X-ray production

Question 1

Where are electrons emitted from in an X-ray tube?

Answer 1

a tungsten filament within the cathode

Question 2

What is used to accelerate the electrons towards the anode?

Answer 2

A large accelerating potential difference is applied between the cathode and a positively charged dense target (anode) in an evacuated glass envelope (i.e. in a vacuum). Free electrons released from the cathode are repelled by the negative charge of the cathode and accelerated to high velocity towards the positively charged anode.

Question 3

Why are the electrons accelerated?

Answer 3

The high velocity of each electron gives it a relatively large amount of kinetic energy (energy of movement) which is converted to heat (about 99%) and x-rays (about 1%) through interactions within the anode.

Question 4

Why does the the filament need a high resistance?

Answer 4

This allowed it to generate heat - When the filament is white hot (incandescent) thermionic emission occurs.

Question 5

What is thermionic emission?

Answer 5

Thermionic emission is the emission of electrons from a metal surface caused by thermal vibrational energy overcoming the electrostatic forces holding electrons to the surface.

Question 6

Why is tungsten used for the cathode filament?

Answer 6

tungsten is a good thermionic emitter. Tungsten has a low vapour pressure (which gives the filament longevity) and may be manufactured into thin wire. Tungsten has a very high melting point (about 3370°C).

Question 7

What is the focusing cup made from and why?

Answer 7

The focusing cup is made from materials of high melting point but poor thermionic emission. If electrons were thermionically emitted from the focusing cup, the effect would be to degrade image spatial resolution due to the apparent increase in filament size

Question 8

Why is copper used in the anode?

Answer 8

Most of the kinetic energy of the electrons is converted into heat within the target (and very little into x-rays). Therefore the target material and the anode itself must be able to dissipate heat effectively (to limit thermal damage).

Copper (Cu) is a better conductor of heat than tungsten (W). In a stationary anode x-ray tube, the tungsten target is embedded in a copper anode. Heat is conducted along the copper and into an oil reservoir outside the tube and surrounding it.

Question 9

What controls how many electrons are released by the cathode?

Answer 9

The filament current as a higher current induces more thermionic emission - up to around 10A.

Question 10

What is the tube current?

Answer 10

The flow of free electrons constitutes the x-ray tube current (expressed in mA) which is much less than the filament current

Question 11

What is filament voltage?

Answer 11

The filament voltage (required to cause current flow in the filament circuit) is of the order of 10 V

Question 12

What is the tube voltage?

Answer 12

voltage of 20-150 kV applied between the cathode and the anode (to cause the tube current).

Question 13

Why is using the electrical mains an issue for generating xrays?

Answer 13

It has an alternating voltage (at a frequency of 50 hertz (Hz) in the UK) which has alternate positive and negative half-cycles which is less steady than e.g. a DC current from a large battery.

Question 14

What is done to the AC mains current for use in generating xrays?

Answer 14

The amplitude of the mains voltage may be increased using a step-up transformer and the amplified voltage rectified, using diodes for example, so that either only the positive half-cycles are retained (half-wave rectification, one pulse per cycle), or the negative half-cycles are made positive (full-wave rectification, two pulses per cycle).

Question 15

What issue does rectification of AC current produce?

Answer 15

a voltage ripple which is expressed as a percentage of the maximum voltage, also known as the peak kilovoltage (kVp). For both half-wave and full-wave rectification with a single phase mains supply, the ripple is 100%

The addition of more electrical components can yield 12 pulses per cycle and a reduction in the ripple down to 3-10%

Question 16

In modern xrays is voltage ripple an issue?

Answer 16

Modern x-ray generators are of either the ‘constant potential’ or ‘high-frequency’ design. In these types of generator, the voltage ripple is small or negligible

Question 17

What would the bremsstrahlung curve look like without filtration?

Answer 17

A straight line from Y axis down to the X axis at the kV of the tube.

Question 18

At what point in the Xray spectrum is the intensity highest?

Answer 18

bewteeen 1/3 and 1/2 of the maximum energy

Question 19

What is roughly the average photon energy?

Answer 19

50-60% of maximum energy

Question 20

What are the characteristic radiation lines dictated by?

Answer 20

binding energies in the atoms of the target

Question 21

What is the maximum energy of an Xray output defined by?

Answer 21

The kV of the tube

Question 22

How do you calculate the total x-ray intensity of a spectrum?

Answer 22

the area under the curve

Question 23

What is the low-energy cut off of an xray spectrum defined by?

Answer 23

the filtration used

Question 24

What is beam quantity?

Answer 24

Beam quantity is the total intensity of the beam; this is equal to the total area between the spectrum curve and the horizontal axis (representing x-ray photon energy).

Question 25

What is beam quality?

Answer 25

Beam quality describes its penetrating power; it may be expressed as the beam half value thickness (HVT), sometimes in combination with the tube kV (or kVp). A hard beam is very penetrating and has a high HVT, whereas the reverse is true for a soft beam.

Question 26

What is the effect of increasing kV on the Xray spectrum?

Answer 26

increases the peak intensity, the total intensity, the photon energy at which the peak intensity occurs, the average photon energy and the maximum photon energy.

Question 27

What is total xray intensity proportional to?

Answer 27

kV^2

Question 28

What is the K-edge of tungsten?

Answer 28

69.5 keV

Question 29

Why are the L and M characteristic lines of tungsten rarely seen?

Answer 29

The low-energy L and M series are not normally seen in the output spectra of diagnostic tubes because they are severely attenuated by filtration.

Question 30

True or false
A. The total intensity of x-rays produced in an x-ray tube is approximately proportional to kV6
B. If all other factors are constant in an x-ray tube, doubling the tube current doubles the total x-ray intensity
C. A target made of low Z material is more efficient at producing x-rays than a target of high Z material
D. Generator ripple does not affect beam quality
E. One use of tube filtration is to preferentially remove soft x-rays from the spectrum

Answer 30

A. False. Both the width and the height of the x-ray spectrum are increased as the kV is increased. Thus the area under the spectral curve is increased and this is equal to the total intensity of the x-ray beam. In fact, x-ray intensity (tube output) is roughly proportional to kV2. This relationship varies slightly for different target materials but is never kV6.

B. True. As the tube current increases, more electrons interact with the anode per unit time. Therefore the total intensity of x-rays increases in proportion to the tube current.

C. False. The greater the atomic number (Z) of the target atoms, the more likely is an electron interaction through bremsstrahlung and hence the greater is the production efficiency of x-rays. The intensity of bremsstrahlung is proportional to Z.

D. False. The presence of x-ray generator ripple means that the kV across the tube is not constant; it fluctuates because the kV oscillates with the ripple. For the same kVp, the electron energy for a constant potential tube is thus greater than the average electron energy for a tube with generator ripple. Reduced average electron energy leads to a reduced average x-ray energy (but not to a reduced maximum x-ray energy). Reduced average photon energy changes the beam quality by making it softer; the ripple has reduced the penetrating power of the beam.

E. True. Soft (low-energy) x-rays within the spectrum contribute to patient x-ray dose without contributing to the radiograph simply because most of them are absorbed within the body. By using a filter to absorb the low-energy x-rays as they exit the x-ray tube, the patient is saved from the burden of unnecessary radiation dose. In the UK there is a recommended minimum total filtration of 2.5 mmAl (for an x-ray tube operating above 70 kVp).

Question 31

T or F
Regarding the emission of x-rays:
A. Tungsten is used for the filament because it has a high melting point
B. Tungsten is used for the filament as it has a very high electrical resistivity
C. Thermionic emission is the release of electrons from the surface of a metal which is incandescent
D. The rate of thermionic emission decreases as the filament current is increased

Answer 31

True
False
True
False

Tungsten is used for the filament because a material is needed that will not melt and that will not evaporate. Evaporation would lead to some tungsten vapour in the vacuum and the deposition of a thin layer of tungsten on the inside of the x-ray tube. This could cause arcing (a form of electrical conduction) within the tube.

