1. FRCR: Physics Lectures Diagnostic Radiology
An introduction to radiography with X-rays and the X-ray tube
Dr Tim Wood
Clinical Scientist

2. Lecture aims
Revision of the physics of x-ray interactions and production
Describe how the x-ray tube is constructed and understand the importance of each component
Basic understanding of x-ray generators
Explain what factors affect the output of an x-ray tube

3. Introduction

4. A little bit of history…
Wilhelm Röntgen discovered X-rays on 8th Nov 1895
Took first medical X-ray of wife’s hand (22nd Dec 1895)
Used to diagnose Eddie McCarthy’s fractured left wrist on 3rd Feb 1896 (20 min exposure)
Awarded first Nobel Prize in Physics in 1901 for his discovery of ‘Röntgen rays’

5. A little bit of history…

6. Thankfully, things improved!…

7. What is diagnostic radiology?
The science dealing with X-rays and other high-energy radiation, especially the use of such radiation for the diagnosis and treatment of disease
Origin: 
1895–1900; radio-  + -logy
Related forms:
ra·di·ol·o·gist, noun

8. What is diagnostic radiology?
The underlying principle of the majority of diagnostic radiological techniques is that X-rays display differential attenuation in matter
When the X-ray beam is targeted at a patient, the different tissues in the body will remove a different number of X-rays from the beam
The resulting modified X-ray flux can then be ‘captured’ by some form of detector to produce a latent image or radiation measurement
Detection may be through film, phosphor screens, digital detectors, etc

9. X-ray Properties Electromagnetic photons of radiation
February 6, 2007
X-ray Properties
Electromagnetic photons of radiation
Emitted with various energies & wavelengths not detectable to the human senses
Travel radially from their source (in straight lines) at the speed of light
Can travel in a vacuum
Display differential attenuation by matter
The shorter the wavelength, the higher the energy and hence, more penetrating
Can cause ionisation in matter
Produce a ‘latent’ image on film/detector 9 9 9

10. Planar or three-dimensional?
Planar imaging is the most common technique used in diagnostic radiology
General radiography e.g. PA chest
Mammography screening
Intra-oral dental radiography
Fluoroscopy (but some modern ones can do 3D)
The anatomy that is in the path of the beam is all projected onto a single image plane
Tissues will overlap and may not be clearly visible
Contrast is generally poorer than in 3D imaging techniques

11. Planar or three-dimensional?

12. Planar or three-dimensional?1 1 1
Subject contrast
7:1 1 7 1 1 1 1
2D detector
2D image contrast
3:1 3 9 3

13. Planar or three-dimensional?
3D imaging offers superior contrast to 2D
More techniques are becoming available
Computed Tomography (CT), Cone beam CT, Tomosynthesis, etc
Compromise is that doses tend to be much higher than the planar image
e.g. CT chest = 6.6 mSv c.f. PA chest = 0.02 mSv (a factor of 330 difference!)
Hence, despite being less common, they account for a significant proportion of the UK populations exposure to medical radiation
CT accounts for 11% of examinations, but 68% of dose (HPA 2008 review)

14. X-ray interactions with matter
It is the physics of the interactions with matter that determine how each imaging technique works, and how it is used in clinical practice
So, a bit of revision…

15. Attenuation & imaging
Contrast in an image is generated by the differential attenuation of the primary X-ray beam
Scatter occurs in all directions, so conveys no information about where it originated – can degrade image quality, if it reaches film/detector

16. Attenuation For a mono-energetic photon beam:
where, I = final intensity, I0 = incident intensity, µ = attenuation coefficient, x = thickness
Equal thicknesses of material reduce the intensity by the same fraction (half-value thickness).

17. Attenuation
Attenuation coefficient, µ, decreases with increasing photon energy (except for absorption edges)
Increases with atomic number of material, Z
Increases with density of material, ρ
Transmission of 70 kVp;
1 cm of soft tissue  66% transmitted
1 cm bone  17% transmitted
1 cm tooth  6% transmitted

18. Forward vs. Back-scatter
Forward scatter is most likely, but ...
Forward scatter is attenuated by the patient, and
Deeper layers receive a smaller intensity, so there are fewer scattering events
Overall, see more back scatter.
Advantage for image quality (less scatter, but more attenuation at the detector), but may pose a risk in terms of radiation protection

19. Forward vs. Back-scatter

20. Interaction Processes
Elastic scattering
Photoelectric effect
Compton effect

21. Elastic Scatter Photon energy smaller than BE
Causes e- to vibrate – re-radiates energy
No absorption, only scatter
< 10% of total interactions in diagnostic range i.e. not significant

