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SUMMARY: Although the concentration of N-acetylaspartate (NAA) is often used as a neuronal integrity marker, its normal temporal variations are not well documented. To assess them over the 1–2 year periods of typical clinical trials, the whole-brain NAA concentration was measured longitudinally, over 4 years, in a cohort of healthy young adults. No significant change (adjusted for both sex and age) was measured either interpersonally or intrapersonally over the entire duration of the study.

Although most mild traumatic brain injury (mTBI) patients suffer any of several post-concussion symptoms suggestive of thalamic involvement, they rarely present with any MRI-visible pathology. The aim here, therefore, is to characterize their thalamic metabolite levels with proton MR spectroscopy (1H-MRS) compared with healthy controls.T1-weighted MRI and multi-voxel 1H-MRS were acquired at 3 Tesla from 20 mTBI (Glasgow Coma Scale score of 15-13) patients, 19-59 years old, 0-7 years post-injury; and from 17 age and gender matched healthy controls. Mixed model regression was used to compare patients and controls with respect to the mean absolute N-acetylaspartate (NAA), choline (Cho) and creatine (Cr) levels within each thalamus.The mTBI-induced thalamic metabolite concentration changes were under +/- 13.0% for NAA, +/- 13.5% for Cr and +/- 18.8% for Cho relative to their corresponding concentrations in the controls: NAA: 10.08 +/- 0.30 (mean +/- standard error), Cr: 5.62 +/- 0.18 and Cho: 2.08 +/- 0.09 mM. These limits represent the minimal detectable differences between the two cohorts.The change in metabolic levels in the thalamus of patients who sustained clinically defined mTBI could be an instrumental characteristic of 'mildness'. 1H-MRS could, therefore, serve as an objective laboratory indicator for differentiating 'mild' from more severe categories of head-trauma, regardless of the presence or lack of current clinical symptoms.

Fast, high-resolution, longitudinal relaxation time (T1) mapping is invaluable in clinical and research applications. It has been shown that two spoiled gradient recalled echo (SPGR) images acquired in steady state with variable flip angles is an attractive alternative to the multi-image sets previously acquired with inversion or saturation recovery. The known sensitivity of the two-point method to transmit radiofrequency field (B1) inhomogeneity exacerbated at 3 T and above, however, mandates its combination with an additional, time-consuming and possibly specific-absorption-rate-intensive B1 measurement, preventing direct migration of the method to these fields. To address this, we introduce a method designed to be free of systematic errors caused by B1 inhomogeneity in which the value of T1 is extracted from three SPGR images acquired with echo planar imaging (EPI) readout. The precision of the T1 maps produced is found to be comparable to the two-point method, while the accuracy is greatly improved in the same time and spatial resolution. A welcome byproduct of the method is a map of B1 that can be used to correct other acquisitions in the same session. Tables of the optimal acquisition protocols are provided for several total imaging times.

The transverse relaxation times, T(2), of N-acetylaspartate (NAA), total choline (Cho), and creatine (Cr) obtained at 3T in several human brain regions of eight healthy volunteers are reported. They were obtained simultaneously in 320 voxels with three-dimensional (3D) proton MR spectroscopy ((1)H-MRS) at 1 cm(3) spatial resolution. A two-point protocol, optimized for the least error per given time by adjusting both the echo delay (TE(i)) and number of averages, N(i), at each point, was used. Eight healthy subjects (four males and four females, age = 26 +/- 2 years) underwent the hour-long procedure of four 15-min, 3D acquisitions (TE(1) = 35 ms, N(1) = 1; and TE(2) = 285 ms, N(2) = 3). The results reveal that across all subjects the NAA and Cr T(2)s in gray matter (GM) structures (226 +/- 17 and 137 +/- 12 ms, respectively) were 13-17% shorter than the corresponding T(2)s in white matter (WM; 264 +/- 10 and 155 +/- 7 ms, respectively). The T(2)s of Cho did not differ between GM and WM (207 +/- 17 and 202 +/- 8, respectively). For the purpose of metabolic quantification, these values justify to within +/-10% the previous use of one T(2) per metabolite for 1) the entire brain and 2) all subjects. These T(2) values (which to our knowledge were obtained for the first time at this field, spatial resolution, coverage, and precision) are essential for reliable absolute metabolic quantification.

