Diagnostic Imaging Workstation Display Characteristics

The American College of Radiology began to publish standards for the electronic display of diagnostic images in 1994 and has updated these standards regularly. [1]

LCD displays have largely replaced CRTs in diagnostic imaging because they offer higher luminance which is more stable over time, more stable image geometry, lower power consumption and heat generation, automatic calibration, a smaller footprint, and longer service life. [2]

Certain characteristics of displays are considered to be important determinants of perceived image quality. [1, 3] These include:

• Luminance: ACR guidelines suggest a maximum luminance of at least 171 candelas per meter squared (cd/m2). Most modern LCD's produce at least 300 cd/m2, with some medical grade monitors producing 600 cd/m2 or more. The difference between minimum and maximum luminance should be at least 171 cd/m2.

• Matrix size: determines spatial resolution. ACR guidelines recommend 2.5 lp/mm at a minimum, which requires 5 pixels per mm. A 14" x 17" (356 mm x 432 mm) adult chest xray digitized at 5 pixels/mm would therefore require a display matrix of 1780 x 2160 = 3.8 megapixel (MP). Current displays range up to 6MP.

• Bit depth: 8 bits (256 shades of gray) per pixel is recommended at a minimum. Ten or 12 bit monochrome displays are desirable, but expensive. Color displays are typically 8 bits per color subpixel (3 x 8 = 24 bits per pixel). The apparent bit depth is increased by choosing 8 bits from a 10 or 12 bit look up table (LUT).

• Response time: time required for an individual pixel to complete a change from black to white to black again. Slow response times lead to smearing and ghosting of moving images. 5 ms is an excellent response time. [4]

• Contrast ratio: the luminance difference between the blackest black and the whitest white discernible on the screen simultaneously. Higher contrast ratios lead to greater ability to distinguish shades of gray on monochrome monitors and to an increase in the number of shades of colors visible on color monitors.

• Noise. Pixel to pixel luminance variation when displaying a uniform gray target. Noise limits fine contrast and spatial resolution.

• Veiling glare. Stray light from within the display which reaches the observer's eye, reducing image contrast.

References

1. American College of Radiology. "ACR Technical Standard for Electronic Practice of Medical Imaging". http://www.acr.org/SecondaryMainMenuCategories/quality_safety/guidelines/med_phys/electronic_practice.aspx. Accessed 5/23/2009.
2. Krupinski, EA, and M Kallergi. “Choosing a Radiology Workstation: Technical and Clinical Considerations.” Radiology 242.3 (2007): 671.
3. American Association of Physicists in Medicine. "Assessment of Display Performance for Medical Imaging Systems". http://www.aapm.org/pubs/reports/OR_03.pdf. Accessed 5/23/2009.
4. Wikipedia. "Response time (technology)". http://en.wikipedia.org/wiki/Response_time_(technology).

Submitted by Philip Stalker