22. Photoelectric Effect Process of complete absorption
~30% of interactions in diagnostic range
Energy is transferred to bound e-, which is ejected at a velocity determined by difference in photon and BE
e- dissipates energy locally, and is responsible for biological damage
Hence, main source of radiographic contrast (and dose), and why Lead is used in protection

23. Photoelectric Effect

24. Photoelectric Effect
Leaves atom in unstable state – electronic reconfiguration results in emission of X-ray or Auger electron
Auger emission more probable for low Z material – short range in tissue (= more biological damage)
Low energy X-rays reabsorbed locally
Rapid fall-off with increasing energy

25. Compton Effect
Process of scatter and partial absorption – inelastic scattering
Photon collides with a free electron (photon energy >> BE)
Loses small proportion of its energy and changes direction
Energy loss depends on scattering angle and initial photon energy
Photon free to undergo further interactions until completely absorbed (Photoelectric)

26. Compton Effect

27. Compton Effect
Compton scatter mass attenuation coefficient almost independent of energy over diagnostic range
Ratio of Z/A similar for most elements of biological interest (~0.5) – offers little in terms of radiographic contrast

28. The Mass Attenuation Interaction Coefficient
Each process is independent – can add the interaction coefficients to give the total mass attenuation coefficient
Z dependence is the source of contrast in radiographic imaging

29. The Mass Attenuation Interaction Coefficient

30. The Mass Attenuation Interaction Coefficient

31. Maximising Radiographic Contrast
Maximise contrast due to Photoelectric absorption – use lower energy photon beams (note, it is the mean energy of the beam, not kVp that is important)
Use scatter rejection techniques such as scatter grids and air gaps
Limit beam to smallest area consistent with diagnostic task to minimise amount of scatter generated
BUT…

32. Maximising Radiographic Contrast
More Photoelectric absorption means higher patient dose
Scatter rejection techniques attenuate the primary beam, so a higher patient dose is required for acceptable image receptor dose
NEED TO BALANCE IMAGE QUALITY WITH PATIENT DOSE!!!
Hence, the principle of ALARA (As Low As Reasonably Achievable)
Use the highest energy beam that gives acceptable contrast, consistent with the clinical requirements

33. The X-ray tube

34. Generating x-rays
X-rays are produced when highly energetic electrons interact with matter
Require;
An electron source (cathode)
A target (anode)
Evacuated path to accelerate across
High voltage ( kV)
However, most of the energy is given off as heat, and x-rays are thrown off in all directions
Require cooling and shielding
The x-ray tube insert
The generator
The tube housing

35. The physics of x-ray production
A bit of revision…
The physics of x-ray production

36. The physics of X-ray production
Electron reaches the anode with kinetic energy equivalent to the accelerating potential (kVp)
Electrons penetrate several micrometres below the surface of the target and lose energy by a combination of processes
Large number of small energy losses to outer electrons of the atoms = heat
Account for about 99% of all energy dissipated from e- beam in the diagnostic range
Relatively few, but large energy loss X-ray producing interactions with inner shell electrons or the nucleus

37. Heat generating processes
When an electron (e-) strikes the target, most likely interaction is with loosely bound e-s that surround nuclei
Relatively weak interactions – slight deflection, ionisation or excitation
Small amount of energy transfer (per interaction) – observed as heat
However, accounts for ~99% of all energy dissipated from e- beam in the diagnostic range

38. Bremsstrahlung

39. Bremsstrahlung
Radiates energy in all directions, up to a maximum equivalent to kVp = continuous spectrum
High energy cut-off (≡ kVp) due to release of all energy in head on collision
Low energy cut-off due to self-attenuation by x-ray tube
>80% of X-rays produced are Bremsstrahlung (except for mammography)

40. Bremsstrahlung

41. Characteristic x-rays
Interactions with tightly bound e- (typically K-shell)
If energy of e- exceeds binding energy (BE) → ionisation
Vacancy leaves atom unstable
e- from higher state drops down, releasing X-ray photon (energy = difference in BE)
Gives characteristic peaks on X-ray spectrum that are specific to the target
For Tungsten target, Kα = keV and Kβ = 67 keV
Not observed below 70 kVp

