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### QUESTIONS» Image Creation

How is the MRI image created? This section explores slice selection, spatial encoding and the FFT.

# Phase Difference

## What is a phase difference?

The technical solution in 1 in 5 Star Trek™ plots.

A change in frequency.

The difference between two points in the time of a cycle.

INCORRECT. That's not what I was getting at.

Try again.

INCORRECT. There may be a phase difference between two oscillations (or movements) even though the frequency of the movements is the same. For example, two clocks may tick alternately whilst both ticking exactly one time per second. These clocks have the same frequency (1 Hz) but are out of phase.

If the frequency of one of the clocks is changing, the phase difference between them changes (sometimes they tick together—in phase, sometimes alternately—out of phase). Thus it is a changing phase difference (not just a difference in phase) which is equivalent to a change in frequency.

Try again.

CORRECT. For example, two clocks which both tick exactly one time per second may not be ticking simultaneously. In this instance, the clocks have the same frequency (1 Hz) but there is a phase difference between their ticking.

Phase is measured in degrees (0-360°) or radians (0-2π) and repeats (361° is indistinguishable from 1°; they are the same). Thinking as phase as the angle on a clock face may be helpful.

Books: Q&A in MRI p17, MRI From Picture to Proton p107
Online: Basics of MRI