Use of nuclear spin noise spectroscopy to monitor slow magnetization buildup at millikelvin temperatures

At ultralow temperatures, longitudinal nuclear magnetic relaxation times become exceedingly long and spectral lines are very broad. These facts pose particular challenges for the measurement of NMR spectra and spin relaxation phenomena. Nuclear spin noise spectroscopy is used to monitor proton spin polarization buildup to thermal equilibrium of a mixture of glycerol, water, and copper oxide nanoparticles at 17.5 mK in a static magnetic field of 2.5 T. Relaxation times determined in such a way are essentially free from perturbations caused by excitation radiofrequency pulses, radiation damping, and insufficient excitation bandwidth. The experimental spin-lattice relaxation times determined on resonance by saturation recovery with spin noise detection are consistently longer than those determined by using pulse excitation. These longer values are in better accordance with the expected field dependence trend than those obtained by on-resonance experiments with pulsed excitation

Non-Linear Signal Detection Improvement by Radiation Damping in Single-Pulse NMR Spectra

When NMR lines overlap and at least one of them is affected by radiation damping, the resonance line shapes of all lines are no longer Lorentzian. We report the appearance of narrow signal distortions, which resemble hole-burnt spectra. This new experimental phenomenon facilitates the detection of tiny signals hidden below the main resonance. Theoretical analysis based on modified Maxwell–Bloch equations shows that the presence of strong transverse magnetization creates a feedback through the coil, which influences the magnetization of all spins with overlapping resonance lines. In the time domain this leads to cross-precession terms between magnetization densities, which ultimately cause non-linear behavior. Numerical simulations corroborate this interpretation

Radiation damping in High Resolution NMR

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Macroscopic and microscopic fields in high-resolution liquid NMR

The microscopic magnetic-induction field "seen by each nucleus" in a material medium and which is generated by a rapidly time-dependent spin magnetization gives rise to surprising new features in high-resolution nuclear magnetic resonance experiments. The purpose of the present paper is to show how the relations between the macroscopic average fields, the magnetization, and the microscopic fields (which were studied and clarified long ago at thermal equilibrium) can be extended to the present NMR context in which the magnetization can become rapidly time dependent and unrelated to thermal equilibrium properties. © 1990 Academic Press, Inc.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Unified derivation of the dipolar field and relaxation terms in the Bloch-Redfield equations of liquid NMR

The standard theory of NMR relaxation in liquids (with molecular motion described as a classical Brownian motion, and including intermolecular spin-spin couplings) is re-examined, taking great care not to drop significant contributions from the dipolar coupling between distant molecules. This results in "modified Bloch-Redfield equations" for the spins in a single molecule, valid at all spin temperatures, which contain both the usual relaxation terms and a coupling of each spin with a classical average dipolar field. Delicate issues raised in this derivation, like the neglect of quantum correlations between spins on different molecules at (repeated) initial times, are discussed with the help of exact calculations (for all spin temperatures) performed on a simplified model which includes equal couplings between all N spins of a system. The same model is used to compare the merits of different forms of "high temperature" approximation. We also propose an iterative scheme for solving the "modified Bloch-Redfield equations," in which the starting point is the well understood solution of the problem without dipolar field. Finally, a short discussion is given of the relation between "quantum correlations" and "quantum coherences" in the perspective of multiple-pulse and multiple-quantum experiments. These two notions are very simply related in the strict first approximation of weak order, and have often not been clearly distinguished. However, in the second order approximation, which is required whenever dipolar field effects manifest themselves, unrelated spins may display observable coherences although they are not coupled (hence not correlated). © 1995 American Institute of Physics.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Radiation damping in high resolution liquid NMR: A simulation study

Radiation damping generates surprising new features in two-dimensional and spin-echo experiments. The theoretical interpretation of the unexpected results has led to some controversy. The purpose of the present paper is to show, by computer simulations, that the experimental results can be completely understood by describing radiation damping through a classical magnetic field caused by the current induced in the sample coil (resonantly enhanced by the high quality factor of the tuned circuit), and acting back on the spins. We examine a variety of two-dimensional and "multiple quantum" experiments in homogeneous fields, and spin-echo experiments in a fixed field gradient (with and without molecular diffusion). © 1995 American Institute of Physics.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Optimizing radiation dose parameters in MDCT arthrography of the shoulder: illustration of basic concepts in a cadaveric study.

