# Investigations In Stroke by QCT277

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```									 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
Cormack
   First system devised by Dr. Godfrey
Hounsfield
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
Scanner
Still Scanners!
   Third Generation
both rotate in step with each other

   Fourth Generation
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
images.
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-
scale

   Areas of low density, with low CT
numbers appear darker on the grey-
scale
Windowing
   Relationships between the CT
numbers and the grey-scale can be
   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
1000

0

-1000
Image Quality
    Greater contrast sensitivity than

1.   All tissues are viewed directly, as opposed to
looking at deep tissues obscured by superficial
tissues
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
Noise
   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)
Artifacts
   Most commonly appear as streaks
   Causes:

- patient moving during scanning

- metal objects in field of view
   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
   Different areas record different values
Dose Distribution
   Direct exposure is limited to the
tissue within the slice
   A small amount will scatter to