Selective homonuclear polarization transfer for spectroscopic imaging of GABA at 7T

We develop and implement a selective homonuclear polarization transfer method for the detection of 3.0 ppm C-4 GABA resonance by spectroscopic imaging in the human brain at 7T. This single shot method is demonstrated with simulations and phantoms, which achieves comparable efficiency of detection to that of J-difference editing. The macromolecule resonance that commonly co-edits with GABA is suppressed at 7T through use of a narrow band preacquisition suppression pulse. This technique is implemented in humans with an eight channel transceiver array and high degree B(0) shimming to measure supplementary motor area and thalamic GABA in controls (n = 8) and epilepsy patients (n = 8 total). We find that the GABA/N-acetyl aspartate ratio in the thalamus of control volunteers, well controlled and poorly controlled epilepsy patients are 0.053 ± 0.012 (n = 8), 0.090 ± 0.012 (n = 2), and 0.038 ± 0.009 (n = 6), respectively.

J-refocused coherence transfer spectroscopic imaging at 7 T in human brain

Short echo spectroscopy is commonly used to minimize signal modulation due to J-evolution of the cerebral amino acids. However, short echo acquisitions suffer from high sensitivity to macromolecules which make accurate baseline determination difficult. In this report, we describe implementation at 7 T of a double echo J-refocused coherence transfer sequence at echo time (TE) of 34 msec to minimize J-modulation of amino acids while also decreasing interfering macromolecule signals. Simulation of the pulse sequence at 7 T shows excellent resolution of glutamate, glutamine, and N-acetyl aspartate. B(1) sufficiency at 7 T for the double echo acquisition is achieved using a transceiver array with radiofrequency (RF) shimming. Using an alternate RF distribution to minimize receiver phase cancellation in the transceiver, accurate phase determination for the coherence transfer is achieved with rapid single scan calibration. This method is demonstrated in spectroscopic imaging mode with n = 5 healthy volunteers resulting in metabolite values consistent with literature and in a patient with epilepsy.

Hippocampal correlates of pain in healthy elderly adults: a pilot study

BACKGROUND

Few neuroimaging investigations of pain in elderly adults have focused on the hippocampus, a brain structure involved in nociceptive processing that is also subject to involution associated with dementing disorders. The goal of this pilot study was to examine MRI- and magnetic resonance spectroscopy (MRS)-derived hippocampal correlates of pain in older adults.

METHODS

A subset of 20 nondemented older adults was drawn from the Einstein Aging Study, a community-based sample from the Bronx, NY. Pain was measured on 3 time scales: 1) acute pain right now (pain severity); 2) pain over the past 4 weeks (Short Form-36 Bodily Pain); 3) chronic pain over the past 3 months (Total Pain Index). Hippocampal data included volume data normalized to midsagittal area and N-acetylaspartate to creatine ratios (NAA/Cr).

RESULTS

Smaller hippocampal volume was associated with higher ratings on the Short Form-36 Bodily Pain (r(s) = 0.52, p = 0.02) and a nonsignificant trend was noted for higher ratings of acute pain severity (r(s) = -0.44, p = 0.06). Lower levels of hippocampal NAA/Cr were associated with higher acute pain severity (r(s) = -0.45, p = 0.05). Individuals with chronic pain had a nonsignificant trend for smaller hippocampal volumes (t = 2.00, p = 0.06) and lower levels of hippocampal NAA/Cr (t = 1.71, p = 0.10).

CONCLUSIONS

Older adults who report more severe acute or chronic pain have smaller hippocampal volumes and lower levels of hippocampal N-acetylaspartate/creatine, a marker of neuronal integrity. Future studies should consider the role of the hippocampus and other brain structures in the development and experience of pain in healthy elderly and individuals with Alzheimer disease.

Short echo spectroscopic imaging of the human brain at 7T using transceiver arrays

Recent advances in magnet technology have enabled the construction of ultrahigh-field magnets (7T and higher) that can accommodate the human head and body. Despite the intrinsic advantages of performing spectroscopic imaging at 7T, increased signal-to-noise ratio (SNR), and spectral resolution, few studies have been reported to date. This limitation is largely due to increased power deposition and B(1) inhomogeneity. To overcome these limitations, we used an 8-channel transceiver array with a short TE (15 ms) spectroscopic imaging sequence. Utilizing phase and amplitude mapping and optimization schemes, the 8-element transceiver array provided both improved efficiency (17% less power for equivalent peak B(1)) and homogeneity (SD(B(1)) = +/-10% versus +/-22%) in comparison to a transverse electromagnetic (TEM) volume coil. To minimize the echo time to measure J-modulating compounds such as glutamate, we developed a short TE sequence utilizing a single-slice selective excitation pulse followed by a broadband semiselective refocusing pulse. Extracerebral lipid resonances were suppressed with an inversion recovery pulse and delay. The short TE sequence enabled visualization of a variety of resonances, including glutamate, in both a control subject and a patient with a Grade II oligodendroglioma.

Hippocampal extracellular GABA correlates with metabolism in human epilepsy

As the major inhibitory neurotransmitter in human brain, GABA is an important modulator of hyperexcitability in epilepsy patients. Given the high energetic cost of neurotransmission and synaptic activity, GABA concentrations may be hypothesized to correlate with metabolic function. We studied human epilepsy patients undergoing intracranial EEG monitoring for seizure localization to examine microdialysis measures of extracellular GABA (ecGABA), pre-operative MR spectroscopic measures of neuronal mitochondrial function (NAA/Cr), and wherever possible, neuropathology and hippocampal volumetry. Two groups undergoing intracranial monitoring for seizure localization were studied: surgically treated hippocampal epilepsy (MTLE) and neocortical (non-hippocampal seizure onset) epilepsy. All data are hippocampal and thus these groups allow comparisons between the epileptogenic and non-epileptogenic regions. ecGABA was measured using in vivo microdialysis performed during intracranial monitoring. Pre-operative in vivo MR spectroscopic imaging was performed to measure the ratio of N-acetyl aspartate (NAA) to creatine. Standard methods for neuropathology and hippocampal volumetry were used. In the neocortical group, increased ecGABA correlated with greater NAA/Cr (R = +0.70, p < 0.015, n = 12) while in the MTLE group, increased ecGABA linked with decreased NAA/Cr (R = -0.94, p < 0.001, n = 8). In MTLE, ecGABA (increased) and NAA/Cr (decreased) correlated with increased glial cell numbers (R = +0.71, p < 0.01, n = 12, R = -0.76 p < 0.03 respectively). No relationship was seen between ecGABA and hippocampal volumes in either group. In epilepsy, ecGABA increases occur across a range of metabolic function. Outside the seizure focus, ecGABA and NAA/Cr increase together; in contrast, within the seizure focus, ecGABA increases with declining mitochondrial function.

