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At least 757 records · Page 42Linked to original sources
PubMed2000

Ethanol in human brain by magnetic resonance spectroscopy: correlation with blood and breath levels, relaxation, and magnetization transfer.

BACKGROUND: Proton magnetic resonance spectroscopy (1H MRS) allows measurement of alcohol in the human brain after alcohol consumption. However, the quantity of alcohol that can be detected in the brain by 1H MRS pulse sequences has been controversial, with values ranging from about 24% to 94% of the temporally concordant blood alcohol concentrations. The quantitation of brain alcohol is critically affected by the kinetics of alcohol uptake and elimination, by the relaxation times of the protons that give rise to the brain alcohol signal, and by the specifics of both pulse sequence timing and radio frequency pulse applications. METHODS: We investigated these factors in 20 light-drinking subjects after oral administration of approximately 0.85 g/kg body weight of alcohol by localized 1H MRS and measurements of blood and breath alcohol concentrations obtained at the same time. Specifically, we measured transverse and longitudinal relaxation times of brain alcohol and its signal saturation on application of on- or off-resonance radio frequency pulses. All 1H MRS measurements were performed at a time after brain and blood alcohol concentrations had equilibrated. RESULTS: 1H MRS measures of brain alcohol were correlated highly with both breath and blood alcohol concentrations after equilibration in brain tissue. The measured 1H MRS relaxation times of brain alcohol were shorter than given in previous reports that were limited by smaller subject numbers, improper use of 1H MRS methods, and estimates rather than measurements. The brain alcohol signal decreased by about 30% on application of on- or off-resonance saturation pulses. CONCLUSIONS: 1H MRS allows direct measurement of brain alcohol, formerly only possible indirectly through inferences from breath alcohol levels. Quantitation of brain alcohol levels need to take into account measured relaxation times and alcohol signal attenuation due to presence and timing of standard radio frequency MRS pulses. Saturation experiments give evidence for the existence of more than one compartment of brain alcohol characterized by different molecular environments. They suggest that a fraction of brain alcohol is invisible to 1H MRS.

Adult↗
PubMedDate not supplied

[Some changes in bioelectrical activity of the brain during short regulated exposure to pulsed electrical fields].

It was shown that short (10 s) exposures of white rats being at rest to impulse (1 ms) electromagnetic fields (170 A/m) at a fixed frequency of pulse sequence in the range of the theta-rhythm do not change the contribution to the electrocorticogram (ECoG) of the harmonic whose frequency corresponds to the frequency of pulse sequence. By contrast, treatments with a frequency-pulse modulation at which the interpulse intervals corresponding to the frequency of the harmonic being studied were not observed throughout the range of exposure but at which their high correlation was provided, assisted in decreasing the contribution of the harmonic to the ECoG spectrum. If the functional state of the central nervous system was changed by drugs leading to the predominance of the short-wavelength activity in the ECoG, both regimes of exposure significantly affected the contribution of the harmonic in the ECoG, whose direction and dynamics depended on the excitability of neurons.

Animals↗
PubMed1990

MR of the paranasal sinuses.

The purpose of this project was to examine the anatomy and pathology of the paranasal sinuses as seen by MR imaging. This was accomplished through correlations of MR images of normal volunteers with matched cadaver whole organ cryosections. The information obtained by MRI was compared to that of plain films and CT in the detection of a variety of conditions affecting the paranasal sinuses. The majority of the pathological processes could be quite adequately imaged by T1 weighted pulsing sequences. When more tissue specific information was required in some infiltrating malignant lesions, T2 weighting pulsing sequences are quite helpful for tumors that crossed the subarachnoid space into the central nervous system or in characterizing tissues in airless sinuses. Other than the single case of osteoid osteoma where X-ray studies were superior, magnetic resonance provided equal or superior information to the X-ray examinations.

Cadaver↗
PubMed1996

17O-decoupled 1H detection using a double-tuned coil.

