## 2 Materials and Methods  
A fiber-optic system implementing both fluorescence spectroscopy and laser Doppler flowmetry methods was developed (Figure 1a) especially for performing experimental measurements from approximately one diagnostic volume of biotissue (1-3 mm3). The main units of device were designed in cooperation with SPE \"LAZMA\" Ltd. (Moscow, Russia).  
A distinctive feature of the system is laparoscopic optical probe (30 cm in length and 3 mm in diameter), designed to access organs under investigation through standard instruments for minimally invasive manipulations. The probe contains 6 optical fibers (Figure 1b). A single-mode laser module with a radiation wavelength of 1064 nm was used in LDF channel, sources of 365 nm and 450 nm wavelengths were used for fluorescence excitation. The wavelengths were selected due to the fluorescent properties of such main endogenous fluorophores as NADH, FAD and stromal collagen. Namely, reduced NADH has maximum of emission at approx. 490 nm under excitation with UV  
Figure 1: Schematic presentation of installation for experimental studies (**a**) and location of fibers in the laparoscopic probe (**b**).  
radiation (365 nm). Oxidized FAD fluoresces with peak of emission at approx. 520-540 nm under excitation radiation with wavelength of 450 nm [18].  
The probing fiber of LDF channel has diameter of 6 um, 2 receiving fibers have a diameter of 400 um. The source-detector distance for LDF channel is 1.5 mm. The diameters of the probing and receiving fibers of FS channel are 400 um. The numerical aperture of the fibers is 0.22. For safety reasons, as well as to minimize photobleaching effect, the radiation power for a wavelength of 365 nm at the fiber probe output does not exceed 1.5 mW. To assess the safety of the optical probe, effective tissue irradiance was calculated and its value was about 1.4 W/m\({}^{2}\). According to the guidelines of the International Commission on Nonionizing Radiation Protection (ICNIRP), the limiting UV exposure duration per day for this level of effective irradiance is at least 10 s. In the study, the actual time of the UV-irradiation did not exceed 2 s. [19]. Therefore, the using of proposed source wavelength was deemed safe. The output power for an excitation wavelength of 450 nm does not exceed 3.5 mW. The distance between the radiation source and receiver in FS channel is 1 mm.  
A spectrometer of 350-820 nm detection range is used to record the fluorescence spectra. Data from the device is transferred to a personal computer for storing and further processing.  
The studies were conducted at the department of interventional radiology of Orel Regional Clinical Hospital (Orel, Russia). This part of the work involved 42 patients aged 68 \(\pm\) 12. Areas of interest included three types of organs with following pathologies: common bile duct (obstructive jaundice caused by pancreatic cancer or cholelithiasis, 29 patients); gallbladder (acute destructive cholecystitis, 5 patients); liver abscess (8 patients). All studies were approved by the local Committee for Human Biomedical Research Ethics (record of the meeting No10 of 16.11.2017) and were carried out in accordance with the principles outlined in the 2013 Declaration of Helsinki by the World Medical Association. The patients signed an informed consent indicating their voluntary willingness to participate in the study. The measurements were performed during diagnostic and therapeutic interventions under ultrasound and X-ray examination. The fibre optical probe was inserted into the lumen of the hollow organs (gallbladder, common bile duct) via minimally invasive approach. After that, the double excitation wavelengths autofluorescence has been recorded in several places from both affected and unaffected by the pathology areas.