Brain-metabolite transverse relaxation times in magnetic resonance spectroscopy increase as adenosine triphosphate depletes during secondary energy failure following acute hypoxia-ischaemia in the newborn piglet

Brain MRI after therapeutic hypothermia in asphyxiated newborns: Predictive value at one year of imaging features

PURPOSE

To compare early brain MRI using a composite imaging score and outcome at one year in asphyxiated newborns treated by hypothermia.

METHODS

This retrospective study included for two years consecutive asphyxiated term newborns treated by hypothermia for hypoxic-ischemic encephalopathy, and who had brain MRI before day 8. Therapeutic hypothermia was initiated within the first 6 h of life and continued for 72 h. Imaging protocol included T1 and T2 sequences, diffusion-weighted imaging (DWI), evaluated with a specific composite score, and spectroscopy. Clinical evaluation was performed at one year of age, outcome was classified as favorable/unfavorable. The primary endpoint was the correlation between our MRI score and outcome with the definition of a threshold. The secondary endpoints were to find the most relevant criteria within the score and to evaluate objective signal measurements to support subjective criteria.

RESULTS

Among the 33 included patients, 9 died during the first days of life, 20 had a favorable outcome, 4 an unfavorable one. MRI score was correlated to a poor clinical outcome (p < 0.001). Most of the criteria within the score and spectroscopy results were relevant (p < 0.05). Cerebral edema was objectively assessed by the signal intensity ratio of white matter compared to cerebrospinal fluid (CSF) on T2-weighted images (p < 0.001).

CONCLUSION

MRI score was predictive of neurodevelopmental outcome at one year. The most relevant criteria within the score were DWI abnormalities in basal ganglia and thalami and loss of white-cortical grey matter differentiation. Signal intensity ratio between white matter and CSF higher than 0.75 supports the presence of edema.

Rapid multi-echo measurement of brain metabolite T₂ values at 7 T using a single-shot spectroscopic Carr-Purcell-Meiboom-Gill sequence and prior information

We present a method for the robust and accurate estimation of brain metabolite transverse relaxation times (T2 ) from multiple spin-echo data acquired with a single-shot Carr-Purcell-Meiboom-Gill (CPMG) spectroscopic sequence. Each acquired echo consists of a small number of complex time-domain data points. The amplitudes of the spectral components in each echo are calculated by solving a set of linear equations in which previously estimated frequencies and linewidths serve as prior information. These priors are obtained from a short MRS experiment in which a large number of time-domain data points are acquired, and are subsequently estimated using linear prediction with singular value decomposition (LPSVD) processing. We show that this process can be used to accurately and rapidly measure the T2 values for the main singlet resonances in single-volume MRS measurements in the brain. The proposed method can be generalized to any set of MRS experiments comprising repeated measurements of amplitude changes, e.g. as a function of an experimental parameter, such as TE, inversion time or diffusion weighting.

Improved reproducibility of MRS regional brain thermometry by 'amplitude-weighted combination'

Brain temperature is important in stroke and trauma. In birth asphyxia, hypothermia improves outcome, but local brain temperature information is needed to optimise therapy. The proton MRS water chemical shift (δ(water) ) is temperature dependent, and the in vivo brain temperature has often been estimated by measuring δ(water) relative to the N-acetylaspartate (NAA) singlet methyl resonance. However, the NAA peak amplitude may be reduced if cerebrospinal fluid occupies part of the MRS voxel and because of the lower concentration in immaturity, pathology and neonatal white matter. These factors can increase random and systematic δ(NAA) errors and also, therefore, MRS brain temperature errors. The aim of this study was to improve MRS brain temperature reproducibility and resilience to pathological, developmental and regional peak amplitude variations by amplitude-weighted combination (AWC) of brain temperatures (T(Cho) , T(Cr) and T(NAA) ) determined using the prominent choline (Cho), total creatine (Cr) and NAA resonances separately as chemical shift references. δ(water) - δ(Cho) , δ(water) - δ(Cr) and δ(water) - δ(NAA) were calibrated against tympanic temperature in piglet brain at 7 T (2.5-cm-diameter surface coil over the parietal lobes; binomial water suppression spin-echo sequence; TE = 540 ms; TR = 5 s). Eight normal human infants underwent thalamic region (Thal) and five occipito-parietal (OP) cerebral MRS at 2.4 T [point-resolved spectroscopy (PRESS) localisation; cubic voxel, 8 mL; water suppression off; TE = 270 ms; TR = 2 s]. AWC with T(Cho) , T(Cr) and T(NAA) weighted by the squared Cho, Cr and NAA peak amplitudes provided the smallest intersubject standard deviations: Thal, 0.45°C; OP, 0.33°C (for T(NAA) values of 0.65°C and 1.12°C, respectively). AWC provided resilience against simulated pathological alterations in Cho, Cr and NAA peak amplitudes, with Thal and OP T(AWC) changing by less than 0.04°C. AWC improves both intersubject reproducibility of MRS temperature estimation and resilience against pathological, anatomical and developmental variation of Cho, Cr and NAA peak amplitudes.

