Estimation of brain iron in vivo by means of the interecho time dependence of image contrast

An imaging protocol for a quantitative estimation of disease-induced variations in brain iron is proposed and then validated, first, on a phantom and second, on a group of 11 healthy volunteers. The relative estimate of brain iron is achieved from a rate difference image that measures the enhancement, delta R2app, of the transverse relaxation rate of water protons brought about by the heterogeneous accumulation of iron in the glial cells. At 1.5 T, the phantom study demonstrates, over the range 0-6 A/m, a linear dependence of delta R2app on the magnetization difference between microspheres and a paramagnetic gel, with a sensitivity of approximately 2 s-1 A-1 m. In the group of healthy volunteers (mean age 33 +/- 7 years) devoid of disease-related or appreciable age-related accumulations of iron, the precision of delta R2app was still sufficient to distinguish the globus pallidus and the putamen from all of the other iron-containing brain structures in a manner that was significant at the 99% confidence level.

The effect of radiation on normal human CNS as detected by NMR spectroscopy

In a prospective study, proton (1H) and phosphorus (31P) nuclear magnetic resonance spectroscopy were used to search for effects of brain tumor radiotherapy on normal human central nervous system. Phosphorus spectroscopy data at 1.5 T seems to suggest that any radiation induced damage that may occur as a result of therapeutic brain irradiation, does not alter the relative concentrations of phosphorus metabolites or the intracellular pH beyond the limits of normal variation (approximately +/- 20%). Proton spectroscopy, on the other hand, detects post radiation changes in the ratios of certain nuclear magnetic resonance visible metabolites following radiotherapy, particularly choline/N-acetylaspartate, and especially in regions of brain receiving high doses of radiation. Such changes may be indicative of the release of membrane bound choline during radiation induced demyelination of brain. Of interest, we have found elevated metabolite ratios of 31P in normal central nervous system prior to radiotherapy, which persisted throughout the time span of the study in both the ipsilateral and contralateral cerebral hemispheres.

The response of the strongly coupled AB system of citrate to typical 1H MRS localization sequences

The response of the strongly coupled AB system of citrate to the STEAM and PRESS spatial localization sequences is developed analytically and then verified experimentally at 2.35 T. At this field strength, the response predicted by the complete strong-coupling calculation is shown to differ greatly from that of the simpler weak-coupling approximation. It is shown that the strong-coupling effects induce a significant field-strength dependence in the time-dependent modulations of the response to either sequence. These effects are illustrated for the STEAM and PRESS responses at field strengths of 1.5, 2.35, and 4.7 T. Independent of the chosen sequence, the increase in the signal-to-noise (S/N) of the frequency response with increasing field strength is shown to be significantly less for the citrate system than for uncoupled spins. The factors which weaken the S/N dependence of the citrate AB signal with increasing field strength are quantified and discussed.

Relaxation times and microstructures

A discussion is presented of the evaluation of multiple relaxation components from water protons in biological tissue. The principal focus is to draw attention to the way in which limitations in the raw NMR data, such as signal-to-noise ratio, data sampling density and acquisition window width, affect the precision and resolution in the processed multiple component solution of the return to thermal equilibrium. The second issue discussed is the interpretation of these multiple components in terms of microstructural compartments of the biological sample and, thirdly, we outline some of the successes in determining regional and pathological variations in microstructure in the human body in-vivo, using the technique of multiple relaxation components.

An in vitro evaluation of the effects of local magnetic-susceptibility-induced gradients on anisotropic water diffusion in nerve

It has been suggested that the anistropy of the water-diffusion coefficient measured in nerve and in white matter could arise from locally anisotropic background gradients induced by the static field, B0. By utilizing 1) pulse sequences, which minimize the effects of background gradients, and 2) changes in sample orientation, which would maximize the change in the magnitude of these gradients if present, it is shown that in four excised nerves the background gradients do not play a measurable role in the anisotropy of the water-diffusion coefficient at a field strength of 2.35 T. The excised nerves evaluated were the olfactory, trigeminal, and optic nerves of the garfish and the sciatic nerve of the frog.

