Ex-vivo cellular MRI with b-SSFP: quantitative benefits of 3 T over 1.5 T

INTRODUCTION: The use of MRI with iron-based magnetic nanoparticles for imaging cells is a rapidly growing field of research. We have recently reported that single iron-labeled cells could be detected, as signal voids, in vivo in mouse brains using a balanced steady-state free precession imaging sequence (b-SSFP) and a customized microimaging system at 1.5 T. METHODS: In the current study we assess the benefits, and challenges, of using a higher magnetic field strength for imaging iron-labeled cells with b-SSFP, using ex vivo mouse brain specimens imaged with near identical systems at 1.5 and 3.0 T. RESULTS: The substantial banding artifact that appears in 3 T b-SSFP images was readily minimized with RF phase cycling, allowing for banding-free b-SSFP images to be compared between the two field strengths. This study revealed that with an optimal 3 T b-SSFP imaging protocol, more than twice as many signal voids were detected as with 1.5 T. CONCLUSION: There are several factors that contributed to this important result. First, a greater-than-linear SNR gain was achieved in mouse brain images at 3 T. Second, a reduction in the bandwidth, and the associated increase in repetition time and SNR, produced a dramatic increase in the contrast generated by iron-labeled cells.     

Subjective perception of safety in healthy individuals working with 7 T MRI scanners: a retrospective multicenter survey

Objective

To retrospectively assess perception of safety of healthy individuals working with human 7 Tesla (T) magnetic resonance imaging (MRI) scanners.

Materials and methods

A total of 66 healthy individuals with a mean age of 31 ± 7 years participated in this retrospective multicentre survey study. Nonparametric correlation analysis was conducted to evaluate the relation between self-reported perception of safety and prevalence of sensory effects while working with 7 T MRI scanners for an average 47 months.

Results

The results indicated that 98.5 % of the study participants had a neutral or positive feeling about safety aspects at 7 T MRI scanners. 45.5 % reported that they feel very safe and none of the participants stated that they feel moderately or very unsafe while working with 7 T MRI scanners. Perception of safety was not affected by the number of hours per week spent in the vicinity of the 7 T MRI scanner or the duration of experience with 7 T MRI. More than 50 % of individuals experienced vertigo and metallic taste while working with 7 T MRI scanners. However, participants’ perceptions of safety were not affected by the prevalence of MR-related symptoms.

Conclusions

The overall data indicated an average perception of a moderately safe work environment. To our knowledge, this study delineates the first attempt to assess the subjective safety perception among 7 T MRI workers and suggests further investigations are indicated.

     

High resolution morphologic imaging and T2 mapping of cartilage at 7 Tesla: comparison of cartilage repair patients and healthy controls

Object

Our objective was to use 7 T MRI to compare cartilage morphology (thickness) and collagen composition (T2 values) in cartilage repair patients and healthy controls.

Materials and methods

We scanned the knees of 11 cartilage repair patients and 11 controls on a 7 T MRI scanner using a high-resolution, gradient-echo sequence to measure cartilage thickness and a multi-echo spin-echo sequence to measure cartilage T2 values. We used two-tailed t tests to compare cartilage thickness and T2 values in: repair tissue (RT) versus adjacent cartilage (AC); RT versus healthy control cartilage (HC); AC versus HC.

Results

Mean thickness in RT, AC, HC were: 2.2 ± 1.4, 3.6 ± 1.1, 3.3 ± 0.7 mm. Differences in thickness between RT–AC (p = 0.01) and RT–HC (p = 0.02) were significant, but not AC–HC (p = 0.45). Mean T2 values in RT, AC, HC were: 51.6 ± 7.6, 40.0 ± 4.7, 45.9 ± 3.7 ms. Differences in T2 values between RT–AC (p = 0.0005), RT-HC (p = 0.04), and AC–HC (p = 0.004) were significant.

Conclusion

7 T MRI allows detection of differences in morphology and collagen architecture in: (1) cartilage repair tissue compared to adjacent cartilage and (2) cartilage repair tissue compared to cartilage from healthy controls. Although cartilage adjacent to repair tissue may be normal in thickness, it can demonstrate altered collagen composition.     

Quantitative assessment of trabecular bone micro-architecture of the wrist via 7 Tesla MRI: preliminary results

Object  

The goal of this study was to determine the feasibility of performing quantitative 7 T magnetic resonance imaging (MRI) assessment of trabecular bone micro-architecture of the wrist, a common fracture site.     

