We have observed in a clinical scanner that the center frequency of water MRI changed with the 1H spin flip angle. The amount of the change could be explained by the tip-angle dependent nuclear spin paramagnetic susceptibility, much the same way as the usual, tissue electronic para- and dia-magnetic susceptibility induces B0 shift in MRI. The observed shift, corresponding to nuclear susceptibility of +0.004 ppm in water, may affect the ultimate accuracy of MR-based tissue magnetic susceptibility measurements if not properly accounted for.
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