Imaging with X-rays Flashcards
Compton Scatter
Increases with:
- increasing denisty
- increasing electron density
- lower x-ray beam energy
Increasing beam energy results in lower scatter (more energy goes to recoil electron which gets absorbed)
At 50keV Compton & Photoelectric interactions occur equally
What is Elastic Scatter (also known as Coherent or Rayleigh Scatter)
Occurs where photon energy is less than binding energy
Photon bounces off electron
No contribution to dose
More likely to occur with high atomic number elements (as electrons more tightly bound) and low photon energy
- No ionisation takes place
- No energy lost
- No absorption takes place
Bremmstrahlung Radiation
What is bremmstrahlung proportional to ?
Probability of bremmstrahlung is proportional to:
Atomic number2
Curve Shape
What effect on the curve does changing target do?
Changing target:
- changes average beam energy
- changes the characteristic peaks
- does NOT affect peak beam energy
X-ray Quantity (Number of photons)
What is x ray quantity proportional to?
Directly proportional to the tube current
Proportional to the peak kv2
Dependent on the choice of target material (higher atomic number = more photons)
(Target material does not affect energyO
Computed Radiography
What colour light stimulates and what colour is released?
Red light laser stimulates electrons
Blue light released
Barium fluorohalide doped with europium is schintillator layer
- The log of the light output from the screen is directly proportional to the log of the x-ray dose
Electromagnetic vs Sound waves
How do both waves move?
Electromagmentic waves move as TRANSVERSE WAVES
Sound waves are longitudinal waves that go through periods of rarefaction and compression
Geometric Unsharpness
What effect does exposure time have on geometric unsharpness?
Shortening of exposure time causes INCREASED geometric unsharpness
(shorter exposure means higher mA and blooming in size of focal spot)
Grids
What is usual line density?
How thick are lead strips?
What is grid ratio?
Line density of a grid usually 30-80cm (number of lead strips per cm)
Lead strips are 0.05 - 0.07mm
Grid ratio is depth divided by width
What is tube output proportional to?
The square of the peak kV
(in lower dose systems like mammography, it is equal to the peak kV cubed)
Tube kV tested with?
Digital kV meter
Cathode Filament voltage?
10v
4A
(Power dissipation of 40 watts)
What are the Focal Spot sizes?
Mammography
Radiography
Fluoro
- 1mm - magnification mammography
- 3mm - normal mammography and magnification radiography
- 6mm - conventional radiography (small focal spot) and fluoro
- 2mm - conventional radiography (large focal spot)
DSA
What matrix size used in DSA?
Image intensifiers are used to reconstruct images
DSA matrix size is 1024 x 1024
Spatial resolution is 2-3lp/mm
Target angle
What is target angle?
Do large or small target angles improve fine detail?
Usually 7-20 degrees
Smaller target angles improve fine detail imaging (reduces geometric unsharpness)
Smaller target angle used in mammography
Heat Dissipation in the tube
How is heat dispersed from tube?
Radiation is the primary method
Convection currents flow in the oil
Wave Rectification
How many diodes needed?
AC or DC current?
Converts AC to DC
Uses diodes
Full wave rectification needs at least 4 diodes
How to calculate Beam Intensity?
What is it proportional to ?
Does filtration affect beam intensity?
Number of photons per unit area x mean photon energy
/
Unit time
Beam intensity is directly proportional to the tube current (number of electrons flowing from cathode to anode)
Approximately proportional to kV squared
Intensity will DECREASE after filtration as some of the photons are filtered out
Unwanted Radiation
Made up of
- Leakage radiation - is what is transmitted throught tube housing
- Scattered radiation - is what has changed direction after leaving tube
Leakage radiation should not exceed 1mGy/hr at 1m from the focus
Heat Loading - what are units of measurement?
How is it calculated?
Measured in joules or heat units (old term)
For constant three phase potential it is equal to:
kV x mA
For single phase its is equal to:
0.7 x kV x mA
X-ray Spectrum
What effect on curve will increasing kV and mA have?
