X-Ray Lectures & Lab Flashcards
Where on the EM radiation spectrum do x-rays lie?
Between UV light and gamma radiation
What is the energy range of x-rays?
Wavelength?
Frequency?
100 eV to 1 MeV
10-0.01 nanometers
30x1015 - 30x1018 Hz
What is the name for light emission evoked by scratching or rubbing?
triboluminescence
What is the significance of the rotation in a rotating anode x-ray tube?
rotation allows the large amount of heat generated from the electrons striking the anode to dissipate over the entire surface of the anode
How are electrons produced in a cathode tube?
By applying a high voltage to a cathode within a rarified (evacuated) medium, which accelerates electrons across the gap between the electrodes.
Draw the diagram of an x-ray cathode.
What is Bremsstrahlung?
EM radiation produced by deceleration of a charged particle when it is deflected by another charged particle
In Bremsstrahlung, what is the relationship between the degree of direction change of a deflected electron and the energy of the resulting x-ray?
the more the electron is deflected from its original path, the greater the x-ray energy
What is a characteristic X-Ray?
How are they formed?
X-rays emitted when an outer-shell electron fills an inner-shell vacancy
- Incident electron strikes a core electron, removing it
- This creates a “core hole”
- Outer shell electrons relax to fill the core hole
- This relaxation releases energy in the form of x-rays
Why are characteristic x-rays called “characteristic”?
Relaxation of a certain element’s outer shell electrons emits a distinct quantum of energy because every element has unique electron energy levels.
What special phenomenon can occur when an electron relaxes from an outer shell to fill a core hole?
What does it produce?
- the energy emitted from the relaxation can be transfered to another outer shell electron, causing it to escape the atom
- this creates an Auger electron
What is a characteristic x-ray called when the core hole is filled by an electron from the adjacent higher-energy shell?
What about an electron from 2 shells out?
3?
And which has the greatest energy?
Alpha Type X Ray
Beta Type
Gamma Type
etc.
The further out the electron comes from, the higher the energy output of its relaxation.
How does a graph of a characteristic x-ray spectrum look?
- background of Bremsstrahlung
- peaks for alpha and beta characteristic x-rays
- beta is smaller wavelength, and thus higher E, because it comes from a further out shell
Why can an x-ray spectrum only reveal elemental structure and not molecular?
Because outer shell electrons in molecules are tied up in bonding, so only inner shell electrons participate in characteristic x-radiation.
What is the main x-ray effect in diagnostic use of x-rays?
Describe it.
X-Ray Photoeffect
- similar to photo-effect with gamma radiation
- incident x-ray transfers its energy to an inner shell electron
- the electron is emitted in a single step
Why would a graph such as this one with curves for several different accelerating voltages only show characteristic x-ray peaks above a certain voltage?
Because lower voltages do not produce x-rays with high enough energy to remove tightly bound k shell electrons and make the electron holes necessary for characteristic x-radiation.
What is the Duane Hunt Law?
the minimum wavelength of Bremsstrahlung radiation is inversely proportional to accelerating voltage and can be determined via the equation:
λmin = k / U
k is a constant … K = h ᐧ c/e ≅ 1230 pmᐧkV
What 3 processes contribute to the attenuation of x-rays?
2 of them are more important than the 3rd… why?
- Compton Scatter - photon strikes outer shell e-, ejects lower energy Compton e- and is scattered
- **Photoeffect **- photon strikes inner shell e-, transfers most of its energy, ejecting photoelectron (dependent on absorber atomic #)
- Pair Production - photon strikes near nucleus, creates electron and positron pair
- pair production is irrelevant except in high energy therapeutic x-radiation because softer x-rays don’t have enough energy
What is the equation for mass attenuation coefficient for photoeffect?
τ m = C⋅λ3 ⋅ Z3
Where Z is atomic number and C is a proportionality factor
What takes the place of thickness (x) in the radiation attenuation law, when using a mass (μm) rather than linear attenuation coefficient (μ)?
Surface Density (Xm)
- the product of thickness x times density ρ
- has a unit of mass/area … g/cm2 in this case
What is the half value thickness equivalent when using mass attenuation coefficient?
Half Value Mass (Dm)
- mass of material behind 1 cm2 surface area that decreases incident intensity by half
Dm = 0.693/μm
What is the mass attenuation coefficient?
a density-independent radiation attenuation coefficient obtained by dividing linear attenuation coefficient by density of the absorber
unit is cm2/g
How can the atomic number of multi-atomic systems be approximated when trying to determine the mass attenuation coefficient for photoeffect?
Effective Atomic Number or Zeff
For the different absorption processes (including Rayleigh scatter), what is the dependence of attenuation coefficients on atomic number (Z)?
And what is their dependence on E of the incoming radiation?
What kind of x-ray is used in diagnostic imagine?
Bremsstrahlung
What processes use characteristic x-ray?
- bone densitometry
- ID of materials (toxic elements, etc.)
- x-ray therapy
- structural analysis (ex: DNA by Watson-Crick)
At the lower photon energies found in diagnostic x and gamma rays, how are atomic number of the absorbing material and the dominant absorption process related?
low atomic number - Compton effect
high atomic number - photoeffect
If higher atomic number (Z) means greater attenuation, why are elements with greater Z than lead not used for radiation protection?
because the higher elements tend to be radioactive themselves
Why must x-rays be “filtered”?
How are they filtered?
Bremsstrahlung x-rays have many wavelengths.
Lower wavelengths would be too greatly absorbed in the body, increasing risk of damage.
A 1-2 mm layer of aluminum is placed between the x-ray source and the body to absorb the lower wavelength radiation.
What is a “summation image”?
- an x-ray image in which all layers between the source and absorber take part in the absorption of x-radiation such that the image indicates the “sum” of the absorption by all layers
- creates the 2D images seen in standard x-ray pictures
(ex: multiple different organs all absorbing to a different degree)
What are the two types of contrast materials, their effects, usage and examples of them?
What if both of them are used?
- **Positive **- absorbs more than environment, barium sulfate in GI, iodine intravenously
- Negative - absorbs less than environment, gases often used
- if both positive and negative are used, this is **dual contrast **imaging
How can blood vessel lumens be visualized via x-ray?
Digital Subtraction Angiography
- x-ray is taken without contrast material
- positive contrast material is injected into blood, to create an x-ray in which the vessesl stand out
- image 1 is subtracted from image 2 to produce an image of only the vessels
What is an x-ray image amplifier use for and how does it work?
- decreases exposure and increase image quality
- radiation passes from source through body
- attenuated radiation hits luminescent screen
- light from screen hits photocathode
- e- ejected from cathode is accelerated thru electrodes
- accelerated e- hits 2nd luminescent screen, forming much brighter image
Explain traditional tomography.
a special x ray method used to negate the shadowing effects of layers other than the intended, observed layer
- angled x-ray source moves on semi-circular path
- detector parallel to body axis moves in opposite direction, always remaining within path of source
- the center of this circular path is somewhere within the body, producing a sharp image of only that layer
- used in pulmonology to remove rib shadowing and dentistry for dental panoramas
What is x-ray CT?
How does it differ from traditional tomography?
And spiral CT?
X-Ray Computed Tomography
- the density of different body parts are calculated by a computer
- spiral CT involves slow movement of the patient along the body axis through a rotating x-ray tube
What is the Hounsfield-Scale?
a scale which relates the attenuation of a material to that of water via the equation:
What is “windowing” in relation to xray?
relating the entire visible grey scale to a smaller part of the Hounsfield scale to exclude more or less attenuative tissues from the image
