Differences In Magnetic Field Strength Research Articles

Harmonized Z-Scores Calculated from a Large-Scale Normal MRI Database to Evaluate Brain Atrophy in Neurodegenerative Disorders.

Alzheimer’s disease (AD), the most common type of dementia in elderly individuals, slowly and progressively diminishes the cognitive function. Mild cognitive impairment (MCI) is also a significant risk factor for the onset of AD. Magnetic resonance imaging (MRI) is widely used for the detection and understanding of the natural progression of AD and other neurodegenerative disorders. For proper assessment of these diseases, a reliable database of images from cognitively healthy participants is important. However, differences in magnetic field strength or the sex and age of participants between a normal database and an evaluation data set can affect the accuracy of the detection and evaluation of neurodegenerative disorders. We developed a brain segmentation procedure, based on 30 Japanese brain atlases, and suggest a harmonized Z-score to correct the differences in field strength and sex and age from a large data set (1235 cognitively healthy participants), including 1.5 T and 3 T T1-weighted brain images. We evaluated our harmonized Z-score for AD discriminative power and classification accuracy between stable MCI and progressive MCI. Our procedure can perform brain segmentation in approximately 30 min. The harmonized Z-score of the hippocampus achieved high accuracy (AUC = 0.96) for AD detection and moderate accuracy (AUC = 0.70) to classify stable or progressive MCI. These results show that our method can detect AD with high accuracy and high generalization capability. Moreover, it may discriminate between stable and progressive MCI. Our study has some limitations: the age groups in the 1.5 T data set and 3 T data set are significantly different. In this study, we focused on AD, which is primarily a disease of elderly patients. For other diseases in different age groups, the harmonized Z-score needs to be recalculated using different data sets.

Bootstrap approach for meta-synthesis of MRI findings from multiple scanners

BackgroundNeuroimaging data from large epidemiologic cohort studies often come from multiple scanners. The variations of MRI measurements due to differences in magnetic field strength, image acquisition protocols, and scanner vendors can influence the interpretation of aggregated data. The purpose of the present study was to compare methods that meta-analyze findings from a small number of different MRI scanners. MethodsWe proposed a bootstrap resampling method using individual participant data and compared it with two common random effects meta-analysis methods, DerSimonian-Laird and Hartung-Knapp, and a conventional pooling method that combines MRI data from different scanners. We first performed simulations to compare the power and coverage probabilities of the four methods in the absence and presence of scanner effects on measurements. We then examined the association of age with white matter hyperintensity (WMH) volumes from 787 participants. ResultsIn simulations, the bootstrap approach performed better than the other three methods in terms of coverage probability and power when scanner differences were present. However, the bootstrap approach was consistent with pooling, the optimal approach, when scanner differences were absent. In the association of age with WMH volume, we observed that age was significantly associated with WMH volumes using the bootstrap approach, pooling, and the DerSimonian-Laird method, but not using the Hartung-Knapp method (p < 0.0001 for the bootstrap approach, DerSimonian-Laird, and pooling but p = 0.1439 for the Hartung-Knapp approach). ConclusionThe bootstrap approach using individual participant data is suitable for integrating outcomes from multiple MRI scanners regardless of absence or presence of scanner effects on measurements.

Signal intensity of lanthanum carbonate on magnetic resonance images: phantom study

Lanthanum carbonate (LC) is used to treat hyperphosphatemia. The purpose of this study was to investigate the signal intensity (SI) of LC on magnetic resonance imaging (MRI) scans of phantoms. LC tablets were thoroughly ground and mixed with distilled water or edible agar (0.05, 0.25, 0.5, and 2.5 mg/ml) in plastic bottles. Four intact tablets were placed in plastic bottles that did or did not contain distilled water or agar. Two radiologists consensually evaluated T1- and T2-weighted images (WIs) obtained with 1.5- and 3.0-T MRI systems for the SI of unground and ground tablets. On T1- and T2WI, the SIs of the LC suspensions and the solvents alone were similar; the SIs of unground tablets alone and of the air were also similar. Unground tablets in phantoms filled with solvent exhibited lower SI than the solvent. Ground tablets in suspension were not visualized on MRI or computed tomography. These results remained unchanged regardless of differences in magnetic field strength or the solvent used. Ground LC had no contrast enhancement effect on T1WI; on T2WI it did not affect the SI of the solvent. Unground LC tablets may be visualized as a "filling defect" on MRI.