Greater hypoxia-induced cell death in prenatal brain after bacterial-endotoxin pretreatment is not because of enhanced cerebral energy depletion: a chicken embryo model of the intrapartum response to hypoxia and infection

Infection is a risk factor for adult stroke and neonatal encephalopathy. We investigated whether exposure to bacterial endotoxin increases hypoxia-induced brain cell death and impairs cerebral metabolic compensatory responses to hypoxia. Prehatching chicken embryos (incubation day 19) were exposed to bacterial lipopolysaccharide (LPS) (3 mg Salmonella typhimurium LPS per egg) or hypoxia (4% ambient O(2) for 1 h), alone or in combination with LPS, followed 4 h later by hypoxia. Cerebral cell death and glial activation were assessed histologically. Further, chicken embryo brains were studied by magnetic resonance imaging (MRI) and spectroscopy (MRS) to assess haemodynamic and metabolic responses. In most brain areas, combined LPS/hypoxia resulted in a 30- to 100-fold increase in terminal deoxynucleotidyl transferase dUTP nick end labelling -positive cells, compared to control and single-insult groups. Glial activation correlated with the severity of cell death and was significantly greater in the combined-insult group (P<0.05). Hypoxia was associated with a 10-fold increase in lactate/N-acetyl-aspartate (NAA), an approximately 20% increase in total creatine/NAA, rapid decreases in T2 and T2(*), and a reduction in direction-averaged brain-water diffusion (D(av)) by approximately 15%. Liposaccharide pretreatment did not alter the magnitude or timing of these responses, but engendered baseline shifts (increased Cho/NAA, Cr/NAA, and Dav, and reduced T2(*)). In conclusion, LPS greatly increased hypoxia-induced brain damage in this model and induced changes in baseline haemodynamics and metabolism but did not affect the magnitude of the glycolytic response to hypoxia. The damage-enhancing effects of LPS are not because of additional energy depletion but because of a synergistic toxic component.

Advances in optical imaging of the newborn infant brain

New methods of imaging the oxygenation, hemodynamics, and metabolism of the newborn infant brain are being developed, based on illumination of the head with near-infrared light. Techniques known as optical topography and optical tomography have the potential to provide valuable information about the function of the normal brain, and about a variety of cerebral pathology such as hypoxic-ischemia. Optical methods provide a unique means of monitoring brain oxygenation safely in an intensive care environment without interference with the normal handling of the infant. Studies on infants have focused on the assessment of steady-state regional cerebral perfusion and tissue oxygenation, as well as monitoring hemodynamic changes in response to sensory stimulation. Recent technological and methodological advances in this research field are reviewed, and the likely impact of optical imaging methods on the care of newborn infants is assessed.

Magnetic resonance proton spectroscopy and diffusion weighted imaging of chick embryo brain in ovo

Metabolic compensatory mechanisms may partly account for the decreased vulnerability to hypoxia observed in the developing brain. We used proton magnetic resonance spectroscopy and diffusion-weighted imaging to measure the cerebral concentrations of lactate and other metabolites, as well as the apparent diffusion coefficient (ADC) of tissue water, before, during and after hypoxia in anaesthetised chick embryos in ovo. Reducing the inspired oxygen concentration to 8% for 40 min caused a significant rise in both mean (+/-S.D.) lactate:creatine and alanine:creatine ratios from 0.58 (0.41) to 1.56 (0.56) and 0.14 (0.14) to 0.29 (0.17), respectively. Under similar hypoxic conditions, ADC did not change from a mean baseline value of 0.95 (0.09), but did fall to 0.40 (0.12) x 10(9) mm(2) s(-1) with further stepwise reductions in oxygenation. Moderate hypoxia increases lactate concentration in the developing chick brain without compromising cellular energy metabolism.

