Production of X-rays

Please answer all questions True or False. There is

no negative marking.

1. The cathode of the X-ray tube

a. Is commonly made of tungsten

b. Has a high melting point

c. Has a low resistance

d. Is positively charged in relation to the anode

e. Can exceed temperatures of 2200°C

The cathode of a typical X-ray tube is commonly made of a coiled wire

of tungsten. This metal is chosen because it has a high melting point and

low vapour pressure (few electrons are evaporated off the surface). The

process of producing electrons is called Thermionic Emission. Tungsten

has a relatively high resistance and this is needed to reach the high

temperatures required to achieve thermionic emission. The temperature

can exceed 2200°C. The cathode is negatively charged in relation to the

anode.

1a. True – the cathode is commonly made from a coiled wire of

tungsten

1b. True – it must have a high melting point

1c. False – the resistance needs to be high

1d. False – the cathode is negatively charged

1e. True – the temperature can exceed 2200°C

2. Concerning the anode

a. It is positively charged relative to the cathode

b. Molybdenum is used in most X-ray tubes

c. Rhodium is often added to tungsten to reduce pitting and

cracking that can be caused by overheating

d. The distance between the cathode and anode affects the

quality of the X-rays produced

e. 90% of the energy from the electrons striking the anode is

dissipated as heat

The anode in most X-ray tubes is composed of a tungsten/rhenium alloy.

The rhenium is added to prevent pitting and cracking of the anode caused

by the high temperatures, which would decrease the life expectancy of

the tube, the effi ciency of X-ray production and the quality of the beam.

Molybdenum is used in mammography tubes. The distance between the

anode and cathode has no effect on the quality of the beam. 99% of the

energy is dissipated as heat and only 1% is converted to X-rays.

2a. True – the anode is positively charged in relation to the cathode

2b. False – the anode is a tungsten/rhenium alloy in most tubes

2c. False – rhenium is added

2d. False – this has no effect

2e. False – 99% is dissipated as heat

3. The X-ray tube

a. Envelope is housed in oil to help lubricate the motor

bearings

b. Envelope contains a vacuum

c. Housing does not contain shielding to prevent radiation

leakage

d. Envelope is generally constructed from thick lead

e. Housing oil can be used as a temperature sensitive switch

The X-ray tube envelope is commonly made of thick walled glass. In it

are the cathode, anode and vacuum. Metal envelopes are also available.

The vacuum is needed to prevent the electrons produced at the cathode

from striking gas molecules as they travel towards the anode. This would

signifi cantly impede the effi ciency of the X-ray tube. The housing has

several functions that include the following:

Ó Lead-lined and so shields against leakage/stray radiation

Ó Allows the mounting of the envelope and the tube itself

Ó Electrical insulation by means of oil

Ó Provides electrical contact terminals

Ó Contains a window through which the X-rays pass after being produced.

Some attenuation occurs at this point

The oil that is found in the housing serves several purposes:

Ó Electrical insulation

Ó Dissipation of heat via convection

Ó It can be used as a switch. As the heat is absorbed by the oil it will

expand. This expansion will continue with further heating. A switch –

Expansion Diaphragm – can be triggered when the oil has expanded

a certain amount and so can prevent over-heating of the system.

3a. False – the oil is not used to lubricate the bearings

3b. True – this is needed to prevent the electrons from striking gas

molecules

3c. False – contains a lead lining to act as shielding

3d. False – the envelope is commonly thick walled glass

3e. True – the housing oil can be used as a temperature dependant

switch

4. Regarding the X-ray tube fi lament

a. Typically, it has a current of 4A

b. It has a typical power dissipation of 40W

c. There is a surrounding space charge of positive charge that

repels the electrons away from the cathode and towards the

anode

d. Tube fi lament current and tube current are the same

e. It is surrounded by a focusing cup

The cathode fi lament is typically made of tungsten, with a current of 4A

and voltage of 10V. It therefore has a typical power dissipation of 40W.

Two fi laments may be found within the same X-ray tube and these can be

used to produce different-sized focal spots. The fi lament has a focusing

cup which acts like a lens to target the electrons onto a small spot on the

anode. If two fi laments are present then two focusing cups are needed.

The tube fi lament current and the tube current are NOT the same. The

fi lament current is that present across the fi lament itself. The tube current

is the current fl owing from anode to cathode.

Surrounding the fi lament is the space charge. This is a cloud of negatively

charged electrons that act to prevent the further emission of electrons.

As a result of this space charge, below a certain tube voltage electrons

will not travel from the cathode to the anode and so no X-rays will be

produced – a ‘space charge limited’ operation. Hence a certain minimum

operating voltage is required.

4a. True – this is a typical current

4b. True – this is the typical power dissipation

4c. False – the space charge is with negatively charged electrons

4d. False – these are not the same. Tube fi lament current is the current

across the fi lament, and tube current is the current between anode and

cathode

4e. True – the focusing cup acts as a lens to target the electrons

5. Rotating anodes

a. Use induction motors

b. Are mounted on rods with a high conductivity

c. Use oil-lubricated bearings to minimise heat conduction

d. Have an anode diameter of approximately 10 cm

e. Increase the area over which heat is dissipated

Rotating anodes are now commonly used in X-ray tubes. They allow a

much larger area to be bombarded by electrons because the disc is rotating.

This increase in the area of bombardment also increases the area of heat

loss, that in turn results in an increase in the Tube Rating. To maintain

the vacuum in the envelope the motors use electrical induction to rotate.

Silver, never oil, is often used as the lubricant for the bearings. To prevent

the bearings from overheating and hence seizing up, the anode stem must

be made of a low-conductivity metal such as molybdenum.

5a. True – induction motors are used in rotating anodes

5b. False – they are mounted on low conductivity rods

5c. False – oil is not used to lubricate the bearings

5d. True – this is a common size for a rotating anode

5e. True – rotating the anode increases the area over which electrons

strike it and hence generates heat

6. The following focal spot size and clinical application are

correctly paired

a. 0.01 to 0.015 mm for magnifi cation mammography

b. 0.1–0.15 mm for magnifi cation mammography

c. 0.6 mm for fl uoroscopy

d. 0.3 mm for magnifi cation radiography

e. 1.2 mm for conventional radiography with small focal

spot

The following focal spot sizes and applications are found:

Ó 0.1–0.15 mm magnifi cation mammography

Ó 0.3 mm mammography & magnifi cation radiology

Ó 0.6 mm conventional radiography (small focal spot) &

fl uoroscopy

Ó 1.2 mm conventional radiography (large focal spot)

6a. False – 0.1–0.15 mm

6b. True – 0.1–0.15 mm

6c. True – 0.6 mm

6d. True – 0.3 mm

6e. False – 0.6 mm

7. Concerning the actual focal spot

a. It is the area on the anode over which heat is produced

b. Increasing the target angle will decrease the actual focal

spot

c. Increasing mA decreases the effectiveness of focusing

d. It can be measured using a pinhole camera technique

e. Increasing the actual focal spot decreases the tube loading

The Actual Focal Spot is the area on the anode over which heat is produced.

It determines the tube rating. Since a larger actual focal spot can dissipate

heat over a larger area the tube rating will be higher. However, this will

alter the Effective Focal Spot and hence unsharpness etc. Increasing the

target angle will increase the actual focal spot size. Increasing the mA

will decrease the effectiveness of focusing. This is called Blooming, and

it is caused by the electrostatic repulsion of the electrons. This effect will

cause an increase in the focal spot size. Focal spot sizes can be measured

using pinhole cameras, star or bar test patterns and slit cameras.

7a. True – this is the defi nition

7b. False – increasing the target angle increases the actual focal spot

sizec.

7c. True – this is called Blooming

7d. True – this is one way to test the actual focal spot size

7e. False – increasing the focal spot size increases the tube loading

8. Concerning the Target Angle

a. It is generally between 70 and 200

b. It is the angle between the target surface and the central

beam

c. Steeper angles result in larger fi elds of coverage

d. Mammography uses a steep target angle

e. Smaller target angles improve fi ne detail imaging

The target angle is the angle between the target surface and the central

beam. It is usually between 7° and 20°. A steeper target angle (smaller

angle) results in a narrower effective focal spot. This means a smaller

fi eld of coverage but improvement in the geometrical unsharpness.

Mammography requires fi ne detail imaging and only needs a narrow fi eld

of view; therefore it is suited to steeper target angles.

8a. True – it is normally between 7° and 20°

8b. True – this is how the target angle is measured

8c. False – as steeper angle means a narrower effective focal spot and

smaller fi eld of coverage

8d. True – mammography does use a steep target angle

8e. True – by improving geometrical unsharpness

9. In diagnostic X-ray tubes

a. The rotating anode may rotate at up to 10000 rpm

b. Convection currents are set up in the housing oil

c. Conduction is the primary method of heat dissipation

d. 10% of the electrical energy used is converted to X-rays

e. An ionisation chamber is fi tted between the anode and

cathode

In modern X-ray tubes with rotating anodes the anode may rotate at

between 3000 and 10000 rpm. This allows the heat produced at the target

to be spread over a larger area. Heat generated at the target surface must

be dissipated. The three ways that heat can be dissipated are radiation,

convection and conduction. All three methods are used within the X-ray

tube. Heat radiation to the insulating oil is the main method for heat

dissipation. This is aided by blackening the anode surface. This loss of

heat by radiation increases as the temperature of the anode increases.

Heat loss is proportional to Temp 4 (Kelvin). This heat is then transferred

by convection to the tube housing. Conduction also occurs within the

anode. Some of this heat is conducted along the anode stem. This must

be kept to a minimum to prevent the rotors from seizing, hence poorly

conducting materials are chosen for the stem. Ionisation chambers are

not fi tted between the anode and cathode. Only 1% of the energy is

converted to X-rays.

9a. True – it may rotate between 3000–10,000 rpm

9b. True – convection currents fl ow in the oil

9c. False – radiation is the main method for heat dissipation

9d. False – 1% of the electrical energy used is converted into X-rays

9e. False – there is a vacuum between the anode and cathode

10. With regard to voltage waveform rectifi cation

a. It converts DC to AC

b. It uses capacitors in parallel for rectifi cation of the current

c. Rectifi cation has no effect on X-ray beam quality

d. Full wave rectifi cation requires a minimum of 4 diodes

e. Unrectifi ed current can produce X-rays

The current from the national grid is Alternating Current (AC) – it fl ows

both ways. Rectifi cation is the process by which AC is converted to Direct

Current (DC), in which the current fl ows one way. This is achieved by

using diodes. At least 4 diodes are needed to achieve full wave rectifi cation.

Whilst unrectifi ed current can produce X-rays, it is highly ineffi cient.

Rectifi cation will affect the quality of the beam.

10a. False – it converts AC to DC

10b. False – it uses diodes not capacitors

10c. False – rectifi cation affects the beam quality

10d. True – full wave rectifi cation needs at least 4 diodes

10e. True – unrectifi ed current can produce X-rays

11 Regarding the X-ray beam quality

a. X-ray beams are monochromatic

b. Quality is unaffected by the tube kVp

c. Increasing peak kVp increases mean beam energy

d. Decreasing voltage waveform ripple improves beam

quality

e. Filtration has no effect on beam quality

X-ray beam quality relates to the effective photon energy of the X-rays.

The effective X-ray energy is an average of the photon energies of a typical

polychromatic X-ray beam, and it is typically one third to one half of the

maximum photon energy. Things that will affect the quality of the beam

are those that will affect the mean energy. Hence:

Ó Increasing kVp increases the beam quality

Ó Reducing voltage waveform ripple will increase the average energy of

the X-ray photons. This will increase beam quality. For a three-phase

12-pulse system the ripple is approximately 4%. For a three-phase

6-pulse system the ripple is approximately. 14%. So the 12-pulse

system will have increased beam quality.

Ó Appropriate tube fi ltration should selectively remove low energy

photons from the beam as these generally contribute little to the image

and increase patient dose. Removing the lower energy photons will

increase the mean energy of the beam and hence increase the quality.

This is known as Beam Hardening.

Increasing the beam quality will increase the penetrating power of the

beam. This equates to an increase in the Half-Value Layer (HVL) of a

material. Increasing the beam quality will also reduce patient dose since

fewer lower energy Bremsstrahlung X-rays are produced, which contribute

more to patient absorbed dose than they do to image formation.

