Radiofrequency (RF) pulse

a major constituent of an MR pulse sequence. In any pulse sequence the patient is subject to repetitive RF or excitation pulses (nutation pulses), whose frequency is such that they satisfy the Larmor equation and thus cause the magnetization vectors (spins) of each voxel to change their state. The radiofrequency power (which is proportional to the square of the amplitude) of the pulse is proportional to the flip angle through which the spins are tilted under its influence. Flip angles between 0 and 90 are typically used in a gradient echo pulse sequence, 90 and a series of 180 pulses in a spin echo pulse sequence and an initial 180 pulse followed by a 90 and 180 pulses in an inversion recovery IR pulse sequence.

Most frequently,RF pulses are used in the spin preparation phase (module) of a pulse sequence which prepare the spin system for the ensuing measurements. In many sequences, RF pulses are also applied to the volumes outside the one to be measured. This is the case when spatial presaturation techniques are used to suppress flow artefacts. Many preparation pulses are required in MR spectroscopy to suppress signal from unwanted spins. The simplest preparation pulse making use of spectroscopic properties is a fat saturation pulse, which specifically irradiates the patient at the fat resonant frequency, so that the magnetization coming from fat protons is tilted into the xy-plane where it is subsequently destroyed by a strong dephasing gradient.

The frequency spectrum (i. e. the relative amplitude of the frequencies) of RF pulses is critical as it determines the spatial extension and homogeneity over which the spin magnetization is influenced while a gradient field is applied. For example, a sinc pulse will excite a slice with a rectangular profile (see excitation pulse (I), Fig. 1). In magnetization transfer imaging off-resonance RF pulses are sent into the spin system, exciting the spins present in macromolecular structures, which in turn transfer their energy to the water protons.

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