Diagnostic X-rays Flashcards
What does a beam of high energy electrons from a heated filament bombard?
A positively charged heavy metal targets anode
What do electrons mostly react with?
Target’s orbital electrons producing heat (995), remaining electrons interact with target nuclei
How are X-rays formed?
Electrons changing direction and velocity within vicinity of . a heavy nucleus
What is deflection dependent on?
Nuclear charge (atomic number) of anode
What happens when there is direction and velocity change?
The electrons lose energy in the form of electromagnetic radiation producing a continuous x-ray spectrum
What is important in generation of X-rays?
Bremsstrahlung
What is the process of Bremsstrahlung?
A high speed electrons travelling in a material is slowed or completely stopped by forces of any atom it encounters
What happens as high speed electron approaches an atom?
It will interact with negative force from electron of atom, and it may be slowed or completely stopped
What happens if the electrons is slowed down?
It will exit material with less energy
What will be radiated as x-radiation of equal energy?
The energy used to slow electrons is excessive to atom
What is created at the surface of target?
A small proportion of x-ray
What type of x-ray are absorbed?
Those formed deeper within target material
Why does the energy transformation that yield x-rat radiation vary?
Bombarding electrons approach nuclei differently
What does the resulting x-radiation take the form of?
Continuous X-ray spectrum
What does the maximum beam energy Emax depend on?
Applied high voltage (Kv peak or Kvp) between cathode and anode
What results in x-ray spectrum curve with maximum?
The lowest energy photon are removed from spectrum since beam of x-ray leaving x-ray tube is filtered by glass envelope of x-ray tube
What creates orbital vacancies?
Collision between beam electrons and inner orbital electrons (K+L shells) of target
Energy of electrons > Binding energy of K-shell (BeK) or L shell (BEL)
What happens when electron vacancies are quickly filled?
Emission of characteristic radiation in form of line spectra
What is energy of characteristic radiation?
Difference between binding energies of electronic shell
BeK- BeL
How is characteristic X-ray emitted?
L electrons fills K-shell
K shell electrons is ejected
What is the most common material for anode target?
Tungsten
What range does line spectra occur at?
Roughly between the energies of 59keV and 69keV
What do majority of x-ray tubes employ?
Rotating anode to spread heat from electronic beam on to a larger surface for more effective anode cooling
How does x-rays originate?
Rectangular area (focal spot) of electron beam
What is x-ray tube?
vacuum tube that converts electrical input power into X-rays
What does the focal spot size influence?
Sharpness of x-ray image
Why are two focal spot necessary?
A second smaller spot is used for higher resolution
What does increasing high voltage Kvp increase?
Overall intensity of the continuous spectrum
What is intensity of x-ray beam proportional to?
Kilovoltage squared
What doubles X-ray beam intensity?
30% increase in Kvp
What increases with peak kilovoltage?
The effective energy, Eeff, of x-ray spectrum
What does the choice of kilovoltage determine?
Patient penetration and Image quality
Low kilovoltage
High contrast image (distinguish between soft tissue differenc)
What are easily absorbed?
Low energy x-ray photons
Higher kilovoltage
Increased penetration X-rays
Reduce contrast difference
What does the characteristic radiation not change with?
Kilovoltage but its increasing in intensity, contributing about 10% for a tungsten anode
What doesn’t alter the shape of x-ray spectra but intensity of x-ray beam increased proportionally to tube currents?
Increasing tube current
does not change effective energy
What is the x-ray beam intensity Q a product of?
- Anode material atomic number
- The square of applied kilovoltage kvp
- Tube current mA
Q=ZKVP^2mA
What is the intensity value Q influenced by?
Degree of beam filtration
How does effective energy increase?
Adding filters which remove the lower x-ray energies that change the proportion of high to low energy photons in spectrum
What significantly reduces the patient entrance dose?
Removing low energy radiation from beam
What happens when x-ray beam goes through attenuator?
Mean or effective energy is increased due to prefential loss of low energy x-rays
What does beam hardening cause?
Edges of object to appear brighter than centre, even if the material is same throughout
What is a collimator?
A device which narrows a beam of particles or waves
What are adjustable collimators used for?
Varying the overall x-ray beam size for different fields of view
What is anode heated considerably by?
Electron beam
What can x-ray photons be transmitted through?
Absorbing material unchanged
What does the arrival of x-ray photon at imaging detector depend on?
- Photon absorption
2. photon scatter
What goes to inner orbital electrons?
Incident photons
K-electron
Closest to atomic nucleus
L-electron
Orbit furthest away from nucleus
What happens to the electron involved in photoelectric absorption?
Ejected from atom with a kinetic energy Ee=photon E
What is required to free electrons from its original position?
Less energy
Ee=E-BE
What is radiation dose?
Ejected electrons which expend energy close to place of origin and energy imparted to these is energy truly given to absorber
What is empty K shell filled by?
Electrons from L or M electron orbit
What is maximum absorption?
K-electron binding energy
What is a contrast agent in vascular studies?
Iodine
What has a mean energy of diagnostic X-ray spectrum?
K-shell BE
33keV
What is photoelectric effect?
A form of interaction of x-ray/gamma photons with matter
What does a low energy photon interact with?
Electron in the atom and removes it from its shell
What does photoelectric effect contribute to?
Attenuation of x-ray beam as it passes through matter
What is photoelectric absorption related to?
- Atomic number of attenuating medium
- The energy of the incident photon (E)
- physical density of attenuating medium (p)
What is equation of PEA?
Z^3P/E^3
Beam collimators
Light and X-ray field match each other
When does compton effect occur?
Interaction of x-ray or gamma photon with free electrons or loosely bound valence shells electrons
Compton effect
Incident x-ray photons interact with orbital electrons from outer shell which has binding energy which is negliably small
What does the energy of scattered photon decrease with?
Increased scattered photon angle
What does electron density show?
Difference between dense materials
What does the direction of scattered radiation depend on?
Energy of incident beam
What is scatter proportional to?
Number of free electrons + photon energy E
What are examples of less dense material?
Water
Soft tissue
What is the probability of compton effect directly proportional to?
- Number of outer shell electrons i.e. the electron density
2. Physical density of the material
What is the probability of compton effect inversely proportional to?
Photon energy
What does the probability of compton effect not depend on?
Atomic number
As the photon energy increases
A forward angle of scatter is favoured
What do scattered electrons produce?
Biological damage
What does compton interaction dominate?
Photoelectric absorption for photon energies > 50kEV
When is photoelectric effect mostly common at?
Low photon energies
What relies to a greater extent on most diagnostic investigations?
Compton interactions
When is the contrast of image better?
Imaging with lower energy X-rays
What is the contrast C dependent on?
Thickness of target tissue
Difference in linear attenuation coefficients of target and uniform regions
What do most x-ray interactions produce?
Energetic electrons
What results in radiation dose?
Energy loss
What are more likely to interact?
Low energy X-rays
What is absorbed dose taken as ?
Physical indicator of biological tissue damage by ionizing radiation
What does the exposure of x-ray lead to?
radiation-induced cancer
