T1rho dispersion of rat tissues in vitro.

The purpose of this study was to demonstrate T1rho dispersion in different rat tissues (liver, brain, spleen, kidney, heart, and skeletal muscle), and to compare the 1/T1rho data to previous 1/T1 data and magnetization transfer of rat tissues at low (0.1 T) B0 field. The 1/T1rho dispersion showed a fairly similar pattern in all tissues. The highest 1/T1rho relaxation rates were seen in liver and muscle followed by heart, whereas the values for spleen, kidney, and brain were quite similar. Compared to 1/T2 relaxation rate, the greatest difference was seen in liver and muscle. The rank order 1/T1rho value at each locking field B1 was the same as the transfer rate of magnetization from the water to the macromolecular pool (Rwm) for liver, muscle, heart, and brain. The potential value T1rho imaging is to combine high T1 contrast of low field imaging with the high signal to noise ratio of high static field imaging. When the T1rho value for a given tissue is known, the contrast between different tissues can be optimized by adjusting the locking time TL. Further studies are encouraged to fully exploit this. Targets for more detailed research include brain infarct, brain and liver tumors.     

Tissue characterization using R1rho dispersion imaging at low locking fields

Measurements of the variations of spin-locking relaxation rates (R_1ρ) with locking field amplitude allow the derivation of quantitative parameters that describe different dynamic processes, such as slow molecular motions, chemical exchange and diffusion. In some samples, changes in R_1ρ values between locking frequency 0 and 200 Hz may be dominated mainly by diffusion of water in intrinsic field gradients, while those at higher locking fields are due to exchange processes. The exchange and diffusion effects act independently of each other, as confirmed by simulation and experimentally. In tissues, the relevant intrinsic field gradients may arise from the magnetic inhomogeneities caused by microvascular blood so that R_1ρ dispersion over weak locking field amplitudes (≤ 200 Hz) is affected by changes in capillary density and geometry. Here we first review the theoretical and experimental background to the interpretation of R_1ρ dispersions caused by intrinsic magnetic susceptibility variations within the tissue. We then provide new empirical results of R_1ρ dispersion imaging of the human brain and skeletal muscle at low locking field amplitudes for the first time and identify potential applications of R_1ρ dispersion imaging in clinical studies.     

NMR at very low fields

Although nuclear magnetic resonance in low fields around or below the Earth's magnetic field is almost as old as nuclear magnetic resonance itself, the recent years have experienced a revival of this technique that is opposed to the common trend towards higher and higher fields. The background of this development is the expectation that the low-field domain may open a new window for the study of molecular structure and dynamics. Here, we will give an overview on the specific features in the low-field domain, both from the technical and from the physical point of view. In addition, we present a short passage on the option of magnetic resonance imaging in fields of the micro-Tesla range.     

Influence of magnetic fields on electron-Ion recombination at very low energies

Radiative recombination (inverse photoionization) is believed to be well understood since the beginning of quantum mechanics. Still, modern experiments consistently reveal excess recombination rates at very low electron-ion center-of-mass energies. In a detailed study on recombination of F6+ and C6+ ions with magnetically guided electrons we explored the yet unexplained rate enhancement, its dependence on the magnetic field B, the electron density n(e), and the beam temperatures T( perpendicular) and T( ||). The excess scales as T(-1/2)( perpendicular) and, surprisingly, as T(-1/2)( ||), increases strongly with B, and is insensitive to n(e). This puts strong constraints on explanations of the enhancement.     

Variable-range hopping conduction in doped germanium at very low temperatures and high magnetic fields

The conductivity of doped Ge below the metal-insulator transition is measured at temperatures between 4 K and 40 mK and in magnetic fields up to 7 Tesla. In zero field the resistivity exponent diverges asT ^–1/2. In weak fields the magnetoresistance increases asB ² and becomes exponentially large in strong fields and at low temperatures. The results can be described quantitatively in terms of variable-range hopping between localized states having a Coulomb gap in the density of states at the Fermi level. The magnetoresistance is calculated for arbitrary fields by means of a quasi-classical method. A fit to the data gives the radius of the localized states and the density of states. The sample is found to be very close to the metal-insulator transition. A small increase of the binding energy is observed in strong fields.     

A comparison of high-T c superconductors: Specific heat in magnetic fields at very low temperatures

We discuss specific-heat,C(T), results on YBa₂Cu₃O_7–(=0.13, 0.99) and Bi₂Sr₂CaCu₂O₈. For completion, we compare these with literature data on La_1.85Sr_0.15CuO₄ and La₂CuO₄ by emphasizing the data at lowest temperatures,T0.45K. The strong upturn inC(T)/T found atB=0 is predominantly associated with a splitting of nuclear states by quadrupolar interactions. The shift of this upturn towards higher temperatures, when magnetic fields up to 8 T are applied, indicates additional hyperfine splitting. For YBa₂Cu₃O_6.87 and Bi₂Sr₂CaCu₂O₈, subtraction of the nuclear specific heat from the raw data reveals a spin-glass derived contribution ^*(T,B), with ^* being strongly depressed upon both warming and applying externalB-fields. The probable source for ^* T are impurity phases (in YBa₂Cu₃O_7–) and/or frustrated Cu²⁺ moments created by lattice defects. Any intrinsic linear specific heat, ⁱ , due to, e.g., spinon excitations or normal electronic carriers, would have to imply ⁱ <1 mJ/K² mole_f.u. for each of these two compounds.Dedicated to Prof. Dr. G. von Minnigerode on the occasion of his 60th birthday     

