摘要:

AbstractThe mathematical tools that were developed in Part I are used to show quantitatively how the signal‐to‐noise ratio and the ringing time depend on Q factors, and thus how overall performance can be improved. The noise factor of a model preamplifier also is considered, and the minimum noise factor is derived in terms of the impedances of the probe and of the preamplifier. The optimization of the noise factor in realistic signal reception systems is discus

ISSN:1043-7347    

DOI:10.1002/cmr.1820040402

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractGenerating a magnetic resonance image requires that all three spatial dimensions of a spin system somehow get encoded into NMR signals. Although there are many ways this can be done, by far the most common is to restrict the excitation to a slice with the use of frequency‐selective radio frequency (rf) pulses in the presence of a magnetic field gradient and to encode the remaining two spatial dimensions into echo signals with two mutually orthogonal „in‐plane”︁ gradients. In the standard scheme, spins along one in‐plane dimension are localized by receiving the echo signal in the presence of a „read‐out”︁ gradient. Spins along the other in‐plane dimension are localized with the use of „phase‐encoding”︁ gradients of different amplitudes applied for short periods prior to each readout. Just how these processes yield a data set whose two‐dimensional (2D) Fourier transformation represents the spin density within the selected slice is a major topic of this discussion. A topical extension, to rf echo planar methods in which the usual phase‐encoding process is accelerated with the use of multiple echoes, is also presented. The effects on image contrast and artifact when different echoes are used for distinct phase‐encoding steps in the presence ofT2decay (the spin‐spin relaxati

ISSN:1043-7347    

DOI:10.1002/cmr.1820040403

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractNMR rotating‐frame spin‐lattice relaxation (Tip) is a potentially powerful tool for the investigation of chemical exchange processes in the microsecond time range, a time range that is not readily examined by the usual solution NMR methods. This article deals with on‐resonance rotating‐frame‐relaxation measurements. The relaxation mechanism is analyzed to provide a theoretical basis for accurate evaluation of the contribution of chemical exchange toT1prates. The procedure described permitsT1pmeasurements for medium‐sued and large molecules in solution. Interpretation of the exchange contribution provides the dynamic parameters of exchange lifetime and chemical shift difference between exc

ISSN:1043-7347    

DOI:10.1002/cmr.1820040404

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractThe use of selective excitation of individual multiplets or transitions in an NMR spectrum has long been suggested for spectral interpretation, observation of nuclear Overhauser effects, investigation of exchange kinetics, polarization transfer, suppression of unwanted signals, spectral editing, measurement of dipolar cross‐relaxation rates, slice and/or volume inin vivospectroscopy, microscopy, and NMR imaging. A general view of theory and applications of selective excitation techniques are provided. The different ways of achieving the desired frequency selectivity or biselectivity (tailoring of hard pulses, shaping of soft pulses, trains of hard pulses) in the excitation profile are summarize

ISSN:1043-7347    

DOI:10.1002/cmr.1820040405

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

ISSN:1043-7347    

DOI:10.1002/cmr.1820040406

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

ISSN:1043-7347    

DOI:10.1002/cmr.1820040401

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY