Pitfalls and advantages of different strategies for the absolute quantification of N-acetyl aspartate, creatine and choline in white and grey matter by 1H-MRS

Intra- and inter-site reproducibility of human brain single-voxel proton MRS at 3 T

INTRODUCTION

Clinical trials that involve participants from multiple sites necessitate standardized and reliable quantitative MRI outcomes to detect significant group differences over time. Metabolite concentrations measured by proton MRS (¹ H-MRS) provide valuable information about in vivo metabolism of the central nervous system, but can vary based on the acquisition and quantitation methods used by different MR sites. Therefore, we investigated the intra- and inter-site reproducibility of metabolite concentrations measured by ¹ H-MRS on MRI scanners from a single manufacturer across six sites.

METHODS

Five healthy controls were scanned twice within 24 h at six participating 3 T MR sites with large single-voxel PRESS (TE/TR/NSA = 36 ms/4000 ms/56) and anatomical images for voxel positioning and correction of partial volume relaxation. Absolute metabolite concentrations were calculated relative to the T₁ and T₂ relaxation corrected signal from water. Intra- and inter-site reproducibility was assessed using Bland-Altman plots and intra- and inter-site coefficient of variation (CoV) as well as intra- and inter-site intra-class correlation coefficient.

RESULTS

The median intra-site CoVs for the five major metabolite concentrations ([NAA], [tCr], [Glu], [tCho] and [Ins]) were between 2.5 and 5.3%. Inter-site CoVs were also low, with the median CoVs for all metabolites between 3.7 and 6.4%. Metabolite concentrations were robust to small inconsistencies in voxel placement and site was not the driving factor in the variance of the measurement of any metabolite concentration. Between-subject differences accounted for the majority of the concentration variability for creatine, choline and myo-inositol (42-65% of the variance).

CONCLUSION

A large single-voxel ¹ H-MRS acquisition from a single manufacturer's MRI scanner is highly reproducible and reliable for multi-site clinical trials.

Cerebral Mitochondrial Microangiopathy Leads to Leukoencephalopathy in Mitochondrial Neurogastrointestinal Encephalopathy

BACKGROUND AND PURPOSE

Mitochondrial neurogastrointestinal encephalopathy is a rare disorder due to recessive mutations in the thymidine phosphorylase gene, encoding thymidine phosphorylase protein required for mitochondrial DNA replication. Clinical manifestations include gastrointestinal dysmotility and diffuse asymptomatic leukoencephalopathy. This study aimed to elucidate the mechanisms underlying brain leukoencephalopathy in patients with mitochondrial neurogastrointestinal encephalopathy by correlating multimodal neuroradiologic features to postmortem pathology.

MATERIALS AND METHODS

Seven patients underwent brain MR imaging, including single-voxel proton MR spectroscopy and diffusion imaging. Absolute concentrations of metabolites calculated by acquiring unsuppressed water spectra at multiple TEs, along with diffusion metrics based on the tensor model, were compared with those of healthy controls using unpaired t tests in multiple white matters regions. Brain postmortem histologic, immunohistochemical, and molecular analyses were performed in 1 patient.

RESULTS

All patients showed bilateral and nearly symmetric cerebral white matter hyperintensities on T2-weighted images, extending to the cerebellar white matter and brain stem in 4. White matter, N-acetylaspartate, creatine, and choline concentrations were significantly reduced compared with those in controls, with a prominent increase in the radial water diffusivity component. At postmortem examination, severe fibrosis of brain vessel smooth muscle was evident, along with mitochondrial DNA replication depletion in brain and vascular smooth-muscle and endothelial cells, without neuronal loss, myelin damage, or gliosis. Prominent periependymal cytochrome C oxidase deficiency was also observed.

CONCLUSIONS

Vascular functional and histologic alterations account for leukoencephalopathy in mitochondrial neurogastrointestinal encephalopathy. Thymidine toxicity and mitochondrial DNA replication depletion may induce microangiopathy and blood-brain-barrier dysfunction, leading to increased water content in the white matter. Periependymal cytochrome C oxidase deficiency could explain prominent periventricular impairment.

Comparison of human brain metabolite levels using 1H MRS at 1.5T and 3.0T

Proton magnetic resonance spectroscopy (MRS) of the human brain has proven to be a useful technique in several neurological and psychiatric disorders and benefits from higher field scanners as signal intensity and spectral resolution are proportional to the magnetic field strength.

Imaging of endogenous exchangeable proton signals in the human brain using frequency labeled exchange transfer imaging

PURPOSE

To image endogenous exchangeable proton signals in the human brain using a recently reported method called frequency labeled exchange transfer (FLEX) MRI.

METHODS

As opposed to labeling exchangeable protons using saturation (i.e., chemical exchange saturation transfer, or CEST), FLEX labels exchangeable protons with their chemical shift evolution. The use of short high-power frequency pulses allows more efficient labeling of rapidly exchanging protons, while time domain acquisition allows removal of contamination from semi-solid magnetization transfer effects.

