Xray (Erin) Flashcards
What happens to the quantity of x-ray photons when the kV is increased?
Quantity of xray photons increases
What happens to the average energy of xray photons when the kV is increased?
Average energy increases
What happens to the maximum xray photon energy when the kV is increased?
Maximum energy increases
What happens to the average photon energy in a beam when mA is increased?
No change
What happens to the minimum photon energy in a beam when mA is increased?
No change
What happens to the maximum photon energy in a beam when mA is increased?
No change
What effect does filtration have on the average energy of photons?
Increases the average energy (removes the lower energy photons)
What effect does filtration have on the total number of photons?
Decreases the total number of photons (as lower energy photons are removed)
What happens to wavelength as frequency increases?
It decreases (they are indirectly proportional to each other)
What happens to frequency as wavelength increases?
It decreases (they are indirectly proportional to each other)
What effect does increasing the atomic number of the anode target have on the quantity of x-ray photons?
Increases the quantity of x-ray photons
What effect does increasing the atomic number of the anode target have on the energy of the characteristic x-rays produced?
Increases the characteristic energies (higher atomic number = more positive nucleus = higher electron binding energies)
Does the photoelectric or compton effect predominate at lower kVp?
Photoelectric
Does the photoelectric or compton effect predominate at higher kVp?
Compton
What effect does decreasing the anode angle have on the focal spot size?
Decreases the focal spot size
What effect does increasing the anode angle have on the focal spot size?
Increases the focal spot size
What effect does decreasing the anode angle have on the anode heel effect?
Larger anode heel effect
What effect does increasing the anode angle have on the anode heel effect?
Smaller anode heel effect
What effect does increasing mA have on noise?
Decreases noise
What effect does making the image receptor thicker on noise?
Decreases noise
What effect does decreasing kV have on image contrast?
Increases contrast
What effect does increasing kV have on image contrast?
Decreases contrast
What effect does increased scatter have on image contrast?
Decreases image contrast
What effect does increasing the atomic number difference in objects have on image contrast?
Increases contrast
What effect decreasing the atomic number difference in objects have on image contrast?
Decreases image contrast
What is the photoelectric effect equation?
ρZ3 / E3
What is the effect of using an image receptor with a greater attenuation coefficient on noise?
Decreases noise (more photons are absorbed and converted into signal)
What is the effect of using an image receptor with a lesser attenuation coefficient on noise?
Increases noise (less photons are absorbed and converted into signal)
What is the effect of using a thicker image receptor on noise?
Decreases noise (as more photons are absorbed)
What is the effect of using a thinner image receptor on noise?
Increases noise (as less photons are absorbed)
What effect does a smaller focal spot size have on the energy of the xrays emitted?
No effect
Does compton scattering usually involve outer or inner shell electrons?
Usually outer shell electrons because they are weakly attached to the atom
In compton scatter as the scattering angle increases what happens to the energy of the scattered xray?
Decreases the energy of the scattered xray
The anode disc has an angle of around…?
5-15 degrees
What 3 parameters can be changed to preserve tube rating?
kV
mA
Exposure time
What is the equation for linear attenuation co-efficient?
μLAC = 0.693* / HVL
What is the equation for mass attenuation co-efficient?
μMAC = μLAC/ρ
How is magnification increased?
Decreasing focus to film distance
Increasing object to film distance
How is magnification decreased?
Increasing focus to film distance
Decreasing object to film distance
What is the effect of using flat plate detectors for magnification views in fluoroscopy on patient dose?
No change in dose - as images are just digitally magnified
What is the effect of using flat plate detectors for magnification views in fluoroscopy on spatial resolution?
No improvement in spatial resolution
As images are just digitally magnified (the same image is taken)
How is GRID RATIO calculated?
Interspace channel height divided by channel width
How is grid selectivity calculated
Primary radiation (non-scattered photons)/secondary radiation (scattered photons)
How is grid factor calculated
exposure necessary with grid/exposure necessary without grid
What is a moving grid often called
Often referred to as a Bucky
In fluoroscopy where in the image is signal used to guide the adjustment of kV and mA
Signal from the centre is used
What is spatial resolution measured in?
Line pairs per mm
lp/mm
Describe vignetting
Term used to describe the phenomenon whereby the centre of an uncorrected image appears brighter than the peripheries
What causes vignetting
Front of image intensifier is dome-shaped however the screen we view the image on is flat. The transposition between these two causes a pincushion type appearance and results in loss of brightness at the edges of the image
What is the fixed focus-film distance in mammography (in cm)
Fixed around 65/66cm
What is the average dose per film in mammography?
1.5-3mGy
Through what process are electrons emitted from the filament at the cathode
Thermionic emission
What is the filament in an X-ray tube commonly made of
Tungsten
Why does Tungsten make a good filament (4)
Good thermionic emitter
High atomic number
Can be manufactured into thin wire
High melting temperature
Why is molybdenum used for the focusing cup in an X-ray tube (2)
High melting point
Poor thermionic emitter (electrons won’t be released that could interfere with filament electrons)
What is the focusing cup made of in an xray tube
Molybdenum
What charge does the focusing cup have
Negative charge
What is the anode typically made of
Tungsten with rhenium added
Why is rhenium added to tungsten at the anode
to prevent cracking
What charge does the anode have
Positive to attract electrons
What is the actual focal spot
Physical area of the focal track that is impacted
What is the effective focal spot
The area of the focal spot that is projected out of a tube
Where are stationary anodes still used
Dental radiology and radiotherapy systems
Why is a rotating anode now used (compared to a stationary anode)
Because it rotates it overcomes heating by having different areas exposed to the electron stream over time
Why is molybdenum used as the stem in the anode
Poor conductor of heat
High melting point
Conducts very little heat to prevent damage to the metal bearings
By what process is heat removed from the anode
Radiation
What is the effect of lengthening exposure time on tube rating
Decreases tube rating
What is the effect of decreasing focal spot size (at the anode) on tube rating
Decreases tube rating
What is the effect of decreasing disc rotation speed on tube rating
Decreases tube rating
What is the effect of decreasing anode tube diameter on tube rating
Decreases tube rating
What is the window in an xray tube Made from
Beryllium with aluminium
Or
Copper
What must be total filtration be equal to (terms of aluminium thickness)
> 2.5mm aluminium equivalent
What is total filtration a combination of
Inherent filtration and added filtration
How much current is usually passed through the filament at the cathode
10A
What is the effect of wave rectification on mean photon energy
Increased mean photon energy
As fever photons of lower energy released
What is the effect of wave rectification on xray output
Increased xray output
As stays closer to the maximum for longer
What is the effect of wave rectification on exposure time
Shorter exposure time
As output higher, can run exposure for shorter time to get same output
What is the effect of wave rectification on patient dose
Lower patient dose
As increased mean energy photons means fewer low energy photons that contribute to the patient dose but not to the final image
What % of energy is converted to heat and photons at the anode
99% heat
1% xray photons
What is the purpose of the glass envelope in an xray tube
Contains vacuum
What 2 methods can full wave rectification be achieved by
Three phase supply
High frequency generator
What % of X-rays are emitted via Bremsstrahlung radiation
80%
What is meant by beam QUALITY
the ability of the beam to penetrate an object or the energy of the beam
What is meant by beam QUANTITY
The number of xray photons in the beam
What is the effect of a more uniform waveform current on average xray energy
Increased average xray energy
What is the effect of a more uniform waveform current on quantity of xray photons
Increased quantity of xray photons
What is the effect of a more uniform waveform current on maximum keV
Same maximum keV
What 3 processes contribute to attenuation
Compton effect
Photoelectric absorption
Elastic scatter
Does Compton scatter mostly occur with inner or outer shell electrons
Outer shell/loosely bound electrons
What is the equation for the probability of the Compton affect (Compton attenuating co-efficient)
