애리조나 대학 아틀라스 xray 강의자료_phys586-lec05-xray

8/2/2019 xray _phys586-lec05-xray

1/60

1

X-rays

Ouch!


2/60

2

X-rays

X-rays are produced when electrons areaccelerated and collide with a target

Bremsstrahlung x-rays

Characteristic x-rays

X-rays are sometimes characterized by thegenerating voltage

0.1-20 kV soft x-rays

20-120 kV diagnostic x-rays

120-300 kV orthovoltage x-rays

300 kV 1 MV intermediate energy x-rays

> 1MV megavoltage x-rays


3/60

3

Bremmstrahlung

Bremsstrahlung x-rays occur whenelectrons are (de)accelerated in theCoulomb field of a nucleus


4/60

4

Bremsstrahlung


5/60

5

Bremsstrahlung

The power radiated from an acceleratingcharge is given by Larmors equation

In the case of an electron in the Coulomb fieldof a nucleus

3

22

3

2

c

aeP

Zmr

Zek

m

Fa ~

2

2


6/60

6

Bremsstrahlung

The probability of bremsstrahlung goes as Z

2

,hence high Z targets are more effective thanlow Z

The energy of the x-rays varies from zero to

the maximum kinetic energy of the electron(x-ray tube kVp)

The energy spectrum from a thick target goesas 1/E but inherent (1mm Al eq) plusadditional (few mm Al) filtration removes thelower energy x-rays

Here I am referring to diagnostic x-rays


7/60

7

Bremsstrahlung

The unfiltered energy spectrum isapproximately given by Kramers law whichwas an early application of quantummechanics

ETKZEI e


8/60

8

Bremsstrahlung


9/60

9

Characteristic x-rays

After excitation, ionswith a vacancy intheir inner shell cande-excite

Radiatively throughx-ray fluorescence

Non-radiativelythrough the emission

of Auger electrons


10/60

10

Characteristic X-raysThus an x-ray spectrum will also show

characteristic x-rays arising from L to K and Mto K transitions after ionization of a K electron

Usually transitions to higher shells

absorbed by the filtration or are not x-rays


11/60

11

Characteristic X-rays

The probability of K shell fluorescenceincreases with Z


12/60

12



13/60

13


Sometimes the characteristic x-rays areemphasized using the same material fortarget and filter

Characteristic x-rays from molybdenum are

effective in maximizing contrast in mammography


14/60

14


Mo target, filter, and result


15/60

15

Directionality

For MeV electrons, bremsstrahlung x-rays are preferentially emitted in theelectrons direction

For keV electrons, bremsstrahlung x-

rays are emitted at larger anglesCharacteristic x-rays are emitted

isotropically since there is no angular

correlation between the incidentelectron that causes the ionization andthe fluorescent photon


16/60

16

X-ray Tube

A simplified x-ray tube (Coolidge type)shows the idea behind most x-ray tubestoday


17/60

17

X-ray Tube

In addition to bremsstrahlung and

characteristic x-ray production, electrons alsoloose energy through collisions

Collision losses dominate in this energy region

For 100 keV electrons in W

Thus >99% of the electron energy goes intoheating the target rather than x-rays

Removing heat from the anode in a vacuum is an

issue

MeVin820losscollisionlossradiation

EEZ

009.0820

741.0

losscollision

lossradiation


18/60

18

X-ray Tube

Efficiency of x-ray production depends on thetube voltage and the target material

W (Z=74) in this example

ZVP

P

IZVP

VIP

deposited

radiated

radiated

deposited

9

29

109.0Efficiency

109.0

kVp

(V)

Heat(%)

X-rays(%)

50 99.7 0.3200 99 1

6000 65 35


19/60

19

X-ray Tube

X-ray tubes


20/60

20

X-ray Tube

More detail


21/60

21

X-ray Tube

Housing for shielding (Pb) and cooling(oil)


22/60

22

X-ray Tube

More detail


23/60

23

X-ray Tube

The main parts of the x-ray tube are Cathode/filament

Typical electron current is 0.1-1.0 A for shortexposures (< 100 ms)

