BMEN90021 Medical Imaging
© 2022, The University of Melbourne
Radiography
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Dr Kathryn Stok
(Bldg 261, Biomedical Engineering)
Consultation hours:
by appointment
My Research
Micro-computed Tomography Image Processing Mechanobiology Mechanics of Materials (bio/TE)
© 2022, The University of Melbourne
Radiography: x-rays
• Atransmission-basedtechnique
• X-rays from a source travel through an object (patient), to be detected by a film or ionising chamber on other side.
• Howdoesimagecontrastarise?
• Differential attenuation of x-rays in the body.
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• Whatdo“x-rays”and“Student’st-distribution”have in common?
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• 27th March 1845 – 10th February 1923
• X-rays originally known as Röntgen rays
• Nobel prize in 1901
• Mechanical engineering @ University of Zurich
• Refused to patent work, as he believed all should benefit from the fruits of his scientific career.
• 1895, experimented with current though gas in vacuum tubes to produce cathode rays.
• Sealed tubes in cardboard cartons ∴ no external light sources
• Put wife’s hand in path of rays, photographic film the other side.
• Wife’s quote: “I have seen my death!”
• By 1896, x-ray machines were everywhere…
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Uses of x-ray
• Intravenouspyelography(IVP)
• Genitourinary tract including kidney stones
• Abdominalradiography
• Liver, bladder, abdomen, pelvis
• Chestradiography
• Lungs & ribs
• X-rayfluoroscopy
• Genitourinary & gasterointestinal diseases
• Mammography
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Disadvantages of x-rays
• Ionisingradiation
• Can cause tissue damage
• Particular concern for pediatric & obstetric imaging
• Planarprojectionoftissue
• Overlapping soft tissues and bones can be very difficult to interpret in x-ray images
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The electromagnetic spectrum
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What are x-rays?
• X-raysareelectromagneticwaves
h: Planck’s constant
f: frequency
c: speed of light in a vacuum λ: wavelength
• Note that 1eV = 1.602 x 10-19J
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X-ray source = X-ray “tube”
Shape of x-ray beam is related to beam of electrons leaving cathode. Therefore careful design required.
A potential difference of 15-150 kV applied between anode and cathode, as a rectified alternating voltage. Max voltage = “kilovolts peak” kVp
Anode: positively charged metal target
Cathode: negatively charged electron source
tungsten wire: 200μm diameter Coil: 2mm diameter, 1cm height
Electric current passes through cathode, causing heating to 2200 ̊C, at which point thermal energy is sufficient for some electrons to move away = thermionic emission
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Focusing cup
The larger the negative potential, the narrower the beam.
If large enough (~2kV), beam switches off entirely. This is the basis for pulsed CT operation.
f = F sin(θ)
f = 0.3 mm for mammography
f = 0.6 – 1.2 mm for other forms
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Approximately:
coverage = 2 * (source-to-patient distance) * tan(θ)
Electrons to x-rays
• Metalanodeabsorbssomekineticenergy from electrons, producing x-rays.
• Thereforetypeofmetalisimportant.
• Thehighertheatomicnumber,themore efficient the production of x-rays.
• Tungstenmostcommon:Atomicnumber74
• Platinum (78) and gold (79) have lower melting points, therefore not suitable.
• Mammography uses molybdenum, lower energy x-rays
• Tungsten:stillonly1%energyintox-rays
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Why tungsten?
• High atomic number (74)
• High melting point (3370°C) • Good thermal conductivity
• Low vapour pressure
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X-rays so far…
• Thusfar,thex-raytubehasbeenintroduced.
• Wenowneedtolearnabout:
• the x-ray tube in more detail,
• the x-ray energy spectrum,
• how x-rays interact with tissue, and
• X-ray instrumentation.
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The X-ray tube current
• Currentinthex-raytubedefinedasthenumberofelectrons
per second that travel from the tungsten cathode to the anode.
• 50 – 400 mA for planar x-ray
• (Up to 1000 mA for CT)
• Lower currents in continuous imaging, eg. fluoroscopy
• “Tubeoutput”ismeasuredinwatts
• Tube output also depends on the strength of vacuum.
• Q: What effect on current would a stronger vacuum have?
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Tube power rating
• Tubepowerratingisdefinedasthemaximumpower
dissipated in an exposure time of 0.1secs.
