Impact of fasting on human brain acid-base homeostasis using natural abundance (13) C and (31) P MRS.

PURPOSE: To use (13) C magnetic resonance spectroscopy (MRS) and (31) P MRS to develop a direct assay for regional [HCO3-] in the human brain and to define brain pH and physiological response of [HCO3-] to fasting. MATERIALS AND METHODS: Seven healthy subjects underwent MRS examinations on a 1.5T MRI scanner. Subjects were well fed with repeated examinations performed after 4 and 12 hours of fasting. Proton noise decoupling (13) C MRS were acquired using pulse and acquired acquisition while (31) P MRS were acquired using a 2D chemical shift imaging method with relaxation time (TR) of 2 seconds. RESULTS: Fasting brain bicarbonate concentrations (6.7 ± 2.5 mM for 12-hour fasting, P = 0.002 and 8.3 ± 2.1 mM for 4-hour fasting, P = 0.015) are significantly reduced compared to fed state (11.6 ± 1.3 mM). However, no significant difference in brain pH was observed, confirming the critical role of pCO2 in intracerebral pH homeostasis. CONCLUSION: We demonstrated that the intracellular HCO3- in human brain is readily modified by diet but appears to have no measurable effect on cerebral pH. Natural abundance (13) C can provide useful information relevant to human brain pH homeostasis by providing information for HCO3-.

Minimally invasive biomarker confirms glial activation present in Alzheimer's disease: a preliminary study.

We applied (13)C magnetic resonance spectroscopy (MRS), a nonradioactive, noninvasive brain imaging technique, to quantify the oxidation of [1-(13)C] acetate in a conventional clinical magnetic resonance imaging (MRI) scanner in five consecutive elderly subjects at various clinical stages of Alzheimer's disease (AD) progression. [1-(13)C] acetate entered the brain and was metabolized to [5-(13)C] glutamate and glutamine, as well as [1-(13)C] glutamate and glutamine, and the final glial oxidation product, (13)C bicarbonate, at a linear rate. Calculation of the initial slope was similar in a single subject, examined twice, 1 month apart (test-re-test 8%). Mean rate of cerebral bicarbonate production in this elderly group was 0.040 ± 0.01 (n = 5). Assuming that the rate of conversion of acetate to bicarbonate is a reflection of glial metabolic rate and that glial metabolic rate is a surrogate marker for 'neuroinflammation', our preliminary results suggest that [1-(13)C] MRS may provide biomarkers for diseases, believed to involve microglia and other cells of the astrocyte series. Among these is AD, for which novel drugs which ameliorate the damaging effects of neuroinflammation before symptoms of dementia appear, are in advanced development. The value of (13)C MRS as an early, noninvasive biomarker may lie in the conduct of cost-effective clinical trials.

Swift Acetate Glial Assay (SAGA): an accelerated human ¹³C MRS brain exam for clinical diagnostic use.

We demonstrate a robust procedure for the quantitative characterization of glial metabolism in human brain. In the past, the slope of the uptake and production of enriched label at steady state were used to determine metabolic rates, requiring the patient to be in the magnet for 120-160 min. In the present method, (13)C cerebral metabolite profiles were acquired at steady state alone on a routine clinical MR scanner in 25.6 min. Results obtained from the new short method (SAGA) were comparable to those achieved in a conventional, long method and effective for determination of glial metabolic rate in posterior-parietal and frontal brain regions.

Fast volumetric spatial-spectral MR imaging of hyperpolarized 13C-labeled compounds using multiple echo 3D bSSFP.

PURPOSE: The goal of this work was to develop a fast 3D chemical shift imaging technique for the noninvasive measurement of hyperpolarized (13)C-labeled substrates and metabolic products at low concentration. MATERIALS AND METHODS: Multiple echo 3D balanced steady state magnetic resonance imaging (ME-3DbSSFP) was performed in vitro on a syringe containing hyperpolarized [1,3,3-2H3; 1-(13)C]2-hydroxyethylpropionate (HEP) adjacent to a (13)C-enriched acetate phantom, and in vivo on a rat before and after intravenous injection of hyperpolarized HEP at 1.5 T. Chemical shift images of the hyperpolarized HEP were derived from the multiple echo data by Fourier transformation along the echoes on a voxel by voxel basis for each slice of the 3D data set. RESULTS: ME-3DbSSFP imaging was able to provide chemical shift images of hyperpolarized HEP in vitro, and in a rat with isotropic 7-mm spatial resolution, 93 Hz spectral resolution and 16-s temporal resolution for a period greater than 45 s. CONCLUSION: Multiple echo 3D bSSFP imaging can provide chemical shift images of hyperpolarized (13)C-labeled compounds in vivo with relatively high spatial resolution and moderate spectral resolution. The increased signal-to-noise ratio of this 3D technique will enable the detection of hyperpolarized (13)C-labeled metabolites at lower concentrations as compared to a 2D technique.

