OTS: Zign Medical’s unique method for medical light transmission testing

Explanation of the measuring method of the MedZense LG20(-e)

Zign Medical aims to increase general knowledge and create specific insights in the quality of your surgical instruments, specifically surgical instruments used in endoscopic surgery such as light cables, rigid and flexible endoscopes, and videoscopes. The MedZense LG20(-e) has been specifically designed to measure the light transmission of these surgical instruments, while aiming to provide more insight in the quality of your instruments compared to the state of the art in light transmission testers. The MedZense IQM platform is a software platform developed to document and analyze the measurement results produced by the MedZense LG20(-e) and other instrument quality test devices, and provides the opportunity to actively increase the quality of the surgical instruments your surgeons work with every day.

 

MedZense LG20(-e) Measurement Method

As the input, the MedZense LG20 sends a calibrated light pulse produced by an RGB LED through the light cable. On the other side at the output, light sensitive sensors measure the amount of light that is received through the light cable. The LG20 starts with sending a white light pulse through. The measured output results in the light transmission score (OTS). After this, the LG20 measures the light transmission through the visible light spectrum, sending red, green and blue light pulses to determine the difference in throughput between the colors. Precise calibration of the device is needed to correct for all differences in LED’s, sensors and connectors. The PT10 probe for testing rigid endoscopes includes multiple light sensitive sensors and uses a calculation to determine the light transmission of all rigid endoscopes regardless of their orientation and viewing angle when inserted in the PT10 probe.

Reference

The MedZense LG20/LG20-e uses a reference to calculate a score for the quality of the light cable regarding light transmission. This reference is based on new, good quality, 3.5mm light cables with a length of 2.5 meter. This reference is the industry standard and forms the basis of all light transmission measurements of light cables. Due to the physical nature of light, there is always a loss of light when transmitting light through any medium. The loss of light for the reference light cable described above is 55% (industry standard). In other words, the reference assumes a cable performs perfectly when the light transmission measured at the output is 45% of the input.

Example: When a light cable scores 100% compared to the reference, the actual measured light transmission is: 100% x 45% = 45% When a light cable scores 70% compared to the reference, the actual measured light transmission is: 70% x 45% = 31,5%

Light cables are all different

It is important to understand that no two light cables are identical, even if they have the same model number and produced by the same manufacturer. The light transmission of a cable is influenced by the exact amount of fibers in the fiber bundle, the quality of the fibers used and other factors during the production process. Therefore, brand new light cables could perform better, or worse compared to this reference described above. The reference loss of light of 55% was determined based on a 3.5mm diameter light cable, but this 55% loss of light also applies to light cables with a larger diameter (for example the 4.8mm diameter cables). However, compared to the 3.5mm diameter cables the 4.8mm diameter cables have a larger opening for light to enter the fiber bundles, and thus when a 3.5mm diameter cable and a 4.8mm diameter cable are connected to the same lightsource, the 4.8mm diameter cable will transmit more light in absolute terms. The larger diameter allows more light to enter the fiberoptic bundle, and although each individual fiber still has an average loss of light of 55%, a higher value for light can be measured at the output of the 4.8mm diameter cable compared to the 3.5mm cable. The length of a light cable influences the measurement results only marginally, and this is therefore not taken into account in the measurement calculation.

MedZense LG20 Light Transmission Measurements

The MedZense LG20 compares the measured output with the known input and calculates the actual light transmission as a percentage. The MedZense LG20 then divides the measured light transmission percentage by a factor 0.45, and displays this outcome as the Optical Throughput Score (OTS). Although the calculations are based on the industry standard, using the OTS calculation allows the measurement result to be above 100 OTS. This results in an accurate measurement result of the actual light transmission of a light cable.
The following table provides an overview how the measurement results of the MedZense LG20 compare to the industry standards for 3.5mm diameter cables:

Actual Light
Transmission (%)
Industry Standard
measurement compared
to the reference (%)
Optical Throughput
Score (OTS) with
MedZense LG20
10% 22% 22 OTS
20% 44% 44 OTS
30% 67% 67 OTS
40% 89% 89 OTS
45% 100% 100 OTS
50% 100% 111 OTS
60% 100% 133 OTS
70% 100% 156 OTS
80% 100% 178 OTS
90% 100% 200 OTS
100% 100% 222 OTS
     

A brand-new, high quality 4.8mm diameter light cable will score on average 150 OTS with the MedZense LG20(-e), with exceptional quality cables reaching 170-180 OTS. For 4.8mm diameter cables a measurement score of 150 OTS equals 45% actual light transmission. 

