chapter 3: x-ray production - wordpress.com

PHYSICS FOR RADIOGRAPHERS 2 (HDR202)

CHAPTER 3:

X-ray ProductionPREPARED BY:MR KAMARUL AMIN BIN ABDULLAH

SCHOOL OF MEDICAL IMAGINGFACULTY OF HEALTH SCIENCES

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CHAPTER 3: X-RAY PRODUCTION

CONTENTS

LEARNING OUTCOMES

At the end of the lesson, the student should be able to:-

Explain the principle of x-ray production.

Explain the Bremsstrahlung and Characteristic radiation.

Explain the X-ray Quality and Intensity including the definition,

measurement, and significance in medical imaging and the factors

affecting them.

Describe the Basic x-ray circuit including each of the components with

their functions.

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CONTENTS

TOPIC OUTLINES

INTRODUCTION

3.1 History of X-ray 3.7 Factors Affecting the X-ray Emission Spectrum

3.2 Properties of X-ray 3.8 X-ray Quantity

3.3 Principle of X-ray Production 3.9 X-ray Quality

3.4 Interactions with Target

3.5 Conditions for X-ray Production

3.6 X-ray Emission Spectrum

3.6.1 Characteristic X-ray Spectrum

3.6.2 Bremsstrahlung X-ray Spectrum

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CONTENTS

3.1 History of X-Ray

In 1895, Wilhelm Roentgen, German

Physicist, was studying high voltage

discharges in vacuum tubes in

Crookes tube, then he noticed

fluorescence of barium

platinocyanide screen lying several

feet from tube end.

These rays where named X-rays

which means invisible penetrating

radiation.

X represent unknown in

mathematics.

Crookes Tube

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CONTENTS

3.1 History of X-Ray

Wilhelm Conrad Roentgen

In 8th November 1895, he has

produced and detected

electromagnetic radiation in a

wavelength range today known as X-

rays or Röntgen rays.

The German physicist won a Nobel

Prize for his discovery of the X ray in

1901.Wilhelm Roentgen, 1895

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CONTENTS

3.2 Properties of X-ray

X-rays are high energy waves, with very short wavelengths, and travel at the

speed of light.

X-rays have no mass (weight) and no charge (neutral). You cannot see x-rays;

they are invisible.

X-rays travel in straight lines; they can not curve around a corner.

An x-ray beam cannot be focused to a point; the x-ray beam diverges

(spreads out) as it travels toward and through the patient. This is similar to a

flashlight beam.

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CONTENTS

3.2 Properties of X-ray

X-rays are differentially absorbed by the materials they pass through. More

dense materials will absorb more x-rays than less dense material (like skin

tissue). This characteristic allows us to see images on an x-ray film.

X-rays will cause certain materials to fluoresce (give off light). We use this

property with intensifying screens used in radiography.

X-rays can be harmful to living tissue. Because of this, you must keep the

number of films taken to the minimum number needed to make a proper

diagnosis.

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CONTENTS

3.3 Principle of X-Ray Production

The x-ray production is caused by the interaction of electrons from the

cathode (filament) with the target anode.

The electrons are emitted by filament will be targeted to the anode. Then,

they will bombard the anode target to produce an x-ray.

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CONTENTS

3.4 Conditions for X-Ray Production

A heated filament, which releases negative electrons by thermionic emission.

A positive anode, which attracts them.

A high tension supply, which accelerates the electrons to very high speeds.

A target (part of the anode), whose job is to force the fast moving electrons

to deviate very rapidly, thus causing x-rays to be emitted.

An x-ray tube must be in vacuum condition to allow the electrons travel in

high speed.

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CONTENTS


There are TWO major interactions in anode target for x-ray production:-

a) Bremsstrahlung Radiation

b) Characteristic Radaition

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CONTENTS


3.5.1 Bremsstrahlung Radiation

Also known as braking radiation or general radiation.

Bremsstrahlung x-rays are produced when high-speed electrons from the

filament are slowed down as they pass close to, or strike, the nuclei of the

target atoms.