In vivo NMR of hyperpolarized 3He in the human lung at very low magnetic fields

We present NMR measurements of the diffusion of hyperpolarized 3He in the human lung performed at fields much lower than those of conventional MRI scanners. The measurements were made on standing subjects using homebuilt apparatus operating at 3mT. O(2)-limited transverse relaxation (T(2) up to 15-35s) could be measured in vivo. Accurate global diffusion measurements have been performed in vivo and in a plastic bag; the average apparent diffusion coefficient (ADC) in vivo was 14.2+/-0.6mm(2)/s, whereas the diffusion coefficient in the bag (3He diluted in N(2)) was 79.5+/-1mm(2)/s. 1D ADC mapping with high SNR ( approximately 200-300) demonstrates the real possibility of performing quality lung imaging at extremely low fields.     

On the mobility of very pure semiconductors at very low temperatures

The mobility μ of a very pure semiconductor at very low temperatures is investigated in terms of a model where electrons are scattered by charged impurities distributed uniformly in space, and the electron-electron interaction is taken into account by the Debye-Hueckel screening in the interaction potential. The equation for the current relaxation rate Γ, derived previously by the proper connected diagram expansion, incorporates the quasi-particle effect in a self-consistent manner. The solution of this equation at high carrier concentrations n yields the so-called Brooks-Herring formula. At lower concentrations, the solution deviates significantly from the latter. The solution is in general smaller than the standard expression for the rate based on the Boltzmann equation; and this is consistent with the existing conductivity data available. At the very low concentrations e.g. n = n₃ = 10¹³cm^?3 or lower for Ge, the mobility calculated is inversely proportional to the square-root of the impurity concentration n_s, and has a $\text{T}{}^{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}$-dependence (T: temperature).

$\text{μ = 0.3597\&z.xl;h}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{k(k}{}_{\mathrm{B}}\text{T)}{}^{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}\text{(ze)}{}^{\mathrm{?1}}\text{n}{}_{\mathrm{s}}{}^{\mathrm{<mtext>?1</mtext><mtext>2</mtext>}}\text{m}{}^1{}^{\mathrm{<mtext>?3</mtext><mtext>4</mtext>}}$

, where k is the dielectric constant. The conductivity data directly comparable with this formula are not available at present. However, the quasi-particle effect which led to this peculiar concentration-dependence should also show itself in the cyclotron resonance width; there, experiment and theory both show the ${}_{\mathrm{n}}{}_{\mathrm{s}}$-dependence for very pure semiconductors.     

Changes of relaxation times T1 and T2 in rat tissues after biopsy and fixation

NMR spectroscopical measurements of relaxation times were conducted on muscle, intestine, fatty tissue and cerebral cortex and white matter of the rat at various time intervals following removal of the tissue. It appeared that most tissues can be stored at 4 degrees C up to 24 hours without noticeable effects on NMR relaxation parameters. Exceptions are the T2 of muscle and the T1 and T2 of intestine, which tended to change in the first hour after biopsy. Relaxation parameters change considerably after fixation of the tissues. Therefore the effects of fixation have to be taken into account when carrying out NMR measurements on fixed tissues.     

Spin relaxation in metals at very low temperatures

A general theory of spin relaxation in metals is developed from a statistical mechanical point of view. The theory is valid for all temperature domain and the multiple-time characteristics of the relaxation process are completely determined: The relaxation times are strongly dependent on the temperature and magnetic field. At very low temperatures, behaviours of the relaxation times are quite different from the usual ones showing a saturation effect. Temperature varations of the relaxation times for I ? 1 (I the magnitude of spin) are qualitatively different from those for $\text{I =}\text{1}\text{2}$. Namely, in the former case, the largest relaxation time has a maximum as a function of inverse temperature.     

Superconductivity in an organic insulator at very high magnetic fields

We investigate by electrical transport the field-induced superconducting state (FISC) in the organic conductor lambda-(BETS)2FeCl4. Below 4 K, antiferromagnetic-insulator, metallic, and eventually superconducting (FISC) ground states are observed with increasing in-plane magnetic field. The FISC state survives between 18 and 41 T and can be interpreted in terms of the Jaccarino-Peter effect, where the external magnetic field compensates the exchange field of aligned Fe3+ ions. We further argue that the Fe3+ moments are essential to stabilize the resulting singlet, two-dimensional superconducting state.     

