Cerebral phosphoester signals measured by 31P magnetic resonance spectroscopy at 3 and 7 Tesla

Assessment of sodium (23Na) brain MRI at 3T - preliminary results

Purpose

The purpose of this work was to establish a database of tissue sodium concentration (TSC) in the normal brain of healthy volunteers. Tissue sodium concentration can be used as a sensitive marker of tissue viability in stroke or radiation therapy monitoring.

Material and methods

Thirty-seven volunteers were scanned with a 23Na protocol in the span of one year; within this group, 29 studies were of acceptable quality. The study was approved by the Local Bioethics Committee. Data were acquired during a single magnetic resonance (MR) scanner session. The single scanner session consisted of 23Na 3D radial gradient echo (GRE) acquisition, MPRage, SPACE-FLAIR, and Resolve-DTI. MPRage images were segmented to obtain masks of the grey matter (GM), white matter (WM), and cerebrospinal fluid (CSF), which were registered to the sodium image space for image analysis. Images were transformed into TSC maps - a signal calibration curve obtained from the reference phantom of known sodium concentration and known relaxation time.

Results

The collected data were analysed in 2 different ways: volunteers were divided by sex and by age. No significant differences in TSC were found between sexes. In all comparisons there was a significant difference in TSC between younger and older volunteers. In healthy volunteers mean TSC were as follows: GM 33.21 ± 4.76 mmol/l, WM 28.41 ± 4.03 mmol/l and for CSF 41.3 ± 6.69 mmol/l.

Conclusions

This preliminary work is a base for further work with sodium imaging in brain lesions. The entirety of the col-lected data will be useful in the future as a baseline brain TSC for comparison to values obtained from pathologies.

Magnetic Field Dependence of Spectral Correlations between 31P-Containing Metabolites in Brain

Spectral correlations between metabolites in ³¹P magnetic resonance spectroscopy (MRS) spectra of human brain were compared at 3 and 7 Tesla, the two commonly used magnetic field strengths for clinical research. It was found that at both field strengths, there are significant correlations between ³¹P-containing metabolites arising from spectral overlap, and their downfield correlations are markedly altered by the background spectral baseline. Overall, the spectral correlations between ³¹P-containing metabolites are markedly reduced at 7 Tesla with the increased chemical shift dispersion and the decreased membrane phospholipid signal. The findings provide the quantitative landscape of pre-existing correlations in ³¹P MRS spectra due to overlapping signals. Detailed procedures for quantifying the pre-existing correlations between ³¹P-containing metabolites are presented to facilitate incorporation of spectral correlations into statistical modeling in clinical correlation studies.

Monte Carlo study of metabolite correlations originating from spectral overlap

Monte Carlo simulations and a mathematical model of spectral fitting were used to study the correlations between metabolites with overlapping resonances. The dependence of the polarity and the magnitude of cross-correlation coefficients between overlapping metabolites on the spectral patterns of MRS signals was investigated. The results demonstrate the importance of quantifying metabolite correlations originating from spectral overlap as they may confound determination of correlations of biological origin. The findings also indicate that it is possible to minimize unwanted metabolite correlations by altering spectral patterns in the presence of significant spectral overlap.

Accumulation of 3-aminopropylphosphonate in the ex vivo brain observed by phosphorus-31 nuclear magnetic resonance

3-aminopropylphosphonate (3-APP) is known for its use as an exogenous indicator of extracellular volume and pH in phosphorus-31 nuclear magnetic resonance (³¹ P NMR) studies. We used 3-APP for estimating the extracellular volume in NMR studies of several ex vivo preparations including retrograde perfused mouse heart (n = 4), mouse liver slices (n = 2), xenograft breast cancer tumors (n = 7, MCF7), and rat brain slices (n = 4). In the former three preparations, the 3-APP signal was stable in lineshape and intensity for hours and the chemical shift of the signal in the presence of the biological sample was the same as in the perfusion medium without the biological sample. However, in studies of brain slices, the 3-APP signal appeared split into two, with an upfield component (0.7 ± 0.1 ppm to the left) increasing with time and showing a wider linewidth (66.7 ± 12.6 vs. 39.1 ± 7.6 Hz, the latter is of the perfusion medium signal). This finding suggests that 3-APP inadvertently accumulated in brain slices, most likely as a membrane bound form. This observation limits the use of 3-APP as an inert biochemical indicator in brain preparations and should be taken into account when using 3-APP in vivo.

In Vitro 31P MR Chemical Shifts of In Vivo-Detectable Metabolites at 3T as a Basis Set for a Pilot Evaluation of Skeletal Muscle and Liver 31P Spectra with LCModel Software

Most in vivo ³¹P MR studies are realized on 3T MR systems that provide sufficient signal intensity for prominent phosphorus metabolites. The identification of these metabolites in the in vivo spectra is performed by comparing their chemical shifts with the chemical shifts measured in vitro on high-field NMR spectrometers. To approach in vivo conditions at 3T, a set of phantoms with defined metabolite solutions were measured in a 3T whole-body MR system at 7.0 and 7.5 pH, at 37 °C. A free induction decay (FID) sequence with and without ¹H decoupling was used. Chemical shifts were obtained of phosphoenolpyruvate (PEP), phosphatidylcholine (PtdC), phosphocholine (PC), phosphoethanolamine (PE), glycerophosphocholine (GPC), glycerophosphoetanolamine (GPE), uridine diphosphoglucose (UDPG), glucose-6-phosphate (G6P), glucose-1-phosphate (G1P), 2,3-diphosphoglycerate (2,3-DPG), nicotinamide adenine dinucleotide (NADH and NAD⁺), phosphocreatine (PCr), adenosine triphosphate (ATP), adenosine diphosphate (ADP), and inorganic phosphate (Pi). The measured chemical shifts were used to construct a basis set of ³¹P MR spectra for the evaluation of ³¹P in vivo spectra of muscle and the liver using LCModel software (linear combination model). Prior knowledge was successfully employed in the analysis of previously acquired in vivo data.