33. CT: COLLIMATORS Flashcards
1
Q
- What are Collimators?
A
- they are used in X-Ray imaging
- they are found at the exit of the X-Ray Tube
- they are used to find the size and the shape of the
X-Ray beam
2
Q
- What do CT units make use of?
A
- they make use of Collimators
3
Q
- What does the interaction of X-Rays with tissue create?
A
- it creates randomly scattered photons
- these create image noise
- this clouds the image
- this decreases the image quality
4
Q
- What role do Collimators play when it comes to X-Rays?
A
- they are found at the front of the detector
- they act as an anti-scatter grids
5
Q
- What do these anti-scatter grids do?
A
- they eliminate X-Ray photons
- these deviate from a straight source-detector path
- they reduce the beam size
- they limit the beam thickness
6
Q
- What does the presence of Collimator on the front of the detector result in?
A
- it results in a small apparent focal spot
7
Q
- What are Artefacts?
A
THEY ARE THE DISCREPANCIES THAT ARE FOUND BETWEEN:
- the CT numbers represented in the image
- AND the expected CT numbers (in HU units)
8
Q
- What are 5 types of Common Artefacts?
A
- Beam Hardening
- Partial Volume Effect
- Bad Detectors
- Metal
- Patient Motion
9
Q
- What 2 types of Collimators do we have when it comes to CT Scanners?
A
- THE ONE THAT IS PLACED AT THE FRONT OF THE
X-RAY TUBE:- this is known as the Pre-Collimator
- this reduces the beam size
- this limits the beam thickness
- this results in a smaller focal spot
- results in better image quality
- THE ONE THAT IS IN CONTACT WITH THE DETECTOR:
- this is known as the Post Collimator
- this one acts as an Anti- Scatter Grid
- it absorbs the Scatter Radiation
- results in better image quality
10
Q
- What is the Beam Hardening Effect?
A
- this is when the X-Ray beam is hardened by passing
through an object - this happens specifically in denser objects
- specific areas of the image will then appear darker
than they should be
11
Q
- What happens in extreme Beam Hardening cases?
A
- a detector element may fail
- this results in a constant output signal
12
Q
- What is a primary source of image noise?
A
- detectors
- their associated signal amplifiers
13
Q
- What reduces the amount of image noise?
A
- Electronic Noise Suppression Filters
- Software Noise Suppression
14
Q
- Who can reduce the noise in an image?
A
- the operator
- provided that this loss of detail is acceptable
15
Q
- What kind of beams can X-Rays not produce?
A
- Monochromatic beams
- this is why an artefact related to beam hardening is
common
16
Q
- What does Beam Hardening result in?
A
- the image can have streaks and shading
- the image can be interpreted inaccurately
THE STREAKS:
- will be specifically seen in regions that are very dense
- such as the bone
17
Q
- When are lower energy X-Rays absorbed?
A
- when the energy peak will shift towards higher
energies - the absorption values will be underestimated
18
Q
- How can we reduce Beam Hardening?
A
- by reducing the pre-hardening of the beam
- this is done by placing a thin metal plate in front of the
tube
19
Q
- What materials can the thin metal plate made out of?
A
- Molybdenum
- Tungsten
20
Q
- What can compensate for the Beam hardening in the projections?
A
- applying appropriate algorithms in the reconstruction
21
Q
- What does this image show?
A
- The Beam Hardening Effect
22
Q
- When does the Partial Volume Effect occur?
A
- it occurs whenever a pixel represents more than one
kind of tissue - it can also occur when a single voxel contains a variety
of tissue - this occurs in CT and Nuclear Imaging
23
Q
- When is the Partial Volume Effect particularly relevant?
A
- when a tissue boundary lies within a CT splice
- this means that one splice contains information from
the previous splice
24
Q
- What does the Partial Volume Effect result in?
A
- it blurs the intensity distinction
- this happens between the adjacent tissues
25
Q
- What can reduce the Partial Volume Effect?
A
- higher resolutions
- repositioning of the patient
- we can use thinner slices
26
Q
- What do these images show?
A
- the Partial Volume Effect