Scatter And Grids Flashcards
What 3 things can happen to an x-ray beam?
It can be transmitted
It can be absorbed
It can be scattered
What does it mean if an x-ray beam is transmitted?
The x-ray photons pass straight through the patient and interact with the image reception, which produces a black image
What does it mean if an x-ray beam is absorbed?
The x-ray photons are completely absorbed by the patient and don’t reach the image receptor. The image is then given contrast
What does it mean if an x-ray beam is scattered?
The x-ray photons collide with the atoms in the patients and then travel in different directions
Out of the 3 things that can happen to an x-ray beam, which are important for image production?
(2)
Transmitted
Absorbed
Out of the 3 things that can happen to an x-ray beam, which one is unwanted and degrades the image?
Scattered
What happens in scatter?
(2)
Some incoming photons aren’t absorbed.
They lose energy during interactions with the atoms within the tissues
Why is it important to reduce scatter?
(2)
Degrades the image (image looks noisy)
Increases dose to the patient
What are the 2 types of scatter?
Coherent scatter
Compton scatter
Does coherent scatter occur at very high or low energies?
Very low energies
What happens in coherent scattering?
(3)
The incoming photon interacts with the atom
The photon doesn’t have enough energy to release any electrons from their bond state
The photon is absorbed by the atom and re-emitted in a random direction using the same energy
What are the features of coherent scatter?
(4)
The energy of the photons coming in is equal to the energy of the photons coming out
There’s no energy loss
Occurs with low energy x-rays typically below the diagnostic range
Could occur within the diagnostic range of x-rays, but only in a very small amount
What happens in Compton scattering?
(6)
The incoming photon interacts with an orbital electron (electron on the outside)
The electron is removed from its shell
The electron is ejected from the atom, as is known as a Compton electron
The photon loses energy in the process and is deflected in a new direction
Both the photon and the electron have enough energy to interact with he tissues within the patient or exit the patient
If a scattered photon interacts with the image receptor, it doesn’t provide any useful information
What are the factors that affect scatter?
(3)
kVp
Thickness of body part
Field size (collimation)
What are the factors that affect scatter?
(3)
kVp
Thickness of body part
Field size (collimation)
How does kVp affect scatter?
(2)
Increasing the kVp will increase scatter
As the energy of the beam increase, more Compton scatter effects will occur and cause more collisions
How does thickness of body part affect scatter?
(3)
Increasing the thickness of the body part will increase scatter
The photons will have more distance to travel through the body part
Increasing the amount of Compton scatter events
What does thicker body parts do to the kV?
They increase the kV
What has to happen since increasing the thickness of a body part increases scatter?
A Bucky has to be used for larger body parts- the Bucky contains the grid and the grid contains scatter
How does field size (collimation) affect scatter?
(3)
Increasing the field size will increase scatter
More tissue is exposed to the x-ray beam
This increases the amounts of scatter events
What is the difference between characteristic x-rays and brehmstrahlung, and Compton scattering and photoelectric effect?
Characteristics x-rays and brehmstrahlung occurs in the x-ray tube
Compton scattering and photoelectric effect occurs in the patient
How does reducing the kVp reduce the scatter?
(3)
Lowering the kVp will reduce the energy of the x-ray beam
This will reduce the amount of Compton scatter effects occurring
Therefore, decreasing the kVp will decrease scatter
What can be used to reduce the thickness of the body part?
Compression
How does reducing the thickness of the body part affect scatter?
Reducing the thickness reduces the amount of tissues that the photons must pass through, resulting in less Compton scatter events
When is reducing the thickness of the body part used?
In mammography- the breasts are compressed
How does reducing the field size affect scatter?
(2)
Reducing the field size of the x-ray beam will result in a smaller area that’s exposed to the patient
This reduces the amount of scatter that can occur
What are other scatter reducing techniques?
(2)
Air gap technique
Grids
How does the air gap technique reduce scatter?
Creating a gap between the patient and the image receptor will prevent some of the scattered photons from reaching the receptor
What kVp is the air gap technique most effective for?
Projections requiring a kVp below 90
Why isn’t the air gap technique effective for projections with a kVp above 90?
With a kVp above 90, there’s too many interactions occurring, so it doesn’t have much of an impact on reducing the scattered photons
What is a limitation of the air gap technique?
The increase in the object to image receptor distance will result in image unsharpness and magnification. The source to image distance should be decreased to compensate for this
What is the main way to reduce scatter?
Grids
When are grids used?
For dense areas of the body (over 10cm thick), such as the abdomen, pelvis and skull
What do grids do?
They reduce the effects of scatter
Where are the grids found?
In the Buckys
What are the functions of a grid?
(3)
Transmit the primary beam
Absorb the scatter
Improve the image quality
How is a grid constructed?
(2)
It’s made up of parallel strips of a highly attenuating material, such as lead
These strips are uniformly spaced out and are separated by an interspace material such as aluminium
How do grids work?
(2)
Primary photons will pass through the grid’s plates
While secondary photons that cause scatter will be absorbed by the strips
What are the types of grids?
(2)
Parallel
Focused
What would happen if the grid was at an angle?
The primary beam would also be absorbed by the grid
What is a parallel grid?
Where is it used?
(2)
The lead strips run parallel to each other
It’s used mainly in fluoroscopy and parallel imaging
What is a focused grid?
(2)
The lead strips re angled to match the divergence of the x-ray beam
This allows more of the primary beam to be transmitted than parallel grids
Which grid is most common?
Focused grid
What does the SID mean?
Source to image distance
= distance from x-ray tube to detector
Why is it important to that the correct grid is used for the SID when using a focused grid?
Because the incorrect SID could result in the lead strips absorbing the primary beam, which results in grid cut off
When can grid cut off happen?
If the grid is misaligned or the SID is incorrect grid, cut off can happen
What happens when grid cut off happens?
(2)
Less of the primary beam will reach the detector
This results in an increase in noise on the image (which causes the whiteness in the image)
What are the factors to consider when using a grid?
(3)
The mAs should be increased (because more photons will be absorbed)
The grid must be perpendicular (at right angles) to the x-ray beam
The grid must be the correct way up (so that the lead strips are angled correctly)
What happens if we use a grid on a thinner area compared to a thicker area?
(2)
We have to increase the mAs, which increases the dose to the patient
Whereas on a thicker area, increasing the dose mAs and dose has less of an impact as the material is thicker
What is a virtual grid?
It’s software that replicates a grid. It uses algorithms to remove scatter. It recognises and removes the scatter, but it’s nit as good as usual an actual grid.
