Magnetic resonance imaging Spin echo sequence

The basic principles of NMR can be exploited to generate magnetic resonance images of the brain. One widely used MRI technique is the spin echo sequence. A 90° radiofrequency pulse is repetitively applied to the brain with a constant repetition time (TR ms) between consecutive pulses. Following each 90° pulse, protons are excited and then relax. The dephasing component of relaxation can be reversed by applying a second 180° pulse some time (TE/2 ms) after the 90° pulse. We can imagine that following the first 90° pulse, protons immediately begin to precess idiosyncratically and the emitted signal decays. By reversing this process, the 180° pulse causes rephasing and an increase in emitted signal which has a maximum or echo at TE ms (time to echo) after the initial 90° pulse ( Fig 2).

Fig. 2 Spin echo pulse sequence. A 90° excitation pulse of radiofrequency energy is immediately followed by exponential decay of the T2-weighted signal. A 180° pulse TE/2 ms later causes rephasing of proton spins and an exponential increase in T2-weighted signal with maximum (echo) TE ms after the 90° pulse. Images are acquired at TE (thick arrow). The protons are allowed to relax completely before the next 90° pulse, TR ms after the previous excitation.

The spatial location of the radiofrequency signal emitted by free induction decay in a given volume of the brain is encoded in three spatial dimensions by slice-selective radiofrequency irradiation combined with frequency- and phase-encoding gradients. To improve scan time, multiple slices can be excited in an interleaved fashion (multislice acquisition). This means that after the output signal is detected from one slice, and while the net magnetization vector is relaxing back to its equilibrium state, other slices can be excited. The in-plane resolution (voxel size) of the image is determined by the field of view and the number of voxels in the image. Typically, in-plane resolution at 1.5 T is in the order of 0.5 to 2 mm, and slice thickness is 2 mm or more.

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