10 Questions
What is required to achieve magnification in imaging?
Increasing the OID and FID
How does magnification affect spatial resolution?
It improves spatial resolution
What is the effect of increasing OID on scatter fraction?
It decreases scatter fraction
What is the consequence of using magnification techniques?
Improved spatial resolution and increased dose
What is the advantage of magnification in imaging?
It improves spatial resolution and contrast
How does magnification affect dose?
It increases dose
What is the effect of magnification on image sharpness?
It reduces image sharpness
What is required for technique selection in screen-film systems?
KV setting is based on required contrast
What is the effect of increasing OID on focal spot size?
It decreases focal spot size
How does magnification vary with patient thickness?
It varies with patient thickness
Study Notes
Magnification Imaging
- Magnification is achieved by increasing the Object Image Distance (OID), which generally requires an increase in the Focus Image Distance (FID) as well.
- The actual magnification achieved varies with depth in the patient, e.g., from 1.4 at the Exit side to 1.75 at the Entrance side for a patient thickness of 20 cm, FID 140 cm, and Focus Screen Distance (FSD) 80 cm.
Image Receptor Requirements
- Magnification requires the use of a larger image receptor.
- However, for large body regions, this may not be possible.
Consequences of Magnification
- Increased OID affects:
- Dose
- Spatial Resolution
- Signal-to-Noise Ratio (SNR)
Dose Considerations
- Increasing the OID reduces the scatter fraction at the image receptor, requiring an increase in mAs and hence patient dose to maintain the dose to the image receptor.
Unsharpness and Focal Spot Size
- An increase in OID leads to a reduction in image sharpness due to the geometric blur of the focal spot.
- Magnification techniques require a significant reduction in focal spot size compared to contact methods.
Spatial Resolution
- The overall sharpness of the complete system can be improved due to the increase in image size compared to the unsharpness of the image receptor.
- This is because of the reduction in effects such as:
- Light Spread for screen-film systems
- Pixel Size for digital systems
Technique Selection
- With Screen-Film systems, technique selection is relatively straightforward:
- The choice of kV setting is based on the required contrast.
- The mAs is chosen to produce a suitable optical density for the region of clinical interest.
This quiz covers the concepts of magnification in imaging, including how it's achieved and how it varies with patient depth.
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