1 to 1 AECs and scatter radiation

Question 1

Request card for a 75 year old male patient states: x-ray pelvis, follow up left THR, ?loosening. The patient is average size. He walks into the x-ray room and you decide to x-ray him on the table.

The pre-set exposure factors for an AP pelvis examination are:
80kV
150mA
2 outside chambers

a) Explain why 150mA is stated, rather than an mAs (1 mark)
b) State the exposure factors that you would use for the AP pelvis on this patient. Explain your answer (4 marks)
c) Explain how and why these exposure factors would differ if you were to image this patient on a trolley (2 marks)

Answer 1

a) Explain why 150mA is stated, rather than an mAs (1 mark)
The AEC is being used which detects the exposure, manipulating time, therefore only an mA is needed.

b) State the exposure factors that you would use for the AP pelvis on this patient. Explain your answer (4 marks)
80kV, 150mA, right side chamber only. (1 mark)
The prosthesis on the left side will attenuate more x-rays than bone (1 mark) leading to less x-rays reaching the left side chamber and thus increasing the time required to terminate the exposure (1 mark).
The right hip has no prosthesis so the right chamber will terminate with the correct mAs

c) Explain how these exposure factors would differ if you were to image this patient on a trolley (2 marks)
An mAs would need to be set (1 mark)
As the AEC cannot be used on a trolley, requiring a manual exposure(1 mark).

Question 2

Your departmental protocol indicates that you also need to undertake a turned lateral hip for this clinical history.
Using the pre-set lateral hip exposure factors for an average patient, you produce the image shown.
d) Explain possible reasons for the high deviation index (3 marks)
+3.5 deviation index
Hip x-ray looks diagnostic

Answer 2

d) Explain possible reasons for the high deviation index (3 marks)

Likely that the pre-set exposures use the AEC, with the centre chamber selected (1 mark)
This will position the THR over the chamber, increasing the time required to terminate the exposure(1 mark).
The image is therefore overexposed, leading to an increased DI, though the algorithm corrects the resultant brightness (1 mark)

Question 3

What are AECs?

Answer 3

Designed to produce images with optimal density by controlling the amount of radiation reaching the IR
A specific amount of radiation WILL produce a specific density
“Density” refers to the raw image prior to the algorithm amplifying the output signal
“Brightness” is the intensity of individual pixels on the resultant image

Question 4

Using an AEC
Still need to select:
IR
kVp
mA (this is usually pre-determined but can be changed)
Grid
Density setting
Maximum time (on some equipment only)

An AEC controls the TIME of an exposure

Answer 4

Still need to select:
IR
kVp
mA (this is usually pre-determined but can be changed)
Grid
Density setting
Maximum time (on some equipment only)

An AEC controls the TIME of an exposure

Question 5

For AECs to work properly…..
Centring must be correct

Detector selection must be correct

Collimation must be accurate
Will affect amount of scatter
Scatter will contribute to ionisation of chamber

Minimum response time
Shortest exposure time system can deliver
Usually longer with an AEC than without
May be an issue for paeds, uncooperative patients etc

Answer 5

For AECs to work properly…..
Centring must be correct

Detector selection must be correct

Collimation must be accurate
Will affect amount of scatter
Scatter will contribute to ionisation of chamber

Minimum response time
Shortest exposure time system can deliver
Usually longer with an AEC than without
May be an issue for paeds, uncooperative patients etc

Question 6

Safety features of AEC

Back up time
150 – 200% of expected exposure time

Allows larger exposures if required due to patient size but protects from excessive exposure if there are other issues
Incorrect centring
Incorrect detector selected

Check back up time, especially for larger patients

Answer 6

Safety features of AEC

Back up time
150 – 200% of expected exposure time

Allows larger exposures if required due to patient size but protects from excessive exposure if there are other issues
Incorrect centring
Incorrect detector selected

Check back up time, especially for larger patients

Question 7

The patient can affect ionisation…
Gassy abdomen
Contrast media
Pathology e.g. pleural effusion
Prostheses

Gonad protection also affects AEC exposures
One of the many reasons for change to protocols regarding gonad protection in 2020

Answer 7

The patient can affect ionisation…
Gassy abdomen
Contrast media
Pathology e.g. pleural effusion
Prostheses

Gonad protection also affects AEC exposures
One of the many reasons for change to protocols regarding gonad protection in 2020

