XR production Flashcards
Generally, how do the EMR types, XRs and gamma rays differ?
- XRs: originate from interactions between fast-moving electrons and atoms.
- Gamma rays: originate from the nuclei of an atom.
In nucs, why is plastic used to shield beta emitters?
- Beta emitters (classic = 90 Yttrium) can travel further than alpha, i.e., through paper, but not plastic.
- Plastic has a low Z, so fewer Brems are produced.
- If lead was used, way more Brems would be produced as the Z is much higher.
What is Bremsstrahlung radiation?
- This is created when incident electrons are slowed down (brake) by the nucleus.
- As they slow down they give off energy as other energy XRs.
- These other energies are not the characteristic XRs, and are represented by the numbers under the Brems curve.
mA vs. kVp: draw the table of differences re:
- What it controls in the tube.
- Effect on the Brems spectrum.
- Effect on XR quantity/intensity.
What makes heel effect worse?
- Smaller target angle
- Smaller source to image distance (SID)
- Large film size/field of view (the field is less uniform when spread out)
List 3 modifications for performing XR on a newborn.
- Do not use a grid
- Keep the current the same: so same # of XRs.
- Half the tube voltage: to 65 kVP, which will decrease skin dose.
- What does it mean if radiation is ionizing?
- How much energy does it take to make radiation ionizing?
- It means that the radiation can remove electrons from an atom.
- A photon must have at least 15eV to remove an electron from an atom.
In an exam setting, how would you know if the anode target material has changed and you’re given a Brems spectrum?
- The characteristic peaks will shift energy.
What is the proper way to orient the anode/cathode in CXRs to avoid heel effects?
- Vertically: this will ensure that XRs of similar intensity hit similar/paired structures at similar levels.
What is the special relationship b/w incident electron energy and the resultant photon?
No photon can have an energy greater than that of the incident electron.
List the 5 factors that change the XR (Brems) spectrum.
- mA
- kVp
- filtration
- target material:
- Changes the characteristic peaks w/higher Z material having peaks at higher energies/greater quality.
- Increasing the Z will also increase XR quantity.
- generator type:
- improving the efficiency will increase XR beam quality & quantity
How do K-shell characteristic XR energies compare to the K-shell binding energy?
- The XR energies will always be a little less than the binding energy.
What % of incident electrons are converted to XRs?
- Only 1%!!!
- 99% converted to heat.
What is the relationship of EMR velocity to its frequency and wavelength?
velocity = frequency x wavelength
- Since EMR velocity is fixed, frequency & wavelength have an inverse relationship
HVL:
- Define HVL.
- If a beam is filtered, what happens to the average XR photon energy?
- What happens to average XR/photon energy with each HVL?
- How does the HVL for a mono-energetic beam compare to that of a poly-energetic beam?
- Define 10th HVL.
- HVL = the amount of material required to attenuate an XR to 1/2 its original output.
- It increases.
- It increases: the more HVLs applied, the more energetic the average photon will be.
- Higher.
- Thickness of material required to kill an XR so that 90% is gone. (Used for shielding.)
DEXA:
- Basically, how is this done?
- What is the DEXA dose vs. spine XR dose?
- Measurements are made at 2 different photon energies: 40 & 70 keV.
- DEXA = 0.001 mSv; spine XR = 1.5mSv.
- What is the velocity of electromagnetic radiation?
3 x 108 m/s
What is the relationship b/w atomic number (Z) and K-shell binding energy?
K-shell binding energy (eV) is proportional to Z2
- So a higher atomic # element will give much higher keV XRs.
- This makes sense, since as the nucleus becomes larger, there are more protons in it, which binds the K-shell electrons all the more tightly.
What is the name of the innermost electron orbit?
K-shell.
This vs. that: XR housing vs. enclosure.
Housing:
- The shell around the tube.
- Purpose: to reduce XR leakage.
- XRs are produced isotropically, i.e., in all directions.
- The housing (made of lead) absorb the XRs flying in not so useful directions.
- Decreases dose to patient.
Enclosure:
- The bubble of the tube that is the shell around the anode, cathode, etc.
- Purpose: to maintain a vacuum.
- If the vacuum wasn’t there, then the electrons would be stopped by air atoms.
- There are a range of Brems energies produced by incident electrons. What determines the max energy Brems?
- At what approximate energy is the peak/hump of the Brems spectrum?
- The Max kVp applied.
- Brems can have any energy from 0 up to the max KE of the incident electrons.
- At ~1/3 Emax.
What happens if you drop the kVp below the K-shell energy threshold?
- You won’t generate any characteristic XRs.
- On the Brems spectrum, you won’t see the characteristic XR energy peaks.
Explain, in terms of K-shells, how XRs are produced.
- An electron moves in and strikes an inner shell (K-shell) electron, displacing it.
- In order to displace it, a constant amount of energy is needed.
- For tungsten, the electron must have 69.5keV.
- The K-shell electron is ejected, leaving a vacancy in the K-shell.
- An outer shell electron from that atom moves into the K-shell vacancy & a characteristic XR is released.
- Define off-focus radiation.
- What is its effect?
- This is scatter from the anode outside the actual focal spot.
- It increases pt exposure & image blurring.
What is the difference b/w actual and apparent/effective focal spot?
Actual: this is where the electrons actually land on the anode.
Apparent: this is where the XRs land on the pt & determines the amount of blur.
What is the Z and symbol for tungsten?
Z=74
Tungsten = W
Auger electrons:
- What are they/how are they made?
- Which kinds of elements are more likely to emit Auger electrons instead of XRs?
- If the energy from an incident electron frees a K-shell electron, but then an outer shell electron receives that energy and is ejected instead of an XR, then this is an Auger electron.
- No XRs are emitted by doing this!!!
- Smaller Z-number elements.
What is the K-shell binding energy of tungsten?
-69.5 keV
What is the difference in structure of an alpha vs. beta particle?
Alpha:
- Essentially a He nucleus: 2 protons, 2 neutrons.
- It attracts electrons b/c of its 2+ charge.
- It’s got a lot of energy, but fat and slow and can’t penetrate far, as it’s too fat.
- It’s stopped by paper.
Beta:
- An electron that has been ejected from an atom.
- It has less energy, but lighter, and can travel very fast and penetrate deeply.
- It’s stopped by plastic.
- Define target angle (re: anode).
- How does target angle relate to focal spot?
- How does target angle relate to heel effect?
- Target angle = the angle used on the anode to create the effective focal spot.
- The smaller the angle, the smaller the effective focal spot.
- The smaller the angle the greater the heel effect.
What does “thermionic emission of electrons” mean?
- This is how XRs are made in an XR tube for our purposes.
- Essentially, a current is run through a filament (made of tungsten) and heats it up, so boils the electrons off.
- The hotter a filament gets, the more electrons are released.
Draw the design of an XR tube.
- Tungsten filament which is the negatively-charged cathode (negative Cathy).
- Current is applied through the tungsten filament.
- This generates electrons from the filament.
- The electrons are attracted to the + charged anode (usually also made of tungsten).
- As the electrons accelerate they gain kinetic energy (keV).
- When they strike the anode target, they lose their kinetic energy in 3 ways:
- Excitation
- Ionization
- Radiative losses
Bremsstrahlung continuum:
- What do the peaks mean?
- They are the energies of the characteristic XRs.
- Each peak is the energy difference b/w the inner vacated shell & the outer shell that is vacated.