Quantitative characterization of the eddy current fields in a 40-cm bore superconducting magnet

The temporal and spatial dependence of the eddy current fields, generated by switching off x, y, and z gradients in a 40-cm bore Bruker superconducting magnet, have been studied by measuring the offset frequency of the proton FID obtained from a small spherical sample. The measurements were made with the pre-emphasis unit deactivated. The data obtained at each location were well fitted to a sum of four exponentially decaying components. The shortest decay is, in each case, associated with the decay of the current in the gradient coil itself, while the other decays are associated with three eddy currents with decay times ranging from 13 to 480 ms. Each of the three eddy currents generates an essentially uniform gradient for at least 4 cm on either side of the isocenter. However, when the eddy currents are generated by either the vertical transverse gradient or the longitudinal gradient, the eddy current gradients are also accompanied by a field shift.

Simultaneous 31P MRS of the soleus and gastrocnemius in Sherpas during graded calf muscle exercise

The observation that the amount of lactate formed during hypobaric hypoxia decreases with the severity of hypoxia has become known as the "lactate paradox." We used noninvasive 31P magnetic resonance spectroscopy (MRS) to further probe this problem and explore the nature of muscle metabolism during rest-exercise-recovery transitions in Sherpas indigenous to the high Himalayas of Nepal. MRS data were obtained using a whole body 1-m bore, 1.5-T Phillips Gyroscan spectrometer. Muscle-specific localization of MRS data acquisition was achieved by means of a modified image-selected in vivo spectroscopy sequence (ISIS). The spectra acquired from the medial and lateral gastrocnemius muscle, rich in fast-twitch fibers, were well constrained by selective excitation and by the boundary of the leg. The spectra from a third region contained signals predominantly from the soleus, a muscle formed mainly of slow-twitch fibers. We quantified relative concentration changes in phosphocreatine (PCr), Pi, and ATP during a series of calf muscle work bouts; free ADP concentrations were calculated on the assumption that the creatine phosphokinase reaction was always essentially at equilibrium. Hydrogen ion concentrations were calculated from the chemical shift of Pi, which represents the equilibrium between mono- and diprotonated phosphate. Plantar flexion was quantified using a calf muscle ergometer designed for operation within a 1-m whole body magnet. We found that the concentration of ATP was rigorously regulated and thus did not change despite large changes in ATP turnover rates required through exercise. The relative concentrations of PCr and Pi were linear functions of the percent maximum work rate of the lateral and medial gastrocnemius, but on transition to exercise the fractional concentration changes in these metabolites were much less than the fractional change in muscle ATP turnover rates. The relationship between muscle ATP turnover rate and free ADP concentration was complex; again, a kinetic order of 1 was not observed. In contrast to the gastrocnemius, the soleus muscle sustained much smaller changes in the concentrations of these crucial metabolites during rest-work-recovery transitions. Unlike the situation in most other muscles rich in fast-twitch fibers characterized by lactate-associated acidosis during muscle work, the intracellular pH in gastrocnemius of Sherpas was stable through these protocols, which is consistent with the low lactate production (i.e., with the lactate paradox) observed in indigenous highlanders.

Metabolite-specific NMR spectroscopy in vivo

An outline is presented of metabolite-specific in vivo NMR spectroscopy (particularly in brain). It reviews from a physical spectroscopist's perspective, the need for and the methods of observation of, individual metabolite resonances.

Nuclear magnetic resonance spectroscopy and sensitizer-adduct measurements of photodynamic therapy-induced ischemia in solid tumors

Dunning R3327-AT prostate carcinomas growing in Fischer X Copenhagen rats were treated with interstitial photodynamic therapy (PDT--15 mg/kg Photofrin II 4 hours before illumination with 630-nm light via four parallelly implanted optical fibers) at different light intensities. Forty to 60 minutes after treatment, 31P-nuclear magnetic resonance spectra of tumors in anesthetized animals were obtained at 2.35 Tesla using surface coil localization. Areas under resonance peaks were normalized to the area under the peak of a phosphorus standard positioned at a fixed distance on the opposite side of the surface coil. Tumor concentrations of phosphomonoesters and phosphodiesters showed no change after tumor light doses up to 3000 J. Phosphocreatine, alpha-adenosine triphosphate (ATP), beta-ATP, and gamma-ATP signals decreased and inorganic phosphate signals increased with increasing light doses. The intratumor pH did not change significantly at these short times after PDT. In other R3327-AT and R3327-H tumor-bearing animals, [3H]misonidazole was administered 30 minutes prior to PDT treatments of both tumors. Twenty-four hours later, the tumors were resected in toto, and levels of retained [3H]misonidazole were determined in lased tumor specimens by liquid scintillation procedures. The amount of [3H]misonidazole activity in tumor tissue (covalently bound after hypoxic reduction) increased with light doses up to 3000 J. Sensitizer-adduct formation was found to correlate with the ratio of the concentration of inorganic phosphate to that of beta-ATP, both of which are presumed measures of tumor oxygenation status. These measurements have high-lighted the heterogenous nature of the oxygenation status of these experimental tumors. The precision of each assay for estimating tumor oxygenation is discussed.

