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Optimum shadow-casting illumination for endoscopic task performance

RK Mishra, GB Hanna, SI Brown, A Cuschieri

Department of Surgery and Molecular Oncology

Ninewells Hospital & Medical School

Dundee, Scotland, United Kingdom

Original Research of Dr. R.K. Mishra in United Kingdom

Click here for brief overview of this research of laparoscopic shadow to enhance task performance (47 MB) | On YouTube

Key words:  Shadow, laparoscopy, depth cues, ergonomics, imaging

Click here to see this research published in Archives of Surgery (Arch Surg.2004; 139: 889-892).

Abstract

We hypothesize that task performance improves with the use of (i) balanced degree of shadow and illumination compared to no or maximum shadow contrast and (ii) shadow-casting illumination from above compared to the side of the operative field. The standard task entailed touching target points on an undulating surface using a surgical hook Each run consisted of 13 target points in a random sequence. The end points for each run were the execution time and number of errors.  Five settings were investigated: no shadow, 22%, 42%, 65% shadow contrast created by above illumination and 22% shadow contrast produced with side illumination. Ten surgeons participated in the study and each surgeon performed three runs with each setting in a random order. Shadow contrast settings had a lower number of errorenty-two percent shadow contrast had a lower number of errors with above illumination compared to the side (p<0.0001). With the above illumination, 22% and 42% shadow contrast had a lower number of errors compared to maximum shadow contrast of 65% (p<0.005). Optimum endoscopic task performance is obtained with above shadow-casting illumination and a balanced degree of illumination and shadow contrast. 

Introduction

Restrictions in video endoscopic imaging accounts for degraded task performance compared to human eye [6].  Standard video endoscopic imaging systems present two-dimensional depth (pictorial) cues of the operative field and thus, the surgeon has to reconstruct a three-dimensional picture from a two-dimensional image.  Several three-dimensional video-endoscopic systems have been introduced into the market to improve depth perception of the operative field but without significant improvement in endoscopic task performance [3, 6, 9, 10]. The current video-endoscopic systems have a single disparity, which yields different magnitudes of depth depending on the viewing distance.  By contrast, the human visual system perceptually rescales disparity information for different distances to produce valid depth, a process called stereoscopic depth constancy [4].  The depth perceived by the current three-dimensional systems is valid if it corresponds to the predicted depth by geometry [5].  As a result, there is a limited operational distance in which three-dimensional effect is obtained, but outside this range, the surgeon operates from non-valid depth information. 

An alternative approach for improving depth perception is to enhance monocular depth cues available in the current two-dimensional endoscopic image. The coaxial alignment of the lens system and optical light fibres in rigid endoscopes does not permit shadow production in the operative field [11]. Our earlier research has shown that endoscopic task performance significantly improves when a shadow producing arrangement (separate ports for imaging and illumination) is used [8]. The employment of various illumination sources produces different degrees of shadow. The aim of this study was to optimise shadow production in terms of the degree of shadow contrast and the location of shadow-casting illumination. We hypothesize that endoscopic task performance improves with the use of (i) balanced degree of shadow and illumination compared to no or maximum shadow contrast and (ii) shadow-casting illumination from above compared to the side of the operative field. 

Materials and Methods

Ergonomics of the set up

Experiments were conducted in a wooden trainer box (480 X 360 X 250mm) using two-dimensional video-endoscopic system (Karl Storz, Tuttingen, Germany). Two illumination sources were used; the primary endoscope provided both illumination and imaging while a second endoscope provided shadow-casting illumination. Figure 1 shows the experimental apparatus. The primary endoscope (10mm of forward-viewing direction) was introduced with an optical-axis-to-target angle of 30° and at a distance of 100 mm from a fixed point on the trainer base (set up reference point). The primary endoscope was attached to a telecam camera to display the endoscopic field on a Sony monitor (model PVM-1443MD Sony, Tokyo, Japan). The shadow-casting illumination was provided by 10 mm forward-viewing endoscope at a distance of 100mm from the set up reference point. The shadow-casting illumination was positioned with light direction either perpendicular or at an angle of 30° to the horizontal plane at the reference point to provide the above or sidelight. Two 450W halogen light sources were connected to the endoscopes using fibre optic light cables of 10mm diameter. The light sources were adjusted independently to change the level of illumination. An endoscopic instrument was introduced with an elevation angle of 60° (between the instrument axis and horizontal plane) and the tip of the instrument at the reference point. The angle between the instrument and the centre of shadow-casting illumination was 30° whereas the angle between the instrument and the optical axis of primary endoscope was 30 degree. The endoscope viewed the instrument from below. 