Primary objective: To investigate the relationship between the number of dilated Virchow-Robin spaces (VRS) and neurocognitive findings in patients with traumatic brain injury (TBI).Research design: Thirty-eight patients with TBI and 21 controls were studied.Methods and procedures: Fifteen patients underwent MRI within a mean interval of 5.4 (range 1–12) days from the brain injury and 23 after an average period of 5.5 (range 0.2–31) years. All subjects were examined with a battery of 13 neuropsychological tests (NP).Main outcomes and results: The average number of VRS was significantly higher in patients than in controls. There were no significant differences between patients and controls in terms of NP tests. The number of VRS showed a significant inverse correlation with processing speed and a positive correlation with visual perceptual of attention only in patients studied within a short delay of trauma.Conclusions: VRS are not directly associated to neurocognitive findings, suggesting that they may re...

Summary: We report the findings from whole-brain proton MR spectroscopy, quantifying the neuronal marker N-acetylaspartate (NAA), for 2 presurgical meningioma patients and 10 healthy controls. The patients’ whole-brain NAA (WBNAA) concentrations were considerably elevated (3+ SDs) compared with healthy controls when excluding the tumors from brain volume; WBNAA levels normalized following correction to approximate “preneoplastic” brain size. These results suggest global neuronal preservation in these 2 patients while their brains were compressed by large, slowly growing, extra-axial masses.

Object. Diffuse axonal injury (DAI) is a major complication of traumatic brain injury (TBI) that leads to functional and psychological deficits. Although DAI is frequently underdiagnosed by conventional imaging modalities, it can be demonstrated using diffusion tensor imaging. The aim of this study was to assess the presence and extent of DAI in patients with mild TBI. Methods. Forty-six patients with mild TBI and 29 healthy volunteers underwent a magnetic resonance (MR) imaging protocol including: dual—spin echo, fluid-attenuated inversion recovery, T2-weighted gradient echo, and diffusion tensor imaging sequences. In 20 of the patients, MR imaging was performed at a mean of 4.05 days after injury. In the remaining 26, MR imaging was performed at a mean of 5.7 years after injury. In each case, mean diffusivity and fractional anisotropy were measured using both whole-brain histograms and regions of interest analysis. No differences in any of the histogram-derived measures were found between patients and control volunteers. Compared with controls, a significant reduction of fractional anisotropy was observed in patients' corpus callosum, internal capsule, and centrum semiovale, and there were significant increases of mean diffusivity in the corpus callosum and internal capsule. Neither histogram-derived nor regional diffusion tensor imaging metrics differed between the two groups. Conclusions. Although mean diffusivity and fractional anisotropy abnormalities in these patients with TBI were too subtle to be detected with the whole-brain histogram analysis, they are present in brain areas that are frequent sites of DAI. Because diffusion tensor imaging changes are present at both early and late time points following injury, they may represent an early indicator and a prognostic measure of subsequent brain damage.