17O-decoupled proton MR spectroscopy imaging with a double-tuned radiofrequency (RF) coil at 2 T was used to detect and quantify H2 17O in tissue containing various concentrations of 17O-enriched water in 5% gelatin. The pulse sequence used in these experiments consisted of a conventional proton spin-echo sequence with RF irradiation at the 17O resonance frequency applied between the proton 90 degrees pulse and the signal acquisition window. The double-tuned coil provided several advantages over systems using separate RF coils for 17O decoupling and proton excitation/detection, including ensuring that the same (or similar) sample volumes are excited and decoupled and permitting accurate calibration of the 17O decoupling pulse amplitude. The efficiency of 17O decoupling as a function of decoupling RF amplitude, decoupling duration, and decoupling resonance offset was investigated. Finally, the specific absorption rate of the 17O decoupled pulse sequence was investigated and found to lie within federal guidelines at 1.5 T.

Hydrogen↗
PubMed1984

Magnetic resonance imaging of the thyroid, thymus, and parathyroid glands.

Magnetic resonance (MR) images of the neck were obtained in 16 patients with use of a variety of spin echo and inversion recovery pulse sequences. Anatomic resolution was best with high-resolution spin echo images obtained with the pulse sequence repetition rate equal to 2.0 seconds and the echo delay equal to 28 msec because this imaging technique offered excellent contrast between normal tissues and had the highest signal noise ratio. The spatial resolution of MR was nearly as good as state of the art computerized tomography (CT). However, streak artifacts caused by motion and x-ray beam hardening often limited CT but did not affect MR. Tumors and lymph nodes were more easily differentiated from muscle and blood vessels with MR than with CT because of the superior soft-tissue contrast of MR. Tissue characterization allowed MR differentiation of thyroid nodules, thyroid cysts, and parathyroid tumors from normal thyroid tissue. Thyroid cyst fluid had the greatest water content and longest T1 and T2 relaxation times of all tissues studied. However, nonspecifically increased T1 and T2 relaxation times overlapped for a variety of neoplastic and inflammatory conditions. With further experience, MR imaging is likely to become a useful technique for the evaluation of neck masses.

Adenoma↗
PubMed1995

Breakthrough of single-quantum coherence and its elimination in double-quantum filtering.

Breakthrough of single-quantum coherence is shown to occur after application of a double-quantum filter with the conventional four-step phase-cycling scheme. This single-quantum breakthrough is due to the intersequence stimulated echo which has been generated by the radiofrequency pulses in the preceding pulse sequence and appears at the same time as the double-quantum coherence signal in the current pulse sequence. Moreover, the phase of the intersequence stimulated echo is the same as the phase of the double-quantum coherence signal; i.e., the phase of the intersequence stimulated echo is twice the phase change of the radiofrequency pulses in the creation period when their phase is rotated in accordance with the conventional four-step phase-cycling scheme. Consequently, the intersequence stimulated echo passes through the double-quantum filtration in the conventional four-step phase-cycling scheme and gradient pulses. A new phase-cycling scheme which can filter out the single-quantum breakthrough signal is proposed here and its effectiveness is verified experimentally and by computer simulations.

Computer Simulation↗
PubMed1997

Double quantum coherence of quadrupolar I = 1 nuclei: 14N in NH4ClO4.

We have calculated optimal conditions for observing the double quantum coherence of quadrupolar nuclei I = 1 in single crystals and powders after the two and three pulse sequences (theta1)x-tau1-(theta2)alpha-tau2 [-(theta3)beta-tau3] Here alpha and beta are the phase-cycling angles, which were incremented properly to select the desired coherence transfer pathways. Experiments for double quantum excitation and detection were performed on the single crystal and powder samples of NH4ClO4. Because of a fast decay of the signal from the powder after the two-pulse sequence, we employed the third pulse to produce an echo related to the double quantum coherence.

Chlorates↗
PubMed2005

A standardised method for measuring magnetisation transfer ratio on MR imagers from different manufacturers--the EuroMT sequence.

