Exploiting the aiming beam to increase the safety of laser lithotripsy: Experimental evaluation of light reflection and fluorescence

Abstract

Background and Objectives In Holmium laser lithotripsy, usually, the surgeon is guided by a visible beam superimposing the infrared (IR) treatment radiation. It has been shown that a green aiming beam excites stone autofluorescence. This fluorescence signal can be used for calculi detection to check the correct fiber position before triggering the IR laser, thus preventing damage to soft tissue and application devices. However, also the directly reflected green light from the fiber tip gives valuable information on fiber position and its surface condition. Materials and Methods An external fiber‐fiber‐coupling‐box (fiber core diameter 365 µm) for pulsed holmium laser radiation (2.1 µm) was set up containing a green diode laser module (520 nm, average power on the sample <0.5 mW) and optics and detectors for measuring the reflected light of this aiming beam as well as the fluorescence excited with it. Measurements were done via a lock‐in technique with more than 20 human calculi samples and porcine calix in vitro. After the implementation of automatic data storage signals during ongoing in vitro lithotripsy procedures were recorded with the fiber positioned on tissue, stone, or in/on medical equipment (working channel of an endoscope, stone retrieval basket). Results Stone fluorescence signals measured were a factor of 7 to >100 higher than those of tissue. Stone fluorescence was detectable in “non‐contact mode” with a linear signal decrease over a distance up to ~1 mm in front of the fiber tip (core diameter 365 µm) and with severely damaged fibers (max. decrease: 75% with pinched off fiber). Reflection signals of the fiber tip surface in air and water surrounding decreased significantly when the fiber was damaged; measured ratios of intact to damaged fiber found in the air were (5–17):1 and in water (1.6–3.7):1. Surfaces in front of the fiber aggravated the evaluation of fiber condition due to reflections but enabled to detect, for example, the working channel of a flexible endoscope in combination with the (missing) fluorescence signal. Conclusions Autofluorescence induced by a green aiming beam can be exploited for stone detection in laser lithotripsy. A reflection measurement can give further information on fiber condition and position. Implementing this kind of safety features for an automatic block of IR laser emission in case of weak or missing fluorescence and un‐normal reflections can assist the surgeon by avoiding tissue perforation, and damage to medical devices such as endoscopes. Lasers Surg. Med. © 2019 Wiley Periodicals, Inc.

Laser-guided real-time automatic target identification for endoscopic stone lithotripsy: a two-arm in vivo porcine comparison study

“…Lange et al [12] obtained similar findings regarding the usefulness of autofluorescence for stone detection in an in vitro study. However, we took it one step further: This innovative technology was now-after having completed in vitro studies-successfully tested in large animal models, and extensive preclinical data were collected [10].…”

A novel laser lithotripsy system with automatic target recognition: from bench to bedside

“…The increasing clinical need to prevent tissue damage from laser light emitted from laser systems has inspired research on novel photonic technologies. Lange et al was the first group to demonstrate that autofluorescence is capable of distinguishing between urinary stones and human tissue [6]. Other research groups also proved that tissue and stone differentiation is feasible via target autofluorescence spectra [11,12].…”

“…Beyond this direct effect, insufficient irrigation during an endourological laser intervention might cause an uncontrolled temperature rise that damages adjacent tissue indirectly and delayed in time [5]. Photonic technologies have recently been used to experimentally identify urinary stones to increase safety while employing the Ho:YAG laser [6]. More recently, Winfree et al applied an autofluorescent imaging method to facilitate identification of Randall's plaque [7].…”

Theoretical and experimental evaluation of the distance dependence of fiber-based fluorescence and reflection measurements for laser lithotripsy

“…Because fiber damage leads to a change in the backscattering of light, its detection can be exploited for this purpose [25,27]. However, when a surface in front of the fiber partly reflects the aiming beam used for monitoring, this change in the signal can cause an incorrect assessment of the condition of the end face of the fiber [28]. Therefore, it is necessary to investigate the extent to which external reflections contribute to the overall reflection signal for varying distances from the surface to the fiber.…”

“…The course of the signal drop with increasing distance is also relevant for fluorescence, which is exploited for stone detection and needs to work in non-contact mode. Corresponding considerations and calculations can be found in numerous articles in the field of fiber sensors, and initial results regarding fluorescence/reflection signals measured using a laser lithotripsy setup have been published [28].…”