42. Characteristic x-rays

43. The X-ray spectrum
Combination of Bremsstrahlung and characteristic x-rays yields characteristic spectrum

44. The X-ray spectrum
The peak of the continuous spectrum is typically one third to one half of the maximum kV
The average (or effective) energy is between 50% and 60% of the maximum
e.g. a 90 kVp beam can be thought of as effectively emitting 45 keV X-rays (NOT 90 keV)
Area of the spectrum = total output of tube
As kVp increases, width and height of spectrum increases
For kVp, intensity is approximately proportional to kVp2 x mA

45. Controlling the X-ray spectrum – Exposure factors
Increasing kVp shifts the spectrum up and to the right
Both maximum and effective energy increases, along with the total number of photons
Increasing mAs (the tube current multiplied by the exposure time) does not affect the shape of the spectrum, but increases the output of the tube proportionately
kV waveform – three-phase or high frequency generators will have more high energy photons than single phase. Hence, output and effective energy are higher

46. Quality & Intensity Definitions:
Quality = the energy carried by the X-ray photons (a description of the penetrating power)
Intensity = the quantity of x-ray photons in the beam
An x-ray beam may vary in both its intensity and quality 46

47. Quality
Describes the penetrating power of the X-ray beam, and is governed by the kilo-voltage (kVp)
Usually described by the Half-Value Thickness
i.e. the thickness (in mm) of Al required to half the beam intensity for a given kVp
Typically >2.5 mm Al for general radiography
Changing the quality of the beam will change the contrast between different types of tissue.
A highly penetrating beam is referred to as ‘Hard’ and a poorly penetrating beam as ‘Soft’ 47

48. Intensity
Intensity - is the quantity of energy flow onto a given area over a given time; the ‘brightness’ of an x-ray beam
The tube current (mA) is a measure of X-ray beam intensity
Intensity is directly proportional to mA.
i.e. Double the mA, double the dose (quality not affected)
Intensity is also affected by kVp 48

49. The mechanics of making an x-ray
The x-ray tube

50. X-ray tubes – an overview
The x-ray tube insert is where we make the x-rays used in x-ray imaging systems
Cathode, anode, glass/metal enclosure under vacuum
The x-ray tube housing supports and shields the tube insert
The x-ray generator supplies the power
kV, mA and exposure time are selectable by the operator

51. The x-ray tube insert

52. The cathode The negative electrode
Composed of a filament (source of electrons) and a focussing cup
Often have two filaments for broad and fine focus
Filament heating current applied (approx. 10 V, 7 A)
Process of thermionic emission releases electrons from the surface of the filament
Heat up to ~2200°C
‘Free’ electrons in the metal gain enough energy to overcome the binding potential
Tungsten metal is ideal material
Need to apply high voltage to move these electrons across to the anode

53. Electron production in the X-ray tube
Applied voltage chosen to give correct velocity to the electrons kV + - mA
Filament
(heats up on prep.)‏
Target

54. The anode
Metal target electrode, held at a large positive potential difference relative to the cathode
Electrons strike the anode and convert kinetic energy to mostly heat, with relatively few x-rays released
So to get an acceptable output from the tube, a lot of heat must be generated that has to be dealt with
Don’t want to blow the tube up!
Limits are placed on exposure factors to avoid damage
Choice of material important
Alloys of 10% rhenium and 90% tungsten are resistant to surface damage – good for general x-ray tubes
Also other design features used to limit heat damage…

55. Stationary or rotating?
Simple x-ray tubes (e.g. dental) have tungsten insert on a solid copper block
Conducts heat away from focal spot
Limited to low tube currents as heat builds quickly
Rotating anodes used for most diagnostic applications
Bevelled disc on a rotor assembly
Spins 3,000-10,000 rpm
Spread the heat out over a large area
Allows greater loading and higher x-ray output
Tube does not energise until the disc is up to speed
Cooled via radiative emission transferring heat to tube insert and surrounding oil bath

56. The rotating anode

57. The rotating anode

58. Anode angles
The x-ray generating surface of the anode is angled (7-20⁰)
Actual focal spot is the area of anode struck by electrons
Apparent focal spot size is much smaller due to geometry
Heat spread out over a large area without affecting resolution

59. Anode angles
Smaller angles allow greater tube loading and finer resolution
BUT, limits maximum field size due to cut-off on anode side
Focal spot size varies across the image field
Anode side = shorter effective focal spots
Cathode side = elongated effective focal spot
Optimal target angle depends on clinical application

60. The focal spot Typical focal spot sizes are
mm for mammography
mm for general radiography
0.6 mm for fluoroscopy
mm for CT