To determine in a cadaveric study the lowest achievable radiation dose and optimal tube potential generating diagnostic image quality in multidetector computed tomography (MDCT) arthrography of the shoulder. Six shoulders from three human cadavers were scanned using a 256-MDCT system after intra-articular injection of diluted iodinated contrast material. Using six decreasing radiation dose levels (CTDI <sub>vol</sub> : 20, 15, 10, 8, 6, and 4 mGy) and for each dose level, four decreasing tube potentials (140, 120, 100, and 80 kVp), image noise and contrast-to-noise ratio (CNR) were measured. Two independent and blinded observers assessed the overall diagnostic image quality, subjective amount of noise, and severity of artifacts according to a four-point scale. Influence of those MDCT data acquisition parameters on objective and subjective image quality was analyzed using the Kruskal-Wallis and Wilcoxon signed-rank tests, and pairwise comparisons were performed. Multidetector CT protocols with radiation doses of 15 mGy or higher, combined with tube potentials of 100 kVp or higher, were equivalent in CNR to the reference 20 mGy-140 kVp protocol (all p ≥ 0.054). Above a CTDI <sub>vol</sub> of 10 mGy and a tube potential of 120 kVp, all protocols generated diagnostic image quality and subjective noise equivalent to the 20 mGy-140 kVp protocol (all p ≥ 0.22). Diagnostic image quality in MDCT arthrography of the shoulder can be obtained with a radiation dose of 10 mGy at an optimal tube potential of 120 kVp, corresponding to a reduction of up to 50% compared with standard-dose protocols, and as high as 500% compared with reported protocols in the literature

Suitability of helical multislice acquisition technique for routine unenhanced brain CT: an image quality study using a 16-row detector configuration.

Subjective and objective image quality (IQ) criteria, radiation doses, and acquisition times were compared using incremental monoslice, incremental multislice, and helical multislice acquisition techniques for routine unenhanced brain computed tomography (CT). Twenty-four patients were examined by two techniques in the same imaging session using a 16-row CT system equipped with 0.75-width detectors. Contiguous "native" 3-mm-thick slices were reconstructed for all acquisitions from four detectors for each slice (4x0.75 mm), with one channel available per detector. Two protocols were tailored to compare: (1) one-slice vs four-slice incremental images; (2) incremental vs helical four-slice images. Two trained observers independently scored 12 subjective items of IQ. Preference for the technique was assessed by one-tailed t test and the interobserver variation by two-tailed t test. The two observers gave very close IQ scores for the three techniques without significant interobserver variations. Measured IQ parameters failed to reveal any difference between techniques, and an approximate half radiation dose reduction was obtained by using the full 16-row configuration. Acquisition times were cumulatively shortened by using the multislice and the helical modality

Observation and selective suppression of the dipolar-field effects in 2D NMR in liquids in homogeneous fields

Intense and numerous satellite peaks caused by the dipolar field have been observed in COSY experiments performed in homogeneous magnetic fields on a concentrated sample (proton NMR at 600 MHz on a dilute solution of acetone in water). Radiation damping was made negligible by the use of a very small sample. As shown by numerical simulations, these results are predicted in great detail by the simple, traditional theory based on Bloch equations incorporating relaxation and the classical average dipolar field caused by all spins in the sample. A simple and efficient decoupling scheme has also been demonstrated by which dipolar-field effects caused by a concentrated solvent can be suppressed for all peaks in the spectrum, except for a narrow spectral range around the NMR frequency of the solvent itself. This scheme should prove very useful for experiments in homogeneous fields as soon as radiation damping is routinely eliminated by the techniques which are presently under development. C 1996 Academic Press, Inc.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Evaluation of biventricular ejection fraction with ECG-gated 16-slice CT: preliminary findings in acute pulmonary embolism in comparison with radionuclide ventriculography.

This study aimed to assess the feasibility of cardiac global function evaluation during a whole-chest multi-slice CT (MSCT) acquisition in patients referred for suspicion of pulmonary embolism (PE), and to compare the results with planar equilibrium radionuclide ventriculography (ERNA). Ten consecutive haemodynamically stable patients (six female, four male; mean age 69.7 years; heart rate 65-99 bpm) with suspicion of PE underwent an MSCT and ERNA within a 6 h period. CT acquisition was performed after contrast medium injection by using 16x1.5 mm collimation and retrospective ECG gating. Left ventricular (LVEF) and right ventricular (RVEF) ejection fractions were calculated using dedicated three-dimensional software. Relationships between measurements obtained with MSCT and ERNA were assessed using linear regression analysis and reliability of MSCT was assessed with intra-class correlation coefficient. Bland-Altman analysis was performed to calculate limits of agreement between MSCT and ERNA. MSCT was performed successfully in ten patients with a mean acquisition time of 16.5+/-2.8 s. Functional cardiac evaluation was possible on CT for all patients except for one due to poor opacification of right ventricle. Linear regression analysis showed a good correlation between MSCT and ERNA for the LVEF (R=0.91) and the RVEF (R=0.89) measurements. Intra-class correlation was superior for LVEF (0.92) than for the RVEF (0.68). Bland-Altman plots demonstrated that MSCT substantially overestimated the ERNA RVEF. Morphological CT data demonstrated PE in four of ten of patients and alternative diagnoses in five of ten patients. Our study reveals that MSCT with retrospective ECG gating may provide in one modality a morphological and a functional cardiopulmonary evaluation. Comparison with ERNA demonstrated a good correlation for both ventricular ejection fractions