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Questions and Answers
What is required to achieve magnification in imaging?
What is required to achieve magnification in imaging?
How does magnification affect spatial resolution?
How does magnification affect spatial resolution?
What is the effect of increasing OID on scatter fraction?
What is the effect of increasing OID on scatter fraction?
What is the consequence of using magnification techniques?
What is the consequence of using magnification techniques?
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What is the advantage of magnification in imaging?
What is the advantage of magnification in imaging?
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How does magnification affect dose?
How does magnification affect dose?
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What is the effect of magnification on image sharpness?
What is the effect of magnification on image sharpness?
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What is required for technique selection in screen-film systems?
What is required for technique selection in screen-film systems?
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What is the effect of increasing OID on focal spot size?
What is the effect of increasing OID on focal spot size?
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How does magnification vary with patient thickness?
How does magnification vary with patient thickness?
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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.
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Description
This quiz covers the concepts of magnification in imaging, including how it's achieved and how it varies with patient depth.