A 3.5mm diameter light cable scoring 100 OTS equals to the industry standard of a cable scoring 100% (45% actual light transmission), and our recommendation is that it should be discarded when the measurement value drops below 65 OTS. 
A 4.8mm diameter light cable scoring 150 OTS equals to the industry standard of a cable scoring 100% (45% actual light transmission), and our recommendation is that it should be discarded when the measurement value drops below 90 OTS.

 

MedZense LG20: Simple and Advanced mode

The MedZense LG20 can be used in two modes. Changing the mode is possible through the settings on the device.

Simple mode
When using the MedZense LG20 in the simple mode, the measurement results are shown with a maximum of 100 OTS. If the tested light cable actually performs better compared to the reference, the LG20 will still show 100 OTS as a result. The MedZense LG20 does not differentiate between 3.5mm and 4.8mm diameter cables, and by default the rejection level is set to 70 OTS. The rejection level can be manually altered in the settings menu on the device.

Advanced mode
When using the MedZense LG20 in the advanced mode, measurement results higher than 100 OTS are displayed with their actual score. The MedZense LG20 now differentiates between 3.5mm and 4.8mm diameter cables. The device calculates the OTS score and checks for both diameter cables if the score is above or below the rejection level set for that diameter. The rejection levels can be manually altered in the settings menu on the device.

 

Testing Rigid Endoscopes

With the MedZense LG20-e (LG20 + PT10 probe) it is possible to measure the light transmission of the fibers of a rigid endoscope. During the testing process of rigid endoscope, the MedZense LG20-e starts by performing a reference measurement of a light cable. During this measurement, the performance of the cable is measured and recorded. After this reference measurement the MedZense LG20-e will measure the total system of the tested light cable attached to the rigid endoscope. The performance of the cable is then used as a correction on the measurement with the rigid endoscope connected. Thus, the result displayed by the MedZense LG20-e is the light transmission of the rigid endoscope alone. The test results for rigid endoscopes are calculated with the same reference of 55% light loss.

Rigid Endoscopes: Diameter and amount of fibers
There is a wide variety of rigid endoscopes available and all of them are different. The light transmission measurement results are generally dependent on the diameter of the endoscope. A larger diameter endoscope has more light fibers and therefore can transmit more light from the calibrated lightsource in the LG20.

The table below contains our recommendations for testing rigid endoscopes. As these recommendations are currently based on a relatively small sample size, we highly encourage our customers to validate these rejection levels over time with the actual users of the rigid endoscopes (often OR staff/surgeons) based on their working experience with these rigid endoscopes. This table will be updated continuously based on our ongoing research into the light transmission quality of rigid endoscopes. 

Endoscope Diameter Average Fiber Core OTS Rejection Level Average OTS Score Measured
2.7 mm 1.2 mm 30 OTS 35 OTS
4 mm 1.8 mm 35 OTS 50 OTS
5 mm 2.6 mm 45 OTS 60 OTS
10 mm 5.2 mm 75 OTS 95 OTS

Please note: It is important to measure rigid endoscopes in combination with the correct diameter light cable! A 10mm diameter endoscope tested with a 3.5mm diameter cable will give inaccurate results, similar to testing a 2.7mm diameter endoscope with a 4.8mm diameter cable.

 

Light Color Spectrum

Next to the OTS score for white light transmission, the MedZense LG20(-e) calculates the transmission of light through the visible light spectrum by measuring the transmission of red, green and blue light. When the quality of the light fibers deteriorates over time, the light output through a cable or endoscope may become more red. Red light has longer wavelengths compared to green (medium) and blue (short) wavelengths. Damages to the fibers therefore may result in blue and green light not able to pass through but red light still able to pass through.

In the simple mode, a score (1 to 10) is displayed. In the advanced mode, a percentage per primary color is displayed. The percentages are relative to each other. The best performing color (red) is always displayed as 100%, and the other two have a percentage relative to the best performing color.

Lets look at the following example:

Red: 100%
Green: 91%
Blue: 82%

Red is here the best performing color. Green performs at 91% relative to red. Blue performs at 82% relative to red.

The point score is calculated as follows: The deviation between the best performing color (always 100%, often red) and the other two colors are added together and deducted from 100%. The result is divided by 10 and rounded to a whole number. The theoretical minimum score is -10. However, we have set a minimum of 1 out of 10.

In the example above, the point score would be:
Deviation from 100%:
Green: 9%
Blue: 18%
Total: 27%

Score: (100% – 27% = 73%) -> 7