Glial dysfunction in abstinent methamphetamine abusers.

Persistent neurochemical abnormalities in frontal brain structures are believed to result from methamphetamine use. We developed a localized (13)C magnetic resonance spectroscopy (MRS) assay on a conventional MR scanner, to quantify selectively glial metabolic flux rate in frontal brain of normal subjects and a cohort of recovering abstinent methamphetamine abusers. Steady-state bicarbonate concentrations were similar, between 11 and 15 mmol/L in mixed gray-white matter of frontal brain of normal volunteers and recovering methamphetamine-abusing subjects (P>0.1). However, glial (13)C-bicarbonate production rate from [1-(13)C]acetate, equating with glial tricarboxylic acid (TCA) cycle rate, was significantly reduced in frontal brain of abstinent methamphetamine-addicted women (methamphetamine 0.04 micromol/g per min (N=5) versus controls 0.11 micromol/g per min (N=5), P=0.001). This is equivalent to 36% of the normal glial TCA cycle rate. Severe reduction in glial TCA cycle rate that normally comprises 10% of total cerebral metabolic rate may impact operation of the neuronal glial glutamate cycle and result in accumulation of frontal brain glutamate, as observed in these recovering methamphetamine abusers. Although these are the first studies to define directly an abnormality in glial metabolism in human methamphetamine abuse, sequential studies using analogous (13)C MRS methods may determine 'cause and effect' between glial failure and neuronal injury.

Ornithine transcarbamylase deficiency with persistent abnormality in cerebral glutamate metabolism in adults.

PURPOSE: To determine cerebral glutamate turnover rate in partial-ornithine transcarbamylase deficiency (OTCD) patients by using carbon 13 ((13)C) magnetic resonance (MR) spectroscopy. MATERIALS AND METHODS: The study was performed with approval of the institutional review board, in compliance with HIPAA regulations, and with written informed consent of the subjects. MR imaging, hydrogen 1 ((1)H) MR spectroscopy, and (13)C MR spectroscopy were performed at 1.5 T in 10 subjects, six patients with OTCD and four healthy control subjects, who were in stable condition. Each received intravenous (13)C-glucose (0.2 g/kg), C1 or C2 position, as a 15-minute bolus. Cerebral metabolites were determined with proton decoupling in a parieto-occipital region (n = 9) and without proton decoupling in a frontal region (n = 1) during 60-120 minutes. RESULTS: Uptake and removal of cerebral glucose ([1-(13)C]-glucose or [2-(13)C]-glucose) were comparable in healthy control subjects and subjects with OTCD (P = .1). Glucose C1 was metabolized to glutamate C4 and glucose C2 was metabolized to glutamate C5 at comparable rates, both of which were significantly reduced in OTCD (combined, P = .04). No significant differences in glutamine formation were found in subjects with OTCD (P = .1). [2-(13)C]-glucose and its metabolic products were observed in anterior cingulate gyrus without proton decoupling in one subject with OTCD. CONCLUSION: Treatments that improve cerebral glucose metabolism and glutamate neurotransmission may improve neurologic outcome in patients with OTCD, in whom prevention and treatment of hyperammonemic episodes appear to be insufficient.

PASADENA hyperpolarization of 13C biomolecules: equipment design and installation.