Compton attenuating co-efficient = density/energy
What effect does increased material density have on the probability of the Compton effect
Increases probability
What effect does increased photon energy have on the probability of the Compton effect
Decreases probability
However note at higher energies the proportion of Compton to photoelectric effect is higher
Hence in CT Compton effect predominates
But overall Compton goes down as energy increases
What effect does increasing atomic number have on the probability of the Compton effect
NO EFFECT
Compton effect is INDEPENDENT OF ATOMIC NUMBER
What is the effect of decreased xray energy on the probability of the Compton effect
Increases probability of the Compton effect
(although note photoelectric effect does predominate at lower keV)
Does the photoelectric effect occur due to interaction with a inner or outer shell electron
Typically inner shell (bound) electron
How much of the incoming photon energy is transferred to the bound electron in the photoelectric effect
All the energy
What is the equation for the probability of the photoelectric effect
τ = ρZ3 / E3
As the energy of the photon increases what happens to the probability of the photoelectric effect
Decreases
As the mass density of a material increases what happens to the probability of the photoelectric effect
Increases
As the atomic number of a material increases what happens to the probability of the photoelectric effect
Increases
When is the probability of the photoelectric effect the highest in relation to binding energies
When the xray photon energy is slightly above the electron binding energy (L edge and K edge)
What is the k edge of iodine
33 keV
As the mass density of a material increases what happens to the probability of the photoelectric effect
Increases
What is HVL a measure of
The penetrating power of the xray beam
What is the equation for calculating the factor of reduction by HVL
2 (to power of number of HVL)
E.g if beam has gone through 4 HVL is have been attenuated by a factor of 16
(2 to power of 4 = 16)
What is the linear attenuation coefficient (LAC)
The probability of the material to attenuate the beam
How is the LAC calculated
0.693/HVL
What unit is LAC measured in
cm-1
What is MAC (mass attenuation coefficient) a measure of?
Rate of energy loss by a photon beam as it travels through an area of material
How is MAC calculated
MAC = LAC/density
What is MAC independent of
Density
What is the unit for MAC
cm2 g-1
What 2 assumptions does attenuation make about a beam
Monoenergetic beam
X-rays coming from a point source
What causes beam hardening
The lower energy photons are attenuated proportionally more than the higher energy photons and are removed
Leaving behind higher energy photons (i.e beam hardening)
What is the effect of beam hardening on the HVL value?
Increased HVL
Why does a wider xray beam increase the measured HVL
Due to increased scatter
What is the equation for image subject contrast
c ∝ (μ1 – μ2) x t
where:
c = contrast
μ = attenuation coefficient of object 1 and 2 in the material being imaged
t = thickness of the structure
What is the effect of a thicker structure being image on contrast
Increases contrast
What is the contrast in an image due to
Difference in attenuation between tissues
What is the effect of a greater attenuation difference between two objects on image contrast
Improves image contrast
What is the effect of decreasing tube potential kV on image contrast
Improves image contrast
What is the effect of increasing the difference between atomic number (Z) between two objects on image contrast
Improves image contrast
What is the effect of increasing the difference in density between two objects on image contrast
Improves image contrast
What 5 factors improve image contrast
Decreasing kV
Thicker structure
Greater attenuation between two objects
Increasing difference in Z of objects
Increasing difference in density of objects
What is the effect of scatter on contrast
Reduced contrast
What is the effect of overlying tissue on image contrast
No effect
What is spatial resolution a measure of
How far apart two objects must be before they can be seen as separate details in the image
What is Modulation Transfer Function (MTF)?
How well a system is able to represent the object with all its detail (contrast, resolution etc). Basically how perfect is the imaging system.
