Discussion
Why is phase cycling used in pulse sequences?
Answer: D
Repeating a sequence with the phases of pulses and receiver stepped in a set pattern means unwanted signals add destructively while the wanted pathway adds constructively, removing quadrature images, axial peaks and much instrumental imperfection. It is the reason many sequences require multiples of eight or sixteen scans even on a strong sample. Flip angle depends on pulse power and duration, the relaxation delay is set by T1, and bandwidth by pulse length. Gradient selection now achieves some of the same ends in a single scan.
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