Medical Display Calibration and QA

8/2/2019 Medical Display Calibration and QA

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Calibrating the most demanding professional displays

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Why calibratemedical displays?

Hospitals and clinics have quality programs for medical equipment.However, often the displays used for diagnostics and reviewing are notpart of those quality assurance programs.

No monitor, including medical ones, displays images perfectly. Poorimage quality results in fatigue of the doctors, more time spend onreadings and in some cases still wrong diagnostics. Life critical decisions,based on computer display readings, imply a need for Calibration andQuality Assurance.
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DICOM calibration

of displays in medicalenvironment

- What is light and how does it influence image?

- What is DICOM GSDF Part14?- Why calibrate displays to DICOM?- How does calibration work?- What displays can be calibrated?- What are the required tools?
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What is light?

Visible light is the electromagnetic radiation that can be seen bythe human eye.Light is visible in the wavelengths from 380 to740 nm. When light is broken by a prism, different colors of thewhite light become visible.
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How do we see light?

Rod and con cells in the retina of the human eye are sensible tolight and can distinguish up to 10 million different color andluminance shades. These cells form the images.

Non image building cells are the photosensitive ganglion cells,

that are responsible for adjusting the size of the pupil.
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What is DICOM Part14?

The DICOM Standard is developed and published by

the US organisation NEMA (National ElectricalManufacturers Association).

The Part 14 of this standard discusses how pixel

values should be interpreted and displayed.

Part 14 also specifies a function that relates pixelvalues to displayed luminance values. This function iscalled GSDF (Gray Scale Display Function).
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GSDF

Originally developed by the researcher Peter G.J. Barten and published as "ContrastSensitivity of the Human Eye and Its Effects on Image Quality". He created a function that

describes the sensitivity of the human eye to luminance levels.

The human eye can distinguish between two dark colors even if the difference is very low, like1 or 2 cd/m2, but is unable to distinguish that difference at high brightness levels.

The function precisely describes the various Just Noticeable Difference levels and theLuminance level in relation to it. This function is included in the DICOM standard and called GrayScale Display Function.
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Why calibrateto DICOM GSDF?

Adapt display to human eye. Human eye is not an digital device like acomputer is. The computer display, as the interface between the human andthe digital machine, must be adapted to the characteristics of the human eye.

Re-calibrate over time. Computer displays tend to change over time, likeany other light source. Backlight lamps and RGB filters change. That is whyeven a factory calibrated display needs to be re-calibrated over time tocontinue matching the sensibility of the human eye.

Make sure displays look the same. Consistency of images on variousdisplays is important for diagnostics. Calibration iensures that an image looksthe same in all services and on all workstation.

Standardise display outputs. Calibrated displays meet the key qualityassurance standards in medical industry.

Safety and quality. Increase diagnostic safety and improve quality

assurance.
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Why calibrate

color of a display? human eye perceives

various color shades in adifferent way

a diagnostic image on acolor display is influencedby color hues: reddishgray shade will lookdifferent from blueishgray, so adjustingluminance is not enough

color calibration ensuresthe same colortemperature on all drivinglevels (different graylevels) of the display
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Why calibrate a display

regularly?

display white level degrades

display back light changes overtime

display color filters change overtime

regular verification and adjustmentof the white level of the displayis necessary
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How does calibration work?

1. Measurements taken.Reference colors and gray levels aredisplayed on the monitor and measured with a colorimeter or luminancemeter.

2. LUT generated. From the actual measured values and the knownBarten's formula target values a correction table called Look Up Table(LUT) is generated.

3. LUT applied. This LUT is loaded and applied in the graphic board or

inside the display.
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Almost any LCD, LED, or OLED display can be calibrated to DICOM GSDF tocompete with traditional medical displays.

What displays can be calibratedto DICOM GSDF?
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Standardised Quality Assurance

In some countries the legal bodies enforce acceptance testing of diagnosticand reviewing displays when they are installed, and constancy testingfor quality assurance on a regular basis.

Major QA standards are AAPM TG18, DIN 6868-57, DIN 6868-157,JESRA X-0093 , IEC 62563-1

The tests performed within those standards consist of visual tests andmeasurement tests, which verify conformance to the requirements ofthe standards.
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Controlled Environment

Apart from calibration and constant verification of the display it is importantto control the working environment:

ambient light must be stable and remain within the acceptable range

no direct reflection on the display panel should occur

walls of the reading rooms should be painted neutral gray
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PerfectLum calibrates displays and projectors to DICOM GSDF on Windows and

Mac operating systems and uses DDC/CIcommunication to adjust the display.

calibrates any displayto DICOM GSDF.
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performs

acceptance and constancy tests

to key medical standards
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Download

A free 15-day trial of PerfectLum display calibrationand quality assurance software is available atwww.perfectlum.com/downloads
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Medical Display Calibration and QA

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