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QUESTIONS
» Image Creation

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

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Simplest Pulse Sequence

Answer this...

What's the simplest pulse sequence diagram you can draw?

A single RF pulse.

A gradient echo pulse sequence.

A spin echo pulse sequence.


An RF pulse produces an FID (echo).

CORRECT. The simplest pulse sequence is a single RF pulse. An oscillating magnetic field is applied for a short time (the RF pulse). The net magnetisations of spin isochromats are rotated away from their alignment with the external magnetic field and they precess, inducing a signal in detection equipment. That signal is sometimes called an echo.

The problem with this situation is that the signal which we can measure is the sum of all the signals from all the precessing net magnetisations from the whole patient. An image cannot be created. By varying the magnetic field across the patient we can do two things:

  1. Cause precession (and thus, signal) from only a section of the patient. This is slice selection.
  2. Encode the signal from within a slice by frequency, so that the frequency of signals relate to position in the scanner. This is frequency encoding.

A gradient echo pulse sequence.

INCORRECT. A gradient echo pulse sequence includes slice selection magnetic field gradients, a phase encoding magnetic field gradient, and frequency encoding magnetic field gradients. These are not required to produce a NMR signal. (They are of course required for spatial encoding in MRI). What is the most essential part of a gradient echo pulse sequence?

Try again.


A spin echo pulse sequence.

INCORRECT. A spin echo pulse sequence includes slice selection magnetic field gradients, a phase encoding magnetic field gradient, and frequency encoding magnetic field gradients. These are not required to produce a NMR signal. (They are of course required for spatial encoding in MRI). What is the most essential part of a spin echo pulse sequence?

Try again.

Further reading on this topic:
Books: MRI The Basics p75

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