CT Terminology
Imaging Planes
Imaging Planes
Axial (transverse) CT
Axial (transverse) CT
Coronal CT
Coronal CT
Sagittal CT
Sagittal CT
Image Orientation
Hounsfield Unit (HU)
Hounsfield Unit (HU)
CT images are density maps based on ease of x-ray penetration through the body
Hounsfield unit is a dimensionless unit used to measure CT density
Named after Sir Godfrey Hounsfield, inventor of CT
Scale is defined so water density = 0HU and air = -1000HU
HU Examples
HU Examples
Different densities from top down
Lung (mostly air)
Bone (rib)
Soft tissue (chest wall muscle)
Fat (subcutaneous)
How densities help
How densities help
Low density fluid (near pure water) consistent with ascites
Low density fluid (near pure water) consistent with ascites
High density fluid, consistent with blood/hemoperitoneum
High density fluid, consistent with blood/hemoperitoneum
Windowing
Windowing
Controls brightness and contrast of the image
Window width = range of displayed HU
Wide window used for structures with very different densities (e.g. lung, which contains air and soft tissue)
Narrow window used for structures with similar density (e.g. soft tissue)
Window level = center point of the displayed range