Frequency shifts in parametrically enhanced low-field MR detection.
A high-amplitude, high-frequency readout field has previously been proposed for use with low-field MR. Because the resulting modulation sidebands are at higher frequencies than the low-field steady precession, improved detection sensitivity results. However, if the ac readout field is inhomogeneous, it will necessarily have transverse components resulting in frequency shifts and broadening of the MR signal. Numerical solutions of Bloch's equations are compared to the Bloch-Siegert result to assess the size of the frequency shifts. A formula is derived by the average Hamiltonian method and provides an excellent fit to the numerically obtained shifts.
Sinkovits
Daniel W
DW
Department of Physics, Washington University, CB 1105, St. Louis, MO 63130-4899, USA.
Conradi
Mark S
MS