Investigations In Stroke by QCT277


									 CT Scans -
the principles
  Craig Douglas
             General Info.
   It is an X-ray imaging method
   Formation of an image is a distinct
    two stage process
   Principles described by Dr. Alan
   First system devised by Dr. Godfrey
                  The Process
   Step 1- Scanning:
    - A very thin X- ray beam is passed through the
    edges of a slice of body tissue
    - The beam is rotated around the body ( takes up
    to 15 seconds )
    - Radiation penetrates to different degrees,
    depending on the density of the tissues they pass
    through ( dependent on the atomic numbers of
    the elements within the tissues )
    - Electronic radiation detectors detect penetration
   Step 2- Image Re-Construction
             Still Scanners!
   Third Generation
    -X-ray tube and radiation sensors
    both rotate in step with each other

   Fourth Generation
    -X-ray tube rotates and radiation
    sensors form a continuous circle
   No real indications of superiority
           Image Formatting
   A CT image appears to be a
    continuous display of a slice of tissue

   But, No!

   A CT scan is actually a matrix of
    individual elements
           Image Formatting
   The slice of tissue is formatted as an
    array of small volumes of elements,
    these are known as VOXELS
   Each pixel on the monitor will
    represent a VOXEL
   One pixel is displayed as uniform
    brightness, the elements within it are
    blurred together
           Voxels and Pixels
   The operator selects the number of
    pixels in an image, therefore this
    dictates the size of the voxels

   As pixel number increases, voxel size
    decreases, allowing for better quality
              CT Numbers
   Each pixel can be represented by a
    CT number
   The value is related to the physical
    density of the tissue in the
    corresponding voxel
   Measured in Hounsfield Units
   Water is the reference and assigned
    a value of zero
               CT Numbers
   Tissues more dense than water have
    a positive number
   Same for opposites

   CT Number = density (tis) – density (H2O)
                        density (H2O)

               = ‘a’ X 1000
             Image Display
   Areas of high density, with high CT
    numbers appear white on the grey-

   Areas of low density, with low CT
    numbers appear darker on the grey-
   Relationships between the CT
    numbers and the grey-scale can be
    adjusted by the system viewer
   Viewer controls can set upper and
    lower CT number window limits
   CT numbers > upper limit = white
   CT numbers < lower limit = black
   Easier distinction between soft tissues


                Image Quality
    Greater contrast sensitivity than
     plain radiography because:

1.   All tissues are viewed directly, as opposed to
     looking at deep tissues obscured by superficial
2.   The relatively thin X-ray beam produces
     relatively less scatter
3.   The ability to express a small range of CT
     numbers over the entire grey scale
   This is a mottled appearance on CT
   Its presence decreases the visibility
    of low contrast features
   Produced due to the random manner
    in which x-ray photons are absorbed
    in the tissue
   Decrease by: > x-ray dose (careful)
                  or, > voxel size (poor)
   Most commonly appear as streaks
   Causes:

    - patient moving during scanning

    - metal objects in field of view
            Radiation Dose
   Measure absorbed radiation dose to
    the tissue within the scanned slice
   Rad = 100 ergs per 1g tissue

   Gray =1 joule per 1kg tissue
   Represents radiation at a specific
    point, not total radiation applied
   Different areas record different values
           Dose Distribution
   Direct exposure is limited to the
    tissue within the slice
   A small amount will scatter to
    adjacent tissues
   Total radiation energy increases with
    the no. of slices imaged and this
    must be considered
   Generally even distribution, due to
    rotation of X-ray beam

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