11a. False – X-ray beams are polychromatic

11b. False – increasing the kVp increases the beam quality

11c. True – increasing peak kVp increases the mean beam energy

11d. True – reducing the voltage ripple increases average beam energy

and improves beam quality

11e. False – appropriate fi ltration increases beam quality

12. X-ray beam intensity

a. Refers only to the number of photons produced

b. Is approximately proportional to kV2

c. Is directly proportional to mAs

d. Is unaffected by beam fi ltration

e. Is unaffected by voltage waveform

If a point source produces photons, then the number of photons from

that source passing through unit area is called the Photon Fluence. These

photons will all have varying energies and the sum of these energies

per unit area is called Energy Fluence. The beam intensity is the energy

fl uence per unit time

Intensity = No. of photons per unit area ¥ mean photon energy

Unit time

Beam intensity is directly related to the tube current (the number of

electrons fl owing from the cathode to the anode) and the exposure time;

therefore beam intensity is directly related to mAs. It is approximately

proportional to kV2 but at lower voltages it is closer to kV3. Since fi ltration

will absorb some of the photons the intensity will decrease. Other factors

that will affect the intensity include the voltage waveform (increased

intensity with lower waveform ripple) and distance from the anode focal

spot (Inverse Square Law).

12a. False – it is energy fl uence per unit time

12b. True – it is approximately equal to kV2

12c. True – beam intensity is directly proportional to mAs

12d. False – it will be affected by fi ltration

12e. False – lower voltage waveform ripple increases intensity

13. A three-phase generator has the following advantages over a

single-phase generator

a. It has a lower voltage waveform ripple

b. Reduced dose to the patient

c. Longer exposure times are needed for same film

blackening

d. Higher peak voltage

e. Does not require rectifi cation

Three phase generators receive current from three different lines. These

current sources are 1200 out of phase with each other. Rectifi cation of

this supply will give either a 6-pulse or a 12-pulse voltage waveform

(dependent on the degree of rectifi cation that is performed); the 6-pulse

having 14% voltage waveform ripple and the 12-pulse having 4% voltage

waveform ripple. A single-phase supply has 100% ripple after rectifi cation.

Since the three-phase supply provides a much more constant supply, the

quality of the X-ray beam produced is increased. This means that shorter

exposure times are needed for three-phase supplies, the patient dose is

reduced and the penetrating power of the beam is increased.

13a. True – this is an advantage over a single phase supply

13b. True – improved beam penetration and shorter exposure times

13c. False – shorter exposure times are needed

13d. False – if the same voltage is used for both systems the peak voltage

will not be different

13e. False – rectifi cation is required

14. Concerning X-ray beam fi ltration

a. It occurs at the glass window of general X-ray tubes

b. Beryllium may be used in mammography X-ray tubes

c. Filtration will always reduce X-ray output (other factors

constant)

d. Filters are designed solely to stop high energy photons

e. The K-edge for molybdenum is approx. 30 keV

X-ray beam fi ltration occurs in all X-ray tubes as the beam passes through

the window. It is often quoted in terms of mm of aluminium. The inherent

fi ltration that occurs with diagnostic tubes is approximately 0.5–1 mm

of aluminium. In general radiographic machines this is negligible and so

extra fi lters may be required. These are used to absorb the low energy

Bremssrahlung radiation, as this is mainly absorbed by the patient and

hence increases patient dose whilst contributing little to the image.

In general radiology units the total fi ltration should be at least 2.5 mm of

aluminium (or equivalent). For high kV techniques (chest radiography,

etc) further fi ltration may be needed and a combination fi lter of aluminium

and copper is used. Molybdenum may be used as a fi lter in mammography

units. It has a K-edge of approx. 20 keV. It will be used in conjunction

with a molybdenum anode.

It is important to remember that fi lters will NOT affect the maximum energy

of the X-ray spectrum but WILL affect the mean energy of the beam. They

will also increase the half-value layer for a particular material.

14a. True – some beam fi ltration will occur at the window

14b. True – a beryllium window is used in mammography units

14c. True – any fi lter will decrease output

14d. False – fi lters are designed to reduce lower energy photons that

increase dose

14e. False – approximately 20 keV

15. Beam hardening

a. Occurs after fi ltration

b. Results in increased penetrating power of the beam

c. Lowers the maximum photon energy of the beam

d. Has no effect on the mean energy of the X-ray beam

e. Lowers the half-value layer (HVL)

After fi ltration the lower energy photons are removed. Thus the mean energy

of the beam is increased but the maximum energy of the photons in the

beam remains unchanged. ‘Beam hardening’ refers to this preferential loss

of lower energy photons. Since the mean energy of the beam is increased,

the penetrating power of the beam is increased and so is the HVL (Half

Value Layer.) HVL is defi ned as that thickness of a given material that

will reduce the intensity of a radiation beam to one half of its original

value. Beam hardening does NOT occur with monochromatic beams.

