Shizhe Li

In vivo 13C magnetic resonance spectroscopy of human brain on a clinical 3 T scanner using [2-13C]glucose infusion and low-power stochastic decoupling.

This study presents the detection of [2‐13C]glucose metabolism in the carboxylic/amide region in the human brain, and demonstrates that the cerebral metabolism of [2‐13C]glucose can be studied in human subjects in the presence of severe hardware constraints of widely available 3 T clinical scanners and with low‐power stochastic decoupling. In the carboxylic/amide region of human brain, the primary products of 13C label incorporation from [2‐13C]glucose into glutamate, glutamine, aspartate, γ‐aminobutyric acid, and N‐acetylaspartate were detected. Unlike the commonly used alkanyl region where lipid signals spread over a broad frequency range, the carboxylic carbon signal of lipids was found to be confined to a narrow range centered at 172.5 ppm and present no spectral interference in the absence of lipid suppression. Comparison using phantoms shows that stochastic decoupling is far superior to the commonly used WALTZ sequence at very low decoupling power at 3 T. It was found that glutamine C1 and C5 can be decoupled using stochastic decoupling at 2.2 W, although glutamine protons span a frequency range of ≈700 Hz. Detailed specific absorption rate analysis was also performed using finite difference time domain numerical simulation. Magn Reson Med, 2009.

13C MRS of occipital and frontal lobes at 3 T using a volume coil for stochastic proton decoupling

Previously, we devised a novel strategy for in vivo 13C MRS using [2‐13C]glucose infusion and low‐power proton decoupling, and proposed that this strategy could be used to acquire 13C MR spectra from the frontal lobe of the human brain. Here, we demonstrate, for the first time, in vivo 13C MRS of human frontal lobe acquired at 3 T. Because the primary metabolites of [2‐13C]glucose can be decoupled using very‐low‐radiofrequency power, we used a volume coil for proton decoupling in this study. The homogeneous B1 field of the volume coil was found to significantly enhance the decoupling efficiency of the stochastic decoupling sequence. Detailed specific absorption rates inside the human head were analyzed using the finite difference time domain method to ensure experimental safety. In vivo 13C spectra from the occipital and frontal lobes of the human brain were obtained. At a decoupling power of 30 W (time‐averaged power, 2.45 W), the spectra from the occipital lobe showed well‐resolved spectral resolution and excellent signal‐to‐noise ratio. Although frontal lobe 13C spectra were affected by local B0 field inhomogeneity, we demonstrated that the spectral quality could be improved using post‐acquisition data processing. In particular, we showed that the frontal lobe glutamine C5 at 178.5 ppm and aspartate C4 at 178.3 ppm could be spectrally resolved with effective proton decoupling and B0 field correction. Because of its large spatial coverage, volume coil decoupling provides the potential to acquire 13C MRS from more than one brain region simultaneously. Copyright

Detection of glutamate, glutamine, and glutathione by radiofrequency suppression and echo time optimization at 7 tesla

To achieve detection of glutamate (Glu), glutamine (Gln), and glutathione (GSH) by minimizing the N‐acetyl‐aspartate (NAA) multiplet signals at 2.49 ppm using a echo time (TE) ‐optimized PRESS pulse sequence and a novel J‐suppression radiofrequency pulse.

Quantitative measurement of N-acetyl-aspartyl-glutamate at 3 T using TE-averaged PRESS spectroscopy and regularized lineshape deconvolution.

This article introduces regularized lineshape deconvolution in conjunction with TE‐averaged PRESS spectroscopy to measure N‐acetyl‐aspartyl‐glutamate (NAAG). Averaging different echo times suppressed the signals of multiplets from strongly coupled spin systems near 2 ppm; thus, minimizing the interfering signals to detect the acetyl proton signal of NAAG. Signal distortion was corrected by lineshape deconvolution, and Tikhonov regularization was introduced to reduce noise amplification arising from deconvolution; as a result, spectral resolution was enhanced without significantly sacrificing signal‐to‐noise ratio (SNR). This new approach was used to measure NAAG in the two regions of interest of healthy volunteers, dominated by gray matter and white matter, respectively. The acetyl proton signal of NAAG was directly quantified by fitting the deconvoluted spectra to a Voigt‐lineshape spectral model function, yielding the NAAG–N‐acetyl‐aspartate (NAA) ratios of 0.11 ± 0.02 for the gray matter voxels (n = 8) and 0.18 ± 0.02 for the white matter voxels (n = 12). Magn Reson Med, 2011.