Tungsten does not have a particularly high electrical resistivity (it is between that of copper and iron). The necessary high electrical resistance is achieved through drawing the tungsten into thin wire, i.e. a filament.

The rate of thermionic emission increases with the filament temperature as electrons receive enough thermal energy to escape from the surface of the metal.

Question 32

T or F
Which of the answers below describe accurately the characteristic x-rays from an x-ray tube with a tungsten target?
A. They all have the same energy
B. They are absent from the spectrum if the tube voltage is lower than 50 kV
C. They have energy that depends on the tube voltage
D. They are produced in greater quantity than bremsstrahlung x-rays

Answer 32

False
True
False
False

The energies of the characteristic x-ray peaks are equal to the differences in binding energies of electron shells in tungsten. If a directly ionising collision creates a vacancy in the K-shell and this is filled by a bound electron from the L-shell, a Kα characteristic x-ray at 58 keV is emitted. If the vacancy is filled by an M-shell-bound electron, a Kβ characteristic x-ray at 68 keV is emitted. Both of these are members of the K series of characteristic photons.

The binding energy of a tungsten K-shell electron is 69.5 keV. At a tube voltage of 50 kV, the accelerated electrons bombarding the target have a kinetic energy of 50 keV which is insufficient to liberate a K-shell electron. Therefore, the K series is not produced.

The binding energy of an L-shell electron is about 11.5 keV while that of an M-shell electron is 1.5 keV, so an x-ray tube operated at
50 kV can produce both the L and M series of characteristic photons. However, the energies of these radiations are so low that they are severely attenuated by filtration and do not contribute to the spectrum.

As regards the number and intensity of emitted x-rays, bremsstrahlung is the dominant production mechanism within a standard x-ray tube.

Question 33

What are the three different ways to select tube factors?

Answer 33

Manual selection
Automatic exposure control (AEC)
Anatomically programmed radiography (APR)

Question 34

How are tube factors selected in manual mode?

Answer 34

The voltage (kV) setting will be selected by one such control. The tube current (mA) and exposure time (seconds) may be selected as separate values, or as the combined value of current multiplied by time known as milliampere-seconds (mAs). If mAs is selected, the x-ray set will select the most appropriate values of time and current:

Separate selection of mA and time settings is known as 3-button control (kV, mA, seconds)
Combined selection of current multiplied by time is known as 2-button control (kV, mAs)

If the selected exposure is within the range permitted by the x-ray set, the display will indicate that the exposure can be made, usually by displaying a green light.

Question 35

How are tube factors selected in AEC?

Answer 35

Automatic exposure control uses a radiation detector, positioned directly in front of the image receptor, to determine when the receptor has had sufficient radiation exposure to produce adequate image quality. In standard radiography, the detector is usually an ionisation chamber. The operator selects the kV setting, and the x-ray set determines the mA setting.

Question 36

How do AECs differ in mammography?

Answer 36

in mammographic equipment, the AEC is much more sophisticated, and may control not only the exposure, but also selection of target, filtration and kV settings.

Question 37

What is a typical AEC set up and can you alter this?

Answer 37

Generally, there are three ionisation chambers, and the operator may select any combination of them. The selection will depend on the collimation of the x-ray beam and on the anatomy that needs to be displayed.

Question 38

In what situations may the AEC cause issues?

Answer 38

The ionisation chamber might be shielded from the x-ray beam, for example by the collimator

The x-ray beam might not be pointed at the ionisation chamber

The AEC might be faulty

In such cases, the ionisation chamber would never receive enough radiation dose to terminate the exposure, the patient would receive a large radiation dose, and the image would be overexposed.

Question 39

What safeguards are in place for the AEC?

Answer 39

A back-up timer, which limits the maximum exposure that the x-ray set can give. Note that this is usually a device to protect the x-ray tube from damage, rather than a device to protect the patient

Facility for the operator to set an exposure (mAs) greater than that likely to be needed, but much shorter than the back-up time. The exposure will be terminated at this mAs value whether or not the AEC has reached the required dose

A dose rate detector incorporated into the AEC. If this does not detect radiation when the exposure is started, the exposure is terminated immediately

Question 40

What is APR mode?