To quantify, with three-dimensional proton magnetic resonance (MR) spectroscopy, metabolic characteristics of normal-appearing white matter and nonenhancing lesions in patients with relapsing-remitting multiple sclerosis (MS).Institutional review board approval and informed patient consent were obtained. Nine patients with relapsing-remitting MS (six women, three men) and nine age-matched control subjects (seven women, two men) were studied with T1- and T2-weighted MR imaging and three-dimensional proton MR spectroscopy at spatial resolution less than a cubic centimeter. Absolute N-acetylaspartate (NAA), creatine (Cr), and choline (Cho) levels were obtained from 171 voxels: 66 from lesions on T2-weighted MR images (43 hypointense and 23 isointense on T1-weighted MR images), 31 from normal-appearing white matter, and 74 from analogous normal white matter regions on images in control subjects.Mean NAA level in hypointense lesions (5.30 mmol/L +/- 2.27 [standard deviation]) was significantly lower (Por = .05) than that in isointense lesions (7.82 mmol/L +/- 2.28), normal-appearing white matter (7.37 mmol/L +/- 1.71), and normal white matter in control subjects (8.89 mmol/L +/- 1.54). Cho (1.79 mmol/L +/- 0.65) and Cr (5.64 mmol/L +/- 1.50) levels in isointense lesions were indistinguishable from those in normal-appearing white matter (1.74 mmol/L +/- 0.46 and 4.99 mmol/L +/- 0.97, respectively) but were significantly higher (Cho, 20%; Cr, 24%) than those in normal white matter in control subjects (1.44 mmol/L +/- 0.40 and 4.30 mmol/L +/- 1.32, respectively). NAA, Cho, and Cr levels in normal-appearing white matter were significantly different than those in normal white matter in control subjects (NAA, 20% lower; Cho, 14% higher; and Cr, 17% higher).Abnormal metabolic activity persists in all MS tissue types. Increased Cr and Cho levels suggest (a) ongoing gliosis and attempted remyelination in isointense lesions on T1-weighted MR images and (b) membrane turnover (de- and remyelination), in addition to increased cellularity (gliosis, inflammation) in normal-appearing white matter.

The metabolic changes in the deep gray matter (GM) nuclei, thalamus, and basal ganglia of patients with relapsing-remitting multiple sclerosis were investigated with quantitative, multivoxel, three-dimensional proton MR spectroscopy. This technique facilitated the study of several bilateral structures in a single session at sub-cubic centimeter spatial resolution. Compared with 9 matched control subjects, the deep GM nuclei of 11 patients showed 7% lower N-acetylaspartate and 14% higher choline levels (p = 0.02 for both).

Global brain atrophy estimated using MRI and whole brain N-acetylaspartate (WBNAA) concentration measured with proton MR spectroscopy were obtained in 42 patients with relapsing-remitting multiple sclerosis and 41 matched control subjects. Patients exhibited cross-sectional atrophy (0.5%; p = 0.033) and WBNAA decline (1.8%/y; p = 0.005) vs disease duration. The 3.6-fold rate disparity between the two processes suggests that neuronal/axonal dysfunction (N-acetylaspartate decline) precedes parenchyma loss, not its consequence (i.e., is an earlier, more sensitive specific metric of the ongoing disease activity).

Multiple sclerosis (MS) has traditionally been viewed as an inflammatory demyelinating white matter (WM) disease of the central nervous system. However, recent pathology and MRI studies have shown lesions in the gray matter (GM) as well. To ascertain the extent of GM involvement, we obtained with nonlocalizing proton MR spectroscopy the concentration of N-acetylaspartate (NAA), a metabolite found almost exclusively in neuronal cells, T2-lesion loads, and GM and WM fractions in the entire brain of 71 relapsing-remitting (RR) MS patients (51 women, 20 men, 25-55 years old) and 41 healthy controls (27 women, 14 men, 23-55 years old). The average whole-brain NAA (WBNAA) difference between the patients and the controls was -2.9 mM (-22%,P

