X ray physics - BMEbme.elektro.dtu.dk/31545/notes/X-ray_physics_dtu_08.pdfX ray physics Lectures DTU...
Transcript of X ray physics - BMEbme.elektro.dtu.dk/31545/notes/X-ray_physics_dtu_08.pdfX ray physics Lectures DTU...
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X ray physics
LecturesDTU
Mikael Jensen oct.2008
.
Why use x-rays ?
Non invasive, very high resolutionquick
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Electromagnetic spectrum
X-ray
gamma-rays
1 nm
1 fm
SkullX ray image is shadow image
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Hand x-ray
Chest
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X-rays give rapid, high resolution anatomical information
(many photons, good S/N)
Not much soft tissue contrast
But muchcan begainedfrom hgh S/N
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Generation of X-rays
X-ray emission from Wolfram Anode X-ray tube
0
200
400
600
800
1000
1200
0 20 40 60 80 100 120 140 160
Photon energy (keV)
Rel
ativ
e in
tens
ity a
t fix
ed e
lect
ron
curr
ent
V=50kVV=90kVV=130kV
W olfram
KαKβ
1 mm Al f ilter cut-off
Specify kV and mAs !
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Radiation interaction
•Ionization• Direct kinetic energy transfer• Atomic and molecular exitation• Radiative processes• Nuclear reactions
Ionising radiation
• Releases energy throughIonisation
• But also Recombination• And through Secondary
radiation• Allways ending as heat• And perhaps chemical
change
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A biological relevant measure for energy transfer
• LET = Linear Energy Transfer.Measured in keV/μm
• Dose = Energy deposited per unit massMeasured in Gray (Gy)= J/Kg
x
LET=-dE/dx
D=dE/dM M
Macroscopic Description
• Range
• Extinction
I(x)=
I(x)= Io exp(-μx)
{Io for x<R
0 for x>R
α, β
γ
Not valid for X-rays
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To measures of thickness
• Linearx measured in meter (μm, mm, cm)
• Area weightin gram/m2
(g/cm2, mg/cm2)
Area weight= x•ρ
Two types of interaction
A single event removes the particle(the wawe).Constant ”probabiblity ofremoval” per unit length
The single event ”retards” the ionisingparticle slightly. Results in a welldefined enrgy loss per unit length.
γ
E= 24 MeV
E=23 MeVα
Al folie 3 mg/cm2
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Exponential decrease γx dx
No
NN-dN
dN= - μ•N • dx ⇒ N(x)= No • exp(- μx)
N(x) is the number of photons per unit area, ”intensity, flux”
x
N
Microscopic interactionProbability measuredin barns
(10-24 cm2)
and milliBarn(mB, 10-27 cm2)
10-10 m
10-15 m
?
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Number of Reactions P
P=σNtarget Nbeam
Ntarget numberNbeam number per area
Ntarget number per areaNbeam number
Energy loss betaparticles(electrons)
Range ?
R= 407 E 1,38 mg/cm2
Cave: electrons arenormally not monoenergetic
0,15 MeV<E<0,8 MeV
R= (542 E-133) mg/cm20,8 MeV<E<3 MeV Glendenin formulas
”decceleration”
Bremsstrahlung
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Interaction of gamma and X-rays
3 important ways of interaction•Photoelectric effect•Compton scattering•Pair production (Eγ >1022 keV)
σtot=σfoto+σcompton+σpair !
Photoelectric effect = σfoto konstant Z5 Eγ
( ),-3 5
E=Eγ-EbE γ
Einstein
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Compton effect
Compton
= σcomptonkonstant Z
E
Compton scattering
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Klein-Nishina formula for Compton scattering
θ Is the angle of deflection for the photonα Is the energy of the primary gamma ray relative to 511keV (mec2)
Angles
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Compton angular description
A very likely event
Primary X ray
Secondary,
”δ” rays
Scattered X ray
Compton electron
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Pair production (no importance for medical x-ray use)
= σpair konstant Z2 ⎛
⎝⎜⎜
⎞
⎠⎟⎟ −
289
( )ln 2 α21827
Two types of geometryGood geometry
”poor” geometri
Contributions from scattered radiation
Detector or film
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Loss of energy (macroscopic description)
E = E0 exp(-μenx)=E0 exp(-μen/ρ·ρx) x
Loss of intensity
I = I0 exp(-μx) = I0 exp(-μ/ρ·ρx) x
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Stopping of X rays
• Photo proces
• Compton proces
• (Pair effect)
Lead
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water
Mass attenuation coefficient µ⁄ρ
0,1310,1860,424Bone
0,1370,1710,227Water
0,1360,1690,226Musle
0,9995,5498,041Lead
0,1360,1690,212Fat
0,1220,1540,208Air
200 keV100 keV50 keV
Data from http://physics.nist.gov/PhysRefData/XrayMassCoef/cover.html
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Why pay interest in theseinteractions ?
• Some of the photonsshould pass the sample
• Some should be stopped in the sample.
• The radiation should bestopped in the detector(film, plate, screen….)
• The radiation should be becollimated
Biological damage from ionisation
1. Ionisation
2. Free radicals
3. DNA change
4. Lack of repair
H-OH-
4 steps:
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Consequence of DNA damage
• Single events: Most likely DNA repair
• No repair ? Cell death
• No repair, cell survives ? Small chance it is chenged to a cancer cell.
Cells and tissue under rapid cell division most radiation sensitive
Stochastical effect
Damage riskproportional to dose
No known lower limit
Effect can show up late
Dose
Damage
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Deterministic effect
• Threshold value
• Rapid onset
• Often local
• Cell deathLD50 humans: 5 Gy
Dosis1 Gy
A biological relevant measure for energy transfer
• LET = Linear Energy Transfer.Measured in keV/μm
• Dose = Energy deposited per unit massMeasured in Gray (Gy)= J/Kg
x
LET=-dE/dx
D=dE/dM M
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Unit of dose
• Gray ( J/kg)
• With important ”biological” weight factors linked to Sievert (Sv) (still J/kg)
X ray doses
• Single exposure, small area, short path• -limb, teeth, chest• few micro Sievert
• Multiple exposures whole body, low energyto enhance contrast (CT….)
• several milli Sievert
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Stocastic Risc
• ICRP says:
4 -5% / Sievert (Sv) total risc fatal cancer
Dose
HiroshimaNagasaki