Week 6 - Techniques For Image Optimisation

Question 1

What is Compton scattering?

Answer 1

A dominant interaction between diagnostic x-rays and the human body.
It completes with the photoelectric absorption process.

As energy increases, the probability of Compton interaction increases relative to photoelectric interaction.

Compton scattering doesn’t depend on atomic number.

Scattering occurs int he patient.

Question 2

Scatter doesn’t add any patient information too the image.
Just gives a fog wash over the image.
Which means there is a loss of contrast.
But there is no loss of resolution.

Question 3

Scatter increases patient dose.
And to radiographers.

Question 4

Scatter is detrimental to diagnostic image quality.
Doesn’t give the image any useful information about anatomical areas of interest.

Question 5

To produces a diagnostic image - we need to control scatter radiation produced int he patient and what reaches the image receptor.

Question 6

An image which only transmits unscattered x-rays = a sharp image.
But there is less variation of greys.

Question 7

An image that has no absorption only full transmission with scattered radiation = a dull grey image.

Question 8

But in reality our images will have some contrast.
There will be some transmission and some absorption.

But some contrast may be lost due to the presences of scattered x-rays.

There will be more variation in greys.

Question 9

What is scattered radiation a result of?

Answer 9

Compton scatter interaction.

Incoming x-ray photons lose energy and change direction.

Question 10

What 3 factors iunfluyenec the amount of scatter produced on an image?

Answer 10

1. Increased patient thickness (will need a higher kVp to penetrate through - which means more Compton scatter occurs).
2. Increased x-ray field size (if is an extremity).
3. kVp (this upping of energy can mean more Compton scatter occurs).

Question 11

What does collimation affect?

Answer 11

Dose.
Image quality.

It is a very powerful and important concept.

Question 12

Increasing collimation is bringing the field size in (making it smaller).

Answer 12

This increases contrast too.

Question 13

Increasing collimation will -

Answer 13

1. Decrease patient dose.
2. Decrease scatter produced in the patient.
3. Reduce scatter on the image receptor exposed.
4. In grease radiographic contrast.

Question 14

What is bad about decreasing the collimation, and how to we compensate for this effect?

Answer 14

Making collimation tighter will reduce the amount of radiation reaching the image receptor.
In digital imaging, this means there will be increased Quantum Mottle / Noise - this is bad for image viewing.

To counteract this effect - exposure factors, usually mAs, need to be increased.

Question 15

What does the DAP meter in the x-ray tube measure?

Answer 15

It measures all of the x-rays that come out of the x-ray tube.

Question 16

What are the units for measuring with the DAP meter?

Answer 16

Gray/cm2.
It is a unit of the area of the beam.

Question 17

By collimating (bringing in the field), DAP meter value decreases.
Because less is emerging through it.

Question 18

Patient dose is very sensitive to the area of interest.

Question 19

As beam collimation increases (field size gets smaller), the quantity of scatter radiation decreases and radiographic contrast increases.

Question 20

At high kVp levels, scatter radiation has more higher energy to penetrate the image receptor.

Question 21

Radiation emits in a forward direction.

Question 22

Both Compton and Photoelectric Effect become weaker when a higher kVp is used.

Answer 22

But Compton interaction is more dominant than the PE.

Question 23

If we are to use a higher kVp, then our primary consideration is to ensure adequate penetration of the area of interest, to produce good subject contrast.
Need to not give high radiation dose to an anatomical area that is not in interest.

Answer 23

But we can’t make kVp too low - because we need some x-rays to actually pass through the thicker structures and not be fully absorbed - otherwise no image at all would be produced.

Question 24

Higher kVP is more preferable than increasing the mAs to a higher level.
Why?

Answer 24

Because it gives lower patient dose.

If mAs was simultaneously adjusted, we need to do the 15% rule -
Would have to halve mAs to jeep exposure unchanged.

Question 25

What would a larger collimation field size create?

