Concentric Ring Trajectory Sampling With k-Space Reordering Enables Assessment of Tissue-Specific T1 and T2 Relaxation for 2H-Labeled Substrates in the Human Brain at 7 T.

Abstract

Deuterium metabolic imaging (DMI) is an emerging Magnetic Resonance technique providing valuable insight into the dynamics of cellular glucose (Glc) metabolism of the human brain in vivo using deuterium-labeled (²H) glucose as non-invasive tracer. Reliable concentration estimation of ²H-Glc and downstream synthesized neurotransmitters glutamate + glutamine (Glx) requires accurate knowledge of relaxation times, but so far tissue-specific T₁ and T₂ relaxation times (e.g., in gray and white matter) have not been determined. Such measurements are time-consuming and particularly challenging in the presence of dynamically changing metabolite levels (e.g. ²H Glc and ²H Glx). This study aimed to assess T₁ and T₂ relaxation times of deuterated resonances, i.e., water, Glc and Glx in human gray and white matter using inversion recovery and Hahn spin-echo ²H MRSI (magnetic resonance spectroscopic imaging), respectively, with non-Cartesian concentric ring trajectory readout (CRT) including specific k-space reordering at 7 T. The sequence was validated using phantom measurements and all results were compared to unlocalized acquisitions. Thirteen healthy volunteers participated in the study, with 10 of them scanned ~90 min after oral administration of 0.8 g/kg [6,6'-²H]-glucose. Significantly different T₁ and T₂ relaxation was observed between GM and WM for ²H water (T₁ ^{GM/WM/unlocalized} = 358 ± 21/328 ± 12/335 m ± 6 ms, p = 0.01) and ²H Glx (T₂ ^{GM/WM/unlocalized} = 37 ± 2/35 ± 2/33 ± 3 ms, p = 0.02), respectively, consistent with unlocalized acquisitions. No significant regional differences were found for ²H water (T₂ ^{GM/WM/unlocalized} = 36 ± 2/34 ± 2/31 ± 2 ms, p = 0.08), ²H Glc (T₁ ^{GM/WM/unlocalized} = 70 ± 5/73 ± 4/80 ± 5 ms, p = 0.13; T₂ ^{GM/WM/unlocalized} = 36 ± 1/34 ± 2/34 ± 2 ms, p = 0.24) and Glx (T₁ ^{GM/WM/unlocalized} = 172 ± 15/172 ± 12/165 ± 11 ms, p = 1.00). Knowledge of tissue-specific relaxation times can enhance the accuracy of concentration estimation and metabolic flux rates in future studies, potentially improving our understanding of various brain diseases such as cancer, neurodegenerative diseases or diabetes, which are often linked to impaired glucose metabolism.