Introduction

Ureteroscopy is currently the most frequent intervention for renal stones in many countries worldwide, and is an established procedure commonly done in urology practice [1, 2], with a plethora of applications [3]

Imaging characteristics of ureteroscopes impacting on clarity, visibility, contrast, color accuracy and resolution are important parameters that have been extensively evaluated [4,5,6,7,8,9,10] due to their importance in treatment efficacy and safety. Image clarity allows accurate identification of pathology, precise stone lasering, basketing, biopsy and ablation of urothelial carcinoma.

Vision and clarity of images in ureteroscopy are dependent on many factors, including illumination and distribution of light from the tip of ureteroscopes. With newer and single-use ureteroscopes, homogenous and comparable illumination characteristics would be expected from scopes available for use in daily clinical routine. On the contrary, there have been previous reports on undesirable artefacts from poor light source design causing shadowing and dark corners in the endoscopic field of view [8], which ideally should not arise in scopes that were already released for use in humans.

Only few studies so far have evaluated light source of ureteroscopes, and all were performed at fixed distances with scopes held in air instead of saline [6, 7, 11,12,13,14,15]. One study looked at light intensity distribution characteristics in prototype ureteroscope tip models, but exact illuminance was not measured [16]. Illumination properties depend on the light source from the scope and the medium it is projected across. To the best of our knowledge, no prior study on ureteroscope optics was done in saline (NaCl 0.9%), which is in fact the native ureteroscopy medium used in clinical routine. The refraction index differs between air and saline [17], and illumination properties arguably may vary accordingly.

The aim of this present study was to evaluate illumination properties of contemporary ureteroscopes, including comparisons in air vs. saline. Light sources from ureteroscopes were also evaluated in an analysis of the tip design.

Material and methods

We evaluated a series of currently available flexible ureteroscopes at our institution, including the Flex-Xc and Flex-X2s (Karl Storz SE & Co. KG, Tuttlingen, Germany), URF-P7 and URF-V3 (Olympus, Centre Valley, PA, USA), Uscope 7.5F PU3033A and Uscope 9.2F PU3022A (Zhuhai Pusen Medical Technology Co. Ltd. Guangdong, China) as well as the OTU WiScope (OTU Medical Inc, CA, USA). To get as close as possible to real working conditions, the single-use scopes (Pusen 7.5F, Pusen 9.2F and OTU WiScope) were brand new models from sealed sterilized packages. Reusable scopes (Storz and Olympus scopes) had all been rinsed, disinfected, and decontaminated after use in humans, with no record of the number of prior interventions.

For the Storz Flex-X2s, the Power LED 175 light source (unit used < 100 h) was used with a corresponding 230 cm and 3.5 mm fiberoptic cable, as well as with the IMAGE1 S HX-P HDTV 1-Chip pendular camera (Karl Storz SE & Co. KG, Tuttlingen, Germany). For the Olympus URF-P7 and URF-V3, the VISERA elite CLV-S190 light source (Xenon short-arc lamp used < 100 h) was used together with a corresponding WA03310A 300 cm and 4.3 mm fiberoptic light cable, as well as the CH S190 08 LB camera head (Olympus, Centre Valley, PA, USA). Both fiberoptic cables were brand new.

A color spectrometer incorporating the Vishay VEML 6040 color sensor (RGBW200, ELV Elektronik AG, Leer, Germany) was used for lux and color temperature measurements, as well as for red, green and blue (RGB) light measurements. The Vishay VEML6040 color sensor senses RGB as well as white light with peak sensitivities for RGB at 645 nm, 575 nm, and 460 nm, respectively, according to the manufacturer information [18]. The range of spectral bandwidth for RGB is ± 45 nm, ± 45 nm, and ± 35 nm, respectively. The high sensitivity and wide dynamic range of the sensor allows for light intensity measurements between < 0.01 lx up to 16’496 lx. RGB values are provided as normalized values relative to the reading settings of the sensor (% of maximum). The sensor was waterproofed with a transparent polyvinylidene chloride film to allow experiments to be performed in saline (NaCl 0.9%).

A 3D printed black model was used to hold the ureteroscopes at a fixed distance of 20 mm from the center of a target board in a dark room (Fig. 1A). The target board included one center opening, as well as eight inner and eight outer ring openings distributed in circles at 4 mm and 8 mm from the center opening respectively, resulting in a 19 mm diameter target circle. The ureteroscope was maintained in a non-deflected, straight position, with the center of the endoscopic view aligned to the center opening of the target board. The size of the target field and distance from the light sensor was chosen with reference to dimensions of models constructed on data from endocasts [19] used in studies testing scopes in the setting of laser lithotripsy [20], to reflect in vivo settings. The tray holding the set-up was colored black to minimize noise from scattered light and all measurements were performed in a black chamber.