Hippocampal neurochemistry, neuromorphometry, and verbal memory in nondemented older adults

BACKGROUND

Characterization of the behavioral correlates of neuromorphometry and neurochemistry in older adults has important implications for an improved understanding of the aging process. The objective of this study was to test the hypothesis that a measure of hippocampal neuronal metabolism was associated with verbal memory in nondemented older adults after controlling for hippocampal volume.

METHODS

4-T MRI, proton magnetic resonance spectroscopy ((1)H MRS), and neuropsychological assessment were conducted in 48 older adults (23 women; mean age 81 years). Average hippocampal N-acetyl aspartate/creatine ratios (NAA/Cr) and hippocampal volumes were obtained. Neuropsychological evaluation included tests of verbal memory (Buschke and Grober Free and Cued Selective Reminding Test-Immediate Recall [FCSRT-IR], Wechsler Memory Scale-Revised Logical Memory subtest) and attention and executive function (Trail Making Test Parts A and B).

RESULTS

Linear regression analysis indicated that after adjusting for age, hippocampal NAA/Cr was a significant predictor of FCSRT-IR performance (beta = 0.38, p = 0.01, R (2) = 0.21). Hippocampal volume was also a significant predictor of FCSRT-IR performance after adjusting for age and midsagittal area (beta = 0.47, p = 0.01, R (2) = 0.24). In a combined model, hippocampal NAA/Cr (beta = 0.33, p = 0.03) and volume (beta = 0.35, p = 0.03) were independent predictors of FCSRT-IR performance, accounting for 30% of the variance in memory.

CONCLUSIONS

These findings indicate that nondemented older adults with smaller hippocampal volumes and lower levels of hippocampal N-acetyl aspartate/creatine ratio metabolites perform more poorly on a test of verbal memory. The integrity of both the structure and metabolism of the hippocampus may underlie verbal memory function in nondemented elderly.

A subcortical network of dysfunction in TLE measured by magnetic resonance spectroscopy

OBJECTIVE

The goal of this work was to evaluate the relationship between neuronal injury/loss in the hippocampus, thalamus, and putamen in temporal lobe epilepsy (TLE) patients using (1)H magnetic resonance spectroscopic imaging.

METHODS

(1)H spectroscopic images from the hippocampus and thalamus of controls and patients with TLE were acquired at 4 T. The spectroscopic imaging data were reconstructed using an automated voxel-shifting method based on anatomic landmarks providing four, six, and three loci for the hippocampus, thalamus, and putamen, respectively. For correlation analysis, the hippocampal and striatal loci were averaged to provide single estimates of the entire structure, whereas the thalamus was divided into two regions, an anterior and posterior measure, using the average of three loci each.

RESULTS

The ratio of N-acetyl aspartate to creatine (NAA/Cr), a measure of neuronal injury/loss, was significantly reduced in both the ipsilateral and contralateral hippocampi and thalami. NAA/Cr in the ipsilateral hippocampus was significantly correlated with the ipsilateral and contralateral anterior and posterior thalami, putamen, and contralateral hippocampus. In control subjects, the hippocampi were only correlated with each other.

CONCLUSIONS

The data demonstrate that there is significant neuronal injury/loss in both the ipsilateral and contralateral thalami in temporal lobe epilepsy patients, with greater impairment in the anterior portions of the ipsilateral thalamus. The degree of injury/loss in the ipsilateral and contralateral thalamus and putamen is directly correlated with that of the ipsilateral hippocampus. This is consistent with the hypothesis that the impairment and damage associated with recurrent seizures as measured by N-acetyl aspartate originating in the hippocampus results in injury and impairment in other subcortical structures.

Spectroscopic imaging of the pilocarpine model of human epilepsy suggests that early NAA reduction predicts epilepsy

Reduced hippocampal N-acetyl aspartate (NAA) is commonly observed in patients with advanced, chronic temporal lobe epilepsy (TLE). It is unclear, however, whether an NAA deficit is also present during the clinically quiescent latent period that characterizes early TLE. This question has important implications for the use of MR spectroscopic imaging (MRSI) in the early identification of patients at risk for TLE. To determine whether NAA is diminished during the latent period, we obtained high-resolution (1)H spectroscopic imaging during the latent period of the rat pilocarpine model of human TLE. We used actively detuneable surface reception and volume transmission coils to enhance sensitivity and a semiautomated voxel shifting method to accurately position voxels within the hippocampi. During the latent period, 2 and 7 d following pilocarpine treatment, hippocampal NAA was significantly reduced by 27.5 +/- 6.9% (P < 0.001) and 17.3 +/- 6.9% (P < 0.001) at 2 and 7 d, respectively. Quantitative estimates of neuronal loss at 7 d (2.3 +/- 7.7% reduction; P = 0.58, not significant) demonstrate that the NAA deficit is not due to neuron loss and therefore likely represents metabolic impairment of hippocampal neurons during the latent phase. Therefore, spectroscopic imaging provides an early marker for metabolic dysfunction in this model of TLE.

4 T Actively detuneable double-tuned 1H/31P head volume coil and four-channel 31P phased array for human brain spectroscopy

Typically 31P in vivo magnetic resonance spectroscopic studies are limited by SNR considerations. Although phased arrays can improve the SNR; to date 31P phased arrays for high-field systems have not been combined with 31P volume transmit coils. Additionally, to provide anatomical reference for the 31P studies, without removal of the coil or patient from the magnet, double-tuning (31P/1H) of the volume coil is required. In this work we describe a series of methods for active detuning and decoupling enabling use of phased arrays with double-tuned volume coils. To demonstrate these principles we have built and characterized an actively detuneable 31P/1H TEM volume transmit/four-channel 31P phased array for 4 T magnetic resonance spectroscopic imaging (MRSI) of the human brain. The coil can be used either in volume-transmit/array-receive mode or in TEM transmit/receive mode with the array detuned. Threefold SNR improvement was obtained at the periphery of the brain using the phased array as compared to the volume coil.

Regional energetic dysfunction in hippocampal epilepsy

OBJECTIVES

There is increasing evidence for a dysfunctional metabolic network in human mesial temporal lobe epilepsy (MTLE). To further describe this, we evaluated the bioenergetic status in unilateral MTLE inter-regionally and in relation to neuropathology.

MATERIALS AND METHODS

We used whole brain high field (4 T) 31P MR spectroscopic imaging to determine in vivo PCr and ATP, studying n=22 patients (all candidates for hippocampal resection) and n=14 control volunteers. The degree of bioenergetic impairment was assessed by calculating the ratio of PCr to ATP.

RESULTS

Compared to controls, patients demonstrated significant decreases in PCr/ATP from the ipsilateral amygdala and pes (0.84 +/- 0.14, 0.87 +/- 0.10, respectively, patients vs 0.97 +/- 0.15, 0.98 +/- 0.16, controls). In patients, the ipsilateral thalamic energetics positively correlated with contralateral hippocampal energetics. In addition, the ipsilateral thalamic and striatal energetics negatively correlated with hippocampal total glial counts.