The piglet as an animal model for hypobaric hypoxia

The objective of this study was to evaluate the piglet as a suitable animal model for human diseases of high altitude. We studied 12 piglets, 4-10 weeks old, in a hypobaric chamber under conditions of high altitude at a pressure of 1/2 atmosphere (to approximately 320 Torr) for various periods of time (12, 24, 36, 48, and 72 hours) with continuous monitoring. The animals were decompressed every 24 hours for grooming and feeding. Two animals were studied as nonexposed controls, and one was studied as a control in the chamber without decompression. The animals were euthanized after the exposure, and a complete autopsy was performed. The tissues were then analyzed with light and electron microscopy. The animals all exhibited clinical features of ataxia, tachypnea with Cheyne-Stokes respiration, and lethargy. One animal vomited. The histologic and ultrastructural analysis showed normal organs, particularly lung and brain. The piglet may be a suitable animal model for the study of high altitude-related diseases in humans, but prolonged uninterrupted exposure and a delay in euthanasia after exposure to high altitude may be necessary for the development of reactive pathologic changes.

Biexponential transverse relaxation (T(2)) of the proton MRS creatine resonance in human brain

Differences in proton MRS T(2) values for phosphocreatine (PCr) and creatine (Cr) methyl groups (3.0 ppm) were investigated in studies of phantoms and human brain. Results from phantom studies revealed that T(2) of PCr in solution is significantly shorter than T(2) of Cr. Curve-fitting results indicated that the amplitude-TE curves of the total Cr resonance at 3.0 ppm in human brain (N = 26) fit a biexponential decay model significantly better than a monoexponential decay model (P < 0.006), yielding mean T(2) values of 117 +/- 21 ms and 309 +/- 21 ms. Using a localized, long-TE (272 ms) point-resolved spectroscopy (PRESS) proton MRS during 2 min of photic stimulation (PS), an increase of 12.1% +/- 3.5% in the mean intensity of the total Cr resonance in primary visual cortex (VI) was observed at the end of stimulation (P < 0.021). This increase is consistent with the conversion of 26% of PCr in VI to Cr, which is concordant with (31)P MRS findings reported by other investigators. These results suggest a significantly shorter T(2) for PCr than for Cr in vivo. This difference possibly could be exploited to quantify regional activation in functional spectroscopy studies, and could also lead to inaccuracies in some circumstances when the Cr resonance is used as an internal standard for (1)H MRS studies in vivo.

Early increases in brain myo-inositol measured by proton magnetic resonance spectroscopy in term infants with neonatal encephalopathy

Our aim was to assess brain myo-inositol/creatine plus phosphocreatine (Cr) in the first week in term infants with neonatal encephalopathy using localized short echo time proton magnetic resonance spectroscopy and to relate this to measures of brain injury, specifically lactate/Cr in the first week, basal ganglia changes on magnetic resonance imaging (MRI), and neurodevelopmental outcome at 1 y. Fourteen term infants with neonatal encephalopathy of gestational age (mean +/- SD) 39.6 +/- 1.6 wk, birth weight 3270 +/- 490 g, underwent MRI and magnetic resonance spectroscopy at 3.5 +/- 2.1 d. Five infants were entered in a pilot study of treatment with moderate whole-body hypothermia for neonatal encephalopathy; two were being cooled at the time of the scan. T(1)- and T(2)-weighted transverse magnetic resonance images were graded as normal or abnormal according to the presence or absence of the normal signal intensity of the posterior limb of the internal capsule and signal intensity changes in the basal ganglia. Localized proton magnetic resonance spectroscopy data were obtained from an 8-cm(3) voxel in the basal ganglia using echo times of 40 and 270 ms, and the peak area ratios of myo-inositol/Cr and lactate/Cr were measured. Outcome was scored using Griffith's development scales and neurodevelopmental examination at 1 y. MRI and outcome were normal in six infants and abnormal in eight. myo-Inositol/Cr and lactate/Cr were higher in infants with abnormal MRI and outcome (p < 0.01, p < 0.01, respectively). myo-Inositol/Cr and lactate/Cr were correlated (p < 0.01) and were both correlated to the Griffith's developmental scales (p < 0.01, p < 0.01, respectively). In conclusion, these preliminary data suggest that early increases in brain basal ganglia myo-inositol/Cr in infants with neonatal encephalopathy are associated with increased lactate/Cr, MRI changes of severe injury, and a poor neurodevelopmental outcome at 1 y.

