Biomedical subjects

Heidi A Ward

Publications and source records attributed to Heidi A Ward.

4 recordsLinked to original sources

PubMed2006

Imaging artifacts at 3.0T.

Clinical MRI at a field strength of 3.0T is finding increasing use. However, along with the advantages of 3.0T, such as increased SNR, there can be drawbacks, including increased levels of imaging artifacts. Although every imaging artifact observed at 3.0T can also be present at 1.5T, the intensity level is often higher at 3.0T and thus the artifact is more objectionable. This review describes some of the imaging artifacts that are commonly observed with 3.0T imaging, and their root causes. When possible, countermeasures that reduce the artifact level are described.

Artifacts↗

PubMed2004

Rapid T2 estimation with phase-cycled variable nutation steady-state free precession.

Variable nutation SSFP (DESPOT2) permits rapid, high-resolution determination of the transverse (T2) relaxation constant. A limitation of DESPOT2, however, is the presence of T2 voids due to off-resonance banding artifacts associated with SSFP images. These artifacts typically occur in images acquired with long repetition times (TR) in the presence of B0 inhomogeneities, or near areas of magnetic susceptibility difference, such that the transverse magnetization experiences a net phase shift during the TR interval. This places constraints on the maximum spatial resolution that can be achieved without artifact. Here, a novel implementation of DESPOT2 is presented incorporating RF phase-cycling which acts to shift the spatial location of the bands, allowing reconstruction of a single, reduced artifact-image. The method is demonstrated in vivo with the acquisition of a 0.34 mm3 isotropic resolution T2 map of the brain with high precision and accuracy and significantly reduced artifact.

Brain↗

PubMed2002

Spherical navigator echoes for full 3D rigid body motion measurement in MRI.

We developed a 3D spherical navigator (SNAV) echo technique that can measure rigid body motion in all six degrees of freedom simultaneously by sampling a spherical shell in k-space. 3D rotations of an imaged object simply rotate the data on this shell and can be detected by registration of k-space magnitude values. 3D translations add phase shifts to the data on the shell and can be detected with a weighted least-squares fit to the phase differences at corresponding points. MRI pulse sequences were developed to study k-space sampling strategies on such a shell. Data collected with a computer-controlled motion phantom with known rotational and translational motions were used to evaluate the technique. The accuracy and precision of the technique depend on the sampling density. Roughly 2000 sample points were necessary for accurate detection to within the error limits of the motion phantom when using a prototype time-intensive sampling method. This number of samples can be captured in an approximately 27-ms double excitation SNAV pulse sequence with a 3D helical spiral trajectory. Preliminary results with the helical SNAV are encouraging and indicate that accurate motion measurement suitable for retrospective or prospective correction should be feasible with SNAV echoes.

Humans↗

PubMed2002

Real-time autoshimming for echo planar timecourse imaging.

Head motion within an applied magnetic field alters the effective shim within the brain, causing geometric distortions in echo planar imaging (EPI). Even if subtle, change in shim can lead to artifactual signal changes in timecourse EPI acquisitions, which are typically performed for functional MRI (fMRI) or diffusion tensor imaging. Magnetic field maps acquired before and after head motions of clinically realistic magnitude indicate that motion-induced changes in magnetic field may cause translations exceeding 3 mm in the phase-encoding direction of the EPI images. The field maps also demonstrate a trend toward linear variations in shim changes as a function of position within the head, suggesting that a real-time, first-order correction may compensate for motion-induced changes in magnetic field. This article presents a navigator pulse sequence and processing method, termed a "shim NAV," for real-time detection of linear shim changes, and a shim-compensated EPI pulse sequence for dynamic correction of linear shim changes. In vivo and phantom experiments demonstrate the detection accuracy of shim NAVs in the presence of applied gradient shims. Phantom experiments demonstrate reduction of geometric distortion and image artifact using shim-compensated EPI in the presence of applied gradient shims. In vivo experiments with intentional interimage subject motion demonstrate improved alignment of timecourse EPI images when using the shim NAV-detected values to update the shim-compensated EPI acquisition in real time.

Artifacts↗