CONCLUSIONS

These data are consistent with a view that in MTLE, the bilateral hippocampi, ipsilateral thalamus and striatum are linked in their energetic depression, possibly reflecting the propagation of seizures throughout the brain.

Metabolic differences between multiple sclerosis subtypes measured by quantitative MR spectroscopy

We used quantitative magnetic resonance (MR) spectroscopic imaging with T1-based image segmentation to evaluate the subtypes of multiple sclerosis (MS) (eight patients each group of relapsing-remitting [RR], secondary progressive [SP] and primary progressive [PP]). There was no significant difference in age between the PP group with the RP, SP or control group. We found that the metabolite ratio of choline/NA from the periventricular white matter region was not significantly different between the RR and SP groups. Using an ANOVA, the ratios of periventricular choline/NA or creatine/NA of these combined groups were significantly higher than the PP and control groups. Quantification of these data suggest that the major cause of the elevation of these parameters is due to an increase in choline and creatine in the RR group while NA is decreased in the SP group. Thus, early PP disease appears to be relatively intact with respect to neuronal loss.

Measurement of beta-hydroxybutyrate in acute hyperketonemia in human brain

We report the measurement of D-beta-hydroxybutyrate (BHB) in the brains of six normal adult subjects during acute infusions of BHB. We used high field in vivo (1)H magnetic resonance (MR) spectroscopy in the occipital lobe in conjunction with an acute infusion protocol to elevate plasma BHB levels from overnight fasted levels (0.20 +/- 0.10 mM) to a steady state value of 2.12 +/- 0.30 mM. At this level of hyperketonemia, we determined a tissue BHB level of 0.24 +/- 0.04 mM. No increases in brain lactate levels were seen in these data. The concentrations of BHB and lactate were both considerably lower in comparison with previous data acquired in fasted adult subjects. This suggests that up-regulation of the monocarboxylic acid transporter occurs with fasting.

Differentiation of glucose transport in human brain gray and white matter

Localized 1H nuclear magnetic resonance spectroscopy has been applied to determine human brain gray matter and white matter glucose transport kinetics by measuring the steady-state glucose concentration under normoglycemia and two levels of hyperglycemia. Nuclear magnetic resonance spectroscopic measurements were simultaneously performed on three 12-mL volumes, containing predominantly gray or white matter. The exact volume compositions were determined from quantitative T1 relaxation magnetic resonance images. The absolute brain glucose concentration as a function of the plasma glucose level was fitted with two kinetic transport models, based on standard (irreversible) or reversible Michaelis-Menten kinetics. The steady-state brain glucose levels were similar for cerebral gray and white matter, although the white matter levels were consistently 15% to 20% higher. The ratio of the maximum glucose transport rate, V(max), to the cerebral metabolic utilization rate of glucose, CMR(Glc), was 3.2 +/- 0.10 and 3.9 +/- 0.15 for gray matter and white matter using the standard transport model and 1.8 +/- 0.10 and 2.2 +/- 0.12 for gray matter and white matter using the reversible transport model. The Michaelis-Menten constant K(m) was 6.2 +/- 0.85 and 7.3 +/- 1.1 mmol/L for gray matter and white matter in the standard model and 1.1 +/- 0.66 and 1.7 +/- 0.88 mmol/L in the reversible model. Taking into account the threefold lower rate of CMR(Glc) in white matter, this finding suggests that blood--brain barrier glucose transport activity is lower by a similar amount in white matter. The regulation of glucose transport activity at the blood--brain barrier may be an important mechanism for maintaining glucose homeostasis throughout the cerebral cortex.

Regional differences in intramyocellular lipids in humans observed by in vivo 1H-MR spectroscopic imaging

Regional differences in the content of intramyocellular lipids (IMCL), extramyocellular lipids, and total creatine (TCr) were quantified in soleus (S), tibialis posterior (TP), and tibialis anterior (TA) muscles in humans using in vivo 1H proton spectroscopic imaging at 4 T. Improved spatial resolution (0.25-ml nominal voxel resolution) made it feasible to measure IMCL in S, TP, and TA simultaneously in vivo. The most significant regional difference was found in the content of IMCL compared with extramyocellular lipids or TCr. The concentrations of TCr were found to be 29-32 mmol/kg, with little regional variation. IMCL content was measured to be 4.8 +/- 1.6 mmol/kg tissue wt in S, 2.8 +/- 1.3 mmol/kg tissue wt in TP, and 1.6 +/- 0.9 mmol/kg tissue wt in TA in the order of S > TP > TA (P < 0.05). It is likely that these IMCL values are consistent with the known fiber types of these muscles, with S having the greatest fraction of type I (slow-twitch, oxidative) fibers and TA having a large fraction of type IIb (fast-twitch, glycolytic) fibers.

Alkaline pH changes in the cerebellum of asymptomatic HIV-infected individuals

Human immunodeficiency virus (HIV) infection of the brain causes a complex cascade of cellular events involving several different cell types that eventually leads to neuronal cell death and the manifestation of the AIDS-associated dementia complex (ADC). Upon autopsy HIV-infected individuals show lesions within subcortical regions of the brain, including the cerebellum. Previously we have demonstrated, in primary and cell culture models of rat and human astrocytes, a change in intracellular pH (pH(i)) due to increased Na(+)/H(+) exchange following exposure to inactivated virus or gp120, the major HIV envelope glycoprotein. To further investigate whether any such in vivo pH(i) changes occur in human brains subsequent to HIV infection, we measured the pH(i) of the cerebellum in eight HIV-positive individuals and nine healthy volunteers using (31)P magnetic resonance spectroscopy imaging (MRSI) at high field strength (4.1 T). The results showed a significant difference between the age-adjusted mean pH(i) in the cerebellum in control group and patient groups (7.11 +/- 0.03 vs 7.16 +/- 0.04), and further HIV-infected individuals displayed a significant increase in the number of cerebellar volume elements that were alkaline. We hypothesize that this propensity towards alterations in cerebellar pH(i) may portend later neurological involvement resulting from HIV infection.

Quantitative (31)P spectroscopic imaging of human brain at 4 Tesla: assessment of gray and white matter differences of phosphocreatine and ATP

This report describes the implementation and application of a multicompartment analysis of (31)P spectroscopic imaging data to determine the tissue-specific heterogeneities in metabolite content in the human brain and surrounding tissue. Using this information and a multicompartment regression analysis the phosphocreatine and ATP content of "pure" cerebral gray and white matter, the cerebellum, and skeletal muscle was determined in a group of 10 healthy volunteers. The data were converted to mM units using previously reported values for the T(1)s of phosphocreatine and ATP at 4 T, the water content of human brain, and an external reference for absolute quantification. The phosphocreatine concentration in cerebral gray and white matter, the cerebellum, and skeletal muscle was 3.53 +/- 0.33, 3.33 +/- 0.37, 3.75 +/- 0.66, and 25.8 +/- 2.3 mM, respectively. The ATP concentration in cerebral gray and white matter, the cerebellum, and skeletal muscle was 2.19 +/- 0.33, 3.41 +/- 0.33, 1.75 +/- 0.58, and 8.5 +/- 1.9 mM, respectively. Magn Reson Med 45:46-52, 2001.

