Lactate discrimination incorporated into echo‐planar spectroscopic imaging

Abstract

A technique for discriminating a lactate signal from overlapping lipid signals in 1 H spectroscopic imaging is presented. It is based on J-coupling between lactate protons and on the broad spectral bandwidth of lipid signal. Measurement parameters used in the technique are determined so that TE is separated from n/J (n: a natural number, J: J-coupling constant) enough to suppress the lipid signal at the time when the lactate signal is strongest. Data processing is used to calculate the lactate signal intensity from the reconstructed spectra. This technique enables lactate to be discriminated in a single measurement and enables spectra of other metabolites to be acquired simultaneously. However, it necessitates a homogeneous magnetic field, long TE, and supplementary lipid suppression. Imaging of lactate distributions can provide useful information about anaerobic metabolism in tissues, enabling an estimate of the oxygen shortfall in ischemic tissue to be obtained. Lactate imaging usually uses the 1 H chemical shift to separate the lactate signal from other metabolites, but contamination from overlapping lipid signals sometimes makes it difficult to accurately evaluate the lactate concentration. Additional discrimination of the lactate signal from the lipid signal is therefore of great benefit for obtaining accurate lactate images, and several lactate-discriminating techniques have been reported that use the difference between the relaxation times of lactate and lipid (1,2) or homonuclear coupling in lactate protons (3-17). Application to spectroscopic imaging has also been described (2-8). The lactate-discriminating techniques using homonuclear coupling can be classified as four kinds: selective irradiation (3), spectral editing (4 -6), multiple quantum coherence (7,8), and polarization transfer (9). These techniques demonstrate the efficiency of lactate discrimination, but there remain some disadvantages with regard to measurement time and to obtaining the spectra of other metabolites. At least two measurements are generally required for selective irradiation, spectral editing, or polarization transfer (3-5,9); and images of other metabolites cannot be acquired simultaneously when multiple quantum coherence are used (7,8). Although the spectral editing technique using multi-spin-echo allows the images of the other metabolites to be simultaneously acquired in a single measurement, sufficient discrimination is sometimes not provided by the T 2 variation of the lipid signal (6).In this work we present a fast lactate-discriminating technique for 1 H spectroscopic imaging. This technique enables lactate-discrimination in a single measurement and makes possible the simultaneous acquisition of the spectra of other metabolites. It is based on homonuclear coupling between lactate protons and on the broad bandwidth of lipid spectrum. By using TE values determined by n/J (n: a natural number, J: J-coupling constant), the lipid signal can be decreased at the time when the lactate signal is strongest. The technique includes...

Multi-spin-echo J-resolved spectroscopic imaging without water suppression: application to a rat glioma at 7 T

“…This method can be compared to editing methods [30,31], which were developed to discriminate lactate with SI. In the experiment performed by Bito et al [31], with a digital resolution of about 35 µl, the acquisition was performed in 16 min at 4.7 T. However, it is difficult to compare directly the SNR in the different studies: both the field strengths and the manner of presenting the data are different.…”

Achieving sufficient spectral bandwidth for volumetric 1H echo‐planar spectroscopic imaging at 4 Tesla

“…The data can then be acquired with a frequency offset such that the limited bandwidth available is centered in the upfield region rather than on water. This concept was implicitly used before for 2D EPSI on a 4.7 Tesla animal system (6), as well as for 3D EPSI on a 4 Tesla whole‐body system (7); however, it has never been explicitly rationalized. Alternatively, in this study, the data were acquired without the frequency offset and shifted in the spectral domain by the same frequency value during post‐processing to unwrap the spectral aliasing.…”

Strongly coupled versus uncoupled spin response to radio frequency interference effects: application to glutamate and glutamine in spectroscopic imaging

“…Coupled spins have not been investigated as thoroughly, because complex coupling patterns can lead to difficulty in detection using standard spectroscopic imaging techniques. Nevertheless, some studies of coupled spins have examined glutamate (Glu) (5)(6)(7), glutamine (Gln) (5,7,8), myo-inositol (6,7), g-aminobutyric acid (GABA) (9,10) and lactate (11)(12)(13). In spectroscopic imaging, usually a two-or threedimensional volume of interest (VOI) is defined by slice selection, and phase encoding is used to divide the VOI into localized voxels.…”