BACKGROUND AND PURPOSE: Persistent T1-hypointense lesions (“black holes”) are thought to represent permanent damage of brain parenchyma. We attempted to ascertain whether the metabolic profiles of these hypointense areas support this hypothesis and whether these profiles correlate with these hypointense findings. METHODS: Four patients with relapsing-remitting multiple sclerosis and four matched control volunteers underwent MR imaging and 3D proton MR spectroscopy. Absolute levels of N-acetylaspartate (NAA), creatine, and choline (Cho) were obtained in 0.19 cm(3) voxels containing 14 T1-hypointense lesions (average volume, 0.4 cm(3); range, 0.2–1.0 cm(3)) in patients. Metabolite levels were analyzed, by using Pearson correlation, against their respective lesions’ hypointensity relative to the surrounding normal-appearing white matter. RESULTS: Moderate correlation, r = 0.56, was found between the NAA level and MR imaging hypointensity. Of the 14 lesions studied, 12 were deficient in NAA and 11 had excess Cho compared with corresponding brain regions in control volunteers. Only one lesion was significantly deficient in all three metabolites, indicative of total damage or matrix loss. CONCLUSION: No relationship was found between the hypointensity of the lesions and their metabolic profile. Specifically, lesions with the same hypointensity on T1-weighted MR images were metabolically variable (ie, displayed disparate metabolite levels and behavior). Also, although 86% of the lesions exhibited abnormally low NAA, 71% also had increased Cho. This indicates that although neuronal damage had already occurred (lower NAA), these lesions were still “smoldering” with active membrane turnover (high Cho), most likely because of de- and remyelination, indicative of shadow plaques (remyelinated lesions). Consequently, relapsing-remitting hypointense lesions represent neither final-stage nor static pathologic abnormality.

Although axonal pathology is recognized as one of the major pathological features of multiple sclerosis, it is less clear how early in its course it occurs and how it correlates with MRI-visible lesion loads. To assess this early axonal pathology, we quantified the concentration of whole-brain N-acetylaspartate (WBNAA) in a group of patients at the earliest clinical stage of the disease and compared the results with those from healthy controls. Conventional brain MRI and WBNAA using unlocalized proton magnetic resonance spectroscopy were obtained from 31 patients at presentation with clinically isolated syndromes suggestive of multiple sclerosis and paraclinical evidence of dissemination in space, and from 16 matched controls. An additional conventional MRI scan was obtained in all patients 4-6 months later to detect dissemination of lesions in time. The mean WBNAA concentration was significantly lower in patients compared with the controls (P < 0.0001). It was not significantly different between patients with and without enhancing lesions at the baseline MRI or between patients with and without lesion dissemination in time. No correlation was found between WBNAA concentrations and lesion volumes. Widespread axonal pathology, largely independent of MRI-visible inflammation and too extensive to be completely reversible, occurs in patients even at the earliest clinical stage of multiple sclerosis. This finding lessens the validity of the current concept that the axonal pathology of multiple sclerosis is the end-stage result of repeated inflammatory events, and argues strongly in favour of early neuroprotective intervention.

Multiple sclerosis (MS) is the most common demyelinating disease of the central nervous system, affecting nearly 350 000 Americans, 100 000 Britons, and over 2 million people worldwide. It is the leading cause of nontraumatic neurological disability in young and middle-aged adults [1]. Roughly 85% of MS patients, two-thirds of whom are women, experience acute symptoms over short (weeks) periods, followed by variable, unpredictable lengths (months to years) of partial or complete remission, entering the relapsing-remitting (RR) stage. These cycles continue, leading to accumulating clinical disability from incomplete remissions. After 10 years, 50% will enter the secondary progressive phase of the disease [2]. This progression entails chronic clinical deterioration, increasing motor, sensory, and cognitive deficits, but not a significant reduction of life expectancy [2].

BACKGROUND AND PURPOSE: The T2-weighted MR imaging total lesion volume and Expanded Disability Status Scale (EDSS) score are two common measures of relapsing-remitting multiple sclerosis disability and pathologic abnormality. Because the whole-brain N-acetylaspartate concentration is considered to be a new marker of the disease burden, the purpose of this study was to evaluate the relationship among these three measures. METHODS: The whole-brain N-acetylaspartate concentration and T2-weighted lesion volume were quantified by using MR imaging and proton MR spectroscopy in 49 patients with relapsing-remitting multiple sclerosis (36 female and 13 male patients; average age, 39 years; age range, 24–55 years; average EDSS score, 2; range of EDSS scores, 0–6). Correlations among whole-brain N-acetylaspartate concentrations, T2-weighted lesion volumes, and EDSS scores were obtained. RESULTS: No correlation was found between whole-brain N-acetylaspartate levels and either T2-weighted lesion volumes or EDSS scores. A weak correlation was found between the EDSS scores and T2-weighted lesion volumes (P = .043, r(s) = 0.292). CONCLUSION: Despite the lack of correlation between whole-brain N-acetylaspartate concentration and the clinical disability reflected in the EDSS score, only the former evaluates the global neuronal cell disease in the entire brain, including those lesions that are occult to conventional imaging techniques.

