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X-ray Imaging

H. Ghadiri -PhD 1

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X-ray imaging chain

x-ray Production

x-rayAttenuation

x-ray Detection

• In transmission-based imaging, it is requires that the energy

penetrates the bodys tissues and interacts !ith them.• I" the tra#ersed energy experiences some type o" interaction $e.g.,absorption or scattering%, then the detected energy !ou&d containuse"u& in"ormation regarding the interna& anatomy, and thus it !ou&dbe possib&e to construct an image o" the anatomy using thatin"ormation.

•  'here is a genera& trend in the a&& x-ray imaging machines( X-rayProduction, X-ray Attenuation, and X-ray Detection.

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1X-ray Production

)

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X-ray Production

X-rays are produced !hen high&y energetice&ectrons interact !ith matter, con#erting some o"their *inetic energy into e&ectromagnetic radiation

+

 'hree essentia& conditions "or the production o" x-rays(1. source o" "ree e&ectrons.

. acce&erating the e&ectrons to extreme

speeds.

). sudden&y dece&erating the e&ectrons.

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X-ray Production

• &ectron source/athode

•  'argetAnode 

• #acuated path "or the e-s to tra#e& through0acuumed insert

• xterna& energy source to acce&erate the e-s H0 Generator

A de#ice that produces x-rays in the diagnostic energy range typica&&ycontains an e&ectron source, an e#acuated path "or e&ectronacce&eration, a target e&ectrode, and an externa& po!er source topro#ide a high #o&tage $potentia& dierence% to acce&erate thee&ectrons.

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- Cathode: source o" "ree e&ectrons(ermionic emission "rom heated 2&ament $34445 /%

6pace charge ore&ectron c&oud"ormed around

the 2&ament

X-ray Production

 'he numbers o"e&ectrons that are

a#ai&ab&e are ad7ustedby the 2&ament currentand 2&amenttemperature

•  'he cathode is the negati#e

e&ectrode in the x-ray tube,comprised o" a flament or2&aments and a ocusing cup.

• A 2&ament is made o" tungsten !ire!ound in a he&ix, and is e&ectrica&&yconnected to the 2&ament circuit,

!hich pro#ides a #o&tage o"approximate&y 14 0.

• 8hen energi9ed, the 2&amentcircuit heats the 2&ament throughe&ectrica& resistance, and theprocess o" thermionic emission

re&eases e&ectrons "rom the2&ament sur"ace at a rate

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X-ray Production

:

- Cathode: source o" "ree e&ectrons(ermionic emission "rom heated 2&ament $34445 /%

• In most x-ray tubes, the "ocusing

cup is maintained at the samepotentia& dierence as the 2&amentre&ati#e to the anode, and at theedge o" the s&ot, an e&ectric 2e&dexists that repe&s and shapes thec&oud o" emitted e&ectrons "rom the

2&ament sur"ace.

• As a &arge #o&tage is app&iedbet!een the cathode and anode inthe correct po&arity, e&ectrons areacce&erated into a tight distribution

and tra#e& to the anode, stri*ing asma&& area ca&&ed the "oca& spot.

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- Anode: 'he anode is a meta& targete&ectrode that is maintained at a &argepositi#e potentia& dierence re&ati#e to thecathode

;< 0

-<0

6peed = />

/athode

Anode

X-ray Production

• Acce&erating the e&ectrons toextreme speeds( !ith extreme&yhigh-#o&tage app&ied.

• &ectrons stri*ing the anodedeposit most o" their energy as

heat, !ith on&y a sma&& "ractionemitted as x-rays•  'ungsten $8, ? @ :+% is the most

!ide&y used anode materia&because o" its high me&ting pointand high atomic number.

• A tungsten anode can hand&esubstantia& heat deposition !ithout

;<0

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- 6udden&y dece&erating the e&ectrons.

• meta&s !ith #ery high atomic-numbers

• sudden&y dece&erating e&ectrons

• radiation o" e&ectromagnetic !a#es(o in"rared !a#e $heat%o #isib&e &ighto u&tra#io&et !a#es

o x-rays $4.B to 1B%

$3CCB%

Anode

X-ray Production

X-rays are a "orm o" e&ectromagneticradiation, as are radio !a#es, in"raredradiation, #isib&e &ight, u&tra#io&etradiation and micro!a#es. ne o" themost common and bene2cia& uses o"

X-rays is "or medica& imaging. X-raysare a&so used in treating cancer and in

&ectrons stri*ing the anodedeposit most o" theirenergy as heat, !ith on&y a

sma&& "raction emitted as x-rays

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• &ectron interactions !ith the anode $target%produce(

 – Heat  the *inetic energy $<% o" the e&ectron depositsits energy in the "orm o" heat $3CCB%

 – X-ray  n&y 31B o" the e&ectron interactions resu&t inx-rays production

• Fremsstrah&ung  continuous energy spectrum – /ou&omb interactions

• /haracteristic x-rays  discrete energies – Incident e- co&&ision !ith orbita& e- $< incident e-  F orbita& e-%

X-ray Production

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 – nergy o" the e-s is expressed in *e0 $*i&oe&ectron #o&t%

 – 'he < o" the e- is proportiona& to *0p $pea**i&o#o&tage%

X-ray Production$bra*ing radiation%

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X-ray Production$bra*ing radiation%

• /ontinuous spectrum o" radiation isproduced by abrupt dece&eration o"

charged partic&e.

