Nuclear paramagnetism-induced MR frequency shift and its implications for MR-based magnetic susceptibility measurement.

PURPOSE: To investigate the 1 H spin contribution (0.004 parts per million (ppm)) to the water magnetic susceptibility and discuss its implications for high-precision phase mapping and tissue susceptibility measurement. METHODS: Free induction decay (FID) signals were acquired at 3 Tesla (T) and 9.4T from thin square phantoms at a range of tip angles. The FID frequency shift was examined at a high resolution ( < 0.01 Hz) for different phantom orientations relative to the main magnetic field (B0 ). B0 maps on an axial and a coronal slice of a spherical phantom were obtained at 3T to examine the tip angle and orientation dependence at the 0.001 ppm level. RESULTS: A frequency shift of about 0.3 Hz was observed between tip angles of 10 ° and 90 ° when the thin phantom was normal to B0 at 3T, whereas the shift changed sign and was halved in magnitude when the phantom's face was parallel to B0 . At 9.4T, the effect size increased proportionately. The orientation-dependent frequency shift was also observed in the B0 map experiment. These observations agree with theoretical frequency shift due to longitudinal 1 H spin polarization. CONCLUSION: Magnetic susceptibility contribution from the nuclear paramagnetism should be taken into account in the interpretation of high-precision phase and susceptibility mapping in MRI. Magn Reson Med 77:848-854, 2017. © 2016 International Society for Magnetic Resonance in Medicine.