Retrospective image correction in the presence of nonlinear temporal magnetic field changes using multichannel navigator echoes

Abstract

Spatio-temporal magnetic field changes in the brain caused by breathing or body movements can lead to image artifacts. This is especially a problem in T(2)(*)-weighted sequences. With the acquisition of an extra echo (navigator), it is possible to measure the magnetic field change induced frequency offset for a given slice during image acquisition. However, substantial local variation across a slice can occur. This work describes an extension of the conventional navigator technique that improves the estimation of the magnetic field distribution in the brain during strong field fluctuations. This is done using the combination of signals from multiple coil elements, the coil sensitivity profiles, and frequency encoding: termed sensitivity-encoded navigator echoes. In vivo validation was performed in subjects who performed normal breathing, nose touching, and deep breathing during scanning. The sensitivity-encoded navigator technique leads to an error reduction in estimating the field distribution in the brain of 73% ± 16% compared with 56% ± 14% for conventional estimation. Image quality can be improved via incorporating this navigator information appropriately into the image reconstruction. When the sensitivity-encoded navigator technique was applied to a T(2)(*)-weighted sequence at 7 T, a ghosting reduction of 47% ± 13% was measured during nose touching experiments compared with no correction.

Utility of real-time field control in T2 *-Weighted head MRI at 7T

“…Versatile improvement of image quality in T 2 * imaging at high field holds promise for a range of applications. Field control is expected to be particularly useful in patients that breathe deeply and irregularly as has been observed in Alzheimer's disease patients . In both Alzheimer's and Huntington's disease, T 2 *‐weighted imaging is increasingly used to visualize cortical changes in the early stages of the pathology .…”

“…Since susceptibility effects scale with field strength, these distortions are particularly prominent at high field. In particular, high‐field MRI of the head has been found to be affected by respiratory motion, swallowing, and limb motion, which perturb spatial encoding and can thus give rise to image artifacts . Sensitivity to this mechanism is greatest in gradient‐echo scans with long echo times such as T 2 *‐weighted imaging, susceptibility‐weighted imaging , quantitative susceptibility mapping , and BOLD‐fMRI time series.…”

Retrospective correction of physiological field fluctuations in high-field brain MRI using concurrent field monitoring

“…Breathing, in particular, involves motion of the chest wall, diaphragm, and abdomen, giving rise to field changes that have been shown to affect brain imaging at high fields . Likewise, limb motion, speaking and swallowing have been shown to influence the quality of high‐field neuroimaging . Field perturbations are most detrimental to gradient‐echo (GRE) sequences with long echo times and multiple‐shot acquisition, in which they cause shot‐to‐shot inconsistency of phase accrual.…”

Time‐efficient measurement of multi‐phase arterial spin labeling MR signal in white matter

“…To minimize motion artifacts, the volunteer's head was carefully immobilized using foam padding. Navigators or optical tracking might be additional solutions to limit the effects of motion, but these were considered to be beyond the scope of the current study. Another limitation of the current study is the use of EPI as the imaging readout, whereas three‐dimensional readout approaches are considered to be the more optimal strategy.…”