25/03/22 19:00 A-LEVEL PHYSICS A-LEVEL PHYSICS Medical Physics
Dec 28, 2015
19/04/23 15:53
A-LEVEL PHYSICSA-LEVEL PHYSICS
X-rays
• Basic concept
• X-rays can expose photographic film
• X-rays can pass through soft tissue
• X-rays can not pass through bone
• Put a body over some photographic film and then “shine” X-rays on them
• Develop the film
• Photograph of the inside of the patient
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A-LEVEL PHYSICSA-LEVEL PHYSICS
Major applications
• Radiography– Diagnostic imagery
• Radiotherapy– Destroying dangerous cells
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A-LEVEL PHYSICSA-LEVEL PHYSICS
Properties of X-rays
• Can not be reflected• Can not be refracted• Can not be focused
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A-LEVEL PHYSICSA-LEVEL PHYSICS
Attenuation
• Attenuation– Intensity of X-rays reduced– Passage through matter
xeII 0
• Linear attenuation coefficient
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A-LEVEL PHYSICSA-LEVEL PHYSICS
Half-value thickness HVT
• The thickness of material needed to reduce intensity to 50% of its initial value
x
II0
0 X
IxeI
I 00
2
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A-LEVEL PHYSICSA-LEVEL PHYSICS
693.0
2
1log
2
12 0
0
x
x
e
eII
e
x
x
Half-thickness value X1/2
• Half the original intensity
• Cancel I0
• Take natural logs
• Make x the subject
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A-LEVEL PHYSICSA-LEVEL PHYSICS
X-ray spectrum
0 2 4 6 8 10
Wavelength [x10exp11]
Intensity [arbitary
units]
0
2
6
4
8
10
Continuous radiation Bremsstrahlung
Line spectrum
min
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A-LEVEL PHYSICSA-LEVEL PHYSICS
Continuous radiation Bremsstrahlung
• Energy of the emitted X-ray is equal to the energy lost by the electron– Some…why there is a range– All…why there is a minimum
wavelength
qV
hc
hcqV
hcE
hfE
qVE
min
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A-LEVEL PHYSICSA-LEVEL PHYSICS
Line spectrum
• High energy incident electron
• Removes DEEP-LYING orbiting electron
• Outer electron falls into the space
• Emits X-ray photon– Much more energy than visible light– Light photons use higher orbital transitions
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A-LEVEL PHYSICSA-LEVEL PHYSICS
X-ray spectrum
0 20 40 60 80 100
Photon energy [keV]
Intensity [arbitary
units]
0
2
6
4
8
10
Range of energies corresponding to
frequency of X-ray
Maximum value corresponding to min
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A-LEVEL PHYSICSA-LEVEL PHYSICS
Quality & Intensity: Different supply voltage
0 20 40 60 80 100
Photon energy [keV]
Intensity [arbitary
units]
0
2
6
4
8
10
100kV
50kV
More energy available to create X-rays from each incident electron
19/04/23 15:53
A-LEVEL PHYSICSA-LEVEL PHYSICS
Quality & Intensity: Different filement current
0 20 40 60 80 100
Photon energy [keV]
Intensity [arbitary
units]
0
2
6
4
8
10
Top 20mA
Lower 10mA
Greater number of electrons striking the
anode so more X-rays produced
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A-LEVEL PHYSICSA-LEVEL PHYSICS
Interaction with matter
• Simple scattering• Photoelectric absorption• Compton scattering• Pair production
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A-LEVEL PHYSICSA-LEVEL PHYSICS
Simple scattering
• Low energy• Can’t remove an electron• Incident x-ray is deflected• No loss of energy• Material scatter X-ray• Without absorbing energy
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A-LEVEL PHYSICSA-LEVEL PHYSICS
Photoelectric absorption
• Similar to the production of line spectra• Incident high energy electron replaced by X-ray
photon• X-ray loses ALL energy as an inner electron is
ejected– Photoelectron…ionizes other atoms
• Lower energy photon released as an electron falls into the vacant orbit
19/04/23 15:53
A-LEVEL PHYSICSA-LEVEL PHYSICS
Pair production
• High energy X-ray photons– 1.022MeV
• [rest mass of electron + positron]
• Nucleus interaction– X-ray vanishes– Electron-positron pair created
• Electron ionizes atoms• Positron annihilated by electron
– Two photons created
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A-LEVEL PHYSICSA-LEVEL PHYSICS
Filtration
• Limit X-rays to the “useful” energies– Remove unwanted low energy X-rays– Heterogeneous
• Range of values
• Absorbing filter material– Photoelectric absorption– All energy of incident X-ray absorbed– Low energy photon emitted
19/04/23 15:53
A-LEVEL PHYSICSA-LEVEL PHYSICS
Effect of filtering
0 20 40 60 80 100
Photon energy [keV]
Intensity [arbitary
units]
0
2
6
4
8
10
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A-LEVEL PHYSICSA-LEVEL PHYSICS
Filtration
• HVT increases• Higher proportion of high-energy X-ray photons
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A-LEVEL PHYSICSA-LEVEL PHYSICS
Beam size and alignment
• Mirror reflects light that is transparent to X-rays
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A-LEVEL PHYSICSA-LEVEL PHYSICS
X-ray image quality
• Focal spot
• Angled target– Enlarged focus
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A-LEVEL PHYSICSA-LEVEL PHYSICS
Scattering
• To produce an image only use X-rays that have passed directly through the body
• Grid– Vertical lead strips
• 5mm deep 0.5mm apart 0.05mm thick
– Moves from side to side• prevent grid from casting
its own shadow
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A-LEVEL PHYSICSA-LEVEL PHYSICS
Contrast media
• How to take an X-ray of the stomach or other organ?– Barium meal– High Z [atomic number]– Absorb X-rays– So get a better contrast on the image
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A-LEVEL PHYSICSA-LEVEL PHYSICS
Intensifying screens
• Intensifying cassette– Reduces patient’s exposure time– White plastic– Fluorescent crystals
• Absorb X-rays• Give out light
• Film much more sensitive to light• Metal prevents back scattering
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A-LEVEL PHYSICSA-LEVEL PHYSICS
Fluoroscopy
• Real time images• X-ray to light• Nice idea but…
– X-ray dose too high– Patient dies!
• Image intensifier
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A-LEVEL PHYSICSA-LEVEL PHYSICS
Computer Tomography
• Need a powerful computer• Compares signals from 2000+ tiny detectors• X-ray source rotated around the patient
– pules of X-rays produced
• Complex mathematical algorithm used to determine the attenuation at each point in the body
• Excellent resolution• Able to be used on soft tissue
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A-LEVEL PHYSICSA-LEVEL PHYSICS
Computer Tomography
• The NET for…– CT scanning– Principles – Images
• Encarta for…– Uses– Treatment