The impetus for the development and implementation of proton magnetic resonance spectroscopy (1H-MRS) in multiple sclerosis (MS) comes from the desire to understand the biochemical nature of the disease. Magnetic resonance (MR) is a highly sensitive but rather nonspecific window on brain pathology. MR specificity is of critical importance to successful correlation of imaging and histopathology. The diverse pathology of MS provides opportunities for multiple treatment strategies. Thus, identifying particular biochemical abnormalities may be viewed as the initial step in categorizing the disease process. MS lesions are disseminated over the entire central nervous system, yet until recently (1996) MRS methodology involved just single-voxel experiments. Although important data can be obtained in this way, much abnormal brain is excluded from the measurement. The following descriptions represent an approach that facilitates metabolic measures on a larger scale. The rationale for three-dimensional (3D) 1H-MRS and whole-brain N-acetylaspartate (WBNAA) quantification is that the diffuse nature of MS necessitates measures that take into account as much brain as possible. Additionally, the 3D 1H-MRS technique enables us to focus on lesions and activity (defined by gadolinium enhancement) in a prospective manner. This chapter will briefly review the current methodology and discuss two new methods utilizing a more global approach to MRS, in vivo brain 1H-MRS methods.

The intra-individual and inter-individual variations of the global N-acetylaspartate (NAA) concentration were measured in a cohort of five 42+/-5 year-old normal females. The total NAA signal from the whole head was obtained with non-localized non-echo proton spectroscopy (1H-MRS) and converted into absolute mole amounts using phantom replacement. Since NAA is assumed to be present only in neurons, its concentration was obtained by dividing these mole amounts with the brains' volume, calculated from high resolution MRI. The key feature of the procedure is its near-complete suppression of the intense subcutaneous and bone marrow lipids' signals, whose chemical shifts neighbor and underlay the NAA. This was achieved by exploiting the lipids' much shorter T1s, compared to that of NAA, for destructive interference of their signals in co-addition following alternating, nonselective 180 degrees inversions. The average global, inter-individual NAA concentration in that group was found to be 10.63 mM with a 95% confidence interval of 10.43-10.82 mM.

Multivoxel 3D localized proton spectroscopy using a hybrid of 1D 8th-order transverse Hadamard spectroscopic imaging (HSI) with 2D chemical shift imaging (CSI) is demonstrated in human brain. The spatially selective HSI pulse incorporates naturally into the PRESS sequence (TE = 135 ms), which then both excites an 8 x 8 x 6 cm parallelepiped volume of interest (VOI) and subdivides it into eight slices. The planes of these slices are further partitioned into 16 x 16 voxel arrays using 2D CSI to yield 8 x 8 x 8 voxels within the VOI. Simultaneous 3D coverage yields good voxel signal-to-noise (8, 12, and 22 for choline, creatine, and N-acetylaspartate, respectively) from these 0.75-ml voxels, in approximately 45 min. The high spatial isolation allows localization to within less than 1 cm from the skull without fat contamination.

Refocused insensitive nucleus enhancement by polarization transfer (RINEPT) from protons (1H) to a J-coupled phosphorus (31P) has been incorporated into three-dimensional (3D) chemical-shift-imaging (CSI) sequence on a clinical imager. The technique is demonstrated on a phantom and in in vivo human brain. The polarization-transfer efficiency (approximately 1.2) is lower than the theoretical maximum of gamma1H/gamma31P approximately 2.4 resulting from 1H-1H homonuclear J couplings of similar magnitude competing with the 1H --> 31P transfer. Nevertheless, compared with direct 31P Ernst-angle excitation, signal gains of up to x1.8 were obtained mainly as a result of T1 differences between 31P and the 1H. Spectral interpretation is simplified by editing out all non-proton-coupled 31P signals. The duration, approximately 50 min, and power deposition, approximately 1 W x kg(-1), make the application suitable for human studies.

