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Measuring image quality of ENT endoscopes

On July 25, Nicolas Cediey and Noor Everts visited a clinical physicist, Geert Geleijnse, at Erasmus MC. Geert spends most of his time in the ENT department. Here he does research on the image quality of endoscopes. In addition to his current research, Geert also examined the image quality of an endoscope disinfected with the D60. We will discuss the research and the results of this test here.

Research on the image quality of ENT endoscopes

Why is it important to measure image quality of endoscopes?

An endoscopic examination is stressful for patients. For the ENT, it is not so bad when you compare it to an endoscopic examination at the MDL or Urology. In ENT, an endoscope is brought up through the nose to finally look into the throat. This type of examination only takes place if there is a reason for it with the goal of forming a better picture of what is going on. Imaging is the primary purpose of an endoscope. But how do you know if the image quality of this camera is good?

Now, if a hospital wants to purchase new endoscopes, information about image quality is missing from the manufacturers' brochures. Of course, the brochures do show beautiful examples, but these images cannot be used for comparison. There is no choice but to visually assess the image quality yourself by, for example, checking what the image looks like on a hand or in the throat of a test subject. This test is subjective and it is difficult to perform it again identically with another endoscope for a fair comparison.

Images can be compared if it can be measured according to a standard. With a standard it is possible to compare different types of endoscopes in a procurement process, or to follow the image quality of an endoscope over time. So far, no standard has been developed for this. For the photographic camera industry, standards have been developed in recent decades that we can use to our advantage. For consumer purposes, for example, it is desirable that the images appear slightly sharper and the colors slightly more vivid, while a camera for photographing paintings should be as true to life as possible. A doctor makes an image recording with the aim of making a diagnosis and that is something else: the image does not have to be 'prettier' or 'true to life'. For making a diagnosis it is important that he or she can properly distinguish between healthy and unhealthy. The methods of the photo camera industry cannot simply be adopted one to one, but they can be used as a basis.

In Erasmus Medical Centrum, many endoscopes are used in the department. Therefore, with his research, Geert wants to create a standard for objective measurements that provides more insight into which aspects of image quality are important for diagnostics and useful for comparing products during a procurement process or for quality controls during the user phase. These points include:

  • The smallest detail that can be seen on screen
  • How much noise is present in the image
  • Whether the colors are rendered true to life

How can you measure image quality?

For the study, a test setup was created where a test card is used: the Rez Checker Target. This is a small square with several boxes(see Figure 1). On it, the image quality can be assessed for sharpness, noise and color accuracy, among other things. The endoscopes are positioned in a test setup where the test chart is placed in a dark room and the tip of the endoscope is held at a fixed position. The distance from tip to test chart is thus always the same. In this way, pictures are taken and then analyzed by a custom written script (developed by Geert). More info can be found in this article.

Figure 1:
Rez Checker Target

How was the influence of UV-C light on image quality measured?

Geert also deployed this test setup on a Pentax VNL9-CP endoscope that passed a compatibility test with the UV Smart D60. Pentax Netherlands then checked the exterior of the product and its operation and the endoscope passed the compatibility test well: except for some discoloration, the material and mechanics of the endoscope were not affected. Geert was curious to see if there was also no effect on image quality to be measured. So Geert received the same endoscope from Pentax and measured it 10 times. These measured values were compared to the measured values of 45 other endoscopes of the same type that are used clinically at the ENT outpatient clinic of Erasmus MC(see Figure 2).

Figure 2: Sharpness and noise

This included looking at sharpness: what is the smallest detail you can see. This is unchanged, and falls within the range with a "healthy" endoscope(see Figure 2). Also the noise and color accuracy show no difference. So that's good news. In retrospect, it also makes sense, because to possibly damage the chip, UV light has to get to the chip. The UV-C light does not get beyond the surface of the endoscope. After all, UV light doesn't go through glass.

Yet, the primary function of an endoscope is imaging to make diagnoses, and even after disinfecting so many times with the UV Smart, no negative impact on image quality is measurable.

One thing to note is that Geert checked one endoscope. It would be better to repeat this study on a number of endoscopes that are actually in use.

Read more about the compatibility test and of the effect of UV-C light on endoscopes here.

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