CT Principles - Computed Tomography - Lecture Slides, Slides of Computed Tomography

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Principles of

Computed

Tomography

Radiography vs. CT

 Both based on differential attenuation

of x-rays passing through body

 Radiography

 “Shadowgraph” using x-ray light source

 CT

 Cross-sectional image  Image computed from pencil beam intensity measurements through only slice of interest

Limitations of Radiography

 Optical density dictated by

total attenuation

encountered by beam

 Thin highly-attenuating

objects appear to be same

density as thicker low-

attenuating object.

Patient

X-ray Beam

Film

Thin dense object Thick less dense object

Early Solution: Conventional Tomography

 Tube and film move

 Rotate around fulcrum

 Image produced on film

 Objects above or below fulcrum

plane change position on film &

thus blur

CT Advantages

 View anatomy without looking through

underlying / overlying structures

 improves contrast

 Uses tightly collimated beam

 minimizes scattered radiation  improves contrast

 Demonstrates very small contrast

differences reliable & repeatedly

CT X-ray Beam

Conventional X-ray Beam

Film as a Radiation Detector

 Analog

 not quantitative

 Not sensitive enough to

distinguish small

differences in incident

radiation

 Applications

 film badges  therapy dosimetry

Data Aquisition

 Slice by slice

 One slice at a time

 Volume acquisition

 data for an entire volume collected  patient moves in axial direction during scan  tube traces spiral-helical path through patient

Scanning

 X-ray tube rotates around patient

 detectors also rotate for 3rd generation CT

 Detectors measure radiation

transmitted through patient for

various pencil beam projections

 Relative transmissions calculated  Fraction of beam exiting patient

Patient

X-Ray beams

Photon Phate

 What can happen to an x-ray photon passing

through a material (tissue)?

Material Incoming X-ray Photon

Photon Phate #1: Nothing

 Photon exits unaffected  same energy  same direction

Material Incoming X-ray Photon

Outgoing X-ray Photon

Photon Phate #3: Scatter

 Lower energy photon emerges  energy difference deposited in material  Photon usually emerges in different direction

Material Incoming X-ray Photon

Outgoing X-ray Photon

Photon Beam Attenuation

 Anything which removes original photon from

beam

 absorption  scatter Material Incoming X-ray Photon

Incoming X-ray^ Material Photon

Outgoing X-ray Photon

Attenuation Equation for

Mono-energetic Photon Beams

I = Ioe-mx

I = Exiting beam intensity

Io = Incident beam intensity

e = constant (2.718…)

m = linear attenuation coefficient

• property of
  • absorber material
  • beam energy

x = absorber thickness

Material Io I

x

For photons which are neither absorbed nor scattered

Example Beam Attenuation

 Using equation to calculate beam intensity for various

absorber thicknesses (m = .223)

1cm 100 80

I = Ioe-mx

100*e-(0.223)(1)^ = 80