Spectroscopic imaging of glutamate C4 turnover in human brain

One-dimensional spectroscopic imaging of (13)C-4-glutamate turnover is performed in the human brain with a 6 cc nominal voxel resolution at 4T. Data were acquired with an indirect detection approach using a short spin echo single quantum (1)H-(13)C heteronuclear editing method and a 7 cm surface coil with quadrature (13)C decoupling coils. To analyze the data as a function of tissue type, T(1)-based image segmentation through the surface coil was performed to determine the gray and white matter contributions to each voxel. The tricarboxylic acid (TCA) cycle rate in gray and white matter was then determined using a two-compartment model with the tissue fractionation derived from the image segmentation. The mean values for the TCA cycle rate for occipital gray and white matter from three volunteers was 0.88 +/- 0.12 and 0.28 +/- 0.13 respectively, in agreement with literature data.

Human brain beta-hydroxybutyrate and lactate increase in fasting-induced ketosis

Ketones are known to constitute an important fraction of fuel for consumption by the brain, with brain ketone content generally thought to be low. However, the recent observation of 1-mmol/L levels of brain beta-hydroxybutyrate (BHB) in children on the ketogenic diet suggests otherwise. The authors report the measurement of brain BHB and lactate in the occipital lobe of healthy adults using high field (4-T) magnetic resonance spectroscopy, measured in the nonfasted state and after 2- and 3-day fasting-induced ketosis. A 9-mL voxel located in the calcarine fissure was studied, detecting the BHB and lactate upfield resonances using a 1H homonuclear editing sequence. Plasma BHB levels also were measured. The mean brain BHB concentration increased from a nonfasted level of 0.05 +/- 0.05 to 0.60 +/- 0.26 mmol/L (after second day of fasting), increasing further to 0.98 +/- 0.16 mmol/L (after the third day of fasting). The mean nonfasted brain lactate was 0.69 +/- 0.17 mmol/L, increasing to 1.47 +/- 0.22 mmol/L after the third day. The plasma and brain BHB levels correlated well (r = 0.86) with a brain-plasma slope of 0.26. These data show that brain BHB rises significantly with 2- and 3-day fasting-induced ketosis. The lactate increase likely results from ketones displacing lactate oxidation without altering glucose phosphorylation and glycolysis.

Measurements of human brain ethanol T(2) by spectroscopic imaging at 4 T

Previous MRS measurements of ethanol in human brain have yielded a range of transverse relaxation times for ethanol methyl resonance at 1.5 T (200-380 ms). To determine the T(2) of the methyl proton resonance of ethanol in human brain, 8 x 8 spectroscopic images were acquired at 16 different TE values. A frequency-selective refocusing pulse was used to suppress J-modulation of the ethanol triplet, permitting nonintegral multiples of 1/J to be used for TE values. The measured T(2) values for the methyl resonances of ethanol, creatine, and N-acetyl aspartate in mixed tissues were 82 +/- 12, 148 +/- 20, and 227 +/- 25 ms, respectively. Regression analysis of the measured T(2) as a function of gray matter content indicates a shorter T(2) value for ethanol in pure white matter compared to that in pure gray matter. Magn Reson Med 44:35-40, 2000.

Statistically driven identification of focal metabolic abnormalities in temporal lobe epilepsy with corrections for tissue heterogeneity using 1H spectroscopic imaging

1H spectroscopic imaging of N-acetyl-aspartate, creatine, and choline has proven to be a sensitive indicator for the lateralization of seizure foci in temporal lobe epilepsy. Previous studies have used right-left comparisons to identify the epileptogenic tissue assuming that alterations due to the disease process outweigh the effects of tissue heterogeneity. To evaluate the effectiveness of tissue heterogeneity corrected analyses, we evaluated three criteria for lateralization of the seizure focus: 1) a statistically driven method adjusted for tissue composition, 2) a single valued threshold, and 3) a single global index of the hippocampus. The statistically driven analysis lateralized all eight patients correctly, whereas the single threshold method incorrectly lateralized one case and the global index failed to identify a significant difference in two cases. These findings indicate that increased accuracy and sensitivity can be obtained by correcting for tissue heterogeneity when analyzing spectroscopy studies of temporal lobe epilepsy.

31P MRS measurement of mitochondrial function in skeletal muscle: reliability, force-level sensitivity and relation to whole body maximal oxygen uptake

The reliability, relation to whole-body maximal oxygen uptake (VO(2max)), and force-level sensitivity of (31)P MRS markers of mitochondrial function were studied in 39 normal-weight women. Following 90 s isometric plantar-flexion exercises at 45, 70 and 100% of maximum voluntary contraction, skeletal muscle mitochondrial function was determined from the phosphocreatine recovery time constant (TC(PCr)), the ADP recovery time constant (TC(ADP)), and the rate of change in PCr during the first 14 s of recovery (OxPhos). VO(2max) was measured on a treadmill. Test-retest measurements were obtained in a subset of seven women. Overall, TC(PCr), TC(ADP) and OxPhos were reproducible for all exercises (coefficients of variation = 2.3-19.3%). With increasing force-level, TC(PCr) was prolonged (29.0 +/- 8.2, 31.9 +/- 9.0 and 35.4 +/- 9.5 s), OxPhos was increased (0.159 +/- 0.081, 0.247 +/- 0.090 and 0.310 +/- 0.114), and TC(ADP) was shortened (22.4 +/- 7.9, 21.3 +/- 6.2, and 19.5 +/- 6.7; p < 0.01). All MRS markers of mitochondrial function were correlated with VO(2max) (r = 0.41-0.72; p < 0.05). These results suggest that measurements of TC(PCr), TC(ADP) and OxPhos yield reproducible results that correlate with whole-body VO(2max) and that vary in force-level sensitivity.

Effect of weight reduction, obesity predisposition, and aerobic fitness on skeletal muscle mitochondrial function

We used (31)P magnetic resonance spectroscopy to measure maximal mitochondrial function in 12 obesity-prone women before and after diet-induced weight reduction and in 12 matched, never-obese, and 7 endurance-trained controls. Mitochondrial function was modeled after maximum-effort plantar flexion from the phosphocreatine recovery time constant (TC(PCr)), the ADP recovery time constant (TC(ADP)), and the rate of change in PCr during the first 14 s of recovery (OxPhos). Weight reduction was not associated with a significant change in mitochondrial function by TC(PCr), TC(ADP), or OxPhos. Mitochondrial function was not different between postobese and never-obese controls by TC(PCr) [35.1 +/- 2.5 (SE) vs. 34.6 +/- 2.5 s], TC(ADP) (22.9 +/- 1.8 vs. 21.2 +/- 1.8 s), or OxPhos (0.26 +/- 0. 03 vs. 0.25 +/- 0.03 mM ATP/s), postobese vs. never-obese, respectively. However, TC(ADP) was significantly faster (14.5 +/- 2. 3 s), and OxPhos was significantly higher (0.38 +/- 0.04 mM ATP/s) in the endurance-trained group. These results suggest that maximal mitochondrial function is not impaired in normal-weight obesity-prone women relative to their never-obese counterparts but is increased in endurance-trained women.