Question 8

Problems with AEC images
Incorrect positioning of patient
Poor exposure
Noisy image
Increased patient dose
Poor collimation
Scatter contributes to ionisation leading to overall under exposure
Reduced contrast
Noisy image

Answer 8

Incorrect positioning of patient
Poor exposure
Noisy image
Increased patient dose
Poor collimation
Scatter contributes to ionisation leading to overall under exposure
Reduced contrast
Noisy image

Question 9

How is the exposure determined when more than one chamber is selected?
There are 7 combinations of chamber available:
Each chamber individually (3)
All 3 chambers (1)
Any 2 chambers (3)

When selecting more than 1 chamber, the signal is “averaged”

The amplifier (see slide 6) sums the charge and divides it by the number of cells selected

Exposure is terminated when the optimal charge is achieved

Answer 9

How is the exposure determined when more than one chamber is selected?
There are 7 combinations of chamber available:
Each chamber individually (3)
All 3 chambers (1)
Any 2 chambers (3)

When selecting more than 1 chamber, the signal is “averaged”

The amplifier (see slide 6) sums the charge and divides it by the number of cells selected

Exposure is terminated when the optimal charge is achieved

Question 10

For example:
3 chambers selected
One chamber is over a metal prosthesis,
Only two chambers will contribute to the signal as the one over the prosthesis will not receive much radiation
The average will be measured across all three
This will lead to overexposure

Answer 10

For example:
3 chambers selected
One chamber is over a metal prosthesis,
Only two chambers will contribute to the signal as the one over the prosthesis will not receive much radiation
The average will be measured across all three
This will lead to overexposure

Question 11

What AEC do you use when there is bilateral prosthesis for a pelvis?

Answer 11

Centring lower to include the length of the prostheses brings the chambers over the hip joint and thus metal.

Using the two outside chambers will lead to over-exposure (see slide 16).

Using the centre chamber will lead to under exposure as this is positioned over the SP and soft tissue.

No chamber then

Question 12

ABDOMEN
Though in this case, it is possible that the gas on the left side of the abdomen may lead to underexposure.

It is impossible to know this in advance of taking the image.

Answer 12

ABDOMEN
Though in this case, it is possible that the gas on the left side of the abdomen may lead to underexposure.

It is impossible to know this in advance of taking the image.

Question 13

Size setting
Allows you to change exposure to match patient body size
Increases / decreases kVp
May increase mA
Increases backup time
Has no effect on overall resultant image density
This needs to be same for all patients regardless of size/age

Answer 13

Size setting
Allows you to change exposure to match patient body size
Increases / decreases kVp
May increase mA
Increases backup time
Has no effect on overall resultant image density
This needs to be same for all patients regardless of size/age

Question 14

Density setting
Adjusts the amount of radiation required to terminate the exposure
Increases or decreases time
And therefore mAs
No impact on kV
Brightness of resultant image still managed by processing algorithm
Change to EI/DI
Change to DAP
Has little use with DR systems

Answer 14

Density setting
Adjusts the amount of radiation required to terminate the exposure
Increases or decreases time
And therefore mAs
No impact on kV
Brightness of resultant image still managed by processing algorithm
Change to EI/DI
Change to DAP
Has little use with DR systems

Question 15

Density setting
+4 +100
+3 +75
+2 +50
+1 +25
0 0
-1 -25
-2 -50
-3 -75
-4 -100

Answer 15

Density setting
+4 +100
+3 +75
+2 +50
+1 +25
0 0
-1 -25
-2 -50
-3 -75
-4 -100

Question 16

You are undertaking a pelvis x-ray, ?# right NOF
The female patient is average size and on a trolley.

For 6 marks, describe how you will minimise the impact of scatter on the AP projection. Explain your answers?

Answer 16

Any 3 of:
Centre correctly (1 mark) and collimate closely (1 mark)
Correct centring enables close collimation (1 mark)

Limiting the size of the beam, limits the amount of attenuation interactions that cause scatter, thus reducing the amount of scatter produced. (1 mark)

Use the correct exposure factors (1 mark), ensuring that any grid used is compatible with the exposure factors chosen (1 mark).