Magnetisation transfer ratio (MTR) is increasingly used to evaluate neurological disorders, especially those involving demyelination. It shows promise as a surrogate marker of disease progression in treatment trials in multiple sclerosis (MS) but the value measured is highly dependent on pulse sequence parameters, making it hard to include the technique in large multi-centre clinical trials. The variations can be reduced by a normalisation procedure based on the flip angle and timing of the presaturation pulse, but correction for parameters such as saturation pulse shape, amplitude, duration and offset frequency remains problematic. We have defined a standard pulse sequence, to include a standard presaturation pulse and set of parameters, which can be implemented on scanners from both General Electric and Siemens, and has also been used on Phillips scanners. To validate the sequence and parameters, six European centres measured MTR in the frontal white matter of normal volunteers. It was possible to measure MTR values in controls which were consistent to within approximately +/-2.5 percentage units across sites. This degree of precision may be adequate in many situations. The remaining differences between sites and manufacturers are probably caused by B1 errors.

Brain↗
PubMed1994

Application of spin-spin relaxation to measurement of surface area and pore size distributions in a hydrating cement paste.

Nuclear magnetic resonance (NMR) relaxation analysis has been applied to interpret the evolution of microstructure in a cement paste during hydration. Measurements of transverse magnetic relaxation were made in fully and partially filled white cement pastes for hydration times between 1 h and 6 mo. It was found that only the evaporable water molecules contributed to the echo amplitude in the NMR measurement, while both the evaporable and the hydrated water contribute to the amplitude of the free induction decay. The spin-spin relaxation rate was found to increase markedly during hydration and can be directly related to the total surface area of the CSH gel accessible to evaporable water. The amount of water consumed during the hydration process was determined independently from the amplitude of the NMR echo signal extrapolated to the beginning of the pulse sequence. A numerical inversion of the full spin-spin relaxation profile, determined with a CPMG pulse sequence, was interpreted as a pore volume distribution function. The measurements of total surface area, amount of water consumed, and pore distributions were performed in situ nondestructively, throughout the hydration period for a single sample of cement paste.

Construction Materials↗
PubMed2005

PITANSEMA-MAS, a solid-state NMR method to measure heteronuclear dipolar couplings under MAS.

A 2D NMR method is presented for the measurement of the dipole-dipole interaction between a proton and a low-frequency nuclear spin species in the solid state under the magic angle spinning. It employs the time averaged nutation concept to dramatically reduce the required radio frequency (rf) power on the low γ nuclear channel and spin exchange at the magic angle is used to suppress (1)H-(1)H dipolar interactions and chemical shifts. The flexibility in choosing the spinning speed, rf power and the scaling factor of the pulse sequence are of considerable importance for the structural studies of biological solids. The performance of the pulse sequence has been numerically and experimentally demonstrated on several solids.

Journal Article↗
PubMed2006

[MRI safety aspects and artifacts of atrial septal defect and patent foramen ovale occluders at 1.5 tesla: a phantom study].

PURPOSE: To investigate the magnetic resonance safety and magnetic resonance artifact intensities of patent foramen ovale and atrial septal occluders in a phantom study. MATERIALS AND METHODS: Seven different commercial occluder models (Amplatzer ASD, Angel Wing ASD, Helex Septal Occluder, Cardia PFO-Star Generation I & III, Sideris Buttoned Device, Starflex ASD) were tested in a 1.5 Tesla whole-body MR scanner. Following a deflection test, the non-magnetic devices were imaged using different MRI pulse sequences including conventional spin echo, spoiled gradient echo as well as fast refocused steady-state pulse sequences (trueFISP) commonly used for cardiac MRI. Additionally, device temperatures were measured in the MR scanner during two imaging protocols (trueFISP, HASTE) with an MR-compatible fiber-optical thermometer. RESULTS: One ferromagnetic device (Sideris Buttoned Device) did not pass the deflection test and was excluded from further experiments. For typical cardiac imaging protocols, the remaining non-ferromagnetic devices showed only minor image artifacts with artifact sizes not exceeding 2 mm next to a strut. Heating was not observed in the devices. CONCLUSION: Non-ferromagnetic occluder models used in this study can safely be imaged with typical cardiac MRI protocols at 1.5 Tesla. The device artifact sizes are small enough to allow anatomical and functional MRI even in the immediate vicinity of the occluders.

Artifacts↗
PubMed1991

Dynamic gadolinium-enhanced MR imaging of the spleen: normal enhancement patterns and evaluation of splenic lesions.

The authors studied the ability to improve detection of splenic lesions during suspended respiration with dynamic gadolinium-enhanced T1-weighted spin-echo magnetic resonance (MR) imaging. In the first phase of the study, normal splenic contrast material enhancement patterns were assessed in 10 control patients without splenic lesions. A heterogeneous signal intensity pattern was observed in 11 patients with splenic lesions during bolus injection of gadopentetate dimeglumine, with conversion to homogeneous enhancement 1 minute later. Mean splenic enhancement was 321% during bolus injection, with a rapid return toward baseline signal intensity thereafter. In the second phase, evaluation of 18 splenic lesions detected with contrast-enhanced computed tomography in 11 patients revealed that dynamic gadolinium-enhanced MR pulse sequences significantly improved lesion conspicuity and detectability compared with conventional T1-and T2-weighted pulse sequences. Contrast-to-artifact ratio measurements were 0.5, 3.7, and 9.3 for conventional T1-weighted, T2-weighted, and dynamic gadolinium-enhanced MR images, respectively.

Adult↗
PubMed2000

Asymmetry of brain functional activation: fMRI study under language and music stimulation.

OBJECTIVE: To determine the asymmetry of the human brain functional activation. METHODS: With the help of GE Signa Horizon MRI system, 14 cases of right-handed volunteers were examined and the blood oxygenation level dependent method was used. The T1-weighted images were obtained with spin echo pulse sequence and the functional imaging (T2*-weighted) was performed using a single shot echo planar imaging pulse sequence. Data analysis was done with Sun Sparc Workstation and by the method of student t test or correlation analysis. RESULTS: Most of activation areas were in the left hemisphere under language stimulation, while they were in the right side under music stimulation. Besides, a few brain areas in the contralateral cerebral cortex were also activated under both stimulations. CONCLUSION: The present study supported the hypothesis of the asymmetry of brain functional activation and many brain areas of the cerebral cortex as well as both hemispheres worked in coordination. In addition, it also proved that fMRI is a feasible method in the study of human brain in vivo.

Acoustic Stimulation↗
PubMed1993

MRI diffusion-weighted imaging of the brain: contributions to image contrast from CSF signal reduction, use of a long echo time and diffusion effects.

The contributions of CSF signal reduction, use of a long echo time and diffusion weighting to the appearance of pulsed gradient spin echo (PGSE) images are analysed by reference to T2-weighted spin echo and T2-weighted fluid attenuated inversion recovery (FLAIR) pulse sequences. Both PGSE and T2-weighted FLAIR sequences reduce CSF signal and produce very heavy T2 weighting allowing the specific additional contribution produced by the diffusion weighting of PGSE sequences to be recognized. Considerable advantage accrues from CSF suppression with both PGSE and FLAIR sequences through reduction in partial volume effects and artefacts. The very heavy T2 weighting with both these pulse sequences highlights certain white matter tracts and provides high sensitivity to disease. The additional diffusion weighting with PGSE sequences can enhance or reduce white matter tract signals and may enhance or reduce lesion conspicuity relative to the FLAIR sequences. Many of the benefits attributed to the diffusion-weighted PGSE sequence may result from the reduction of the CSF signal and the heavy T2 weighting of the sequence without a contribution from diffusion effects. However, additional anatomical detail, sensitivity to myelination and increased lesion conspicuity may result from the diffusion weighting.

Brain↗
PubMed1998

Five quantum coherence of I=5/2 nuclei: 27Al in polycrystalline AlCl3.