Glutamate in cerebral tissue of asphyxiated neonates during the first week of life demonstrated in vivo using proton magnetic resonance spectroscopy

We tested the hypothesis that glutamate (Glx) levels as demonstrated by proton magnetic resonance spectroscopy ((1)H-MRS) are elevated in brain tissue of neonates with severe hypoxic-ischemic encephalopathy (HIE). Studies were performed in 26 neonates (median gestational age 40.5 weeks, range 36.7-42.4 weeks; median birth weight 3,360 g, range 2,180-4,200 g). The median postnatal age at the time of testing was 2.5 days (range 1-7 days). HIE was scored according to Sarnat as grade I (n = 4), grade II (n = 15) or grade III (n = 7). Results for neonates with mild to moderate HIE (group 1) were compared to those with severe HIE (group 2). After magnetic resonance imaging, (1)H-MRS was performed in a single volume of interest including the basal ganglia. An echo time of 31 ms was used. After curve-fitting procedures, peak area ratios of different brain metabolites were calculated. The median total Glx/N-acetylaspartate ratio was 1.21 (range 0.64-3.25) in group 1 versus 1.55 (range 1.10-2.75) in group 2 (p = 0.035). The median total Glx/choline ratio was 1.33 (range 0.71-2.52) in group 1 versus 2.14 (range 1.21-3.55) in group 2 (p = 0.019). We concluded that during the first days of life, Glx was elevated in the basal ganglia of neonates with severe HIE.

Value of (1)H-MRS using different echo times in neonates with cerebral hypoxia-ischemia

Previous studies have shown altered brain metabolism after cerebral hypoxia-ischemia, using magnetic resonance spectroscopy with echo times (TE) of 272 and 136 ms, based on peak-area or peak-height ratios. The present study examined the additional value of proton magnetic resonance spectroscopy with a short TE (31 ms) to predict a poor outcome in neonates with brain hypoxia-ischemia. Studies were performed in 21 full-term neonates with perinatal asphyxia in a 1.5 tesla magnetic field. Proton magnetic resonance spectroscopy was performed in a single volume of interest including the basal ganglia. TE of 272, 136 and 31 ms were used. After curve-fitting procedures, peak-areas as well as peak-height ratios of different brain metabolites were calculated, comparing patients with a poor versus a good outcome. Seven neonates out of 21 had a poor outcome. Neonates with a poor outcome showed a significantly lower N:-acetylaspartate/choline (NAA/Cho) and a significantly raised lactate/NAA (Lac/NAA) ratio using TE of 272 and 136 ms. Using a TE of 31 ms, no differences were found in glutamate/NAA (Glx/NAA), Glx/Cho, myo-inositol/NAA (mI/NAA), and mI/Cho ratios between neonates with a good and those with a poor outcome. Highest predictive values could be achieved for NAA/Cho with a TE of 136 ms. We conclude that low NAA/Cho and high Lac/NAA ratios predict a poor outcome in neonates with cerebral hypoxia-ischemia. TE of 272 and 136 ms have a better predictive value than a TE of 31 ms.

Proton magnetic resonance spectroscopy of human basal ganglia: response to cocaine administration

BACKGROUND

Proton magnetic resonance spectroscopy was used to determine the effects of intravenous cocaine or placebo administration on human basal ganglia water and metabolite resonances.

METHODS

Long echo time, proton magnetic resonance spectra of water and intracellular metabolites were continuously acquired from an 8-cm(3) voxel centered on the left caudate and putamen nuclei before, during, and after the intravenous administration of cocaine or a placebo in a double-blind manner.

RESULTS

Cocaine, at both 0.2 and 0.4 mg/kg, did not alter the peak area for water. Cocaine at 0.2 mg/kg induced small and reversible increases in choline-containing compounds and N-acetylaspartate peak areas. Cocaine at 0.4 mg/kg induced larger and more sustained increases in choline-containing compounds and N-acetylaspartate peak areas. No changes in either water or metabolite resonances were noted following placebo administration.

CONCLUSIONS

These increases in choline-containing compounds and N-acetylaspartate peak areas may reflect increases in metabolite T2 relaxation times secondary to osmotic stress and/or increased phospholipid signaling within the basal ganglia following cocaine administration. This is the first report of acute, drug-induced changes in the intensity of human brain proton magnetic resonance spectroscopy resonance areas.

Prenatal and perinatal striatal injury: a hypothetical cause of attention-deficit-hyperactivity disorder?

Experimental data indicate a particular vulnerability of striatal neurons in the developing brain, and together with the idea that the striatum is important for context recognition and behavior, these data have led the author to search for subtle striatal lesions, in the form of biochemical changes, in children who have suffered perinatal adverse events. Evidence is presented to demonstrate that the composition of metabolites in the striatum is altered, primarily in the form of an elevated level of lactate, in human neonates who have suffered various perinatal disorders, such as germinal matrix hemorrhage, intrauterine growth retardation, and asphyxia. An elevated level of lactate suggests tissue hypoxia, which may interfere with the formation of frontostriatal circuits and may play a role in the pathogenesis of the behavioral disturbances observed in a proportion of children with a history of perinatal adverse events.

