Rapid Fourier imaging using steady-state free precession.

Abstract

Reversal of the read gradient in a SSFP imaging experiment allows a full spin echo to be collected in the interval tau between successive rf pulses. Orthogonal gradient pulses are used to dephase and subsequently rephase the transverse magnetization each tau enabling 2D or 3D Fourier techniques. The minimum data collection time per slice in the 3D technique is 3.1 s (128 X 256). For a 2D data collection, an oscillating bipolar sawtooth gradient is used to select the slice. Each phase-encode value must be averaged over an equivalent portion of the oscillating slice-selection gradient and this condition gives a minimum of 25 s for 2D data collection. Excellent slice selection is achieved with less than 5% of the signal lying outside the slice profile central lobe. Images at 0.14 T show tissue contrast may be manipulated by changing the rf pulse angle, an example of which is the presence or absence of gray/white matter contrast at rf pulse angles of 30 and 90 degrees, respectively. The pulse angle theta dependence of five samples with different values of T2/T1 was measured and numerically calculated with good agreement between theory and experiment for theta less than or equal to 90 degrees.