Investigating the reliability of depth monitoring using optical coherence tomography during laser osteotomy Arsham Hamidi 1, Yakub A. Bayhaqi 1, Ferda Canbaz 1, Alexander A. Navarini 2, Philippe C. Cattin 3, and Azhar Zam 1 1 Biomedical Laser and Optics Group (BLOG), Department of Biomedical Engineering, University of Basel 2 Digital Dermatology, Department of Biomedical Engineering, University of Basel 3 Center for medical Image Analysis and Navigation (CIAN), Department of Biomedical Engineering, University of Basel Introduction Laser osteotomy is a contactless and potentially minimally invasive technology for bone surgery, providing freedom of cutting geometry and high precision. One technical drawback of using lasers for osteotomy is the lack of depth feedback. Optical coherence tomography (OCT) is a non-invasive, high- resolution, and high-speed interferometric imaging system. Following the first report of the integrated OCT and laser ablation system, an extensive effort has been made to improve this integrated setup [1]. An irrigation system is commonly used together with laser ablation systems to keep the tissue hydrated and the surface temperature at a fixed level which is, below the damage/carbonization threshold. However, accumulation of water inside the laser-induced cut can lead to a wrong depth measurement, and consequently, damage the critical structure in the vicinity and/or excessive depth of cuts. Thus, a proper depth monitoring system is required to obtain correct depth measurements. Materials and Methods Figure 1 shows a schematic of the integrated system. The swept-source OCT (SS-OCT) system operates at 110 kHz with a central wavelength of 1314 nm, and spectral bandwidth of 64.5 nm. The imaging range was measured as 26.2 mm in the air. The Er:YAG laser (LITETOUCH by Syneron) was integrated with the OCT system with a dichroic filter [2]. A shutter was responsible for controlling the ablative laser, it stopped the laser pulses after reaching the desired depth. Accumulated water inside the laser-induced cut leads to the wrong measurement of the depth of incision. By detection of a less depth of cut in comparison to the measured depth in the previous laser pulse, the shutter was adjusted to be closed, and in the meantime, accumulated water extracted using high air-pressure. Results Figures 2(a)-(c) demonstrate the errors associated with OCT depth monitoring during laser osteotomy. Figure 2(a) shows the actual depth of laser-induced incision (2.6 mm). Although Fig. 2(b) represents the same depth of the cut as Fig. 2(a), the OCT system detects the air-water interface (0.96 mm) instead of the actual depth of the cut. Shooting the next Er:YAG laser pulse will lead to water explosion (Fig. 2(c)), which may cause imaging artifacts. Conclusions This study demonstrates one of the errors accompanied by the OCT system as a visual feedback system during laser osteotomy. An excessive amount of water that accumulated inside the laser-induced cuts can both lead to a wrong depth measurement and inducing imaging artifacts for the real-time OCT system. The results show the possibility of using the OCT system as visual feedback for laser osteotomy. This system can also potentially be used as a feedback system for the irrigation system. An Introduced method shows that this method can be used to control the irrigation system in order to prevent wrong depth measurements. References: [1] Boppart, Stephen A., et al. "High-resolution optical coherence tomography-guided laser ablation of surgical tissue." Journal of Surgical Research 82.2 (1999): 275-284. [2] Hamidi, Arsham, et al. “Long-range optical coherence tomography with extended depth-of-focus: A visual feedback system for smart laser osteotomy." Journal of Biomedical Optics Express, 10.1364/BOE.414300. Fig.1 : Integration of OCT system with laser ablation (Er:YAG laser). Fig.2 : OCT images of different conditions, (a) actual depth of cut, (b) crater filled with water, (c) water