Introduction

The global prevalence of stone disease is increasing [1,2,3]. In tandem, there has been a huge increase in ureteroscopy procedures worldwide over the last two decades, which is currently the most frequent intervention for renal stones [2].

Clarity of vision during ureteroscopy is important and ensures accurate identification and precise targeting of pathology [4,5,6,7,8]. Visibility typically becomes of utmost relevance for tumor detection, safe stone lasering or future implementation of artificial intelligence [9, 10].

Many factors play a role in vision and clarity of images for ureteroscopy. These include illuminance and light distribution from the ureteroscope tip to the target object, and environmental factors such as the effects of direct and indirect light in an enclosed kidney cavity.

To the best of our knowledge, distribution of illuminance from light sources of ureteroscopes in an enclosed cavity in saline has not been evaluated so far. Of the prior studies that have evaluated light source of ureteroscopes, most were done in open systems [6, 7, 11,12,13,14,15]. Only one study in literature incorporated an enclosed system, but a diffuser plate was applied to the incoming light [16]. Another study looked at the effects of an integrating sphere on light transmission and measured only the integrated light output from one single opening [17]. No prior study looked at light measurements at different points in an enclosed sphere. More importantly, all these prior studies were performed in air, while illumination properties have been shown to be different with air and saline in a recent study, saline being the medium used in clinical routine during ureteroscopy [15].

The aim of the study was to evaluate illuminance and color temperature from ureteroscopy light sources within a closed in-vitro kidney calyx model in saline, including distribution of direct and indirect light.

Material and methods

A series of ureteroscopes available at our institution were evaluated, including the Flex-Xc and Flex-X2s (Karl Storz SE & Co. KG, Tuttlingen Germany), the URF-P7 and URF-V3 (Olympus, Center Valley, PA, USA), and single use scopes Uscope 7.5F PU3033A, Uscope 9.2F PU3022A (Zhuhai Pusen Medical Technology Co. Ltd. Guangdong, China) and WiScope (OTU Medical Inc., CA, USA). The single use scopes (Pusen 7.5F, Pusen 9.2F, OTU WiScope) were brand new from sealed sterile packages, to reflect real world working conditions. Reusable scopes (Storz and Olympus scopes) had all undergone rinsing, disinfection, and decontaminated after clinical use, with no record of the number of prior case usage.

Together with the Storz Flex-X2s, the Power LED 175 light source (unit used < 100 h) was used with the corresponding 230 cm and 3.5 mm fiberoptic cable, as well as 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) with the corresponding WA03310A 300 cm and 4.3 mm fiberoptic light cable, as well as the CH S190 08 LB camera head (Olympus, Center Valley, PA, USA) were used. The fiberoptic cables were brand new.

A color spectrometer housing the Vishay VEML 6040 color sensor (RGBW200, ELV Elektronik AG, Leer, Germany) was used for illuminance and color temperature measurements in saline, as previously described [15].

A 3D-printed pink kidney calyx model was used to hold the ureteroscopes at a fixed distance of 20 mm from the concave surface of the calyx model in a dark room (Fig. 1). This consisted of a closed spherical cavity to replicate the human kidney calyx and included three openings located at the center, 45°and 90° to the axis of the scope. The model was rotated in successive 90° steps relative to the axis of the scope to obtain measurements at nine points across the sphere. Measurement openings were chosen to measure the direct light (center), combined direct and indirect light (45°) and indirect light only (90°). Peripheral illuminance and peripheral color temperature were defined as measurements from opening positions at 45° and 90°, as opposed to measurements from the center opening position. Pink material was chosen for the kidney calyx model to replicate human urothelial mucosa. The ureteroscope was kept in a non-deflected position, with the center of the scope view aligned to the center sensor opening. To reflect in vivo settings, the size of the target field and distance from the light sensor was chosen based on dimensions of models constructed on data from endocasts [18] used for in-vitro studies evaluating laser lithotripsy [19, 20]. All measurements were performed in saline to replicate the usual conditions found in ureteroscopy clinical routine.

