Magnetic field gradients and coherence-pathway elimination.
The amount of residual transverse magnetization after the application of a pulsed magnetic-field gradient (PFG) is calculated for a homogeneous volume of interest (VOI) with three orthogonal pairs of sides and for VOIs that deviate from this idealized case. Many of the results can be understood in terms of the Fourier-transform relationship between the VOI intensity profile as a function of position, I(x), and the residual signal as a function of net PFG strength, S(k). Dephasing of transverse magnetization is more effective with imperfect slice-selection profiles, a circular sample cross section, or decreased B1 intensity near the edges of a sample than in the idealized case. When two or more orthogonal PFGs are applied, the net amount of dephasing depends on the source of intensity variation (sample geometry or slice-selective radiofrequency pulse profiles) and the orientation of the PFGs. If a gradient is parallel to one side of a VOI with rectangular cross sections, dephasing is more effective if a second, orthogonal gradient of equal intensity is added than if the original intensity is doubled. In contrast, if a PFG is orthogonal to the axis of an NMR tube with a circular cross section, addition of a second gradient orthogonal to the tube axis provides only slightly greater dephasing and is less effective than doubling the original intensity.
Department of Diagnostic Imaging, St. Jude Children's Research Hospital, Memphis, Tennessee 38105-2794, USA.
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Kingsley
D005583:Fourier Analysis; D009682:Magnetic Resonance Spectroscopy; D008280:Magnetics