Multi-frequency selective pulses

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.

Hadamard spectroscopic imaging (HSI) is a technique to obtain localized spectroscopic information from n regions of interest in n scans [Boll, Hafl, Goel, Goe2, Goe4, MU14]. It is a straightforward extension of the multi-frequency selective-pulse technique... 

The measured responses to the combinations of multi-frequency selective pulse excitation can be unscrambled for each volume element by transformation with a super-Hadamard matrix. The dimension of this matrix equals the product of the dimensions of the Hadamard matrices used for encoding each space axis. 

We have implemented the principle of multiple selective excitation (pulse sequence II in fig. 1) thereby replacing the low-power CW irradiation in the preparation period of the basic ID experiment by a series of selective 180° pulses. The whole series of selective pulses at frequencies /i, /2, , / is applied for several times in the NOE build-up period to achieve sequential saturation of the selected protons. Compared with the basic heteronuclear ID experiment, in this new variant the sensitivity is improved by the combined application of sequential, selective pulses and the more efficient data accumulation scheme. Quantitation of NOEs is no longer straightforward since neither pure steady-state nor pure transient effects are measured and since cross-relaxation in a multi-spin system after perturbation of a single proton (as in the basic experiment) or of several protons (as in the proposed variant) differs. These attributes make this modified experiment most suitable for the qualitative recognition of heteronuclear dipole-dipole interactions rather than for a quantitative evaluation of the corresponding effects. 

A closely related technique can be used for multi-slice imaging (Fig. 6.2.7) [Fral]. The scheme of Fig. 6.2.5(c) is appended by further slice-selective 90° pulses with different centre frequencies, so that the magnetization of other slices is selected [Fral]. In this way, the otherwise necessary recycle delay can effectively be used for acquisition of additional slices. However, the contrast in each slice is affected by a different Ty weight, because is different for each slice. The technique can readily be adapted to line-scan imaging by applying successive slice-selective pulses in orthogonal gradients [Finl]. 

Fig. 6.2.7 [Fral] Timing diagram for multi-slice imaging by the STEAM method. The third rf pulse is used for slice selection. It is repeated with different centre frequencies for acquisition of different slices.

A pulsed dye laser pumped with flash lamps is used in most cases, as here the selection of different dyes, frequency doubling, and Raman shifting, which is possible as a result of the high energies available, allows the whole spectral range down to 200 nm to be covered. However, diode lasers, particularly for multi-step excitation, and dye lasers pumped with an excimer or a nitrogen laser can also be applied. 

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