Temporal and anatomical variations of brain water apparent diffusion coefficient in perinatal cerebral hypoxic-ischemic injury: relationships to cerebral energy metabolism

Cerebral apparent diffusion coefficients (ADCs) were determined in nine newborn piglets before and for 48 h after transient hypoxia-ischemia. Phosphorus MRS revealed severely reduced cerebral energy metabolism during the insult and an apparently complete recovery 2 h after resuscitation commenced. At this time, mean ADC over the imaging slice (ADCglobal) was 0.88 (0.04) x 10(-9) m2 x s(-1) (mean (SD)), which was close to the baseline value of 0.92 (0.4) x 10(-9) m2 x s(-1). In seven of the animals, a "secondary" failure of energy metabolism then evolved, accompanied by a decline in ADCglobal to 0.64 (0.17) x 10(-9) m2 x s(-1) at 46 h postresuscitation (P < 0.001 versus baseline). For these seven animals, ADCglobal correlated linearly with the concentration ratio [phosphocreatine (PCr)]/[inorganic phosphate (Pi)] (0.94 < r < 0.99; P < 0.001). A nonlinear relationship was demonstrated between ADCglobal and the concentration ratio [nucleotide triphosphate (NTP)]/[Pi + PCr + 3 NTP]. The ADC reduction commenced in the parasagittal cortex before spreading in a characteristic pattern throughout the brain. ADC seems to be closely related to cerebral energy status and shows considerable potential for the assessment of hypoxic-ischemic injury in the newborn brain.

Diffusion- and T2-weighted increases in magnetic resonance images of immature brain during hypoxia-ischemia: transient reversal posthypoxia

Hypoxic-ischemic changes in brain are detected earlier with diffusion-weighted (DW) than with T2-weighted magnetic resonance (MR) imaging techniques in adults, whereas the response in immature brain is not known. We investigated MR imaging changes prior to, during, and/or after 2 h of hypoxia-ischemia (right carotid artery occlusion + 2 h of hypoxia) in 7-day-old rats anesthetized with isoflurane. In general, within the first 45 min of hypoxia-ischemia there were no changes in the DW or T2-weighted images. By the second hour of hypoxia-ischemia there were marked areas of increased intensity in both the T2 and the DW images, with cortex and striatum being affected prior to thalamus and hippocampus. The area of DW exceeded that of T2 hyperintensities. In the first hour after hypoxia-ischemia there was a transient recovery of hyperintensities on both T2 and DW images. Between 24 and 72 h the hyperintense area on DW images decreased, whereas that on T2-weighted images increased. The distribution of pathological damage assessed histologically correlated with the areas of hyperintensity on the MR images. In contrast to adult brain, early hypoxic-ischemic injury in immature brain is detected as an increase in intensity in both diffusion- and T2-weighted images, indicating a unique alteration in brain water dynamics in this neonatal model of hypoxia-ischemia. These imaging changes and alterations in brain water can rapidly but transiently reverse upon the start of normoxia and reperfusion, suggestive of secondary energy failure or delayed neuronal death.

Proton T2 relaxation of cerebral metabolites during transient global ischemia in rat brain

Putative changes of metabolite T2 relaxation times were investigated before and after a 20-min period of global ischemia in rat brain in vivo (n = 10) using localized proton MRS at different echo times (2.35 T). Neither absolute T2 relaxation times (TE = 20-270 ms) nor time courses of T2-weighted metabolite signals (TE = 135 ms) revealed statistically significant changes during the occlusion or early reperfusion relative to pre-ischemic baseline. These findings are in line with reports of relaxation changes at much later stages and further demonstrate that altered T2 relaxation is not a confounding factor in diffusion-weighted long-TE proton MRS during early ischemic events.

Measurement of cytochrome oxidase and mitochondrial energetics by near-infrared spectroscopy

