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X ray physics

Lectures DTU

Mikael Jensen oct.2017 .

Two types of x-ray imaging,- and two lectures

•  Planar x-ray •  CT

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Why use x-rays ?

Non invasive, high resolution Quick, widespread

Konrad Røntgen, 1896

X-rays give rapid, high resolution anatomical information

(many photons, good S/N) Rapid introduction, simple technology

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Electromagnetic spectrum

X-ray

gamma-rays

1 nm

1 fm ν·λ = c E = h · ν

Planar x-ray imaging

•  Direct projection ( image) : 2D shadow! ”Black” is low attenuation: Air, lung… ”White” is high attenuation : Bone…. •  Geometry factors: FilmFocusDistance,

FilmObjectDistance, FocusObjectDistance

Geometrical enlargement

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Skull X ray image is shadow image

Hand x-ray

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Chest

And possibility of tomography

Much can be gained from 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

Kα

Kβ

1 mm Al f ilter cut-off

Specify kV and mAs !

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https://www.oem-xray-components.siemens.com/x-ray-spectra-simulation

Spectrum simulation:

Parameters: Anode, kV, mAs, filter

Radiation interaction

• Ionization • Direct kinetic energy transfer •  Atomic and molecular exitation •  Radiative processes •  Nuclear reactions

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Ionising radiation

•  Releases energy through Ionisation

•  But also Recombination •  And through Secondary

radiation •  Allways ending as heat •  And perhaps chemical

change

A biological relevant measure for energy transfer

•  LET = Linear Energy Transfer. Measured in keV/µm

•  Dose =

Energy deposited per unit mass Measured in Gray (Gy)= J/Kg

x

LET=-dE/dx

D=dE/dM M

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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

Two measures of thickness

•  Linear x measured in meter (µm, mm, cm)

•  Area weight

in gram/m2

(g/cm2, mg/cm2)

Area weight= x•ρ

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Two types of interaction

A single event removes the particle (the wawe).Constant ”probabiblity of removal” per unit length

The single event ”retards” the ionising particle slightly. Results in a well defined enrgy loss per unit length.

γ

E= 24 MeV

E=23 MeV α

Al folie 3 mg/cm2

Exponential decrease γ

x dx No

N N-dN

dN= - µ•N • dx ⇒ N(x)= No • exp(- µx)

N(x) is the number of photons per unit area, ”intensity, flux”

x

N

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Microscopic interaction Probability measured in barns

(10-24 cm2)

and milliBarn (mB, 10-27 cm2)

10-10 m

10-15 m

?

Number of Reactions P

P=σNtarget Nbeam

Ntarget number Nbeam number per area

Ntarget number per area Nbeam number

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Energy loss betaparticles (electrons)

Range ?

R= 407 E 1,38 mg/cm2

Cave: electrons are normally not monoenergetic

0,15 MeV<E<0,8 MeV

R= (542 E-133) mg/cm2 0,8 MeV<E<3 MeV Glendenin formulas

”decceleration” Bremsstrahlung

Interaction of gamma and X-rays

3 important ways of interaction • Photoelectric effect • Compton scattering • Pair production (Eγ >1022 keV)

σtot=σfoto+σcompton+σpair !

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Photoelectric effect = σ

foto konstant Z5 Eγ( ),-3 5

E=Eγ-Eb E γ

Einstein

Compton effect

Compton

= σcompton

konstant ZE

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Compton scattering

Klein-Nishina formula for Compton scattering

θ Is the angle of deflection for the photon α Is the energy of the primary gamma ray relative to 511keV (mec2)

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Angles

Compton angular description

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Lead

water

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Why pay interest in these interactions ?

•  Some of the photons should pass the sample

•  Some should be stopped in the sample.

•  The radiation should be stopped in the detector (film, plate, screen….)