What is meant by an MTF = 1
Same range is obtained in the image
What is meant by an MTF <1
Lower range in the image
What is meant by an MTF = 0
No information in the image
What is the Nyquist frequency
The highest frequency that can be imaged
What does Nyquist criterion state
The sampling frequency must be at least twice the highest signal frequency
Sampling frequency = 2 x Nyquist frequency
What effect does increasing mA have on noise
Decreases noise
What is the effect of using an image receptor with a greater attenuation coefficient on noise
Decreases noise
What is the effect of making the image receptor on noise
Decreases noise
What is the effect of using larger detector elements on noise
Decreases noise
What are the 4 causes of unsharpness
Geometrical unsharpness
Movement unsharpness
Edge unsharpness
Image receptor unsharpness
What 3 things improve geometrical unsharpness
Fine focal spot size
Increased focus to detector distance
Reduced object to detector distance
What is the cause of image unsharpness
If the detector element lies across the border between a light and a dark area the pixel displayed will be an average of these two values creating a blurred border
What causes movement unsharpness
If object moves during acquisition
Edges will be blurred
What causes edge unsharpness
If an object has a tapering edge the attenuation will gradually decrease along the object
What 3 things worsen geometrical unsharpness
Larger focal spot size
Decreased focus to detector distance
Increase object to detector distance
What is the effect of smaller detector elements on noise and SR
Improved SR
Increased noise
What is the effect of smaller distance between detector elements on SR
Improved SR
Likelihood of Bremstrahlung radiation equation?
Probability = EZ (energy and atomic number)
Tungsten atomic number?
74
For tungsten targets, X-rays tube voltage is set at above…?
70kVp
Tungsten characteristic X-rays are what energies?
58keV and 68keV
The X-ray tube has a ??? Shield to protect from ??? Radiation
Lead
Leaking
Rayleigh scatter has how much energy transfer?
No energy is transferred
At about ???kV, in soft tissue, the number of PE interactions = the number of Compton interactions
25kEv
At about ???kEV, in bone, the number of PE interactions = Compton interactions
45keV
LAC equation
pZ / E
At a K edge, X-rays with energy above the k edge are preferentially…
Attentuated
At a K edge, X-rays with energy below the k edge are preferentially…
Transmitted
In iodine contrast studies, typically the k edge is 33 so a tube voltage of ??? Is used
75kEv
So that the average photon has a higher chance of being attenuated
How does CR work? Step by step
- X-rays stored in phosphor
- Red light used to read, which releases blue light
- PMT tubes convert light into electric signal
- White light used to “clean” the plate
Computer radiography: how is the sample digitised?
An ADC samples at regular intervals
Computed Radiography: what is sampling pitch? What is sampling frequency?
Pitch - pixel size
Frequency- samples per mm,
ie 1/pixel size
For Computed radiography: spatial resolution depends on…? (4)
- Small pixels/high frequency
- Thinner readout laser
- Thinner phosphor layer
- Smaller crystals (less scatter)
Computed radiography: typical spatial resolution? Highest resolution?
Typical = 2-3 lp /mm
Highest is 5 lp/mm
Indirect Digital radiography: how does it work
- Caesium iodide scintillation layer turns X-rays to light
- Light is amplified and detected by photodiodes
- This is converted into voltage by TFT switches
- The voltage is digitised for images
Indirect Digital radiography: spatial resolution typically?
2-3.5 lp/ mm
Direct Digital radiography: how does it work steps?
- X-rays directly hit a photoconductor to release electrons (no light)
The conductor is amorphous selenium - The electron charges are stored and read out to make an image
Direct Digital radiography: pixel size depends on..?
Size of the transistors in the array
Advantage of direct vs indirect Digital radiography?
Better spatial resolution in direct
4 ways to reduce scatter in X-rays
- Reduce the volume of tissue hit with X-rays (compression or collimating)
- Reduce the kvP
- Use anti scatter grid
- Increased air gap between patient and image receptor
How is intensity related to kvP?
Intensity proportional to kVP SQUARED
How is mA related to intensity?
Intensity is proportional to mA
What is the definition of spatial resolution?
The ability to perceive fine detail in an image
OR
The ability to resolve objectd as separate entities
In an X-ray machine what are the rotor bearing lubricated with?
Silver
What is a typical scatter grid ratio?
8:1
Thermal rating has units of…?
kW per 0.1 second
Intensity is proportional to…? (Equation)
mA x kVP squared
High pass filters have what effect in spatial resolution? Why?
High pass filters are edge enhancement filters that improve resolution (but increase noise)
