Longitudinal Analysis of COVID-19 Impacts on Mobility: An Early Snapshot of the Emerging Changes in Travel Behavior.

The COVID-19 pandemic has caused a huge disruption worldwide with direct and indirect effects on travel behavior. In response to extensive community spread and potential risk of infection, during the early stage of the pandemic many state and local governments implemented non-pharmaceutical interventions that restricted non-essential travel for residents. This study evaluates the impacts of the pandemic on mobility by analyzing micro panel data ( = 1,274) collected in the United States via online surveys in two periods, before and during the early phase of the pandemic.

Design strategies for improved velocity-selective pulse sequences.

Over the years, a variety of MRI methods have been developed for visualizing or measuring blood flow without the use of contrast agents. One particular class of methods uses flow-encoding gradients associated with an RF pulse sequence to distinguish spins in flowing blood from stationary spins. While a strength of these particular methods is that, in general, they can be tailored to capture a desired range of blood flow, such sequences either do not provide a sharp transition from stationary spins to flowing spins, or else are long, generating relaxation losses and undesirable SAR, and have limited immunity to resonance offsets and to RF inhomogeneity.

Quantitative framework for prospective motion correction evaluation.

Purpose: Establishing a framework to evaluate performances of prospective motion correction (PMC) MRI considering motion variability between MRI scans.

Methods: A framework was developed to obtain quantitative comparisons between different motion correction setups, considering that varying intrinsic motion patterns between acquisitions can induce bias. Intrinsic motion was considered by replaying in a phantom experiment the recorded motion trajectories from subjects.

Accurate Measurement of Magnetic Resonance Imaging Gradient Characteristics.

Recently, gradient performance and fidelity has become of increasing interest, as the fidelity of the magnetic resonance (MR) image is somewhat dependent on the fidelity of the gradient system. In particular, for high fidelity non-Cartesian imaging, due to non-fidelity of the gradient system, it becomes necessary to know the actual k-space trajectory as opposed to the requested trajectory. In this work we show that, by considering the gradient system as a linear time-invariant system, the gradient impulse response function (GIRF) can be reliably measured to a relatively high degree of accuracy with a simple setup, using a small phantom and a series of simple experiments.

Radiofrequency pulse designs for three-dimensional MRI providing uniform tipping in inhomogeneous B₁ fields.

Although high-field MRI offers increased signal-to-noise, the nonuniform tipping produced by conventional radiofrequency (RF) pulses leads to spatially dependent contrast and suboptimal signal-to-noise, thus complicating the interpretation of the MR images. For structural imaging, three-dimensional sequences that do not make use of frequency-selective RF pulses have become popular. Therefore, the aim of this research was to develop non-slice-selective (NSS) RF pulses with immunity to both amplitude of (excitation) RF field (B(1) ) inhomogeneity and resonance offset.