XR Flashcards
Heel effect is improved with
- larger anode angle
- larger focus to film distance
- smaller film size (field of view) - the field is more uniform in the center
quality and quanity trivia
Increased target Z =
Increased kVp =
Increased mAs =
Increased Voltage Ripple =
Added Filtration =
Increased target Z = inceases quality and quantity
Increased kVp = increases quality and quantity
Increased mAs = increased quantity only, no effect to quality
Increased Voltage Ripple = decrease in quantity and quality
Added Filtration = increased quality, decreased quantity
Is the probability of compton scatter dependent on Z of the atom or density of the material?
DENSITY OF THE MATERIAL
probability of compton scatter does NOT depend on the Z of an atom
probabilty of the photoelectric effect with change in atomic number
probability of the photoelectric effect is proportional to the atomic number cubed
Z3
equation for the probability of photoelectric effect in matter
probability of the photoelectric effect is inversely proportioal to the incident photon energy cubed:
1/E3
what is the TVL?
TVL = 10th half value length
it is the thickness of a material that can attenuate an xray 90%
Contrast media is used to increase the likelihood of a photon interaction due to an increase in the probability of:
Photoelectric effect Contrast media is used to increase the likelihood of photoelectric absorption.
what is the difference between the two curves?
Characteristic peaks changed, which is how you know the TARGET marterial is changed.
The green curve is a lower Z element. K shell energy increases with Z
What determines highest energy Xray, and what determines the lowest energy Xray
Highest energy xray depends on the selected kVp
Lowest energy xray depends on the placement of the xray filter which filters out lower enregy xrays
Photon interactions with tissue in the diagnostic energy range are dominated by:
Compton scatter
what happens to the heel effect as you increase the focus to film distance?
Larger focus to film distance = less heel effect
at shorter distances the heel effect is more pronounced
what is the effect on the bremsstrahlung spectrum if mAs is increased from 200 mAs to 400 mAs?
Coherent scattering is significant in diagnostic imaging because:
Coherent scatteringis significant in diagnostic imaging because it can degrade image quality. In coherent scattering reactions, no ionization occurs and no deposition of energy into the atom occurs. Coherent scattering events account for less than 5% of photon interaction in soft tissue at photon energies above 70 keV.
what is the apparent (effective) focal spot?
where the xrays land on the patient
Does compton scattering occur at higher or lower energies?
higher energies
In chest radiograhy, what is the orientation of the anode-cathode?
anode up cathode down, oriented vertically
Soft tissues incrase over the upper abdomen so you want the cathode side which has a more intense beam
How is Bremsstrahlung radiation generated?
Bremsstrahlung radiation is electromagnetic radiation emitted from the slowing down of electrons due to interactions between the negative charge of the electron and the positive charge of the nucleus. The energy of the bremsstrahlung photon is equal to the energy lost by the electron after it slowed compared to its initial energy.
Why are there are range of energies of Xrays given off from Bremststrahlung interactions?
as electrons slow down and approach the nucleus, the distance from the nucleus determines the energy given off.
there is a spectrum energies of xrays emitted
Amount of k-shell characteristic xrays at the following kVps
kVp manipulation trivia:
below 69.5 kVp =
Between 80-150 kVp =
Between 150-300 kVp =
Above 300 kVp =
below 69.5 kVp = Zero k-shell characteristic xrays
Between 80-150 kVp = 10-25% K shell characteristic xray contribution
Between 150-300 kVp = Progressive decrease in contribution
Above 300 kVp = negligible contribution
Photon with energy > _______eV is ionizing
15 eV
What is the effect on the heel effect when you make a steeper target angle?