Modeling heating effects in nanoscale devices: the present and the future

In this review paper we give an overview on the present state of the art in modeling heat transport in nanoscale devices and what issues we need to address for better and more successful modeling of future devices. We begin with a brief overview of the heat transport in materials and explain why the simple Fourier law fails in nanoscale devices. Then we elaborate on attempts to model heat transport in nanostructures from both perspectives: nanomaterials (the work of Narumanchi and co-workers) and nanodevices (the work of Majumdar, Pop, Goodson and recently Vasileska, Raleva and Goodnick). We use our own simulation results which we have used to examine heat transport in nanoscaling devices to point out some important issues such as the fact that thermal degradation does not increase as we decrease feature size due to the more pronounced non-stationary transport and ballistic transport effects in nanoscale devices. We also point out that instead of using SOI, if one uses Silicon on Diamond technology there is much less heat degradation and better spread of the heat in the Diamond material. We also point out that tools for thermal modeling of nanoscale devices need to be improved from the present state of the art as 3D tools are needed, for example, to simulate heat transport and electrical transport in a FinFET device. Better models than the energy balance equations for the acoustic and optical phonons what we presently use in our simulators are also welcomed. The ultimate goal is to design the tool that can be efficient enough but at the same time can simulate most accurately both electrons and phonons within the particle pictures by solving their corresponding Boltzmann transport equations self-consistently. Investigations in integration of Peltier coolers with CMOS technology are also welcomed and much needed to reduce the problem of heat dissipation in nanoscale devices and interconnects.     

Modeling drive currents and leakage currents: a dynamic approach

The dynamics of electrons and holes propagating through the nano-scaled channels of modern semiconductor devices can be seen as a widespread manifestation of non-equilibrium statistical physics and its ruling principles. In this respect both the devices that are pushing conventional CMOS technology towards the final frontiers of Moore’s law and the upcoming set of alternative, novel nanostructures grounded on entirely new concepts and working principles, provide an almost unlimited playground for assessing physical models and numerical techniques emerging from classical and quantum mechanical non-equilibrium theory. In this paper we revisit the Boltzmann as well as the Wigner–Boltzmann equation which offers a valuable platform to study transport of charge carriers taking part in drive currents. We focus on a numerical procedure that regained attention recently as an alternative tool to solve the time-dependent Boltzmann equation for inhomogeneous systems, such as the channel regions of field-effect transistors, and we discuss its extension to the Wigner–Boltzmann equation. Furthermore, we pay attention to the calculation of tunneling leakage currents. The latter typically occurs in nano-scaled transistors when part of the carrier distribution sustaining the drive current is found to tunnel into the gate due the presence of an ultra-thin insulating barrier separating the gate from the channel region. In particular, we discuss the paradox related to the very existence of leakage currents established by electrons occupying quasi-bound states, while the (real) wave functions of the latter cannot carry net currents. Finally, we describe a simple model to resolve the paradox as well as to estimate gate currents provided the local carrier generation rates largely exceed the tunneling rates.     

Reproducibility and regional variations of an improved gagCEST protocol for the in vivo evaluation of knee cartilage at 7 T

Objective

The objective was to establish a gagCEST protocol that would enable robust and reproducible assessment of the glycosaminoglycan (GAG) content in knee cartilage at 7 T within a clinically feasible measurement time.

Materials and methods

Ten young healthy volunteers (mean age 26 years, range 24–28, five males, five females) were examined on a 7 T MR system. Informed consent was obtained from all individual participants prior to enrollment into the study. Each volunteer was measured twice for reproducibility assessment. The examined knee was immobilized using a custom-made fixation device. For the gagCEST measurement, a prototype segmented 3-D RF-spoiled gradient-echo sequence with an improved saturation scheme employing adiabatic pulses was used in a scan time of 19 min. The asymmetry of the Z-spectra (MTR_asym) in selected regions of interest in knee cartilage was calculated. Differences in MTR_asym between different regions were evaluated using ANOVA and the Bonferroni corrected post hoc test.

Results

The improvement of the saturation scheme reduced the influence of field inhomogeneities, resulted in more uniform saturation, and allowed for good reproducibility in a reasonable measurement time (19 min), as demonstrated by an intraclass correlation coefficient of 0.77. Improved fixation helped to reduce motion artifacts. Whereas similar MTR_asym values were found for weight-bearing and non-weight-bearing femoral cartilage, lower values were observed in the trochlear groove (p = 0.028), patellar (p = 0.015) and tibial cartilage (p < 0.001) when compared to non-weight-bearing femoral cartilage.

Conclusion

Reasonable reproducibility and sensitivity to regional differences in GAG content suggests that the improved gagCEST protocol might be useful for assessing the biochemical changes in articular cartilage that are associated with early stages of cartilage degeneration.

     

Reliability options: Can they deliver on their promises?

Capacity mechanisms have been controversial in theory as well as practice. Lessons from experience with different capacity mechanisms led to the development of the reliability options. This mechanism promises two advantages over other types of capacity mechanisms. Firstly, it ensures the availability of capacity contracted via the capacity mechanism during scarcity. Secondly, the reliability option mechanism limits any energy market distortion due to its implementation and provides the consumer a hedge from high prices. We assess the ability of reliability options in delivering the two promises by analysing the reliability option designs in Italy and Ireland. We find that they deliver on the first promise but only partly on the second.     

Current status and challenges in PEMFC stacks, systems and commercialization

Polymer electrolyte membrane fuel cells(PEMFCs)operate atlowtemperatures(60~80℃),offer a fast start-up and shut downti me(few minutes),fast response to changing electric loads,andcan sustain unli mited thermal cycles·They are ideally suited fortransport…     

<Emphasis Type="Italic">T</Emphasis><Subscript>2</Subscript> mapping of cerebrospinal fluid: 3 T versus 7 T

Object

Cerebrospinal fluid (CSF) T ₂ mapping can potentially be used to investigate CSF composition. A previously proposed CSF T ₂–mapping method reported a T ₂ difference between peripheral and ventricular CSF, and suggested that this reflected different CSF compositions. We studied the performance of this method at 7 T and evaluated the influence of partial volume and B ₁ and B ₀ inhomogeneity.

