Selective excitation with the DANTE sequence. The baseline syndrome

Abstract

The simplest type of selective radiofrequency pulse is the single long weak pulse (I, 2) which we shall refer to as a soft pulse. If it has a rectangular envelope, the frequency-domain excitation spectrum contains a set of side-lobe responses flanking the ma in response, approximating a sine function. For practical reasons it is often convenient to replace this soft pulse with a repetitive sequence of hard pulses separated by free precession delays, the so-called DANTE sequence (3, 4). If all the hard pulses are identical, the response approximates that of the rectangular soft pulse, except that there are s ideband responses at intervals of the pulse repetition rate 1 /(At). Alternatively the hard pulses can be amp litude modu lated (either in intensity or in pulse width) to tailor the effective pulse envelope according to some suitable shape (4). Gaussian (5)) hyperbolic secant (6)) sine function ( 7)) and half-Gaussian (8) shaping functions have been used for various purposes. The soft pulse and the corresponding DANTE sequence have very similar properties provided that they both have the same total duration and overall flip angle. In a sense, DANTE is merely a digitized version of the soft pulse. When the corresponding magnetization trajectories are calculated from the Bloch equations, starting on the 3-Z axis, they terminate very close to the same point. The soft pulse trajectory is a smooth curve and the DANTE trajectory follows a zigzag path (representing the effects of alternate pulsing and free precession) which crosses and recrosses the smooth curve. When the DANTE sequence contains a large number of hard pulses (fine digitization) then the results obtained with the two kinds of selective pulse are virtually indistinguishable. However, for a DANTE sequence with a lim ited number of pulses (coarse digitization) there are two important discrepancies. In the frequency doma in, they take the form of a positive displacement of the baseline and an oscillatory contribution. Both effects were evident in the first simulations of DANTE excitation spectra (3). For selective excitation experiments, baseline distortions of this kind are particularly unfortunate, leading to weak excitation of the NMR spectrum across the entire frequency range. The purpose of this communicat ion is to point out that these baseline artifacts may be corrected by halving the intensity of the first and last pulses of the DANTE sequence. We take as an example a rectangular soft pulse (Fig. 1 a) compared with a IO-pulse DANTE sequence (Fig. 1 b) of the same total duration ( T = 9At) and the