RF interference suppression in a cardiac synchronization system operating in a high magnetic field NMR imaging system

An electrocardiographic (ECG) unit suitable for cardiac-synchronized nuclear magnetic resonance imaging in high magnetic fields is presented. The unit includes lossy transmission lines as ECG leads in order to suppress radio frequency (RF) interference in the electrocardiogram. The unit's immunity to RF interference is demonstrated.

Estimation of the iron concentration in excised gray matter by means of proton relaxation measurements

To validate their correlation with tissue iron concentration, proton transverse relaxation measurements have been made at 2.35 T (100 MHz) in 25 samples of excised, frozen, but unfixed human gray matter tissue obtained from the globus pallidus, putamen, caudate, thalamus, and cortex of five postmortem brains free of neurological disease. The iron concentration was independently measured, using atomic absorption spectroscopy. The proton transverse relaxation measurements exploited the interecho time dependence of the apparent transverse relaxation rate, R2app, obtained from a Carr-Purcell-Meiboom-Gill (CPMG) sequence. An empirical semilogarithmic relationship between R2app and the interecho time provided a measure of the relaxation enhancement due to iron, namely, a slope p, which demonstrated a significant correlation (r = 0.78, P < 0.001) with tissue iron concentration. Moreover, a simple rate difference, delta R2app, determined between interecho time values of 6 and 60 ms, was also found to correlate significantly with iron concentration (r = 0.81, P < 0.001). Both of the foregoing correlations were better than that of R2app itself. When the tissue samples were subdivided into brain structure groups, the intergroup differences in rho reflected their known differences in iron accumulation and correlated with those of the mean group iron content, determined by atomic absorption spectrometry.

Chronic lithium does not alter human myo-inositol or phosphomonoester concentrations as measured by 1H and 31P MRS

Lithium may act by decreasing intracellular concentrations of myo-inositol. The present study measured the effects of chronic lithium on myo-inositol concentrations in volunteers. Eleven subjects received either lithium (n = 7) or placebo (n = 4) for 7 days in a double-blind study. Myo-inositol concentrations at baseline and day 8 were measured in vivo using 1H magnetic resonance spectroscopy (MRS). The results showed that lithium did not alter brain myo-inositol concentrations compared to placebo. In 5 other subjects we used 1H MRS and 31P MRS to measure changes in both myo-inositol and phosphomonoester concentrations. This second study showed that lithium did not alter myo-inositol or phosphomonoester concentrations. Thus, the present studies do not support the hypothesis that lithium significantly affects the brain concentrations of myo-inositol or phosphomonoesters; however, it is possible these findings represent an inability to detect the changes in myo-inositol and phosphomonoester concentrations that may have occurred following lithium administration.

Increased basal ganglia iron in striatonigral degeneration: in vivo estimation with magnetic resonance

BACKGROUND

As many as 20% of individuals with the clinical diagnosis of Parkinson's disease (PD) do not have the characteristic neuropathologic features of PD at post mortem. The striatonigral degeneration (SND) subtype of multiple system atrophy is one of the categories of pathology which may be incorrectly diagnosed as PD on the basis of clinical presentation. SND may be associated with increased iron deposition in the putamen which can be detected with magnetic resonance imaging.

METHODS

We have estimated regional brain iron content in a patient with probable SND, using a novel imaging method developed in our laboratory, and have compared the results in this patient to those which we have previously reported in patients with PD and in age-matched controls.