Spectroscopic imaging of the uptake kinetics of human brain ethanol

Previous measurements of the ratio of brain to venous blood alcohol have ranged from 21-100%, depending on the experimental model, pulse sequence, and the concentration reference used. The goal of this study was to evaluate the uptake kinetics and visibility of brain ethanol in comparison to venous blood levels using a pulse sequence that minimizes uncertainties due to differences in J-modulation, T(1), and T(2) between ethanol and the concentration standard. This was achieved using a short TE (24 msec) spin echo sequence with a semiselective refocusing pulse to minimize J-modulation losses of the ethanol. Brain ethanol levels were measured with 10-min time resolution using a 16 x 16 spectroscopic imaging matrix with nominal voxels of 1.44 cc. During the course of the study, the brain/blood alcohol ratio declined from a value of 1.54 +/- 0.74 at 35 min after drinking to a final value of 0.93 +/- 0.16 at 85 min postdrinking. Magn Reson Med 42:1019-1026, 1999.

Non-P(i) buffer capacity and initial phosphocreatine breakdown and resynthesis kinetics of human gastrocnemius/soleus muscle groups using 0.5 s time-resolved (31)P MRS at 4.1 T

High-time-resolution (0.5 s) (31)P MRS has been used to evaluate the initial phosphoreatine (PCr) breakdown and resynthesis kinetics, to calculate the non-P(i)(/non-bicarbonate) buffer capacity (betanon-P(i)(/non-bicarb)), and to calculate the constant relating the change in intracellular pH to the muscle's H(+) efflux rate (lambda). The slope of PCr vs time demonstrates that a slope calculated using the first 10 s of recovery underestimates initial PCr recovery rates by up to 56%. A 1-2 s time window is needed to produce a slope that is statistically equivalent to the slope measured using a 0.5 s time window (p = 0.008, one-way RM-ANOVA, Student-Newman-Keuls multiple comparison test). In addition, there was no delay or acceleration in PCr recovery after a 90 s maximum voluntary contraction (MVC) in normal subjects. This demonstrates that oxidative metabolism is occurring at the end of a 90-s MVC in normal individuals. Fitting recovery data has determined that betanon-P(i)(/non-bicarb) = 24.3 +/- 5.4 slyke (mmol/L/pH unit) and that lambda = 0.129 +/- 0.077 mM/(ph s) for human gastrocnemius/soleus muscle. betanon-P(i)(/non-bicarb) is in agreement with measurements in cat biceps, cat soleus and rat gastrocnemius muscles.

Linear projection method for automatic slice shimming

A fast, reliable automatic slice shimming method is described. In-slice shim adjustments are based on one-dimensional phase mapping of four in-slice linear projections through the slice center. For axial, coronal, and sagittal slices it is shown that all in-slice first-, second-, and third-order spherical harmonic terms of B(0) inhomogeneity can be unequivocally determined and corrected. Through-slice shim adjustment is achieved using a one-dimensional projection of the entire slice or ROI along the slice-selection direction. Applications of this method to single-slice in vivo spectroscopic imaging of human brain have resulted in reproducible, high-quality spectroscopic data. Magn Reson Med 42:1082-1088, 1999.

Measurement of the tricarboxylic acid cycle rate in human grey and white matter in vivo by 1H-[13C] magnetic resonance spectroscopy at 4.1T

13C isotopic labeling data were obtained by 1H-observed/13C-edited magnetic resonance spectroscopy in the human brain in vivo and analyzed using a mathematical model to determine metabolic rates in human grey matter and white matter. 22.5-cc and 56-cc voxels were examined for grey matter and white matter, respectively. When partial volume effects were ignored, the measured tricarboxylic acid cycle rate was 0.72+/-0.22 (mean +/- SD) and 0.29+/-0.09 micromol min(-1) g(-1) (mean +/- SD) in voxels of approximately 70% grey and approximately 70% white matter, respectively. After correction for partial volume effects using a model with two tissue compartments, the tricarboxylic acid cycle rate in pure grey matter was higher (0.80+/-0.10 mol min(-1) g(-1); mean +/- SD) and in white matter was significantly lower (0.17+/-0.01 micromol min(-1) g(-1); mean +/- SD). In 1H-observed/13C-edited magnetic resonance spectroscopy labeling studies, the larger concentrations of labeled metabolites and faster metabolic rates in grey matter biased the measurements heavily toward grey matter, with labeling time courses in 70% grey matter appearing nearly identical to labeling in pure grey matter.

Ketosis and epilepsy: 31P spectroscopic imaging at 4.1 T

PURPOSE

To determine whether changes in the high-energy phosphates occur with use of the ketogenic diet in patients with intractable epilepsy.

METHODS

31P magnetic resonance spectroscopic imaging studies were performed at 4.1 T in seven patients with intractable epilepsy (four Lennox-Gastaut syndrome, one absence, one primary generalized tonic-clonic, and one partial complex) before and after institution of the ketogenic diet. Coronal 1H anatomic imaging also was performed to provide correlation to the 31P data.

RESULTS

Taking the patients as a group, the ratio of phosphocreatine (PC)/gamma-adenosine triphosphate (ATP) measured at baseline (regular diet) compared with that measured after the ketogenic diet showed a small but significant increase from 0.61+/-0.08 to 0.69+/-0.08 (p < 0.05). Comparing the ratio of PCr inorganic phosphorus (Pi) measured at baseline with the postketogenic diet, there was a significant increase from 2.45+/-0.27 to 2.99+/-0.44 (p < 0.05).

CONCLUSIONS

As a group, improvement of energy metabolism occurs with use of the ketogenic diet. This is in agreement with the chronic ketosis studies performed earlier in rodents.

Quantitative spectroscopic imaging of the human brain

A method to provide B1 correction and cerebrospinal fluid (CSF) referencing is developed and applied to spectroscopic imaging of the human brain at 4.1 T using a volume head coil. The B1 image allows rapid determination of the spatially dependent B1 that is then used to compensate for the B1 sensitivity of the spectroscopic sequence. The reference signal is acquired from CSF located in a lateral ventricular position using a point-resolved echo spectroscopy (PRESS) acquisition. The CSF spectrum is also corrected for B1 dependence. Together with T2 and T1 corrections, this method is used to provide quantitative values of N-acetylaspartate (NAA), creatine (Cr), and choline (Ch). The metabolite concentrations obtained from a spectroscopic imaging slice through the ventricles in seven normal controls are in good agreement with previously published literature values. This method is applied in a patient with secondary progressive multiple sclerosis, showing separate areas of abnormalities in both NAA and Cr.

