What is x-ray beam hardening? What happens as the beam penetrates tissue and bone?
Something to do with the way the spectrum changes as it passes through matter.
X-rays generated by the tube have a wide range of energies
Lower energy photons are attenuated more rapidly than higher energy photons. (duh)
- As beam penetrates tissue and bone..
- - Shape of spectrum changes
- -Effective energy of beam (Eff) increases which leads to
- -reduced differences in attenuation coeffs (between bone and soft tissue)
- Reduced image contrast
Give some sources of beam hardening
- Inherent x-ray tube filtration eg
- X-ray window
- 1-2mm glass/metal
- Si (z=14), Al(z=14)
- Cut off below 15keV
- Added filtrationUse it (usually Al) to attenuate low energy photons that add to dose but not image
(including the patient) 'hardens' the beam.
What is the meaning of half value layer? What assumptions do you make?
Thickness of a material that reduces beam intensity by half
- Narrow beam geometry
- No scattering
- Used as an approximation for diagnostic beams
- Usually expressed in mm Al
What is the purpose of Half value layer?
- -Quantifies the ability of the beam to penetrate tissue
- -indirect measurement of the energies (quality) of an xray beam
- -normally used to characterise polyenergetic beams
Give some typical x-ray beam HVL values for soft tissue and mammographies
- Tissue ranges between 2.5-3cm
- Mammography is 1cm
Give the percentage of xray beams transmitted through a patient in chest, skull and abdominal rdiographs
- 10% for chest
- 1% for skull
- 0.5% for abdominal
What happens to HVL if you add filtration to xray
HVL increases, but it is not the same thing as real HVL(?)
What is meant by the anode heel effect?
X-rays leaving the anode tangential to the anode surface are reduced in intensity when arriving at the detector
What causes the anode heel effect?
Reduction is due to self absorption
of photons by the anode, caused by microscopic roughness
of anode surface.
- Beam on the anode side has
- lower intensity
- higher effective enrgy
What is meant by the x-ray tube output? What is it proportional to? What are its units?
- x-ray tube output is the dose per unit mAs
- Proportional to Ztarget and kVp^2
Units are mGy/mAs (milli in both cases) at 1m for a set kVp
Can be calculated if dose is known at a different distance using inverse square law
What should the output of xray tube output be proportional to for a set xray target?
What are the overall factors that govern image quality?
Noise, spatial resolution, contrast, scatter
What is meant by noise in X-rays?
Even fr uniform X-ray exposure, adjacent areas have differing photon counts, following a poisson distribution
For N photons reaching the detector, the noise is: Noise ∝ √N
What is meant by quantum mottle? What is it like in diagnositc radiolgy?
Its another word for relative noise, with the formula
Relative noise ∝ (√N/N)*100
In diagnostic radiology this comes to N>105 for every mm2 so thats a relative noies value of 0.3%
What is meant by spatial resolution in X-rays? What are its units?
Abiligy to distinguish smal objects placed very close together. It has units of line pairs per mm (lp/mm)
- A line pair is a pair of dark and light lines (opaque and radiolucent)
- So 1lp means you can see a 0.5mm wide bar with 2 0.25 lines either side of it
- X lp/mm shows that X line pairs can be resolved in 1mm.
- Typical limiting resolution: 6lp/mm
What are the 3 types of contrast?
- Subject contrast,
- Object contrast
- Radiographic contrast
What is object contrast?
The difference in attenuation characteristics of adjacent materials
What is Subject contrast?
The difference in x-ray intensity transmitted through different adjacent materials
What is Radiographic contrast
The difference in density (i.e greyness between areas in the radiograph)
What is the subject contrast of this scenario?
When is object contrast highest?
- It is highest at lower kVp
How does one increase penetration whilst maintaining high contrast?
- Increasing Xray beam intensity (increasing mAs).
- It compensates for low penetration at low kVp
- It increases patient dose
What happens with higher kVp?
- You get a higher transmission/penetration
- You get a decreased contrast
What are the optimal conditions for good contrast?
What are the optimal conditions for low noise?
high kVp, high mAs
What are the optimal conditions for low radiation dose?
What is threshold contrast? What is it dependent on?
The minimum contrast difference that can be distinguished
- Depends on:
- Signal to noise ratio
- Size of object of interest
How does one assess threshold contrast?
- Use test objects
What is scatter?
Secondary photon interactions mainly produced by compton interactions
What factors make scatter increase?
- Increases with kV (Due to reduction in compton scattering, but more photons are scattered forward).
- Increases with thickness of material
- Increases with size of field of view
How does scatter degrade image quality?
- It increases image noise
- Reduces image contrast
How does one calculate subject contrast in general considering scatter?
- where P = no. of primary (unscattered) photons
- S = no. of scattered photons
- R = S/P
What 3 methods can we use to remove scatter?
- Air gap technique
- Anti-scatter grids
- Reciprocating grids
Draw a diagram to explain what is the air gap technique
How do antiscatter grids work?
- Made of narrow lead bars that only allow photons travelling in a specific range of angles.
- The scattered photons at high angles will be absorbed by the lead strips.
What is the grid ratio? What happens when you increase it?
- Ratio of strip height to grid gap between bars.
- Increasing grid ratio:
- Improves image contrast
- Needs more careful alignment (susceptible to artefacts)
- Needs higher expocure
How do reciprocating grids work?
They move during the exposure, spreading the image of the grid lines to render them invisible
What happens if you damage a grid?
You cause artefacts
What are collimators?
- They are adjustable parallel opposed lead shutters
- They define the x-ray beam size and shape emerging from the X-ray window
- It can reduce the total patient mass irradiated and thus reduce patient dose and exposure.
During collimation, how does one define the X-ray field?
By using a light source and mirror to define the X-ray field.
Using a diagram, explain how we define magnification factor (m) in X-rays
What is unsharpness. Explain how one defines geometric unsharpness formula for X-rays with a diagram
- Its the penumbra that results from xrays arriving at slightly different locations in the focal spot since the focal spot is actually an area
What increases the focal spot blur?
- Increased magnification
- Increased focal spot size
How does one get a good magnification
Small focal spot
How does automatic exposure control work?
- Exposure is terminated when the right radiation amount reaches the detectors
- A thin radiation detector is placed in front of the detector and behind the grid
- AEC systems are almost radiolucent
- Level of exposure is set to provide optimal image quality