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Advanced Biomedical Imaging
Dr. Azza HelalDr. Azza HelalA. Prof.A. Prof. of Medical Physics of Medical Physics
Faculty of MedicineFaculty of MedicineAlexandria UniversityAlexandria University
Lecture 6Lecture 6Basic physical principles of computed Basic physical principles of computed
tomographytomography & &
Image formationImage formation
Points to be covered
Linear Tomography
Components of computed tomography unit
Basic data acquisition
CT scanner generations
Principle of CT imaging
CT number & its clinical application
Windowing.
Method of imaging single slice of object parallel
to film and placed at a specific point (fulcrum)
which is adjusted to height of anatomy of interest.
1.1. Linear Tomography: (single slice imaging)Linear Tomography: (single slice imaging)
Tube and film moves from 1st position to the 2nd , all points in focal plane project to same position on the film
Points above or below the focal plane do not project to the same film position and are blurred.
By changing the relative motion of the film and tube the focal plane can be adjusted upward or downward
It is useful in IV urography
Body imaging tomography (Computed tomography) (CT): Reconstruction by a computer of an image of a plane or slices of an object.
55
X-ray TubeX-ray Tube
DetectorsDetectors
CT TableCT Table
X-ray BeamX-ray Beam
Table, scanning gantry (x-ray source & detector array)
x-ray generator, computer and viewing consoles.
2. Components of CT unit2. Components of CT unit
33 . .Basic data acquisitionBasic data acquisition
Probing patient from different directions during 360 rotations with x-ray beam of known intensity (I0) &
measuring it after it has passed the pt (I) using detectors.
Detector (Scintillator / ionization chambers) measures
exiting x-ray beam (I) & converts it into a proportional
signal current.
From I & I0, (U) is calculated (reflect intensity of photon
beam attenuated)
I=I0e-ux
u is different for different tissue density.
Image is developed from multiple measurements of x-ray u detected from exterior of the patient.
By solving a system of linear equations for several
projections, value of u can be computed.
4. CT scanner generations
1st: single beam & detector. Translate - rotate
2nd : narrow fan beam / detector array.
3rd: wide fan beam / detectors array, both rotate
4th: x ray tube alone rotates / stationary detectors.
5th : multiple x ray tubes & multiple stationary
detectors to scan multiple slices simultaneously.
4. CT scanner generations4. CT scanner generations
1st 2nd
3rd4th
4. CT scanner generations4. CT scanner generations
• To allow the computer to present this information with a
large scale, a reference material is used (water). Why? its
µ is close to those of soft tissues.
• CT number is calculated as a relative comparison of x-ray
attenuation of a voxel of tissue to equal volume of water.
• CT number of each voxel (average of individual CT no of
the contents of corresponding voxel) is computed & stored
in computer memory.
55 . .Principle of CT imagingPrinciple of CT imaging
Hounsfield UnitsHounsfield Units
large amount of data presented as grey scale (whiteness is α
average µ of contents /voxel).
Tissues appear as shades of gray from black & white.
Tissues with high u (bone) appear white
Tissues with low u (air) appears black.
bone= 1000 water= 0 air= -1000
.
The Hounsfield scaleThe Hounsfield scale
CT number (reflect density)
Air = -1000, Lung -550 to -950
Fat=-80 to -100
Water = 0Water = 0
White matter = 20-30, Gray matter = 35-45
soft tissue +40 to +60 depends on KV
Acute Hge = 70 to 100 HU
Calcification = 200 to 300 HU
Bone +500 to +1500 Contrast agent +3000
CT Numbers or Hounsfield UnitsCT Numbers or Hounsfield Units
CT tissue characterizationCT tissue characterization
Selective display of a restricted range of gray scale of selected tissues (tissues of interest).
Tissues with CT no outside this window un displayed.
• Manipulated by selection of :
• window center.
• window width
WindowingWindowing
• Window level is CT number selected for centre
of the range of numbers displayed on the image.
•Window width is total range of values selected.
•Width determines contrast.
• A narrow window enhances inherent contrast.
•Window level determines the brightness
W = 80W = 80
C = 50C = 50
W = 80W = 80
C = 20C = 20
W = 200W = 200
C = 30C = 30
W = 80W = 80
C = 30C = 30
W = 50W = 50
C = 30C = 30
Centre average gray<centre lighter gray>centre darker gray
In routine work
•Brain is visualized at level 30 and width 80.
•Soft tissue is visualized at level 40 and width 250.
•Bones are visualized at level 1000 & width 2000.
•Lungs are visualized at level -600 & width 1500.
As CT no reflects u and so different tissues densities,
So it is used to characterize normal tissues & pathologies as calcification & lesion diameter.
QuestionsQuestions
1. Define; window level & width?
2. What are the detectors used in CT unit?
3. What is the CT number of bone, air and water?
4. What is the difference between u and CT
number of the tissue?
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