Fast Spin Echo Imaging

In conventional spin echo (SE) imaging, the longitudinal magnetization (that is generated by the presence of the main magnetic field, B0) is tipped into the transverse plane using a radiofrequency (RF) pulse that tips the magnetization 90 degrees. Once in the transverse plane, the magnetization rotates, or precesses, around the axis of the main magnetic field. As the transverse magnetization precesses, it generates a time-varying signal in the receiver coil. While the transverse magnetization is precessing, it is diminishing. Two phenomena that cause diminution of the transverse magnetization, and the signal generated by it, are interactions with nuclear neighbors (T2 relaxation) and magnetic field inhomogeneities (T2* relaxation). T2 relaxation is desirable in that it permits signal differences from tissues with different T2 values. T2* can be detrimental as it causes loss of signal in regions affected by magnetic field inhomogeneities. T2* effects can be ameliorated by applying an RF pulse that flips the transverse magnetization 180 degrees. When this is done, the magnetization that is lost (dephased) prior to the application of the 180 degree RF pulse is recovered (rephrased) after the application of the 180 degree pulse. When the maximum magnetization is restored at the echo time (TE), the receiver is turned on and the signal is sampled. During the application of the 90 and 180 degree RF pulses, a magnetic field gradient is applied so that the RF pulses tip only magnetization in a given slice. This magnetic field gradient is referred to as the slice-selection gradient.