Discovery, Properties & Production of X-rays Flashcards
Who discovered x-rays and when ?
The German scientist Wilhelm Conrad Roentgen on November 8, 1895
What tube was this person experimenting with, and how did it work ?
The Crookes tube - he discovered that there was a glow on a sheet of barium platinocyanide-coated paper far away from the tube. So he named the mysterious energy ray/ photon x-ray ‘x’ meaning unknown
What is the wave-particle duality ?
The concept by which light/x-ray can behave both as a wave and a particle (photon- particle of light).
Wave properties:
- has oscillating electric and magnetic fields at right angles to each other (electromagnetic waves)
- has an amplitude, wavelength and frequency as a wave - wavelength and frequency varies dependent on colour of light
- can defract as a wave
- have negative and positive interference
Particle properties:
- can exhibit photoelectric effect (
Blue light has high enough frequency to eject an electron off certain metals, whilst red light does not) - travel in straight lines
- electrons behave same way but are charged whilst a photon is neutral
Why can x-rays be described as waves ?
They move in waves that have wavelength (f) and frequency (λ)
What is the symbol for speed of light ?
c = speed of light
What is the equation for speed of light ?
c = f λ
What is the definition of wavelength ?
The distance between 2 successive troughs or crests of a wave
What unit is wavelength measured in ?
Angstroms (Å) (1 Å = 10^ -10 m)
What amount of angstroms does x-rays used in radiography range between ?
0.1 to 1.0 Å
What is the definition of frequency?
The number of waves passing a given point per given unit of time (second)
What are the symbols for wavelength and frequency ?
Frequency : f
or the Greek letter v, pronounced ‘nu’
Wavelength : λ
What units is frequency measured in ?
Hertz (Hz) - (l/s)
What amount of Hertz does x-rays used in radiography range between ?
3x10^18 and 3x10^19 Hz
What describes the relationship between wavelength and frequency?
An inversely proportional relationship
What is the relationship between wavelength and frequency for higher energy-rays?
Decreased wavelength and increased frequency
What is the relationship between wavelength and frequency for lower energy-rays?
Increased wavelength and decreased frequency
What does the symbol E stand for?
E = Photon Energy (1.986 x 10 ^ -25)/ λ
What does the symbol h stand for?
Planks constant (6.62 x 10^ -34 Js)
What are three formulas for E ?
E = hv
E = hc/λ
E = (1.986 x 10^ - 34 Js)
What property of X-rays make it impossible for them to be felt, smelled or heard?
Invisibility
X-rays being electrically neutral make what impossible?
X-rays having neither a positive or negative charge make it impossible for them to be accelerated or made to change direction using a magnet or electrical field
Why do x-rays produce no resistance in being put into motion nor do they produce force?
Because they have no mass
What speed do x-rays travel at?
The speed of light in a vacuum ( 3 x 10^8 m/s) or (186,000 miles per second)
How do x-rays travel?
In a straight line
How can x-rays interact with matter?
They can be absorbed or scattered
How are Bremsstrahlung X-rays emitted?
They are emitted through electron-nuclear field interactions
How are Characteristic X-rays emitted ?
They are emitted when orbital electrons transition from one allowed orbit to another vacancy in another allowed orbit
How does a photon (x-ray) interact with matter in the photoelectric effect?
The photon (incident photon) interacts with a tightly bound orbital electron of an attenuator then disappears, while that orbital electron is ejected from the atom as a photoelectron with kinetic energy
How does a photon (x-ray) interact with matter in Coherent (Rayleigh) scattering?
The incident photon interacts with a bound orbital electron (along with the combined action of the whole atom). The photon loses none of its energy (elastic event) and is scattered through a small angle
- since no energy transfer from photon to charged particle - no role in energy transfer coefficient but contributes to attenuation coefficient
How does a photon (x-ray) interact with matter in Incoherent scattering?
The incident photon interacts with a free and stationary orbital electron. This photon’s energy is much larger than the binding energy of the orbital electron. This photon loses part of its energy to the recoil electron (Compton electron) and is then scattered through a scattering angle
Describe the effect that increasing tube potential (kV) has on the x-ray spectrum
Increased:
- quantity of x-ray photons
- average energy
- maximum energy
- If kV is great enough characteristic energy is produced
What is the effect of filtration on a X-ray spectrum
It produces fewer lower energy photons
- increases average energy of photons (x-ray beam quality)
- decreases total number of photons (reduces AUC - X-ray beam quantity)
- if we increase density of filter it increases photoelectric effect which increases likelihood of X-rays to be attenuated.
*If increased atomic number also more attenuation - Aluminum typical used - Atomic number 13 - If we increase energy of X-rays attenuation decreases likelihood of photoelectric effect to occur (denominator of equation increases)
Inherent filtration - glass envelope, conducting oil, glass window (silicone mainly - atomic number 14, beryllium atomic number 4 or aluminum) , mirror in collimator of X-ray tube
Added filtration - 0.5 to 1.5 mm aluminum equivalent or beryllium or copper
Types of filters - added/compensation/equilibrium filters:
allows more equal distribution of exposure to detector
Wedge filter - attenuate more of beam over thinner part of patient’s body eg. Foot toe to heel
Bow tie filter - CT scanners - filter thicker at sides to attenuate X-rays more - as less tissue to go through at sides
Photoelectric effect Equation:
first symbol (likelihood of effect to occur), p = density, Z = atomic number, E = energy
Trough filter - X-rays attenuated more by thicker part towards thinner periphery of mediastinum (lungs) vs thicker center with sternum, heart and great vessels
What are the effects of having a more uniform (rectified) waveform current on the X-ray spectrum
- increases average energy
- Increases quantity of x-ray photons
- same maximum keV
Describe and illustrate the spectrum of x-ray energies produced by an X-ray tube
The resulting spectrum of x-ray photon en- ergies released is shown in the graph. At a specific photoenergy there are peaks where more x-rays are released. These are at the characteristic radiation energies and are dif- ferent for different materials. The rest of the graph is mainly Bremsstrahlung, in which pho- tons with a range of energies are produced. Bremsstrahlung accounts for the majority of x-ray photon production.
