Phase-encoding Gradients
Why do some gradients change frequency and others change phase? It seems like they should do all work the same way.
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Both frequency-encoding gradients and phase-encoding gradients do work in exactly the same way, but are used for different purposes.
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All imaging gradients temporarily change the resonant frequencies of protons while the gradient is being applied. When the gradient is turned off, the protons go back to their original precession frequencies. However, these protons will have gained or lost phase relative to a reference state though they are now again resonating at the same frequency. They possess "memory" of their historical encounter with the gradient manifest by a permanent phase shift (Φ).
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Signals from a reference proton and a proton subjected to a gradient. The red dot represents the 11th peak of each signal. The gradient causes the lower proton to accumulate more cycles with a resultant permanent phase shift (Φ) even after the gradient has been turned off.
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The magnitude of the accumulated phase shift (Φ) is proportional to the strength of the gradient (G) and the time (t) that gradient is applied, reflected in the equation Φ = γ • G • t. By convention, application of a gradient in an MR pulse timing diagram the gradient strength is often indicated by a rectangle, where the height of the rectangle represents gradient strength (G) and its width reflects time (t) the gradient has been applied. The area under the rectangle (G•t) is therefore proportional to the phase shift.
Related Questions
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