Materials and methods

T ₂-preparation-based CSF T ₂-mapping was performed in seven healthy volunteers at 7 and 3 T, and was compared with a single echo spin-echo sequence with various echo times. The influence of partial volume was assessed by our analyzing the longest echo times only. B ₁ and B ₀ maps were acquired. B ₁ and B ₀ dependency of the sequences was tested with a phantom.

Results

T _2,CSF was shorter at 7 T compared with 3 T. At 3 T, but not at 7 T, peripheral T _2,CSF was significantly shorter than ventricular T _2,CSF. Partial volume contributed to this T ₂ difference, but could not fully explain it. B ₁ and B ₀ inhomogeneity had only a very limited effect. T _2,CSF did not depend on the voxel size, probably because of the used method to select of the regions of interest.

Conclusion

CSF T ₂ mapping is feasible at 7 T. The shorter peripheral T _2,CSF is likely a combined effect of partial volume and CSF composition.

     

Improving fMRI in signal drop-out regions at 7 T by using tailored radio-frequency pulses: application to the ventral occipito-temporal cortex

Objective

Signal drop-off occurs in echo-planar imaging in inferior brain areas due to field gradients from susceptibility differences between air and tissue. Tailored-RF pulses based on a hyperbolic secant (HS) have been shown to partially recover signal at 3 T, but have not been tested at higher fields.

Materials and methods

The aim of this study was to compare the performance of an optimized tailored-RF gradient-echo echo-planar imaging (TRF GRE-EPI) sequence with standard GRE-EPI at 7 T, in a passive viewing of faces or objects fMRI paradigm in healthy subjects.

Results

Increased temporal-SNR (tSNR) was observed in the middle and inferior temporal lobes and orbitofrontal cortex of all subjects scanned, but elsewhere tSNR decreased relative to the standard acquisition. In the TRF GRE-EPI, increased functional signal was observed in the fusiform, lateral occipital cortex, and occipital pole, regions known to be part of the visual pathway involved in face-object perception.

Conclusion

This work highlights the potential of TRF approaches at 7 T. Paired with a reversed-gradient distortion correction to compensate for in-plane susceptibility gradients, it provides an improved acquisition strategy for future neurocognitive studies at ultra-high field imaging in areas suffering from static magnetic field inhomogeneities.

     

Mathematical advances and horizons for classical and quantum-perturbed drift-diffusion systems: solid state devices and beyond

The classical drift-diffusion model employed in semi-conductor simulation is now seen as part of a hierarchy of mathematical models designed to capture the intricate patterns of current flow in solid-state devices. These models include those incorporating quantum mechanical effects. Scientific computation has vastly outpaced our mathematical understanding of these models. This article is restricted in its focus, and describes mathematical understanding achieved during the last few decades primarily in terms of Gummel decomposition, as applied to drift-diffusion models and the closely related family of quantum corrected drift-diffusion models. Drift-diffusion models are being employed once again in organic devices, and in bio-chip devices, and a re-examination is now seen as timely, as such studies proceed beyond solid state devices.     

Nanowire transistor modeling: influence of ionized impurity and correlation effects

This study presents two relevant effects influencing the electronic transport of nanowire transistors. We first focus on the ionized impurity impacts and calculate the current characteristics with a self-consistent three-dimensional (3D) Green’s function approach. The results show the effects of both acceptor and donor impurities on the physical electron properties. In particular, we emphasize that the presence of a donor induces different transport phenomena according to the applied gate bias. In a second part, we report a numerical study of the self-energy correction due to correlation effects from dynamic screening of the moving electron in silicon nanowire transistors. This many-body effect, which is not included in the usual Hartree approximation, is then incorporated self-consistently into a non-equilibrium Green’s function (NEGF) code. The results pinpoint the importance of dielectric confinement whose magnitude can not be neglected compared to its quantum counterpart in ultimate nanowire transistors.     

Monitoring demyelination and remyelination by magnetization transfer imaging in the mouse brain at 9.4 T

OBJECTIVE: The aim of this study was to assess quantitatively structural changes in myelin content occurring during demyelination and remyelination by magnetization transfer imaging (MTI). MATERIALS AND METHODS: In a reversible model of demyelination with no axonal loss, mice intoxicated by cuprizone were studied by MTI in vivo at 9.4 T. MRI data were compared to histopathological examinations. RESULTS: Data revealed that the magnetization transfer ratio (MTR) decreased significantly during demyelination and increased during remyelination with strong correlation to the myelin content (r = 0.79, P = 0.01). CONCLUSIONS: This study demonstrated that MTR is a sensitive and reproducible quantitative marker to assess myelin loss and repair. This may lead to in vivo monitoring of therapeutic strategies promoting remyelination.     

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