Abstract
The full spatially resolved spectral information of an object can be recorded with the aid of magnetic resonance spectroscopic imaging (MRSI) as described in Chap. 28. This of course may be a time-consuming procedure. Therefore one often prefers first to record an ordinary NMR image using a fast-imaging pulse sequence, for instance, and then to define a region of interest from which a localized spectrum can selectively be acquired with the aid of a volume-selective spectroscopy technique. One thus gains fast and accurate access to local spectral parameters at the expense of information from other object regions.
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Notes
Accumulation of coherent-phase signals must not be confused with the acquisition of multiple signals each phase-encoded with different increments of the encoding gradient. Transients of the latter sort do not strengthen the signal-to-noise ratio because the phases of signals originating from different voxels are incoherent (compare Eq. 28.2).
The same situation can also be produced with the aid of combined hard and soft-pulse excitation sequels [15, 125, 374].
Volume-selective spectroscopy is normally performed in order to examine coupled spin systems. Therefore the term “stimulated-echo acquisition mode” (STEAM) originally suggested for imaging schemes [151] and occasionally encountered in the literature is not appropriate for experiments with coupled spins.
A modified pulse sequence for “double-quantum filtered volume-selective editing” (DQFVOSING) will be discussed below.
Note that the phases of the two 90° pulses of the preparation pulse sequence should be in phase in this case when all offsets by gradients, inhomogeneities, and chemical shifts are refocused. The transfer from single-quantum to double-quantum coherences will only then be complete.
Employing considerably more RF pulses another method based on multiple-quantum coherences was suggested in [265] for the same purpose. However, this “split-pathway volumeselective editing” (SP-VOSING) spectroscopy technique requires many more pulses. The principle is to split intermittently the coherence pathways of coupled as well as uncoupled spins. Each pathway leads eventually to single-quantum coherences so that all echo signals appear in superimposed form. Coupled spins follow pathways implying single-quantum and multiple-quantum coherences in parallel. The coherence-transfer echoes into which these pathway terminate are constructively superimposed. This is in contrast to uncoupled spins, which produce stimulated and secondary Hahn echoes cancelling each other.
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© 1997 Springer-Verlag Berlin Heidelberg
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Kimmich, R. (1997). Homonuclear Localized NMR. In: NMR. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-60582-6_37
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DOI: https://doi.org/10.1007/978-3-642-60582-6_37
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