Table of contents:
- Why perform a DICOM compliance test with an external sensor?
- Test patterns in QAWeb Agent versus PACS
- Distance measurement versus on-screen measurement
- How to perform a DICOM compliance test with an external sensor and QAWeb Agent
All currently selling Barco diagnostic displays have a built-in sensor which is capable of doing an intervention-free DICOM compliance test. This feature is very convenient and saves you a lot of time. However, in some cases, there is a need for a test with an external sensor. Here are some examples:
- You have a regulation in your country or region which requires this (e.g. NY State)
- You question the accuracy of the built-in sensor
- Your own QA system requires each sensor to be verified periodically
- A visual test on your display failed, even though the DICOM compliance test with the internal sensor passed.
What do you need for correctly performing a DICOM compliance test with an external sensor?
- A calibrated external sensor which has a luminance range which exceeds the range of the display (e.g. when doing the test on a Coronis Uniti MDMC-12133, you need a sensor which is capable of accurately measuring 1000 Cd/m2)
- TG18-LN12 test patterns, preferably using QAWeb Agent to display the test patterns (see later)
- Stable ambient light in the room which is in line with the calibrated ambient light settings.
- A tool to calculate/evaluate the measurements
- When you use a QAWeb supported sensor, QAWeb can do the calculations automatically for you. The benefit is that your test results are automatically available and are stored in QAWeb for future reference.
- When using an unsupported sensor, then you will need an external calculation sheet. For your convenience, we have added such a sheet to this article.
Several external sensors are supported in QAWeb Agent. You can find the full list in the QAWeb Agent user guide which is linked to this KB article (scroll to the bottom of the page). Make sure to attach the sensor to the workstation and wait for the driver to load correctly before opening QAWeb Agent.
We regularly come across failed DICOM compliance tests where the test patterns were measured in PACS. When we redo the test with QAWeb Agent rendered test patterns, then the test passes. How is this possible?
QAWeb Agent renders the test patterns on the fly, specifically for the resolution of the display(s) attached and most importantly: QAWeb Agent will not apply any correction to the test pattern. In several cases we have come across over the past couple of years, we found that when test patterns are displayed in PACS, an additional correction (which is added on top of the DICOM calibration in the display) is applied by the PACS viewer to the test pattern. Because of this additional correction/calibration of the test pattern, the test can fail. Note that in this case, there is a big chance that this correction is also applied to the medical images which are viewed in the PACS viewer. This means that also the images may not be DICOM compliant. Should this be the case, please contact a customer service representative of the PACS company.
There are two documented ways of measuring display luminance:
- On-screen: The sensor is placed on the display surface. No ambient light influence will be measured. This is the most reliable method for measuring.
- Distance: The sensor is held at a distance (typically 50cm). Any ambient light which reflects on the screen will be measured by the sensor as well. Here, the measurement can be heavily influenced by changing ambient light or for instance moving the sensor during the measurement.
Please check your sensor's documentation to know which measurement methods are supported by this sensor. Not every sensor supports on-screen or distance measurements. Several types of sensors have specific adaptors or accessories for transforming the sensor into a distance or on-screen sensor.
The following steps explain how to do a compliance test using a supported and connected sensor (scroll up to find info about supported sensors):
- Connect the sensor to the workstation. Make sure it is properly detected by the workstation and that the drivers have been installed correctly.
- Open QAWeb Agent
- Click the 'Configuration' button.
- Select the display for which you want to do the compliance test (the icons follow the desktop configuration)
- Click the 'Compliance test' tab
- You will get a popup window which asks you if you want to measure with an external sensor. Click [Yes] (clicking [No] will perform the test with the built-in sensor if the display has a supported front sensor or result in an error if the display does not have a supported front sensor)
- Now you will see a large white square in the center of the display you want to measure. Put your sensor in the middle of that square and press the [Yes] button to start the test. Make sure your sensor does not move during the test.
- Once the test is done, you will see a window with the results.
- Setup your sensor for measuring (zero-calibration, activate the sensor's display or start up the measuring software)
- Open QAWeb Agent
- Click 'Test patterns'
- Starting on page two of the test patterns window, you can find the AAPM TG18-LN test patterns.
- Click the AAPM TG18-LN01 test pattern. It will be displayed full screen on the QAWeb controlled displays. Put your sensor in the middle of the rectangle (or aim at the middle if you are doing a distance measurement) and do your measurement. Wait until the measured value is stable on the display of your sensor. Take note of that measurement.
- Now repeat for the LN02 up to LN18 test patterns.
Now that you have your measurements, you need to think about the ambient light compensation as well. Should you not be familiar with ambient light compensation, we recommend you to read the following article which explains all about ambient light: [KB7842]
To properly calculate the deviation to the ideal DICOM curve, you need to make sure that, if there is any ambient light compensation taken into account during calibration, to also take that ambient light compensation into account when calculating your DICOM compliance. The method will depend on your measurement method:
This is the easiest in terms of calculations. Because of the distance measurement, you already measured the reflected ambient light. In this case, you can use the raw measurements for your calculations.
In this case, the position of your sensor shields the measurements from any reflections on the screen. Therefore you will need to add the reflected ambient light to your measurements in order to have a correct DICOM evaluation.
Here you can find out where you can find the ambient light compensation QAWeb calculates during a calibration: [KB7941]
You measure the first point of the DICOM curve with the TG18-LN01 pattern. The measurement is 0.57 cd/m2. You used the instructions in [KB7941] to find the ambient light compensation and it is 0.07 cd/m2. The actual value you are going to use in your calculation sheet is 0.57+0.07 = 0.64 cd/m2.
To make it easier for you, we have added a calculation sheet which has the option to fill out the raw measurements and has an additional field for adding the ambient light value. You can find the sheet below this article: GSDF_Compliance_Check_Barco_Healthcare_v3.xls
Last updated Oct 31 2018