Introduction To CR And Digital Imaging Detectors Flashcards
What is the intensity of x-rays impacted by?
(2)
The thickness of the material
The type of material
When there’s a thicker material, is attenuation more or less likely to occur?
More likely
What is noise?
Unwanted fluctuations in signal
What are the 2 types of noise we can get?
Electronic noise
Quantum noise
What is quantum noise about?
Poisson distribution
What is Poisson distribution?
The probability of the photons being attenuated or absorbed. (In the same situation, it’s expected that they’d have the same attenuation, but due to fluctuations they don’t)
How does scatter affect an image?
It makes the difference between photons passing through material and no material smaller, which reduces the contrast of the image
If there’s more photons, is there more or less noise?
Less noise, so better image quality
If there’s less photons, is there more or less noise?
More noise
What is a penumbra?
A shadow
Is it more ideal to have a small or large focal spot?
Small focal point. We want to get the spread of x-rays coming from a small source
What are the features of a perfect detector?
(4)
It fully absorbs all of the transmitted photons
It has a good spatial resolution and doesn’t add to the blurring of the penumbra
It operates in standard conditions, so normal day to day life, e.g. it doesn’t only work in extremely low temperatures
It’s durable and cheap
What are the 2 types of radiography?
Computed radiography (CR)
Digital radiography (DR)
Where is computed radiography (CR) used?
In dental systems
What happens in computed radiography (CR)?
(4)
Radiation is absorbed by an imaging plate
(There’s no fixed detector- there’s only an imaging plate that can move)
The energy from the absorption is trapped, forming a latent image
When we put the imaging plate in the reader, the release of the trapped energy is converted into the image
What are examples of CR readers and cassettes?
(3)
Agfa
Fuji
Carestream (Kodak)
For the exam, do we need to know equations?
NO!!!!!
For the exam, will we ever be asked any maths questions?
NO!!!!!
Label the CR plate/detector:
What are the 3 layers in a CR plate/detector?
Conductive layer
Reflective layer
Protective layer
What happens to the CR plate/detector?
The protective layer gets removed so that the conductive layer can be exposed to the detector
What is the conductive layer in the CR plate made out of?
A crystalline structure
What happens in the CR plate?
(6)
The x-ray photons enter the protective layer
The photons interact with the crystal in the reflective layer
The photons then interact with the electrons in the conductive layer.
The electrons get a lot of energy and become excited
The electrons want to get rid of the energy
When the electrons lose the energy, they go to the valence band
What can happen to the electrons in the CR plate?
The electrons can get trapped in the crystal structure of the forbidden zone and get held in place there. However, they can only get trapped if they find a trap in the structure. But they don’t have enough energy to leave the trap
What can we do to release the trapped electrons from the crystal structure in the CR plate?
(2)
We can give the electrons more energy to escape the trap- this is how readers use energy to produce images
This produces light, and the light is proportional to the energy that the electrons absorbed, which can produce an image
What’s an example of how we can give the trapped electrons in the CR plate energy?
We can use a laser, as that’ll give the electrons energy
Where do the electrons get trapped in the CR plate?
In the forbidden zone
What is the difference in energy between the conduction of x-rays and the valence band dependent on?
It’s dependent on what the crystal is made out of
Which band is more stable- the valence band or the conduction band?
Valence band
Where is the ideal place that all electrons in the CR plate want to go?
The valence band
How does a reader produce an image?
(3)
It converts the light it detects into an electrical charge
The electrical charge is converted from a charge to a digital signal
This produces an image
What are problems with using a reader to produce an image?
(3)
Not all trapped radiation is released
The longer the laser dwells, the more trapped electrons are released, which increases the sensitivity
After the plate is read, a white light exposes the whole plate so the electrons can all be released and the plate can be used again. This doesn’t contribute to the image
Why do the electrons in the trap in the CR plate degrading by 25% in the first 8 hours of being there?
