Hounsfield Unit

Aka: Hounsfield Unit, Radiodensity, CT Scan Window Level, CT Scan Window Width

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II. Definitions

  1. Hounsfield Unit
    1. XRay attenuation unit used in CT Scan interpretation
    2. Characterizes the relative density of a substance (amount of X-ray radiation absorbed by each element in tissue)
    3. Each pixel is assigned a value between -1000 (air, black) to Zero (water) to +3000 (dense bone, white)

III. Indications

  1. Computed Tomographic Scanning (CT Scan)

IV. Imaging: Substance densities in Hounsfield Units (Radiodensity)

  1. Air: -1000
  2. Lung: -700
  3. Soft Tissue: -300 to -100
  4. Fat: -50
  5. Water: 0
  6. CSF: +15
  7. Blood: +30 to +45
  8. Muscle: +40
  9. Calculus: +100 to +400
  10. Bone: +1000 (up to +3000 for dense bone)

V. Imaging: CT Window Settings

  1. Window
    1. Gray scale value assigned to tissues of various densities
    2. Materials with densities less than the lower window limit will be displayed as 100% black
    3. Materials with densities greater than the upper limit will be displayed as 100% white
  2. Window Width
    1. Range of Hounsfield Units displayed
    2. Hounsfield Units are divided into 16 gradations to display on screen
    3. Human eye can discern 700-900 shades of gray
      1. Kimpke (2007) J Digit Imaging 20(4): 422–32 [PubMed]
  3. Window Level
    1. Hounsfeld Unit in center of the window width
    2. Mid-Radiodensity in the total range
  4. Window Setting
    1. Displayed as L/W = Level/Width
    2. Abdominal CT = 350/50 (soft tissue Radiodensity)
      1. Range: -125 to +225 HU centered at +50 HU
      2. Gradations: Each 350/16 or 22 HU displays a different shade of gray on the screen
    3. Chest CT = 1500/-500 (air Radiodensity)
      1. Range = -1250 to +250 HU centered at -500 HU
      2. Gradations: Each 1500/16 or 94 HU displays a different shade of gray on the screen
    4. Bone CT = 2000/250 (bone Radiodensity)
      1. Range = -750 to +1250 HU centered at +250 HU
      2. Gradations: Each 2000/16 or 125 HU displays a different shade of gray on the screen
    5. Brain CT = 45/80
      1. Range = 5 to 85
      2. Includes CSF (0 HU), brain parenchyma (20-50 HU) and blood (30-70 HU)
      3. Consider Subdural CT Window = 50/130 or 70-100/150-300
        1. Wider window helps differentiate Subdural Hematomas
  5. Window Adjustments (PACS screen adjustment)
    1. Inverted Image
      1. Flips color gradient, where air is bright white (eases free air visualization)
    2. Window Width (Horizontal adjustments)
      1. Drag mouse right to increase window width
        1. Increases range of HU per each 1/16 shade of gray
        2. Decreases contrast
      2. Drag mouse left to decrease window width
        1. Decreases range of HU per each 1/16 shade of gray
        2. Increases contrast
    3. Window Level (Vertical adjustments)
      1. Drag mouse up to increase the window level
        1. Increases the middle or mean Radiodensity toward bone (or white, +1000 HU)
      2. Drag mouse down to decrease the window level
        1. Decreases the middle or mean Radiodensity toward air (or black, -1000 HU)

VI. Resources

  1. McGonigal (2013) Trauma MedEd, July, 2013
    1. http:\\www.traumameded.com

VII. References

  1. Broder (2021) Crit Dec Emerg Med 35(5): 10-1
  2. Hounsfield (1980) Med Phys 7:283-90 [PubMed]

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