Fig. 1
figure 1

Experimental setup for illumination characteristics of ureteroscopes. A Experimental setup with a 3D printed black model; B and C Top view of the Flex-Xc illumination in air and saline; D Target board layout with center, inner and outer ring openings

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For each measurement, the color spectrum analyzer sensor was placed at one of the 17 openings of the target board and measurements were repeated 5 times. For each repeated measurement, the ureteroscope was withdrawn and reinserted into the 3D printed black model. Illuminance was measured at all 17 target board openings, while color temperature and RGB measurements were only performed for the center opening. We arbitrarily chose the Storz Flex-Xc to compare light properties in air vs. saline at 50% and 100% brightness settings. Afterwards, the entire experiment set-up was performed in saline including all scopes, thus replicating usual conditions found in clinical routine during ureteroscopy.

Whenever the scope setup allowed for light source brightness adjustment, measurements were repeated at light brightness settings of 50% and 100%. For the Storz Flex-Xc this was found available to be adjusted only via the menu buttons on the scope handle. For the Storz Flex-X2s, in addition to brightness settings adjustable on the light source unit, buttons on the camera head also allowed for separate brightness adjustment. Preliminary analysis of the Storz Flex-Xc results led the authors to decide for the Storz Flex-X2s to keep the brightness setting in the camera head at 50% and take only measurements for the 50% and 100% brightness settings adjusted on the light stack. For all other scopes, the brightness setting was adjusted on the light stack. Since light brightness can be set either to automatic or manual mode on the Olympus light stack as well as with the Storz camera head, all measurements were performed using the manual mode to ensure consistent measurements throughout all experiments.

Finally, ureteroscope tip designs were evaluated. Digital still photographs of the tip design of ureteroscopes were taken with an Olympus Pen E-P7 camera and a Panasonic Leica DG Macro-Elmarit 45 mm 2.8 lens. The angulations between the ureteroscope lens, light source(s) and working channel were measured using the computer software ImageJ [21].

Statistical analysis

Illuminance was expressed as mean values of 5 repeated measurement in lux. For skewness, a clock face view of the endoscopic scope image was used for easy reference and direct application to daily clinical practice. For calculation of skewness, the mean value resulting from the inner and outer opening at each clock face direction was used. From there, the result was expressed as percentage of the clock face direction with the highest illuminance, allowing to draw a radar chart for each scope.

An analysis on all scopes at 50% and 100% brightness settings was performed using one-way ANOVA with Tukey post hoc comparisons. Student’s t-test analyses were performed to evaluate the impact of air vs. saline, brightness settings and distance of openings from center on mean illuminance, color temperature and RGB values. For all tests, a two-sided p value < 0.05 was considered statistically significant. All statistical tests were performed with GraphPad Prism 9.5.1 (GraphPad Software, La Jolla CA, USA). Illuminance maps were created using mean illuminance values with the web program Heatmapper [22]. Radar charts of the light illuminance skewness were made with Microsoft Excel for Microsoft 365 MSO Version 2302 Build 16.0.16130.20332 (Microsoft Corporation, Redmond WA, USA).

Results

Air vs. saline

Both overall illuminance and center opening illuminance were found to be significantly higher in saline compared to air (5679 vs. 5205 lx, p = 0.02 and 8600 vs. 6558 lx, p < 0.001, respectively) (Table 1). Additionally, there was a significant illuminance drop between the inner and outer ring measurements in air (−660 lx, p < 0.005), whereas no significant drop was found in saline (p = 0.23). The top view of the light illumination path was noted to be comparatively more dispersed in air, while rather triangular with distinct borders in saline (Fig. 1B and C).

Table 1 Illumination properties of flexible ureteroscopes
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Illumination characteristics in saline

Overall illuminance significantly differed between different flexible ureteroscopes at both 50% and 100% brightness settings (ANOVA p < 0.001 for both settings) (Table 1).

The Flex-Xc had the highest overall illuminance at 100% brightness settings (5679 lx), followed by Pusen 9.2F (5280 lx at 100% brightness settings), Flex-X2s (4613 lx), P7 (4371 lx), V3 (2374 lx), WiScope (582 lx) and finally the Pusen 7.5F with the lowest overall illuminance readings (255 lx). The same ranking was found for 50% brightness settings (Table 1).

