UNIPOLAR FAST SPIN ECHO FOR PERMANENT MAGNET MRI

摘要

A method of reducing artifacts produced during Fast Spin Echo measurements made using permanent magnet NMR instruments. The method includes applying encoding gradients that do not switch signs throughout the experiment. Prior to the 90° RF pulse, a strong RM gradient pulse is given to produce a dominant and constant residual magnetization. The encoding is done through the combination of encoding gradients with the aid of the 180° RF pulses of the echo train. A first constant encoding gradient is given before the first 180 pulse. Then two variable encoding gradients are provided after each 180 pulse; one applied prior to and one applied subsequent to each acquisition in the echo train.

主权项

1. A method for reducing artifacts produced during a Fast Spin Echo (FSE) sequence in a nuclear magnetic resonance instrument comprising a gradient power amplifier and a permanent magnet comprising a plurality of pole pieces, said permanent magnet characterized by a gradient-dependent residual magnetization Bg, said FSE pulse sequence comprising, in order:
a 90° RF pulse 100 characterized by a duration T90; an echo train comprising a first 180° RF pulse 110, a first echo 130, X−1 additional 180° RF pulses 110a and X−1 additional echoes 130a, X≧1; and, a navigator pulse 120;wherein said method comprises:
applying a residual magnetization (RM) pulse 300 prior to said 90° RF pulse, said RM pulse having amplitude GRMpe along a phase axis and either positive or negative sign relative to an origin along said phase axis; applying a shim gradient pulse 200 of amplitude at least sufficient to cancel any net effect on said gradient-dependent residual magnetization Bg produced by said RM pulse 300; applying, prior to said first 180° RF pulse, a single preparation gradient pulse 310 characterized by amplitude Gshiftpe≦GRMpe, whereby for a gradient pulse duration T,kshiftpe=GshiftpeT along said phase axis; applying, for each 180° pulse i (1≦i≦X) in said echo train:
a first encoding gradient pulse 320 applied subsequent to 180° pulse i and prior to echo i, said first encoding gradient pulse characterized by amplitude Gaipe such that kaipe=GaipeT along said phase axis; and,a second encoding gradient pulse 330 applied subsequent to said echo i and prior to 180° pulse i+1, said second encoding gradient pulse characterized by amplitude Gbipe such that kbipe=GbipeT along said phase axis, said second encoding gradient Gbipe having an amplitude of identical sign to the sign of the amplitude of said RM pulse; and, applying, subsequent to said navigator pulse, a final gradient pulse 340, said final gradient pulse having an amplitude of sign identical to the sign of said RM pulse.