- In an NMR experiment, there is no spatial discrimination to allow us to determine the origin of different parts of the signal in space.
- This is a prerequisite for an imaging technique like MRI.
- Spatial encoding of signals is MRI is achieved using magnetic field gradients
- A gradient is an additional static magnetic field applied for a short period of time in the same direction as B0, but varying linearly in strength with position along a chosen axis.
- In the presence of a gradient, the Larmor frequency varies with position along the chosen axis, e.g.
- This phenomenon is used in three different ways to encode spatial position in MRI: slice selection, frequency encoding and phase encoding. Details of these techniques are given in more advanced textbooks.
- The imaging process requires repeated application of a train of RF and gradient pulses, known as a pulse sequence. RF pulses are applied at an interval known as the repetition time, TR, followed in each case by acquisition of an echo signal at the echo time, TE.
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A particular strength of MRI is the ability to alter the appearance and information content of acquired images by changing the pulse sequence. This is not possible for example in x-ray imaging, which essentially depends on the single parameter of attenuation coefficient.
- A proton MR image is essentially a map of (water) proton density, which varies relatively little between tissues.
- Commonly, the pulse sequence is adjusted so that signal intensity is related to the T1 or T2 relaxation time of tissue (known as T1 weighted or T2 weighted images).
- T1 weighting is added by shortening the interval between consecutive excitation pulses (TR). The amount of longitudinal magnetisation available for excitation depends on the degree of recovery that has taken place, and hence on T1.
- T2 weighting is added by lengthening TE. The amount of transverse magnetisation remaining depends on the extent of decay and hence on T2.
- Recall from above that these relaxation times differ between different tissues: this is the origin of signal contrast in most MR images.
- The four images below were collected consecutively from the same slice in the brain of a volunteer. The dramatic differences is signal intensity reflect differences in T1 or T2.
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