Answer 25

1. It would generate scatter radiation - due to greater amounts of exposed tissue. E.g. for the pelvis. 2. More radiation reaching the image receptor. 3. Additional scatter can be produced from using the table top.

Question 26

Additional scatter can be produced form using the table top.

Question 27

What factor can control the 3 previous issues that arise from using larger field sizes?

Answer 27

Collimation. Field size is in the control of the radiographer. It is their primary method of preventing scatter radiation being produced.

Question 28

What is the meaning of Large Soft-Tissue part Thickness?

Answer 28

Larger patients and their body parts present more exposed tissues, which generates more scatter radiation. This means most of the primary beam is attenuated.

Question 29

However the chest is an exception to part thickness as a cause for major scatter radiation. Why?

Answer 29

Because the chest includes the lungs, which contain air. Air has a very low density, so is less susceptible to scatter.

Question 30

Does the radiographer have any control over Part-Thickness?

Answer 30

No control over it. It is just based on what the patient is like.

Question 31

Imaging thicker parts of the body = more scatter. More than the thinner parts of the body.

Question 32

Compression of anatomy, e.g. the breast tissue, can improve contrast and Lowe patient dose. E.g. in mammography.

Question 33

What are 2 methods of scatter control?

Answer 33

1. Beam Collimation. 2. Radiographic Grids.

Question 34

Method of scatter control - 1. Beam Collimation -

Answer 34

Collimation is decreasing the x-ray beam field size. An uncollimated beam is circular - but there is a box underneath the x-ray tube with 2 dials on it, and adjusting these can adjust the field size - making it smaller into more of a square shape. Increased beam collimation = decreased field size = decreased patient dose. Field size increases = relative quantity of scatter radiation increases. More x-rays = more possible scatter radiation.

Question 35

Collimation increases = Field size decreases. Patient dose decreases. Scatter radiation decreases. Radiograohic contrast increases - as there is less scatter. But if digital - quantum noise increases. This occurs if not enough x-rays reach the image receptor.

Answer 35

Collimation decreases = Field size increases. Patient dose increases. Scatter radiation increases. Radiograohic contrast decreases. If digital - quantum noise decreases.

Question 36

What are the collimators?

Answer 36

They are located immediately below the tube window. It is compromised of a set of lead shitter - which can block any unwanted x-ray beam. It has longitudinal and laser blades. It helps produce fields fo various sizes.

Question 37

What do the collimators consist of?

Answer 37

Lead shutters. Longitudinal and laser blades. A white light and mirror - which projects the light field onto the patient - it shines through the lead shutters. This light indicates the location of the primary beam, and it matches with the x-ray field. An x-ray field measurement guide - in cm. A plastic temp,ate with cross-hairs in the centre - this indicates the centring point. Automatic collimators - e.g. for when you can’t see the detector size, I.e. if using the bed. It mechanically adjusts the primary beam and shape of it to the IR.

Question 38

Need to do list of QA - quality assurance checks - to check the alignment of the x-ray field and light field.

Question 39

At the end of the day, don’t actually need to put the x-ray tube to ‘bed’, because this actually damages the plastic window and shift it,m meaning the centring point will be off centre.

Question 40

Summary of collimators -

Answer 40

Scatter radiation is a result of comptons scatter interaction of x-rays with the patient. Scatter reduces image contrast, by dding a uniform exposure to the image. Increased patient thickness = more exposed tissue to the primary beam = greater scatter radiation. Larger field sizes = more scatter radiation, due to more expired tissue. High kVp = more scatter, because Compton dominates at higher energies, over PE. Collimation of primary beam = reduced scatter.

Question 41

Who controls the radiation exposure required to produce a diagnostic image?

Answer 41

The radiographer.

Question 42

The radiographer judges what kVp and mAs needs to be used in relation to the patient size.

Question 43

Extremities - reliable exposure settings achievable.