Answer 40

the operator can choose from a set of pre-programmed exposures.

Use of APR saves the operator from having to look up the exposure settings from a chart. Sometimes, APR settings may include an option to increase or decrease the exposure settings for patients who are larger or smaller than average.

Question 41

T or F
Selecting a smaller focal spot:
A. Will improve the spatial resolution of the image
B. Will reduce the range of exposure factors available
C. Will improve the radiographic contrast
D. May remove the need to use an anti-scatter grid
E. Will require the selection of a lower kV setting

Answer 41

A. True. A smaller focal spot will reduce geometric unsharpness and thus increase the spatial resolution of the image. This is the prime reason for selecting a smaller focal spot.

B. True. With a small focal spot, the heat generated by the electrons striking the target is concentrated into a smaller area, increasing the likelihood of tube damage. Damage is most likely at the highest power (kV x mA) ratings, so the highest values of tube current (mA) will not be available with the smaller focal spot. This might necessitate the use of longer exposure times.

C. False. The focal spot size does not affect contrast.

D. False. The purpose of an anti-scatter grid is to increase contrast. The focal spot size does not affect contrast and therefore has no direct bearing on the need for a grid.

E. False. Selecting a small focal spot does not necessarily mean that a lower kV setting is necessary. Selecting a lower kV does not always reduce tube heating because it might well require an even greater corresponding increase in mAs to obtain the required image quality.

Question 42

Why must the operator press the exposure button throughout the exposure?

Answer 42

This feature is essential in the event of a failure of the timer or the AEC system. Most radiographic exposures are short, and the operator will usually be able to stop the exposure well before the back-up timer.

Question 43

Why is a light beam diaphragm needed in an xray set?

Answer 43

It provides an illuminated field that indicates where the x-ray beam will strike the patient

It allows the operator to adjust the size of the x-ray field. Restriction of the field size is known as collimation. Collimation is important in limiting the radiation exposure of the patient to the area necessary for the examination.

Question 44

What is the light beam diaphragm made up o?

Answer 44

The diaphragm contains a light bulb and a mirror. The mirror is inclined so that the light appears to come from the x-ray focus. This ensures that the light field and the x-ray field coincide at all focus to receptor distances and at all field sizes.

Question 45

T or F
What happens when the field size is reduced by collimation?
A. The entrance surface dose to the patient is significantly reduced
B. The dose-area product is significantly reduced
C. The effective dose to the patient is significantly reduced
D. When using an AEC, the exposure is more likely to be correct
E. The amount of scatter is reduced
F. The correct anatomy is more likely to be seen on the radiograph

Answer 45

A. False. The volume of tissue irradiated will be reduced, and this will reduce both the dose-area product and the effective dose. However, it will not significantly affect the entrance dose, which depends on the dose required by the image receptor and on the transmission through the patient.

B. True. See feedback for A.

C. True. See feedback for A.

D. True. If a larger x-ray field than necessary is used, the AEC chamber may not lie directly under the tissue of interest, so the exposure may not be optimal. However, note that if the x-ray field is too small, it may fail to irradiate the whole AEC chamber, leading to an increased patient dose.

E. True. Reducing the field size will reduce the total number of scattering events. With a large field size, some x-rays would be scattered from outside the area of interest into the area of interest, increasing the effect of scatter in that part of the image.

F. False. If the beam is collimated too tightly, the chances of excluding something that you need to see are increased.

Question 46

What allows the voltage of the AC mains supply to be changed?

Answer 46

A transformer - a step up transformer would increase the voltage.

Question 47

What is a self-rectified xray set?

Answer 47

where they only use the positive half of the waveform.
the current across the tube flows in only one direction, the voltage still fluctuates between zero and a peak. This peak value is known as the peak kilovoltage (kVp).

Self-rectified sets are used only in applications such as intra-oral dental x-ray sets, where only relatively low kilovolts (kV) and milliamperes (mA) are required.