Lateralization of human temporal lobe epilepsy by 31P NMR spectroscopic imaging at 4.1 T

OBJECTIVE

To compare the phosphorous metabolite ratios in the mesial temporal lobe of healthy volunteers (n = 20) with the corresponding ratios in patients with temporal lobe epilepsy (n = 30) using 31P NMR spectroscopic imaging and to lateralize the seizure focus in temporal lobe epilepsy patients using various phosphorous metabolite ratios-phosphocreatine to inorganic phosphate (PCr/Pi), PCr to adenosine triphosphate (PCr/gamma-ATP), and (gamma-ATP/Pi)--and to compare with clinical lateralization results.

METHODS

All 31P NMR spectroscopic imaging studies were performed on a high-field, 4.1 T, whole-body NMR spectroscopic imaging system using a 31P/1H double-tuned volume coil.

RESULTS

We found an average reduction of 15% in the PCr/Pi and gamma-ATP/Pi ratios compared with the corresponding ratios in healthy volunteers in the entire mesial temporal lobe, and more than a 30% reduction in these two ratios in the anterior region of the epileptogenic mesial temporal lobe. These ratios were also reduced significantly in the ipsilateral lobe when compared with their corresponding values in the contralateral lobe. In patients we lateralized the seizure focus, based on these 31P NMR data, and compared the results with the clinical lateralization. The lateralization based on either the PCr/Pi or the gamma-ATP/Pi ratio yielded a correspondence of 70 to 73% with the final clinical lateralization. In the subgroup of patients (n = 9) that needed intracranial EEG for the presurgical lateralization because of inconclusive results from the noninvasive methods, a 78% correspondence was found with the 31P NMR-based lateralization, whereas MRI provided a correspondence of only 33%, and scalp EEG provided a correspondence of only 56%.

CONCLUSIONS

These results suggest the utility of adding the 31P NMR method to the group of noninvasive modalities used for presurgical decision making in temporal lobe epilepsy patients.

A novel k-space trajectory measurement technique

A new k-space trajectory measurement technique is proposed and demonstrated. This technique measures the k-space trajectory, in seconds, using only a few readout lines, using phase values of acquired MR signals. As a result of the technique's efficiency, k-space trajectory measurement using patient data becomes possible. The utility of this techniques is demonstrated in phantom and human studies at 4.1 T.

Evaluation of 31P metabolite differences in human cerebral gray and white matter

31P NMR is commonly used to study brain energetics in health and disease. Due to sensitivity constraints, the NMR measurements are typically made in volumes that do not contain pure gray or white matter. For accurate evaluation of abnormalities in brain metabolite levels, it is necessary to consider the differences in normal levels of 31P metabolites in gray and white matter. In this study, voxels from a three-dimensional spectroscopic image acquisition were analyzed for their dependence on tissue type to assess differences in metabolite levels between gray and white matter. Specifically, gray matter was found to have significantly higher ratios of phosphocreatine (PCr) to gamma-ATP and PCr to the total 31P metabolite signal, whereas pH and the ratio of PCr to inorganic phosphate (Pi) were found to differ insignificantly between gray and white matter. Thus, tissue type can be an important factor to consider for alterations in bioenergetics by 31P NMR spectroscopic studies of the brain.

Measurements of human cerebral GABA at 4.1 T using numerically optimized editing pulses

The goal of this work was to develop and evaluate a numerically optimized inversion pulse to be used with a homonuclear editing sequence to measure human cerebral GABA in vivo at 4.1 T in the occipital lobe. The optimized pulse was constructed using pallindromic symmetry with 30 pulses and 29 delays. The optimized pulse provided greater selectivity than the equivalent bandwidth matched DANTE pulse and sinc shaped DANTE. The improved selectivity reduced the co-editing of the macromolecule resonance, permitting the GABA edited doublet to be resolved in vivo. Using cerebral creatine as a reference, 7.1 mM, the measured GABA level was 1.15 +/- 0.13 mM in the occipital lobe.

Biological and clinical MRS at ultra-high field

The advantages of performing spectroscopic studies at higher field strengths include increased SNR, improved spectral resolution for J-coupled resonances, and improvements in the selectivity of spectral editing schemes. By using pulse sequences that minimize the required echo time, refocus J-evolution, employ low peak B1 requiring pulses and take advantage of spectroscopic imaging methods, these advantages can also be utilized in clinical applications of spectroscopy at high field. In addition to the static measurements measurements of N-acetyl aspartate (NAA), creatine (CR) and choline (CH) which can be performed at 1.5 T, high resolution measurements of glutamate, glutamine, GABA and the incorporation of 13C labeled glucose into glutamate are possible with improved spatial and spectral resolution. These methods have been utilized in patients with seizure disorders and multiple sclerosis to identify, characterize and map the metabolic changes associated with these diseases and their treatment.

A method to measure arbitrary k-space trajectories for rapid MR imaging

A method to measure arbitrary k-space trajectories was developed to compensate for nonideal gradient performance during rapid magnetic resonance (MR) imaging with actively or nonactively shielded gradients at a magnetic field strength of 4.1 T. Accurate MR image reconstruction requires knowledge of the k-trajectory produced by the gradient waveforms during k-space sampling. Even with shielded gradients, residual eddy currents and imperfections in gradient amplifier performance can cause the true k-space trajectory to deviate from the ideal trajectory. The k-space determination was used for spiral gradient-echo imaging fo the human brain. While individual calibrations are needed for new pulse sequences, the method of k-space determination can be used for any sequence of preparation pulses and readout gradient waveforms and should prove useful for other trajectories, including the rastered lines of echo-planar imaging.

A general approach to error estimation and optimized experiment design, applied to multislice imaging of T1 in human brain at 4.1 T

In this report, a procedure to optimize inversion-recovery times, in order to minimize the uncertainty in the measured T1 from 2-point multislice images of the human brain at 4.1 T, is discussed. The 2-point, 40-slice measurement employed inversion-recovery delays chosen based on the minimization of noise-based uncertainties. For comparison of the measured T1 values and uncertainties, 10-point, 3-slice measurements were also acquired. The measured T1 values using the 2-point method were 814, 1361, and 3386 ms for white matter, gray matter, and cerebral spinal fluid, respectively, in agreement with the respective T1 values of 817, 1329, and 3320 ms obtained using the 10-point measurement. The 2-point, 40-slice method was used to determine the T1 in the cortical gray matter, cerebellar gray matter, caudate nucleus, cerebral peduncle, globus pallidus, colliculus, lenticular nucleus, base of the pons, substantia nigra, thalamus, white matter, corpus callosum, and internal capsule.