13C MRS of human brain at 7 Tesla using [2‐13C]glucose infusion and low power broadband stochastic proton decoupling

Carbon‐13 (13C) MR spectroscopy (MRS) of the human brain at 7 Tesla (T) may pose patient safety issues due to high radiofrequency (RF) power deposition for proton decoupling. The purpose of present work is to study the feasibility of in vivo 13C MRS of human brain at 7 T using broadband low RF power proton decoupling.

N-acetyl-aspartyl-glutamate detection in the human brain at 7 Tesla by echo time optimization and improved Wiener filtering.

To report enhanced signal detection for measuring N‐acetyl‐aspartyl‐glutamate (NAAG) in the human brain at 7 Tesla by echo time (TE) ‐optimized point‐resolved spectroscopy (PRESS) and improved Wiener filtering.

Automatic high-order shimming using parallel columns mapping (PACMAP).

This work presents a new automatic high‐order shimming method that maps the B0 field using a group of parallel columns. We found that a pair of four columns in two separate slices could determine an optimal correction field comprising the spherical harmonic terms up to the third‐order. The technique of multiple stimulated echoes was incorporated into the method, allowing the use of at least eight shots to accomplish field mapping. The shim currents were first determined in the logic frame by assuming that the slices were in axial planes, and then uniquely converted into the physical frame where the slices could be at any oblique angle, by using a spherical harmonics rotation transformation. This method thus works regardless of slice orientation. It was demonstrated on a 3T scanner equipped with a complete set of second‐order harmonic shim coils. Both phantom and in vivo experiments showed that this newly introduced high‐order shimming method is an effective and efficient way to reduce field inhomogeneity for a region of imaging slices. Magn Reson Med, 2009.

Simultaneous determination of metabolite concentrations, T1 and T2 relaxation times

To simultaneously measure concentration and T1 and T2 values of metabolites in the human brain in a single scan session.

In vivo detection of 13C isotopomer turnover in the human brain by sequential infusion of 13C labeled substrates

This study demonstrates the feasibility of simultaneously detecting human brain metabolites labeled by two substrates infused in a sequential order. In vivo (13)C spectra of carboxylic/amide carbons were acquired only during the infusion of the second substrate. This approach allowed dynamic detection of (13)C labeling from two substrates with considerably different labeling patterns. [2-(13)C]glucose and [U-(13)C(6)]glucose were used to generate singlet and doublet signals of the same carboxylic/amide carbon atom, respectively. Because of the large one-bond (13)C-(13)C homonuclear J coupling between a carboxylic/amide carbon and an aliphatic carbon (~50 Hz), the singlet and doublet signals of the same carboxylic/amide carbon were well distinguished. The results demonstrated that different (13)C isotopomer patterns could be simultaneously and distinctly measured in vivo in a clinical setting at 3T.

Repeatability of (31) P MRSI in the human brain at 7 T with and without the nuclear Overhauser effect.

An often‐employed strategy to enhance signals in 31P MRS is the generation of the nuclear Overhauser effect (NOE) by saturation of the water resonance. However, NOE allegedly increases the variability of the 31P data, because variation is reported in NOE enhancements. This would negate the signal‐to‐noise (SNR) gain it generates. We hypothesized that the variation in NOE enhancement values is not caused by the variability in NOE itself, but is attributable to measurement uncertainties in the values used to calculate the enhancement. If true, the expected increase in SNR with NOE would improve the repeatability of 31P MRS measurements. To verify this hypothesis, a repeatability study of native and NOE‐enhanced 31P MRSI was performed in the brains of seven healthy volunteers at 7 T. The repeatability coefficient (RC) and the coefficient of variation in repeated measurements (CoVrepeat) were determined for each method, and the 95% limits of agreement (LoAs) between native and NOE‐enhanced signals were calculated. The variation between the methods, defined by the LoA, is at least as great as that predicted by the RC of each method. The sources of variation in NOE enhancements were determined using variance component analysis. In the seven metabolites with a positive NOE enhancement (nine metabolite resonances assessed), CoVrepeat improved, on average, by 15%. The LoAs could be explained by the RCs of the individual methods for the majority of the metabolites, generally confirming our hypothesis. Variation in NOE enhancement was mainly attributable to the factor repeat, but between‐voxel effects were also present for phosphoethanolamine and (glycero)phosphocholine. CoVrepeat and fitting error were strongly correlated and improved with positive NOE. Our findings generally indicate that NOE enhances the signal of metabolites, improving the repeatability of metabolite measurements. Additional variability as a result of NOE was minimal. These findings encourage the use of NOE‐enhanced 31P MRSI. Copyright