SENSE shimming (SSH): A fast approach for determining B₀ field inhomogeneities using sensitivity coding

Abstract

The pursuit of ever higher field strengths and faster data acquisitions has led to the construction of coil arrays with high numbers of elements. With the sensitivity encoding (SENSE) technique, it has been shown that the sensitivity of those elements can be used for spatial image encoding. Here, a proofof-principle is presented of a method that can be considered an extreme case of the SENSE approach, completely abstaining from using encoding gradients. The resulting sensitivity encoded free-induction decay (FID) data are then not used for imaging, but for determining B 0 field inhomogeneity distribution. The method has therefore been termed "SENSE shimming" (SSH Although modern MR scanners produce highly homogeneous fields, this is often more than counterbalanced by the increased susceptibility effects at field strengths of 3T and higher. Therefore, today's scanners are equipped with a set of shim coils that can be used for reversing the effects of the subject-induced inhomogeneities. Strategies for finding optimal field correction parameters, called "shimming," can be classified into static and dynamic, depending on whether they are used before or during the acquisition. Among the most popular static shimming solutions are the line-shape characterization of free-induction decays (FIDs), along with stimulated-echo acquisition mode (STEAM)-based methods (1), projections along selected diagonals (2), and 2D and 3D field maps (3). For the dynamic case, methods acquiring a limited number of projections are popular (e.g., Refs. 4 -6) with every method having a tradeoff between acquisition speed and accuracy.For imaging, the concept of reducing acquisition time by using coil arrays was introduced with the well-known simultaneous acquisition of spatial harmonics (SMASH) (7) and SENSE (8) techniques, where the spatially-varying sensitivities of the individual coil elements are used as additional spatial encoding information. This allows the reduction of the amount of needed phase encoding steps while usually keeping the k-space extent unchanged; thus, the acquisition time can be shortened by a reduction factor R, with the number of coil elements being the theoretical maximum. The resulting foldovers are then resolved using previously measured sensitivities.The method presented here uses a special case of this setup: we abstain completely from using traditional frequency and phase encoding, thus yielding a foldover of all pixels into one; i.e., observing a single discrete point on an FID, which we will call the "FID point" throughout this work. We then address the question, as to whether a limited number of FID points seen by the elements of a coil array, along with the respective sensitivities, can be used to assess B 0 field inhomogeneities. The inhomogeneities are described using reasonable model functions, for example a spherical harmonic set, with a number N inhom of coefficients. It is clear that for the described method the number of coil elements, N coil , will be a theoretical upper limit for N inhom ; however,...

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