Simulation Validation of an 8-Channel Parallel-Transmit Dipole Array on an Infant Phantom: Including RF Losses for Robust Correlation with Experimental Results.

It is crucial to demonstrate a robust correlation between the simulated and manufactured parallel-transmit (pTx) arrays performances to release the currently-used, very restrictive safety margins. In this study, we describe the qualitative and quantitative validation of a simulation model with respect to experimental results for an 8-channel dipole array at 7T. An approach that includes the radiofrequency losses into the simulation model is presented and compared to simulation models neglecting these losses. Simulated S-matrices and individual B1+-field maps were compared with experimentally measured quantities. With the proposed approach, an average relative difference of ~1.1% was found between simulated and experimental reflection coefficients, ~4.2% for the 1st coupling terms, and ~9.4% for the 2nd coupling terms. A maximum normalized root-mean-square error of 4.8% was achieved between experimental and simulated individual B1+-field maps. The effectiveness of the simulation model to accurately predict the B1+-field patterns was assessed, qualitatively and quantitatively, through a comparison with experimental data. We conclude that, using the proposed model for radiofrequency losses, a robust correlation is achieved between simulated and experimental data using the 8-channel dipole array at 7T.

Segmenting electroencephalography wires reduces radiofrequency shielding artifacts in simultaneous electroencephalography and functional magnetic resonance imaging at 7 T.

PURPOSE: Simultaneous scalp electroencephalography and functional magnetic resonance imaging (EEG-fMRI) enable noninvasive assessment of brain function with high spatial and temporal resolution. However, at ultra-high field, the data quality of both modalities is degraded by mutual interactions. Here, we thoroughly investigated the radiofrequency (RF) shielding artifact of a state-of-the-art EEG-fMRI setup, at 7 T, and design a practical solution to limit this issue. METHODS: Electromagnetic field simulations and MR measurements assessed the shielding effect of the EEG setup, more specifically the EEG wiring. The effectiveness of segmenting the wiring with resistors to reduce the transmit field disruption was evaluated on a wire-only EEG model and a simulation model of the EEG cap. RESULTS: The EEG wiring was found to exert a dominant effect on the disruption of the transmit field, whose intensity varied periodically as a function of the wire length. Breaking the electrical continuity of the EEG wires into segments shorter than one quarter RF wavelength in air (25 cm at 7 T) reduced significantly the RF shielding artifacts. Simulations of the EEG cap with segmented wires indicated similar improvements for a moderate increase of the power deposition. CONCLUSION: We demonstrated that segmenting the EEG wiring into shorter lengths using commercially available nonmagnetic resistors is effective at reducing RF shielding artifacts in simultaneous EEG-fMRI. This prevents the formation of RF-induced standing waves, without substantial specific absorption rate (SAR) penalties, and thereby enables benefiting from the functional sensitivity boosts achievable at ultra-high field.

Towards an integrated neonatal brain and cardiac examination capability at 7 T: electromagnetic field simulations and early phantom experiments using an 8-channel dipole array.

OBJECTIVE: Neonatal brain and cardiac imaging would benefit from the increased signal-to-noise ratio levels at 7 T compared to lower field. Optimal performance might be achieved using purpose designed RF coil arrays. In this study, we introduce an 8-channel dipole array and investigate, using simulations, its RF performances for neonatal applications at 7 T. METHODS: The 8-channel dipole array was designed and evaluated for neonatal brain/cardiac configurations in terms of SAR efficiency (ratio between transmit-field and maximum specific-absorption-rate level) using adjusted dielectric properties for neonate. A birdcage coil operating in circularly polarized mode was simulated for comparison. Validation of the simulation model was performed on phantom for the coil array. RESULTS: The 8-channel dipole array demonstrated up to 46% higher SAR efficiency levels compared to the birdcage coil in neonatal configurations, as the specific-absorption-rate levels were alleviated. An averaged normalized root-mean-square-error of 6.7% was found between measured and simulated transmit field maps on phantom. CONCLUSION: The 8-channel dipole array design integrated for neonatal brain and cardiac MR was successfully demonstrated, in simulation with coverage of the baby and increased SAR efficiency levels compared to the birdcage. We conclude that the 8Tx-dipole array promises safe operating procedures for MR imaging of neonatal brain and heart at 7 T.

Evaluation of the whole auditory pathway using high-resolution and functional MRI at 7T parallel-transmit.