Over exposure will lead to more scatter being produced (1 mark);
Under exposure, whilst not contributing to scatter, degrades image quality and leads to increased absorbed dose

Higher kV requires a larger grid ratio to ensure that the more forward directed scatter is attenuated by the grid (1 mark)

Employ a post-processing AI grid replacement algorithm to the image (1 mark)
Can use a manufacturer’s name for this i.e. smart grid, sim grid etc

This replicates the actions of the grid by “removing” scatter from the image whilst enabling a non-grid exposure to be used, leading to lower patient dose (1 mark)

Question 17

Photons
As an X-Ray photon passes through matter it can be:
Attenuated i.e. absorbed (photoelectric) or scattered (Compton);
OR
Transmitted unaltered.

Answer 17

Photons
As an X-Ray photon passes through matter it can be:
Attenuated i.e. absorbed (photoelectric) or scattered (Compton);
OR
Transmitted unaltered.

Question 18

Secondary / scattered radiation
This is X-Radiation which is produced when the primary beam interacts with matter;
Most of it in a direction other than that of the primary beam;
It always has a lower energy level than the primary beam so is more likely to be absorbed.
It also degrades the radiographic image by reducing radiographic contrast and sharpness

Answer 18

Secondary / scattered radiation
This is X-Radiation which is produced when the primary beam interacts with matter;
Most of it in a direction other than that of the primary beam;
It always has a lower energy level than the primary beam so is more likely to be absorbed.
It also degrades the radiographic image by reducing radiographic contrast and sharpness

Question 19

Factors determining whether a photon is absorbed or scattered:
The incident energy (kVp)
Higher energy = more scatter;

The density & atomic number of the matter;
Absorption only affected
No impact on amount of scatter

The volume of matter;
Collimation and thickness
Both absorption and scatter affected

Answer 19

Factors determining whether a photon is absorbed or scattered:
The incident energy (kVp)
Higher energy = more scatter;

The density & atomic number of the matter;
Absorption only affected
No impact on amount of scatter

The volume of matter;
Collimation and thickness
Both absorption and scatter affected

Question 20

How do these factors effect absorption and scattering of the beam?

As incident energy (kVp) increases: absorption decreases and the amount of scattered radiation produced increases but that which is produced is in a more forward direction;
As patient volume increases: absorption increases and amount of scatter produced increases;
As atomic number and thus density of the matter increases: absorption increases but scatter remains constant

Answer 20

How do these factors effect absorption and scattering of the beam?

As incident energy (kVp) increases: absorption decreases and the amount of scattered radiation produced increases but that which is produced is in a more forward direction;
As patient volume increases: absorption increases and amount of scatter produced increases;
As atomic number and thus density of the matter increases: absorption increases but scatter remains constant

Question 21

Three ways of controlling scattered radiation:
Reducing/minimising the amount of scattered radiation produced;

Reducing/minimising the amount of scattered radiation reaching the image receptor.

Post-processing the image using AI to reduce the impact of scatter on the image

Answer 21

Three ways of controlling scattered radiation:
Reducing/minimising the amount of scattered radiation produced;

Reducing/minimising the amount of scattered radiation reaching the image receptor.

Post-processing the image using AI to reduce the impact of scatter on the image

Question 22

Reducing the amount of Scattered Radiation Produced
Close control of extra-focal radiation
Collimating the primary beam;
Using compression to reduce the volume of matter/patient irradiated;
Careful choice of exposure factors

Answer 22

Reducing the amount of Scattered Radiation Produced
Close control of extra-focal radiation
Collimating the primary beam;
Using compression to reduce the volume of matter/patient irradiated;
Careful choice of exposure factors

Question 23

Reducing the Amount of Scattered Radiation Reaching the Image Receptor
Absorbing any back scatter produced behind the image receptor;
Using a secondary radiation grid interposed between the patient and the image receptor
Arranging an air gap between the patient and the image receptor;

Answer 23

Reducing the Amount of Scattered Radiation Reaching the Image Receptor
Absorbing any back scatter produced behind the image receptor;
Using a secondary radiation grid interposed between the patient and the image receptor
Arranging an air gap between the patient and the image receptor;

Question 24

The Secondary Radiation Grid

Answer 24

The principle of a secondary radiation anti-scatter grid is that it absorbs the secondary/scattered x-rays and transmits the primary beam.
Absorbing the secondary/scattered radiation improves the image quality by improving the image contrast
The grid does not affect the amount of scatter produced, it simply limits the amount that reaches the IR

The grid not only attenuates the scattered radiation
It also attenuates some of the primary beam
Therefore less photons reach the IR
Therefore if a grid is used the mAs has to be increased
This increases the number of photons produced

Question 25

kVp and grids

Answer 25

Higher kVp requires a higher grid ratio.
High kVp produces less scatter but the scatter produced is in a more forward direction so if the lead slats are not deep enough the scatter radiation passes through the radiolucent interspace
But the higher the grid ratio used the more primary radiation is absorbed by the grid.
Therefore, the fewer photons reach the IR
The exposure (mAs) has to be increased.

Question 26

What is grid factor?

Answer 26

The Grid Factor is the amount by which an exposure has to be increased if a grid is used compared with the exposure required if a grid is not used.

As the grid ratio increases, more photons are absorbed by the grid
Therefore less photons reach the IR
Leading to reduced density
mAs must be increased
GF = mAs with the grid / mAs without the grid
GF is normally displayed on the grid
But can be calculated from the grid ratio

Question 27

Grid factor in practice
If the exposure for an AP c-spine without a grid uses 5 mAs, what mAs should be used to produce a image of the same density with a 8:1 ratio grid, with a GF of 4?
(This information is found on the grid)
GF = mAs with grid/mAs without grid
4 = mAs with grid / 5
mAs with grid = 4 x 5 = 20

Answer 27

It may be that 20 mAs would lead to the patient dose being too high.
Exposures can be manipulated to reduce patient dose whilst maintaining overall image density
“Rule of 10 (number 2)”
For every 10 kVp added, mAs can be reduced by half and the overall image density will remain the same

AP c-spine with no grid = 70kVp 5mAs

AP c-spine with grid = 70kVp 20mAs
Can manipulate to become 80kVp 10mAs

However, contrast will now be reduced.
A grid is introduced to increase contrast
Need to weigh up which will be best image quality for lowest patient dose
This is optimisation of exposure factors

Question 28

Grid and dose
The use of a secondary radiation grid always results in an increase in exposure and hence radiation dose;

Potential solutions:
Air gap technique
Artificial intelligence processing algorithms

Answer 28

The use of a secondary radiation grid always results in an increase in exposure and hence radiation dose;

Potential solutions:
Air gap technique
Artificial intelligence processing algorithms

Question 29

Principles of Air Gap technique?

Answer 29

This technique employs an air gap instead of a secondary radiation grid;
The air gap must be at least 15cm;
It works on the principle that scattered radiation is produced in all directions and introducing an air gap will mean less reaches the image receptor because the rays are diverging.

Question 30

Artificial intelligence gridless systems
Most manufacturers have developed processing algorithms to replace the need for a physical grid
Software is applied to the image
Can be automatic in pre-settings
Or manual and applied by the radiographer
Compensates for scatter on the image, “removing” it digitally

How does it work?
An enhancement algorithm applied during processing or post-processing
Each manufacturer is slightly different, though the overall principle is the same
The algorithm develops a “scatter distribution image”
A representation of the scatter contained in the image
This is subtracted from the original image

Answer 30

Artificial intelligence gridless systems
Most manufacturers have developed processing algorithms to replace the need for a physical grid
Software is applied to the image
Can be automatic in pre-settings
Or manual and applied by the radiographer
Compensates for scatter on the image, “removing” it digitally

How does it work?
An enhancement algorithm applied during processing or post-processing
Each manufacturer is slightly different, though the overall principle is the same
The algorithm develops a “scatter distribution image”
A representation of the scatter contained in the image
This is subtracted from the original image

Question 31

Benefits of AI “grids”

Answer 31

No need to increase mAs when compared to non-grid exposures
No risk of grid cut-off or exposure errors
No need for extra physical equipment
No additional weight

Question 32

As per department protocol, you next undertake a HBL hip using the wall stand.

As you are about to expose the image, you “pause and check”.
During this you notice that the incorrect detector is selected. You correct this to select the wall stand detector. You confirm that the centre chamber is selected and a grid is in place.

This is the image you produce
For 4 marks, explain why the image quality is so poor and how you will correct this.

Answer 32

There is no evidence of collimation on this image (1 mark)

This has greatly increased the scatter produced (1 mark),

therefore more has reached the IR leading to decreased brightness, contrast and increasing unsharpness (1 mark)
Changing detectors often results in collimation automatically changing
This should always be checked

The image can be improved by repeating with correct collimation (1 mark)
Also need to bring centring more proximal

This will reduce scatter produced, leading to a better quality image (1 mark)