Spoiling of transverse magnetization in gradient-echo (GRE) imaging during the approach to steady state.

ABSTRACT

The signal evolution behaviors and corresponding image appearances for different methods of spoiling or refocusing the transverse magnetization in short TR gradient-echo imaging during the approach to steady state were investigated experimentally and using computer simulations based on the Bloch equations. Specifically, ideally spoiled, gradient-spoiled, gradient-refocused, and RF-spoiled pulse sequence configurations were studied. This study showed that, for the gradient-spoiled configuration, the signal evolution is position and phase-encoding order-dependent and, under typical imaging conditions, can deviate substantially from the ideally spoiled signal evolution at some spatial positions, resulting in intensity banding image artifacts. For the gradient-refocused configuration, the signal evolution oscillates toward the steady state and, generally, does not closely approximate that of ideal spoiling, resulting in different image contrast or image blurring. Using RF spoiling, the signal evolution closely approximates the ideally spoiled case for flip angles less than approximately 20 degrees and T2 values of less than approximately 200 ms and results in relatively artifact-free images. Also, this study showed that, for RF spoiling, an RF-pulse phase-difference increment other than 117 degrees, such as 84 degrees may be optimal for gradient-echo imaging during the approach to steady state.