The diagnosis of neurodegenerative disorders based on clinical and pathological findings using an MRI approach

Diffusion tensor imaging and voxel based morphometry are efficient in the diagnosis of neurodegenerative disorders, however, these technologies are available at a limited number of facilities. Thus, conventional MRI remains important in the diagnosis of neurodegenerative disorders. Dorsolateral putaminal hyperintensity (DPH) and hot cross bun sign discriminate Parkinson's disease from multiple system atrophy (MSA). However, differences in magnetic field strength and sequence affect the sensitivity of DPH, and mild DPH may be observed in normal elderly subjects. Patients with progressive supranuclear palsy (PSP) presenting Richardson's syndrome show midbrain tegmentum atrophy, but Parkinsonian PSP (PSP-P) and pure akinesia with gait freezing, which are other phenotypes of PSP, may not show especially during early course of illness. In patients with corticobasal degeneration (CBD), asymmetrical cerebral atrophy corresponding with corticobasal syndrome (CBS) may be a characteristic finding. However, at autopsy, CBS patients presenting asymmetrical cerebral atrophy have CBD as the underlying pathology in approximately 50% of PSP patients. The sensitivity and specificity of MRI for the diagnosis of MSA, PSP, and CBD is based generally on clinical diagnostic criteria. Diagnosis based on MRI has limitations, and, therefore, we should reconsider our diagnoses, particularly during the early course of illness.

Hard X-ray emission at the footpoints of solar flares

We analyse X-ray flare observations made by the Soft X-ray Telescope and Hard X-ray Telescope on the Yohkoh satellite during period 1999–2001. For the strongest flares, when HXT imaging with high time cadence (0.5–1 s) is possible, clear changes in the asymmetry of the observed footpoints fluxes are revealed. We present four examples of flares which illustrate such asymmetrical time variations the best. Observed asymmetries are usually interpreted as due to a difference in magnetic field strength or field divergence at the footpoints, however this interpretation should lead to the existence of strong magnetic field oscillations or movements of the footpoints’ locations in the presence of strong magnetic gradients. Our analysis indicates rather that either differences in the electron injection or electron distribution in each loop’s leg is the source of the observed asymmetry variations. We discuss possible mechanisms which may be responsible for such effect.

Variations of the hard X-ray footpoint asymmetry in a solar flare

We analysed hard and soft X-ray observations from the SXT and HXT instruments on the Yohkoh satellite of an X5.3 class flare of 25 August 2001. The most prominent features of this flare are: a fast (450 km s-1), hard (up to 90 keV) and compact X-ray moving feature, two clear footpoint sources, and loop-top emission between them. The Yohkoh/HXT observations with high time cadences (0.5–1 s) revealed clear, quasi-periodic changes in the asymmetry of the observed footpoint fluxes. Observed asymmetry is usually interpreted as an effect of the difference in magnetic field strength or the field divergence at the footpoints. Such a traditional interpretation should lead in our case to the existence of strong magnetic field oscillations or movements of the footpoints' locations in the presence of a strong magnetic gradient. Our analysis indicates instead that differences in the injection conditions in each loop's leg are a source of the observed asymmetry variations. We also observed that the emission for each footpoint had a different spectrum, and its evolution was also different, but generally with the spectrum hardening at highest energies. We argue that these effects are connected with the mechanism of energy release and/or electron distribution.

The Structure of Umbral Fluxtubes

Subsurface filamentation of sunspot magnetic fields has been postulated as a source of the visible small-scale structure of sunspot umbrae. We examine this possibility by investigating the magnetohydrodynamic structure of thin, vertical magnetized gas columns embedded in sunspot umbrae. These umbral fluxtubes are assumed to have weaker field strengths than the surrounding umbral atmosphere. The steady-state magnetohydrodynamic equations are solved numerically in the slender fluxtube approximation, thus allowing for stationary internal mass flows. We include the radiative exchange of heat between the umbral fluxtube and the ambient medium using a simple relaxation time approach.The geometric shape of the steady flow solution is a gas column converging with height. We discuss the relationship of our results to observed properties of umbral brightenings (umbral dots). We show that, even if there is a large difference in magnetic field strength between the dot and the ambient medium in deeper layers, the field strengths are nearly equal in the observable layers, a result required by the observations. We also show that either high temperatures at the lower boundary of the dots or strong upflows are needed in order to produce bright continuum structures.

