Spectral Editing

Even though their concentration levels limit the number of detectable metabolites with in vivo spectroscopy, there are still a dozen or more metabolites contributing to the total spectrum. Due to signal overlaps and complicated spectral patterns, most of these metabolites cannot easily be differentiated from each other, and have to be treated as metabolite groups like the Glx-components glutamine, glutamate and GABA, or just as baseline disturbances like most of the macromolecules.

Any technique simplifying or selectively changing the appearance of a spectrum can be considered a spectral editing technique. Most clinical 1H spectroscopy sequences have several of these techniques in common like CHESS water or lipid suppression, or spatially selective excitation. In this chapter, all the techniques that allow to simplify a spectrum by focusing on a subset of specific metabolites will be discussed under spectral editing. Increasing the echo time TE is the simplest approach, where only metabolites with long T2 relaxation times will contribute to the spectrum.

Most of the advanced spectral editing techniques rely on the phenomenon of either homonuclear or het-eronuclear spin coupling. Spin coupling, also known as J-coupling, is responsible for several of the spectral patterns of individual metabolites observable in spectroscopy, like the doublet of lactate or the multiplet of GABA. Even though J-coupling itself is independent of the field-strength B0, several of the editing sequences cannot be applied with clinical 1.5 T scanners. A sufficient spectral resolution and usually high signal when aiming for low concentrated metabolites are prerequisites for successful spectral editing.

In vivo measurements of cerebral GABA for example are limited by its low concentration and by the presence of the significantly overlapping resonances at GABA-2 (2.3 ppm) from Glx, at GABA-3 (1.9 ppm) from NAA and at GABA-4 (3.0 ppm) from the methyl group of creatine. Fortunately, the methyl group of creatine is not subject to the effects of J-coupling. This allows it to be suppressed or separated using a variety of spectroscopic techniques, such as J-editing [43-47], 2D J-resolved spectroscopy [48], longitudinal scalar order difference editing [49], and multiple quantum filtering [50, 51]. These methods selectively prepare GABA-3 and GABA-4 into a steady state while suppressing the dominant overlapping creatine signal at 3.0 ppm. The GABA-4 can be made visible by further advanced processing, which can include signal averaging or subtraction. The result is a single signal assigned to GABA as shown in Fig. 6.8, even though a significant amount of co-edited resonances from macro-molecules, such as glutathione at 2.87-2.94 ppm, are included.

Another application of the J-coupled editing technique is in the detection of glutamate (Glu), which gives rise to a complex proton spectrum characterized by the coupled spins of the C2-C4 hydrogen nuclei. At the moderate field strength of 1.5 T, the in vivo brain spectrum in the respective spectral ranges exhibits poor resolution and, despite the relatively high brain Glu concentration of 7-12 mmol/l, low sensitivity due to substantial contributions by glutamine (Gln). In conventional spectroscopy sequences, the resonances in this range are therefore mostly assigned to a mixture of Glu and Gln (and sometimes GABA), summarized as Glx-components. A method to accurately measure the tissue level of brain glutamate at 3 T is based on a TE-averaged PRESS data acquisition, which gives an unobstructed single line response for glutamate at 2.38 ppm

I I I I I I_I I i I I L I I I I 1 I_I_|_

in vitro pure

GABA spectrum




Fig. 6.8. In vivo GABA spectrum (a) acquired with use of a specific editing sequence. Due to the complicated spectral pattern (b) and the overlay of GABA with other metabolites (c), advanced acquisition techniques are required for successful detection of GABA. (Image courtesy of Ruber International Hospital, Madrid, Spain)

,Besl in vivo probe-p

(Fig. 6.9). The sequence is based on a modification of the standard asymmetric single-voxel PRESS sequence with equidistant TE increments ranging from 35 to 195 ms [52]. This sequence also provides a sensitive method to measure the other metabolites and their effective T2 relaxation rates for uncoupled spins [53,54].

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