OBJECT: The PASADENA method has achieved hyperpolarization of 16-20% (exceeding 40,000-fold signal enhancement at 4.7 T), in liquid samples of biological molecules relevant to in vivo MRI and MRS. However, there exists no commercial apparatus to perform this experiment conveniently and reproducibly on the routine basis necessary for translation of PASADENA to questions of biomedical importance. The present paper describes equipment designed for rapid production of six to eight liquid samples per hour with high reproducibility of hyperpolarization. MATERIALS AND METHODS: Drawing on an earlier, but unpublished, prototype, we provide diagrams of a delivery circuit, a laminar-flow reaction chamber within a low field NMR contained in a compact, movable housing. Assembly instructions are provided from which a computer driven, semi-automated PASADENA polarizer can be constructed. RESULTS: Together with an available parahydrogen generator, the polarizer, which can be operated by a single investigator, completes one cycle of hyperpolarization each 52 s. Evidence of efficacy is presented. In contrast to competing, commercially available devices for dynamic nuclear polarization which characteristically require 90 min per cycle, PASADENA provides a low-cost alternative for high throughput. CONCLUSIONS: This equipment is suited to investigators who have an established small animal NMR and wish to explore the potential of heteronuclear ((13)C and (15)N) MRI, MRS, which harnesses the enormous sensitivity gain offered by hyperpolarization.

Quality assurance of PASADENA hyperpolarization for 13C biomolecules.

OBJECT: Define MR quality assurance procedures for maximal PASADENA hyperpolarization of a biological (13)C molecular imaging reagent. MATERIALS AND METHODS: An automated PASADENA polarizer and a parahydrogen generator were installed. (13)C enriched hydroxyethyl acrylate, 1-(13)C, 2,3,3-d(3) (HEA), was converted to hyperpolarized hydroxyethyl propionate, 1-(13)C, 2,3,3-d(3) (HEP) and fumaric acid, 1-(13)C, 2,3-d(2) (FUM) to hyperpolarized succinic acid, 1-(13)C, 2,3-d(2) (SUC), by reaction with parahydrogen and norbornadiene rhodium catalyst. Incremental optimization of successive steps in PASADENA was implemented. MR spectra and in vivo images of hyperpolarized (13)C imaging agents were acquired at 1.5 and 4.7 T. RESULTS: Application of quality assurance (QA) criteria resulted in incremental optimization of the individual steps in PASADENA implementation. Optimal hyperpolarization of HEP of P = 20% was achieved by calibration of the NMR unit of the polarizer (B (0) field strength +/- 0.002 mT). Mean hyperpolarization of SUC, P = [15.3 +/- 1.9]% (N = 16) in D (2)O, and P = [12.8 +/- 3.1]% (N = 12) in H (2)O, was achieved every 5-8 min (range 13-20%). An in vivo (13)C succinate image of a rat was produced. CONCLUSION: PASADENA spin hyperpolarization of SUC to 15.3% in average was demonstrated (37,400 fold signal enhancement at 4.7 T). The biological fate of (13)C succinate, a normally occurring cellular intermediate, might be monitored with enhanced sensitivity.

Clinical NOE 13C MRS for neuropsychiatric disorders of the frontal lobe.

In this communication, a scheme is described whereby in vivo (13)C MRS can safely be performed in the frontal lobe, a human brain region hitherto precluded on grounds of SAR, but important in being the seat of impaired cognitive function in many neuropsychiatric and developmental disorders. By combining two well known features of (13)C NMR-the use of low power NOE and the focus on (13)C carbon atoms which are only minimally coupled to protons, we are able to overcome the obstacle of SAR and develop means of monitoring the (13)C fluxes of critically important metabolic pathways in frontal brain structures of normal volunteers and patients. Using a combination of low-power WALTZ decoupling, variants of random noise for nuclear overhauser effect enhancement it was possible to reduce power deposition to 20% of the advised maximum specific absorption rate (SAR). In model solutions (13)C signal enhancement achieved with this scheme were comparable to that obtained with WALTZ-4. In human brain, the low power procedure effectively determined glutamine, glutamate and bicarbonate in the posterior parietal brain after [1-(13)C] glucose infusion. The same (13)C enriched metabolites were defined in frontal brain of human volunteers after administration of [1-(13)C] acetate, a recognized probe of glial metabolism. Time courses of incorporation of (13)C into cerebral glutamate, glutamine and bicarbonate were constructed. The results suggest efficacy for measurement of in vivo cerebral metabolic rates of the glutamate-glutamine and tricarboxylic acid cycles in 20 min MR scans in previously inaccessible brain regions in humans at 1.5 T. We predict these will be clinically useful biomarkers in many human neuropsychiatric and genetic conditions.

PASADENA hyperpolarization of succinic acid for MRI and NMR spectroscopy.