Fig. 1
figure 1

Experimental set-up. A Photograph of the 3D-printed enclosed pink model used for measurements with light measurement openings at 0°, 45° and 90 °C from the scope’s axis. The photograph is an outside view of the model. For measurements, obliterating 3D-printed pink counter-pieces were used to leave only one measurement opening accessible to the spectrometer’s sensor (not shown in this photograph to the sake of better representation of the openings from an outside view). B Endoscopic view of the 3D-printed enclosed pink model (still image from a Storz Flex-Xc inserted in the pink model, which was completely immerged in saline). The visible openings for the spectrometer sensor are obliterated with 3D-printed pink counter-pieces in this image, with the open 90° position out of the field of vision. Blue dots indicate the actual and virtual light measurement positions. The actual model was turned 90° relative to the axis of the scope to cover all virtual positions for each 45° and 90° opening positions. C Cross section of the 3D-printed pink model cut in half to display the position of the ureteroscope in relation to the spherical cavity and its opening positions for the spectrometer sensor. The black rubber cylinder was used to ensure solid tightening of the ureteroscope within the model during measurements. The tip of a OTU WiScope is visible and looking out 4 mm from the black rubber to ensure a standardized distance of 20 mm between tip of the scope and the concave surface of the calyx model. Blue dots indicate the actual and virtual light measurement positions. To cover all positions, the calyx model was turned in the axis of the scopes in successive 90° steps

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For each measurement, the color spectrometer sensor was placed at one of the nine opening positions. Five repeated measurements were taken at each position. For each measurement, the ureteroscope was withdrawn and reinserted in the kidney calyx model. Illuminance (lux) and color temperature (Kelvin) were measured at all nine opening positions.

When the scope set-up allowed light source brightness adjustment, measurements were done at light brightness settings of 50% and 100%. This was found for the Storz Flex-Xc 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. Based on our findings with the Storz Flex-Xc, brightness settings for the Storz Flex-X2s in the camera head was kept at 50% and brightness settings only adjusted on the light source unit. The brightness setting for all other scopes was adjusted on the light source unit only. As brightness can be set to automatic or manual mode on the Olympus light stack as well as on the Storz camera head, we performed all measurements using the manual mode to ensure consistency throughout all experiments.

Statistical analysis

Illuminance and color temperature for a given opening position were expressed as mean values of five repeated measurements. Overall illuminance and overall color temperature were defined as mean from all measurement opening positions for a given scope. Illuminance skew was defined as the direction of the highest peripheral illuminance (mean of 45° and 90° position) in a clock face fashion for a given scope. Any other direction with no statistically significant difference compared to that highest peripheral illuminance direction also accounted for illuminance skew. To quantify the extent of skewness for a given scope, the lowest peripheral illuminance direction was considered and reported as percentage relative to the highest peripheral illuminance direction. This reflects the relative intra-scope dominant direction(s) of peripheral illuminance and, therefore, skewness of the scopes, with comparison to the areas of possible relative darkness to quantify the extent of skewness.

Comparisons of continuous values over two groups were evaluated by Student’s t tests. Comparisons of continuous values over multiple groups were evaluated by one-way ANOVA with Tukey post hoc comparisons. All statistical tests were performed with GraphPad Prism 9.5.1 (GraphPad Software, La Jolla CA, USA). 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

Overall illuminance

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

Table 1 Comparison of overall illuminance of flexible ureteroscopes in a pink kidney calyx model
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The Flex-Xc was the scope with the highest overall illuminance at 100% brightness setting (5890 lx), followed by Flex-X2s (2596 lx), P7 (2512 lx), Pusen 9.2F (2093 lx), V3 (1422 lx), WiScope (563 lx) and finally the Pusen 7.5F with the lowest overall illuminance (272 lx). This same ranking was found at 50% brightness setting (Table 1).

Intra-scope comparison between the 50% and the 100% brightness setting revealed significant differences in overall illuminance for all scopes (all p < 0.05), except for the Flex-Xc (p = 0.95) and Pusen 9.2F (p = 0.10). The highest relative intra-scope illuminance increase between 50 and 100% brightness setting was for the V3 (+ 312%), and the lowest illuminance increase for the Flex-Xc (+ 1%).

When considering each opening position separately, maximum illuminance was found at the center opening for all scopes (ranging 516–12,058 lx at 50% brightness and 454–11,871 lx at 100% brightness settings), with a consistent drop in illuminance from the center to 45° and 90° opening positions (peripheral illuminance drop) (Table 2, Fig. 2).

Table 2 Direct and indirect light effects on illuminance in a pink kidney calyx model
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Fig. 2
figure 2

Comparison of center versus peripheral illuminance of flexible ureteroscopes. Data are mean illuminance at center, 45°and 90° opening positions with 95% CI measured in an enclosed pink kidney calyx model. The Y-axes scales are different for each graph

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Illuminance at center opening position

Illuminance at the center opening position significantly differed between flexible ureteroscopes at both 50% and 100% brightness settings (both ANOVA p < 0.001). Tukey’s post hoc analysis revealed that mean differences between each scope were significant compared to any other reference scope at 50% and 100% brightness settings, except for the Pusen 7.5F and OTU WiScope at 50% brightness settings (Table 2).