Simultaneous acquisition of 1H-decoupled 31P and 19F 3D CSI is demonstrated in the liver of a patient undergoing 5-fluorouracil chemotherapy. Both 31P and 19F shared the same voxel size (64 or 27 ml), bi-level 1H-decoupling and 0.35 s TR. The measurements were done in a 1.5 Tesla clinical imager with three radio-frequency (RF) channels and a triple-tuned surface-coil. The overall MRI and MRS examination time was under 90 min. Simultaneous acquisition of 31P and 19F permits localized study of the influence of hepatic metabolism on the uptake and catabolism of fluoropyrimidine drugs without extra measurement time or higher SAR.

A method is proposed for carrying out chemical-shift imaging simultaneously on several nuclei (1H and 31P in this example), using a commercial clinical NMR imager fitted with a second RF channel and a dual-tuned birdcage coil to fit the human head. Nuclei of different gamma are examined at the same field of view by exciting each nucleus successively at times proportional to gamma during the same phase-encoding gradient waveform. Thus, each higher-gamma nucleus is exposed to a smaller area of the gradient. Additionally, since in vivo protons typically have a shorter T1 and roughly an order of magnitude higher sensitivity than phosphorus, it is possible to interleave 1H-only acquisitions between the simultaneous 1H, 31P observations while the lower-gamma nucleus relaxes. Consequently, additional information is obtained with either higher spatial resolution or greater sensitivity (more signal averaging) without lengthening the duration of the examination.

Neuro-axonal damage is a well known sequelae of MS pathogeneses. Consequently, our aim was to test whether the ∼20% of patients with MS exhibiting a clinically benign disease course also have minimal neural dysfunction as reflected by the global concentration of their MR imaging marker NAA.Q(NAA) was obtained with nonlocalizing whole-head (1)H-MR spectroscopy in 43 patients with benign RRMS (30 women, 13 men; mean age, 44.7 ± 7.3 years of age) with 21.0 ± 4.4 years (range, 15-35 years) of disease duration from the first symptom and an EDSS score of 1.9 (range, 0-3). Q(NAA) was by divided by the brain volume (from MR imaging segmentation) to normalize it into WBNAA. All participants gave institutional review board-approved written informed consent, and the study was HIPAA compliant.The patients' lesion load was 12.2 ± 7.7 cm(3). Their 8.3 ± 1.8 mmol/L WBNAA was 35% lower than that in controls (P.001). Individual average loss rates (absolute loss compared with controls divided by disease duration) clustered around 0.22 ± 0.09 mmol/L/year (1.7%/year, assuming monotonic decline). This rate could be extrapolated from that already reported for patients with RRMS of much shorter disease duration. WBNAA did not correlate with lesion load or EDSS.Normal WBNAA is not characteristic of benign MS and is not an early predictor of its course. These patients, therefore, probably benefit from successful compensation and sparing of eloquent regions. Because they may ultimately have a rapid decline once their brain plasticity is exhausted, they may benefit from treatment options offered to more affected patients.

Etude comparative de la polarisation de spin, avec resolution temporelle, a 100 K dans un solvant rigide isotrope et dans un solvant mesomorphe

The fluorescence and force-area behavior of chlorophyll a (Chl a) monomolecular films diluted with hydrocarbon on an aqueous subphase is reported. It is found that the diluent hexadecane, instead of conventional fatty alcohols, results in a substantial increase in the fluorescence yield of Chl a. The luminescence intensity varies nonmonotonically with respect to the intermolecular distance. A model for the mutual orientation of the chlorophyll pigments and the consequent energy transfer processes in terms of a Forster mechanism is proposed. For a random distribution of the Chl a molecules in the monolayer (k2 = 2/3) an average value of 43° is calculated for the angle between the molecular planes and the water surface. This model yields a value of 23 A for the Forster R0. Also, a value of 58° is derived for the angle between the Q-band transition moment and the line of intersection between the molecular plane and water surface. When a mixture of Chl a and Chl b in the monolayers is being excited at the absorption band of the latter pigment, typical fluorescence of Chl a is being detected. This sensitization process can also be interpreted in terms of the orientational model of the Chl molecules as presented in this work.

