Biomedical subjects

Yansong Zhao

Publications and source records attributed to Yansong Zhao.

3 recordsLinked to original sources

PubMed2005

Computer simulation studies of the effects of dynamic shimming on susceptibility artifacts in EPI at high field.

Dynamic shimming in multi-slice imaging aims to achieve optimal magnetic field homogeneity by updating the shim coil currents for each slice in real time. Dynamic shimming may reduce the signal loss and geometric distortion caused by magnetic susceptibility variations between tissues and is likely to be valuable for fast T2*-sensitive imaging techniques like EPI. A computer simulation of dynamic shimming using real image data has been developed to demonstrate the effectiveness of higher order dynamic shimming for echo planar imaging at high magnetic field, and to investigate the potential benefits of different orders of shim coil. Geometric distortions and signal intensities for different degrees of dynamic shimming were simulated and the results are compared with the images obtained with a conventional shimming technique. These results demonstrate the effectiveness, necessity and difficulty of high order dynamic shimming.

Artifacts↗

PubMed2004

New approach for correcting distortions in echo planar imaging.

A new method is described that can correct the distortions due to multiple off-resonance effects in echo planar imaging, including those caused by B(0) field inhomogeneities, eddy currents, and gradient waveform imperfections. The proposed method uses a phase encoded acquisition and is as effective as the method of Chen and Wyricz (Chen and Wyricz, Magn Reson Med 1999;41:1206-1213) in correcting for distortions. Unlike Chen and Wyricz's approach, this new method works directly in distorted space and requires fewer scans. It also avoids the difficulties of phase unwrapping inherent in field mapping methods. Results using this new method with phantoms and human head scans at 3.0 T demonstrate the efficacy of the method in correcting distortions in both spin echo echo planar imaging (EPI) and gradient echo EPI.

Artifacts↗

PubMed2004

MRI simulator with object-specific field map calculations.

A new MRI simulator has been developed that generates images of realistic objects for arbitrary pulse sequences executed in the presence of static field inhomogeneities, including those due to magnetic susceptibility, variations in the applied field, and chemical shift. In contrast to previous simulators, this system generates object-specific inhomogeneity patterns from first principles and propagates the consequent frequency offsets and intravoxel dephasing through the acquisition protocols to produce images with realistic artifacts. The simulator consists of two parts. The input to part 1 is a set of "susceptibility voxels" that describe the magnetic properties of the object being imaged. It calculates a frequency offset for each voxel by computing the size of the static field offset at each voxel in the image based on the magnetic susceptibility of each tissue type within all voxels. The method of calculation is a three-dimensional convolution of the susceptibility-voxels with a kernel derived from a previously published method and takes advantage of the superposition principle to include voxels with mixtures of substances of differing susceptibilities. Part 2 produces both a signal and a reconstructed image. Its inputs include a voxel-based description of the object, frequency offsets computed by part 1, applied static field errors, chemical shift values, and a description of the imaging protocol. Intravoxel variations in both static field and time-dependent phase are calculated for each voxel. Validations of part 1 are presented for a known analytic solution and for experimental data from two phantoms. Part 2 was validated with comparisons to an independent simulation provided by the Montreal Neurological Institute and experimental data from a phantom.

Algorithms↗