Digital Display Device Notes

Chapter 20: Digital Display Device

Objectives

  • Describe factors affecting digital display device performance.
  • Identify AAPM TG 18 and its significance.
  • Explain AAPM TG 18 test patterns for performance assessment.
  • Relate luminance to gray scale and p-values.
  • Discuss quality control tests and schedules.

Performance Assessment Standards

  • Soft copy digital image: displayed on a monitor.
  • Standardization initiatives:
    • SMPTE (Society of Motion Picture and Television Engineers):
      • SMPTE pattern: defines format, dimensions, and contrast for resolution measurements.
      • 5-95% luminance patches: identify luminance deviations.
    • NEMA (National Electrical Manufacturers Association) and ACR: developed DICOM.
      • DICOM (Digital Imaging and Communication in Medicine) Standards:
        • Allows medical images to be transferred and displayed with similar gray scale on any DICOM compatible device.
        • Mandates Grayscale Display Function (GSDF) on all digital display devices.
    • DIN (Deutsches Institut fur Normung) 2001:
      • Developed acceptance testing standards for digital display devices.
    • VESA (Video Electronic Standard Association):
      • Developed Flat Panel Display Measurement standard (FPDM) in 1998.
      • Provides instructions for system performance evaluation.
    • AAPM TG 18 (American Association of Physicists in Medicine):
      • Developed test patterns and procedures for display testing in Task Group Report 18 (TG 18).
      • Recommends patterns for different purposes.
      • AAPM TG 18 measurements should be included routinely for evaluation.
      • ACR recommends AAPM patterns for QC.
      • Vendors provide patterns for display devices.
      • Technologists use some patterns for system performance.
      • Medical physicists use others for comprehensive testing.
      • Radiologic Technologists are responsible for frequent assessment with AAPM TG 18 QC test patterns.

Luminance Meter

  • Luminance: Brightness of a digital display device.
  • Illuminance: Intensity of light on a surface, measured in lumens/square meter or lux.
  • Photometric evaluation and ambient light levels are essential for image QC.
  • Photometer: measures luminance.
    • Near range: used close to the device.
    • Telescopic: used at 1 m from device.
    • Ambient light should be constant to avoid interference.
    • Calibrated consistent with National Institute of Standards and Technology (NIST).
    • Should measure 0.05-1000 cd/m2^2 and have photopic spectral response within 3%.
  • Lux or Illuminance meter: evaluates reflection and ambient light.
    • Also calibrated according to NIST.
  • Colorimeters: measure color coordinates.

Digital Display Device Quality Control

  • Typically uses AAPM TG 18 patterns.
  • Can be quantitative or qualitative.
  • Geometric distortion:
    • Displayed image is geometrically different from original.
    • Affects size and shape of image features.
    • Tested with TG 18 QC tool and TG 18 LPV/LPH test pattern.
    • Look for pin cushioning and barrel-like distortions.
    • More common on CRTs.
    • Measure distances of square areas with a plastic ruler.
    • Primary devices: 2% distortion allowed.
    • Secondary devices: 5% distortion allowed.
  • Reflection:
    • Ideal luminance: only light from the display.
    • Reflection: specular (mirror image) or diffuse (randomly scattered).
    • Specular reflector evaluation:
      • Turn off the monitor and look for a "mirror image" within a 15-degree angle at 30-50cm.
      • Digital displays exhibit very little reflection.
    • TG 18 AD pattern:
      • Low contrast to evaluate diffuse reflection.
      • Observe the threshold of visibility for low contrast under ambient light and in total darkness.
      • Ideally, the threshold of visibility should be the same under both conditions.
      • If not, ambient lighting should be reduced.
  • Luminance response: relationship between displayed luminance and input values.
    • Digital image data (gray scale values) are sent as p-values.
    • P values are transformed into digital driving levels (DDL), then into luminance values via the LUT.
    • Transformation to DDLs is standardized by DICOM.
    • TG 18 CT test pattern: qualitative evaluation.
    • Best image viewing: straight on.
  • Display spatial resolution.
  • Display noise.
  • Luminance response: relationship between displayed luminance and input values.
    • Digital image data collected as gray scale values, are sent to the digital display device as p-values.
    • P values are transformed into digital driving levels (DDL) that are transformed into luminance values through the LUT.
    • Transformation of values to DDLs is standardized by DICOM.
    • TG 18 CT test pattern performs qualitive evaluation on luminance response of a device.
    • Has 16 low contrast targets that should all be visualized at about 30cm.
    • TG 18 LN patterns can be utilized to measure luminance and recorded for 18 different DDLs.
    • Luminance should match manufacturers quoted values.

Digital Display Device Quality Control (Continued)

  • Luminance response:
    • Affected by location on the display and viewing angle.
    • Luminance non-uniformity: maximum luminance variation across the display area.
    • Varies from center to edges in CRT.
    • TG 18 UN test patterns: observe for gross variations between squares.
    • No variations with the area of 1cm or larger should be noted.
    • TG 18 UNL10 and UNL 80 patterns: used with a photometer to measure luminance in 5 locations and evaluate deviation.
    • 200×(L<em>max−L</em>min)/(L<em>max+L</em>min)200 \times (L<em>{max} - L</em>{min}) / (L<em>{max} + L</em>{min})
    • Deviation should be less than 30%.
  • Angle Dependence:
    • TG 18 Cx test pattern
    • Visualize half moon targets straight on, then at different viewing angles to assess visibility with varying angle dependency
    • Region in which display show no variation in viewed patterns should be used clinically
    • Angle limits can be labeled on the display
    • LED displays should be adjusted to optimally face the user
  • Display Spatial Resolution:
    • Obtain quantitative measurement to produce separable images of different points of an object.
    • TG 18 CX and QC patterns can be used for SR evaluation
    • evaluate patterns in middle and corners with magnifying glass and compare
    • TG 18 PX can be evaluated for non uniformity in spatial resolution
  • Display Noise:
    • Any fluctuations that interfere with detection of true signal.
    • Quantified with the TG 18 AFC test pattern.
    • Look for just noticeable luminance difference as a function of size by evaluating the large number of regions with different target positions.
    • Identify the size and contrast that are constant between the four identical areas.

Quality Control by the Radiologic Technologist

  • Medical physicist acceptance testing any new display devices and at least annual testing
  • Routine quality control tests with AAPM TG 18 should be performed by technologist
  • Periodic review of QC program by a qualified medical physicist
  • Annual and post repair medical physics performance evaluations
  • The TG 18-QC test pattern should be viewed regularly.
  • Does not take long/Easy
  • MUST MAINTAIN QC RECORDS!

Summary

  • Scientific organizations provide guidance for digital display characteristics and quality.
  • Assessment is the responsibility of medical physicists and radiographers.
  • AAPM TG 18 testing patterns are common tools for radiographers.