Question 48

What is full wave rectification?

Answer 48

By using a rectification circuit in the supply to the x-ray tube, both halves of the cycle can be used. Note that the voltage still varies from zero to the kVp during a cycle. This variation is called the ‘ripple’. The ripple of a self-rectified or full-wave rectified supply is 100% because the voltage fluctuates between zero and the kVp.

Question 49

In full wave rectified sets why are most of the xrays generated at well below kVp and why is this a problem?

Answer 49

When the potential difference across the x-ray tube is self-rectified or full-wave rectified, the voltage is much less than the peak for most of each cycle. Most x-rays, therefore, are generated using voltages well below the kVp.

This means that the x-ray spectrum will be dominated by lower energy photons. If these are filtered out, as they would normally be, then the output of the x-ray set will be low. This is a limitation of a high voltage generator that uses a single sinusoidally-varying supply, also known as a single-phase supply.

Question 50

What can be used to keep the kVp of a set as close to max as possible?

Answer 50

A 3-phase supply

Question 51

How does 3 phase supply work?

Answer 51

contains three voltage waveforms. Each one has two peaks every 20 ms (1 per 50 seconds), but the peaks occur at different times because the waveforms are out of phase. They can be combined to produce a waveform with six peaks every 20 ms

Question 52

What is the ripple of a 3 phase supply set?

Answer 52

15% (vs 100% of full wave rectified)

Question 53

What can be used to get an almost constant high voltage?

Answer 53

high-frequency generator. This passes the mains supply through several electronic circuits to produce an almost constant high voltage. This has hardly any ripple

Question 54

What is the advantage of high-frequency generators over 3 phase supply?

Answer 54

High-frequency generators have the added advantage that they are much more compact than 3-phase generators. They have now replaced 3-phase generators, though some 3-phase equipment is still in use.

High-frequency generators can also be used with the DC supply from a battery in mobile x-ray equipment.

Question 55

Why is reducing ripple good?

Answer 55

The smaller the ripple, the greater the mean photon energy will be

The smaller the ripple, the greater the x-ray output will be (proportional to the area under each graph)

In general, the use of waveforms with small ripple will mean that, in comparison with kV with greater ripple:
Exposures can be shorter, Patient doses will be lower

Question 56

T or F - Compared with a full-wave rectified supply, a high-frequency supply at the same peak voltage:
A. Produces an x-ray beam with a greater mean photon energy
B. Produces an x-ray beam with a greater peak photon energy
C. Produces an x-ray beam with a greater minimum photon energy
D. Produces a greater x-ray output (dose per mAs)
E. Requires an x-ray tube with greater filtration

Answer 56

A. True. With a high-frequency supply, the voltage is at the peak value throughout the exposure. With a full-wave rectified supply, the voltage is smaller than the peak for much of the time.

B. False. The peak photon energy depends on the kVp, which is the same for both types of rectification.

C. False. The minimum photon energy is determined by the x-ray tube window and the filtration, rather than by the voltage used to produce the x-rays.

D. True. The greater voltage throughout much of the exposure results in greater x-ray output.

E. False. The filtration requirements are determined by the need to remove lower energy photons from the beam.

Question 57

How does mass effect heat removal from x-ray tubes?

Answer 57

Large masses of material can contain much more heat than small masses
For small masses of material it takes only a small amount of heat energy to produce a large temperature rise

Question 58

How does anode angle effect heat transfer in xray production?

Answer 58

We need a small focal spot to produce a sharp image. This means that the electron beam in the x-ray tube strikes a very small area. We can reduce the temperature rise by making this area larger. One way of doing this is by selecting the angle of the anode so as to reduce the apparent size of the focal spot as seen from the receptor.

Question 59

How does changing the angle of the anode effect the image?

Answer 59

Angling the anode has an effect on the x-ray output known as the anode heel effect. Those x-rays with longer paths through the target material are more likely to be absorbed by it than those with shorter paths. Note that this variation occurs only in one direction – parallel to the anode-cathode axis with higher intensity on the cathode side.

