Physics, Techniques and Procedures

Excitation pulse

in an NMR experiment the radiofrequency RF pulse that flips the magnetization - initially parallel to the main magnetic B0 field and the z-axis - by a certain flip angle away from this axis, producing an xy-plane component of the magnetization. The xy-component of spin magnetization precesses in the xy-plane with the Larmor frequency. The envelope (shape) of the excitation pulse and also other radiofrequency pulses used in MR determines, in combination with the magnetic gradient fields, which regions of the imaged object are excited (see MR imaging). As an important and frequently used example, consider the situation where a section of finite thickness needs to be excited by a radiofrequency pulse. Application of a gradient field leads to a linear variation of the Larmor frequency over the body of a patient. Excitation of a slice of finite thickness thus translates into irradiating the object with a narrow range of radiofrequencies. The aim is to excite spins having a frequency range from n to n + Dn, but none outside this range, i. e. the amplitudes of the power of irradiating radio waves should produce a certain flip angle in the frequency interval of interest, but have no effect outside this interval (Fig.1). As frequency and time domain information in MR are related by Fourier transformation FT , the temporal envelope of the excitation pulse is found by Fourier transform of the rectangular frequency profile desired. It happens that the time domain envelope is a so-called sinc function:

sinc wt = sin wt / t

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Fig.1

Amplitude variation of a sinc excitation pulse.
Excitation pulse, Fig.1