Short echo time relaxation-enhanced MR spectroscopy reveals broad downfield resonances

Abstract

Purpose: Most MR spectroscopy (MRS) pulse sequences rely on broadband excitation with water saturation and typically focus on upfield signals. By contrast, the downfield spectrum, which contains many potentially useful resonances, is typically not targeted because conventional water-suppressed techniques indirectly saturate the labile protons through exchange. Relaxation-enhanced MRS (RE-MRS) uses frequency-selective excitation while actively avoiding bulk water perturbation, thereby enabling high-quality downfield spectroscopy. However, RE-MRS typically requires very long (typically >40 ms) echo times (TEs) due to its localization module, which inevitably decreases sensitivity and filters shorter T₂ components. Here, we overcome this limitation by combining RE-MRS and image selected in vivo spectroscopy (ISIS) localization, abbreviated iRE-MRS, which in turn allows very short TEs (5 ms using our hardware).

Methods: Experiments were performed in vitro for validation as well as and in in vivo rat brains at 9.4T.

Results: The new iRE-MRS methodology was validated in phantoms where good performance was noted. When the downfield spectrum was investigated at short TEs in in vivo rat brains, iRE-MRS provided very high sensitivity; the ensuing downfield spectra encompassed numerous broad peaks, as well as a broad baseline. All downfield spectral peaks were highly attenuated by increasing TEs as well as by applying water saturation, although to different extent. The signal ratios also varied between TEs, suggesting that exchange rates are different among the downfield signals.

Conclusions: Short-TE iRE ¹ H downfield MRS opens new directions in the investigation of in vivo downfield metabolites and their role on healthy and disease processes.

Keywords: MRS; downfield spectroscopy; exchange; magnetic resonance spectroscopy; relaxation enhancement.