RESULTS

FLEX-based exchangeable proton signals were detected in human brain over the 1-5 ppm frequency range from water. Conventional magnetization transfer contrast and the bulk water signal did not interfere in the FLEX spectrum. The information content of these signals differed from in vivo CEST data in that the average exchange rate of these signals was 350-400 s(-1) , much faster than the amide signal usually detected using direct saturation (∼30 s(-1) ). Similarly, fast exchanging protons could be detected in egg white in the same frequency range where amide and amine protons of mobile proteins and peptides are known to resonate.

CONCLUSIONS

FLEX MRI in the human brain preferentially detects more rapidly exchanging amide/amine protons compared to traditional CEST experiments, thereby changing the information content of the exchangeable proton spectrum. This has the potential to open up different types of endogenous applications as well as more easy detection of rapidly exchanging protons in diaCEST agents or fast exchanging units such as water molecules in paracest agents without interference of conventional magnetization transfer contrast.

Structural and metabolic damage in brains of patients with SPG11-related spastic paraplegia as detected by quantitative MRI

The goal of this work was to assess brain structural and metabolic abnormalities of subjects with SPG11 and their relevance to clinical disability by using quantitative magnetic resonance (MR) metrics. Autosomal recessive hereditary spastic paraplegia (AR-HSP) with thin corpus callosum and cognitive decline is a complex neurological disorder caused by mutations in the SPG11 gene in most cases. Little is known about the process leading to corticospinal and white matter degeneration. We performed conventional MRI/MR spectroscopic imaging ((1)H-MRSI) examinations in 10 HSP patients carrying an SPG11 mutation and in 10 demographically matched healthy controls (HC). We measured in each subject cerebral white matter hyperintensities (WMHs), normalized global and cortical brain volumes, and (1)H-MRSI-derived central brain levels of N-acetylaspartate (NAA) and choline (Cho) normalized to creatine (Cr). Clinical disability was assessed according to patients' autonomy in walking. Conventional MRI showed WMHs in all patients. Global brain volumes were lower in patients than in HC (p < 0.001). Decreased values were diffusely found also in cortical regions (p < 0.01). On (1)H-MRSI, NAA/Cr values were lower in SPG11 patients than in HC (p = 0.002). Cho/Cr values did not differ between patients and HC. Cerebral volume decreases and NAA/Cr in the corona radiata correlated closely with increasing disability scores (p < 0.05). Quantitative MR measures propose that widespread structural and metabolic brain damage occur in SPG11 patients. The correlation of these MR metrics with measures of patients' disease severity suggests that they might represent adequate surrogate markers of disease outcome.

Applications of chemical shift imaging to marine sciences

The successful applications of magnetic resonance imaging (MRI) in medicine are mostly due to the non-invasive and non-destructive nature of MRI techniques. Longitudinal studies of humans and animals are easily accomplished, taking advantage of the fact that MRI does not use harmful radiation that would be needed for plain film radiographic, computerized tomography (CT) or positron emission (PET) scans. Routine anatomic and functional studies using the strong signal from the most abundant magnetic nucleus, the proton, can also provide metabolic information when combined with in vivo magnetic resonance spectroscopy (MRS). MRS can be performed using either protons or hetero-nuclei (meaning any magnetic nuclei other than protons or ¹H) including carbon (¹³C) or phosphorus (³¹P). In vivo MR spectra can be obtained from single region of interest (ROI or voxel) or multiple ROIs simultaneously using the technique typically called chemical shift imaging (CSI). Here we report applications of CSI to marine samples and describe a technique to study in vivo glycine metabolism in oysters using ¹³C MRS 12 h after immersion in a sea water chamber dosed with [2-¹³C]-glycine. This is the first report of ¹³C CSI in a marine organism.

(1)H-MRS of brain metabolites in migraine without aura: absolute quantification using the phantom replacement technique

OBJECTIVE

Several studies have demonstrated differences in migraine patients when performing (1)H-MRS; however, no studies have performed (1)H-MRS in migraine without aura (MwoA), the most common migraine subtype. The aim of this (1)H-MRS study was to elucidate whether any differences could be found between MwoA patients and controls by performing absolute quantification.

MATERIALS AND METHODS

(1)H-MRS was performed in 22 MwoA patients and 25 control subjects. Absolute quantification was based on the phantom replacement technique. Corrections were made for T (1) and T (2) relaxation effects, CSF content, coil loading and temperature. The method was validated by phantom measurements and in vivo measurements in the occipital visual cortex.

RESULTS

After calibration of the quantification procedure and the implementation of the required correction factors, measured absolute concentrations in the visual cortex of MwoA patients showed no significant differences compared to controls, in contrast to relative results obtained in earlier studies.

CONCLUSION

In this study, we demonstrate the implementation of quantitative in vivo (1)H-MRS spectroscopy in migraine patients. Despite rigorous quantification, no spectroscopic abnormalities could be found in patients with migraine without aura.