PURPOSE: The aim of the present study is to show a MR procedure for the evaluation of simultaneous left and right auditory functions with functional MRI, and high-resolution acquisition of anatomical auditory pathway using parallel-transmit (pTx) methods at 7T. METHODS: The time-efficient MR acquisition included two steps: RF weights were optimized for the regions-of-interest and high-resolution MR images of the inner-ear were acquired for the first 30 min (400 µm-iso resolution) followed by functional MRI acquisitions along the whole auditory pathway during the next 20 minutes. Data was processed with a linear cross-correlation analysis to define frequency preferences for each voxel in the auditory relays. RESULTS: Tonotopic maps revealed ordered bilateral frequency gradients in the auditory relays whereas at the level of the cochlear nuclei and superior olivary complexes the frequency gradients were less evident. A 21% increase in transmit-field efficiency was achieved over the left/right inner-ear regions and thus its main structures were clearly discernible using the pTx methods, compared to a single transmit RF coil. CONCLUSION: Using 7T pTx allows a fast (less than 60 min in total) and qualitative evaluation of the simultaneous left and right auditory response along the entire auditory pathway, together with high-resolution anatomical images of the inner-ear. This could be further used for patient examination at 7T.

A local multi-transmit coil combined with a high-density receive array for cerebellar fMRI at 7 T.

The human cerebellum is involved in a wide array of functions, ranging from motor control to cognitive control, and as such is of great neuroscientific interest. However, its function is underexplored in vivo, due to its small size, its dense structure and its placement at the bottom of the brain, where transmit and receive fields are suboptimal. In this study, we combined two dense coil arrays of 16 small surface receive elements each with a transmit array of three antenna elements to improve BOLD sensitivity in the human cerebellum at 7 T. Our results showed improved B1+ and SNR close to the surface as well as g-factor gains compared with a commercial coil designed for whole-head imaging. This resulted in improved signal stability and large gains in the spatial extent of the activation close to the surface (<3.5 cm), while good performance was retained deeper in the cerebellum. Modulating the phase of the transmit elements of the head coil to constructively interfere in the cerebellum improved the B1+ , resulting in a temporal SNR gain. Overall, our results show that a dedicated transmit array along with the SNR gains of surface coil arrays can improve cerebellar imaging, at the cost of a decreased field of view and increased signal inhomogeneity.

Effects of indoor air pollution on household health: evidence from Turkey.

Indoor air pollution caused by the use of biomass energy in heating and cooking adversely affects the health status of household members. In Turkey, with the rapid economic growth of the last decade, biomass has been among the most consumed types of household energy for heating and cooking due to inadequate infrastructure, dependence on foreign energy, and high energy prices. This study aims to add empirical evidence to the literature on health status and indoor air pollution in Turkey caused by households' energy choices. This study analyzed these effects with random effects panel discrete ordered models using the Income Living Conditions Micro Longitudinal Data Set for the period 2014-2017. As a result of the analysis, we found that the factors of age, being female, having dependent children, and indoor air pollution have adverse effects on health status. However, education level and income level affect health status positively. The most important observation obtained from this study is that even high-income households are adversely affected by indoor air pollution due to the lack of access to clean energy resources.

Evaluation of factors that may affect disease development and severity in childhood atopic dermatitis.

BACKGROUND: Atopic dermatitis is a chronic, relapsing, inflammatory disease. Data concerning the role of perinatal conditions in the development of the disease are few and inconsistent. The purpose of this study was to investigate the effects of skin care on the severity of the disease, also to evaluate the relations of perinatal conditions with disease development and severity. METHODS: Seventy-five patients with atopic dermatitis aged 2-10 years, their parents and 65 healthy age-compatible volunteers and their parents were included in the study. The Scoring Atopic Dermatitis (SCORAD) score was used to assess the severity of the disease. RESULTS: Emollient use (regular/irregular) was higher in patients with severe disease compared to those with mild and moderate disease, but no significant difference was determined in regular use rates (P=0.029, and P=0.504, respectively). Topical corticosteroid use and sleep disturbance rates in the previous three months in the subjects regularly using emollients were low (P=0.032, and P=0.005, respectively). The incidence of severe disease was higher in the patients born in the winter (P=0.033). Severe disease rates were significantly high in the subjects in the patient group with advanced maternal age, a history of cesarean delivery and incubator use (P=0.017, P=0.046, and P=0.025, respectively). CONCLUSIONS: Low topical corticosteroid requirement and sleep disturbance rates in the subjects regularly using moisturizers emphasize the importance of emollient use in treatment. The association of cesarean delivery, history of incubator, and birth in winter with severe disease suggests that disease severity is related to less exposure to environmental allergens in the hygiene hypothesis.