13C editing of glutamate in human brain using J-refocused coherence transfer spectroscopy at 4.1 T

The method of single quantum 13C editing is analyzed and implemented with water suppressed J-refocused coherence transfer spectroscopy. Analysis of the 13C inversion pulse demonstrates that it is optimally placed into the second echo of the J-refocused sequence. We have used this method to acquire 13C-edited spectra of glutamate from phantoms and in vivo. The turnover of 13C4-labeled glutamate in human brain in vivo was observed in parasagittal gray matter using a volume head coil at 4.1 T with a time resolution of 5.3 min.

Is the intracellular pH different from normal in the epileptic focus of patients with temporal lobe epilepsy? A 31P NMR study

We performed in vivo 31P NMR spectroscopic studies of human brain on a 4.1 T whole-body NMR system. Based on a control group of 20 healthy volunteers, the normal pHi was 7.05 (SD, 0.06; SEM, 0.01) in the left temporal lobe and 7.04 (SD, 0.04; SEM, 0.01) in the right temporal lobe. We also studied a patient group consisting of 13 individuals with unilateral temporal lobe epilepsy. The mean pHi was 7.02 (SD, 0.04; SEM, 0.01) in the ipsilateral lobe and 7.02 (SD, 0.05; SEM, 0.01) in the contralateral lobe. These results clearly show that no statistically significant difference in pHi is observed between the two lobes, either in normal controls or in patients. Also, no significant pHi difference exists between the control group and the patient group. Lateralization in each of the 13 patients with unilateral epilepsy, based on their individual pHi difference between the ipsilateral lobe and contralateral lobe (delta pHi), showed that three patients were nondiagnostic cases because their delta pHis were not significantly different from zero (< or = 0.02), five patients showed small delta pHis consistent with their clinical lateralization, whereas the remaining five patients showed delta pHi-based lateralization opposite to the clinical findings. These results seem to indicate an essentially random distribution around delta pHi = 0 within a very small experimental error of +/-0.02 pH units. pHi obtained from eight different areas in each of the 13 unilateral patients also did not show any significantly nonzero delta pHi values. These results led to the conclusion that even at the excellent spectral resolution and reproducibility of the 4.1 T machine (typical SD of 0.05 pH units), no significant pHi effect, induced by temporal lobe epilepsy, could be detected. Therefore, in this study, delta pHi does not appear to be a clinically useful tool for the lateralization of epileptic foci in patients with temporal lobe epilepsy.

Normalization of contralateral metabolic function following temporal lobectomy demonstrated by 1H magnetic resonance spectroscopic imaging

We studied 10 medically intractable temporal lobe epilepsy (TLE) patients prior to surgery using proton magnetic resonance spectroscopic imaging (MRSI) to localize seizure foci. We found significantly elevated creatine/N-acetylaspartate (Cr/NAA) unilaterally in 8 and bilaterally in 2 patients. Five patients have been studied again 1 year after surgery. In the 2 patients with bilateral temporal seizure onsets, MRSI showed normalization of Cr/NAA in the unoperated contralateral tissue following surgical elimination of seizures. This study suggests that metabolic recovery can occur in contralateral temporal areas following surgical treatment of partial epilepsy.

Quantitative 1H spectroscopic imaging of human brain at 4.1 T using image segmentation

Metabolic differences in the content of N-acetylaspartate (NAA), creatinine (CR), and choline (CH) in cerebral gray and white matter can complicate the interpretation of 1H spectroscopic images. To account for these variations, the gray- and white-matter content of each voxel must be known. To provide these data, a T1-based image segmentation scheme was implemented at 4.1 T. The tissue composition of each voxel was determined using the point-spread function of the spectroscopic imaging acquisition and the segmented anatomical image. Pure gray- and white-matter values for CR/NAA and CH/NAA, and the content of CR, CH, and NAA, were determined using a linear-regression analysis of 984 voxels acquired from 10 subjects using white-matter CR as an internal standard. This information was used to establish means and confidence intervals for CR/NAA and CH/NAA from a voxel of arbitrary tissue composition. Using a single-tailed t test, the extent and locations of the metabolic abnormalities (P < 0.05) in a patient with multiple sclerosis were identified.

Evaluation of multiple sclerosis by 1H spectroscopic imaging at 4.1 T

The authors report on high-field (4.1 T) magnetic resonance 1H spectroscopic imaging studies on eight patients with relapsing remitting multiple sclerosis (mean expanded disability status scale (EDSS) 1.0) and eight normal controls. Using T1-weighted imaging to determine lesion position, the authors found the ratios of choline/N-acetyl (NA) compounds and creatine/NA were increased significantly in the multiple sclerosis (MS) patients relative to controls in lesioned tissue, adjacent to lesion, far removed from lesions as well as in periventricular tissue. The gray matter creatine/NA was mildly increased (P < 0.01) in the MS patients, whereas the elevated gray-matter ratio of choline/NA was of borderline significance (P = 0.13). A more detailed comparison of white-matter and mean gray-matter metabolite values indicates that creatine is increased greatest in areas far from lesions. This is in contrast to choline, which was greatest in lesions, and NA, which was smallest in lesions. It is postulated that the creatine increase may reflect an astrocytic (gliotic) or oligodendrocytic remyelinating process. The increased choline most likely reflects varying levels of inflammation and membrane turnover, whereas the NA decrease is representative of axonal dysfunction or loss.

Spectroscopic imaging of human brain glutamate by water-suppressed J-refocused coherence transfer at 4.1 T

The authors reported the development and implementation of a water-suppressed J-refocused coherence transfer sequence to observe glutamate in human brain at 4.1 T. The sequence is modeled for I2S2 and I2S2M spin systems analytically and plotted for a range of echo times. In this sequence, water suppression and refocusing of J-coupled resonances are achieved through a brief multiple quantum step without significant loss of signal. Phantom data are shown. Human brain spectroscopic imaging of glutamate, acquired with a total echo of 36 ms, demonstrates the application of the sequence to observe gray and white matter differences in glutamate content.

Numerically optimized experiment design for measurement of grey/white matter metabolite T2 in high-resolution spectroscopic images of brain

T2 relaxation measurements for choline (Cho), total creatine (Cr = creatine + phosphocreatine), and N-acetylaspartate (NAA) were made separately in eight healthy volunteers using an average of forty 0.5 cc volumes (20 from grey matter and 20 from white matter) in spectroscopic images with a 32 x 32 resolution and a 240 mm field of view. In grey matter, the means and standard deviations of the T2 values were 186 +/- 23, 149 +/- 10, and 232 +/- 15 ms for Cho, Cr, and NAA, respectively, and in white matter, the mean T2 values were 178 +/- 16, 143 +/- 8, and 228 +/- 16 ms, respectively, with no significant differences between grey and white matter. The high-resolution measurements of T2 values were possible because of experimental planning based on the minimization of predicted fitting uncertainties. Explicit expressions were derived to estimate the uncertainties in T2 values, and it was found that two spectroscopic images with echo times of 50 and 250 ms, respectively, would yield sufficient precision for T2 measurements. The derivation of the expressions, a discussion of their behavior, and the experimental planning and verification are presented.