Beyond the T1 limit: singlet nuclear spin states in low magnetic fields

Low-field nuclear spin singlet states may be used to store nuclear spin order in a room temperature liquid for a time much longer than the spin-lattice relaxation time constant T1. The low-field nuclear spin singlets are unaffected by intramolecular dipole-dipole relaxation, which is generally the predominant relaxation mechanism. We demonstrate storage of nuclear spin order for more than 10 times longer than the measured value of T1. This phenomenon may facilitate the development of nuclear spin hyperpolarization methods and may allow the study of motional processes which occur too slowly for existing NMR techniques. This is the first time that the memory of nuclear spins has been extended well beyond the T1 limit in a system lacking intrinsic magnetic equivalence.     

Methodology of1H NMR spectroscopy of the human brain at very high magnetic fields

An ultrashort-echo-time stimulated echo-acquisition mode (STEAM) pulse sequence with interleaved outer volume suppression and VAPOR (variable power and optimized relaxation delays) water suppression was redesigned and optimized for human applications at 4 and 7 T, taking into account the specific requirements for spectroscopy at high magnetic fields and limitations of currently available hardware. In combination with automatic shimming, automated parameter adjustments and data processing, this method provided a user-friendly tool for routine¹H nuclear magnetic resonance (NMR) spectroscopy of the human brain at very high magnetic fields. Effects of first- and second-order shimming, single-scan averaging, frequency and phase corrections, and eddy currents were described. LCModel analysis of an in vivo¹H NMR spectrum measured from the human brain at 7 T allowed reliable quantification of more than fifteen metabolites noninvasively, illustrating the potential of high-field NMR spectroscopy. Examples of spectroscopic studies performed at 4 and 7 T demonstrated the high reproducibility of acquired spectra quality.     

Exciton absorption in silicon at low electric fields

This paper reports for the first time high-resolution transverse EA spectra of Si in low electric fields at liquid-helium temperature. The data conclusively demonstrate that the observed effect is related to the discrete levels of the Wannier exciton and that for the n = 1 level it can be accounted for both qualitatively and quantitatively by means of a Stark approximation where field-induced broadening is phenomenologicaly introduced to allow for weak tunneling-like contributions. The exciton energy levels determined here are in good agreement with the results of wavelength-modulation spectroscopy and all the other parameters of the absorption process are consistent with previous absolute absorption experiments.     

Effects of hydrogen transitions in niobium at very low temperature

The presence of hydrogen in Nb containing O and N interstitial impurities gives rise to a relaxation process at liquid helium temperature. The effect, studied by exciting two vibration modes of the specimen (20 and 75 kHz), can be described by a single Debye curve and is characterized by an apparent activation energy of 1.8 meV and by the rise of the relaxation strength with increase of peak temperature.     

In vivo determination of 31P spin relaxation times (T1, T2, T1 rho) in rat leg muscle. Use of an off-axis solenoid coil

A probe using a solenoid coil tilted 45 degrees off-axis has been used to study the 31P NMR relaxation characteristics of the resonances arising from phosphorus metabolites in rats in vivo. T1, T1 rho and T2 values have been determined for phosphocreatine and ATP in leg muscle. The ratio of 31P T1(1700ms) to T2(12ms) for ATP was in excess of 200:1 compared with a ratio of 5:1 for 1H T1:T2. Of major significance was the observation that T2 values for phosphocreatine (230ms) were markedly longer than T2 values for ATP (12ms). Thus by use of appropriate delay times in spin echo sequences ATP signals can be nulled, and discrete 31P imaging of phosphocreatine in muscle may be possible provided the overall signal-to-noise is satisfactory.     

Superconductivity of compressed platinum powder at very low temperatures

Superconductivity of compressed, high-purity platinum powder (average grain size 2–3 μm) was found by measurements of resistivity, AC susceptibility and magnetization. The transition temperature into the superconducting state T_c and the critical magnetic field B_c strongly depend on the packing fraction f of the samples: we found 0.62T_c(0)1.38 mK and 6.6B_c(0)67 μT for 0.8f0.5, respectively. The temperature dependence of the critical magnetic fields can be described by B_c(T)=B_c(0)(1−(T/T_c)²). The discussion of these results includes possible explanations for the origin of superconductivity in this new superconducting material.     

Dispersion theory of parity non-conservation in pion-nucleon scattering at low energies

We examine the possibility for testing the weak parity non-conserving (pnc) interaction in pion-nucleon scattering at low energies. We start out analysis by assuming the existence of a pnc πNN coupling which conserves time reversal invariance and has the isospin selection rule ΔI = 1, and the existence of fixed-t dispersion relations obeyed by the pnc invariant amplitudes. By using the nonrelativistic expansion, we then derive dispersion relations for the partial-wave amplitudes.We calculate parity-odd observables in π^?p → π^?p and in π^?p → π⁰n, e.g., the up-down asymmetry α in the total cross section for a polarized target and the longitudinal polarization P of the recoil nucleon. In the conventional Cabibbo model for the weak interaction P and α are found to be of order 10^?7 to 10^?8.We have also examined the t-channel pnc ?-exchange process which has the isospin selection rule ΔI = 0, 2 and found it to contribute dominantly to P in the charge-exchange channel π^?p → π⁰n.