Brain NAD Is Associated With ATP Energy Production and Membrane Phospholipid Turnover in Humans.

The brain requires a large amount of energy, mostly derived from the metabolism of glucose, which decreases substantially with age and neurological diseases. While mounting evidence in model organisms illustrates the central role of brain nicotinamide adenine dinucleotide (NAD) for maintaining energy homeostasis, similar data are sparse in humans. This study explores the correlations between brain NAD, energy production and membrane phospholipid metabolism by 31-phosphorous magnetic resonance spectroscopy (31P-MRS) across 50 healthy participants including a young (mean age 27.1-year-old) and middle-aged (mean age 56.4-year-old) group. The analysis revealed that brain NAD level and NAD+/NADH redox ratio were positively associated with ATP level and the rate of energy production, respectively. Moreover, a metabolic network linking NAD with membrane phospholipid metabolism, energy production, and aging was identified. An inverted trend between age and NAD level was detected. These results pave the way for the use of 31P-MRS as a powerful non-invasive tool to support the development of new therapeutic interventions targeting NAD associated phospho-metabolic pathways in brain aging and neurological diseases.

A combined 32-channel receive-loops/8-channel transmit-dipoles coil array for whole-brain MR imaging at 7T.

PURPOSE: Multichannel receive arrays provide high SNR and parallel-imaging capabilities, while transmit-only dipole arrays have been shown to achieve a large coverage of the whole-brain including the cerebellum. The aim of this study was to develop and characterize the performances of a 32-channel receive-only loop array combined with an 8-channel dipole coil array at 7T for the first time. METHODS: The 8Tx-dipoles/32Rx-loops coil array was characterized by the SNR, g-factors, noise correlation matrix, accelerated image quality, and B1+ maps, and compared with a commercial 1Tx-birdcage/32Rx-loops array. Simulated and measured B1+ maps were shown for the 8Tx-dipoles/32Rx-loops coil array and compared with the 8Tx/Rx dipole array. RESULTS: The in-house built 32-channel receive coil demonstrated a large longitudinal coverage of the brain, particularly the upper neck area. G-factors and accelerated MR acquisitions demonstrated robust performances up to R = 4 in 2D, and R = 8 (4 × 2) in 3D. A 83% increase in SNR was measured over the cerebellum with the in-house built 8Tx/32Rx coil array compared to the commercial 1Tx/32Rx, while similar performances were obtained in the cerebral cortex. CONCLUSIONS: The combined 32-channel receive/8-channel transmit coil array demonstrated high transmit-receive performances compared to the commercial receive array at 7T, notably in the cerebellum. We conclude that in combination with parallel transmit capabilities, this coil is particularly suitable for whole-brain MR studies at 7T.

A human cerebral and cerebellar 8-channel transceive RF dipole coil array at 7T.

PURPOSE: Dipole antennas that provide high transmit field penetration with large coverage, and their use in a parallel transmit setup, may be advantageous in minimizing B 1 + -field inhomogeneities at ultra-high field, i.e 7T. We have developed and evaluated an 8-channel RF dipole coil array for imaging the entire cerebral and cerebellar regions in man. METHODS: A coil array was modeled with seven dipoles: six placed covering the occipital and temporal lobes; one covering the parietal lobe; and two loops covering the frontal lobe. Center-shortened and fractionated dipoles were simulated for the array configuration and assessed with respect to B 1 + -field at maximum specific absorption rate averaged over 10 g tissue regions in human brain. The whole-brain center-shortened dipoles with frontal loops coil array was constructed and its transmit properties were assessed with respect to MR images, B 1 + -field, and homogeneity. RESULTS: In simulations, the dipole arrays showed comparable performances to cover the whole-brain. However, for ease of construction, the center-shortened dipole was favored. High spatial resolution anatomical images of the human brain with the coil array demonstrated a full coverage of the cerebral cortex and cerebellum. CONCLUSIONS: The 8-channel center-shortened dipoles and frontal loops coil array promises remarkable efficiency in highly challenging regions as the cerebellum, and phase-only RF shimming of whole-brain could greatly benefit ultra-high field magnetic resonance imaging of the human brain at 7T.

