Fundamental pulse sequences

As we have seen in the previous chapters we can rotate the spins - and the magnetization - from their equilibrium state with an arbitrary angle by applying an appropriate Larmor-frequency RF field for a certain length of time. We have also learned that if the magnetization has transverse component then it will precess with Larmor frequency and induce measureable electromotive force in a nearby coil. Therefore with a suitable sequence of RF fields and coil voltage measurements we can investigate the magnetization of the sample. In MR technology the RF fields that rotate the magnetization are called "pulses" while the detection, demodulation and digitalization of the induced voltage signal is referred to as "acquisition". The whole process containing the RF pulses and acquisitions is called "pulse sequence". In this section two basic pulse sequency is presented, free induction decay, usually referred to as FID, and spin echo, sometimes abbreviated as SE. The sequences will be graphically presented by the so-called sequence diagrams, which represent the RF pulses and acquisitions in a schematic form.

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