When considering only the center opening instead of overall illuminance, the order of illuminance changed with the Flex-X2s falling two marks in the ranking. The Flex-Xc was still the scope with highest illuminance at 100% (center illuminance 11,871 lx), followed by P7 (10,215 lx), Pusen 9.2F (8986 lx), Flex-X2s (7470 lx), V3 (5336 lx), WiScope (1115 lx) and finally the Pusen 7.5F with the lowest illuminance readings (454 lx). The same ranking was found for 50% brightness settings. The scope with highest illuminance (Flex-Xc) had 42 times and 26 times higher illuminance compared to the scope with lowest illuminance (Pusen 7.5Fr), at 50% and 100% brightness settings, respectively.

Peripheral illuminance

There was a significant peripheral illuminance drop for all scopes at both 50% and 100% brightness settings (all p < 0.01) (Table 2) (Fig. 2). When comparing illuminance at center vs 45° opening positions, the difference ranged from − 49 to − 88% at 50% brightness setting and − 48% to − 85% at 100% brightness setting (all p < 0.01). When comparing illuminance at center vs 90° opening positions, the difference ranged from − 43 to − 92% at 50% brightness setting and − 43% to − 88% at 100% brightness setting (all p < 0.01).

The scopes with the highest peripheral illuminance drop were the P7 (− 88% for center vs 45° and − 92% for center vs 90° at 50% brightness setting; − 83% and − 87% at 100% brightness setting) and the Pusen 9.2F (− 86% and − 89% at 50% brightness setting; − 85% and − 88% at 100% brightness setting). The scopes with the least peripheral illuminance drop were the Pusen 7.5F (− 49% and − 43% at 50% brightness setting; − 48% and − 43% at 100% brightness setting) and the WiScope (− 51% and − 50% at 50% brightness setting; − 56% and − 55% at 100% brightness setting).

Illuminance skew

There was a significant illuminance skew for all scopes (ANOVA p < 0.001), except for the V3 (Fig. 3) (Supplementary Table S1). The scope with the highest illuminance skewness was the P7 with dark areas of illuminance as low as 62% relative to the highest peripheral illuminance direction, followed by the Pusen 9.2 F (67%), Flex-Xc (71%), Pusen 7.5F (72%), Flex-X2s (80%), WiScope (85%).

Fig. 3
figure 3

Illuminance skew of flexible ureteroscopes in a pink kidney calyx model under saline. Skew of ureteroscopes are represented by radar charts, comparing mean peripheral illuminance (45° and 90° opening positions) at each 9, 12, 3 and 6 o’clock directions, expressed as percentage of illuminance from the maximum direction. Exact values are found in Supplementary Table S1

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Color temperature

Inter-scope comparisons revealed significantly different overall color temperatures at both 50% and 100% brightness settings (ANOVA p < 0.001) (Supplementary Fig. S1, Supplementary Table S2). The two scopes with the warmest (lowest Kelvin values) overall color temperatures were the V3 and P7 (range 2922 to 3160 Kelvin). The two scopes with the coolest (highest Kelvin values) overall color temperatures were the Pusen 7.5F and PUSEN 9.2F (range 4045 to 4592 Kelvin).

Intra-scope comparison between the 50% and the 100% brightness setting revealed no significant difference in overall color temperature for any scope (all scopes p > 0.05).

For each scope, the center opening always had cooler color temperatures compared to the periphery, with a peripheral drop to warmer (lower) Kelvin values. At 50% brightness setting, the difference of color temperature between the position “center” vs. “45°” ranged from − 24 to − 50%, and “center” vs. “90°” from − 27% to − 55%. At 100% brightness setting, “center” vs. “45°” ranged from − 23 to − 44%, and “center” vs. “90°” from − 31 to − 50% (all p < 0.01).

Discussion

Overall illuminance varied widely between flexible ureteroscopes. More importantly, all ureteroscopes showed significant peripheral illuminance drops as well as significant illuminance skew.