• Dece&eration is caused by deection o"

e&ectrons in the /ou&omb 2e&d o" the

nuc&ei

• ost o" the energy is con#erted intoheat, J31B is x-ray

•  'he energy o" the generated xrayphoton is gi#en by energy conser#ation

•  'he maximum energy "or the producedphoton is gi#en by app&ied H0

e-

Kuc&eus

h ν3

h ν2

h ν1

h ν1< h ν

2 < h ν

3

h ν3 =maximum energy

$Impact !ith nuc&eo&us%

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X-ray Production$bra*ing radiation%

• The x-ray energy depends on the interaction distance between the electron and the

nucleus, it decreases as the distance increases.• 6o, 'he amount o" energy &ost by the e&ectron and thus the energy o" the

resu&ting x-ray are determined by the distance bet!een the incident e&ectronand the target nuc&eus, since the /ou&ombic "orce is proportiona& to thein#erse o" the square o" the distance.

• $electron no. 3%@At re&ati#e&y &arge distances "rom the nuc&eus, the/ou&ombic attraction is !ea*L these encounters produce &o! x-ray energies.

• $electron no. 2% At c&oser interaction distances, the "orce acting on thee&ectron increases, causing a greater dece&erationL these encounters producehigher x-ray energies.

• $electron no. 1% A near&y direct impact o" an e&ectron !ith the targetnuc&eus resu&ts in &oss o" near&y a&& o" the e&ectrons *inetic energy. In thisrare situation, the highest x-ray energies are produced.

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probabi&ity o" passing !ithin the inner #o&ume ( B, $higher x-ray energy%

probabi&ity o" a passing !ithin the outer #o&ume ( 4B. $&o!er

X-ray Production$bra*ing radiation%

max

As a resu&t(

 'he un2&tered energy spectrum "rombra*ing radiation is polychromatic $mu&ti-energy%.

Direct impact on the nuc&eus $rareste#ent% determines the maximum x-ray $

max%

 'he spectrum ↓ approximate&y&inear&y as photon ↑ due to thehigher probabi&ity o" a &arge impactparameter distance

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Mi&ters(1. anode itse&",. g&ass !indo! or bery&&ium

!indo!). surrounding oi&+. added a&uminum 2&ter

bremsstrah&ung x-ray beamspectrum

!ith no 2&tration

 'ypica& bremsstrah&ung x-ray spectrum !ith

2&tration

X-ray Production$bra*ing radiation%

Mi&tration in thiscontext re"ers to theremo#a& o" x-rays byattenuation inmateria&s that areinherent in the x-ray

tube

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• In addition to the continuous bremsstrah&ung x-rayspectrum, discrete x-ray energy pea*s ca&&edNcharacteristic radiationO can be present

• &ectrons in an atom are distributed in she&&s, each

o" !hich has an e&ectron binding energy. 'heinnermost she&& is designated the < she&& and has thehighest e&ectron binding energy, "o&&o!ed by the ,, and K she&&s, !ith progressi#e&y &ess binding

energy

X-ray Production$/haracteristic radiation%

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•  'he e&ectron binding energies are NcharacteristicOo" the e&ements.

1:

X-ray Production$/haracteristic radiation%

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K

L

M

• An empty ho&e in a she&& is 2&&ed bye&ectron "rom outer she&& !ith an emissiono" characteristic radiation.

• discrete energyh ν=EK - EL

X-ray Production$/haracteristic radiation%

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/haracteristic narro!&ines o" intense x-ray aresuperimposed on thecontinuousbremsstrah&ungspectrum.

X-ray Production$/haracteristic and Fra*ing radiation%

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x-ray Tubes

•  'he x-ray tube pro#ides an en#ironment "or theproduction o" bremsstrah&ung and characteristicx-rays.

• a7or tube components are the cathode, anode,rotor/stator, glass or metal envelope, tube port,cable sockets, and tube housing

• Pro#iding( mA, *0

• 1 mA @ Q.+x141A e&ectrons>s

4

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Mactors Aecting X-ray mission

Ruantity @ number o" x-rays in beam  ?target S $*0p%

 S mAs

Rua&ity @ penetrabi&ity o" x-ray beam anddepends on(*0p

tube 2&tration $mm A&%

xposure depends on both quantity andqua&ity/hanges in *0p can be compensated by changes

in mAs to maintain the same exposure(

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• X-ray beam qua&ity – Penetrabi&ity

 – 6pectrum energy

 –  H0

• X-ray beam quantity – Intensity

 – Kumber o" photons

• xposure

 – nergy o! – Ruantity ; Rua&ity

Mactors Aecting X-ray mission

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2X-ray Attenuation

)