30 Questions
What is required to achieve magnification in imaging?
Increasing the OID
How does the actual magnification achieved vary with depth in the patient?
It varies with depth in a non-linear manner
What is a consequence of using magnification in imaging?
Consequences for dose, spatial resolution, and SNR
What happens to the scatter fraction at the image receptor when the OID is increased?
It decreases substantially
Why is an increase in mAs required when the OID is increased?
To maintain the dose to the image receptor
What is the effect of increasing the OID on image sharpness?
It reduces the image sharpness due to geometric blur
Why is a significant reduction in focal spot size required when using magnification techniques?
To compensate for the geometric blur
How does magnification improve spatial resolution?
By increasing the size of the image compared to the unsharpness of the image receptor
What is the basis for choosing the kV setting in screen-film systems?
Required contrast
What is the purpose of choosing the mAs in screen-film systems?
To produce a suitable optical density for the region of clinical interest
What is the primary reason for using a larger image receptor in magnification imaging?
To accommodate larger body regions
How does the patient thickness affect the magnification achieved?
It varies the magnification between the exit and entrance sides
What is the effect of increased OID on geometric blur?
It increases the geometric blur
What is the purpose of reducing the focal spot size in magnification techniques?
To improve the image sharpness
How does magnification affect the overall sharpness of the complete system?
It improves the overall sharpness
What is the primary factor in choosing the kV setting in screen-film systems?
Required contrast
What is the purpose of choosing the mAs in screen-film systems?
To produce a suitable optical density
How does increasing the OID affect the scatter fraction at the image receptor?
It decreases the scatter fraction
Why is an increase in mAs required when the OID is increased?
To maintain the dose to the image receptor
What is the effect of magnification on the spatial resolution compared to a simple zoom of a digital image?
It improves the spatial resolution
What is the main advantage of using a larger image receptor in magnification imaging?
To accommodate larger body regions
What happens to the dose to the image receptor when the OID is increased?
It increases significantly
What affects the spatial resolution in digital systems?
Pixel size
Why is the scatter fraction reduced at the image receptor when the OID is increased?
Because the scattered rays are directed away from the receptor
What is the effect of increasing the OID on geometric blur?
It increases the geometric blur
What is the primary factor in choosing the mAs in screen-film systems?
Suitable optical density
What is the result of magnification on the overall sharpness of the complete system?
It improves the overall sharpness
What is the basis for choosing the kV setting in screen-film systems?
Required contrast
What is the effect of magnification on the spatial resolution compared to a simple zoom of a digital image?
It improves the spatial resolution
What is the consequence of increasing the OID on the focal spot size?
It requires a significant reduction in focal spot size
Study Notes
Magnification Imaging
- Magnification is achieved by increasing the OID, which generally requires an increase in the FID as well.
- The actual magnification achieved varies with depth in the patient.
- Example: Patient thickness is 20cm, FID 140cm, and FSD 80cm, the magnification varies between 1.4 at the Exit side to 1.75 at the Entrance side.
Magnification Requirements
- Magnification requires the use of a larger image receptor.
- For large body regions, this may not be possible.
Consequences of Magnification
- The use of magnification has consequences for:
- Dose
- Spatial Resolution
- SNR
Dose
- Increasing the OID leads to a substantial reduction in the Scatter Fraction at the image receptor.
- To maintain the dose to the image receptor, an increase in mAs and hence the patient dose would be required.
Unsharpness
- An increase in the OID leads to a reduction in image sharpness due to the geometric blur of the focal spot.
- Use of magnification techniques requires a significant reduction in focal spot size compared to contact methods.
- Improvement in the overall sharpness of the complete system is generally due to the increase in size of the image compared to the Unsharpness of the image receptor.
- From effects such as Light Spread for screen-film systems and Pixel Size for digital systems, magnification can improve Spatial Resolution.
Technique Selection
- With Screen-Film systems, technique selection is relatively straightforward:
- The choice of kV setting is based largely on the required contrast.
- The mAs is chosen to produce a suitable optical density for the region of clinical interest.
Quiz about magnification imaging in radiology, including how it's achieved and how it varies with depth in the patient.
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