Actual imaging slice profile of 2D MRI

It has been known that the actual slice profile of 2D MRI is often far form that of a perfect rectangular shape especially when the flip angle is large. This can be the source of error and image artifact in many MRI experiments. To carefully study this imperfection in RF excitation for various numerically optimized RF pulses, we have implemented an efficient numerical algorithm for simulating the evolution of the magnetization during a MR experiment. The scheme solves the Bloch equation via a numerical procedure that involves only the successive matrix multiplications, which are initialized by the values of RF amplitude and the frequency offset due to the companioning magnetic gradient field. It permits the consideration of the magnetization relaxation processes. The actual slice profile was numerically simulated by solving the Bloch equation for a given RF pulse shape and slice selection field gradient. The simulation results shows that the slice profile is often far from perfect for many commonly used frequency selective RF pulses. As a result of this, the apparent image intensity is an integrated signal over the slice envelope for the transverse magnetization for each pixel, which is an averaged contribution over the entire slice thickness, and can be significantly different than that predicted by the ideal case. As expected, this discrepancy worsens when the flip angle exceeds the flip angle for which the RF pulse was optimized. Furthermore, in the routine diagnostic imaging, because of the non-uniform excitation angle along the slice profile, it generates non-uniform steady state profile and can even alter the resulting image contrast.