Maximum illuminance for all ureteroscopes in this study was at the center of the 3D kidney calyx model. Prior evaluation in a non-enclosed system had shown that the maximum illuminance for the same set of ureteroscopes was actually mostly off center [15]. This underlines the importance of the present evaluation in an enclosed spherical pink kidney calyx model, where the effects of direct and indirect light arguably resulted in maximum illuminance readings at the center opening for all scopes, akin to an integrating sphere. This finding also seems to be independent from the various number of light sources and relative positions thereof on the ureteroscope tip that had been described before [15]. The center opening illuminance is the most clinically relevant in endourology, as most surgeons would by default aim to keep the target of interest in the center of the endoscopic field of view. This finding has potential implications on ureteroscope tip design for future developments in the field of endoscopy.

It is striking that all scopes suffered from a significant peripheral illuminance drop as high as − 92% between the center and 45° or 90° opening positions. This varied between ureteroscopes. In endourology, the target of interest may inadvertently be off center despite the best efforts of the surgeon, usually due to limitations of deflection combined with environmental situations in the kidney. In these situations, it is desirable to have a scope with as little peripheral drop as possible. Peripheral illuminance drop may become particularly problematic for scopes with a wide angle lens (“fisheye lens”), since such scopes would suffer from particularly poor peripheral illuminance compared to the center of the field of view [8].

For non-center illuminance, urologists need to be aware of the illuminance skew and potentially darker areas for each scope. This has possible implications on visibility in different parts of the visual field, especially in situations of poorer visibility due to bleeding, larger hydronephrotic systems, or working at further distances. While illuminance skew was present for most scopes, the enclosed calyceal system seems to have mitigated some of the more pronounced illuminance skew that was found in a prior analysis in a black non-enclosed model [15].

For two out of the seven scopes, no significant change in overall illuminance was found between 50 and 100% brightness settings: the Flex-Xc and the Pusen 9.2F. For the Flex-Xc, this absence of difference is explained by the fact that regulating brightness settings from 50 to 100% only impacts on the post-processing of the image, and not on the light source per se, as described before [15]. For the Pusen 9.2F, the high peripheral illuminance drop resulted in a larger range and distribution of illuminance readings from the nine opening positions of the model, resulting in larger 95% confidence intervals for the overall illuminance calculations and, therefore, not reaching the significance level when comparing 50% and 100% brightness settings.

The Kelvin scale for color temperature ranges from warm (lower value) to cool (higher value), and all the ureteroscopes were found within the warmer part of the Kelvin scale, in comparison to the readings in a prior study in a non-enclosed system [15]. This is arguably explained by the difference in light reflection and absorption between the pink 3D kidney calyx model of the present study and the black model that was used in our prior study. Whether warmer or cooler color temperature is more favorable in various situations could be further explored in future studies. The peripheral drop in color temperature when compared to the center opening measurements underlines the validity of the current model which seems to adequately differentiate direct light (center opening position) from combined direct and indirect light (45°) and indirect light only (90°).

Some potential limitations need to be addressed. First, the present study is an in-vitro attempt to assess ureteroscope properties that may impact on in vivo settings and interpretation of the results must be done carefully. Compared to the 3D-printed model in this study, the mucosa of human kidney collecting systems may vary in exact shade of pink, with additional impact of underlying structures such as blood vessels, and possibly different absorption and reflection properties. Second, environmental factors arguably may impact on clinical translation of the findings of this study. For example, blood [21], urine and stone dust may substantially impact on illuminance characteristics during surgery, and should be further evaluated in future studies. Third, there may be some variations to the actual size of the calyceal cavity during ureteroscopy. Fourth, the reusable scopes used in the present study were not brand new, with possible degradation of the optical systems (illumination and imaging [22]) with prior usage. However, this reflects real-world clinical conditions where reusable scopes are barely ever used brand new. Finally, it is not clear how illuminance properties may impact on the final video projection of the surgical site from ureteroscopes on operative monitors.

Future studies should evaluate how environmental factors including blood, urine and stone dust may affect the illumination properties of flexible ureteroscopes, including effects of partial light obstruction in an enclosed cavity, and further explore how our findings may impact on overall efficiency and safety aspects of the surgery, as well as on image-based technologies such as artificial intelligence in ureteroscopy [9, 10].

Conclusions

Illumination properties vary widely between ureteroscopes within an enclosed cavity in saline, and for each scope illuminance differs as well at center vs 45° and 90° positions. Scopes can have a peripheral illuminance drop as high as − 92% compared to the center and the presence of illuminance skew, which are undesirable properties possibly impacting diagnostic purposes in ureteroscopy. Urologists should be aware of this as it may affect the choice of ureteroscopes, and corresponding choice of light brightness setting used in surgery.