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
absorption edge of a contrast agent is a determined by:
binding energy of the k-shell electon The absorption edge of a contrast agent is determined by the binding energy of the K-shell electron. An absorption edge also exists at each energy level corresponding to the binding energy of each electron shell (e.g,. L-shell, etc). However, in most filtered x-ray beams, these lower energy absorption edges are not observed due to filtration of low energy x-ray from the beam.
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
effect of mA and kVp on focal spot
1) high mA, low kVp
2) High kVp
1) high mA, low kVp = Wider spot “blooming”
2) High kVp = smaller spot “thinning”
velocity of electromagnetic radiation
3 x 10^8 m/s
how does filtration of the xray beam affect half value layer
more filtration = higher HVL
less filtration = lower HVL
What does xray contrast refer to?
the ability to tell that two things are different, the more contrast, the easier to tell them apart.
This does NOT refer to administered IV or oral contrast. But, barium and iodine both help increase xray contrast because they have high Zs and therefore more photoelectric effect. More PE = more contrast.
What is the “k-edge”
k-edgerefers to abrupt increase in the photoelectric affect corresponding to the k-shell binding E peak of the matter the xray hits
what is the k-edge of barium?
37 keV
what is the k-edge of iodine?
33 keV
Why choose a kVp between 65-90 when using contrast agents such as barium and iodine?
The xray beam is polychromatic with the kVp representing the PEAK (a minority of the photons). The average energy is going to e between 1/3 to 1/2 the max energy, placing the k edge right in the meat of the band (barium kedge is 37keV and iodine k edge is 33 keV)- maximizing the spike in attenuation of the photoelectric effect.
If mAs is held constant, how much should you raise the kVp to double the intensity of the spectrum?
raise kVp by 15%
kVp is increasing the amount of xrays produced AND increasing the average energy of the beam
If kVp is held constant, how much should you raise the mAs to double the intensity of the spectrum?
double mAs. mAs is directly proportional to beam intensity
How should you alter mAs and kVp to visualize low contrast objects/tissue?
keep kVp constant
increase mAs
If you want to lower dose but maintain a constant exposure, how should you alter kVp and mAs?
raise kVp by 15% then lower mAs by 50%
higher kVp xrays penetrate more easily and delivers a lower dose (skin dose) due to less PE interactions (PE interactions are far worse for dose than compton)
If a radiologic technologit uses 80 kV for the AP projection of the lumbar spine, what is the predominant interaction with BONE
Photoelectric effect
80 kV represents the maximum energy, so the average will be 1/2 to 1/3 of that. Bone having a high Z will predominantly absorb photons via photoelectric effect. Remember the probability of PE is proportional to Z3
What interaction is primarily responsible for dose in the low diagnostic energy range?
Photoelectric effect
The predominant interaction of Tc-99m photons with a sodium iodide crystal is:
Photoelectric effect
What is the unit for linear energy transfer?
keV per micrometer
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.
Attenuation in tissue depends on 3 things:
- effective atomic number in Tissue
- x ray beam quality
- Tissue density
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.
What is a direct result of adding filtration to a diagnostic xray beam?
Patient dose is reduced
image contrast is reduced due to increase in kVp
What always increases as focal spot increases?
Geometric unsharpness
Will lower kV or increased filtration reduced a patient skin dose?
increased filtration with reduce a patient’s skin dose
fewer xray photons interact with patients with increased filtration (lower dose photons)
lower kV photons will be stopped/interact with the patient’s skin and significantly contribute to dose
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
An inner shell vacancy is filled with an electron with simultaneous release of an electron from the same atom, resulting in 2 vacancies:
Definition of an Auger electron
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
What is a bucky grid?
A moving grids which wiggles back and forth rapidly
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 difference between resolution and sharpness?
sharpness = ability of the xray sytem to define an edge
resolution = ability of the xray system to differentiate two closely approximated things
Modulation transfer function describes the relationship between sharpness and resolution
What is the modulation transfer function?
The modulation transfer function(MTF) is the spatial frequency response of an imaging system or a component.
determinant of spatial resolution in a digital detector?
pixel size and spacing (pixel pitch)
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.
What is detective quantum efficiency?
detective quantum efficiency (DQE) refers to the efficiency of a detector in converting incident x-ray energy into an image signal.
High DQE values indicate that less radiation is needed to achieve identical image quality
use of collimation does what to contrast resolution?
collimation Improves contrast resolution.
collimation reduces scatter –> image contrast improved
How does higher kVp affect contrast?
decreases contrast!
Higher kVp –> smaller attenuation coefficients –> less contrast
In digital detectors what determines contrast resolution
number of bits
DR only deals in absolutes, either off or on (0 or 1). Each 1 or 0 is called a bit and refers to the computer’s basic unit of info. These binary digits are used to display the shades of gray in a digital image. the greater the number of bits the more shades of gray available
does width or level determine brightness?
level
In general, does a narrow window width increase or decrease contrast?
narrow window width increases contrast
primary factor influencing image contrast in a digital system:
look up table or “LUT”
why is kVp not as important in DR for contrast compared to film?
DR have a much wider dynamic range and rescaling is done in post processing using a histogram known as a look up table (LUT)
what is the “30% rule”?