Because normal heating and other things are happening to the electrons whilst in the trap
What does the resolution of CR depend on?
The thickness of the phosphor layer
The size of the layer
The use of a reflective layer
What will happen if we use a thicker phosphor layer in CR?
The image will be higher sensitivity but poorer resolution (more blurry)
What type of conversion does CR use?
Indirect conversion
What type of conversion does DR use?
(2)
Indirect conversion
Direct conversion
How does DR use indirect conversion?
It uses an indirect detector
How is an indirect detector used in DR?
(5)
X-rays are converted into light by the material
This causes multiple light photons
This causes the spread of light
The light is turned into a current
The current is tuned into a signal
Do we want the scintillator to be small or big?
Why?
As small as possible
To prevent blurring
What is the disadvantage of having a small scintillator?
It causes less absorption of photons- so we have to decide which is more important: resolution or attenuation?
What is the scintillator made out of?
Caesium iodide doped with thallium
What are the features of the scintillator?
(3)
The caesium iodide crystals are grown in columns and act as a light guide
The caesium iodide and thallium have a K edge of 36 keV and 33keV
The caesium iodide and thallium have atomic numbers of 55 and 53
Why is a benefit that the caesium iodide crystals are grown in columns and act as a light guide?
Because this minimises the spread of light
Why is it a benefit that the caesium iodide and thallium have a K edge of 36 KeV and 33 KeV?
Because t causes good absorption
Why is it a benefit that the caesium iodide and thallium have atomic numbers of 55 and 53?
Because it causes good photoelectric effects to occur
What is the oldest method of indirect conversion DR?
CCD indirect detectors
What are CCD indirect detectors mainly used for?
(2)
Mammography
Dental
What is the current method of indirect conversion CR?
Flat panel indirect detect
How does the flat panel indirect detector work (DR)?
(4)
The x-rays interact with the scintillator
This produces visible light
The photodiodes convert this to a current
The current is then converted into a signal
How is direct conversion done in DR?
Using direct detection
How does direct detection work in DR?
(4)
The x-rays interact with the material
This produces electrons
The electrons are directed towards the detector
This prevents blurring
What are limitations of direct detection in DR?
It uses amorphous-selenium, which has a lower atomic number than caesium. So it’s less likely to absorb x-rays
When would direct detection in DR be useful?
Why?
Mammography
Because they use low k-electrons
What does each detector consist of?
An array of detector elements
What does each detector element consist of?
(2)
Why?
Photoconductor
Electronics
To read signal
What do smaller detector elements have to do?
Why?
Sacrifice the photoconductor area
Because the electronic size is fixed
What happens if we increase the number of detector elements?
The detector efficiency changes, causing a better resolution but less absorption of x-rays, due to having a smaller photoconductor size
Compare direct and indirect conversion in DR:
(3)
Direct: uses a semi-conductor material
Indirect: uses a scintillator material
Direct: produces an image with better resolution (since it doesn’t have a light stage)
Indirect: produces an image with poorer resolution (since it has a light stage
Direct: it has a poorer sensitivity at high energy (it can’t absorb as much energy)
Indirect: it has a greater sensitivity at diagnostic (it can absorb more energy)
What does absorption efficiency mean?
How well does a detector absorb the photons that interact with it?
What does conversion efficiency mean?
It’s the conversion of what you’re detecting from light/starting molecule
What does capture efficiency mean?
How good the image produced is from the signal we’re getting
What are the characteristics of a radiation detector?
(5)
Absorption efficiency
Conversion efficiency
Capture efficiency
Dose efficiency
Reproducibility
How does the detector material impact absorption?
If you want to produce an image with the highest optimum, you need a higher absorption
Is the mA is doubled, how many electrons will interact with the target?
Twice as may electrons
What does intensity mean?
The energy passing through an area at a time
Instead of increasing the tube current, what else can we do?
We can expose the patient for a longer duration
What is the problem with longer exposure times?
The image is at risk of motion artefacts- voluntary and involuntary