Optimal conditions were calculated for the excitation and detection of the five quantum coherence of quadrupolar nuclei with I = 5/2 in powder samples, observed by the two-pulse sequence (theta1)x - tau1 - (theta2)alpha - tau2, where alpha is the phase cycling angle. We varied the pulse lengths and the relative values of the nutation frequency omega1 = gammaB1 and the quadrupolar frequency omegaQ. Also, the effect of the resonance offset was studied under optimal conditions. Besides, the conditions for obtaining the maximal echo amplitude after the two-pulse sequence with alpha = y were found. Theoretical results were compared with experiments on 27Al nuclei in polycrystalline AlCl3.

Aluminum Chloride↗
PubMed2006

Effective Hamiltonians by optimal control: solid-state NMR double-quantum planar and isotropic dipolar recoupling.

We report the use of optimal control algorithms for tailoring the effective Hamiltonians in nuclear magnetic resonance (NMR) spectroscopy through sophisticated radio-frequency (rf) pulse irradiation. Specifically, we address dipolar recoupling in solid-state NMR of powder samples for which case pulse sequences offering evolution under planar double-quantum and isotropic mixing dipolar coupling Hamiltonians are designed. The pulse sequences are constructed numerically to cope with a range of experimental conditions such as inhomogeneous rf fields, spread of chemical shifts, the intrinsic orientation dependencies of powder samples, and sample spinning. While the vast majority of previous dipolar recoupling sequences are operating through planar double-or zero-quantum effective Hamiltonians, we present here not only improved variants of such experiments but also for the first time homonuclear isotropic mixing sequences which transfers all I(x), I(y), and I(z) polarizations from one spin to the same operators on another spin simultaneously and with equal efficiency. This property may be exploited to increase the signal-to-noise ratio of two-dimensional experiments by a factor of square root 2 compared to conventional solid-state methods otherwise showing the same efficiency. The sequences are tested numerically and experimentally for a powder of (13)C(alpha),(13)C(beta)-L-alanine and demonstrate substantial sensitivity gains over previous dipolar recoupling experiments.

Algorithms↗
PubMed1989

Use of Gd-DTPA and fast gradient-echo and spin-echo MR imaging to demonstrate renal function in the rabbit.

The paramagnetic magnetic resonance (MR) imaging contrast agent gadolinium diethylenetriaminepentaacetic acid (DTPA) is freely filtered at the glomerulus and is neither secreted nor reabsorbed by the renal tubules. Fast MR imaging techniques, either gradient-echo or spin-echo, can be used to document the passage of Gd-DTPA through the renal tubules, as reflected by alteration in the MR signal intensity within the different anatomic regions of the kidney. Gradient-echo (repetition time of 35 msec, echo time of 7 msec, flip angles of 10 degrees-100 degrees) and spin-echo (repetition time of 35 msec, echo time of 8 msec) pulse sequences were used to acquire 20 consecutive images, one every 12 seconds, of the rabbit kidney. Both pulse sequences depicted the time course of Gd-DTPA distribution through the kidney but with distinctly different patterns of MR signal change. These dynamic MR images provide an MR nephrogram that directly demonstrates renal morphology and indirectly reflects the functional status of the renal vasculature, renal perfusion, and tubular concentrating ability.

Animals↗
PubMed1991

Magnetic resonance imaging of the pediatric abdomen.

In many ways, MRI of the pediatric abdomen is in its infancy. Motion will be a problem with most of the current pulse sequences, but the problem appears manageable given many of the newer software options for motion suppression. Faster pulse sequences (approaching CT scan times) will serve to improve image quality and reduce the need for sedation. Identification of a reliable enteric contrast agent probably will expand the current utility of MRI beyond evaluation of solid abdominal viscera. Although preliminary results appear promising, controlled comparative studies are necessary to determine the exact role of MRI in diagnosis and staging of common pediatric abdominal malignancies. The addition of MRI intravenous contrast agents and MR spectroscopy are anticipated to add depth and specificity to the diagnostic potential of this exciting modality.

Abdomen↗