Pure-Shift-Based Proton Magnetic Resonance Spectroscopy for High-Resolution Studies of Biological Samples

Abstract

Proton magnetic resonance spectroscopy (¹H MRS) presents a powerful tool for revealing molecular-level metabolite information, complementary to the anatomical insight delivered by magnetic resonance imaging (MRI), thus playing a significant role in in vivo/in vitro biological studies. However, its further applications are generally confined by spectral congestion caused by numerous biological metabolites contained within the limited proton frequency range. Herein, we propose a pure-shift-based ¹H localized MRS method as a proof of concept for high-resolution studies of biological samples. Benefitting from the spectral simplification from multiplets to singlet peaks, this method addresses the challenge of spectral congestion encountered in conventional MRS experiments and facilitates metabolite analysis from crowded NMR resonances. The performance of the proposed pure-shift ¹H MRS method is demonstrated on different kinds of samples, including brain metabolite phantom and in vitro biological samples of intact pig brain tissue and grape tissue, using a 7.0 T animal MRI scanner. This proposed MRS method is readily implemented in common commercial NMR/MRI instruments because of its generally adopted pulse-sequence modules. Therefore, this study takes a meaningful step for MRS studies toward potential applications in metabolite analysis and disease diagnosis.

Keywords: ISIS localization; biological samples; magnetic resonance spectroscopy; pure shift; spectral congestion.

Figure 1

Pulse-sequence diagram of PSYCHE-ISIS MRS experiments. Sinc-shaped pulses denote π slice-selection RF pulses, along with three orthogonal slice-selective gradients, G_sx, G_sy, and G_sz, used for the spatial volume localization. Thin and fat black bars are π/2 RF non-selective pulses. The trapezoids, including double opposed-direction arrows describing two frequency-sweep directions, indicate small-flip-angle (β) saltire chirp pulses; G₁ and G₂ are coherence selection gradients; G₃ is a weak gradient matching with chirp pulses; t₁ is the indirect evolution period, and t₂ is the direct acquisition period; and 2τ the time interval of pure-shift chunks.

Figure 2

MRS experiments on a two-compartment phantom containing 1.0 M Prop (inner bottle) and 1.0 M GABA (outer bottle) aqueous solutions. (A) Axial and coronal spin-echo MRI images of the two-compartment phantom, black dash squares show the localized volume of 5 × 5 × 5 mm³ in the inner bottle, blue dash squares show the localized volume of 5 × 5 × 5 mm³ in the outer bottle, and red dash squares show the localized volume of 5 × 10 × 5 mm³ in both inner and outer bottles. (B–D) 1D PRESS spectra acquired from localized volumes in the inner bottle (B), the outer bottle (C), and both bottles (D). (E–G) 1D PSYCHE-ISIS spectra acquired from localized volumes in the inner bottle (E), the outer bottle (F), and both bottles (G). Spectral resolution is calculated according to the full widths at half maximum (FWHM) of selected peaks marked by blue arrows in all spectra.

Figure 3

MRS experiments on brain metabolite phantom. (A) Axial and coronal spin-echo MRI images for this phantom sample; red rectangles indicate the smaller localized volume of 5 × 5 × 5 mm³, and green rectangles show the larger localized volume of 12 × 12 × 12 mm³. (B,C) 1D MRS spectra acquired from the smaller localized volume by PRESS and PSYCHE-ISIS, respectively. (D,E) 1D MRS spectra acquired from the larger localized volume by PRESS and PSYCHE-ISIS, respectively. Assigned metabolites are shown in all 1D MRS spectra, and some indistinctly assigned peaks in 1D PRESS spectra (B,D) are marked by blue. The asterisks (*) denote the unassigned resonances or spectral artifacts.

Figure 4

MRS measurements on intact pig brain tissues. (A) Axial and coronal spin-echo MRI images for this pig brain tissue sample and the spatially localized volume of 10 × 10 × 10 mm³ shown in the red square. (B) 1D PRESS MRS spectrum. (C) 1D PSYCHE-ISIS MRS spectrum acquired from the spatially localized volume. Assigned metabolites are shown in both 1D MRS spectra, and some indistinctly assigned peaks in 1D PRESS spectrum (B) are marked by blue. And some almost indistinguishable peaks in the brown spectral region of 1D PRESS MRS are recovered and assigned in 1D PSYCHE-ISIS MRS. The asterisks (*) denote the unassigned resonances or spectral artifacts.