, while tissue volume inside the blue line under‐ goes severe thermal damage and tissue volume within the white line experiences coagula‐ tion necrosis [22, 45] (**Figure 1**).

**Figure 1.** (a–d) show the individual components of NeuroBlate® System including the bolts (b), laser probes (c) and robotic motor drive (d). Figures 1e–f depict the integration of robotic motor drive with the MRI scanner. *(Images used by permission from Monteris Medical Corporation, Plymouth, MN, USA. The use of any Monteris Medical photo or image does not imply Monteris' review or endorsement of any article or publication).*

**Disclosures:** Drs. Gene Barnett and Alireza Mohammadi are consultants of Monteris Medical Company (NeuroBlate System). Figure 1 is provided by Monteris Medical Company and is the only contribution of this company in this chapter.
