Understanding Windowing for CT

• HU values:

  • air = - 1000
  • water = 0
  • lung = - 500
  • CSF = 15
  • soft tissue = 100 - 300
  • dense bone = 3000
• Window: contrast - stretching process where the CT image greyscale component of an image is manipulated

  • the greyscale is used across a smaller range of HU
  • doing this changes appearance of the image to highlight particular structures
  • all HU outside the range are displayed as white or black but greater contrast within range
• Windowing: selecting a small range of HU and displaying them over the full brightness range (dark black to bright white) of the image

  • increase contrast of CT scan
  • can show different tissues to their advantage
• The range of HU to be displayed in the image is called the window
• Level: midpoint of the window range in HU, brightness of image is adjusted via window level
• Width: width of window in HU, contrast is adjusted via window width
• Abdomen:

  • window: - 125 to 225
  • level: 50
  • width: 350
• Bone:

  • window: - 400 to 800
  • level: 200
  • width: 1200
• Breast:

  • window: - 250 to 150
  • level: - 50
  • width: 400
• Lung:

  • window: - 1000 to 0
  • level: - 500
  • width: 1000
• Pelvis:

  • window: - 160 to 240
  • level: 40
  • width: 400