Anode/target

Glass/metal envelope

Accelerating voltage

Typical voltage is 20-150 kVp


24/60

24

Cathode

Cathode consists of Low R tungsten wire for thermionic emission

Tungsten has a high melting point (3370C) and minimumdeposit on the glass tube

Tube current is controlled by varying the filament currentwhich is a few amps

A focusing cup Uses electric field lines to focus the electrons

Typically there are two filaments Long one: higher current, lower resolution

Large focal spot

Short one: lower current, higher resolution Small focal spot


25/60

25

Cathode

Dual focus filament is common


26/60

26

Anode

Usually made of tungsten in copper becauseof high Z and high melting point Molybdenum and rhodium used for soft tissue

imaging

Large rotating surface for heat distribution

and radiative heat loss Rotation of 3k-10k revolutions/minute Resides in a vacuum (~10-6 torr) Thermally decoupled from motor to avoid

overheating of the shaft

Target is at an tilted angle with respect toaxis Bremsstrahlung is emitted at ~ right angles for

low energy electrons

Determines focal spot size


27/60

27

Anode


28/60

28

Anode


29/60

29

AnodeThe heating of the anode limits the voltage,

current, and exposure timeAn exposure rating chart gives these limits


30/60

30

Anode

Power = V x I (watts)

Energy = Power x time = V x I x s(joules)

HU (Heating Unit) ~ J Damaged anodes


31/60

31

AnodeThe angle determines the projected focal spot

The smaller the angle the better the resolution Typically 7-20 degrees

Angle

Incident electronbeam width

Apparent focal spot size

Actual focal

spot size

Film

Angle

Incident electronbeam widthIncreasedapparent

focal spot size

Actual focal

spot size

Film


32/60

32

X-rays

The energy of the photons depends onthe electron energy (kVp) and thetarget atomic number Z

The number of photons depends on thethe electron energy (kVp), Z, and thebeam current (mA)

A typical number / area is ~ 1013 / m2

About 1% will hit the film ~ 1011 / m2

Absorption and detection efficiency willfurther reduce this number


33/60

33

Automatic Exposure Control

AEC detectors can ionization chambers or solid-state detectors

X Ray tube

Collimator

Beam

Softtissue

BoneAir

Patient

Table

Grid

Cassette

AEC detectors


34/60

34

Automatic Exposure Control

Most modern x-rays machines are equippedwith automatic exposure control also called aphototime

The AEC sets the technical parameters of the

machine (kV, mA, time, ) in order to avoidrepeated exposures

AEC is used to keep the radiographic quality(film density) equal on all patients

AEC detectors can be ionization chambers orsolid state detectors


35/60

35

Grid

To reduce the number of secondaryscattered photons making it to the film,a grid between the patient and film isused


36/60

36

GridDetails

Grid bars are usually lead whereas the gridopenings are usually made of aluminum orcarbon

Grid thickness is typically 3 mm

Grid ratio is H/W and 10/1 is typical Grid frequency of 60 lines / cm is typical

B/W/H on the figure might be 0.045, 0.120,1.20 in mm

The Bucky factor is the entrance exposurew/wo the grid while achieving the samefilm density 4 is average


37/60

37

Accelerating Voltage

The potential difference betweencathode and anode must be generatedby 60 Hz 220V AC power

High voltages are produced using atransformer


38/60

38


Electrons are accelerated when thefilament is at a negative potential withrespect to the target

Diode circuits can be used to providerectification (AC to DC voltage)

Three phase power (6 pulse or 12 pulse)can be used to reduce ripple

Constant potential operation can beachieved by using constant potential(voltage regulations) or high frequency x-ray generators


39/60

39

Half-wave Rectifier

Not very efficient


40/60

40

Full-wave Bridge Rectifier

This circuit allows the entire inputwaveform to be used


41/60

41


100%

13%

4%

Line voltage

Single phase single pulse

Single phase 2-pulse

Three phase 6-pulse

Three phase 12-pulse

0.02 s

0.01 s

kV ripple (%)


