Electrical engineering

Motion during diffusion-weighted imaging (DWI) introduces phase errors that can cause significant artifacts in brain images. One method of correcting these errors uses additional navigator data to measure the phase corruptions. Standard navigator methods correct for rigid-body motion but cannot correct for nonrigid deformations of the brain related to the cardiac cycle. This work derives a generalized reconstruction that corrects for nonrigid motion based on a least-squares formulation. Since this reconstruction has the disadvantage of being computationally expensive, an approximation is presented, called a refocusing reconstruction. The refocusing reconstruction is both efficient and straightforward. Each readout is multiplied in image space by the phase conjugate of the navigator image, and these rephased readouts are then summed. The conditions under which the refocusing reconstruction is sufficient are considered and methods to improve the quality of refocused images are discussed. In particular, synchronization of the acquisition to the cardiac cycle can provide data that is well-conditioned to the refocusing reconstruction without incurring the large time penalty traditionally associated with cardiac gating. These methods are applied to steady-state DWI, a promising pulse sequence that is particularly sensitive to motion-induced phase artifacts. The refocusing reconstruction is shown to significantly improve SS-DWI over standard rigid-body corrections. Magn Reson Med 50:343–353, 2003. © 2003 Wiley-Liss, Inc.

In functional MRI (fMRI) the resonance frequency shift induced from respiration is a major source of physiological noise. In transition-band SSFP fMRI the respiration-induced resonance offset not only increases noise interference, it also shifts the activation band. This leads to a reduction in the contrast-to-noise ratio (CNR) and the potential for varying contrast levels during the experiment. A novel real-time method that compensates for the respiration-induced resonance offset frequency is presented. This method utilizes free induction decay (FID) phase information to measure the resonance offset. For compensation, one can update the resonant frequency in real time by changing the transmit RF pulse and receiver phases to track the measured offset. The results show decreased signal power in the respiration frequency band and increased numbers of activated voxels with higher Z-scores compared to uncompensated experiments. Magn Reson Med, 2006. © 2006 Wiley-Liss, Inc.

Diffusion-weighted steady-state free precession (DW-SSFP) accumulates signal from multiple echoes over several TRs yielding a strong sensitivity to diffusion with short gradient durations and imaging times. Although the DW-SSFP signal is well characterized for isotropic, Gaussian diffusion, it is unclear how the DW-SSFP signal propagates in inhomogeneous media such as brain tissue. This article presents a more general analytical expression for the DW-SSFP signal which accommodates Gaussian and non-Gaussian spin displacement probability density functions. This new framework for calculating the DW-SSFP signal is used to investigate signal behavior for a single fiber, crossing fibers, and reflective barriers. DW-SSFP measurements in the corpus callosum of a fixed brain are shown to be in good agreement with theoretical predictions. Further measurements in fixed brain tissue also demonstrate that 3D DW-SSFP out-performs 3D diffusion weighted spin echo in both SNR and CNR efficiency providing a compelling example of its potential to be used for high resolution diffusion tensor imaging. Magn Reson Med 60:405–413, 2008. © 2008 Wiley-Liss, Inc.

Magnetic resonance elastography (MRE) is a noninvasive method of measuring tissue compliance. Current MRE methods rely on custom-built hardware to elicit vibrations that are tracked by MR imaging. Knowledge of the wave propagation can be used to calculate the local shear stiffness of the tissue. We sought to determine whether the vibrations of the patient table that result from low-frequency switching of the imaging gradients could be used as an alternative mechanical driving mechanism for MRE. We designed a pulse sequence that includes a gradient lobe specifically for the excitation of mechanical resonance, allowing control of the time between the onset of the vibrations and the velocity-encoding of the readout. Data collected from a gelatin phantom with stiff cylindrical gelatin inserts demonstrated that wave propagation can be imaged with this method. Postprocessing to estimate the local spatial frequency of the waves also allows estimation of the local shear stiffness, where the stiff inserts are clearly identifiable. Data collected on the brain of a normal healthy volunteer showed clear rotational waves propagating from the skull inwards, also allowing generation of shear stiffness maps. Magn Reson Med, 2009. © 2009 Wiley-Liss, Inc.