摘要:

AbstractFears have been voiced that excessive tissue heating could occur in the event that first, a surface coil is placed with its axis parallel to the transmitting rffield leading to a maximal coupling of the two coils and second, the decoupling circuit of the surface coil breaks down. To avoid an rf coupling of the transmitting body coil to the receive‐only surface coil, conventionally applied surface coils are equipped with an active electronic rf decoupling circuit. In extensive worst‐case experiments on phantoms we have shown that no tissue heating occurs for surface coils which are equipped with semiconductor varicap diodes for tuning and matching. These coils should be safe for patient applications even if the decoupling circuit fails. Surface coils equipped with mechanically variable capacitors are generally passively decoupled. To simulate the worst‐case situation phantom experiments were performed in which a surface coil of this type having no passive decoupling circuit was coupled to the transmitter coil by its geometric position. Theoretical calculations, in agreement with the experimental results achieved during a 15‐min measurement in a 1.5‐T MRI whole‐body imager, show that a significant rfpower deposition in the tissue underneath the coil wire occurs, leading typically to a local specific absorption rate of 24 W /kg and a local temperature rise of 52°C. © 1989 Academ

ISSN:0740-3194    

DOI:10.1002/mrm.1910090112

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

年代:1989

数据来源: WILEY

摘要:

AbstractWhen NMR signal amplitudes achieved from spin‐echo sequences with several different repetition times are used to calculate T1, this can be done rapidly by calculating the time coordinate of the center of gravity of the data and determining which T1 should theoretically give this value. In particular, when a large number of similar calculations are made, the time gain may be massive. Simulations suggested that this procedure is only slightly less accurate than least‐squares fitting, and far more rapid. The procedure may also be useful in other types of T1 determination. © 1989 Academic Press,

ISSN:0740-3194    

DOI:10.1002/mrm.1910090113

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

年代:1989

数据来源: WILEY

摘要:

AbstractThe combination of a frequency nonselective excitation suppression method (1331sequence) with selective excitation followed by gradient‐induced dephasing of water transverse magnetization yielded suppression ratios of greater than 10,000:1. The need for gradient preemphasis and correction of Bo field shifts is discussed. The suppression efficiency of this method compared favorably to results obtained using the CPMG spin‐ echo technique to observe metabolite resonances in a urine sample. © 1989 Academic Press,

ISSN:0740-3194    

DOI:10.1002/mrm.1910090114

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

年代:1989

数据来源: WILEY

摘要:

AbstractLocalized proton NMR spectroscopy at 1.5 T using stimulated echoes has been applied to study metabolic alterations in the postischemic phase of patients with acute cerebral infarction. A complete depletion ofN‐acetyl aspartate in the area of infarction has been observed in a patient studied 4 days after stroke. This finding was paralleled by a dramatic increase in the concentration of lactic acid to about 16 mMwithin the lesion, indicating continued anaerobic glycolysis. The diluting effect of the edema has been estimated to reduce average metabolite concentrations by about a factor of 3. © 1989 Academic Press, I

ISSN:0740-3194    

DOI:10.1002/mrm.1910090115

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

年代:1989

数据来源: WILEY

摘要:

AbstractA method based on zero‐quantum coherence transfer for spectral editing of metabolites in aqueous solution in a one‐dimensional experiment is described. Water suppression factors of approximately 8000 were achieved by the use of pulsedB0field gradients and excellent editing was obtained in a single acquisition. The methyl resonances of both lactate and acetaldehyde were readily observed in a mouse brain homogenate by generat‐ ing zero‐quantum coherence using Gaussian pulses for selective excitation of the respective CH resonances. © 1989 Academic Pr

ISSN:0740-3194    

DOI:10.1002/mrm.1910090116

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

年代:1989

数据来源: WILEY

摘要:

AbstractBipolar flow‐encoding gradients can be used in a three‐dimensional magnetic resonance imaging procedure to provide a noninvasive measure ofin vivoblood flow. The resulting volume angiogram is a three‐dimensional data matrix which can be retrospectively analyzed and displayed in a variety of ways. This angiographic technique provides good suppression of signals arising from stationary tissue, thereby permitting the visualization of small vessels having relatively slow flow. This suppression is obtained by modulating the amplitude of the flow‐encoding gradient pulse to either cancel the stationary tissue signal or displace it relative to the flow signal in the volume image. © 1989 Academic Pr

ISSN:0740-3194    

DOI:10.1002/mrm.1910090117

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

年代:1989

数据来源: WILEY

ISSN:0740-3194    

DOI:10.1002/mrm.1910090119

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

年代:1989

数据来源: WILEY

ISSN:0740-3194    

DOI:10.1002/mrm.1910090101

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

年代:1989

数据来源: WILEY