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

Abstract

Two-dimensional J-resolved spectroscopy may be used to separate resonances which overlap in 1D NMR spectra. Coupled with spectroscopic imaging (SI), it would give unequivocal information on the distribution of such resonances. Multi-echo acquisition decreases the minimum experimental time of such 4D experiments. The water peak may be used for phase and chemical-shift reference. This study aimed to demonstrate the feasibility of J-resolved SI based on a multi-echo sequence and without water suppression, and its ability to separate the peaks for lactate and mobile lipid in a rat glioma. Experiments were performed on rat brain, without water suppression, at 7 T. The water signal was used for correcting the phase of the echoes. A FOCSY-like acquisition was used to collect the first part of the echoes at short echo times. Two different data processing methods were tested to overcome the problem of contaminations of metabolite signals by the intense water signal. Maps of N-acetylaspartate, choline, creatine, lactate and mobile lipids were obtained in vivo on a rat glioma in 70 min. The in-plane resolution was 2 mm2. The 2D spatially resolved, 2D J-resolved spectra enabled the separate mapping of lactate and mobile lipids.

Improved two‐dimensional J‐resolved spectroscopy

“…This leads to both an increase in sensitivity and a tilt of the peak tails in the spectra. The experiment with the maximum-echo sampling scheme was further modified to acquire multiple echoes, all sampled as long as possible, in rat brain at 7 T (26).…”

In vivo 2D magnetic resonance spectroscopy of small animals

“…For a given pulse scheme, the sensitivity and the spectral power of resolution did not increase as expected between 1.5 and 3 T, because the decrease of the T2 relaxation time of the metabolite protons results in a significant loss of signal at long echo times [33]. At higher field values, the effect of strong coupling is limited, and 2D J-resolved spectroscopy can be used to investigate efficiently metabolic processes in small animals [27,28,31].…”

“…Decreasing the water signal also decreases the intensity of the neighboring signals, and to avoid this loss of signal, F1 oversampled J-resolved spectroscopy without water suppression has been developed at 1.5 T for studies of human brain [30]. Further improvements have been made to yield spatially resolved, 2D J-resolved spectra, without water suppression, at 7 T for rat brain post-mortem [31]. The 2D J-resolved methods have been applied in vivo at 1.5 T [17,23,29,32], 3 T [26,33], 4.7 T [24] and 7 T [27,28].…”

Ultra-short echo time spectroscopic imaging in rats: implications for monitoring lipids in glioma gene therapy

“…A disadvantage of the current approach is the long acquisition time. If shorter measurement times are needed, echo planar or spiral acquisition methods could be combined with the present localization method (21,22). The present work was performed at 4.7 T, which is a relatively low B 0 for experimental MRSI studies.…”