Download Windowing - Pharos University in Alexandria

Advanced Biomedical Imaging
Lecture 6
Basic physical principles of computed
tomography
&
Image formation
Dr. Azza Helal
A. Prof. of Medical Physics
Faculty of Medicine
Alexandria University
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.
1.
Linear Tomography: (single slice imaging)
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.

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.
2. Components of CT unit
X-ray Tube
X-ray Beam
CT Table
Detectors
Table, scanning gantry (x-ray source & detector array)
x-ray generator, computer and viewing consoles.
5
3. Basic 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. 4.
CTCT
scanner
generations
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 generations
2nd
1st
3rd
4th
5. Principle of CT imaging
• 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.
Hounsfield 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.
The Hounsfield scale

bone= 1000
water= 0
air= -1000
.
CT number (reflect density)
Air = -1000,
Lung -550 to -950
Fat=-80 to -100
Water = 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 tissue characterization
CT Numbers or Hounsfield Units
Windowing

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
selection of :
by
• window center.
• window width
• 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
Centre
<centre
>centre
average gray
lighter gray
darker gray

W = 80
C = 30
W = 80
W = 80
W = 200
W = 50
C = 50
C = 20
C = 30
C = 30
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.
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?