Magnetic resonance in medicine : official journal of the Society of Magnetic Resonance in Medicine
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A magnetic resonance spectroscopic imaging (MRSI) pulse sequence based on proton-echo-planar-spectroscopic-imaging (PEPSI) is introduced that measures two-dimensional metabolite maps in a single excitation. Echo-planar spatial-spectral encoding was combined with interleaved phase encoding and parallel imaging using SENSE to reconstruct absorption mode spectra. The symmetrical k-space trajectory compensates phase errors due to convolution of spatial and spectral encoding. ⋯ LCModel fitting enabled quantification of inositol, choline, creatine, and N-acetyl-aspartate (NAA) in vivo with concentration values in the ranges measured with conventional PEPSI and SENSE-accelerated PEPSI. Cramer-Rao lower bounds were comparable to those obtained with conventional SENSE-accelerated PEPSI at the same voxel size and measurement time. This single-shot MRSI method is therefore suitable for applications that require high temporal resolution to monitor temporal dynamics or to reduce sensitivity to tissue movement.
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MR susceptometry-based blood oximetry relies on phase mapping to measure the difference in magnetic susceptibility between intravascular blood and surrounding tissue. The main source of error in MR susceptometry is the static field inhomogeneity caused by an interface between air and tissue or between adjacent tissue types. ⋯ We propose an alternative method that acquires data without scanner-implemented default shimming, and fits, after appropriate weighting and masking, the static field inhomogeneity to a second-order polynomial. Compared to shimming the retrospective correction technique improved agreement between hemoglobin saturations measured in different segments of a vessel (femoral versus popliteal artery and vein) from three standard errors to less than one.
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A modification of the Stejskal-Tanner diffusion-weighting preparation with a single refocusing RF pulse is presented which involves three gradient lobes that can be adjusted to null eddy currents with any given decay rate to reduce geometric distortions in diffusion-weighted echo-planar imaging (EPI). It has a very similar compensation performance as the commonly used double-spin-echo preparation but (i) is less sensitive to flip angle imperfections, e.g. along the slice profile, and B(1) inhomogeneities and (ii) can yield shorter echo times for moderate b values, notably for longer echo trains as required for higher spatial resolution. It therefore can provide an increased signal-to-noise ratio as is simulated numerically and demonstrated experimentally in water phantoms and the human brain for standard EPI (2.0 x 2.0 mm(2)) and high-resolution EPI of inner field-of-views using 2D-selective RF excitations (0.5 x 1.0 mm(2)). Thus, the presented preparation may help to overcome current limitations of diffusion-weighted EPI, in particular at high static magnetic fields.