• eg.AtubeoperatesatkVp=80kVwithtubecurrent= 1.25A for 0.1secs. What is its power rating?
a)10kW b)80kW c)100kW
• Tubeoperationlimitedbyanodeheating
• Anoderotatestoincreaseeffectivesurfacearea
• Reduces the amount of power deposited per unit area per unit time
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Intensity of the x-ray beam
• Theintensity,I,definedaspowerperunitarea(J/m2)
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Energy of x-rays
Number of x-rays
The x-ray energy spectrum
• Thex-rayenergyspectrumisaplotofenergyvs. intensity:
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X-ray sources
• Whenhigh-energyelectronsstriketheanode, x-rays can be created in two ways:
1. Bremmstrahlung = general radiation 2. Characteristic radiation
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Bremmstrahlung
• Anelectronpassingneara tungsten nucleus deflects due to positive force.
• Results in loss of kinetic energy.
• X-rayemitted,conserving energy
emitted x-ray
• Eachelectronhasmanysuchinteractions
• Therefore a wide range of x-ray energies are emitted
from anode.
• Q: What is the maximum possible energy of these x-rays?
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Bremmstrahlung in the spectrum
• Alineardecreaseinx-rayintensitywithx-ray energy.
This drop-off caused by low energy x-rays being absorbed by the tube housing
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Characteristic radiation
Electrons orbit in shells
• K shell: max 2 electrons
• L shell: max 8 electrons
• M shell: max 18 electrons
• Tightest binding energy in K shell.
• Steptogeneratinganx-ray:
1. Electron from cathode hits tightly bound electron in K shell
2. K shell electron is ejected, leaving a hole
3. Hole is filled by an L (or further out) shell electron
4. Drop in potential energy due to different binding energies is radiated as a single x-ray.
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Facts about characteristic rad’n
• Electron in K-shell has binding energy of 69.5keV
• Electron from cathode must have energy greater than 69.5keV to eject a K-shell electron
• Electron in L-shell has binding energy of
• Electron falls from L-shell to K-shell Therefore x-ray emitted with 59.3keV energy
• In reality, far more complicated shells (sub-shells etc)
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Characteristic rad’n in the spectrum • No characteristic radiation below kVp = 69.5kV
• kVp = 80 – 150kV, ~ 30-50% of intensity in spectrum is characteristic radiation
• Ejection from higher shells possible, but so rare considered negligible.
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Characteristic radiation: elements
• Every element has a unique set of binding energies
• “Characteristic” because the spectrum is unique to the element
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Interaction of x-rays and tissue
Differential attenuation of x-rays in tissues. • This produces
Primary radiation passes straight through tissue
Absorbed radiation absorbed by the tissue
This diagram is too simple, as a third category is scattered by tissue = secondary radiation.
Scattering has two types: coherent and Compton scatting.
© 2022, The University of Melbourne
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Coherent scattering
• Non-ionizinginteractionbetweenx-rays and tissue
• Theincidentx-rayenergyisconverted to simple harmonic motion of electrons in atom
• no ejection of electrons
• Atomthenre-radiatesenergyinformof a scattered x-ray
• Random direction
• Thereforenumberofx-raysreaching detector is reduced.
• Forthosethatreachthedetector,their trajectories have been altered.
Although it appears in this diagram that the scattered x- ray angle is related to incident x-ray angle, this is not the case. The scattering angle is random.
© 2022, The University of Melbourne
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Probability of coherent scattering • Theprobabilityofacoherentscatteringeventis:
• E = energy of incident x-rays
• Zeff = effective atomic number of tissue
• Muscle Zeff = 7.4
• Bone Zeff ~ 20
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Compton scattering
• Interactionbetweenincidentx-rayandaloosely
bound electron in outer shell of tissue atom.
• Theangleofdeflectionrelatedtohowmuchenergy is transferred to ejecting the electron.
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Probability of Compton scattering • Probabilityofx-rayundergoingComptonscatteringis:
1. independent of effective atomic number of tissue
• Therefore little tissue contrast in scattered x- rays
2. linearly proportional to tissue electron density • Therefore, a small amount of image contrast
due to tissue electron density differences. 3. weakly dependent on incident x-ray energy
•Compton scattering dominates at high x-ray energy
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Absorbed radiation
Absorbed radiation absorbed by the tissue
• Predominantmechanismbywhich x-rays are absorbed in tissue is the photoelectric effect:
1. X-rays hits tightly bound electron in K shell
2. K shell electron is ejected, leaving a hole
3. Hole is filled by an L (or further out) shell electron
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Drop in potential energy due to different binding energies is radiated as characteristic radiation (another x-ray).