RESULTS

We observed that putamenal iron content was increased in our SND patient, beyond the 95% confidence limit for inclusion in the PD group, even when considering clinical severity. In contrast, pallidal and thalamic iron were within the PD range.

CONCLUSIONS

The demonstration of increased putamenal iron content may be a useful adjunctive investigative procedure in patients with suspected SND.

Development of an in vivo 19F magnetic resonance method for measuring oxygen deficiency in tumors

A 19F magnetic resonance spectroscopy (MRS) approach to measuring hypoxia in experimental tumors in rats at a field strength of 2.35 T has been investigated in a combined study of in situ and excised tumors. The detection of tumor hypoxia is based on the hypoxia marker approach which depends on the selective, covalent binding of a fluorinated 2-nitroimidazole to hypoxic cells. The 19F MRS measurement of in vivo hypoxia marker binding was made at a fixed postinjection time when unbound, circulating marker molecule concentrations had dropped below detectable levels. A correlation between 19F MRS and scintillation counting measures of tumor-bound, tritium-labeled hypoxia marker was observed. There was no correlation between integrated 19F MRS hypoxia marker signals and the in vivo 31P MRS parameters of hypoxia which have been developed to measure normal tissue ischemia. Radiolabeling studies and previous immunohistochemical studies with the fluorinated hypoxia marker support the conclusion that the 19F MRS approach has promise as a physically noninvasive guide to the use of hypoxia-dependent therapies at clinically usable MRS field strengths.

Acute metabolic responses of postpartal dairy cows to subcutaneous glucagon injections, oral glycerol, or both

This study examined the effects of multiple subcutaneous glucagon injections with or without co-administration of oral glycerol on energy status-related blood metabolites and hormones of Holstein dairy cows in the first 2 wk postpartum. Twenty multiparous cows were fed a dry cow ration supplemented with 6 kg of cracked corn during the dry period to increase the likelihood of developing postpartal fatty liver syndrome. Cows with a body condition score of >or=3.5 points (1- to 5-point scale) were assigned randomly to 1 of 4 treatment groups: saline, glucagon, glycerol, or glucagon plus glycerol. Following treatment, serial blood samples were collected over an 8-h period to determine the effects of glucagon and glycerol on blood metabolites and hormones. Treatment effects were determined by comparing the concentrations of metabolites and hormones during the first 4-h period and the entire 8-h period after treatment administration (time 0) with the concentration of the same compounds at time 0 on d 1, 7, and 13 postpartum. Administration of glucagon alone increased concentrations of plasma glucagon and insulin on d 1, 7, and 13 and increased plasma glucose and decreased plasma nonesterified fatty acids (NEFA) on d 7 and 13 postpartum relative to the saline group. Administration of glycerol alone increased plasma glucose on d 7 and plasma triacylglycerols on d 1 postpartum. Glycerol administration also decreased plasma glucagon and NEFA on d 1, 7, and 13 and plasma beta-hydroxybutyrate (BHBA) on d 1 postpartum relative to the saline group. Administration of glucagon plus glycerol increased and sustained concentrations of plasma glucagon, glucose, and insulin on d 1, 7, and 13 and decreased plasma NEFA on d 1, 7, and 13 and BHBA on d 1 and 7. Early postpartal treatment of dairy cows with glucagon plus glycerol increased plasma glucose and insulin, decreased plasma NEFA and BHBA, and increased secretion of liver NEFA as plasma triacylglycerols. This suggests that glucagon and glycerol, when co-administered, act to decrease the likelihood of metabolism-related syndrome development in dairy cows.

Residual dipolar coupling of the Cr/PCr methyl resonance in resting human medial gastrocnemius muscle

This paper presents a detailed analysis of the TE dependence of the creatine methyl proton signal at 3.02 ppm, resulting from a symmetric PRESS sequence applied to the resting human gastrocnemius muscle. The analysis shows that a two-component decay of the central peak of the dipolar-coupled-methyl triplet should be interpreted as the superposition of a rapid ( approximately 34 msec) dipolar dephasing and a less rapid ( approximately 162 msec) transverse relaxation. These data do not support a two-pool hypothesis for TE dependence of this signal. Magn Reson Med 42:421-424, 1999.

