PACS Monitor

Nel Leung

• Grayscale Standard Display Function (GSDF)

• Look-up Table (LUT)

• LCD vs CRT

• Requirement of a PACS Monitor

Digital Imaging and Communications in Medicine

(DICOM)

Part 14: Grayscale Standard Display Function (GSDF)

GSDF

• provide a standard for equipment manufacturers such that images look similar on different display devices

• enhance image interpretation

Multiple Factors Affecting Display Quality

• luminance response • reflection • glare • spatial resolution • noise • geometrical distortions • temporal response (e.g. temporal noise-

and flicker)

DICOM standard currently covers only the luminance response

aspect of display quality through the DICOM GSDF

Grayscale Standard Display Function

• based on human Contrast Sensitivity

• non-linear : human eye is relatively less sensitive in the dark areas of an image than it is in the bright areas of an image

• easier to see small relative changes in luminance in the bright areas of the image than in the dark areas of the image

• GSDF adjusts the pixel brightness resulting in the same level of perceptibility at all luminance levels → “perceptually linearized”

• with a non-linearized display, the observers took longer to reach a decision and ended up being mistaken more often

Just-Noticeable Difference (JND)

• the luminance difference of a given target under given viewing conditions that the average human observer can just perceive

• JND = kL0LB where k is constant, L0 is the object luminance, and LB is the background luminance

L1 is perceived as the same contrast difference as L2

Look-up Table (LUT)

8 to 8 bit LUT

This gives not only a high color precision, but also a high color variety to exactly reproduce the image.

More of the initial image information is maintained.

LCD CRTMuch Lighter and requires much less Space

Much Heavy and Bulky

Perfect Geometry Geometric Corrections needed

Perfect Modulation Transfer Function (MTF = 1)

Imperfect Modulation Transfer Function (MTF < 1)

Uniform Sharpness Less Uniform Sharpness

1000:1 Contrast Ratio 3000:1 Contrast Ratio

Contrast Ratio dependent on Viewing Angle

Contrast Ratio independent of Viewing Angle

Low Reflection of Ambient Light High Reflection of Ambient Light

Lower Response Speed Instantaneous Response Speed

Backlight Aging Phosphor Aging

LCD CRTLow Power Consumption High Power Consumption

No Image Flicker Image Flicker Present

Narrow Viewing Angle Wider Viewing Angle

Usually produce only crisp images in their "native resolution"

Can switch between Multiple Resolution Settings without a significant loss in Sharpness

More Expensive at initial purchase Less Expensive at initial purchase

Image is always Perfectly “Focused” over the entire screen

Has Clarity or Focus issues at the screen edge

Emission-Free, causes no Electromagnetic Interference

Can generate Electric, Magnetic and even X-Ray Emissions

Less Stable More Stable

Luminance Degradation of LCD

• Temperature changes, particular at startup (may take more than half hour to stabilize)

• The luminance slowly degrades because the phosphors used in the lamps wear out

(Ambient Light Compensation)

Requirement of a PACS Monitor

a) Minimal Resolution Requirement

CT/MR 1248 x 1024

Angio/DSA 1248 x 1024

DF 1248 x 1024

US 1248 x 1024

Thorax 2048 x 1536

Skeletal 2048 x 1536

Mammo 2560 x 2048

b) DICOM GSDF Support

• With a non-linearized display, the observers took longer to reach a decision and ended up being mistaken more often.

c) Brightness

• Diagnostic-review monitors have been defined by the American College of Radiology as requiring a minimum brightness of 170 Cd/m2.

• 300 and 500 Cd/m2 is recommended for use

in a properly darkened diagnostic reading room.

d) Color

• Some applications (CT/MR/US) require Color display

• Performance of Monochrome is 25%–30% better than Color displays

• Color displays calibrated to the DICOM GSDF are preferred.

END