For each scope, there was a significant increase in mean overall illuminance when comparing the 50% and 100% brightness settings, with the highest relative overall illuminance increase for the V3 (+ 258%), and the lowest relative overall illuminance increase with the Flex-X2s (+ 42%). One exception was the Flex-Xc where no significant difference between the 50% and 100% setting was found.

Only two out of seven scopes had their point of maximum illuminance consistently at the center opening of the target board: the Flex-X2s and P7 (Table 2, Fig. 2). All other scopes had maximum illuminance off-center at 50% and/or 100% brightness setting.

Table 2 Illuminance spread over a target board in saline
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Fig. 2
figure 2

Illuminance maps, skewness in saline and tip design of flexible ureteroscopes. Illuminance and skewness maps are represented from an endoscopic point of view, with center of the endoscopic view aligned to the black model center opening. Each white point on the illuminance map corresponds to an opening on the black model. Ureteroscope tip configurations are front facing (according to endoscopic view) and side facing photographs (scope rotated to illustrate beveled tip

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There was a significant drop in mean overall illuminance between inner ring (4 mm away from center) and outer ring (8 mm away from center) for all scopes, except for the Flex-Xc where no significant differences were found (Table 2, Fig. 2). The highest relative drop between inner and outer ring measurements was found for the V3 (−83% at 50% brightness setting, and −77% at 100% brightness setting, respectively). There was presence of different illuminance skewness patterns across scopes (Fig. 2).

Color temperature

Color temperature measurement results are shown in Supplementary Fig. 1. Intra-scope comparisons between 50 and 100% brightness revealed significant differences for three out of seven scopes: + 838 Kelvin (95% CI 40 to 1637; p = 0.04) for the V3 from 50 to 100% light source brightness settings, −223 Kelvin (95% CI −408 to −39; p = 0.02) for the P7 and −510 Kelvin (95% CI −912 to −108; p = 0.02) for the WiScope.

Inter-scope comparisons revealed significant differences of mean color temperature measurements (ANOVA p < 0.001). The lowest color temperature readings were found for the V3 (3322 Kelvin at 50%), which was significantly lower compared to all other scopes in the Tukey’s post hoc analysis (all p < 0.001). The two scopes with the highest color temperature reading ranges were the Pusen 7.5F (7502 Kelvin at 50%) and the WiScope (8159 Kelvin at 50%), which were both significantly higher compared to all other scopes in the Tukey’s post hoc analysis (all p < 0.001).

Red green blue (RGB) normalized values

The maximum channel for the normalized RGB mean measurements was consistently green for all scopes, except for red with the V3 (Supplementary Table 1). The minimum channel was blue for all scopes across 50% and 100% brightness settings. When comparing across scopes, the respective RGB values for all scopes were significantly different at both 50% and 100% brightness settings (both ANOVA p < 0.001).

When comparing individual Red, Green and Blue channel differences at 50% vs 100% brightness settings within each scope, all RGB measurements were significantly different except for the Red and Blue readings for the Flex-Xc (Supplementary Table 1).

Ureteroscope tip design

Ureteroscope tip designs differ widely between scopes (Fig. 2). Of relevance to illumination, three scopes had two light sources (Flex-Xc, Pusen 9.2F, and the WiScope), while the rest had only one light source (Table 3). The light source position relative to the optical lens varied widely as well. Interestingly, two scopes (P7 and WiScope) had transparent tips, with the end of the light sources contained within the transparent part of the tip. All the rest of the scopes had the light source level with the end of the scope tip. All scopes were found to have fiberoptic light sources except two scopes with light emitting diode (LED) sources at the tip of the scope (Pusen 9.2F and WiScope).

Table 3 Characteristics of flexible ureteroscopes’ tips and light sources
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Discussion

The difference in illumination properties of ureteroscopes in air and saline is likely due to the respective differing refractive indexes, in combination with absorption and scattering effects [17]. The findings of this study are important in suggesting that studies on ureteroscope illuminance should ideally not be done in air. The authors suggest that all such future studies should ideally be done in saline instead, to replicate the real performance conditions in endourological surgery.

We note that the Flex-Xc scope handle menu brightness settings of 50 and 100% had illuminance values which were not significantly different. We conclude that the actual illuminance of the light source did not change with different brightness settings, and this is a feature of the scope to increase the exposure of the image on the viewing screen.

