CSI and sense CSI

Chemical shift imaging (CSI) is a magnetic resonance spectroscopy (MRS) method of localizing spectra from multiple locations at the same time. For localization, the principle of phase encoding as used in MRI is applied. Because the applied phase-encoding gradients are strong and short, CSI localization does not suffer from image warping and chemical shift displacement artifacts. The main disadvantage of CSI is the long acquisition time. For example, acquiring the minimum of a single encoding step per phase encode leads to an acquisition time of more than 34min for a twodimensional acquisition with a 32 × 32 matrix and a typical 2 s repetition time. The actual spatial resolution can be described with a point spread function, which exhibits strong side lobes due to the limited extent of the encoding matrix and tissue heterogeneity, leading to contamination from neighboring voxels. Filtering reduces the side lobes and their associated contamination at the cost of resolution and signal-to-noise ratio (SNR) per unit volume. Acquisition weighting can be used to simulate a filter during acquisition and thus optimize SNR. Inhomogeneities in the static magnetic field cause shifts in the frequency domain of spectra from voxels at different locations. Those shifts can be corrected using a field map or the water signal from an additional CSI acquisition without water suppression.Water and lipid suppression are performed with the same approaches as in single volume acquisitions. As MRS is challenged by the low metabolite concentrations, phased-array coils are often used to improve the SNR compared to volume coils. Because of the nonuniform sensitivity profiles of each coil, the signal amplitudes and phases from different coils vary among voxels from different locations. Combining data from different channels can be done using the initial points of the FIDs from each voxel. An SNR-optimized coil combination can be achieved using sensitivity maps, and the signals in the different voxels scaled to display a homogeneous sensitivity distribution. Acquisitions with phased arrays can also be accelerated with parallel imaging methods, typically shortening the above-mentioned 34min acquisition to 9min or less.