INSTITUTE OF LASER MEDICINE

Prof. Dr. P. Hering  

The aim of this work was to develop a reliable and versatile technique for evaluating the spatial distribution of the fluence rate, the temperature rise, and the resulting thermo-induced damage in tissue exposed to laser irradiation. The developed numerical technique combines a Monte Carlo method for the calculation of the fluence rate and absorbed power density distribution with a finite differences method for the solution of the heat transfer equation. This combination allows to take into account:
 

• finite dimensions and exact geometry of the laser radiation delivery system (applicator), 
• arbitrary 2D heterogeneities within the tissue, 
• tissue perfusion, 
• changes of the optical/thermal properties during thermal coagulation, 
• changes in the temporal regime of the laser irradiation during the session;
• option for active cooling. 

The implementation of the Monte Carlo method features the automated control of the computational accuracy and makes use of several variance reduction techniques. The developed software package enables us to use MRI-data (Fig 1.) for initial mapping of the optical properties at the treatment site (Fig. 2.). The technique is used to determine the optimal regimes of irradiation prior to laser treatment and to improve the design of delivery systems. Clinical applications include laser-induced interstitial thermotherapy (LITT) of brain tumors using a Nd:YAG laser. 

 
Fig. 1. An example of a pre-LITT MR image obtained from a patient with Astrocytoma Grade WHO II. The framed rectangle contains the tumor (dark).

 
Fig. 2. The region selected for treatment/simulation, corresponds to the framed rectangle in Fig 1. Also shown is an ITT(TM) applicator which delivers the laser irradiation to the treatment site.

 
Fig. 3. Simulated fluence rate distribution after 6 min of laser irradiation (intermediate result). 

Fig. 4. Simulated temperature distribution after 6 min of laser irradiation (intermediate result).

Fig. 5. Simulated tissue perfusion after 6 min of laser irradiation (intermediate result).

 
Fig 6. Simulated damage distribution after 20 min of laser irradiation (final result). The calculated damage distribution is overlapped with a post-LITT MR image of the selected region. The white rim is due to a post-dosed contrast agent and confines the laser-induced lesion. The AA and BB markers specify the maximal radial and axial extent of the lesion, respectively.

References