Quantitative magnetic resonance imaging phantoms: A review and the need for a system phantom.

The MRI community is using quantitative mapping techniques to complement qualitative imaging. For quantitative imaging to reach its full potential, it is necessary to analyze measurements across systems and longitudinally. Clinical use of quantitative imaging can be facilitated through adoption and use of a standard system phantom, a calibration/standard reference object, to assess the performance of an MRI machine. The International Society of Magnetic Resonance in Medicine AdHoc Committee on Standards for Quantitative Magnetic Resonance was established in February 2007 to facilitate the expansion of MRI as a mainstream modality for multi-institutional measurements, including, among other things, multicenter trials. The goal of the Standards for Quantitative Magnetic Resonance committee was to provide a framework to ensure that quantitative measures derived from MR data are comparable over time, between subjects, between sites, and between vendors. This paper, written by members of the Standards for Quantitative Magnetic Resonance committee, reviews standardization attempts and then details the need, requirements, and implementation plan for a standard system phantom for quantitative MRI. In addition, application-specific phantoms and implementation of quantitative MRI are reviewed. Magn Reson Med 79:48-61, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Response of water to electric fields at temperatures below the glass transition: a molecular dynamics analysis.

The electric field dependence of the structure and dynamics of water at 77 K, i.e., below the glass transition temperature (136 K), is investigated using molecular dynamics simulations. Transitions are found at two critical field strengths, denoted E(1) and E(2). The transition around E(1)≈3.5 V/nm is characterized by the onset of significant structural disorder, a rapid increase in the orientational polarization, and a maximum in the dynamical fluctuations. At E(2)≈40 V/nm, the system crystallizes in discrete steps into a body-centered-cubic unit cell that minimizes the potential energy by simultaneous superpolarization of the water molecular dipoles and maximization of the intermolecular hydrogen bonds. The stepwise and discontinuous increase of the orientational polarization with the increasing electric field indicates that the dipole relaxation in the electric field is highly cooperative.

Diffusion simulation-based fiber tracking using time-of-arrival maps: a comparison with standard methods.

OBJECT: We propose a new tracking method based on time-of-arrival (TOA) maps derived from simulated diffusion processes. MATERIALS AND METHODS: The proposed diffusion simulation-based tracking consists of three steps that are successively evaluated on small overlapping sub-regions in a diffusion tensor field. First, the diffusion process is simulated for several time steps. Second, a TOA map is created to store simulation results for the individual time steps that are required for the tract reconstruction. Third, the fiber pathway is reconstructed on the TOA map and concatenated between neighboring sub-regions. This new approach is compared with probabilistic and streamline tracking. All methods are applied to synthetic phantom data for an easier evaluation of their fiber reconstruction quality. RESULTS: The comparison of the tracking results did show severe problems for the streamline approach in the reconstruction of crossing fibers, for example. The probabilistic method was able to resolve the crossing, but could not handle strong curvature. The new diffusion simulation-based tracking could reconstruct all problematic fiber constellations. CONCLUSION: The proposed diffusion simulation-based tracking method used the whole tensor information of a neighborhood of voxels and is, therefore, able to handle problematic tracking situations better than established methods.

Comparison of gradient encoding directions for higher order tensor diffusion data.

Recently, higher order tensors were proposed for a more advanced representation of non-Gaussian diffusion. These advanced diffusion models have new requirements for the gradient encoding schemes used in the diffusion weighted image acquisition. The influence of the gradient encoding schemes on the estimated standard second order diffusion tensor was previously investigated. Here, we focus on the suitability of different encoding scheme types for higher order tensor models. Two quality measures for the gradient encoding schemes, the condition number of the estimation matrix and a new measure that evaluates the signal deviation on simulated data, are used to determine which gradient encoding is suited best for higher order tensor estimations. Six different gradient encoding scheme types were investigated. A certain force-minimizing scheme type gave the best results in the evaluations presented here.

White matter fiber tractography via anisotropic diffusion simulation in the human brain.

A novel approach to noninvasively tracing brain white matter fiber tracts is presented using diffusion tensor magnetic resonance imaging (DT-MRI). This technique is based on successive anisotropic diffusion simulations over the human brain, which are utilized to construct three dimensional diffusion fronts. The fiber pathways are determined by evaluating the distance and orientation from the fronts to their corresponding diffusion seeds. Synthetic and real DT-MRI data are employed to demonstrate the tracking scheme. It is shown that the synthetic tracts are accurately replicated, and several major white matter fiber pathways can be reproduced noninvasively, with the tract branching being allowed. Since simulating the diffusion process, which is truly a physical phenomenon reflecting the underlying architecture of cerebral tissues, makes full use of the diffusion tensor data, including both the magnitude and orientation information, the proposed approach is expected to enhance robustness and reliability in white matter fiber reconstruction.

Top-down attentional processing enhances auditory evoked gamma band activity.

In contrast to animal studies, relatively little is known about the functional significance of the early evoked gamma band activity in humans. We investigated whether evoked and induced 40 Hz activity differentiate automatic, bottom-up aspects of attention from voluntary, top-down related attentional demands. An auditory novelty-oddball task was applied to 14 healthy subjects. As predicted, more evoked gamma was found for the target condition than in the two task-irrelevant conditions. Since gamma band activity was not enhanced for novel stimuli, the evoked gamma response cannot be explained with a simple concept of stimulus arousal. Neither induced gamma nor the degree of 40 Hz phase-locking were different between the experimental conditions. Taken together, our data emphasize the role of evoked gamma band activity for top-down attentional processing.

Repetitive transcranial magnetic stimulation induces different responses in different cortical areas: a functional magnetic resonance study in humans.

Repetitive transcranial magnetic stimulation (TMS) for 1 s at 4 Hz and 150% of the individual motor threshold was applied to primary motor cortex and adjacent cortical regions where no motor response could be produced. The hemodynamic reaction was measured using an event-related functional magnetic resonance setup. While all volunteers showed a typical signal increase beneath the coil during motor cortex stimulation, no consistent signal changes were present during frontal or parietal stimulation apart from activation of auditory cortex. The results suggest that neuronal stimulation by TMS is followed by an inhibitive phase that compensates for the effect of an initial neuronal activation. It is further concluded that the signal increases during motor cortex fit a sensory feedback from the moving body parts.

Evolution of Athletic Records: Statistical Effects versus Real Improvements.

Athletic records represent the best results in a given discipline, thus improving monotonically with time. As has already been shown, this should not be taken as an indication that the athletes' capabilities keep improving. In other words, a new record is not noteworthy just because it is a new record, instead it is necessary to assess by how much the record has improved. In this paper we derive formulae that can be used to show that athletic records continue to improve with time, even if athletic performance remains constant. We are considering two specific examples, the German championships and the world records in several athletic disciplines. The analysis shows that, for the latter, true improvements occur in 20-50% of the disciplines. The analysis is supplemented by an application of our record estimation approach to the prediction of the maximum body length of humans for a specified size of a population respectively population group from a representative sample.

Trends and random fluctuations in athletics.

Improvements in the results of athletic competitions are often considered to stem from better training and equipment, but elements of chance are always present in athletics and these also contribute. Here we distinguish between these two effects by estimating the range into which athletic records would have fallen in the absence of systematic progress and then comparing this with actual performance results. We find that only 4 out of 22 disciplines have shown a systematic improvement, and that annual best results worldwide show saturation in some disciplines.