EMR Spectrum
EMR is comprised of rapidly alternating magnetic and electric fields that are at 90 deg to each other, where both are perpendicular to the direction of wave propagation. EMR travels at the speed of light through a vacuum. The EM spectrum describes these waves in based on wavelength, with lower energy/longer wavelength EMR (e.g. radio waves) at one end of the spectrum and high energy/shorter wavelength EMR at the higher end of the spectrum (e.g. gamma rays)
EMR spectrum components
Low to high:
- Electric waves
- radiowaves
- infrared
- light
- ultraviolet
- x-rays
- gamma rays
Particle wave duality
EMR can exhibit properties of both waves and particles in different situations.
For example:
Photons of light exhibit particle-like behaviour during the photoelectric effect.
When visible light is refracted or reflected, it exhibits wave-like behaviour.
Production of X-rays
X-rays are EMR produced when applied voltage across tungsten anode and cathode cause the thermionic emission of electrons from the cathode to strike the tungsten anode. The electrons interact with the atoms of the anode to produce predominantly heat, bremsstrahlung radiation and characteristic radiation.
Bremsstrahlung radiation occurs when the emitted electron passes near the nucleus of an atom in the anode to experience attractive coulombic forces. The electron slows down due to these forces, and emits a photon of equivalent energy to the amount of kinetic energy lost in keeping with the law of conservation of energy. The trajectory of the electron is deviated, which can go on to interact with further atoms/electrons.
Characteristic radiation occurs when the electron with sufficient energy strikes an inner shell electron of the anode material atom. When this occurs, the electron in the shell is ejected and a vacancy within the inner electron shell is produced. Electrons from outer shells fill in this gap, and release energy in the form of photons equivalent to the energy difference between the shells. Since the binding energies of each shell are fixed and unique to each element, characteristic radiation is unique to each element. The electron emitted from the cathode must have energy just above the binding energy of the electron shell.
X-ray tube spectrum
An x-ray tube produces a continuous spectrum of x-rays of different energies, with the majority of x-rays at lower energies and decrease in intensity of x-rays with higher energies. Within the spectrum there are ‘spikes’ in the x-ray intensities at the energies that coincide with the characteristic radiation produced by the x-ray tube.
The majority of electrons will interact with the nuclei of the anode atoms at a distance (since the nucleus is so small compared with the size of the atom) and hence the majority of bremsstrahlung radiation is of lower energies.
The voltage produced by an x-ray generator is not always constant, and thus there will be a fluctuation in x-ray energies.
The characteristic radiation produced by an x-ray tube with tungsten anode is 70keV.
kV - X-ray Tube parameter
- increasing kV will increase both the x-ray intensity and the maximum energy of the x-ray spectrum.
- The x-ray beam intensity is proportional to the square of the kV
- Increasing the kV will increase the patient dose
- It will also increase the energy of the photons, and increase compton’s scatter
- Note that increasing the kV by 15% will result in incident radiation on receptor equivalent to doubling the mAs.
- Increasing kV will decrease contrast and increase scatter, but will increase beam penetrance
mA - X-ray tube parameter
- increasing the mA will increase the x-ray intensity, i.e. will result in proportional increase in number of x-rays at all x-ray energies
- no increase in E-max of the x-ray energy spectrum
- will increase the patient dose
- x-ray beam intensity it proportional to mA x s (time of exposure)
- will not increase contrast with increase mAs
- increased mA will decrease the exposure time required and will reduce the potential for movement artefact
Filtration: X-ray tube parameter
- selectively absorbs lower-energy x-rays by radio-opaque material
- will reduce the intensity of lower-energy photons
- filtering will reduce patient dose
Waveform: X-ray tube parameter
- x-ray generators vary in their ability to produce constant voltage to apply across x-ray tube
- unrectified single phase single pulse generators =100% ripple, low average voltage
- rectified single phase 2 pulse = 100% ripple
- rectified 3 phase 6 pulse = 13-25% ripple
- 3 phase 12 pulse = <10%
- constant potential generator <2% ripple
- the more constant the voltage applies is, the more useful energies of photons will be produced
Anode material: X-ray tube parameter
- tungsten anodes produce characteristic x-rays of 70keV
- molybdenum anodes produce characteristic x-rays of 18keV
- rhodium anode procudes characteristic xrays of around 22keV
- must use the appropriate anode for the type of radiography
- tungsten anodes useful for general radiography (CXR, AXR etc)
- Mo and Rhodium anodes used in mammography, where lower energy photons are required to maximise soft tissue contrast from the photoelectric effect (rather than compton’s scatter).