15a. True – beam hardening does occur after fi ltration

15b. True – the mean energy is increased and so is the HVL

15c. False – the maximum energy of the beam is unaffected

15d. False – the mean energy of the beam is increased

15e. False – the HVL is increased

16. With regard to unwanted radiation from an X-ray tube

a. Leakage radiation is the sum of the stray and scattered

radiations

b. The tube housing eliminates all unwanted radiation

c. Leakage radiation should be < 1 mGy/hr at 1 m from the

focus

d. Increasing the tube kV will increase the amount of scattered

radiation in the forward direction

e. Scattered radiation is primarily caused by photoelectric

interactions

Unwanted radiation from a source is made up of scattered radiation and

leakage radiation. Leakage radiation is that which is transmitted through

the tube housing. The components of the housing and the lead lining all

help to minimise it, but it is not completely eliminated. It is currently

recommended that leakage radiation from the tube does not exceed 1 mGy

per hour at a distance of 1 m from the focus.

Scattered radiation is that which has changed direction after leaving the

tube. It can be scattered in the patient, at the couch, at the receptor, etc. The

sum of leakage and scattered radiation equals stray radiation. Scatter occurs

because of Compton interactions more so than photoelectric interactions.

Coherent (also called Rayleigh) interactions will also produce scatter but

to a much lesser degree. At higher kV the amount of radiation travelling

in the forward direction will increase. There will also be a concomitant

increase in the amount of forward scatter.

16a. False – leakage radiation is transmitted through the tube housing

16b. False – it does not eliminate all unwanted radiation

16c. True – this is a standard requirement

16d. True – increasing the kV increases the amount of forward scatter

16e. False – Compton is primarily involved

17. Concerning the anode heel effect

a. It is caused by varying attenuation of the beam within the

cathode

b. It has no practical use

c. It is related to the target angle

d. It can be compensated for with the use of fi lters

e. It causes the beam intensity to vary but not the beam

hardness

The anode heel effect causes an asymmetry in the production of X-rays.

It is caused by the fact that X-rays produced by interactions deep in the

anode are attenuated more by the anode as they travel through it than those

which are produced by interactions nearer the surface. The heel effect only

occurs along the anode/cathode line. The variation in beam attenuation

causes the beam to change in hardness and intensity, and results in higher

X-ray intensity at the cathode end and lower X-ray intensity at the anode

end. The heel effect varies with the target angle. Decreasing the target angle

decreases the spread of the beam and so restricts the fi eld of view.

The heel effect is used in mammography. The higher beam intensity on the

cathode side is directed towards the chest wall where greater penetration

is required. Some compensation for the heel effect can be achieved by

tilting the fi lter so that different parts of the beam pass through different

amounts of the fi lter.

17a. False – varying attenuation in the anode

17b. False – it can be used, and is readily used in mammography

17c. True – the heel effect varies with the target angle

17d. True – fi lters can be used to compensate for the heel effect

17e. False – both the beam intensity and hardness vary

18. The heel effect can be minimised

a. By increasing the target angle

b. Using a tungsten/rhenium alloy instead of pure tungsten

c. Using a smaller fi eld size

d. Using shorter source-to-fi lm distances

e. Using tilted fi lters

The heel effect can be useful as well as detrimental. It is used in

mammography. In other situations it may be necessary to minimise its

effects. Increasing the target angle will reduce its effects, as will using a

smaller fi eld size, where there is a decreased area over which the variation

is detected and only the central portion of the beam is utilised. Using a

longer source-to-fi lm distance reduces the heel effect because only the

central portion of the beam is used. Tilted fi lters are also used to attenuate

the heel effect.