The photoelectric effect
SOUND FAMILIAR?
• Familiar, but importantly different.
This is not to be confused with characteristic radiation as an x-ray source
Here, the characteristic radiation is very low energy and is absorbed almost immediately.
• Therefore, net effect is that incident x-ray does not reach
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Probability of photoelectric effect
• Atlowx-rayenergies, ejection of electrons from L and M shells.
• At“K-edge”,justhigher than binding energy of K shell electrons, probability of photoelectric interactions increases rapidly.
• Higherprobabilitiesfor calcium than oxygen indicate more absorption of bone than air.
Above K-edge:
© 2022, The University of Melbourne
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Attenuation of x-rays in tissue
• Howdoestheintensityofthex-raybeamchange as it moves through tissue?
Object being imaged
X-ray source
Intensity at x
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Linear attenuation coefficient
• Theconstantμiscalledthelinearattenuationcoefficient. • Theunitsofμarecm-1
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Linear attenuation coefficient
• Allabsorptionandscatteringmechanismscombine:
𝜇𝜇=𝜇𝜇 +𝜇𝜇 +𝜇𝜇 𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝 𝐶𝐶𝑝𝑝𝐶𝐶𝑝𝑝𝑝𝑝𝑝𝑝𝐶𝐶 𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝐶𝐶𝑝𝑝
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Photoelectric effect dominates at low x-ray energies
Compton scattering found more at higher x-ray energies
Linear attenuation coefficient
• Allabsorptionandscatteringmechanismscombine:
Photoelectric effect dominates at low x-ray energies
© 2022, The University of Melbourne
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Compton scattering found more at higher x-ray energies
Mass attenuation coefficient
• Insteadofthelinearattenuation coefficient, μ, in cm-1:
Often use a “mass attenuation coefficient” which is μ divided by the tissue density.
Units of mass attenuation coefficient: cm2/g
• Why? © 2022, The University of Melbourne
The reason why it is difficult to image soft- tissue contrast with x- rays.
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Half-value layer
• AparameterusedtocharacteriseX-rayattenuation:
• Half-value layer = thickness of tissue that attenuates
half the x-ray intensities.
© 2022, The University of Melbourne
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More on attenuation coefficients
X-ray intensity attenuation: 𝐼𝐼𝑥𝑥 = 𝐼𝐼0𝑒𝑒-μx
• This basic relationship is valid only in ideal case of: • homogeneous tissue
• single energy x-ray
• Inreality,μisafunctionofboththetissuetypeand the x-ray energy:
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Generalised intensity expressions
• Forasingle-energyx-raythroughanon- homogeneous medium:
𝐼𝐼 =𝐼𝐼𝑒𝑒−�𝑥𝑥 𝜇𝜇𝑥𝑥′ d𝑥𝑥′ 𝑥𝑥 0 𝑥𝑥0
• Forabeamofx-raysofdifferentenergiesdescribed by distribution σ(E):
𝐼𝐼 = �∞𝜎𝜎 𝐸𝐸 𝑒𝑒− �𝑥𝑥 𝜇𝜇 E,𝑥𝑥′ d𝑥𝑥′ d𝐸𝐸 𝑥𝑥0 𝑥𝑥0
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• Ifthethicknessofchestis20cm,whatisthepercentageofX-rays that are transmitted through the chest at an incident energy of 70 keV, assuming HVL values of 3.5 cm for muscle and 1.8 cm for bone? The bone thickness is 4 cm and the tissue (muscle) is 16 cm.
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• Ifthethicknessofchestis20cm,whatisthepercentageofX-rays that are transmitted through the chest at an incident energy of 70 keV, assuming HVL values of 3.5 cm for muscle and 1.8 cm for bone? The bone thickness is 4 cm and the tissue (muscle) is 16 cm.
• Solution:
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X-ray instrumentation • Backtothebasicset-up:
• The“collimator”restrictsthedivergenceofthebeam.
• Makes the beam more “column”-like.
• Consists of lead sheets moved to open the appropriate field-of-view.
