Real‐time guidance of thermal and ultrashort pulsed laser ablation in hard tissue using inline coherent imaging

Abstract

Background and Objective: During tissue ablation, laser light can be delivered with high precision in the transverse dimensions but final incision depth can be difficult to control. We monitor incision depth as it progresses, providing feedback to ensure that material removal occurs within a localized target volume, reducing the possibility of undesirable damage to tissues below the incision. Materials and Methods: Ex vivo cortical and cancellous bone was ablated using pulsed lasers with center wavelengths of 1,064 and 1,070 nm, while being imaged in real-time using inline coherent imaging (ICI) at rates of up to 300 kHz and axial resolution of $6 mm. With realtime feedback, laser exposure was terminated before perforating into natural inclusions of the cancellous bone and verified by brightfield microscopy of the crater crosssections accessed via side-polishing.Results: ICI provides direct information about incision penetration even in the presence of intense backscatter from the pulsed laser and plasma emissions. In this study, ICI is able to anticipate structures 176 AE 8 mm below the ablation front with signal intensity 9 AE 2 dB above the noise floor. As a result, the operator is able to terminate exposure of the laser sparing a 50 mm thick layer of bone between the bottom of the incision to a natural inclusion in the cancellous bone. Versatility of the ICI system was demonstrated over a wide range of light-tissue interactions from thermal regime to direct solid-plasma transition. Conclusions: ICI can be used as non-contact real-time feedback to monitor the depth of an incision created by laser ablation, especially in heterogeneous tissue where ablation rate is less predictable. Furthermore, ICI can image below the ablation front making it possible to stop laser exposure to limit unintentional damage to subsurface structures such as blood vessels or nervous tissue.

Modeling the Laser Ablation Process

“…Although different from other methods found in the literature (e.g. confocal microscopy [13,14]) Optical Coherence Tomography [15], Inline Coherent Imaging [16]), the protocol used in our research work presents a suitable resolution for the measurement of incision depths in the range of tenths of millimeters. The proposed estimation method does not require any additional sensing device, thus it is appropriate for Transoral Laser Microsurgery (TLM).…”

Review of intraoperative optical coherence tomography: technology and applications [Invited]

“…Intraoperative OCT was used to monitor the ablation depth in bone tissue [150] using M-scans at the incision site during laser application. Another reported OCT system was used to guide laser-based cochleostomy, in which OCT image-based active tracking was implemented for motion compensation and improved 3D positioning of the laser application site [151].…”

Application of self-mixing interferometry for depth monitoring in the ablation of TiN coatings

“…16 Being an offline measurement strategy, this method does not give the possibility to compensate the processing deviations. More recently, faster techniques based on Fourier domain optical coherence tomography (OCT) [17][18][19] and self-mixing interferotmetry (SMI) [20][21][22] have been proposed as solutions for ablation depth monitoring. Both of the techniques allow high speed measurement by providing data over only a single dimension, which is depth.…”