Multi-slice excitation

Most types of selective excitation can be modified for simultaneous excitation of n slices or volume elements. Such an approach is advantageous when a limited number of slices or volume elements, but not the entire 3D object, needs to be investigated. By suitable coding of the volume information in n experiments an improvement in signal-to-noise ratio of can be gained [Boll, Miill]. Compared to 3D volume imaging, multislice and multi-volume techniques (cf. Section 9.1) suffer from the lack of achieving well-defined boundaries. 

Fig. 5.3.13 Hadamard encoding and decoding for simultaneous four-slice imaging. The encoding is based on four experiments, A-D. In each experiment, all four slices are excited by a multi-frequency selective pulse. Its phase composition is determined by the rows of the Hadamard matrix H2. The image response is the sum of responses for each individual, frequency selective part of the pulse. Thus, addition and subtraction of the responses to the four experiments separates the information for each slice. This operation is equivalent to Hadamard transformation of the set of image responses. Adapted from [Miil21 with permission from Wiley-Liss. Inc., a division of John-Wiley Sons, Inc.

A solid-state variant of the DANTE sequence (Fig. 5.3.15) is obtained by replacing the rf pulses and the free precession periods of the original sequence by line-narrowing multi-pulse sequences [Carl, Corl, Flepl, Hep2J. Such DANTE sequences can be used for selective excitation in solid-state spectroscopy (cf Fig. 7.2.8) and for slice selection in solid-state imaging (Fig. 5.3.16). 

Ramadan et alP have modified the sequence for localised COSY (L-COSY) by the use of a non-selective adiabatic half passage pulse and two pairs of adiabatic full passage inversion pulses for excitation and spatial refocusing along two orientations. The new sequence, called AL-COSY, has improved slice selection and chemical shift artefacts. Andronesi et al have performed multi-voxel 2D COSY and 2D TOCSY experiments in vivo for the first time. Spectra were obtained from 2D COSY with an 8 x 8 matrix and 64 ty increments at 3 T in six healthy human volunteers and three patients with glioblastoma multiforme spectra from 2D TOCSY were obtained at... 

---