20. X-RAY IMAGE QUALITY Flashcards
1
Q
- Why are poor quality images easier to define?
A
- they have a poor signal-to-noise ratio
- they have poor spatial resolution
- they detract from the process of extracting information
2
Q
- How must radiological images be produced?
A
- with the lowest radiation dose that is consistent with the
diagnostic quality
3
Q
- What does an optimum quality image enable the user to do?
A
- it enables them to make an accurate diagnosis
4
Q
- What can happen to specific characteristics of an image?
A
- they can be evaluated
- this enables us to determine the diagnostic quality of
the image
5
Q
- List the characteristics that can be evaluated.
A
- Patient Positioning
- X-Ray beam and detector positioning
- Collimating and centring the beam to the area of
interest - Minimising the patient movement
- Detector data that is acquired
- Quantity and Quality of photons collected
(brightness and contrast) - Scattered Photons
(noise) - Image viewing display
- Monitor and image matrix size
- Software processing applied to the raw data
- Viewing conditions
(background illumination)
6
Q
- What can be said about the sizes of the radiographic images?
A
- all radiographic images are larger than the objects
being X-Rayed - the magnification is due to the geometry of the
imaging
7
Q
- What is the ideal image geometry situation?
A
- THE OBJECT THAT IS BEING IMAGED
- is parallel to the X-Ray beam and the image receptor - THE RADIATION BEAM
- is at right angles to the object
3.THERE IS A LONG FOCUS TO RECEPTOR DISTANCE
- there is a small object to receptor distance
NB:
- these conditions minimise the distortion of the image
- they minimise the magnification of the unsharpness in
the image
8
Q
- What can be said about the distance between the object and the image receptor?
A
- the object should be as close as possible to the image
receptor
AS THE OBJECT MOVES AWAY FROM THE IMAGE RECEPTOR
- the magnification will increase
- this makes the object bigger
- this magnifies any unsharpness in the image
9
Q
- How do we mathematically describe the relation between the object distance and the magnification?
A
NB:
- the positioning of the patient to produce the image
has a direct effect on the quality of the image
10
Q
- In practice, how much Geometric Sharpness (Ug) are we dealing with?
A
- the amount of Geometric Sharpness is small
- it may be much less than 0.4 mm
AT 0.4 MM:
- is the point at which we begin to perceive
unsharpness in an image
- this is due to the geometric unsharpness
11
Q
- How do we work out the Geometric Unsharpness?
A
12
Q
- Name 2 other factors that increase the level of Unsharpness?
A
- Movement
- The resolution of the monitor
13
Q
- When will an object be magnified?
A
- if the object is not parallel to the image receptor
- different aspects of the object will be magnified
differently - this causes distortion
14
Q
- What are 2 examples of Distortion?
A
- Elongation of the image
- Foreshortening of the image
15
Q
- What are deliberately elongated images used for?
A
- they are used to aid in the diagnosis of a fracture
16
Q
- What can be said about the distance between the patient and the image receptor?
A
- this distance is abbreviated as ORD
- this distance should be as short as possible
- the object should be in contact with the image receptor
if possible
17
Q
- How long is the FRD usually?
A
- it is about 110 cm for techniques on the X-Ray table
- it is about 180 cm for erect chest and cervical spine
work
18
Q
- How can we assess the image quality?
A
- we can assess it using the signal-to-nose ratio
- this is abbreviated to: SNR
19
Q
- What is the SNR?
A
- it is the useful information from the patient
- this information is being imaged
- the noise is anything that detracts from accessing the
information
20
Q
- Where is the useful information derived from?
A
- from the Photoelectric Interactions within the patient
21
Q
- Where is the noise derived from?
A
- Compton Scatter
22
Q
- What does the image receptor not have the ability to do?
A
- it does not have the ability to determine the origin of
scattered photons - there is also electrical noise from the system that
interferes with this process
23
Q
- What can be said about the signal level of Radiographic images?
A
- they have a signal level that is high compared to the
noise - this enables structures to be seen clearly
24
Q
- When will the structures disappear?
A
- when the signal level is similar to or less than the noise
25
Q
- What can be said about the images produced by
X-Rays?
A
- they are often a compromise between obtaining a
perfect signal and reducing the noise
26
Q
- What exists when we image radiosensitive areas?
A
- there is a constraint of minimising the radiation dose
to that area
27
Q
- What can be said about the extremities of the human body?
A
- they are less radiosensitive
- images with higher doses and better definition will be
required for X-Ray purposes
28
Q
- What is the purpose of a Bucky (grid)?
A
- it will enhance the contrast
- it does this by removing the scattered photons before
they reach the detector
29
Q
- Why do we need to justify the increased contrast by the use of the grid?
A
- this is because we are using a grid that will increase the
dose
30
Q
- What happens when there is an air gap left between the patient and the image receptor?
A
- there are fewer scattered X-Rays that will be
intercepted
31
Q
- Why are there fewer scattered X-Rays that are intercepted?
A
- the scattered X-Rays emerge from the body with a
greater range of angles than the transmitted photons
