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The application of multiple fibres for the conformal irradiation of tumours by laser-induced interstitial thermotherapy (LITT) has been investigated. A study was performed to evaluate the coagulated zones produced in porcine muscle tissue in vitro. For delivering specified powers into the tissue, a multifibre system was developed which allows the simultaneous use of up to four fibres. A new quantitative method of magnetic resonance imaging (MRI) has been applied for real-time thermometry. It is based on the temperature dependence of the T1 relaxation time and the equilibrium magnetisation. The MR results were compared with the measurements of fibreoptic thermometers. Since the acquisition time of the selected MR sequence takes only 3 s per slice and the calculation of the temperature measurement could be realised within a few seconds, the temperature mapping works closely to real time. The accuracy of the temperature measurements in muscle tissue was 1.5°C. Whereas single-fibre applications induced convex-shaped isotherms, concave structures were generated by a multifibre LITT.

Although lasers are nowadays widely accepted as a popular scalpel of minimally invasive surgery (MIS), one of the most common orthopedic surgeries-the replacement of the knee joint-is still performed using an ordinary oscillating saw. Since ultra-short laser pulses are usually considered to be inefficient regardless of their high precision, the newest development of femtosecond laser systems has not yet been clinically applied to any mass ablation situation. However, thin disk Yb:KYW lasers meanwhile provide sufficient output power to ablate bone tissue within a reasonable time frame. Our results mainly focus on ablation rates obtained at different spot distances, repetition rates and pulse energies. It is shown that femtosecond laser pulses at high repetition rates are a promising tool for orthopedic surgery.