Physics, Techniques and Procedures

Pulse sequence

the complex sequence of events occurring during MR data acquisition by switching on radiofrequency RF pulses and magnetic gradient fields. Any MR pulse sequence can be divided into three parts, the "preparation" or "excitation module", the "acquisition module" or "readout module" and a third "end module" which is used to modify or abolish remaining xy-plane magnetization prior to a succeeding spin excitation. All three modules are repeated multiple times to form an entire imaging pulse sequence during which all the image points of the two-dimensional image or three-dimensional volume are measured. In magnetization-prepared gradient echo pulse sequences or in echo planar imaging EPI parts of the preparation module may be used either only once or a few times during the entire imaging pulse sequence. How this repetition of events in a pulse sequence occurs further determines the type of imaging sequence and is linked to other measures such as measures to suppress motion artefacts as in cardiac gating or respiratory gating.

During the spin preparation module the sample magnetization is disturbed from its equilibrium magnetization using radiofrequency pulses and some localizing gradient fields. The radiofrequency pulses are used to tilt either part or all of the sample magnetization (and thus the spins) into the xy-plane where it starts to precess. After a certain period of precession, additional radiofrequency pulses and gradients may be applied to the system to produce an echo of the spins during a process called echo rephasing. The spin preparation module determines whether a sequence is a spin echo pulse sequence, a gradient echo pulse sequence or an inversion recovery IR pulse sequence.

During the data acquisition module, the signal of the spins is measured. This is achieved by acquiring either a single or multiple image lines, where the acquisition of multiple image lines after the preparatory "module" without additional radiofrequency pulses is referred to as some type of echo planar imaging EPI .

Depending on the specific type of pulse sequence, the "end module" suppresses or "spoils" signals which may affect the next repetition of events unfavourably.

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