•  The radiation should be be collimated

Mass attenuation coefficient µ⁄ρ 50 keV 100 keV 200 keV

Air 0,208 0,154 0,122

Water 0,227 0,171 0,137

Fat 0,212 0,169 0,136

Musle 0,226 0,169 0,136

Bone 0,424 0,186 0,131

Lead 8,041 5,549 0,999 Data from http://physics.nist.gov/PhysRefData/XrayMassCoef/cover.html

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Soft tissue contrast

Mainly due to the Compton effect - Because Z is tpically low - And - E xray is ”high”

The image is really ”electron density”

The film / detector

Should stop the x-rays ! ( Silver is good ) but old- fashioned

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Late 1990’s

•  A new approach to imaging appeared

•  DR or DDR or Direct Capture imaging

•  Too early to tell which system will prevail

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IMAGE CAPTURE

CR – PSP – photostimulable phosphor plate – REPLACES FILM IN THE CASSETTE

DR – NO CASSETTE – PHOTONS –  CAPTURED DIRECTLY – ONTO A TRANSISTOR – SENT DIRECTLY TO A MONITOR

CR vs DR CR •  imaging plate

•  processed in a Digital Reader

•  Signal sent to computer

•  Viewed on a monitor

DR •  transistor receiver (like

bucky)

•  directly into digital signal

•  seen immediately on monitor –

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Active Matrix Array (AMA) Pixels are read sequentially, one at a time

•  Each TFT and detector represents a pixel

•  DEL = charge collecting detector element

DEL collects e-

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DDR only using amorphous selenium (a-Se)

•  The exit x-ray photon interact with the a-Si (detector element/DEL). Photon energy is trapped on detector (signal)

•  The TFT stores the signal until readout, one pixel at a time

Active matrix array of silicon photodiodes

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Image Resolution

Signal Sampling Frequency Good sampling under sampling

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DR

•  Initial expense high •  very low dose to pt – •  image quality of 100s using a 400s

technique •  Therfore ¼ the dose needed to make the

image

Flat Panel TFT Detectors •  Have to be very careful with terminology •  One vendor claims: “Detector has sharpness of 100

speed screen” •  May be true: TFT detectors can have very sharp edges

due to DEL alignment •  But ! •  Spatial resolution is not as good as 100 speed screen. •  TFT detector = 3.4 lp/mm •  100 speed screen = 8 – 10 lp/mm

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TFT Array Detectors

•  Detector is refreshed after exposure •  If no exposures are produced. . . detector

refreshed every 30 – 45 sec •  Built in AEC, An ion chamber between

grid and detector

Patient Dose

•  Important factors that affect patient dose •  DQE: when using CsI systems •  Both systems “fill factor”

– The percentage of the pixel face that contains the x-ray detector.

– Fill factor is approximately 80%

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DDR has all the advantages of CR imaging techniques

•  Post processing & PACS

Questions ?

DR , original and processed

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Damage

due to ionisation

proportional to energy loss

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 risk proportional 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 death LD50 humans: 5 Gy

Dosis 1 Gy

Stocastic Risk

•  ICRP says: 4 -5% / Sievert (Sv) total

risc fatal cancer

Dose

Hiroshima Nagasaki

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A biological relevant measure for energy transfer

•  LET = Linear Energy Transfer. Measured in keV/µm

•  Dose =

Energy deposited per unit mass Measured in Gray (Gy)= J/Kg

x

LET=-dE/dx

D=dE/dM M

Unit of dose

•  Gray ( J/kg)

•  With important ”biological” weight factors linked to Sievert (Sv) (still J/kg)

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X ray doses

•  Single exposure, small area, short path •  -limb, teeth, chest •  few micro Sievert

•  Multiple exposures whole body, low energy to enhance contrast (CT….)

•  several milli Sievert

X –ray doses

Note that this is given as effective dose - In Sievert ! - -Why ?

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Radiation sources (a) •  X-ray (diagnostic)

Dose rate is high ! Dose depends on: • Distance • Area of collimation • HV • mAs • Filtering

Surface dose is always higher than depth dose.

Doses 10-100 µGy per image

Justified? •  Some procedures are not justified

“Acceptability”

Outcome

Dose