X-ray tube: Components
Glass housing:
- vacuum in glass housing allows thermionic emission to occur
Cooling oil:
Between the glass and metal housings, used to cool the x-ray tube
Metal Housing:
- metal housing to shield radiation, with window allowing transmission of x-ray beam
Cathode:
- tungsten filament within a focussing cup
- filament is heated to cause thermionic emission of e from the filament
- focussing cup focuses e beam towards the anode
- modern cathodes can have 2 filament sizes to allow changing of focal spot size
Anode:
- made of tungsten for general radiology
- round tungsten anode rotates to allow cooling for prolonged use of tube (rotated by rotor and stator) by increasing the effective target area
- tungsten anode surface is bevelled to allow manipulation of anode angles and focal spot size
Window:
- general radiology tube: thinner part of glass
- MMG: beryllium window, absorbs less low energy photons.
Filters and collimators can be used to alter the x-ray beam.
Line focus principle
- Describes the apparent decrease in focal spot size as viewed by the patient due to the anode angle when compared with the length of the irradiated tungsten target.
- The smaller the anode angle, the smaller the focal spot size
- The larger the anode angle, the larger the focal spot size
- The focal spot size is dependent on:
- Filament length
- Anode angle
- Normal anode angles:
- General radiology: ~10-15 degrees
- MMG/fluoroscopy angiography use smaller anode angles for increase the spatial resolution but decreases field of view ~<10
- Focal spot size of general radiography: 1.2mm, while in MMG = 0.3mm
Heel effect and effect on image quality
The heel effect describes the attenuation of x-ray beam on the anode side of the beam due to increased distance travelled through the anode.
This means that the x-ray beam produced on the anode size is less than that on that cathode side, resulting in varied irradiation of the subject that can lead to uneven exposure of images.
The heel effect can be used advantageously by placing the thicker part of the patient on the cathode side of the x-ray beam and the thinner part to the anode side. This will result in a more even exposure. E.g. in MMG, the thicker chest wall is positioned over the cathode side of the beam.
Factors that affect heel effect:
- Anode angle: larger anode angle, smaller heel effect
- Source to image detector (SID) distance: the close the detector to the source, the greater the heel effect
- Field size: the larger the field size, the larger the heel effect
X-ray generators
X-ray generators produce the voltage required to apply across the x-ray tube to produce x-rays. The ideal voltage output from an x-ray generator is constant voltage with little ripple (difference between the peak and trough voltages).
Single phase, unrectified x-ray generators will produce 100% ripple and low average voltages, resulting in low energy x-ray beam. 3 wave, 6/12 phase generators produce less ripple and higher average voltages. Medium-high frequency generators typically produce ripple < 15%, while constant potential generators produce <2% ripple.
The higher the ripple, the lower the average voltage output and lower the energies of the x-ray beam. This will lead to poor image quality and increased patient dose.
AEC
The AEC (photodetector) is a device that detects the amount of incident radiation on the image detector and automatically stops the x-ray tube when a certain amount of radiation is reached that will give a well-exposed image. It theoretically prevents over- and underexposure that would be a result of varying attenuation e.g. patient body habitus.
It consists of a radiation detector that measures the radiation incident on the image detector (photodiode, SSD, gas ionisation chamber). When a certain present threshold is reached for an exposure, it will shut down the x-ray tube. A backup timer is also in place in case of failure of the AEC.
IOnisation vs excitation
- Excitation occurs when energy absorbed by an electron within an atom is less than the binding energy for that electron. The electron becomes excited and subsequently de-excited, releasing energy in the form of a photon. Alternatively, the electron can remain in an excited state (e.g. photostimulable phosphors) and release the energy at a later time.
- Ionisation occurs when energy absorbed by an electron exceeds its binding energy and it is ejected from the atom. The electron shell vacancy is filled by electrons moving form higher shells, releasing characteristic radiation in the process. The atom becomes a cation, until is finds an outside electron to fill the vacancy in the outer shell.