The closer the electrons are to the nucleus, the more they will be slowed

down.

The higher the speed of the electrons crossing the target, the higher the

average energy of the x-rays produced.

The electrons may interact with several target atoms before losing all of their

energy.

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CONTENTS


+High-speed

electron from

filament enters

tungsten atom.

Electron slowed

down by positive

charge of nucelus;

energy released in

form of x-ray.

Electron continues on in

different direction to interact

with other atoms until all of its

energy is lost.

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CONTENTS


3.5.2 Characteristic Interactions

Characteristic x-rays are produced when a high-speed electron from the

filament collides with an electron in one of the orbits of a target atom; the

electron is knocked out of its orbit, creating a void (open space).

This space is immediately filled by an electron from an outer orbit.

When the electron drops into the open space, energy is released in the form

of a characteristic x-ray.

The energy of the high-speed electron must be higher than the binding energy

of the target electron with which it interacts in order to eject the target

electron.

Both electrons leave the atom.

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CONTENTS


Characteristic x-rays have energies “characteristic” of the target material.

The energy will equal the difference between the binding energies of the

target electrons involved.

For example, if a K-shell electron is ejected and an L-shell electron drops

into the space, the energy of the x-ray will be equal to the difference in

binding energies between the K- and L-shells. The binding energies are

different for each type of material; it is dependent on the number of protons

in the nucleus (the atomic number).

K-shell

M-shell

L-shell

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CONTENTS


LK

MHigh-speed electron

with at least 70 keV of

energy (must be more

than the binding

energy of k-shell

Tungsten atom) strikes

electron in the K

shell, knocking it out

of its orbit.

Ejected electron

leaves atom.

Recoil electron

(with very little

energy) exits

atom.

vacancy

X-ray with 59

keV of energy

produced. 70

(binding energy

of K-shell

electron) minus

11 (binding

energy of L-

shell electron)

= 59.

Electron in L-shell

drops down to fill

vacancy in K-shell.

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CONTENTS

3.6 X-ray Energy

Characteristic x-rays have very specific energies. K-characteristic x-rays

require a tube potential of a least 70 kVp

Bremsstrahlung x-rays that are produced can have any energy level up to the

set kVp value. Brems can be produced at any projectile e- value

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CONTENTS

3.7 X-ray Spectrum

A display or graph of the intensity of x-rays, produced when electrons strike a

solid object, as a function of wavelengths or some related parameter;

It consists of a continuous bremsstrahlung spectrum on which are

superimposed groups of sharp lines characteristic of the elements in the

target.

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CONTENTS

3.7 X-ray Spectrum

3.7.1 Discrete Spectrum

Contains only specific values.

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CONTENTS

3.7 X-ray Spectrum

3.7.2 Continuous Spectrum

Contains all possible values.

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CONTENTS

3.7 X-ray Spectrum

3.7.3 Characteristic X-ray Spectrum

The discrete energies of characteristic x-rays are characteristic of the

differences between electron binding energies in a particular element.

A characteristic x-ray from tungsten, for example, can have 1 of 15 different

energies and no others.

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CONTENTS

3.7 X-ray Spectrum

A plot of frequency with which

characteristic x-rays are emitted

as a function of their energy.

This is called characteristic x-ray

emission spectrum.

Characteristic x-rays have

precisely fixed (discrete) energies

and form a discrete emission

spectrum.

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CONTENTS

3.7 X-ray Spectrum

3.7.4 Bremsstrahlung X-ray Spectrum

Bremsstrahlung x-rays have a range of energies and form a continuous

emission spectrum.

The general shape of the bremsstrahlung x-ray spectrum is the same for all x-

ray imaging systems.

The maximum energy (in keV) of a bremsstrahlung x-ray is numerically equal

to the kVp operation.

The greatest number of x-rays is emitted with energy approximately one third

of the maximum energy.

The number of x-rays emitted decreases rapidly at very low energies.