Discovery of a Magnetic/Nonmagnetic Double-degenerate Binary System

view Abstract Citations (56) References (30) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Discovery of a Magnetic/Nonmagnetic Double-degenerate Binary System Liebert, James ; Bergeron, P. ; Schmidt, Gary D. ; Saffer, Rex A. Abstract We report the discovery of an unresolved binary system consisting of two degenerate stars: one a polarized object with an inferred magnetic field among the largest yet found on a white dwarf; the other a normal DA with no detectable field. We have deconvolved the composite 1250 Å-2.2 microns energy distribution into individual spectra and find that, with the exception of the graphic difference in magnetic field strength, the two stars are remarkably similar. For the normal DA, Teff ≍ 14,500 K, log g ≍ 8.5, and M = 0.91±0.07 Msun, while the magnetic component has Teff ≍ 16,000 K, log g ≍ 8.5, and 0.76 ≤ M/Msun < 1.00. The parameters of the latter are less accurately determined since it appears to have an atmosphere dominated by an element other than hydrogen. At a distance of about 40 pc, the projected separation of the stellar components could be as large as 20 AU. The binary might therefore be a wide pair in which two stars of similar initial mass evolved into two very different end states. On the other hand, it could be an unusual product of close binary evolution, in which case this would be the first such example whose combined mass appears to exceed the Chandrasekhar limit. Potential implications and future observations to clarify the evolutionary and magnetic status of the system are suggested. Publication: The Astrophysical Journal Pub Date: November 1993 DOI: 10.1086/173403 Bibcode: 1993ApJ...418..426L Keywords: STARS: BINARIES: CLOSE; STARS: INDIVIDUAL ALPHANUMERIC: LB 11146; STARS: MAGNETIC FIELDS; STARS: WHITE DWARFS; ULTRAVIOLET: STARS full text sources ADS | data products SIMBAD (8) MAST (1)

Evidence of Complex Field Structure in the Magnetic White Dwarf in EXO 033319–2554.2

AbstractWe present detailed theoretical models for polarisation and spectroscopic data from EXO 033319–2554.2 that suggest the presence of two cyclotron emission regions of field strengths B ∼ 5.6 × 107 G and B ∼ 2.8 × 107 G, and which are separated by about 90° on the surface of the white dwarf. The large difference in magnetic field strength and the proximity of the two regions imply a more complex field structure than that of a centred dipole field distribution if it is assumed that the regions are connected by closed field lines.

Significance of proton relaxation time measurement in brain edema, cerebral infarction and brain tumors

We examined the proton relaxation times in vitro in various neurological diseases using experimental and clinical materials, and consequently obtained significant results for making a fundamental analysis of magnetic resonance imaging (MRI) as followings. 1) In the brain edema and cerebral infarction, T 1 prolonged and T 2 separated into two components, one fast and one slow. Prolongation of T 1 referred to the volume of increased water in tissue. The slow component of T 2 reflects both the volume and the content of increased edema fluid in tissue. 2) In the edematous brain tissue with the damaged Blood-Brain-Barrier (BBB), the slow component of T 2 became shorter after the injection of Mn-EDTA. Paramagnetic ion could be used as an indicator to demonstrate the destruction of BBB in the brain. 3) After the i.v. injection of glycerol, the slow component of T 2 became shorter in the edematous brain with the concomitant decrease of water content. The effects of therapeutic drug could be evaluated by the measurement of proton relaxation times. 4) Almost all tumor tissue showed a longer T 1 and T 2 values than the normal rat brain, and many of them showed two componnts in T 2. It was difficult to determine the histology of tumor tissue by the relaxation time alone because of an overlap of T 1 and T 2 values occurred among various types of brain tumors. 5) In vivo T 1 values of various brain tumor were calculated from the data of MRIs by zero-crossing method, and they were compared with the in vitro T 1 values which were measured immediately after the surgical operation. Though the absolute value did not coincide with each other due to differences in magnetic field strength, the tendency of the changes was the same among all kinds of tumors. It is concluded that the fundamental analysis of proton relaxation times is essentially important not only for the study of pathophysiology in many diseases but slso for the interpretation of clinical MRI.

Differences in auroral intensity at conjugate points

Conjugate auroral all-sky camera data obtained in 18 flights through the auroral zone near the College, Alaska, magnetic meridian show hemispherical differences in auroral frequency and intensity. Conjugate auroras on the equatorward boundary of the display (the equatorward arc system) are found to be consistently brighter in the northern hemisphere by a factor of approximately 1.3. At higher latitudes (in the poleward arc system) the auroras exhibit varying degrees of conjugacy, but the auroras in the north tend to occur more frequently and to be brighter. The hemispherical difference is attributed to the 8000-γ difference in magnetic field strength between the conjugate areas. Simplified theoretical calculations of the difference in precipitation at conjugate points from idealized cases of pitch angle diffusion and from a small hemispherical potential difference show that pitch angle diffusion may explain observations in the equatorward arc system but not in the poleward arc system. The evidence for hemispheric differences in auroral particle precipitation adds substantially to earlier indications of variation with longitude of auroral precipitation patterns.

On apparent differences in magnetic field strengths measured from Zeeman splittings of molecular lines and Fe i ? 5250.2 �

On apparent differences in magnetic field strengths measured from Zeeman splittings of molecular lines and Fe i ? 5250.2 