11. Digital Radiography Continued Flashcards
Define post-processing:
Anything that can be added, changed, ect. to a digital image after it has been acquired. (annotation, adding text)
What is the biggest controlling factor for how the processed image will turn out?
The algorithm is the biggest controlling factor
Define windowing:
What happens when we use windowing?
-Adjusting the brightness and contrast (adjusting the values on the pixels)
-As we adjust the windowing, the system reassigns pixels different values that correspond to different brightness’s
Define window Level (WL) (Leveling):
- Controls image brightness
As you increasing the WL, what happens to brightness typically?
The higher the WL the brighter the image
Define window width:
- Controls image contrast
As you increase WW, what happens to contrast?
Decrease or reduce image contrast
More shades of grey
As you decrease WW, what happens to contrast?
increase image contrast
Describe in detail how a narrowing in WW increases contrast:
Narrowing the WW will decrease the amount of values represented on Image contrast, which increases contrast due to the fact it can only produce only a few values
Why would we want to magnify an image?
2
1.To see fractures
2.Looking at detail within image.
Describe image inversion
Areas that were dark are bright and bright areas become dark
Define DQE:
○ How efficient the detector is at converting the incident x-ray energy into an image signal (electronic signal)
How is mAs affected by DQE?
The higher the DQE of a detector, the less mAs that is required for a specific projection
What are 2 disadvantages of digital radiography?
- Lower spatial resolution compared to film radiography
- Dose creep
Describe the concept of dose creep:
○ The system is really good at fixing over exposures compared to under exposures. As a result, we tend to use mAs values that are higher than required.
Describe what it means to have a high DQE value compared to a low DQE:
A higher DQE value would mean that less radiation is required to achieve identical image quality compared to the detector with the lower DQE.
List some things that contribute to the overall quality of the image:
- SC
- IC
- Noise (Quantum model/mottle)
- Signal
- SR
- Pixel size
What is SC affected by?
Scatter Radiation-anything that affects scatter affects subject contrast
What is SC determined by?
five
- Differential Absorption
- Atomic Number
- Physical/Mass Density
- Thickness Differences
- kVp
Without SC, can we have IC?
No
What is the purpse of the algorithm?
Puts our brightness values and contrast where we would like them to be for a specific projection
What is noise (QM) determined by?
Determined by the number of photons hitting the detector to create the signal/image. Not every pixel will recieve the same amount of photons.
QM can be called nosie
What is noise caused by?
1.X-ray photons (inadequate technique)
2.Attenuation
With a Low kVp and mAs, what happens to singal and noise?
- Few photons hit the detector, we get a low signal, more noise
With a high kVp and mAs, what happens to singal and noise?
Many photons hit the detector, we get a high signal, less noise
Remnant radiation can also be called what?
Signal
What happens to singal when more photons hit the IR?
Signal increases
With more more lp/mm, what does that mean in terms of spatial frequency?
- More lp/mm = greater spatial frequency
Do all CR IP’s have the same pixel size?
Not always. If there is a difference, typically the smaller IP’s have smaller pixels compared to the larger IP sizes.
Where is the display pixel/display matrix located?
Is located on the monitor, and varies depending on the monitor that we use
Define pixel pitch:
The distance from the centre of one pixel to the centre of the pixel next to it.
In relation to SR, how does pixel pitch increase the SR?
The smaller the pixel pitch, the smaller the pixel, and therefor the better the spatial resolution.
In relation to SR, how does pixel size, and pixel density increase SR?
The greater the pixel density, the smaller the pixels and therefore the better the spatial resolution
For the same field of view, the greater the number of pixels the greater the what?
The greater the pixel density, the greater the SR
As you increase decrease PP and increase PD, what happens to SR?
Increases SR
What type of aquesition offers a higher DQE? What is the benefit of that?
Indirect aquesition-lower technqies per exposure
Why does direct aquesition have better SR?
Because there is less spreading of the e- as they are being pulled down the amorphus se
When comparing DQE’s of different detectors or systems, how should it be compared and why?
It must be compared at the same kVp so the beam has the same penetration.
What is SNR?
How much noise can be tolerated in the image
As SNR increases, what happens to noise?
Noise decreases
- Different detectors will require more or less exposure to the IR to achieve the same SNR
Why is it important to record more values than we can see?
- Through post processing, we can amplify those differences
Can we record 75 different values?
Yes provided we have a great enough bit depth
Define matrix
Number of pixels
If you decrease the FOV, does pixel size decrease?
No
How can you increase the SR with FOV, and matrix size?
For two different IR’s the smaller the FOV and the larger the matrix size, the better the SR
Define QM:
**Not enough information is getting to the IR, **not fixable through post-processing, flux in the amount of photons for unit area, will always be there.
What is the relationship between QM and SR?
QM has nothing to do with SR
Does mAs control contrast resolution?
- mAs doesn’t affect contrast, however when we reach the saturation point, it does affect contrast.
Why cant we fix saturation through post processing?
- A specific area saw more photons than the other area. As we increase our technique, the system can no longer tell a difference between two different pixels since they have both taken the maximum amount of radiation.