Question 60

How is temperature rise limited in a stationary anode tube?

Answer 60

embedding the tungsten target in a block of copper. The copper forms the largest part of the anode The heat produced by the electrons is conducted into the anode. The anode extends outside the vacuum tube into the tube casing, which contains oil. Cooling fins on the anode help to transfer heat to the oil, which convects the heat to the outside of the tube.

Question 61

How is temperature rise limited in a rotating anode tube?

Answer 61

the temperature rise is limited by constructing the target on a rotating molybdenum disc. A rotating anode loses heat principally by radiation.

Question 62

What are spiral groove bearings and why are they better than standard bearings?

Answer 62

Spiral groove bearings are liquid metal bearings that allow heat conduction along the anode stem. Heat loss during x-ray exposures, when the temperature of the target is high, is still principally by radiation. However, during the interval between exposures, when the anode temperature is lower, conduction becomes the main process of heat loss. The result is that the x-ray tube has a much higher heat rating.

Spiral groove bearings are expensive, so their use is restricted to applications where the x-ray set must run for long periods at high ratings – cardiology and computed tomography

Other advantages are:
The anode can be rotated continuously. You can keep it going between exposures
The tube is much quieter

Question 63

What are xray tubes rated according to?

Answer 63

The maximum mA and mAs that can be used at a particular kV

How much heat can be stored in the tube before an unacceptable temperature is reached either in megajoules (MJ) or heat units (HU)

How quickly the stored heat can be dissipated (MJ/min or HU/min)

Question 64

What is a thermal cut-out?

Answer 64

This may be a simple thermal cut-out in the form of bellows operated by thermal expansion of the oil in the tube to prevent excessive heat build-up.

Question 65

What can help avoid focal spot damage?

Answer 65

The focal spot of an x-ray tube is most likely to be damaged if a large exposure at high kV is made when the tube is cold, for example, at the start of the working day. Tube life can be prolonged by making a number of low kV exposures to warm-up the tube more gradually, before making large exposures.

Question 66

In an x-ray tube with a rotating anode, what is the anode stem made from?

Answer 66

molybdenum

Question 67

Why is molybdenum used for the anode stem in rotating anode x-ray tubes?

Answer 67

It has a low thermal conductivity to reduce the heat transferred to the bearings

Question 68

In a stationary anode x-ray tube, what is the anode made of?

Answer 68

Copper, because its high thermal conductivity will enable rapid heat removal

Question 69

What fills the space between the filament and the target in an x-ray tube?

Answer 69

A vacuum, to allow unimpeded electron flow

Question 70

T or F - In a rotating anode x-ray tube, which of the following are reasons for rotating the anode?

A. Preventing the focal spot from reaching too high a temperature during exposures
B. Reducing the overall temperature of the x-ray tube
C. Cooling the focal spot through additional convection of heat
D. Reducing the anode heel effect
E. Allowing larger mAs values to be used for a single exposure

Answer 70

A. True. The rotating anode spreads the heat out over a larger area, limiting the temperature rise over any one part.

B. False. The heat input is still the same, and the energy used in rotating the anode might even introduce a bit more, so that the overall temperature rise is not reduced.

C. False. The target is in a vacuum, so convection cannot occur.

D. False. The rotation does not change the target angle, and so does not influence the heel effect.

E. True. The reduction in temperature rise in the target area means that larger exposures can be made without damaging the target.

Question 71

T or F - Regarding a rotating anode x-ray tube:
A. The anode stem is made of tungsten
B. The effective focal spot size depends on the anode angle
C. Heat is removed from the anode mainly by thermal conduction
D. The anode heel effect occurs in a direction parallel to the anode-cathode axis

Answer 71

A. False. The stem is made of molybdenum to reduce heat conduction to the bearings.

B. True. Angling the anode produces foreshortening of the target, making its apparent size smaller.

C. False. Most heat is removed by radiation.

D. True. The variation in thickness of target material that photons must penetrate in order to escape, occurs in this direction.