Static Spins Research Articles

Flow-velocity imaging from linear regression of phase images with techniques for reducing eddy-current effects

Proton NMR velocity images are obtained, in which the intensity is proportional a component of flow velocity, by velocity encoding the phase images. These velocity images of flowing water are constructed by a voxel-by-voxel linear regression of image phase versus m 1, a parameter which is proportional to the first moment of the magnetic field gradient and which is used to encode the constant but spatially varying velocity. At each image voxel the standard deviation of the velocity estimate is calculated and plotted as an image. This standard-deviation image provides a check on the accuracy of velocity calculations. The errors in the velocity calculations caused by eddy-current-induced phase shifts are discussed and two methods for reducing these errors are proposed. A pulse sequence with lower eddy-current sensitivity can be chosen and, in addition, static spins can be used for phase referencing. A pulse sequence consisting of time-shifted π pulses is shown to have lower eddy-current sensitivity than other pulse sequences considered. Phase referencing can be used with all methods of velocity encoding and reduces the eddy-current-induced velocity errors.

4731583 Method for reduction of MR image artifacts due to flowing nuclei by gradient moment nulling

A technique has been developed which reduces blood-flow artifacts in multi-echo, multi-slice magnetic resonance (MR) imaging by rephasing spins which have arbitrary velocity as well as static spins. The technique requires tailoring of the gradient structure along at least one of the three axes so as to null out in the preferred embodiment the zeroth and first moments of the gradient distributions.

Comments on57Fe Mössbauer spectra for Kramers doublets: A phenomenological interpretation

A static effective spinS=1/2 Hamiltonian is usually specified by a phenomenonological parameter, set, but the typical choice of parameters could be inappropriate. Two main issues are addressed: (i) Which spin Hamiltonian parameters are observable at a given applied field strength, and (ii) how should one deal with skewsymmetric components of the electronic Zeeman and magnetic hyperfine tensors that can be present for spin-coupled multinuclear complexes. Certain Mossbauer experiments were selected, somewhat arbitrarily, as examples; this analysis can be applied to other spectroscopic techniques as well.