61. The anode heel effect
Ideally, the X-ray beam would be uniform across whole image
However, this is not the case due to the anode heel effect
The steeper the target angle, the worse the effect
The electrons penetrate a few micrometres below the surface of the anode before generating X-rays
Hence, the X-rays that are generated in the target may be attenuated on their way out
X-rays travelling towards the anode edge of the field (A) will pass through more of the target before exiting the tube
Hence, attenuation will be greater on this edge, and beam intensity will be lower than on the Cathode side of the field (B)
Roughening of the anode surface as the tube ages make this worse
Generally not noticeable on most images
Can be minimised by using greater focus-to-detector distances, smaller fields and shallower target angles

62. The anode heel effect

63. Geometric unsharpness and the focal spot
Spatial resolution is dependent upon :
Geometrical unsharpness
Motion unsharpness
Absorption unsharpness
Geometric unsharpness is related to the fact that we cannot (and in fact do not want to) produce an ideal point source of X-rays
The focal spot of the X-ray tube has a finite size that results in blurring across the edge of structures
Can be reduced by using a smaller focal spot, decreasing the object-film distance (OFD) or using a longer focus-to-film distance (FFD)

64. Geometric unsharpness – The ideal point source
Ideal point source of X-rays
FFD
Object
OFD
Film/detector

65. Geometric unsharpness – A ‘real’ focal spot
Focal spot of finite size, f
FFD
Object
OFD
Film/detector
Penumbra

66. Geometric unsharpness and the focal spot
So, to minimise geometric unsharpness
The smallest focal spot should be used, especially if magnification imaging is to be performed (but be careful not to blow the tube!)
The patient should be positioned as close to the detector as possible (unless magnification imaging)
Largest possible focus-to-detector distances to reduce magnification and blurring (within practical limits!)

67. Geometric unsharpness – A ‘real’ focal spot
Focal spot of finite size, f
FFD
Object
Film/detector

68. X-ray tube design – dual focus

69. The x-ray tube housing

70. The tube housing Supports, insulates and protects the tube insert
Oil surrounds the insert for cooling and electrical insulation
Expansion bellows often fitted to prevent exposure if the oil (and hence tube) gets too hot
Some tubes have heat exchangers to actively cool the oil
Lead shielding is fitted around the tube housing to attenuate all x-rays not directed at the tube port
A small fraction of these will escape – known as leakage radiation
Legal limits on how much leakage is allowed

71. Collimators Adjust the size and shape of the x-ray beam
Two pairs of parallel blades of high attenuation material that can be adjusted to define the required rectangular field size
Has a light beam system for visualisation

72. Filtration
Filtration is the removal of x-ray photons as the beam passes through material
The x-ray tube has inherent filtration due to the anode, glass/metal envelope, cooling oil, etc
Added filtration is the sheets of metal deliberately inserted in the beam to reduce the number of low energy photons in the beam
Reduce the dose to the patient
Minimum standards on how much should be present in the beam (depending on application)
Can also have beam shaping filters to compensate for patient shape and to equalize the signal on the detector e.g. ‘bow-tie’ filters in CT

73. X-ray generators

74. Transformers
The x-ray generator provides the high voltage to the x-ray tube (20, ,000 Volts)
Transformers are used to convert low voltage (from a mains supply) into a high voltage through electromagnetic induction
Input AC power on the primary windings, induces a voltage on the secondary windings that is proportional to the input voltage and ratio of the number of turns

75. Transformers
However, to accelerate electrons across the tube, need anode to always be positive relative to cathode
AC waveforms change polarity every half-cycle
Rectifiers can prevent this
The high frequency generator is the now the most common type of generator
Takes a low voltage, low frequency input and converts to low voltage DC via rectifier and smoothing
An inverter creates a high frequency AC waveform from this
A transformer creates the high voltage
Rectified and smoothed again to provide high voltage DC to tube
A number of feedbacks are continuously checked during exposure to ensure appropriate kV and mA

76. High frequency x-ray generator

77. Ripple Ideal voltage would be constant
Real voltages tend to vary, and this depends on the type of generator used
Ripple describes variation in kV during exposure
Single phase = 100% ripple
High frequency = low ripple
Ripple reduces the output of the x-ray tube compared with what would otherwise be expected

78. The operator console
The operator can select tube voltage (kV), tube current (mA), exposure time (s) or the product of mA and time (mAs)
Focal spot selections on some systems
Choice depends on resolution requirements vs damage to x-ray tube
May also have controls for the automatic exposure control
The AEC determines how long exposures should be based on the transmission of x-rays through the subject
Pre-programmed techniques can also be found on the operator console
In fluoroscopy, manual kV and mA control not practical, so automatic control systems are used