Cytochrome oxidase is the terminal electron acceptor of the mitochondrial respiratory chain. It is responsible for the vast majority of oxygen consumption in the body and essential for the efficient generation of cellular ATP. The enzyme contains four redox active metal centres; one of these, the binuclear CuA centre, has a strong absorbance in the near-infrared that enables it to be detectable in vivo by near-infrared spectroscopy. However, the fact that the concentration of this centre is less than 10% of that of haemoglobin means that its detection is not a trivial matter. Unlike the case with deoxyhaemoglobin and oxyhaemoglobin, concentration changes of the total cytochrome oxidase protein occur very slowly (over days) and are therefore not easily detectable by near-infrared spectroscopy. However, the copper centre rapidly accepts and donates an electron, and can thus change its redox state quickly; this redox change is detectable by near-infrared spectroscopy. Many factors can affect the CuA redox state in vivo (Cooper et al. 1994), but most significant is likely to be the molecular oxygen concentration (at low oxygen tensions, electrons build up on CuA as reduction of oxygen by the enzyme starts to limit the steady-state rate of electron transfer). The factors underlying haemoglobin oxygenation, deoxygenation and blood volume changes are, in general, well understood by the clinicians and physiologists who perform near-infrared spectroscopy measurements. In contrast, the factors that control the steady-state redox level of CuA in cytochrome oxidase are still a matter of active debate, even amongst biochemists studying the isolated enzyme and mitochondria. Coupled with the difficulties of accurate in vivo measurements it is perhaps not surprising that the field of cytochrome oxidase near-infrared spectroscopy has a somewhat chequered past. Too often papers have been written with insufficient information to enable the measurements to be repeated and few attempts have been made to test the algorithms in vivo. In recent years a number of research groups and commercial spectrometer manufacturers have made a concerted attempt to not only say how they are attempting to measure cytochrome oxidase by near-infrared spectroscopy but also to demonstrate that they are really doing so. We applaud these attempts, which in general fall into three areas: first, modelling of data can be performed to determine what problems are likely to derail cytochrome oxidase detection algorithms (Matcher et al. 1995); secondly haemoglobin concentration changes can be made by haemodilution (using saline or artificial blood substitutes) in animals (Tamura 1993) or patients (Skov & Greisen 1994); and thirdly, the cytochrome oxidase redox state can be fixed by the use of mitochondrial inhibitors and then attempts make to cause spurious cytochrome changes by dramatically varying haemoglobin oxygenation, haemoglobin concentration and light scattering (Cooper et al. 1997). We have previously written reviews covering the difficulties of measuring the cytochrome near-infrared spectroscopy signal in vivo (Cooper et al. 1997) and the factors affecting the oxidation state of cytochrome oxidase CuA (Cooper et al. 1994). In this article we would like to strike a somewhat more optimistic note--we will stress the usefulness this measurement may have in the clinical environment, as well as describing conditions under which we can have confidence that we are measuring real changes in the CuA redox state.

Proton magnetic resonance spectroscopy of the brain during acute hypoxia-ischemia and delayed cerebral energy failure in the newborn piglet

Studies of the brains of severely birth-asphyxiated infants using proton (1H) magnetic resonance spectroscopy (MRS) have shown changes indicating a rise in cerebral lactate (Lac) and a fall in N-acetylaspartate (Naa). The aim of this study was to test two hypotheses: 1) that these changes can be reproduced in the newborn piglet after transient reversed cerebral hypoxiaischemia, and their time course determined; and 2) that changes in Lac peak-area ratios are related to changes in phosphorylation potential as determined by phosphorus (31P) MRS. Eighteen piglets aged < 24 h were anesthetized and ventilated. Twelve underwent temporary occlusion of the carotid arteries and hypoxemia, and six served as sham-operated controls. 1H and 31P spectra were acquired alternately, both during the insult and for the next 48 h, using a 7-tesla spectrometer. During hypoxiaischemia, the median Lac/total creatine (Cr) peak-area ratio rose from a baseline of 0.14 (interquartile range 0.07-0.27), to a maximum of 4.34 (3.33-7.45). After resuscitation, Lac/Cr fell to 0.75 (0.45-1.64) by 2 h, and then increased again to 2.43 (1.13-3.08) by 48 h. At all stages after resuscitation Lac/Cr remained significantly above baseline and control values. Naa/Cr was significantly reduced below baseline and control values by 48 h after resuscitation. The increases in the Lac peak-area ratios were concomitant with the falls in the [phosphocreatine (PCr)*]/ [inorganic phosphate (Pi)] ratio, during both acute hypoxiaischemia and delayed energy failure. The maximum Lac/Naa during delayed energy failure correlated strongly with the minimum [nucleotide triphosphate (NTP)]/[exchangeable phosphate pool (EPP)] (r = -0.94, p < 0.0001). We conclude that both hypotheses have been confirmed.