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CONTENTS

3.7 X-ray Spectrum

Bremsstrahlung x-ray emission spectrum extends from

zero to maximum projectile electron energy, with the

highest number of x-rays having approximately one-third

the maximum energy. The characteristic x-ray emission

spectrum is represented by a line at 69 kVp.

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CONTENTS

3.7 X-ray Spectrum

3.7.5 Factors Affecting The X-ray Emission Spectrum

There are FOUR factors affecting the shape of x-ray spectrum:-

a) The projectile electrons accelerated from cathode to anode do not all

have peak kinetic energy.

b) The target of a diagnostic x-ray tube that relatively thick causes multiple

interactions of the projectile electrons that it less energy.

c) The low energy x-rays that are more likely to be absorbed in the target.

d) The external filtration that removes low photons energy from the beam.

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CONTENTS

3.8 X-ray Quantity

It is also known as intensity.

It is measured in roentgens (R) or miliroentgens (mR) (mGya) .

Another term used is radiation exposure.

X-ray quantity is the number of x-rays in the useful beam.

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CONTENTS

3.8 X-ray Quantity

3.8.1 Factors Affecting X-ray Quantity

There are 4 main factors affecting it:-

1. mAs

2. kVp

3. Distance

4. Filtration

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CONTENTS

3.8 X-ray Quantity

3.8.1.1 mAs

Miliampere-Seconds

X-ray quantity is directly proportional to the mAs.

When mAs is doubled, the number of electrons striking the tube target is

doubled, and therefore the number of x-rays emitted is doubled.

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CONTENTS

3.8 X-ray Quantity

3.8.1.2 kVp

Kilovoltage Peak

X-ray quantity varies rapidly with changes in kVp.

The change in x-ray quantity is proportional to the square of the ratio of the

kVp

If kVp is doubled, the x-ray intensity would increase by a factor of four.

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CONTENTS

3.8 X-ray Quantity

3.8.1.3 Distance

X-ray intensity varies inversely with the square of the distance from the x-ray

tube target.

This relationship is known as inverse square law.

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CONTENTS

3.8 X-ray Quantity

3.8.1.4 Filtration

X-ray imaging systems have metal filters, usually of 1 to 5 mm of aluminum

(Al), positioned in the useful beam.

The purpose of these filters is to reduce the number of low energy x-rays.

Adding filtration to the useful x-ray beam reduces patient dose because fewer

low-energy x-rays are found in the useful beam.

It reduces the x-ray quantity.

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CONTENTS

3.9 X-ray Quality

It is always related to the penetrability.

As the energy of an x-ray beam is increased, penetrability is also increased.

Penetrability refers to the ability of x-rays to penetrate deeper in tissue.

High energy x-rays are able to penetrate tissue more deeply than low energy

x-rays.

X-rays with high penetrability are termed high quality x-rays.

Those with low penetrability are low quality x-rays.

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CONTENTS

3.9 X-ray Quality

3.9.1 Factors Affecting X-ray Quality

There are two main factors affecting it:-

1. kVp

2. Filtration

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CONTENTS

3.9 X-ray Quality

3.9.1.1 kVp

Kilovolt Peak

As the kVp is increased, so is x-ray beam quality.

An increase in kVp results in a shift of the x-ray emission spectrum toward the

high energy side, indicating an increase in the effective energy of the beam.

The result is a more penetrating x-ray beam.

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CONTENTS

3.9 X-ray Quality

3.9.1.2 Filtration

The primary purpose of adding filtration to an x-ray beam is to remove

selectively low energy x-rays that have little chance of getting to the image

receptor.

It improves the quality of x-ray beam but reduces quantity.

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CONTENTS

3.10 Basic X-ray Circuit

There are two main parts of the circuit, one is the main circuit and the second

is the filament circuit.

A. Main part of X-Ray Circuit: supplies power to the x-ray tube so that x-

rays are produced.

B. Filament Circuit: supplies power to the filament of the x-ray tube so

that the filament supplies enough electrons by thermionic emission.

