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?