Gregory C. Hurst

In-vivo spatially encoded magnetic resonance spectroscopy with solvent suppression

A binomial pulse generator (32) selectively generates binomial radio frequency excitation pulses (60) which induce magnetic resonance only in selected hydrogen dipoles and suppresses resonance in others. An inversion pulse generator (34) generates a first inversion pulse (70) in the presence of a first magnetic field gradient (72) generated by a gradient control (22). The inversion pulse only inverts the magnetization of resonating nuclei in a first plane defined by the first magnetic field gradient. A second inversion pulse (74) applied in the presence of a second magnetic field gradient (76) inverts the magnetization of resonating nuclei in a second planar region defined by the second magnetic field gradient. A third inversion pulse (78) applied concurrently with a third magnetic field gradient (80) inverts the magnetization of resonating nuclei in a third planar region defined by the third magnetic field gradient. Only resonating nuclei in a volumetric element defined at the intersection of the first, second, and third planes are inverted all three times. The magnetization of other dipoles will have dephased differently from the dipoles in the volumetric element. In this manner, only the dipoles in the volumetric element contribute to a spin echo (82) which follows the third inversion pulse. Data acquired during the third spin echo may be spectrographically analyzed to determine the chemical composition within the volumetric element. Alternately, a phase encoding gradient (90) and a read gradient (92) may be applied as part of the sequence to provide the appropriate phase encoding to the acquired data such that the acquired data can be reconstructed into an image representation.

Multiecho multimoment refocussing of motion in magnetic resonance imaging: MEM-MO-RE

Gradient moment nulling techniques for refocussing of spin dephasing resulting from movement during application of magnetic resonance imaging gradients have gained widespread application. These techniques offer advantages over conventional imaging gradients by reducing motion artifacts due to intraview motion, and by recovering signal lost from spin dephasing. This paper presents a simple technique for designing multiecho imaging gradient waveforms that refocus dephasing from the interaction of imaging gradients and multiple derivatives of position. Multiple moments will be compensated at each echo. The method described relies on the fact that the calculation of time moments for nulled moment gradient waveforms is independent of the time origin chosen. Therefore, waveforms used to generate the second echo image for multiple echo sequences with echo times given by TEn = TE1 + (n - 1) * (TE2 - TE1) may also be used for generation of the third and additional echo images. All echoes will refocus the same derivatives of position. Multiecho, multimoment refocussing (MEM-MO-RE) images through the liver in a patient with ampullary adenocarcinoma metastatic to the liver demonstrate the application of the method in clinical scanning.

Some noise properties of 2DFT MR images from asymmetrically sampled data

This report describes noise statistics in 2DFT MR images, expanding the earlier work of Henkelman and others to include variably asymmetric sampling and conjugate synthesis reconstruction. The effects of low-order polynomial and Fourier phase correction used with conjugate synthesis are also explicitly considered. This analysis shows that complex images obtained by conjugate synthesis have an elliptical noise distribution, with the smaller axis corresponding to the imaginary image channel. Derivations and simulations predict a ratio of mean to standard deviation in the background of magnitude images varying from the known value of square root of pi/(4 - pi) (approximately 1.91) for full symmetry to square root of 2/(pi - 2) (approximately 1.32) at fully asymmetric or half-echo sampling; these predictions are validated over a range of asymmetry by experimental measurements. These results are important for predicting and interpreting image noise when using asymmetric sampling.