摘要:

AbstractDesigning gradient coils for permanent magnets requires modeling the interaction between the currents and the ferromagnetic material. We have developed and tested an iterative design approach based on the boundary element method. The power dissipated by the gradient coil is treated as a constraint of the minimization problem. The technique is illustrated on a magnet with a rectangular prismatic cavity and two ferromagnetic plates adjacent to the coil. The procedure improved the linearity of the field from 10.5% to 3.4% and reduced the power dissipation to 63% of the power of the unoptimized coil. The optimized distribution of currents has been transformed to variable locations of coil windings without loss of gradient uniformity. The new approach significantly reduces computation time and memory storage and yields practical designs of gradient coils for permanent magnets.

ISSN:1053-1807    

DOI:10.1002/jmri.1880060142

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1996

数据来源: WILEY

摘要:

AbstractWe developed a statistically based error image for rapid appreciation of unreliable regions in quantitative water proton T1 images. The chi‐squared error and coefficient of variation of the fitted parameter were used to estimate uncertainties in the goodness‐of‐fit to monoexponential T1 relaxation and the reliability of the calculated T1, respectively, for each pixel. Errors exceeding a statistical threshold based on a .1 acceptance criterion were displayed as a color‐coded overlay on the T1 image. Error maps of quantitative T1 images from 31 healthy volunteers showed a characteristic error structure; few pixels within the parenchyma had excessive errors. Clinical cases with stroke and sickle cell disease showed deviations from the normal pattern in the spatial distribution and magnitude of chisquared errors. Disease states may deviate from monoexponential T1 relaxation more than normal brain does. The color‐coded error map is a valuable tool for investigators using quantitative MR imaging to determine tissue relaxation p

ISSN:1053-1807    

DOI:10.1002/jmri.1880060143

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1996

数据来源: WILEY

摘要:

AbstractOocytes fromXenopus laeviswere used as a well‐established model to investigate spatially resolved changes in relaxation time (T1) within a cell during exposure to copper complexes by means of 'H‐NMR‐microscopy. T1 relaxation of intracellular water was shortened in dependence on the complex concentration, the water content, and the water mobility in various cell compartments. A relatively constant T1 decrease was observed in the cytoplasm of the vegetal pole, irrespective of the type and concentration of the permeable complexes used. Since the lowest content of free mobile water molecules was detected at the vegetal pole, we concluded that this quantity was as important for the relaxation time decrease as that of the chelated paramagnetic ions. Experiments using14C‐labeled inulin demonstrated that the paramagnetic metal complexes entered the oocytes without gross injury to their plasma me

ISSN:1053-1807    

DOI:10.1002/jmri.1880060144

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1996

数据来源: WILEY

摘要:

AbstractA meta‐analyses was performed to evaluate the safety of gadodiamide injection (OMNISCAN®) for magnetic resonance imaging in two different age groups (<65 years; ≥ 65 years). Data on vital signs, clinical laboratory parameters, and subjectively experienced adverse events were reviewed for 734 patients included in 19 European Phase II and III trials with gadodiamide injection (0.1 mmol/kg body weight or 0.3 mmol/kg body weight) used in magnetic resonance imaging. One hundred sixty‐four patients were 65 years of age or older. No statistically significant differences were shown between this population and the population younger than 65 years of age with respect to vital signs or clinical laboratory parameters. A total of 48 adverse events, discomfort excluded, were reported, with no significant difference in frequency between the two age groups. Injection‐associated discomfort was significantly (P=.0025) more frequent in the younger (9.2%) than in the older group (2.5%). Gadodiamide injection is in conclusion a safe contrast medium in older as well as in younger

ISSN:1053-1807    

DOI:10.1002/jmri.1880060145

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1996

数据来源: WILEY

摘要:

AbstractThe local magnetic fields from computer monitors are well known. Recently a hypothesis was published that these magnetic fields may lead to a 400% increase of mercury dissolution from dental amalgam fillings. In MRI the exposure to the magnetic field exceeds by far the exposure from computer monitors. Therefore, this study examined the amalgam‐related mercury release for typical MRI conditions, separated for both the static and the variable magnetic fields in a 1.5 T MR‐unit. First, 20 dental cavities were filled with a non‐gamma‐2 amalgam, and the mercury release was measured for 14 days in a non‐magnetic environment. Second, one half of the specimens were exposed to a static magnetic field for 24 hours (group A), while the other specimens were exposed to a repetitive gradient echo sequence for 60 minutes (group B). In both experiments there was no significant increase in mercury release due to MRI. This in vitro study demonstrated no evidence of an elevated mercury dissolution from a nongamma‐2 amalgam during magnetic field exposure by MRI; therefore, there is no increased ris

ISSN:1053-1807    

DOI:10.1002/jmri.1880060146

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1996

数据来源: WILEY

摘要:

AbstractOverhauser enhancement can increase the signal available in low‐field imaging systems. If the signal is enhanced by a factor of 40, low‐field Overhauser‐enhanced imaging may produce contrast equivalent to that achieved in subtraction MR mammography performed at 1.5 T. This article describes experiments in which Overhauser enhancement is performed with nitroxide free radicals at field strengths near 100 G. The enhanced signal is much less than 40, and less than the equilibrium signal at 0.25 T, in spite of absorbed power levels greater than 100 W/kg. Although the signal enhancement for nitroxides is disappointing, it is likely that other free radical contrast agents will be developed that offer higher Overhauser enhanc

ISSN:1053-1807    

DOI:10.1002/jmri.1880060147

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1996

数据来源: WILEY

摘要:

AbstractMagnetic resonance images of patients wearing a surgical halo may have unacceptable artifacts if the halo has a conductive loop structure. This study shows that the observed artifacts are predominantly due to eddy currents generated in the halo by switching field gradients, and that these artifacts can be substantially reduced by adjusting the phase encoding direction in MRI sequences so that it is parallel to the axis of the halo.

ISSN:1053-1807    

DOI:10.1002/jmri.1880060148

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1996

数据来源: WILEY

ISSN:1053-1807    

DOI:10.1002/jmri.1880060149

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1996

数据来源: WILEY

ISSN:1053-1807    

DOI:10.1002/jmri.1880060150

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1996

数据来源: WILEY

ISSN:1053-1807    

DOI:10.1002/jmri.1880060101

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1996

数据来源: WILEY