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Chapter 7 Quality of X-Ray Beams

Half-value layer is used to describe the quality of a beam, the ability to penetrate materials of known composition.

It is used for bremsstrahlung photon beams, which has a spectrum of energies, but not for –ray beams, whose energy is discrete and unique (for example, Co-60 has energies 1.17 & 1.33 MeV photons).

The half-value layer is defined as the thickness of an absorber of specified composition required to attenuate the intensity of the beam to half its original value.

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7.1 Half-Value Layer

For low-energy x-ray beams, beam quality is described in terms of HVL together with kVp. This is because the beam is usually further filtered.

For mega-voltage x-ray beams, the quality is specified by the peak energy alone (e.g. 6-MV). This is because the beam is already filtered through the target and flattening filter so that no additional filtration is used.

(Sometimes TMR ratio such as is also used.)

The average energy is approximately 1/3 of its peak energy.

)1010,10(

)1010,20(2

220

10 cmcmdTMR

cmcmdTMRTMR

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7.2 Filters

Photon energy (keV)

Ene

rgy

flue

nce

per

ener

gy in

terv

alK-characteristic radiation of Tungsten (58-69 keV)

Al (inherent) filtration

Al + Sn + Cu filtration

Al + Sn filtrationK-edge of Tin (29 keV)

50 100 150 200

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7.2 Filters (cont’d)

Thoraeus filters (combination filters) used with orthovoltage x-rays

Filter target (tungsten) composition

Thoraeus I 0.2 mm Sn + 0.25 mm Cu + 1 mm Al

Thoraeus II 0.4 mm Sn + 0.25 mm Cu + 1 mm Al

Thoraeus III 0.6 mm Sn + 0.25 mm Cu + 1 mm Al

Note the order is important:

W (Z=74), Sn (Z=50), Cu (Z=29), Al (Z=13)

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• For cesium and cobalt machines (-rays), filters are not needed because the beams are almost monoenergetic (thus no preferential attenuation by the filters).

• For megavoltage x-ray beams, the beam is hardened by the target (transmission type) and the flattening filter, thus no additional filtration is needed.

7.2 Filters (cont’d)

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7.3 Measurement of Beam Quality Parameters (HVL)

Incident photon fluence

transmitted photon fluence

scattered photons

detector

collimator

1

10

100

0 1 2 3 4 5

Absorber thickness (mm Al)

50

6

50

25

12.5

3rd HVL2nd HVL1st HVL

Tra

nsm

itted

inte

nsity

(%

)

narrow beam ‘good’ geometry

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7.3 Measurement of Beam Quality Parameters (peak voltage)

Direct Methods

R

I

V = R I

Vtot = n×V

Voltage divider

d

Sphere gap

RR R• • •

n

8

  Sphere Diameter (cm)

gap (cm) 5 10 15 25 50 100 200

0.5 17.5 (kv) 16.9 16.5        

1 32.2 31.6 31.3 31      

1.5 46.1 45.8 45.5 45      

2 58.3 59.3 59.2 59      

2.5 69.4 72.4 72.9 73      

3 79.3 84.9 85.8 86      

4   107 111 113 112    

5   128 134 138 138 137 137

7.3 Measurement of Beam Quality Parameters (peak voltage) – sphere-gap method

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7.3 Measurement of Beam Quality Parameters (peak voltage)

Indirect Methods (fluorescent method)

X-ray source

Chamber 1

Chamber 2

filter

attenuatorTube voltage

Tra

nsm

itted

/sc

atte

red

radi

atio

n

K-edge of the attenuator = kVp

Photon energy

atte

nuat

ion

coef

fici

ent

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7.3 Measurement of Beam Quality Parameters (effective energy)

X-ray beams are always heterogeneous because they are produced by bremsstrahlung. The effective energy of an x-ray beam is the energy of a monoenergetic beam that has the same attenuation coefficient or HVL.

(for illustration only)

HVL

693.0

1 2 3 4 5

HVL(mm)

20

40

60

80

100

120

140

160

180

aluminum

copper

effe

ctiv

e en

ergy

(k

eV)

energy

atte

nuat

ion

coef

fici

ent

aluminum

copper

11

10 20 30 40

20

10

0

30

Peak photon energy (MeV)

Hal

f-va

lue

laye

r (m

m P

b or

mm

H2O

)

water

lead

Half-value layer in lead is not the best choice to express beam quality for megavoltage beams. Low atomic number materials such as water are more sensitive to changes in spectral quality in megavoltage x-rays.

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7.3 Measurement of Beam Quality Parameters (mean energy)

The mean energy of a beam may be calculated in two different ways:

max

0

max

0

max

0

max

0

E

E

E

E

E

E

E

E

dE

EdE

dE

EdE

E

E

or

E = photon fluence

E = photon energy fluence

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7.4 Measurement of Megavoltage Beam Energy

The maximum (peak) photon energy can be estimated by measurements such as percent depth dose, tissue air ratios, or tissue maximum ratios. But these methods are relatively insensitive to the maximum energy.

Alternatively, it can also be measured with the photoactivation ratio method. The procedure involves photoactivation of a pair of foils which react differently to different energies.

As the maximum energy itself is not the most important factor for beam quality, it is not routinely measured in the clinic.

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7.5 Measurement of Energy Spectrum

Energy spectrum is needed for certain applications such as accurate dose calculations. It can be measured with a scintillation spectrometer.

Photon energyEne

rgy

flue

nce

per

ener

gy in

terv

al X-ray source

photomultiplier

Pulse height analyzer

scaler

Scintillation crystal

Lead shield