We use the PASADENA (parahydrogen and synthesis allow dramatically enhanced nuclear alignment) method to achieve 13C polarization of approximately 20% in seconds in 1-13C-succinic-d2 acid. The high-field 13C multiplets are observed as a function of pH, and the line broadening of C1 is pronounced in the region of the pK values. The 2JCH, 3JCH, and 3JHH couplings needed for spin order transfer vary with pH and are best resolved at low pH leading to our use of pH approximately 3 for both the molecular addition of parahydrogen to 1-13C-fumaric acid-d2 and the subsequent transfer of spin order from the nascent protons to C1 of the succinic acid product. The methods described here may generalize to hyperpolarization of other carboxylic acids. The C1 spin-lattice relaxation time at neutral pH and 4.7 T is measured as 27 s in H2O and 56 s in D2O. Together with known rates of succinate uptake in kidneys, this allows an estimate of the prospects for the molecular spectroscopy of metabolism.

Towards hyperpolarized (13)C-succinate imaging of brain cancer.

We describe a novel (13)C enriched precursor molecule, sodium 1-(13)C acetylenedicarboxylate, which after hydrogenation by PASADENA (Parahydrogen and Synthesis Allows Dramatically Enhanced Nuclear Alignment) under controlled experimental conditions, becomes hyperpolarized (13)C sodium succinate. Fast in vivo 3D FIESTA MR imaging demonstrated that, following carotid arterial injection, the hyperpolarized (13)C-succinate appeared in the head and cerebral circulation of normal and tumor-bearing rats. At this time, no in vivo hyperpolarized signal has been localized to normal brain or brain tumor. On the other hand, ex vivo samples of brain harvested from rats bearing a 9L brain tumor, 1 h or more following in vivo carotid injection of hyperpolarized (13)C sodium succinate, contained significant concentrations of the injected substrate, (13)C sodium succinate, together with (13)C maleate and succinate metabolites 1-(13)C-glutamate, 5-(13)C-glutamate, 1-(13)C-glutamine and 5-(13)C-glutamine. The (13)C substrates and products were below the limits of NMR detection in ex vivo samples of normal brain consistent with an intact blood-brain barrier. These ex vivo results indicate that hyperpolarized (13)C sodium succinate may become a useful tool for rapid in vivo identification of brain tumors, providing novel biomarkers in (13)C MR spectral-spatial images.

Efficacy of proton magnetic resonance spectroscopy in neurological diagnosis and neurotherapeutic decision making.

Anatomic and functional neuroimaging with magnetic resonance imaging (MRI) includes the technology more widely known as magnetic resonance spectroscopy (MRS). Now a routine automated "add-on" to all clinical magnetic resonance scanners, MRS, which assays regional neurochemical health and disease, is therefore the most accessible diagnostic tool for clinical management of neurometabolic disorders. Furthermore, the noninvasive nature of this technique makes it an ideal tool for therapeutic monitoring of disease and neurotherapeutic decision making. Among the more than 100 brain disorders that fall within this broad category, MRS contributes decisively to clinical decision making in a smaller but growing number. In this review, we will cover how MRS provides therapeutic impact in brain tumors, metabolic disorders such as adrenoleukodystrophy and Canavan's disease, Alzheimer's disease, hypoxia, secondary to trauma or ischemia, human immunodeficiency virus dementia and lesions, as well as systemic disease such as hepatic and renal failure. Together, these eight indications for MRS apply to a majority of all cases seen. This review, which examines the role of MRS in enhancing routine neurological practice and treatment concludes: 1) there is added value from MRS where MRI is positive; 2) there is unique decision-making information in MRS when MRI is negative; and 3) MRS usefully informs decision making in neurotherapeutics. Additional efficacy studies could extend the range of this capability.

Clinical experience with 13C MRS in vivo.

13C MRS was installed on a clinical scanner at 1.5 T in order to facilitate integrated MR examinations of human brain disorders. Using a simplified protocol, (1-(13)C) glucose and/or (1-(13)C) acetate were administered orally or by intravenous infusion. (13)C spectra of diagnostic quality were acquired in more than 100 consecutive studies. Novel (13)C neurochemical data contributed to the understanding of Alzheimer's, Canavan's, mitochondrial and hepatic encephalopathy, epilepsy and normal brain development. (13)C MRS uncovered hitherto unknown disorders of NAA-synthesis, glutamate neurotransmission, TCA-cycle and glycolysis. Despite low inherent signal-to-noise, natural abundance (13)C MRS showed diagnostic promise. (13)C MRS is feasible in a clinical setting, at reasonable cost in neonates, children, adults and elderly patients.