6 Questions
What is the pitch of a helical scan if the table moves 12cm during one tube rotation and the beam width is 8cm?
1.2
What happens when the pitch is greater than 1?
The coils of the helix are separated
If the beam width is 12cm and the table moves 9cm per tube rotation, what happens to the x-ray beams of consecutive helical loops?
They overlap
What is the effect of a pitch less than 1 on the underlying tissues?
They are doubly irradiated
What is the purpose of the pitch in helical scans?
To describe the relationship between table motion and beam width
If the beam width is 8cm and the table moves 6cm per tube rotation, what is the pitch?
0.75
Study Notes
Pitch in Helical Scans
- In helical scans, table motion causes displacement of fan beam projections along the z-axis, dependent on table speed and beam width.
- Pitch is defined as the ratio of table displacement per 360° rotation to section thickness.
- Pitch calculation: pitch = table travel / beam width
- Pitch values:
- Pitch = 1: coils of the helix are in contact, x-ray beams are contiguous
- Pitch < 1: coils of the helix overlap, beams and consecutive loops overlap
- Pitch > 1: coils of the helix are separated, gaps exist between x-ray beams
- Examples:
- Beam width 10cm, table travel 10cm: pitch = 1, x-ray beams are contiguous
- Beam width 10cm, table travel 15cm: pitch = 1.5, gap between x-ray beams
- Beam width 10cm, table travel 7.5cm: pitch = 0.75, beams and consecutive loops overlap
This quiz covers the concept of pitch in helical scans, how it's calculated, and its implications on CT scanning. Learn about the relationship between table speed, beam width, and section thickness.
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