Compensation of offresonance magnetization transfer artifact in SPECIAL at 7T

Introduction: The increase in spectral separation and signal-to-noise ratio at ultra-high field (7T and higher) strength enables the detection of a large number of low concentrated or coupled spin systems aside from the big landmark peaks NAA, creatine and choline. However, their detection is complicated by relatively short T2 relaxation times at ultra-high field strength (7T). The large spectral separation and fast T2 relaxation call for a localization approach that controls the chemical shift displacement artifact while keeping echo-times short. Mlynárik et. al. [1] introduced an elegant approach combining 1D ISIS and 2D spin echo encoding for a localization scheme called SPECIAL. With this approach 3D localization can be reached by using only one slice selective refocusing pulse instead of two as in conventional localization schemes. However, besides the spatial labeling the slice selective inversion pulse used for ISIS encoding prior to the excitation also acts as a magnetization transfer pulse. It was shown among others by Wolf et.al. [2] that magnetization transfer can take place between mobile, observable protons and a bound NMR invisible proton pool. Other authors [3,4,5] demonstrated that magnetization transfer can also lead to significant changes in metabolite signals like creatine, glutamate, glutamine, lactate or myo-Inositol [6]. A similar effect can also be observed for lipids in the skull which can complicate ISIS based localization schemes for H spectroscopy. In this work, it is shown that the fat contamination which occurs in single voxel SPECIAL experiments without outer volumes suppression stems from magnetization transfer due to the ISIS inversion pulse. This leads to different lipid signal intensities in the alternating scans with and without ISIS pulse and therefore results in incomplete cancellation of outer volume fat. An alternative method to the often SAR demanding outer volume saturation is presented to deal with this specific artifact.