79. The operator console

80. Automatic exposure control
AEC measures amount of radiation incident on the detector and terminates exposure when a pre-defined limit is reached
Compensates for different size patients and variations in attenuation
Can have some measure of manual control by using ‘fine density’ controls to force the system terminate a little earlier or later, depending on image quality requirements
Usually have a choice of ion chambers that can be selected in different configurations depending on clinical application
e.g. central chamber, left-right chambers, all three chambers

81. Automatic exposure control

82. Power ratings, heat loading and cooling
Power rating is the maximum power that a tube focal spot can accept or the generator deliver
Small focal spots have lower ratings than large focal spots
Power ratings vary significantly and depend on the modality being used
Heat Units are a simple way of expressing energy deposition and dissipation in the anode
Anode heating and cooling charts show heat loading for various

83. Heat rating
kV, mA and exposure time should be such that the temperature of the anode does not exceed its safe limit
The control system is designed to prevent exposures that exceed the tube rating
Require much higher tube ratings for CT and interventional fluoroscopy units

84. Factors affecting x-ray tube output

85. X-ray tube output
Output of an x-ray tube can be described in terms of;
Quality: Penetrating power of x-ray beam (HVL)
Quantity: Number of photons in x-ray beam
Six major factors determine x-ray tube output;
Anode target material
Tube kilovoltage
Tube current
Exposure time
Beam filtration
Generator waveform

86. Target material
The efficiency of Bremsstrahlung production depends on the atomic number of the anode material
Incident electrons more likely to interact with high Z materials
The anode material also determines the energy of the characteristic x-rays
Target material affects both quantity and quality of the x-ray beam

87. Tube kilovoltage Determines maximum energy of the x-ray beam
Affects quality of beam
Efficiency of x-ray production also affected
Quantity
Exposure ∝ kV2
Increasing kVp increases efficiency (quantity) and quality of the beam
Must change mAs alongside to compensate for changing output of the x-ray tube

88. The X-ray spectrum

89. Tube current and exposure time
Proportional to the number of electrons flowing from cathode to anode
Therefore directly proportional to number of x-rays produced
Quantity
Exposure time is how long the beam is on for
Quantity of x-rays is directly proportional to the exposure factor (mAs)
Beam quality not affected

90. Beam filtration
‘Soft x-rays’ contribute to patient dose without contributing to image production
Placing Al filters in the beam will increase beam quality – this is known as ‘Beam Hardening’
Alternative materials may be used for filtration in specialised applications e.g. mammography (Mo, Rh, Ag) and fluoroscopy (Cu)
Lowest energy photons are most readily absorbed as photoelectric absorption dominates (proportional to the E3)
As the beam passes the Al, the proportion of low energy photons is reduced, and the average photon energy increases

91. Filtration

92. Beam filtration
Modifies the quantity and quality of the x-ray beam by preferential absorption of the low energy x-rays
Reduce the number of x-ray photons
Increases mean energy of the beam

93. Filtration

94. Beam filtration
Hence, Patient dose is reduced with little affect on the radiation reaching the detector
However;
Radiographic contrast is reduced due to the higher mean energy of the beam
Greater exposure factors required to yield satisfactory dose at film/detector (have to drive the tube harder, and hence tube life may be reduced)
The X-ray beam is also filtered by the target that they are produced in, the coolant oil and the window of the housing
‘Inherent filtration’ equivalent to about 1 mm Al

95. Generator waveform Affects the quality and quantity of the beam
A single phase generator has a lower potential difference than a high-frequency generator
High ripple gives lower quality and tube output

96. Focus-to-skin Distance: The Inverse Square Law
For a point source, and in the absence of attenuation, intensity decreases as the inverse of the square of the distance
This is a statement of the conservation of energy

97. The inverse square law
Patient dose can be significantly reduced by increasing the distance to the X-ray tube
FSD < 45 cm should not be used (<60 cm for chests – 180 cm used in practice)

98. Summary

99. Summary X-rays are generated in the x-ray tube
A broad spectrum of energies are produced through Bremsstrahlung and characteristic x-rays
Heat generation is a significant problem in the x-ray tube
A number of factors affect the output of the x-ray tube, and these must all be considered and understood to enable the best possible image to be acquired (for the minimum radiation dose to the patient)