Medical Global Standards – Importance of Quality “Stability” Control (QC)

In our previous blog posting we have explained the difference between Medical Grade monitors and COTS (consumer monitors). In such comparison, we could recognize significant differences and the importance of having a stable device.

Our current article follows up such understanding now: 

  • Why we need medical standards?
  • What exactly is quality control and what will be controlled on medical monitors?
  • What tools we have to use to proceed a quality control and how often is such test required? 

Let us answer such important questions step by step for a better common understanding.

Standards in general are important to describe the technical product limitation and unify the parameters (minimum requirement) and settings we should use to secure the stable accurate image quality. In Medical Image Viewing and Diagnostic DICOM Part 14 Standard was developed to display a greyscale image with all contents smoothly and linear on the monitor.

Such global DICOM standard also is used on medical modalities so whatever image will be captured on the modality will be shown on the medical monitors with same contents. This is independent if the workstation is installed at the hospital side or for Tele-Radiology where doctors work from home or look into images with other doctors located in other countries. By following Global Standards for Image Diagnostic Devices all doctors will be able to see the “same” image for consultation and diagnostic.

Standards are made by experts (different stake holders) including technical experts, vendors of products, government authorities, legal authorities etc., to make sure all aspects will be considered and to follow all technical, economic and scientific aspects and limitations with a neutral view.
The goal of a standard document is always to be seen as recommendation how to use the “state of the art” technology in healthcare environment safely and efficient in combination with existing equipment.

Medical regulations, certifications and classifications, e.g. CE Medical Device Directive defined in Europe or FDA approvals in the USA, embed available local and global standards into the risk management of the device. Device manufacturers must follow such standards strictly and globally.

In case of a medical monitor the following contents will be considered in standard documents:

  • the resolution of the monitor
  • the recommended luminance under consideration of the room ambient light
  • the contrast ratio
  • the pixel pitch size
  • the reflection coefficient
  • the viewing angle definition etc.

It also includes test patterns for visual basic testing and processes how the device itself can be controlled and measured to make sure such settings and parameters will be achieved. It shows the various sensors recommended for control and calibration and their accuracy.

As we have learnt every medical grade monitor deteriorates because of the limitation of the technology, or requires adjustment in case the ambient environment changed. Also such deterioration depends on how the operator uses the product and the level of set brightness. Therefore a “regular” or so called ”periodically testing” is a must. It is called “Constancy Testing”, recommended every quarter where the medical grade monitor will be checked on Luminance, Greyscale and Uniformity by following global medical standard parameters and if the result shows some differences and instability, then the monitor will be calibrated.

We could also define a different name for Quality Control which shows more the importance, we could name it “Stability Control”.

Standards

Another test method is called “Acceptance Testing” – in such test process, the device, in our case the medical grade monitor, will be one time tested in an ideal hospital reading room environment (between 0 and 100lx (Lux) / 0 – 50lx is the recommended one for Mammography) to check if the measured parameters of Luminance, Greyscale and Uniformity can be reached or not. Before doing this, the room ambient light needs to be captured and the medical monitor must be calibrated according to such room light. Some monitor manufacturers already pre-calibrate the medical monitor according to such ideal environment at the factory side, but to be safe it is recommended to do this locally again as the environment could be locally different.

In conclusion of the needed processes we could say Acceptance test is required before the first usage and first installation and only when the monitor will be removed to another environment. Constancy testing is required periodically, in most of the global standards quarterly. After a Constancy test is executed the tester is able to get a test report for the device to compare the performance and acceptance.
Without going into all technical requirements here are some important requirements for medical monitors we have to consider: (there are many others more mentioned in the standard documents):

  • Resolution of the Medical Monitor: depends on the image size, here the target should be that the image resolution should fit equal with the monitor size and resolution
  • Luminance (low / minimum): for General Diagnostic use the minimum luminance is 350cd/m2 and for Mammography the recommended is from 420cd/m2, the low luminance values are also important to start from 1cd/m2 (such value is important to display black and grey levels accurate, if the value is too low a grey will be darken and if it is too high the black will turn to grey which has big impact for doctors to judge medical issues or not on the image)

003360Juergen Heckel

Important and worldwide accepted are the standard documents from AAPM-TG18 and ACR and IEC Standards (e.g. IEC 62563-1) and the German DIN 6868-157.

Last but also very important is to mention that for calibration and QC control and measurements special sensors are needed. Medical monitors nowadays have uniformity control tools embedded and integrated and external sensors which can be used to proceed the testing. For ambient light measurement, tools are available from various manufacturers.

Interesting links to see examples of Global standards:

https://siim.org/page/displays_chapter3
http://dicom.nema.org/dicom/2004/04_14pu.pdf
http://www.din.de/en

In case you have some inputs how you would like to improve the guidlines of global standards please share with us. Your feedback will be discussed and communicated.