42/60

42

ImagesAnalog radiography

Film based still widely used Fluorescent screens are used to convert x-rays into

visible light that is then recorded on film

Screens are more efficient at stopping x-rays than

the film (CaWO4 or Gd2O2S:Tb or other rare earth)


43/60

43

Analog Radiography

The film itself has excellent spatialresolution but

Film detects 0.65% of incident x-rayenergy

Gd2O2S detects 29.5% of incident x-rayenergy

Thus using phosphor screens greatly

reduces the radiation dose to thepatient

And also reduces load on the x-ray tube


44/60

44

Analog Radiography

There are two efficiency considerations

Absorption efficiency or QDE

Fraction of incident x-rays that interactwith the screen

Depends on kVp and screen thickness

Gd2O2S has a QDE of ~ 60% for 80 kVp beam,20 cm patient, 120 mg/cm2 screen thickness


45/60

45

Analog Radiography

Conversion efficiency Fraction of absorbed x-ray energy that is

emitted as light

5% for CaWO4

15% for Gd2O2S

50,000 eV x 0.15 = 7500 eV

7500 eV / 2.7 eV = 2800 photons produced per

absorbed x-ray 50-90% reduction in photon diffusion to film


46/60

46

Analog Radiography

Film is an emulsion containing silver-halide grains (AgBr and AgI) coated onmylar

Body

Film

X-Raysource

DarkLight


47/60

47

Analog Radiography


48/60

48

Film Badge

A film badge consists of a photographic film

with various filtersThe film is a gelatin emulsion containing silver-

halide grains (95% AgBr and 5% AgI) on a

supporting material Grain diameter is ~ 1mm


49/60

49

Film Badge

The film is exposed by light by An electron is released from Br- and moves about

the 1m diameter crystal

The electron may be captured by a trap such as acrystal imperfection or AgS speck

The trapped electron attracts mobile Ag+ ionswhere it is subsequently neutralized

Additional Ag atoms are formed by repeatedtrapping and neutralization

These Ag atoms are called a latent image center The developing process effectively amplifies this

process turning the grains with latent imagecenters into a visible silver deposit


50/60

50

Film Badge


51/60

51

Film Badge

Silver atoms at latent image centers


52/60

52

Images

Digital radiography

Detector based


53/60

53

Digitial Radiography

CCD systems CCD systems use a scintillator like

gadolinium disulphide to convert x-rays tovisible light

Light is collected by optics to demagnifythe 35x45cm2 film to 2-4 cm2 CCD

Well talk about CCDs much later in the

course but essentially visible light isconverted into charge that is amplified andreadout

A negative is the thickness of the detector

system because of the optical system


54/60

54

Digital Radiography

Indirect or direct conversion thin-filmtransistor (TFT) arraysAlso called FPD (flat panel detectors)

Well cover these later in the course as well

probably through a student talk The idea is that charge proportional to the

x-rays received is stored on a capacitor

The charges are conducted out by

transistors one row at a time andsubsequently amplified, multiplexed, anddigitized

The readout is very fast


55/60

55

Digital Radiography

Indirect or direct conversion thin-filmtransistor (TFT) arrays Indirect conversion uses a scintillator layer

(like CsI:Tl) to convert x-rays to visible

light and amorphous silicon photodiodes toconvert visible light into charge

Direct conversion uses an x-rayphotoconductor layer (usually amorphous

selenium) to convert x-rays to chargeAn applied electric field directs the charges

to the charge collection electrodes


56/60

56

Digital Radiography


57/60

57

Digital Radiography


58/60

58

Digital Radiography

Readout


59/60

59

Images

Digital radiography

The battle over image quality, however,may be incomprehensible to anyonewithout a background in high-energy

physics.


60/60

X-rays

For bone tissue, the linear attenuationcoefficient is much greater than that forsoft body tissue

애리조나 대학 아틀라스 xray 강의자료_phys586-lec05-xray

Documents