Measurement of shadow contrast

Illumination was measured at the reference point using a DX200 calibrated light metre (Ins Enterprise, Taipei, Twaiwan). The shadow contrast was measured by taking a snapshot of the operative field shown in figure 2, using a high-resolution video capture card (Trust Corporation, Dordrecht, Netherlands) attached to the endoscopic camera. The captured images were subsequently converted to a grayscale using Corel photo-paint (Corel Corporation, Ontario, Canada) to render the image in 256 separate shades of gray with "0" represents the blackest and "255" the whitest point. A grayscale plot along "A-B" line (figure 2) was made using Scion Image (Scion Corporation, Meryland, USA). Shadow contrast was calculated using the following formula:

Shadow contrast = highest grayscale value on the AB line – lowest value on the shaded area X100 / 256

Settings of illumination and shadow production

There was no shadow with illumination from the primary endoscope as the only source for illuminating the operative field.  Shadow was produced by above and side illumination with the maximum shadow contrast obtained on using the primary endoscope for imaging and another light source for illumination (table 1). Figure 2 shows the shadow produced by above and side illumination. With the above light, the shadow of instrument came from the same side of the monitor frame as the surgeon (real side of the instrument) whereas the instrument's shadow appeared from the side of the monitor frame with side illumination (apparent instrument's side on the monitor). 

Experiment to test the influence of shadow contrast on task performance

The standard task entailed touching target points on an undulating surface using a surgical hook. Those target points were electrodes connected to an LED on a control panel.  Illumination of the LED indicated successful task execution at the corresponding target point. Each run consisted of 13 target points in a random sequence.  A red light in the control panel indicated committed errors. An error was registered on touching wrong electrodes or the surface of the target object.  The end points for each run were the execution time and number of errors. Ten surgeons participated in the study.  Each surgeon performed three runs with each of the five shadow-illumination settings in a random order. All surgeons had 20/20 corrected eyesight and each used the dominant hand to perform the task. Experiments were carried out in the research laboratory with the same background illumination. 

Statistical analysis

The data was not normally distributed and therefore Kruskal-Wallis one-way analysis of variance (ANOVA) and Mann Whitney U-test was used for analysis. Significance level was set at the 5%. 

Results

Table 2 shows the median and interquartile range of the execution time and committed errors. No significant difference in the execution time was found between different settings of shadow contrast. The "no shadow" setting had a higher number of committed errors compared to shadow contrast settings of 22% (p<0.0001), 42% (p<0.0001), and 65% (p=0.009) for above illumination and the shadow contrast setting of 22% (p=0.013) for side illumination. With the above shadow-casting illumination, 22% and 42% shadow contrast had a lower number of errors compared to the maximum shadow contrast (65%) with P<0.0001 and <0.005 respectively. Twenty-two percent shadow contrast had a lower number of committed errors when shadow-casting illumination was located above compared to the side (p<0.0001). 

Legend to figures

Figure 1: Schematic representation of the ergonomics of the set up

Figure 2: Illumination of the operative field to produce 22% shadow contrast from above (fig 2 a) or the side (fig 2 b)

Table 1: Different settings of illumination and shadow production

*Kruskal-Wallis ANOVA

Table 2: Median and interquartile range of the execution time and committed errors on different settings using above or side shadow casting illumination

Figure 2a

Figure 2b

Discussion

The study confirmed our earlier research that shadow depth cues enhance endoscopic task performance [8]. A balanced degree of shadow and illumination is required for optimum task performance as maximum shadow contrast (65%) was accompanied by a higher error rate compared to lower contrast levels (22% and 42%). Low levels of illumination at shaded area may account for poor performance with maximum shadow contrast. The best performance was obtained with a shadow-casting illumination from above compared to the side. Several reports in visual psychology confirmed that the visual system prefers above illumination [1, 2, 12, 15]. The human perception is used to above lightening by the sun and other artificial illumination systems. This develops very early as seven-month old infants have been found to select convex shapes on the basis of shading information [7]. 

In addition to depth enhancement, shadow may have a role in the orientation of endoscopic instruments in the operative field. In the set-up of our study, the instrument was introduced into the trainer box from the same side of the surgeon but the instrument appeared to enter the operative field towards the surgeon from the upper part of monitor frame. The difference between real instrument location and apparent display on the monitor is due to viewing the instrument by the endoscope from below [14]. Nevertheless, with above shadow-casting illumination, the instrument's shadow appeared on the monitor in the same direction as the real location of the instrument. With side illumination, the shadow appeared on the monitor from the same side as the shadow-casting illumination, which was different from real instrument direction. 

Currently, there are two endoscopic systems that generate shadow in the operative field: the illuminating port and shadow-producing endoscope (MGB endoscope Co Ltd, Seoul, Korea) [13, 16]. The shadow produced by both systems is not optimum as the shadow-casting illumination comes from the side by the illuminating cannula and from below by the shadow-producing endoscope. There is a need to develop a system that illuminates the operative field from above in order to create optimum shadow for endoscopic task performance.

COMCLUSION:

Shadow is one of the important depth cue in minimal acceess surgery to improve the task performance of operating surgeon. An optimal shadow casting image with 20 to 30% contrast substantially increases the task performance in Minimal access surgery at it should be used to improve the efficiency of the operating surgeon.  Fibre-optic light bundles can be deployed inside a super-elastic diverging shape memory alloy tubes to provide ceiling shadow-casting illumination. Such system also provides a balance between illumination and shadow contrast. Further research is required to develop the ideal shadow-producing video-endoscopic system. 

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