 Figure 3. Illustrations of nuclear motion in a heterogeneous magnetic field. (A) Any ‘jitter’ at the Larmor frequency of the motion of a nucleus through locally changing magnetic fields results in spin-lattice relaxation. The low frequency wandering of that nucleus through the same fields results in spin-spin relaxation. (B)Field inhomogeneities that are much larger than spin motion cause reversible spin dephasing. |
MR images take time to collect. Typically, spins must be excited many times, each time producing a slightly different measurement. The first relaxation mechanism, namely the longitudinal realignment of the excited spins with B0, must be allowed to occur (to some degree) between each excitation. Otherwise, there is insufficient `realigned' magnetization available for the excitations that follow and the resulting signal is diminished. The rate of this realignment is characterized by the time constant, T1, the time required for 63% of the complete magnetization to form after excitation. The ratio of the time between excitations (the repetition time, TR) and the T1 of spins in any given tissue indicates (but is not equal to) the relative fraction of signal that is produced if complete spin realignment is not allowed.
The cause of this spin realignment is known as spin-lattice relaxation. As nuclei oscillate and tumble in their molecular sea, they pass through locally changing magnetic fields (Fig. 3A). When a component of that motion is at the Larmor frequency through one of these changing magnetic fields, the nucleus `sees' an effective resonant RF pulse that rotates it out of its current state. The collection of these randomly oriented RF pulses serves to speed the nuclear spin states toward their preferred alignment much like shaking a box of balls moves heavy balls to the bottom and light ones to the top.
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