An EPR study of the photoexcited triplet of ZnTPP in the dicostic mesophase of p‐n‐hexa‐hexyloxytriphenylene is reported. The results indicate that (i) the observed spectra are temperature independent (between ∼100–140 K) and typical of axially symmetric guest molecules, with zero field splitting values ‖D‖=0.0308 cm−1, and ‖E‖=0, (ii) the line intensity of the out‐of‐plane canonical orientation of the guest molecules exhibits anisotropic patterns with respect to the sample orientation in the external field. These results are best interpreted by the columnar structure of the host mesogen which forms rodlike structures whose directors are distributed about a plane perpendicular to the applied external magnetic field with a normal distribution and standard deviation of ∼3–8°. The ZnTPP guest molecules are located outside the columns and orient such that the porphyrin planes are parallel to the columns axes. It is also concluded that, in the temperature range studied, the porphyrin molecules are distributed ...

Experiments employing time-sequenced pulsed radiolysis laser photolysis on zinc(II)-tetra-(N-methyl-4-pyridyl)porphyrin chloride (P) in aqueous solutions are presented. This setup enables photoexcitation of the system before, during and after the transient radical formation. Two types of reactions are treated: (1) the reactivity of photoexcited triplet, P T , as compared to its ground state, P, with i -PrOH and Br − 2 radicals in redox reactions; (2) the dynamic and Kinetic effects of photoexcitation on the porphyrins ground state radical (P − + or P· − )system.

Proton NMR has been studied between 0.01 and 1 K in various hydrates of CaSO 4 as part of a program to investigate spin-lattice relaxation mechanisms at low temperatures. All T 1 values were found to be independent of temperature, consistent with the dominance of surface relaxation through interaction with 3 He spins in the heat transfer bath. The recovery of the signal after inversion and after hole burning show different time scales for approach to internal Zeeman equilibrium and for relaxation of Zeeman energy to the lattice.

EPR study of reduced ground and photoexcited triplet state of Photosystem I reaction center in the thermophylic cyanobacterium Mastigocladus laminosus at 8 K is reported. In the reduced ground state preparation, the iron-sulfur EPR spectra are found to be similar to that of Photosystem I reaction center of higher plants. Two types of transient photoexcited triplets are observed and are correlated to the reduction state of the iron-sulfur centers. When electrons can be transferred freely through the acceptors chain, a polarized triplet spectrum is observed, typical of spin-orbit intersystem crossing mechanism with lifetime of approx. 2 ms and is attributed to chlorophyll a, either at the antenna or at A1 in the electron-transport chain. When the iron-sulfur centers are reduced the triplet spectrum is typical of a radical-pair intersystem crossing mechanism with triplet lifetime shorter than 1 ms, and is attributed to P-700. Both species have similar spectroscopic zero field splitting parameters identifying both as chlorophyll a.

We measured the nuclear spin-lattice relaxation time T1, of several surface-bound nuclei, 1H, 19F, 11B, 13C, 29Si, and 2H, immersed in liquid 3He over the temperature range 0.01 K ⩽ T < 1 K. The Larmor frequencies of these nuclei in a 3.39 T field extended from 22 to 144 MHz. All T1 values were temperature-independent and ranged from a few seconds to several hours, depending on the particular nucleus and the surface geometry of the sample. The results indicate that the coupled relaxation of surface spins is a phenomenon occurring in all solids immersed in 3He and thus provides a general mechanism for obtaining high nuclear polarization in solids, that the relaxation is controlled by direct dipole-dipole interactions between the surface spins and 3He in the first surface layer, that the 3He motion dynamics do not change appreciably from one surface to another, and that measurements of T1 may thus be useful for determining the structure of surfaces.

