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

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

Introduction

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’

A little bit of history…

Thankfully, things improved!…

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

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

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

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

Planar or three-dimensional?

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

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)

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…

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

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).

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 radiation @ 70 kVp; 1 cm of soft tissue  66% transmitted 1 cm bone  17% transmitted 1 cm tooth  6% transmitted

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

Forward vs. Back-scatter

Interaction Processes Elastic scattering Photoelectric effect Compton effect

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

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

Photoelectric Effect

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

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)

Compton Effect

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

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

The Mass Attenuation Interaction Coefficient

The Mass Attenuation Interaction Coefficient

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…

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

The X-ray tube

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 (20-150 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

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

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

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

Bremsstrahlung

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)

Bremsstrahlung

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 = 58-59 keV and Kβ = 67 keV Not observed below 70 kVp

Characteristic x-rays

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

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 60-120 kVp, intensity is approximately proportional to kVp2 x mA

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

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

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

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

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

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

The x-ray tube insert

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

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

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…

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

The rotating anode

The rotating anode

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

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

The focal spot Typical focal spot sizes are 0.15-0.3 mm for mammography 0.6-1.2 mm for general radiography 0.6 mm for fluoroscopy 0.6-1.0 mm for CT

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

The anode heel effect

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)

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

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

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!)

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

X-ray tube design – dual focus

The x-ray tube housing

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

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

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

X-ray generators

Transformers The x-ray generator provides the high voltage to the x-ray tube (20,000-150,000 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

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

High frequency x-ray generator

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

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

The operator console

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

Automatic exposure control

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

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

Factors affecting x-ray tube output

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

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

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

The X-ray spectrum

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

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

Filtration

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

Filtration

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

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

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

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)

Summary

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)