A-LEVEL PHYSICS 15/09/2015 13:49 Medical Physics

19/04/23 15:53

A-LEVEL PHYSICSA-LEVEL PHYSICS

Medical Physics

19/04/23 15:53


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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Major applications

• Radiography– Diagnostic imagery

• Radiotherapy– Destroying dangerous cells

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Properties of X-rays

• Can not be reflected• Can not be refracted• Can not be focused

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Rotating-anode X-ray tube

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Attenuation

• Attenuation– Intensity of X-rays reduced– Passage through matter

xeII 0

• Linear attenuation coefficient

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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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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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Measuring

0 X

Log I

cmxy

IxI

0loglog

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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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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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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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Line spectrum

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Line spectrum

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Line spectrum

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Continuous spectrum

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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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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

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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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Interaction with matter

• Simple scattering• Photoelectric absorption• Compton scattering• Pair production

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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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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

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Photoelectric absorption

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Compton effect

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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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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

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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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Filtration

• HVT increases• Higher proportion of high-energy X-ray photons

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Beam size and alignment

• 4 adjustable lead shields

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Beam size and alignment

• Mirror reflects light that is transparent to X-rays

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X-ray image quality

• Focal spot

• Angled target– Enlarged focus

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X-ray image quality

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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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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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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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Fluoroscopy

• Real time images• X-ray to light• Nice idea but…

– X-ray dose too high– Patient dies!

• Image intensifier

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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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Computer Tomography

• The NET for…– CT scanning– Principles – Images

• Encarta for…– Uses– Treatment

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Attenuation…again

• Mass attenuation co-efficient• Consider water

– ice– water– steam

• All have different densities• But ALL have the same MAC

m

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A-LEVEL PHYSICS 15/09/2015 13:49 Medical Physics