Question 44

Larger variation on body sizes (e.g. pelvis, abdomen) = more inconsistency between exposure settings.

Question 45

Can only properly see whether or not the exposure was correct purely from looking at the waukltuiy of the image after it has been produced.

Answer 45

But this doesn’t fulfil as a core part of the role of a radiographer - optimisation.

Question 46

What is optimisation?

Answer 46

It is written into the IR(ME)R17 legislation. So x-ray manufacturers have had to develop 2 pieces of technology to improve image quality and reduce dose - 1. The Automatic Exposure Control. 2. The Grid.

Question 47

What are the Automatic Exposure Controls / Devices (AECs/AEDs)?

Answer 47

It sits embedded into the front surface of the image receptor. It is a device that measures the quality of radiation which reaches the image receptor. It monitors the quality of radiation that reaches the image receptor, and it will then stop the exposure emitted from the x-ray tube at a point in time once the image receptor has received the required amounts of radiation needed. So AEC essentially controls time of exposure. This is not done by the radiographer - can only control the mAs.

Question 48

At the console, the radiographer should select if and which AECs they want the X-ray unit to use for that exam. Radiographer sets mAs at the console.

Answer 48

AECs control exposure time (based on the amount of radiation reaching them). This means it also essentially controls the mAs, because mA x time = mAs. Radiographer also cab set the kVp at the console.

Question 49

Where are the AECs / AEDs located?

Answer 49

Above the imaging receptor. They are the last thing x-rays encounter before the image receptor. They are radiation detectors. When the right amount fo radiation has reached them, they will terminate the exposure.

Question 50

AECs are often confused with the DAP meter. What is the DAP meter?

Answer 50

It measure the amount of x-rays emerging out of the x-ray tube. But AEC measures what makes it through the patient - what reaches the imaging receptor.

Question 51

Most AEC systems use a set of 3 radiation measuring detectors. The radiographer selects what configuration of these they want to use. This determines which of the 3 will actually measure a radiation exposure that reaches the IR. They can select one, two, or all three.

Question 52

How does an AEC actually work?

Answer 52

The radiation measuring detectors are a parallel plate ionisation chamber, which is full of air. As the x-rays interact with it, they ionise the gas it contains. Oncre a certain level of charge form them is reached in the air, then exposure is terminated. So it is dictating exposure time. A predetermined level if radiation must be reached before exposure is terminated. This dictates exposure time, and therefore the amount of radiation to the IR.

Question 53

The AEC contains a capacitor. This is essentially a ‘Charge Bucket’, which stores and measures the amount of charge produced.

Question 54

So an AEC overall contains -

Answer 54

An ionisation chamber, which contains air, enclosed by a thin aluminium shell. A capacitor. An exposure terminating switch.

Question 55

What is the Capacitor?

Answer 55

A device. Used to store a specific quantity of electrical charge. This electrical charge will be in direct proportion to the radiation which the ionisation chamber has been exposed to. Sow hen a rope-determined level is reached, a signal is sent to the termination switch.

Question 56

Imaging the ionisation chamber -

Answer 56

We cans top the ionisation chamber form. Appearing on the image by then being built into the detector where x-rays pass through.

Question 57

What are the ionisation chambers made of?

Answer 57

Radiolucent materials, which transmit x-rays. But the kVp range should be sufficient enough to over-penetrate the ionisation chambers, This prevents artefacts.

Question 58

All of the ionisation chambers are connected by small electrical wires. This is sop they can talk to each other. So can use a combination or all 3 chambers together.

Question 59

What is the Back-Up Time?

Answer 59

It is a safety feature, because of all the risks that can come from working with radiation. The back up time refers to the max. Length of time that the x-ray exposure will continue for when using an AEC system. It is automatically controlled by the radiographic unit. It acts as a safety mechanism unease the AEC system fails or the equipment isn t being used properly. It prevents overexposure to the patient, in case of timer switch failure or radiographer error.