Photostimulable phosphors
- Used in CR to absorb x-ray energies which is later released by laser
- the phosphors are typically barium fluorohalide crystals (BaFI) doped with europium and arranged on the imaging plate
- the europium form traps between the valence and conduction bands
- when x-ray photons strike the plate, the electrons within the crystals are excited and become caught in the ‘traps’
- the electrons can be later released from their traps by scanning the plate with a laser, which releases the electrons from their traps. The electrons de-excite and release visible light as they lose their energy.
Photoelectric effect
- Photoelectric effect occurs when a photon of lower energies strikes an inner shell electron of an atom.
- The photon is completely absorbed by the electron, and is ejected as a photoelectron.
- The inner e shell vacancy is filled by outer shell electrons cascading, and characteristic radiation is produced (which are of such low energy they are absorbed by the surrounding tissue
- Thus the result of the photoelectric effect:
- Photoelectron
- Characteristic radiation
- Cation
- The probability of photoelectric effect occurring is proportional to the cube of Z number of imaged material and inversely proportional to the energy of the photon. Thus low energy photons are used in MMG (e.g. MMG, where 17-22keV photons are used) because there less scatter.
Compton scatter
- Compton’s scatter occurs when a higher energy photo strike a low-energy outer shell electron in the target material
- The electron is ejected, and the photon loses energy as a result of the collision, resulting in a deviation in the path of the photon
- The photon can travel on and produce more interactions
- Result:
- Cation
- Ejected electron
- Photon on deviated path
- The probability of compton’s scatter occurring is independent of the Z number of the imaged material, and is proportional to the electron density of a material.
- increase in photon energy will increase comptons scatter but at very high energies it is inversely proportional
Rayleigh scatter
- Incident photon is scattered by an atom without any loss of energy or change in wavelength.
- Accounts for <5% of interactions in radiography
Scatter
- Increase in field size will increase scatter due to increased number of electrons available for interactions
- Increased kVp will increase both the number of photons and the energy of photons, which will increase the scatter (even though increasing energy will lead to decreased Compton’s scatter).
- Increased patient thickness will increase the amount of scatter due to increased tissue to penetrate (and hence more electrons available for interactions)
Attenuation
= reduction in x-ray beam intensity by interactions with the imaged materials by:
- rayleigh scatter
- photoelectric effect
- compton’s scatter
The linear attenuation coefficient (μ) describes the fraction of photons that is absorbed or scattered by a material per unit distance travelled through it.
It accounts for ALL the causes of attenuation of an x-ray beam as it travels through a material per unit distance.
- The linear attenuation coefficient is dependent on the physical density of a substance
- The fractional loss of photons as it passes through a substance per cm can be calculated by
- No e^-
- No = number of initial photons
- μ = linear attenuation coefficient
- t = thickness of material (cm)
- No e^-
- At small μ values, the value of μ = the fraction of photons passing through
- e.g. μ of 0.01 = 1% of photons lost, or 99% transmission of photons
- At LARGE μ values, the equation e^-μ(t) must be used (t = 1 cm)
- e.g. μ of 0.5 = e^-0.5 = 0.61
- i.e. there is 39% transmission, 61% loss of photons
- e.g. μ of 0.5 = e^-0.5 = 0.61
Note that the linear attenuation coefficient is dependent on density
The mass attenuation coefficient is linear attenuation coefficient / density à μ/p where p = density of the substance
Half value layer
HVL is the thickness of material required to cut transmission of photons by 50%
To calculate the HVL = ln2 / μ = 0.693 / μ
That is:
0.5 = e ^ -μ t
à
- μ t = ln 0.5
t = ln 2/ μ
t = 0.693 / μ
This if a material has a linear attenuation coefficient of 0.5, the HVL = 0.693 / 0.5 = 1.38 cm
Factors increasing attenuation
- increased thickness (distance travelled through the medium) increased attenuation
- increased density of the material increased attenuation
- increasing kV will lead to decreased probability of Comptons scattering but increased number of events of Compton’s scatter, will reduce the probability of Photoelectic effect
- increased atomic number of the imaged material will increase the probability of photoelectric effect
- Increased electron density will increase chances of Compton’s scatter
