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Physics, Techniques and Procedures

Slice rephasing gradient

magnetic gradient field pulse applied after a one-dimensional spatially selective excitation pulse to rephase the transverse magnetization Mxy across the slice. Slice selection is generally accomplished by applying a narrow bandwidth radiofrequency RF pulse in the presence of a constant magnetic field gradient (the slice select gradient). The slice select gradient causes the resonant frequency to depend on position in the direction of this gradient, and therefore defines the location of spins whose resonant frequency is within the bandwidth of the RF pulse. The finite spread of frequencies within the excited slice causes the transverse magnetization at different locations in the slice direction to be at a different phase at the end of the slice. For flip angles of approximately 90 or less, the phase distribution across the slice is approximately linear and can be rephased by a gradient of opposite polarity to that of the Slice selection gradient. The area of this refocusing gradient lobe is approximately equal to the area of the slice select gradient from the centre of the RF pulse until the slice select gradient is turned off. If the RF pulse is symmetrical and centred over the slice select gradient lobe, the area of the refocusing lobe is half that of the slice select gradient.

NP