Question 60

So back-up time protects the x-ray tube and patient.

Question 61

The Back Up Time is usually 150% - 2005 of what the expected exposure time is. This is actually quite a while, but at least there is a limit.

Answer 61

The emergency button can also terminate exposures if needed.

Question 62

What 6 factors can affect AED usage?

Answer 62

1 .Patient positioning. 2. Patient thickness. 3. Chamber selection. 4. Collimation. 5. Minimum response time. 6. Back up timer.

Question 63

How does 1. Patient positioning, affect AED usage?

Answer 63

Positioning of anatomy is relative tot e h chamber positions. Need to look at all of our anatomy, and do our best to fit it on top of all the relative chambers and IR, to get the image. If a chamber is selected but not patient anatomy overlies it, then the timer will cut-off much earlier (reach its limit faster), because x-rays will pass through it quicker compared to patient thick tissue or bone anatomy. It will then mean the whole image will end up looking underexposed - whiter. Incorrect positioning relative to the AEC and chambers can result in under or over exposure - depending on hat is being image - Bone vs soft tissue. Small vs large anatomical structures.

Question 64

How does 2. Patient Thickness, affect AED usage?

Answer 64

The AEC system compensates for changes in patient thickness. If the area of interest is thicker, then exposure time will lengthen to reach the preset exposure to the detectors. 1/3 of UK are overweight / obese - so often need to compensate for these patients very nicely. Some patented and their thickness may need greater technical consideration - Underexposure - If am AEC is superimposed by an area of excessive bowel base, then the timer will cut out much quicker = under exposure. A premature cutout. Overexposure - Destructive pathological conditions, positive contrast media, or prosthetic devises which may superimpose the AEC detector can cause overspure - it is harder for x-rays to pass through so less reaches the IR and AEC chambers so cut out doesn’t happen as soon as expected.

Question 65

How does 3. Collimation, affect AED usage?

Answer 65

X-ray field size is a factor is AEC systems. The additional scatter produced by a larger field size (from failing to restrict the beam) could cause the detectors to terminate the exposure prematurely. The detectors measures both scattered and transmitted radiation emitting from the patient. So the timer will cut of too son is scatter is excessive = this creates an underexposed area of interest. But if the x-ray field size is collimated too closely, then the detector wont receive sufficient exposure = a prolonged exposure time = overexposure.

Question 66

What is an AED? What does it contain?

Answer 66

An exposure measuring device. Consists of an ionisation chamber, a capacitor, and a termination switch. Ionisation chambers are located just above the image receptor. Patient positioning and chamber selection is the 2 most important factors which will affect image quality when using an AED. The AEC chambers should be beneath the anatomical region being imaged. There is a back-up timer - protects patient and x-ray tube.

Question 67

What are some advantages of using an AED for an image?

Answer 67

It maintains consistency and quality in images. It reduces the number of repeats - effectively reducing patient dose. It increases efficiency.

Question 68

What are some disadvantages of using an AED?

Answer 68

The chambers can’t differentiate between transmitted and scattered radiation. If an unknown pathology / metalwork present in the image, it can lead to images of sub-optimal density. It still is dependant in how the radiographer positions the patient.` In digital imaging, we can alter overexposed images and make them better anyway.

Question 69

An AED / AEC is a ionisation chamber.

Question 70

The AEC is situated between the radiographic table and image receptor.

Question 71

AECs control the exposure times. And so also control the mAs.

Question 72

When right amount of radiation has passed through, exposure will cut of.

Answer 72

Patient position is crucial when using AECs.

Question 73

AEC system compensates for patient thickness.

Question 74

The x-ray field size is a consideration in AEC systems.

Question 75

Grids will be used everyday in practise.

Answer 75

If a grid isn’t used when it should be, then a repeat will often need to be done.

Question 76

How do primary x-rays behave when they leave the x-ray tube?

Answer 76

They have a point source. They travel in straight lines. They end up diverging out. However… We have scattered x-rays which exist. And these change direction from a straight line.

