NMR spectroscopy: current status and future possibilities

Advanced In Vivo Heteronuclear MRS Approaches for Studying Brain Bioenergetics Driven by Mitochondria

The greatest merit of in vivo magnetic resonance spectroscopy (MRS) methodology used in biomedical research is its ability for noninvasively measuring a variety of metabolites inside a living organ. It, therefore, provides an invaluable tool for determining metabolites, chemical reaction rates and bioenergetics, as well as their dynamic changes in the human and animal. The capability of in vivo MRS is further enhanced at higher magnetic fields because of significant gain in detection sensitivity and improvement in the spectral resolution. Recent progress of in vivo MRS technology has further demonstrated its great potential in many biomedical research areas, particularly in brain research. Here, we provide a review of new developments for in vivo heteronuclear 31P and 17O MRS approaches and their applications in determining the cerebral metabolic rates of oxygen and ATP inside the mitochondria, in both animal and human brains.

In vivo 31P MRS of human brain at high/ultrahigh fields: a quantitative comparison of NMR detection sensitivity and spectral resolution between 4 T and 7 T

The primary goal of this study was to establish a rigorous approach for determining and comparing the NMR detection sensitivity of in vivo 31P MRS at different field strengths (B0). This was done by calculating the signal-to-noise ratio (SNR) achieved within a unit sampling time at a given field strength. In vivo 31P spectra of human occipital lobe were acquired at 4 and 7 T under similar experimental conditions. They were used to measure the improvement of the human brain 31P MRS when the field strength increases from 4 to 7 T. The relaxation times and line widths of the phosphocreatine (PCr) resonance peak and the RF coil quality factors (Q) were also measured at these two field strengths. Their relative contributions to SNR at a given field strength were analyzed and discussed. The results show that in vivo 31P sensitivity was significantly improved at 7 T as compared with 4 T. Moreover, the line-width of the PCr resonance peak showed less than a linear increase with increased B0, which leads to a significant improvement in 31P spectral resolution. These findings indicate the advantage of high-field strength to improve in vivo 31P MRS quality in both sensitivity and spectral resolution. This advantage should improve the reliability and applicability of in vivo 31P MRS in studying high-energy phosphate metabolism, phospholipid metabolism and cerebral biogenetics in the human at both normal and diseased states noninvasively. Finally, the approach used in this study for calculating in vivo 31P MRS sensitivity provides a general tool in estimating the relative NMR detection sensitivity for any nuclear spin at a given field strength.

Discrepancies between cerebral perfusion and metabolism after subarachnoid haemorrhage: a magnetic resonance approach

INTRODUCTION

There is a variable relation between angiographic vasospasm and delayed ischaemic neurological deficit (DIND). Magnetic resonance (MR) techniques have the potential to investigate the haemodynamic, metabolic, and structural changes occurring with these complications. These techniques have been applied to study DIND in patients recovering from subarachnoid haemorrhage.

METHODS

Fifteen studies were performed on 11 patients, 10 with DIND. Vasospasm was diagnosed angiographically or with transcranial Doppler. The MR protocol consisted of T2 weighted imaging, contrast enhanced dynamic perfusion scanning, TI weighted imaging, and two dimensional localised proton spectroscopy. Relative cerebral blood volume maps were generated from perfusion scans. Metabolite ratios were calculated from proton spectra.

RESULTS

All patients had cortical oedema on T2 weighted images, significantly more pronounced in patients of poor clinical grade (p<0.01). Spectra were normal in good grade patients. Lactate was increased and N-acetyl aspartate decreased in the poor grades, significantly worse in grade 4 compared with grade 3 patients (p<0.05). Spectral changes also correlated with the severity of oedema (p<0.05). Relative blood volumes were significantly higher in oedematous regions of poor compared with good grade patients (p<0.05). Lactate was seen in regions of the brain with increased relative blood volume.

CONCLUSIONS

Despite the paramagnetic effects of haemorrhage, or of the coils and clips used to treat aneurysms, this study demonstrates that patients recovering from subarachnoid haemorrhage can undergo complex MR studies. Oedema, lactate, and increased relative blood volume correlate well with each other and with DIND and poor clinical grade.

What might be the impact on neurology of the analysis of brain metabolism by in vivo magnetic resonance spectroscopy?

In vivo nuclear magnetic resonance spectroscopy (MRS) of the human brain is a recently developed technique which allows to assay noninvasively in vivo key molecules of brain metabolism. After a review of the origin of the signals detected by phosphorus and proton MRS of human brain, the impact of MRS on clinical neurology is examined. MRS of the brain does not purport to be a metabolic "biopsy", but unique applications for brain MRS are (1) quantitating the oxidative state of the brain and defining neuronal death, (2) assessing and mapping neuron damage, (3) evaluating membrane alterations, and (4) characterizing encephalopathies. In the near future brain MRS will be performed routinely after conventional MRI, as a valuable metabolic (and functional) complement to the anatomical evaluation of cerebral pathologies, particularly the toxic, metabolic and infectious encephalopathies.