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89. Computed Tomography

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yaboiemil's version from 2016-02-04 14:50

Section

1. An image in computed tomography is NOT a shadow of an X-ray on film.
2. It is the computer reconstruction of a transversal cross-section of a patients
body, using the attenuation of the x-ray to calculate the density of the tissue
and therefore discover what the internal cross section looks like.
3. There have been many generations of CT scans:

 

Question Answer
First Generation1 thin ray. 1 detector. Moves slowly across to produce 1 cross section.
Second generation1 fanned out ray. 1 longer detector. Moves slowly across to produce 1 cross section.
Third generation1 fanned out ray. 1 arched longer detector. Entire system rotates around patient to produce 1 cross section.
Fourth Generation1 fanned out ray. Detectors completely surround patient. Only x-ray tube rotates around. Shorter exposure.
Spiral CT Scanner Patient moved 5mm longitudinally each cross section. Combined to produce 3D image.
memorize

 

4. The 3D image is produce when the patient is moved longitudinally 5mm, after every cross section is completed. This can
be assembled together to give a 3D image.
5. Hounsfield units are the units used to assess how attenuating a medium is for x-rays.

 

 

Question Answer
Mu TAttenuation coefficient of the medium
Mu WAttenuation coefficient of water
kconstant numerical value of 1000
memorize

Common values for mediums you should know:

Question Answer
Water0
Air-1000
Compact Bone1000
Blood47
White Brain matter30
memorize
Grey Brain matter ; 34
Question Answer
Fat-70
memorize

Clinical importance of CT scans:

6. Provide good resolution of soft tissues
7. Good for planning surgery of tumors
8. Resolution can be improved by contrast agents
9. DISADVANTAGES: High dose (10mSv)

Common doses of Radiation:

 

Question Answer
Natural Sources2mSv/year
Chest x-ray1mSv
Fluoroscopy5mSv
CT Scan10 mSv
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