- What is scatter ?
- Why is it a problem ?
- How can we control the production and
effects of scatter?
What is Scatter ?
Scatter is sometimes known as
as opposed to
• Coherent scattering
• Compton scattering
- Change of direction only
- Scattered photon retains original energy
- Occurs only at low photon energies
- Not a major concern in diagnostic
The scattered photon has a lower energy than the incident photon – but :
100 keV photons
–180 degree scattering, scattered photon = 71.9 keV
–30 degree scattering, scattered photon = 97.5 keV
60 keV photons
–180 0 scattering, scattered photon = 48.6 keV
–30 0 scattering, scattered photon = 59 keV
Hence the scattered photons, though
lower in energy than the primary
photons, are not necessarily, low
- Compton scatter is the major concern
- The scatter can be in any direction
- Compton scattering occurs at all photon
energies – a problem whatever the selected kV
•Scatter can be in any direction
•It can have relatively high energy
Scatter control is required because:
1.Scatter impairs image quality by reducing radiographic contrast
2.Scatter is a potential radiation hazard to staff and others
3.Scatter can increase patient dose
The volume of tissue irradiated is reduced most
commonly by collimation:
2. Light beam diaphragm
Light beam diaphragm
Contains a high intensity light source and a radio-lucent mirror arranged such that the reflected light exactly coincides with the position of the radiation field
Advantages of LBD:
•Visible demonstration of beam centering, field size and shape
•Range of available sizes and shapes – usually rectangular (but sometimes circular iris)
•When lamp fails
•Gets hot (Should have a timer)
•Must be intense light to compete with daylight or room light
Collimation reduces the volume irradiated by reducing the area of the radiation field
Compression reduces the irradiated volume by
reducing tissue thickness
Compression pushes some adipose tissue out of the field
and also reduces the thickness of tissue irradiated
(This may permit a reduction in kV)
Reducing the amount of scatter which reaches the image-receptor:
2.Use of an anti-scatter grid in front of the image-receptor
3.Use of an air gap
1. kV selection
- Compton scattering occurs at all kVs
- The higher the kV, the more energetic the
scatter and the more likely it is to penetrate
the patient and reach the image-receptor
- Especially if there is only a slight angle of
scatter (less loss of energy and traveling in
a more forward direction)
Energy of x-ray beam
2. Anti-scatter grid
•Primary photons are more likely to pass through the inter spaces
•Scatter is more likely to be absorbed by the lead
•BUT, the grid attenuates some of the desired primary x-rays
Types of anti-scatter grid
• Stationary anti-scatter grids
– Used during mobile radiography
– When patients cannot be transferred to x-ray table or
• Moving anti-scatter grids
– Incorporated into tables and erect chest stands
1.Alternate lead foil strips (<1 mm – 0.1 mm) and inter-space material (aluminium, plastic, carbon fibre)
2.Lead has high atomic number and density so ideal absorber (82)
3.Strips may be parallel or focussed (angled about the mid-point of the grid)
4.Covered with plastic or aluminium
•Focused to infinity
•Can be used with various source to image-receptor distances (SIDs)
•Progressively slanted to the periphery to match the x-ray beam divergence from the focal spot
•Requires a set SID
•e.g. 100 or 180 cm
•Tube side and central markings for alignment
Grid Ratio = h/D
•16:1 high kVp >100 (but absorbs more primary beam)
• Higher grid ratio will reject scatter better than a
lower grid ratio, due to the limited angle that is
allowed by the grid structure
• However, a higher ratio grid typically has a
higher dose penalty for its use
Grid line density/Grid frequency
• Number of lines per cm
– Low freq. 40 – 50 (used with systems having a moving grid assembly)
– Medium freq. 50 – 60 (stationary)
– High freq. 60 – 70 (stationary and digital radiography systems)
• Thick lead slat – more primary radiation absorbed
• Many thin slats with thinner inter-space material
will have a higher grid ratio and require a higher
- Specific SID – focussed grid
- Tube must be accurately centred – focussed grid
- Tube side – focussed grid
- Grid must not be tilted
- Tube angled – possible with parallel grid if
angled across its length
- Orientation of grid lines
- Grid ratio (Low kVp = low grid ratio e.g. 8:1)
- Lines per cm
- SID if focussed
- Tube side
- ID number
Moving grid/Bucky mechanism
• Problem of stationary grid is that the internal
structure can be seen on the radiograph
• Grid can move during exposure
•Grid moves quickly in one direction and slowly in the other
•Solenoid and a spring
•Solenoid energised and de-energised during exposure
•No grid lines on images
•Complex mechanism, requires more space and is expensive
•OID is increased
•Starts fast, then slows down during exposure until it stops
•Easy to construct, cheap, compact
•Allows a range of exposure times
•Grid is attached to two pairs of springs, solenoid attracts grid to cause tension in opposite springs, exposure commences and grid is then allowed to swing between the sets of springs
•Until it stops
Considerations for a Bucky mechanism
- Grid can be focussed or parallel
- 10:1 or 12:1 grid ratio
- Movement must start before exposure and continue after
- Speed and distance moved must blur lead slats
- Must only move 2.5 cm either side to prevent grid cut-off
- Smooth and simple movement
- Compact to reduce OFD
Radiation Dose & Grids
•Grid factor ~8 (25/3 mAs)
•Measure of increase in patient dose
•However, increase of image quality is worth the “cost” in additional radiation dose to the patient
Radiation Dose & Grids
• The anti- scatter grid in paediatrics gives
limited improvement in image quality and
increases patient dose
• The smaller irradiated volume (and mass)