Increasing the tube current (mA) will NOT affect the shape of the x-ray spectrum
- will increase amount of characteristic and bremmstrahlung radiation
Increasing the kvP will shift curve up and to the right
Are the max energy of x-ray photons affected by the anode or three phase supply?
NOT affected by:
- anode material
- voltage waveform (this affects mean energy)
How to calculate HVL?
HVL = 0.693/LAC
It is the thickness of material to attenuate a POLYENERGETIC beam by 50%
LAC and MAC are used for monoenergetic beams
What is Mass Attenuation Coefficient ?
What are units?
The fraction of photons removed from a monochromatic energy beam by a homogenous absorber per unit mass
Is LAC/Density
Units are cm squared/kg or cm squared/g
Attenuation of the beam = MAC x mass thickness
Depends on the radiation interaction
Focal Spot Size Quality Control
How to measure focal spot size and resolution?
Focal Spot size measured using Pinhole imaging
Resolution measured using a Star Test Pattern
Tomography
What does narrow angle tomography produce?
Angle?
Blur?
Tomography most useful when imaging structures with High inherent contrast
Contrast depends on slice thickness
- thicker slices produce better contrast
Structures to the right angle of the film appear blurred
- those in line of sight between tube and detector are sharp
In Narrow Angle (thicker slices)
- reduces blur of structures outside focal plane
- preferred for imaging tissues with low contrast
- increased tendance for phantom image formation
How is heat lost from anode??
RADIATION MAINLY
What is Inherent Filtration equal to?
Mainly occurs at glass envelope but includes oil, tube housing, target material
Equivilent to approx 0.5 - 1mm of aluminium
Inherent filtration reduces tissue contrast
What is an Auger Electron ?
Results after the photoelectric effect where an additional electron is released
- Are an important source of dose in radiology
- Occurs as an alternative process to characteristic radiation release
The kinetic energy of auger electron is the binding energy of the K shell minus twice the energy of the L shell
What is normal Bit Depth?
Compression techniques
For DR:
- usually 12 - 16bit depth
- wide the dynamic range the wider the bit depth needed
Compression:
- Lossless
- reversible
- can be restored into an identical version of the original
- can reduce file size by a factor of 2 to 3
- Lossy
- reduces file size by up to 40
- irreversible
Detector Dose indicators (in CR)
Provide a measure of the signal from the image plate AVERAGED over a broad region of the plate
- equivilent to the optical density of the film
- doubling dose does NOT double DDI
What is Noise calculation?
Quantum Noise: proportional to the square root of number of photons
Noise is inversley proportional to the square root of number of photons
SNR = M/ square root of M (M = number of photons)
Signal to noise ratio is PROPORTIONAL to square root of number of photons
What is an electron volt?
When an electron is accelerated through 1 volt the unit used is an electron volt
The rest mass of an electron is 511keV
Three phase supply effect on photon quality and quantity
Increases both quality and quantity
Shifts curve up and to right
What is Isotropic Scatter ?
Where at low energies in compton scatter:
- photons are given off in all directions EQUALLY
What increases contrast in radiography?
- Increasing thickness of structure
Contast is proportional to the difference in LAC of tissues x tissue thickness
Contrast decreases as kV increases
How to reduce scatter and improve contrast?
- Reduce field size
- Decrease patient thickness
- Reduce kV
- Use of antiscatter grid
- Increased Focus to detector distance
What is grid ratio?
Depth of interspace / width
Typical ratio is 8:1
Higher the ratio the better the contrast
What 3 factors are needed for focus grids?
- Must be at specified distance from anode
- Should be tilted progressively to point towards tube focus
- Tube must be accurately angle over the grid
Can result in grid cutoff if above measures arent adhered to
What is standard number of bits used in radiography?
Usually 12 to 14 bits
What is Nyquist frequency?
The max frequency that can be sampled
Equal to half the sampling frequency
What size of anode disc is used in radiography?
Usually 20 - 30cm in diameter