Greater heel effect
Less material for the electron beam to pass through
xray wavelength
0.01 - 10 nm
General radiography focal spot size
0.6 - 1.2 mm
charge of cathode
negative
Three interactions that occur as electrons collide with the tungsten anode
- excitation (just heat, no xray)
- bremsstrahlung (“general” xray)
- ionization (“characteristic xray)
How does keV differ from kVp?
keV describes the energy of a single electron in the beam.
electronvolt (eV) is defined as the energy required to accelerate a single electron at rest through an electron potential difference of 1 volt in a vacuum.
1 eV ≈ 1.603 x 10-19 J
What is the difference between these two spectrums?
Loss of characteristic xrays due to dropping the kVp below the threshold for k shell electrons
probability of photoelectric absorption is approximately proportional to:
the probability of photoelectric absorption is approximately proportional to (Z/E)3, where Z is the atomic number of the tissue atom and E is the photon energy. As E gets larger, the likelihood of interaction drops rapidly.
http://xrayphysics.com/attenuation.html
How will the half value layer differ for a monoenergic and polyenergenic beam with the same peak voltage?
The half-value layer (HVL) for a monoenergetic x-ray beam will be greater than a polyenergetic beam due to greater attenuation of low-energy photons in the polyenergetic beam. The higher attenuation of low-energy photons results in a lower HVL. The 1st HVL for an x-ray beam is found using the linear attenuation coefficient, μ, which varies by photon energy.
synonyms for classical interactions
classical, coherent scattering, Thomson scattering
E of these xrays are <10keV
- does NOT result in ionization
- does NOT result in net transfer of E
- does NOT result in contribution to the image
- only adds a SMALL (TINY) amount of dose to the patient
this has to do with the xray (after leaving the anode) interacting with matter (the human body)
difference between anode in a portable device vs conventional xray tube
portable xray devices often use stationary anodes, limiting tube rating
mammo focal spot size
0.1 - 0.3 mm
What is a delta ray?
delta ray = secondary ionization event Secondary ionization results from ionization occurring from ejected (ionized) electrons that have sufficient energy to cause further ionizations.
How many characteristic Xrays are produced when kVp < 69.5 kVp?
Zero k-shell characteristic xrays
The majority of xrays will be energy levels too low to ionize k-shell e- in tungsten which have binding energy of -69.5keV
What is coherent scattering?
aka classical or Thomson (all same thing)
LOW ENERGY (< 10 keV) incident x-ray photon interacts with an orbital electron of a tissue atom and changes direction.
When such low-energy incident photons interact with tissue atoms, they are not likely to ionize (remove orbital electrons from their shell). Instead, the atom absorbs the energy of this x-ray photon, causing excitation of the atom, and then immediately releases the energy in a new direction (Figure 7-2).
Because the energy is reemitted in a new direction, it is now a scattered photon equal in E to the incident photon but travels in a new direction. Because of its low energy, most classical scatter photons are absorbed in the body through other interactions and do not contribute significantly to the image, but do add slightly to patient dose.
E of these xrays are <10keV
does NOT result in ionization
does NOT result in net transfer of E
does NOT result in contribution to the image
only adds a SMALL (TINY) amount of dose to the patient
In regards to the heel effect, is the cathode or anode side the “strong side”?
cathode side
two primary means by which we can change the x-ray beam produced by the tube:
altering the current (mA) and altering the volage (kV)
How do you make a smaller effective focal spot?
decreasing the target angle
How much energy needs to be transferred to an tungsten k-shell electron to produce a characteristic xray?
> 69 keV
Typically tube set to 80-150 kVp = 10-25%
what is kVp?
kVp = PEAK kilo-voltage
peak potential applied to the x-ray tube, which accelerates electrons from the cathode to the anode in radiography or computed tomography.
An increase in kVp extends and intensifies the x-ray emission spectrum, such that the maximal and average/effective energies are higher and the photon number/intensity is higher.
why is there a vacuum surrounding the anode and cathode?
prevent electrons from interacting with gas atoms
What is an Auger electron?
electron from inner shell is ejected –> atom is ionized –> inner shell is filled by an outer electron –> E is imparted to another electron which is ejected (instaed of E being given off as a characteristic xray)
film size effect on the heel effect
smaller film = less heel effect.
the central beam is the mot uniform. A larger percentage of the central beam will contribute to the exposure if the film is smaller
what is the “actual focal spot”?
where the electrons land on the anode. The area of electron bombardment (and Xray production) on the anode.
