Xrayfund

7/14/1999

CT Scans (computed tomography)

• Dose is about 10-40mGy per slice
• A CT scan image consists of many cells, each assigned a number and displayed as a brightness level on the video monitor
• Each cell is called a pixel
• The numerical information stored in each pixel is CT number or Hounsfield unit (HU)
• The pixel is a 2D representation of a corresponding tissue volume
• The tissue volume is called a voxel and is determined by the pixel size X the slice thickness (1-10mm)
• Gray scale
• CT numbers range from -1000to +1000 HU for each pixel
• Water is defined as 0 HU
• -1000 HU corresponds to air while +1000 HU corresponds to dense bone
• soft tissues range from 0 to +1000 HU
• fat is -100 HU

• enormous waste of information
• actual range of image is 2000HU's or shades of gray
• a video screen can only display 256 shades of gray

• image display

• windowing

• the technologist or radiologist selects a CT number that is about the average CT number of the body tissue being examined
• the computer is then instructed to assign 1 shade of gray to each of the 128CT number below and each of the 128CT number above
• the center CT number is called the window level
• the range of CT numbers above and below the window level is called the window width
• a bone window would have a window level of about +200
• a soft window would be b/w +20 to +40
• the window width may be set at any level w/a maximum of 2000

• gantry--x-ray tube and sensors
• houses the x-ray tube and radiation detectors
• can be angled from 15-30--this is useful in imaging the skull and IVD's
• table automatically positions the pt in exact increments
• CT is 10x's more sensitive in showing soft tissue than plan x-ray

MRI Fundamentals

• The response of polar atoms to discrete radiation frequencies under the influence of a strong magnetic fields
• Polar atoms have unpaired protons or neutrons
• Advantages of MRI over the other modalities
• Best low contrast resolution
• No ionizing radiation—not an x-ray procedure
• 100x better than CT on soft tissue
• direct multiplanar imaging
• no bone or air artifacts—superior in pelvis and EOP
• direct flow measurements—do not need contrast

• totally noninvasive

• precession motion