18a. True – increasing the target angle will minimise the heel effect

18b. False – this will not affect the heel effect

18c. True – a smaller fi eld size will reduce the heel effect

18d. False – using a longer source-to-fi lm distance minimises the heel

effect

18e. True – tilted fi lters can be used to minimise the heel effect

19. The heat loading of an X-ray tube

a. Is measured in Joules

b. Is measured in Heat Units

c. Is always equal to kV ¥ mAs

d. Is independent of the voltage waveform

e. Is directly related to the exposure time

The heat rating of an X-ray tube is a measure of the amount of energy

that is deposited during an exposure. For constant potential or three-phase

supplies it equals kV ¥ mAs. For single-phase supply it is approximately 0.7

x kV ¥ mAs. It is measured in Joules or Heat Units (the old nomenclature)

and 1J = 1.4 HU. Since it is a measure of the amount of energy deposited

during an exposure it will be related to:

Ó Tube voltage

Ó Voltage waveform

Ó Tube current

Ó Number of exposures

Ó Exposure time

19a. True – Joules or Heat Units are used

19b. True – Joules or Heat Units are used

19c. False – it is kV ¥ mAs for constant potential or three-phase supply

only

19d. False – it is dependent on the voltage waveform

19e. True – it is directly related to the exposure time

20. Tube rating may be increased by the following

a. Decreasing the diameter of a rotating anode

b. Using a rotating anode instead of a stationary anode

c. Increasing the focal spot size

d. Using high speed rotating anodes

e. Using active cooling methods

The tube rating is based on the maximum allowable kilowatts for an

exposure time of 0.1 seconds. So the things that will affect heat production

(heat loading) will also affect the tube rating. Some of these include:

Ó Increasing the size of the focal spot so heat is deposited over a larger

area

Ó Using rotating anodes, as this effectively increases the area over which

heat is generated

Ó Increasing the speed of rotation (up to a point) as this will result in

increased area for heat deposition and more even heat spread

Ó Shortening the exposure time

Ó Increasing the diameter of the rotating anode

Ó Using active cooling methods. These may be employed in some

modern angiography systems and those that require prolonged exposure

times

Obviously, since tube rating is calculated from the kV and mA (for 0.1

sec) both of these will be important with regard to the tube rating.

20a. False – increasing the diameter will increase the tube rating

20b. True – using a rotating anode effectively increases the size of the

focal spot

20c. True – increasing the focal spot size deposits heat over a larger

area

20d. True – increasing the speed increases the area for heat deposition

20e. True – allows improved dissipation of heat

21. Concerning Bremsstrahlung radiation

a. It is caused by electron – electron interactions

b. It is limited by the fi lament current

c. The maximum wavelength is related to the kVp

d. Account for the majority of X-ray produced in a X-ray

tube

e. Bremsstrahlung X-ray production is higher for Tungsten than

for Molybdenum if all other factors are equal

Bremsstrahlung radiation (Braking radiation) is caused by the interaction

between electrons (from the cathode) and the nuclei of the atoms that make

up the anode. The electrostatic forces between the incoming electrons and

the nuclei cause the electrons to change direction, and in the process they

lose energy. This loss of energy is released as X-rays.

The low-energy cut-off for these X-rays is because of the heavy attenuation

low-energy X-rays undergo as they exit the tube. The high-energy cut-off

is because the maximum energy of the X-rays can only be equal to the

maximum amount of energy the bombarding electrons had. So if the kVp

is 120 kV, then an electron can have maximum 120 keV energy and hence,

by the principle of energy conservation, the highest energy an emergent

X-ray can have is 120 keV. The high-energy cut-off is therefore limited by

the maximum tube potential and not the fi lament current. The minimum

wavelength of the X-rays corresponds with the maximum energy since:

Wavelength = hc/e

(where h is Planck’s constant, c is the speed of light and e is the energy

of electrons.)

The skewed shape of the spectrum is because of the attenuation the X-rays

undergo and the fact they are not all produced at the anode surface.

Bremsstrahlung radiation accounts for the majority of X-rays produced.

Atoms with higher atomic numbers produce more Bremsstrahlung radiation

because of the stronger force between the electron and the nucleus.

21a. False – caused by interactions between electrons and the anode

nuclei

21b. False – it is limited by the kV not the current

21c. False – the minimum wavelength is related to the kVp

21d. True – they account for the majority of X-rays produced

21e. True – since the atomic number for tungsten is higher than

molybdenum

22. Concerning the spectrum of radiation produced by an X-ray

tube

a. It is not affected by the anode material

b. The minimum X-ray photon frequency is directly related to

the maximum tube potential

c. The tube current alters the shape of the spectrum

d. Increasing tube potential alters the quality of the X-ray

spectrum

e. The characteristic K-shell radiation energy for tungsten is

69.5 keV

The spectrum of radiation produced is related to both the tube potential

and the anode material. These things alter the quality of the X-rays, i.e.

they change the shape of the spectrum. Factors such as exposure time

and tube current will alter the number of X-rays produced, and hence

change the quantity, but they do not alter the shape of the spectrum. The

maximum tube potential is related to the minimum X-ray wavelength

and hence the maximum frequency (as wavelength and frequency are

inversely related to each other).

