Nuclear paramagnetism-induced MR frequency shift and its implications for MR-based magnetic susceptibility measurement
Jinil Park, Jeongtaek Lee, Jang-Yeon Park, Seung-Kyun Lee*
To investigate the 1H 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.
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.
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 1H spin polarization.
Magnetic susceptibility contribution from the nuclear paramagnetism should be taken into account in the interpretation of high‐precision phase and susceptibility mapping in MRI.