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The effect of scattering
• Ideallyallx-raysatfilmwouldbeprimary.
• However,evenwithacollimator,secondaryradiationcan be 50-90% of the x-rays reaching the detector.
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No scatter
Some scatter
Predominantly scatter
Anti-scatter grids
• Anti-scattergridcanbeplacedontopofthex-ray detector.
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Typically from 4:1 to 16:1 Typically 25 to 60 per cm
Intensifying screens
• Tocreatehigh-qualityimages,wouldneedahugepatient dose to deliver sufficient x-rays to the detector.
• Therefore,intensifyingscreensareused.
1. X-rays strike phosphor layers
2. Light emitted from phosphor in all directions
3. Reflective layer reflects light back to film.
Generally double-sided screens. Improves sensitivity by factor of 50!
Therefore can reduce tube current and/or exposure time
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Properties of intensifying screens
• Thethickertheintensifyingscreen,themorediffuse the light beam.
“Light spread function”
• Phosphor layers typically Gadolinium or Lanthanam based. Emit light in green and blue range, respectively.
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Luminescence terminology
• Luminescence=abilityofmaterialtoemitlightafter excitation, either immediately or delayed.
• Fluroescence = Material fluoresces when light emission begins immediately upon excitation, and ceases immediately excitation end.
• Phosphoresence (afterglow) = Continuation of light emission after excitation has ended.
• Inintensifyingscreens,don’twant phosphorescence, as it causes ghosting / “fogging”.
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X-ray film
• Should be maximally sensitive to green or blue light (depending on type of screen used).
• Gel matrix of silver halide particles. When exposed to light and then developed (chemical reduction process) the exposed silver salts turn to metallic silver, which is black.
• Dark regions of image, maximum exposure
• Light regions of image, tissue absorption.
• Thisisa‘negative’image,inconventional photographic film terms.
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© 2022, The University of Melbourne
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Characteristics of x-ray film
• Graininess: Image from silver crystals not continuous
• Speed:Inverselyproportionaltotheamountoflight needed to produce an amount of metallic silver on
development.
• Fact: The number of photons needed to change the grain into metallic silver is independent of grain size.
• Therefore, large grain size = fast film.
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Characteristics of x-ray film • Contrast:PlotofDvsE
• Optical density D: quantifies film blackening via ratio of light incident on film to that passing through. The darker the film, the higher the D.
• Exposure time E
Gamma is max slope of linear region:
The larger the gamma, the smaller the useful exposure range.
log E1 © 2022, The University of Melbourne
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Digital radiography
Computed radiography
• Instead of film, cassette containing an imaging plate
(IP) made of storage-based phosphors
• X-raysexciteatoms,electronstrappedin“electrontraps” • Photonsnotemitteduntilre-stimulated,eg.bylaser.
• After exposure, IP put into a machine to be read • Laserusedtore-stimulatephotons
• Visiblelightisthencapturedbyanopticarray,
sent to a photomultiplier to convert to
analog electrical signal.
• ElectricalsignalputthroughA/Dconverter
(“Analog-to-digital”)
• Resultisadigitalimage(matrix)
© 2022, The University of Melbourne
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Digital radiography
Direct radiography
• Flat panel detectors: Light sensitive active matrix array
• Array of photodiodes on a substrate, coupled to a fluorescent plate
• a photoconductor to cut out the need for fluorescence step, with array of capacitors storing the charge, to later be read out.
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Image quality: SNR
• Factorsaffectingsignal-to-noiseratio:
• Exposure time and tube current
• The kVp value: Higher means more x-rays produced and more x-rays reaching the detector.
• Thickness of patient
• Thickness of phosphor layer
• Quantum mottle: statistical nature of x-rays
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Image quality: Resolution
• Factorsaffectingimageresolution
• Thickness of intensifying screen
• Speed of the x-ray film (fast = large particles)
• Narrowness of x-ray focal spot: anode bevel angle
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Image quality: Contrast
• Factorsaffectingcontrast-to-noiseratio:
Energy of incident x-rays:
• Photoelectric effect dominates at lower energies
• Compton scattering dominates at higher energies
Thickness of body part being imaged
• Thicker means more Compton scattering & less detected
Intensifying screen/film combination
• Detector should amplify differences due to attenuation
Absorption efficiency of detector
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X-ray contrast agents
• Chemicalsintroducedintobodytoincrea
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