Large Scale Dephasing and Spin Echo in MRI

As excited spins move about the molecular landscape, the final phase angle that their magnetism is directed at the time of signal measurement reflects the average frequency at which they have precessed. In turn, this value reflects the average magnetic field strength to which they have been exposed. This magnetic field not only changes on a molecular scale (responsible for T1 and T2 relaxation), it also changes more slowly on a macroscopic scale (Fig. 3B). Thus, any region will have variations in the final phase of its spins, and its composite signal from all of these spins will be reduced. This signal loss is typically referred to as spin dephasing, and its rate, along with the less significant (in this context) mechanisms of spin-lattice and spin-spin relaxation, is characterized by the time constant T2*.

Figure 4.  Signal reduction in a (A) gradient-echo sequence due to spin dephasing over 80 msec.  (B) The signal is recovered in a spin-echo sequence with the application of a radio frequency refocusing pulse 40 msec after the excitation.

Although the loss of signal strength from T1 and T2 relaxation is unavoidable, the loss of signal from spin dephasing (T2*) can be reversed. The same applied resonant RF pulse that excited the spins can reverse the phase of all spins in the transverse plane. This 180º refocusing RF pulse puts the faster-precessing spins behind the slower ones. If 10 msec elapse after the initial spin excitation before this refocusing pulse is applied, the fast spins will catch up to the slow spins 10 msec after the refocusing pulse. Once again a coherent set of spins is created in what is called a spin echo. Data collection schemes with and without this refocusing pulse are called spin-echo and gradient-echo pulse sequences, respectively. The use of this rephasing is very important in regions of magnetic field inhomogeneity if TE is very long (Fig. 4). Rephasing works only if `fast' spins remain fast before and after the refocusing pulse. These RF pulses do not refocus the dephasing that creates spin-spin (T2) relaxation, because it is caused by field changes on a molecular scale, and spins are speeding up and slowing down far too quickly to be refocused coherently.

Figure 5. MR Images taken with (A) proton density weighting: a long TR (4 seconds) and short TE (20 msec), (B) T1 weighting: a reduced TR (500 msec) and short TE (20 msec), and (C) T2 weighting: a long TR (4 seconds) and increased TE (80 msec).