Discuss the process by which X-rays are produced
- Filament current applied through tung- sten filament at cathode.
- Heats up filament to produce enough en- ergy to overcome binding energy of elec- trons (thermionic emission).
- Electrons released from filament.
- Tube voltage is applied across the x-ray tube.
- Electrons, therefore, are accelerated to-
wards positively charged anode, which
gives them a certain energy. - The electrons strike the anode and the
energy released via interaction with the
anode atoms produces x-ray photons. - These x-ray photons leave the x-ray tube through the window in an x-ray beam to-
wards the patient. - They pass through the patient to the de-
tector to produce the x-ray image
Distinguish between Bremstraulang and characteristic x-rays
What is the anode heel effect
Describes the variation in x-ray intensities between the cathode and anode side of an x-ray field.
Due to the distances that the x-rays have to travel when leaving the anode.
Need to travel more distance through anode side. Therefore cathode side has more penetrating beam “
The phenomenon by which more target material is traversed at emission angles perpendicular to the electron beam (closer to the anode) than at those more parallel to it (closer to the cathode).
This increase in material leads to more resorption of the x-rays by the target material resulting in fewer x-rays reaching the field at angles perpendicular to the electron beam.
- therefore cathode side has more penetrating beam - should be placed over area of greatest intensity
- decreased anode angles gives smaller effective focal spot (useful in imaging) but larger anode heel effect - less uniform and more attenuated beam
- Because x-rays are produced deep in the target material they must traverse back out of it before they can proceed to the target field.
Three parameters than can change to manipulate heel effect:
- Anode angle
- SID - source to image distance - reduced effect when increased SID (detector more inferior to X-ray tube) by less variation of X-ray intensities
- Field size - collimate - removes X-rays on cathode an anode side of field
What is the line focus principle
The line-focus principle tells us that a steeper anode angle will reduce the size of the effective focal spot while an anode angle greater than 45° will increase the size of the effective focal spot in comparison to the width of the electron beam.
Application of the line-focus principle allows us to obtain the best possible resolution while spreading out the heat created by the electron interactions on a larger portion of the anode surface
When the angle of the anode becomes more parallel to the central ray, the effective focal spot gets smaller.
- choice of target angle is a compromise between tube loading, geometric unsharpness, and desired area to be covered by the useful beam. For practical purpose, at 40” FFD the anode angle should be no smaller than 15 degrees. A decrease in angle below six degrees will result in anode heel effect
- Steeper target angles create a greater amount of anode-heel effect.
*focus film distance (FFD)
Explain information on the tube rating charts
Describe the process of ionization and excitation of a photostimulable phosphor (PSP)
They are materials that store absorbed energy within excited electrons and release it in the form of light on exposure to laser energy.
Process of X-ray interaction:
- X-ray/ gamma photon interacts with phosphor then releases high energy electrons through PE and Compton scatter
- These secondary electrons excite surrounding electrons from valence to conduction bands through beastie collisions until the secondary electron has lost all kinetic energy
- electrons excited to the conduction band fall into empty electron traps within the forbidden zone, which are created by impurities in the material
- in order to return to the valence band, the electrons require excitation energy to overcome the trap
- in PSP the energy required to return to overcome the trap is high enough that they require laser light energy to do so
- the electron from the conduction band then returns to the valence band and due to the energy difference must lose energy in the form of an emitted photon.
Application
PSPs are used to record and reproduce a latent x-ray image by absorbing the radiation, then releasing the stored energy as light photons when stimulated by a HeNe laser. The emitted photons are detected by a photomultiplier tube, and an electronic signal is produced which is converted to a digital image for viewing on PACS.
Structure
PSP materials are crystal lattices, which give near uniform characteristics to electron bands, and impurities, which alter the electron bands to induce electron traps in the forbidden zone. One of the materials of choice is barium fluorobromide doped with europium (2+ valence), however many others exist with their own unique properties.
The material itself needs certain properties to satisfy a number of criteria in order for it to be useful in image acquisition:
release the stored energy when exposed to a wavelength produced by common lasers
release the stored energy in a photon wavelength readily absorbed by common photomultiplier tubes
retain the latent image without significant signal loss over time due to phosphorescence
Distinguish between atomic excitation vs ionization
Excitation is the addition of a discrete amount of energy to a system such as an atomic nucleus, an atom, or a molecule. It is the result of energy being given to an electron moving it to a higher energy level.
Ionization potential is the amount of energy required to remove the most loosely bound electron from a neutral, gaseous atom. If enough energy is given to the electron to remove it from the atom ionization has occurred.
What is X-ray luminescence in context of a PSP
X-ray luminescence is the physical mecha- nism by which x-ray energy is converted into light in a phosphor screen. It involves two mechanisms that both occur to some degree when a phosphor screen is irradiated:
X-ray fluorescence: the immediate emis- sion of light. This is the mechanism that predominates in screen film radiography
X-ray phosphorescence: this is when the emission of light is delayed over a times- cale of many minutes, hours or days and can be accelerated by shining specific coloured light onto the phosphor. This is the mechanism exploited in CR. It allows x-ray energy to be temporarily stored in a phosphor screen to be read-out later.
