frcr: physics lectures diagnostic radiology lecture 1 an introduction to radiography with x-rays and...
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FRCR: Physics Lectures
Diagnostic Radiology
Lecture 1 An introduction to radiography with X-rays
and the X-ray tube
Dr Tim Wood
Clinical Scientist
Learning Objectives
5.2: Distinguish between different types of diagnostic medical image and understand how such images are created, reconstructed, processed, transmitted, stored and displayed
5.3: Describe the construction and function of medical imaging equipment including the radiation or ultrasound source, image-
forming components and image or signal receptor
5.4: Indicate how imaging equipment is operated and describe the imaging techniques that are performed with such equipment
Learning Objectives
5.5: Identify the type of information contained in images from different modalities
5.6: Distinguish between different indices or image quality, explain how they are inter-related and indicate how they are affected by changing the operating factors of imaging equipment
5.7: Identify agents that are used to enhance image contrast and explain their action
5.8: Explain how the performance of imaging equipment is measured and expressed
Learning Objectives
5.9: Describe the principles of quality assurance and outline how quality control tests of imaging equipment are performed and interpreted
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?
ra·di·ol·o·gy
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• 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
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 11 7
11
1 1 1
3 9 32D detector
Subject contrast7:1
2D image contrast3:1
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…
X-ray interactions with matter (revision)
• Contrast is generated by differential attenuation of the primary X-ray beam
• Attenuation is the result of both absorption and scatter interactions
• Scatter occurs in all directions, so conveys no information about where it originated – can degrade image quality, if it reaches film/detector
• Scatter increases with beam energy, and area irradiated
Pass throughAbsorptionScatter Attenuation
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
E
Z 2yProbabilit
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
3
3
E
ZyProbabilit
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
A
ZyProbabilit
• 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
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
X-ray tube design - basic principles
• Electrons generated by thermionic emission from a heated filament (cathode)
• Accelerating voltage (kVp) displaces space charge towards a metal target (anode)
• X-rays are produced when fast-moving electrons are suddenly stopped by impact on the metal target
• The kinetic energy is converted into X-rays (~1%) and heat (~99%)
X-ray tube designStationary anode – dental X-ray tube
Rotating anode – general X-ray tube
X-ray tube design• Evacuated glass envelope (allow electrons to
reach the target)• Filament (cathode) is source of electrons, with a
focussing cup around it to generate a narrow beam of electrons– Often dual focus to offer finer resolution on diagnostic
sets
Thermionic emission
• Applying a current to the filament causes it to heat up to ~2200°C (‘white hot’ like a light bulb)
• ‘Free’ electrons in the metal gain enough energy to overcome the binding potential – Can overcome the forces holding
them in the metal and escape from the surface
• Tungsten metal is ideal material
Thermionic emission
• Require two sources of electrical energy to generate X-rays– Filament heating current (~10
V, ~10 A)– Accelerating voltage of
between 30-150 kV (30,000-150,000 V); this results in a current of electrons between the anode and cathode (0.5-1000 mA)
Filament(heats up on prep.)
Target
kV
+-
Electron production in the X-ray tubeApplied voltage chosen to give correct velocity to the electrons
mA
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– 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• If e- passes close to nucleus, strong electromagnetic
interaction – decelerates, and deflected• Radiates energy in all directions as X-ray photons,
up to a maximum equivalent to kVp = continuous spectrum
• High energy cut-off (≡ kVp) due to release of all energy in head on collision with heavy nucleus
• Low energy cut-off due to self-attenuation by target, X-ray window and additional filtration
• >80% of X-rays produced are Bremsstrahlung (except for mammography)
Bremsstrahlung
Characteristic X-rays
Characteristic X-rays• Interactions with tightly bound e- (typically K-shell)• If energy of e- exceeds binding energy (BE) of bound
state → ionisation• Vacancy leaves atom unstable• e- from higher state drops down (most often from L- or
M-shell), releasing X-ray photon (energy = difference in BE)
• Gives characteristic peaks on X-ray spectrum that are specific to the target material (BE Z2)
• For Tungsten target, Kα = 58 keV and Kβ = 68 keV– Not observed below 70 kVp
The X-ray spectrum
• Combination of these yields characteristic spectrum.
0.00E+00
5.00E+04
1.00E+05
1.50E+05
2.00E+05
2.50E+05
3.00E+05
3.50E+05
4.00E+05
0 20 40 60 80 100 120 140
Energy (keV)
Inte
nsi
ty
60 kVp80 kVp120 kVp
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
0.00E+00
5.00E+04
1.00E+05
1.50E+05
2.00E+05
2.50E+05
3.00E+05
3.50E+05
4.00E+05
0 20 40 60 80 100 120 140
Energy (keV)
Inte
nsi
ty60 kVp80 kVp120 kVp
The X-ray spectrum
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
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’
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
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