means there is less scattered radiation to
affect the image quality
Air Gap technique
•An air gap can be used in front of the image-receptor instead of a grid
•A gap of 15cm between the patient and the image-receptor
•Some of the obliquely traveling scattered photons will miss the image-receptor
• Radiographic contrast will be improved
• SID must be increased to compensate for the
greater OID which would increase geometric
1. What is the name given to the type of scatter that affects radiographic image quality?
2. What is the difference between the primary x-ray beam and secondary radiation?
3. State THREE reasons why scatter is a problem in diagnostic imaging?
4. Explain why some scattered photons may still have enough energy to reach the image-receptor.
5. What is the role of the primary beam in image production and how does scatter reduce radiographic contrast?
6. Explain how a light beam diaphragm reduces the amount of scatter produced
7. State another method to reduce the volume of tissue being irradiated?
8. Explain how an anti-scatter grid can prevent scattered radiation reaching the image-receptor.
9. State TWO advantages of using a parallel anti-scatter grid as opposed to a focussed anti-scatter grid?
10. How does a grid with a higher grid ratio absorb more scatter?
11. State FOUR reasons for grid cut-off.
12. What is the main reason for using a moving grid?
13. How does the “air-gap technique” reduce scatter reaching the image-receptor?
1)– What is the name given to the type of scatter that affects radiographic image quality?
2) –What is the difference between the primary x-ray beam and secondary radiation?
Primary radiation is un-attenuated whereas secondary radiation is where primary x-ray photons have been scattered.
3)– State THREE reasons why scatter is a problem in diagnostic imaging?
Potential dose to staff, dose to patient outside the area being imaged, reduction in image quality.
4) – Explain why some scattered photons may still have enough energy to reach the image-receptor?
When using a high kV such as 100, for the x-ray photons that are only defected by a small angle such as 30º, very little energy is lost so the scattered x-ray photon energy is still approx.. 97.5 keV and because it is still traveling in a forward direction it is more likely to reach the image-receptor
5) – What is the role of the primary beam in image production and how does scatter reduce radiographic contrast?
When imaging a patient we rely on differential absorption of the primary x-ray beam due to the different tissues in the body.
What is left of the primary beam irradiates our image-receptor and forms a latent image which represents the patient’s body part as a negative image once processed.
Unfortunately, scattered photons can travel in oblique directions and no longer follow the path of the primary beam, thus those that reach the image receptor no longer help to represent the patient’s anatomy and therefore we lose radiographic contrast.
6. Explain how a light beam diaphragm reduces the amount of scatter produced?
It controls the size of the radiation field and if this is reduced fewer x-ray photon interactions take place leading to less scatter.
It is a box that fits onto the tube housing. It contains two pairs of lead leaves, set at right angles to one another which surround a central channel through which the x-ray beam passes.
It contains a high intensity light source and a radio-lucent mirror arranged such that the reflected light exactly coincides with the position of the radiation field. It collimates the beam (reduced radiation field) therefore reducing the volume of tissue irradiated.
7)– State another method to reduce the volume of tissue being irradiated?
8)- Explain how an anti-scatter grid can prevent scattered radiation reaching the image-receptor?
Made up of slats of high attenuating material, inter spaced by slats of low attenuating material.
Primary beam, which is non-deflected, can pass through the low attenuating slats, whilst oblique scatter is absorbed by high attenuating slats.
Construction: alternate lead foil strips (<1 mm – 0.1 mm minimum) and radiolucent inter-space material; lead has high atomic number and density so ideal absorber (82); strips may be parallel or angled about the mid-point of the grid; structure covered with plastic or aluminium.
9)– State TWO advantages of using a parallel anti-scatter grid as opposed to a focussed anti-scatter grid?
The parallel grid is focussed to infinity whereas the focussed grid has a set SID which must be used (such as 100 or 180 cm). You can angle across the length of a parallel grid and not get grid cut off as you would if you used a focussed grid.
10)– How does a grid with a higher grid ratio absorb more scatter?
Grid ratio is the ratio of the height of the lead slats to the distance between them. Therefore a higher grid ratio will increase efficiency due to the limited angle of scatter that is allowed.
11)- State FOUR reasons for grid cut-off.
Specific SID (focussed grid),
tube must be accurately centred (focussed grid),
tube side (focussed grid), grid must not be tilted,
tube angled (possible with parallel grid if angled across its length)
12)–What is the main reason for using a moving grid?
To prevent grid lines showing up on the radiograph.
13)–How does the “air-gap technique” reduce scatter reaching the image-receptor?
A gap of 15cm is used between the patient and the image-receptor, therefore some of the oblique scattered photons will miss the image-receptor.