In softcopy diagnostic systems, displays are used to judge medical images and to take conclusions about patients physical condition.
It is obvious that the image quality is of the utmost importance. Therefore it is necessary to check the quality of the system, and especially the displays image quality, on a regular basis.
QA tests can be performed to check the image quality of all the displays on a medical viewing station. It consists of a number of QA tasks.
Some QA tasks have target values and tolerances that can be set to generate a warning or an error. For other tasks, the user must judge if the result is OK or not. When an error occurs, measures must be taken immediately to restore the image quality.These QA tasks can be scheduled to start automatically on the specified date & time.
When the software is installed, certain pre settings are defined for each display in the viewing station. This settings are the reference for the displays visual behavior. When the display image quality is good, the display is conform to the defined settings.
In many cases, calibration is the remedy for a display that no longer conforms to its pre settings. The calibration routine restores the black luminance, the white luminance and the display function.
A Display Function describes how a display device converts the voltages at the inputs into light. The display function is a graph that shows how the light from the picture tube evolves from minimum to maximum luminance while the data levels at the input of the imaging board go from 0 to maximum.
A digital value which given as input to a Display System produces a Luminance. The set of DDLs of a Display System is all the possible discrete values that can produce Luminance values on that Display System. The mapping of DDLs to Luminance values for a Display System produces the Characteristic Curve of that Display System. The actual output for a given DDL is specific to the Display System and is not corrected for the Grayscale Standard Display Function.
A device or devices that accept DDLs to produce corresponding Luminance values. This includes emissive displays, transmissive hardcopy viewed on light boxes, and reflective hardcopy.
Light from the environment surrounding the Display System which illuminates the display medium. It contributes to the Luminance that is received by an observer from the image display. Ambient Light reduces the contrast in the image.
The Luminance difference of a given target under given viewing conditions that the average human observer can just perceive.
The input value to the Grayscale Standard Display Function, such that one step in JND Index results in a Luminance difference that is a Just-Noticeable Difference.
Luminance is the luminous intensity per unit area projected in a given direction. The Systme Internationale unit (used in PS 3.14) is candela per square meter (cd/m2), which is sometimes called nit. Another unit often used is footlambert (fL). 1 fL = 3.426 cd/m2.
Luminance Range is the span of Luminance values of a Display System from a minimum Luminance to a maximum Luminance.
The Gamma function is a kind of display function, characterizing an apparatus that converts voltages into light or ink or vice versa. Displays, printers, scanners, ... all have their own, unique Gamma function. In a display, this function is exponential. The Gamma value is the exponent that determines the shape of the function.
DICOM stands for Digital Imaging and Communications in Medicine. It is a standard developed by the American College of Radiology (ACR) and the National Electrical Manufacturers Association (NEMA). The standard specifies how digital image data can be moved from system to system. In addition, Supplement 28 Part 14 specifies a function that relates pixel values to displayed Luminance levels and is called Grayscale Display Function Standard.
For further information about DICOM part 14 standards Please refer the following website "http://medical.nema.org/dicom"