How are both the average photon energy and the quatity affected with more filtration?
Quality is improved. Average energy increases due to filtering out low energy photons
Quantity of electrons decreases with more filtration. you are losing electrons when you filter them
In mammo, what part of the breast is aligned with the cathode?
“C”athode on the “C”hest wall
Higher Z has what influence on Bremsstrahlung interactions?
Higher Z = More Bremsstrahlung
charge of anode
positive
“hard xray”
0.2-0.1 mm; more useful in imaging because they penetrate
Big or small anode has better spatial resolution?
smaller anode = better spatial resolution
What is compton scatter?
compton scatterhappens when an xray strikes an OUTER SHELL electron, which removes it from its orbit, the xray loses energy and is deflected in a new direction.
- the ejected electron goes off and interacts with other atoms.
- ionized atom is now positively charged –> potential for biologic harm.
- scattered photon due to deflection never hits the target
COMPTON = BAD (straight out of compton…those guys are trouble)
Compton contributes to dose and fogs up the image
Are more Xrays produced via Bremsstrahlung intractions or the photoelectric effect?
Bremsstrahlung (approx 80%)
Beam hardening occurs when what energy of photons is stripped from the x-ray beam, and what effective energy does this result in?
lower energy; higher effective energy Beam hardening occurs when the lower energy photons are removed from the x-ray beam as the beam passes through matter. The resultant beam has a higher effective energy.
what happens to the average photon energy with each HVL
With each HVL the averge photon energy goes up
3rd HVL > 2nd HVL > 1st HVL
k-shell binding energy of tungsten
-69.5 keV
what is the cathode
negatively charged filament –> super heated –> excited electrons –> thermoionic emission –> electrons shoot towards positively charged anode
photon linear attenuation coefficient is dependent on:
Energy of the photon and density of the material
gamma ray vs Xray
both are ionizing gamma ray: from nucleus, excess energy given off as an atom decays in an attempt to become more stable Xray: emitted by electrons
What is pair production?
high E photon (at least 1.022 MeV) is used on someting with a high Z
(or conversley a photon with around 30 MeV in human tissue)
photon will have enough juice (energy) to shoot past the electrons to interact directly with the nucleus.
Result is complete absorption ofthe photon and the production of a “pair” –> 1 electron and 1 positron
The positron is going to react like it does in a PET scanner –> it will travel a shot distance, collide with an electron and annihilate giving off two 511 keV photons 180 degrees apart.
What is the linear attenuation coefficient?
linear attenuation coefficient: is a constant that describes the fraction of attenuated incident photons in a monoenergetic beam per unit thickness of a material
- more attenuation with denser objects
- more attenuation occurs with high Z-material
- higher attenuation at the K-edge
- lower attenuation with higher kVp
what is the mass attenuation coefficient?
is a constant describing the fraction of photons removed from a monochromatic x-ray beam by a homogenous absorber per unit mass.
It is equivalent to the linear attenuation coefficient divided by the density of the absorber (μ/ρ), and is expressed in cm2/g.
The response curve labeled “A” refers to:
This image shows typical response curves for SFM (A) and FFDM (B). SFM has a limited dynamic range (a.k.a. latitude), which means it is not sensitive over a wide range of exposures. In contrast, digital mammography has a wider dynamic range (i.e., a constant sensitivity over a wider range of exposures) and is one of the several advantages of FFDM over SFM.
https://pubs.rsna.org/doi/pdf/10.1148/rg.246045102
What is quantum mottle?
polyenergetic xray beam is not uniform and at low dose you notice it more
low dose = decreased SNR
purpose of grids?
gridsimprove contrast, reduce scatter
Placed between patient and detector (filters are between tube and patient)
What is a grid ratio?
term to describe how dense a grid is. It’s basically the ratio of height and distance between grid septa.
The higher the ratio, the less scatter, better the contrast
Trade off = dose is increased (more xrays have to hit the detector for the correct exposure increasing the overall amount of dose to the patient due to increased exposure time) which is referred to as the“bucky factor”
what is the bucky factor?
refers to the ratio of mAs required with the grid / mAs without the grid
Grid cut off due to an upside down grid
unwanted absorption of x-rays via an x-ray grid, observed when a grid is employed incorrectly, most often seen with parallel grids. The term cutoff stems from the phenomenon in which the primary x-ray beam is ‘cut off’ by grid lines, leading to an overall decrease in optical density or a decrease in radiographic exposure (more opaque).
Most common reason for grid cut off?
Most commonly observed grid cutoff, most prominent at a short source to image receptor distance (SID), due to the divergent beam geometry, a small SID will lead to undesired absorption of the primary x-ray beam via the grid lines. Taking on the appearance of an overall decrease in optical density or variable optical density across the image (one side appearing less dense than the other).
The distance in which this occurs can be calculated using:
distance to grid cutoff = SID / grid ratio
What is the inverse square law?
The inverse square law describes the principle of dose reduction as the distance from the source increases.
This assumes a point source. If radiation spreads over a spherical area, as the radius increases, the area over which the dose is distributed increases according to
A=4πr2
where A is the area and r is the radius of the sphere.
Therefore, the dose is proportional to the inverse of the square of the radius. Thus if you double the distance you reduce the dose by a factor of four.
calculation of magnification:
What is the modulation transfer function?
The modulation transfer function(MTF) is the spatial frequency response of an imaging system or a component.
If the object is moved closer to the detector, what happens to the magnification?
magnification increases
SID/SOD
If SOD decreases (pt moved closer to the detector) then magnification increases.
For structures with high internal contrast you can use a higher kVp because you don’t need to accentuate the intrinsic contrast as much
kVp for abdominal radiographs DROPS because abdominal soft tissues are closer in density so you need to rely on your photoelectric interactions at lower keV. But notice mA is increased on the abdominal radiograph when kVp is dropped to maintain contrast
Grids aren’t used when the object is <10cm in thickenss
underlying mechanism of automatic exposure control
an ionization chamber placed between the patient and image receptor
what is “bit depth”?
basically the number of bits that determine the number of shades of gray that can be displayed on a computer monitor
calculated by 2x
So a 12 bit depth can display 4096
14 bit depth can display 16384
16 bit depth can display 65536
The more shades of gray available to display on a digital image, the better the contrast resolution (can be thought of as a precision component controlling the exact pixel brightnexx that can be specified)
Moiré type artifact can be seen in computed radiography (CR) systems if grids with low grid frequencies are used that are oriented parallel to the plate readers scan lines.
To avoid this artifact, grids with a frequency of at least 60/cm should be used, oriented perpendicular to the plate readers scan lines.
Upside down, incorrect focal length, and off-center grids would cause grid cut-off artifact, but not a Moiré type pattern.
How do you correct a Moire artifact?
Moiré type artifact can be seen in computed radiography (CR) systems if grids with low grid frequencies are used that are oriented parallel to the plate readers scan lines.
To avoid this artifact, grids with a frequency of at least 60/cm should be used, oriented perpendicular to the plate readers scan lines.
Upside down, incorrect focal length, and off-center grids would cause grid cut-off artifact, but not a Moiré type pattern.
Explain the process of an indirect DR
xray –> light –> charge
photon hits scintillator –> light emitted –> convtered by a photodiode into an electric charge –> electric charge is captured and transmitted by the thin-film-transistor (TFT) array to the workstation