Question 77

To get rid of scattered radiation present in our images…

Answer 77

We use a grid. A grid acts as a scatter radiation filter (from the patient.

Question 78

The grid used should be big enough to cover the largest field size possible.

Question 79

The grid is positioned between he patient and image receptor. Is placed in the Bucky.

Question 80

The grid contains rows of lead strips / slats, which are spaced at fixed intervals.

Answer 80

Between these lead strips are radiolucent spacers, which let through x-rays.

Question 81

We want to keep straight lined x-ray photons. These ones tend not to hit the lead strips, and so they are fully absorbed through to the IR.

Answer 81

But with scatter x-ray photons, they have changed direction. This means they tend to hit at least some of the slanted lead strips. This means they don’t make it fully through to the IR.

Question 82

Grid ratio is how we categorise different grids.

Answer 82

Grid ratio = the ratio between the height of the slat / the distance between them. R = h / d

Question 83

The design of grids is very important. So that it doesn’t appear on the image - as an artefact. Otherwise the slat lines could appear as an artefact.

Answer 83

One design feature top prevent this is the ‘moving grid’ - this is where the grid’s LSAT’s vibrate back and forth.

Question 84

We need to kept the anatomical area of interest over the grid. They set the kVp and mAs (if we aren’t using an AEC system). Then irradiate the patient. Any scatter should be absorbed by the lead slats. So the image will now have improved contrast - as there is less or no scatter in the image.

Answer 84

Have to vibrate the lead slats back and forth - to make sure the lines don’t appear in the image as an artefact.

Question 85

Grids have a lead line orientation.

Answer 85

They can be a Non-Focused Grid - The lead lines run parallel. Or the grid can be Focused - Where the lead lines are angled, to match the angle of divergence of the primary beam. Focused grids allow more transmitted photons (straight line x-rays) to pass through to reach the IR. This means there is an image producing everything - no ends left blank.

Question 86

It is important to use the correct SID for focused grids. This is so that the primary x-rays can precisely pass through the slats.

Answer 86

If further away or closer than optimum, it means some x-rays wont make it through any more as it doesn’t precisely line up. Would get grid cut-off - where the good x-ray photons hit the slats.

Question 87

What is the Grid Factor?

Answer 87

It tells you how to adjust the mAs when changing between grid and no grid. Grid factor can therefore be used to determine how much mAs is needed to be adjusted when changing from using a grid compared to no grid. The % increase required in known as the Grid Factor. Grid Factor = mAs with grid / mAs without grid.

Question 88

Different grids have different grid ratios. It depends on the equipment being used and technique protocols.

Answer 88

A grid better at absorbing scattered photons, so with a higher grid ratio = fewer photons reaching the IR. These slats will be a lot thicker - so harder for primary x-rays to get through - they just hit th lead slats. So would need to increase mAs - to produce a diagnostic image.

Question 89

What are the advantages of using a grid?

Answer 89

It reduces scatter in the patient. So this improves contrast between adjacent anatomical structures. It improves diagnostic quality for techniques that include many tissue types.

Question 90

What are the disadvantages of using a grid?

Answer 90

Increased exposure factors will be required - as mAs needs to be increased. This increases patient dose. If used incorrectly, then the image quality can reduce - the SID is crucial to allow the alignment of the angled slats (to avoid grid cut-off).

Question 91

Most grids are focused on- which allows more transmitted x-rays to reach the IR.

Question 92

The grid allows adjustment of mAs when using a grid - to maintain exposure to IR.

Question 93

Grids increase patient dose.

Question 94

Grids improve diagnostic quality of images.

Question 95

Grids are only necessary for some exams. E.g. larger anatomical body parts. Which would create more scatter. Which needs to be reduced. Like the abdomen or pelvis.

Question 96

Grid improve contrast. Absorbs scattered radiation and prevents it from reaching the IR.