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In the diagram below are the important parts of the circuit. The blue part is

the main x-ray circuit and the tan part is the filament circuit.

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CONTENTS


1. main breaker - this is where the alternating current comes from to power the circuit.

2. exposure switch - when you push the button to start an exposure this switch closes to

start the exposure.

3. autotransformer - this is where you adjust the kVp for the exposure.

4. timer circuit - this part of the circuit stops the exposure.

5. high-voltage step-up transformer - this transformer bumps the voltage up so that the x-

ray tube has very high voltage to make the electrons have enough energy to form x-rays.

6. four-diode rectification circuit - this makes the current only go in one direction through

the x-ray tube.

7. filament circuit variable resistor - this variable resistor adjusts the current going to the

filament.

8. filament step-down transformer - this transformer steps the voltage down and therefore

the current up.

9. x-ray tube - this is where the x-rays are created.

10. rotor stator - this rotates the anode.

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CONTENTS

2.6 References

No. REFERENCES

1 Ball, J., Moore, A. D., & Turner, S. (2008). Essential physics for

radiographers. Blackwell.

2 Bushong, S. C. (2008). Radiologic science for technologists. Canada:

Elsevier.

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CONTENTS

SUMMARY

The x-ray production is caused by the interaction of electrons from the

cathode (filament) with the target anode.

There are TWO major interactions in anode target for x-ray production:

Characteristic and Bremsstrahlung radiation.

X-ray spectrum: A display or graph of the intensity of x-rays.

X-ray quantity (intensity) is the number of x-rays in the useful beam.

X-ray quality (penetrability) refers to the ability of x-rays to penetrate

deeper in tissue.

There are two main parts of the circuit, one is the main circuit and the second

is the filament circuit.

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CONTENTS

NEXT SESSION PREVIEW


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CONTENTS

APPENDIX

FIGURE SOURCE

Figure 1 http://www.universetoday.com/wp-content/uploads/2009/12/Democritus.jpg

Figure 2

Figure 3 http://3.bp.blogspot.com/_nW9kILlRDjU/THYkY9gTwnI/AAAAAAAAAAM/YFmQh

2QLfw4/s1600/John+Dalton.jpg

Figure 4 http://abyss.uoregon.edu/~js/images/atom_prop.gif

Figure 5 http://upload.wikimedia.org/wikipedia/commons/c/c1/J.J_Thomson.jpg

Figure 6 http://2011period6group4.wikispaces.com/file/view/Thomson's_Model.gif/1684

83477/Thomson's_Model.gif

Figure 7 http://www.vias.org/physics/img/rutherford.jpg

Figure 8 http://i54.tinypic.com/n2l3r9.png

Figure 9 http://abyss.uoregon.edu/~js/images/nbohr.gif

Figure 10 http://cdn.timerime.com/cdn-33/users/13890/media/Atom_diagram.jpg

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CONTENTS

APPENDIX

FIGURE SOURCE

Figure 11

Figure 12 http://1.bp.blogspot.com/_SmG3fxIEtUk/TD7yf3eF7XI/AAAAAAAADKQ/uziSUELf

Bwc/s1600/proton.jpg

Figure 13 http://www.cartage.org.lb/en/themes/sciences/chemistry/generalchemistry/a

tomic/BasicStructure/atmparts.gif

Figure 14 http://www.chemistryland.com/ElementarySchool/BuildingBlocks/NeutronProt

onElectronLight.jpg

Figure 15

Figure 16 http://www.medcyclopaedia.com/upload/book%20of%20radiology/chapter03/ni

c_k3_0.jpg

Figure 17 http://www.boluodusmyportfolio.com/Images/radioactivity2.gif

Figure 18 http://www.universetoday.com/wp-content/uploads/2011/04/Radioactive-

Isotopes.jpg

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CONTENTS

Activity

Define or otherwise identify the following:

a) X-ray Quality Answer

b) X-ray Quantity Answer

c) X-ray Spectrum Answer

Describe how kVp affects x-ray quality.

Answer

Explain the properties of x-ray.

Answer

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