Analysis of the temporal and spatial dependence of the eddy current fields in a 40-cm bore magnet

Eddy current fields, generated in an animal-size superconducting NMR magnet by a nominally rectangular pulsed transverse gradient applied in the vertical direction, have been studied by measuring the offset frequency of the proton NMR signal obtained from a small spherical sample. Measurements were made, after various time delays, at nine different locations in the sample space. Analysis of the data shows that the time-dependent fields at all nine locations are quite well accounted for by the superposition of only four independent exponentially decaying components that have time constants in the range from 9 to 400 ms. Two of these were found to be caused by eddy currents generated in the magnet structure. They generate primarily linear gradients, though one of them also produces a B0 shift, indicating a significant asymmetry about the isocenter of the conducting structure in which the eddy current flows. The other two exponentially decaying components, which had very different time constants from the eddy currents and also initial amplitudes of the opposite sign, were generated by the preemphasis unit. This calls into question the procedure used to adjust the preemphasis unit and an alternative method is proposed.

The influence of oxygenation on the 19F spin-lattice relaxation rates of fluosol-DA

The spin-lattice relaxation rates (1/T1) were measured at 94.1 MHz for six peaks in the 19F NMR spectrum of the perfluorochemical blood substitute fluosol-DA, which contains a mixed emulsion of perfluorodecalin and perfluorotripropylamine. Each of these rates increased linearly with the percentage of oxygen dissolved in the emulsion. Relative values of the linear increase for different peaks established that, for perfluorotripropylamine in the mixed emulsion, the oxygen-fluorocarbon interaction is loosely but preferentially oriented in a manner similar to that previously established for other pure fluorocarbons. The uncertainty in the oxygen level estimated from T1 measurements is somewhat less than 5% O2 and it is thus established that quantitative non-invasive oxygenation measurements can be made to sufficient precision by this approach, using fluosol-DA and 19F spin-lattice relaxation.

Solvent proton relaxation of aqueous solutions of the serum proteins alpha 2-macroglobulin, fibrinogen, and albumin

The longitudinal, transverse, and spin-locked rotating frame relaxation rates have been measured for water protons in aqueous solutions of the human serum proteins albumin, fibrinogen, and alpha 2-macroglobulin in the physiological concentration range below 50 g/liter, corresponding to an upper limit for molarity of 725, 147, and 69 microM, respectively. The linear concentration dependence of all the relaxation rates measured at 100 MHz was used to provide the molar sensitivities of each relaxation process for each of the protein solutes. Both the solute dependence and the relaxation-process dependence of the molar sensitivities have been analyzed in terms of a model that has emerged from previous R1 dispersion measurements. This analysis demonstrates consistency between our data and that model for the active motions and their motional rates.

Cardiovascular adaptations in Andean natives after 6 wk of exposure to sea level

Six male Quechua Indians (34.0 +/- 1.1 yr, 159.5 +/- 2.1 cm, 60.5 +/- 1.6 kg), life-long residents of La Raya, Peru (4,350-m altitude with an average barometric pressure of 460 Torr), were studied using noninvasive methods to determine the structural and functional changes in the cardiovascular system in response to a 6-wk deacclimation period at sea level. Cardiac output, stroke volume, and left ventricular ejection fractions were determined using radionuclide angiographic techniques at rest and during exercise on a cycle ergometer at 40, 60, and 90% of a previously determined maximal O2 consumption. Subjects at rest were subjected to two-dimensional and M-mode echocardiograms and a standard 12-lead electrocardiogram. Hemoglobin and hematocrit were measured on arrival at sea level by use of a Coulter Stacker S+ analyzer. After a 6-wk deacclimation period, all variables were remeasured using the identical methodology. Hemoglobin values decreased significantly over the deacclimation period (15.7 +/- 1.1 to 13.5 +/- 1.2 g/dl; P less than 0.01). The results indicate that the removal of these high-altitude-adapted natives from 4,300 m to sea level for 6 wk results in only minor changes to the cardiac structure and function as measured by these noninvasive techniques.

An evaluation of the effect of fasting on the exercise-induced changes in pH and Pi/PCr from skeletal muscle

The changes in the response of skeletal muscle to maximal dynamic exercise were investigated in going from a fasted state to a refed state by means of 31P NMR spectroscopy. It was found that in the fasted state, exercise-induced changes in Pi/PCr and in pH were both inhibited, in comparison with those in the refed state.