Water-suppressed one-dimensional 1H NMR chemical shift imaging of the heart before and after regional ischemia

This study tests the hypothesis that brief periods of ischemia result in an increase in myocardial lipids during early reperfusion. We conducted 1H NMR spectroscopy to serially measure myocardial lipids before and after regional ischemia. Localized 1H NMR spectra (spatial resolution of 1.25 mm) were obtained using a one-dimensional chemical shift imaging technique. Two regions, the subendocardium and the subepicardium, were estimated by summing spectral areas from three slices (3.75 mm). Two groups of dogs that underwent a 45 min ischemia and 4 h reperfusion were studied: a group in which the myocardium beneath the surface coil underwent ischemia and reperfusion; and a group in which the ischemic event was distant from the tissue under the surface coil. Microsphere measurements showed significant blood flow reductions in the subepicardium and subendocardium in the ischemic zones during coronary occlusion. Flow returned to baseline values during reperfusion. In the ischemic zone group, the subendocardium, the triglyceride resonance areas decreased by 24% (p < 0.05) during reperfusion. However, subepicardial triglyceride areas were unchanged. Subendocardial creatine areas were also unchanged. The non-ischemic zone group subendocardial triglycerides decreased by 33% (p < 0.05) following ischemia and reperfusion in the remote region. In contrast to the ischemic group, the subepicardial triglyceride resonance areas decreased by 42% (p < 0.05). Subendocardial creatine areas were unchanged. These data show that triglycerides of the ischemic-reperfused subendocardium do not increase during 4 h of reperfusion. Furthermore, they show that the triglycerides resonance areas of the non-ischemic region decrease following remote ischemia and reperfusion.(ABSTRACT TRUNCATED AT 250 WORDS)

3D 31P spectroscopic imaging of the human heart at 4.1 T

High field (4 Tesla) spectroscopic imaging offers the advantages of increased signal-to-noise ratio and the possibility of acquiring high resolution metabolite images. We have applied a three dimensional spectroscopic imaging sequence using a sparse Gaussian sampling method to acquire phosphocreatine (PCr) images of the human heart with 8-cc voxels. PCr images enabled observation of the septum, left ventricular free wall, apex, and skeletal muscle. Quantitative evaluation of the 50 myocardial voxels acquired from 10 studies of healthy adults revealed a PCr/adenosine triphosphate (ATP) ratio of 1.80 +/- 0.32 after correction for saturation effects. Due to the small size of the voxels and the ability to choose the location of the volumes to minimize inclusion of blood, no correction for blood pool ATP was required. The calculated PCr/ATP ratio is in agreement with other studies at 1.5 and 4.0 T.

High resolution neuroimaging at 4.1T

In this article we report on acquisition of high resolution 512 x 512 images at 4.1T using an inversion recovery gradient-echo sequence and a volume head coil developed for high field applications. The Ti values for cerebral white and grey matter were measured to be 834 and 1282 ms, respectively. The partial saturation inversion recovery sequence (Tir 800 ms and TR 2500 ms) provided excellent contrast-to-noise for white to grey matter. Consequently, the images consistently visualized the thalamic nuclear groups, hippocampal fine structure, as well as small draining vessels of the white matter.

Application of high field spectroscopic imaging in the evaluation of temporal lobe epilepsy

Previous spectroscopic imaging studies of temporal lobe epilepsy have used comparisons of metabolite content or ratios to lateralize the seizure focus. Although highly successful, these studies have shown significant variations within each of the groups of healthy subjects and patients. This variation may arise from the natural differences seen in metabolite concentration in gray and white matter, the complex anatomy seen about the hippocampus, and the large voxels typically employed at 1.5 T. Using a 4.1 T whole body system, we have acquired spectroscopic images with 0.5 cc nominal voxels (1 cc after filtering) to evaluate the regional variation in metabolite content of the hippocampus, temporal gray and white matter, midbrain, and cerebellar vermis. Using a threshold value of 0.90 for CR/NAA, a value 90% of all normal hippocampal voxels lay below, we have correctly identified the presence of epileptogenic tissue in patients with unilateral as well as bilateral seizures. By using comparisons to healthy values of the CR/NAA ratio, this method enables the visualization of bilateral disease and provides information on the extent of gray matter involvement.

Concomitant EEG, lactate, and phosphorus changes by 1H and 31P NMR spectroscopy during repeated brief cerebral ischemia

Pilots of high-performance aircraft are subject to transient loss of consciousness due to cerebral ischemia resulting from sudden high gravitational stress. To assess the effects of gravitational stress-induced blackout on cerebral metabolism and electrical function, we developed an animal model in which global cerebral ischemia is produced repeatedly at short intervals. Rats were prepared by ligation of subclavian and external carotid arteries and the right carotid artery was cannulated bidirectionally to measure circle of Willis and systemic pressures. Ischemia was induced by inflation of an occluder about the left carotid artery. Interleaved 31P and 1H NMR spectra were acquired on a 4.7-T Biospec system simultaneously with EEG recordings. We report results from 20 experiments of 30-min duration in which rats were subject to 30 1-min ischemia:reflow cycles of 10I:50R, 20I:40R, 30I:30R, and 40I:20R [numbers are seconds of ischemia (I) and reflow (R) during each 1-min cycle]. During ischemia the graded delivery of the ischemic insult permitted direct correlations between 2- to 5- and 7- to 20-Hz EEG activity and progressive changes in pH, lactate, ATP, phosphocreatine (PCr) and Pi. The best correlations were found between EEG activity and pH and PCr; correlation coefficients ranged from 0.93 to 0.95. A loss of EEG activity was observed without significant sustained energy loss in all but the most severe cycle.

Evaluation of cerebral gray and white matter metabolite differences by spectroscopic imaging at 4.1T

Using a 4.1T whole body system, we have acquired 1H spectroscopic imaging (SI) data of N-acetyl (NA) compounds, creatine (CR), and choline (CH) with nominal voxel sizes of 0.5 cc (1.15 cc after filtering). We have used the SI data to estimate differences in cerebral metabolites of human gray and white matter. To evaluate the origin of an increased CR/NA and CH/NA ratios in gray matter relative to white matter, we measured the T1 and T2 of CR, NA, and CH in gray and white matter using moderate resolution SI imaging. In white matter the T2s of NA, CR, and CH were 233 +/- 27, 141 +/- 18, and 167 +/- 20 ms, respectively, and 227 +/- 27, 140 +/- 16, and 189 +/- 25 ms in gray matter. The T1 values for NA, CR, and CH were 1267 +/- 141, 1487 +/- 146, and 1111 +/- 136 ms in gray matter and 1260 +/- 154, 1429 +/- 233, and 1074 +/- 146 ms in white matter. After correcting for T1 and T2 losses, creatine content was significantly lower in white matter than gray (P < 0.01, t-test), with a white/gray content ratio of 0.8, in agreement with biopsy and in vivo measurements at 1.5 and 2.0T.