SERA: A technique to improve the performance of the 3D sequence by reducing aliasing artifacts in edge slices

Introduction: It has been shown that the 3D acquisition method can provide better temporal Signal to Noise Ratio (tSNR) for high-resolution fMRI studies (1) compared to the 2D acquisition method. However, there are several practical issues that lower the efficiency of the 3D method and make it less attractive than the 2D method. Among them, an important one is the wrap-around aliasing artifact in the slice-select direction. For 3D acquisition, the thickness of the excited slab is usually set equal to the field of view in the slice-select direction (zFOV) to maintain acquisition efficiency. If zFOV is big enough to cover the whole brain, there is no need to worry about aliasing artifacts. However, using a very large zFOV is not practical in high-resolution fMRI because it requires a large amount of phase encodings in slice-select direction, which cannot be easily fit into an acceptable volume TR. When zFOV is smaller than the brain, aliasing artifacts show up in edge slices. Using a high performance excitation pulse with a sharp transition band can significantly reduce the number of slices affected by aliasing artifacts. However, the highest time-bandwidth product is limited by the maximal B1 that a RF system can provide. Typically, with the use of the high performance excitation pulse, only one slice at each end is severely contaminated by aliasing artifacts. Although throwing away those two slices can easily solve the problem, it surely reduces the efficiency of the 3D method, especially when the number of slices is not large. In this work, a new technique called Slab Excitation with Reduced Aliasing (SERA) is introduced. The SERA technique can be illustrated in Figure 1. It excites and saturates spins in two thin slices (one at each edge) that are just beyond zFOV before the traditional high performance excitation pulse excites the whole slab for 3D acquisition. Since spins outside of zFOV that might be excited by the high performance excitation pulse are saturated at the beginning, they won’t contribute to the signal. Therefore, aliasing artifacts in edge slices can be greatly reduced.