The “30% rule” states you have to increase the mAs by at least 30% if you want to make a visible change in density (density is “blackness” of an image)
what is the “15% rule”?
to maintain density (blackness) you need to decrease the kVp by 15% and double the mA to keep the same degree of “blackness” on the film
For every 4cm of tissue, how does the mA change?
for every 4 cm of tissue, generally double the mA
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
Newborn xray technique:
- do NOT use a grid
- lower the kVp (good technique is around 65 kVp)
- lower mAs (2-4 mAs)
What is a technique to decrease motion blur (as in the case of a wriggly kid)?
Increase kVp slightly to allow for a shorter exposure (decreased mAs)
longer exposure times are more susceptible to motion/blurring
xr technique adjustment for plaster casts
2x the mAs when dry
3x the mAs when wet
xr adjustment for fiberglass cast
no ajustment required
xray technique adjustment for history of splenomegaly
increase the kVp
xray technique adjustment for ascites
increase the kVp
xray technique adjustment for sclerotic phase of paget’s
increase the kVp
xray technique adjustment for osteoporosis
decrease kVp
Which has the potential to lower dose, digital or film radiography?
digital because of post processing options
Does film or digital radiography have higher dose efficiency?
digital. “Higher dose efficiency” is a fancy way to say less radiation. digital has the potential to have lower dose relative to film because of post processing options
How does decreased pixel pitch affect spatial resolution?
pixel pitch is the measurement from the center of one pixel to the next (pixel spacing)
spatial resolution is better for film or digital radiography?
film! it’s not limited by pixel size
underlying mechanism of automatic exposure control
an ionization chamber placed between the patient and image receptor
what parameter does the AEC affect?
mAs
since the AEC controls the QUANTITY of radiation and time
equation for “bit depth”
2x
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.
Does kVp have any effect on blurring of sharp edges?
No! No effect. Increased kVp will increase scatter but has no effect on blurring
things that affect blurring of sharp edges:
- The focal spot of the x-ray tube has a finite size (it is not a point source).
- Increasing geometric magnification (SID/SOD) will also increase blurring; just like the shadow of an object cast against a wall becomes less sharp the farther the object is away from the wall.
How does computed radiography (CR) work?
CR : type of indirect detector system
xr –> light –> charge
Computed radiography (CR) is the use of photostimulable phosphor as an image receptor. The image receptor is held in a similar casing (cassette) to that of the traditional film screen. Computed radiography harnesses the absorption of radiation, trapping electrons at energy levels via the process of photostimulable luminescence.
when plate is exposed to xrays some e- within the phosphor are removed and liberaed e- get trapped in a “conduction band”. To read it out the plate is exposed to a laser which literates the trapped e- and they return to their shells. As the e- return, they release E as BLUE-GREEN light –> photodector which amplifies the light signal and converts it to an electric signal.
what is the sampling pitch in CR?
distance between the laser positions as it’s reading out the plate
In regards to CR, the amount of light detected is proportional:
to the intensity of the incident xray
how are CR plates reset?
exposure to bright white light
if you forget to expose the plate to bright light you get ghosting artifacts
“poster child” DR scintillator
thallium doped cesium iodide
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
What is lateral dispersion?
problem you run into with indirect radiography methods that use phosphors. THat tends to diffuse laterally after it leaves the site of conversion from xray
this creates issues with spatial resolution which get worse with increasing thickness of the crystal
is there lateral dispersion in direct detectors?
NO!
lateral dispersion only happens with indirect systems
What is the “fill factor”?
area of a detector that is sensitive to xrays (in relation to the entire detector area)
pixels typical standard for digital disply:
3 mega pixels
what has better spatial resolution, DR or CR?
DR!
CR has lateral dispersion of light in the phosphor which descreases spatial resolution
which has a higher fill factor, DR or CR?
DR
DR systems are more efficient and nearly 100% of the detector is sensitive to xrays
What can decrease lateral dispersion in CR?
- thinner crystal (thicker crystal = more dispersion)
- columnar crystal structure (with lead)
“Centralized” vs “Decentralized” Workflow
“Centralized” vs “Decentralized” Workflow
- CR is a cassette based “centralized” system = essentially same workflow as conventional film
(CR= Centralized)
- DR allows for a “decentralized” system where the image can be acquired, adjusted and transffered to PACS without the tech leaving the xray room. Each room is its own “center” therefore it is “de-centralized”
(DR= Decentralized)
how does FOV affect geometric distortion in XR?
Geometric distortion or nonuniform magnification is more pronounced at the margins of the image because the beam is not perpendicular to the plane of the detector
(akin to projecting a shadow onto a wall at an angle).
A sphere in the center of the beam will create a circular outline, while at the margins it will become more of a distorted oval. This geometric distortion is more pronounced with larger field of views and closer tube-to-detector distance (larger beam divergence), thus increasing distance of the tube to patients will decrease this distortion.
what is the typical exposure index for an adult body radiograph?
300
what radiographic examination is most likely to make use of AEC?
skull
extremity and infants not grid
AEC not used at bedside
what % of primary x ray photons is typically lost in grids used when performing adult abdominal radiography
30%