22a. False – the anode material affects the radiation spectrum

22b. False – they are inversely related

22c. False – alters the quantity of X-rays but not the spectrum

22d. True – tube potential alters the spectrum of radiation produced

22e. True – characteristic radiation for tungsten is 69.5 keV

23. The characteristic X-rays produced by an X-ray tube

a. Contribute signifi cantly more X-rays than Bremsstrahlung

b. Occur at 17.4 keV and 19.6 keV for molybdenum

c. Are caused by the interaction of bombarding electrons with

unbound electrons in the anode

d. K-shell characteristic X-rays for tungsten will be emitted if

the tube potential is set to 60 kV.

e. L-shell characteristic X-rays are useful in general

radiology

Characteristic X-rays occur when a bound electron (in the anode) is

ejected by a bombarding electron from the cathode. In order to do this

the incident electron must have energy greater than the binding energy of

the anode electron. It follows that in order to eject an electron from the

K shell, an incident electron must have greater energy than the binding

energy of the K shell.

Once the electron is ejected, the vacancy it creates needs to be fi lled.

The electron that fi lls this vacancy may come from a neighbouring shell,

e.g. the L-shell (most likely transition) or even from a free electron. The

different characteristic spectral lines are caused by this fact. For example

the K-series of lines for tungsten range from 58.5 keV (if the vacancy is

fi lled by a neighbouring L-shell electron) up to 69.5 keV (if a free electron

occupies the vacancy).

The vacancies created by the incident electrons can happen in any of the

electron shells, with a similar process to the one described earlier occurring

afterwards. However, since the binding energy of the other shells is lower,

the X-rays produced are of lower energy and are generally absorbed at

the tube window, fi lter etc.

23a. False – Bremsstrahlung contribute more than characteristic

radiation

23b. True – 17.4 keV and 19.6 keV for Molybdenum

23c. False – characteristic radiation occurs when a bound electron in the

anode is ejected by a bombarding electron

23d. False – K-shell characteristic X-rays will not be produced as the

energy requirement is 69.5 keV

23e. False – The L-shell energies are too low to be practically useful

24. With regard to the shape of the X-ray spectrum

a. Increasing tube current alters the shape of the spectrum

b. Increasing the exposure time alters the shape of the

spectrum

c. Increasing the kVp above the K-shell binding energy will cause

new characteristic X-ray lines to appear on the spectrum

d. Increasing the kVp shifts the spectrum upwards and towards

the right

e. The characteristic X-ray lines occur at lower photon energy

for lower atomic number anodes.

The shape of the X-ray spectrum is changed by the kVp. Increasing the

kVp will shift the spectrum upwards and to the right. Changing the tube

current and the exposure time will not affect the shape of the spectrum;

however, they will cause an increase in the amount of Bremsstrahlung

and characteristic radiation. Obviously no new characteristic lines will

appear if the kVp is increased above the K-shell binding energy, as this

is the innermost shell of electrons.

24a. False – this alters the quantity of X-rays produced

24b. False – no effect on the shape of the spectrum

24c. False – this represents the innermost shell of electrons, therefore no

new characteristic radiation will appear

24d. True – increasing the kVp will shift the spectrum to the right

24e. True – since the K-shell binding energies are lower

25. The maximum energy of the X-ray photons in a spectrum

a. Is directly related to the kVp

b. Is affected by the anode material

c. Is unrelated to the distance between the anode and the

cathode

d. Is affected by the voltage waveform

e. Is of the order of 120–140 keV for CT

The maximum energy of the emitted X-ray photons in a spectrum is related

to the kVp. It is unaffected by anode material, anode-cathode distance,

voltage waveform (notice that the question specifi es MAXIMUM energy).

CT uses high energy X-rays and this order of magnitude is correct.

25a. True – the maximum energy is directly related to kVp

25b. False – the maximum energy of the photons is not affected by the

anode material

25c. True – there is no relation between the maximum photon energy and

the distance between the anode and cathode

25d. False – the mean energy is affected but the maximum energy is

not

25e. True – this is the standard range for CT