Metabolic changes in the striatum after germinal matrix hemorrhage in the preterm infant

To investigate the metabolic consequences of germinal matrix hemorrhage (GMH) we used volume-selective 1H magnetic resonance spectroscopy in the striatal region in 12 preterm infants with predominantly small GMH. Both sides of the brain were investigated twice. Metabolite indices were calculated as the metabolite signal, recorded with TR = 1.6 s and TE = 272 ms, divided by the fully relaxed water signal corrected for transverse relaxation time constant (T2) decay. At the first investigation, when the infants were 32.5 +/- 2.0 (mean +/- SD) wk postmenstrual age, the hemorrhage was unilateral or markedly asymmetrical in size in 10 of 12 infants. The lactate index was higher (p < 0.01) and the phosphocreatine + creatine (Cr) (p < 0.05) and N-acetyl-L-aspartate (NAA) (p < 0.05) indices lower in the side with the larger hemorrhage. At the second investigation, 54.1 +/- 2.7 wk postmenstrual age, no sign of a previous GMH could be seen on magnetic resonance imaging in three of 10 infants. Lactate could be detected in two of 10 infants only, and the Cr and NAA indices did not differ between sides. However, the choline index was significantly higher in the side with the larger hemorrhage (p < 0.01). We conclude that GMH is initially followed by lactate accumulation and possibly a delay in maturation as indicated by the transiently low Cr and NAA indices. Moreover, an increased choline index at the corrected age of 3 mo indicates a more persistent metabolic change after small GMH.

Magnesium sulfate after transient hypoxia-ischemia fails to prevent delayed cerebral energy failure in the newborn piglet

Severely birth-asphyxiated human infants develop delayed ("secondary") cerebral energy failure, which carries a poor prognosis, during the first few days of life. This study tested the hypothesis that i.v. magnesium sulfate (MgSO4) after severe transient cerebral hypoxia-ischemia decreases the severity of delayed energy failure in the newborn piglet. Twelve piglets underwent temporary occlusion of the common carotid arteries and hypoxemia. Resuscitation was started when cerebral [phosphocreatine (PCr)]/[inorganic phosphate (Pi)], as determined by phosphorus magnetic resonance spectroscopy, had fallen virtually to zero, and nucleotide triphosphate (NTP) had fallen below a third of baseline. The piglets were randomized to receive, blind, either: 1) three i.v. infusions of 12.5% MgSO4 heptahydrate solution: 400 mg.kg-1 MgSO4.7H2O starting 1 h after resuscitation, and 200 mg.kg-1 12 and 24 h later (n = 6); or 2) three infusions of placebo, 0.9% NaCl (n = 6). Phosphorus and proton spectroscopy were continued until 48 h after resuscitation, and values were compared between the two groups. Mean plasma magnesium levels, 1 h after each of the three doses of MgSO4, were 2.1, 2.0, and 1.9 mmol.L-1, respectively. The severity of the primary insult, determined by the time-integral of depletion of cerebral [NTP]/[exchangeable phosphate pool (EPP)], was similar in the MgSO4-treated and placebo groups. After resuscitation, there was no difference in the progression or severity of delayed energy failure between the two groups, as judged by cerebral [PCr]/[Pi], [NTP]/[EPP], or lactate/creatine and N-acetylaspartate/creatine peak-area ratios. We conclude that MgSO4 did not decrease the severity of delayed cerebral energy failure.

Lactate, N-acetylaspartate, choline and creatine concentrations, and spin-spin relaxation in thalamic and occipito-parietal regions of developing human brain

Previous studies of the brains of normal infants demonstrated lower lactate (Lac)/choline (Cho), Lac/creatine (Cr), and Lac/ N-acetylaspartate (Naa) peak-area ratios in the thalamic region (predominantly gray matter) compared with occipitoparietal (mainly unmyelinated white matter) values. In the present study, thalamic Cho, Cr, and Naa concentrations between 32-42 weeks' gestational plus postnatal age were greater than occipito-parietal: 4.6 +/- 0.8 (mean +/- SE), 10.5 +/- 2.0, and 9.0 +/- 0.7 versus 1.8 +/- 0.6, 5.8 +/- 1.5, and 3.4 +/- 1.1 mmol/kg wet weight, respectively: Lac concentrations were similar, 2.7 +/- 0.6 and 3.3 +/- 1.3 mmol/kg wet weight, respectively. In the thalamic region, Cho and Naa T2s increased, and Cho and Lac concentrations decreased, during development. Lower thalamic Lac peak-area ratios are principally due to higher thalamic concentrations of Cho, Cr, and Naa rather than less Lac. The high thalamic Cho concentration may relate to active myelination; the high thalamic Naa concentration may be due to advanced gray-matter development including active myelination. Lac concentration is higher in neonatal than in adult brain.

