Non-thermal ablation of neural tissue with femtosecond laser pulses

Nonthermal in vitro ablation of bovine neural tissue by using laser-induced optical breakdown generated by ultrashort laser pulses, with durations from 100 fs to 35 ps and pulse energies of up to 165 μJ, has been investigated. The experiments were performed at wavelengths ranging from 630 to 1053 nm by using a femtosecond Ti:Sapphire laser, a femtosecond dye laser, and a picosecond Nd:YLF laser system. Tissue ablations have been achieved by focusing the laser beam on the surface of the tissue, to a spot diameter of 5–20 μm, resulting in the generation of a microplasma. Laser pulses from the Ti:Sapphire laser with 140 fs duration showed a two times higher efficiency of ablation than the longer 30 ps pulses from a Nd:YLF laser with an identical pulse energy. At pulse energies of 140 μJ, single pass excisions deeper than 200 μm were generated by the 140 fs pulses. In addition, the fluence at threshold of the ablation was found to be reduced for shorter pulse durations. For 3 ps laser pulses at 630 nm, we measured the fluence at threshold to be about 5.3 J/cm2; for 100 fs pulses from the same laser the experimental threshold was at 1.5 J/cm2. Histopathological examinations and scanning electron micrographs confirm the high quality of the excisions. No sign of significant thermal damage was observed. PACS: 42.62.Be; 52.50.Jm; 87.50.Hj The application of lasers in neurosurgery has received increasing attention in recent years, although results of early studies were not very promising [1–3]. Investigations primarily by Ascher, Beck, and Heppner discussed the advantages of using lasers in this field of medical therapy, that is, the possibility to perform slightly invasive, noncontact surgery of sensitive neural tissue (e.g., [4, 5]). Recently, Krishnamurthy and Powers provided an extensive review on the application of lasers in neurosurgery [6]. Mainly CO2 and Nd:YAG lasers are used for vaporizing or coagulating the biological material. These free running or Q-switched lasers exhibit primarily a thermal interaction with ∗ To whom correspondence should be addressed the tissue [7, 8]. Therefore, owing to heat diffusion, thermal damage to adjacent tissue is commonly observed [8–11]. Furthermore, a big part of the irradiated tissue remains inside the brain and may lead to edema. A completely different process of laser–tissue interaction is plasma-mediated ablation, demonstrated, e.g., by Puliafito et al. [12] and Stern et al. [13] in ophthalmologic applications. In this process, the extremely high intensity of powerful ultrashort pulses first leads to multiphoton absorption in the material. This results in the ionization of some atoms and molecules, thereby providing initial carriers (“lucky electrons”) for the laser induced optical breakdown (LIOB) [12]. The free electrons and ions absorb energy from the electromagnetic field of the laser radiation by inverse bremsstrahlung, resulting in their acceleration [14]. The subsequent avalanche-like multiplication of free carriers finally leads to LIOB and the generation of a microplasma [15]. In addition, owing to the expansion of the heated plasma, a highpressure transient propagates radially from the LIOB center into the surrounding environment [12, 16]. This shockwave also contributes to the ablation by disruption. Finally, ablation fragments are ejected out of the interaction zone [17]. Advantages of plasma-mediated ablation are the very precise cutting of tissues and the absence of undesired thermal side effects. The latter phenomenon is a result of the ultrashort interaction time: according to the works of Docchio [18] and Zysset et al. [19], a laser pulse with a duration of 30–40 ps (or shorter) generates a microplasma with a luminescence lifetime shorter than 1 ns. Since thermal diffusion is too slow to dissipate the laser energy during this plasma lifetime [20], the thermal energy is confined to the zone of the plasma and no thermal interaction occurs in the adjacent tissue regions. Plasma-mediated ablation of brain tissue using picosecond pulses from a Nd:YLF laser system was first reported by Fischer et al. [21]. In this article, we report the experimental results on the efficiency of plasma-mediated ablation of bovine brain tissue using sub-200 fs laser pulses compared to ablation with pulses from a 30 ps Nd:YLF laser system. We have also investigated the dependence of the threshold of plasma-mediated ablation on the duration of the laser pulses between 100 fs