A study of the transient photoexcited triplet of bacteriochlorophyll-a in a glass matrix at 60 K utilizing light modulation EPR spectroscopy is reported. In view of widely deviating results for the triplet sublevel depopulation rates, this problem was engaged via EPR lineshape analysis rather than direct kinetic measurements. This method also yields the relative population and spin—lattice relaxation rates.

A study of laser photoexcitation‐EPR diode detection of the transient triplet state of the hybrid covalenty linked porphyrin dimer [ZnP–H2P] in a uniaxial liquid crystal and a glass matrix at 100 and 10 K, is reported. This time resolved technique enables monitoring the transient behavior of the magnetization until thermal relaxation sets in. Selective singlet excitation of the dimer components reveals a singlet energy transfer process [ZnP*−H2P]–––ket→ZnP–H2P*, whose rate at 10 K is calculated to be [1.0±0.2]×109 s−1. Triplet line shape analysis of the subunit monomer triplet and the dimer yields: (a) the location of the fine structure tensor axes in the molecular frame (X, Y along the pyrrole nitrogens and Z perpendicular to the porphyrin plane); consequently, the signs of the parameters D and E are found to be positive for the monomeric free base and the dimer; (b) the ratio of the sublevel population rates; and (c) the spin‐lattice relaxation rate.

Using NMR techniques, we have measured the rate at which $^{19}\mathrm{F}$ magnetization is relaxed at the surface of a fluorocarbon solid through contact to liquid $^{3}\mathrm{He}$. These measurements at millikelvin temperatures show that the relaxation rate continues to follow a power-law dependence over a broad range of $^{19}\mathrm{F}$ Larmor frequencies $f (10\ensuremath{-}136 \mathrm{MHz}): \frac{1}{{T}_{1S}}\ensuremath{\propto}\frac{1}{{f}^{1.2}}$. These results complement recent measurements by Maegawa et al. of the high-frequency $^{3}\mathrm{He}$ relaxation rate which is approximately proportional to ${f}^{\ensuremath{-}1}$.

In NMR at low temperatures the steady‐state absorption/dispersion spectrum in general differs from the Fourier transform of the free induction decay. The nature of this difference is discussed and an experimental example is given.

The authors performed {sup 13}C NMR on {sup 13}CO chemisorbed on SnO{sub 2}, a metal oxide oxidizing catalyst, immersed in liquid {sup 3}He between 0.01 and 1 K. The observations confirm IR evidence that the catalyzed reaction CO + {1/2}O{sub 2} {yields} CO{sub 2} proceeds via a stable CO{sub 3} intermediate at the SnO{sub 2} surface. The {sup 13}C spin-lattice relaxation time T{sub 1} is 540 {plus minus} 30 s, temperature independent over 2 decades, indicating the Friedman-Millet-Richardson mechanism is responsible and that all the {sup 13}C are surface bound. The sensitivity gain below 0.1 K is sufficient for > 10 signal-to-noise ratio from the {approximately} 10{sup 18} low {gamma} spins with a single pulse.

The spin-lattice relaxation time ${T}_{1}$ of $^{19}\mathrm{F}$ in undoped ${\mathrm{CaF}}_{2}$ single crystals was measured at temperatures T between 0.010 and 1.2 K in field strengths ${B}_{0}$ from 0.15 to 1.1 T. At the lowest temperatures ${T}_{1}$ was found to be orders of magnitude shorter than expected on the basis of previous experiments (at higher temperatures) and theories. The results fit the empirical expression 1/${T}_{1}$=1.7 exp[-6.8(${B}_{0}$/T${)}^{0.26}$] ${\mathrm{sec}}^{\mathrm{\ensuremath{-}}1}$. The mechanism of the relaxation is unknown.