Illuminance varies widely between ureteroscopes. The Flex-Xc had the highest illuminance across all scopes (both reusable and single-use). Out of the single-use ureteroscopes, the Pusen 9.2F had illuminance readings higher than other reusable ureteroscopes. We note the possibility of single-use scopes to have the capability of high illumination characteristics. On the other hand, the Pusen 7.5F had the lowest illuminance of all scopes, with a factor up to 34 times lower than the scope with the strongest illuminance, reaching merely 3% illuminance compared to the Flex-Xc. How this may impact in clinical routine needs to be evaluated in dedicated in vivo studies. Arguably, such differences in illumination characteristics may impact the overall clarity of the image of ureteroscopes and may partly explain the differences in image quality perception found in a prior study [23].

Urologists need to be aware of the skewness of light from ureteroscopes to prevent missing pathology in darker areas. This is especially valid in situations of poorer visibility including bleeding [23], larger hydronephrotic systems, or working at further distances from the target object. Clock face sectors with highest illuminance may correlate to the choice of scopes in right and left sided surgery, akin to working channel position [24]. The impact of skewness in clinical situations needs to be further evaluated in future studies.

With regards to color temperature readings, the V3 and P7 produced warmer light, while the Pusen 7.5F and WiScope produced cooler light. Clinical relevance of this finding is not clear and deserve dedicated investigation.

It is interesting that individual RGB values change between 50 and 100% brightness settings for all scopes except for the Flex-Xc (likely due to the very homogenous spread of light from its light source). All scopes had green as the maximum channel except for the Olympus V3.

It is important to note that with re-usable equipment, illuminance may deteriorate with time. After each procedure, the quality of vision may progressively worsen due to damage of the optical systems (illumination and imaging [25]), and additionally with fiberoptic ureteroscopes deterioration of the light cable and light source. The light stack should undergo maintenance regularly, and light cables renewed periodically. This is a fundamental difference between re-usable and single-use scopes, where single-use ureteroscopes are always optimal as brand new with each use.

The special transparent tip designs of the WiScope and P7 could be hypothesized to give a “omni-light” with a more homogenous spread of light. However, on the contrary our study showed significant drop in illuminance between inner and outer ring openings for the two scopes. The advantages or limitations of such transparent tip design needs to be further evaluated.

The study has several potential limitations. First, light measurements were taken in a ring-like fashion across the target board, with areas in between openings not measured. The target board area of 19 mm may not capture the entire fall of peripheral light. However, the study setup adequately samples a target field size and ureteroscope distance representative of in vivo ureteroscope use. Measurements wider to this target field are not as clinically relevant. Second, the reusable scopes used for this study were not brand new. However, the usage of the scopes reflects real life clinical scenarios as mentioned, and in fact in some instances had higher illuminance than brand new single-use scopes. Third, the present study is an in vitro attempt to assess ureteroscope properties that may impact in vivo surgery. The interpretation of the data must therefore be taken with care since environmental factors arguably may impact on clinical translation of the findings of this study. Black was used to decrease peripheral light scatter. However, this does not reflect the color of the renal collecting system. The experimental setup was performed as an open setup, which is not reflective of the closed renal collecting system cavity with effects of indirect and direct light. Measurements were also taken at a fixed 20 mm distance, and future studies could further investigate effects on different distances to the ureteroscope tip. Fourth, urine and blood may change illumination characteristics when present in addition to saline irrigation. This may change the refractive index as well as transmission of light across the saline-urine-blood interface and should be further investigated in future studies. Additionally, it is not clear how illuminance properties may impact the final video projection of the surgical site from ureteroscopes on operative monitors. Finally, while there is a wide range of scopes clinically available, we were only able to test a set number of reusable and single-use scopes available to the institution. Future studies could include other newer scopes that have since been made commercially available. Nonetheless, this does not change the main findings of our study: that illuminance varies very widely between different ureteroscopes.

Other aspects of ureteroscope illumination properties should be further evaluated in future studies, including the impact of illuminance and light source design on unwanted interferences (flashes or image artefacts) [26]. Additionally, it remains to be seen if the study findings can be useful in the design of future ureteroscopes with high resolution integrating Artificial Intelligence [27, 28].

Conclusions

In line with previous reports on possible shortcomings of illuminance from flexible ureteroscopes, the present study demonstrates the presence of inhomogeneous light spread as well as large differences in illumination properties of ureteroscopes, possibly impacting on the performance of individual scopes in vivo. Additionally, illumination properties of ureteroscopes are different in air than in saline. The authors suggest that future studies on illumination characteristics of flexible ureteroscopes should ideally be done in saline, and no longer in air. Urologists should be aware of the non-centered maximum illuminance and skewness of some ureteroscopes. Observations from the present study deserve analysis in future studies, including situations in a closed cavity system [29], partial obstruction [30], varying effects of light brightness [31] and account for environmental factors such as urine and blood.