Metabolite concentrations and relaxation in perinatal cerebral hypoxic-ischemic injury

Regional cerebral metabolite concentrations, principally of choline-containing compounds (Cho), total creatine (Cr), N-acetylaspartate (Naa), and lactate (Lac), can be quantified by in vivo proton magnetic resonance spectroscopy. In order to estimate a metabolite concentration, it is often necessary to measure the transverse relaxation time (T2). Metabolite T2s depend on cytosolic viscosity: as [adenosine triphosphate] falls leading to Na+/K+ pump failure, cytosolic water increases and T2s lengthen. In central grey-matter in human infants, Naa may be almost exclusively neuronal: Naa T2 may index neuronal edema and energy generation. In this preliminary report, metabolite concentrations and T2s have been measured in central grey matter in human infants suspected of perinatal hypoxic-ischemic cerebral injury. In infants who developed serious cerebral injury or died, [Cho] and [Naa] were low (the latter suggesting neuronal loss), [Lac] and all metabolite T2s were increased: the Naa T2 increase possibly reflected neuronal edema following failure of energy generation in a fraction of remaining neurons.

Proton magnetic resonance spectroscopy of the brain in normal preterm and term infants, and early changes after perinatal hypoxia-ischemia

The aims of this study were 1) to define normal perinatal maturational changes in proton metabolite peak-area ratios in two regions of the neonatal brain, the thalamic and occipitoparietal regions, and 2) to investigate abnormalities of these ratios after perinatal hypoxia-ischemia. Fifty-four infants were studied: 35 normal control infants at 31-42 wk of gestational plus postnatal age, and 19 "asphyxiated" infants suspected of cerebral hypoxic-ischemic injury. Proton spectra were collected at 2.4 tesla from (2 cm)3 voxels using the point-resolved spectroscopy technique with a 270-ms echo time. Lactate was detected in all infants studied. In the normal infants, lactate relative to N-acetylaspartate (NAA), choline and creatine was significantly greater in the occipitoparietal region than in the thalamus, and fell with increasing maturity in both regions, whereas NAA/ choline increased. The 19 asphyxiated infants were studied on a total of 34 occasions during the 1st wk of life (median age 1.8 d), at gestational plus postnatal ages of 27-41 wk. Maximum lactate/NAA was above 95% confidence limits for the control data in one or both regions in 11 of the 19 infants. Minimum NAA/choline was below 95% confidence limits in only one asphyxiated infants, who was later found to have congenital hypothyroidism. SD scores for lactate, relative to NAA, choline, and creatine, were higher in both regions in the asphyxiated infants compared with the normal infants, particularly in the thalamus. Early results of 1-y follow-up examinations indicate that raised lactate/NAA carries a poor long-term prognosis.

Determination of proton metabolite concentrations and relaxation parameters in normal human brain and intracranial tumours

Quantitative proton spectroscopic studies were performed on 39 volunteers and 16 patients with intracranial tumours. Estimates of T2 were obtained in white matter, grey matter, cerebellum, astrocytomas and meningiomas; T1 was determined in white matter only. White matter values of T2 for trimethylamines, creatine and N-acetyl aspartate (NAA) were 309 +/- 84, 195 +/- 41 and 369 +/- 124 ms, respectively (mean +/- SD, n = 20). Metabolite concentrations in white matter were 2.0 +/- 0.4 mumol/g wet weight for trimethylamines, 7.3 +/- 1.1 for creatine and 11.4 +/- 1.4 for NAA. The mean concentrations of creatine and NAA in grey matter and all of three metabolites in cerebellum were greater than those in white matter. Tumour spectra were characterized by increased trimethylamines, reduced creatine and NAA and occasionally the presence of lactate. Meningiomas were further characterized by the presence of alanine. The mean T2 and concentration of trimethylamines in tumours was significantly greater than in normal brain. Creatine and NAA concentrations were decreased in all tumours. The longer T2 of trimethylamines and presence of alanine in meningiomas indicate that important differences exist in membrane and glucose metabolism within these tumours when compared to either astrocytomas or normal brain tissue.

Quantitative combined phosphorus and proton PRESS of the brains of newborn human infants

Techniques for quantitative, combined phosphorus and proton, point-resolved spectroscopy (PRESS) studies of newborn-infant brain have been developed. Phosphorus PRESS advantages include: voxel-shimming; rapid transmitter-pulse setting; novel use of brain-water as a localized quantitation reference; and reduced broad components. Proton spectra from 1-ml voxels and phosphorus spectra can both be acquired quantitatively within acceptable time. Cerebral lactate was consistently detected by proton PRESS and the normal concentration (approximately 3 mmol/kg wet weight) may be higher than in adult brain. Phosphorus PRESS provided metabolite peak-area ratios and concentrations comparable with those obtained using ISIS.