Nutritional Ketosis Increases NAD+/NADH Ratio in Healthy Human Brain: An in Vivo Study by 31P-MRS.

Ketones represent an important alternative fuel for the brain under glucose hypo-metabolic conditions induced by neurological diseases or aging, however their metabolic consequences in healthy brain remain unclear. Here we report that ketones can increase the redox NAD+/NADH ratio in the resting brain of healthy young adults. As NAD is an important energetic and signaling metabolic modulator, these results provide mechanistic clues on how nutritional ketosis might contribute to the preservation of brain health.

Clinical Neuroimaging Using 7 T MRI: Challenges and Prospects.

The aim of this article is to illustrate the principal challenges, from the medical and technical point of view, associated with the use of ultrahigh field (UHF) scanners in the clinical setting and to present available solutions to circumvent these limitations. We would like to show the differences between UHF scanners and those used routinely in clinical practice, the principal advantages, and disadvantages, the different UHFs that are ready be applied to routine clinical practice such as susceptibility-weighted imaging, fluid-attenuated inversion recovery, 3-dimensional time of flight, magnetization-prepared rapid acquisition gradient echo, magnetization-prepared 2 rapid acquisition gradient echo, and diffusion-weighted imaging, the technical principles of these sequences, and the particularities of advanced techniques such as diffusion tensor imaging, spectroscopy, and functional imaging at 7TMR. Finally, the main clinical applications in the field of the neuroradiology are discussed and the side effects are reported.

Radio-frequency coils for ultra-high field magnetic resonance.

Radiofrequency (RF) coils are key components of magnetic resonance imaging (MRI) systems. The primary purpose of this review is to provide a basic theory of RF coil designs and their characterization by bench measurements, electromagnetic field simulations and MR measurements. With the continuing increase of magnetic field strength in MRI instruments, the RF wavelength in the subject under study becomes comparable to or smaller in size than the anatomical dimensions of the tissue under study, which amplifies the signal inhomogeneity. Also, RF energy increases quadratically with the Larmor frequency, which leads to increased heat deposition in the subject, especially at ultra-high field. Elegant RF coil designs are explored here to address these challenges.

Single acquisition electrical property mapping based on relative coil sensitivities: A proof-of-concept demonstration.

PURPOSE: All methods presented to date to map both conductivity and permittivity rely on multiple acquisitions to compute quantitatively the magnitude of radiofrequency transmit fields, B1+. In this work, we propose a method to compute both conductivity and permittivity based solely on relative receive coil sensitivities ( B1-) that can be obtained in one single measurement without the need to neither explicitly perform transmit/receive phase separation nor make assumptions regarding those phases. THEORY AND METHODS: To demonstrate the validity and the noise sensitivity of our method we used electromagnetic finite differences simulations of a 16-channel transceiver array. To experimentally validate our methodology at 7 Tesla, multi compartment phantom data was acquired using a standard 32-channel receive coil system and two-dimensional (2D) and 3D gradient echo acquisition. The reconstructed electric properties were correlated to those measured using dielectric probes. RESULTS: The method was demonstrated both in simulations and in phantom data with correlations to both the modeled and bench measurements being close to identity. The noise properties were modeled and understood. CONCLUSION: The proposed methodology allows to quantitatively determine the electrical properties of a sample using any MR contrast, with the only constraint being the need to have 4 or more receive coils and high SNR. Magn Reson Med 74:185-195, 2015. © 2014 Wiley Periodicals, Inc.

Simultaneous EEG-fMRI at ultra-high field: artifact prevention and safety assessment.

The simultaneous recording of scalp electroencephalography (EEG) and functional magnetic resonance imaging (fMRI) can provide unique insights into the dynamics of human brain function, and the increased functional sensitivity offered by ultra-high field fMRI opens exciting perspectives for the future of this multimodal approach. However, simultaneous recordings are susceptible to various types of artifacts, many of which scale with magnetic field strength and can seriously compromise both EEG and fMRI data quality in recordings above 3T. The aim of the present study was to implement and characterize an optimized setup for simultaneous EEG-fMRI in humans at 7 T. The effects of EEG cable length and geometry for signal transmission between the cap and amplifiers were assessed in a phantom model, with specific attention to noise contributions from the MR scanner coldheads. Cable shortening (down to 12 cm from cap to amplifiers) and bundling effectively reduced environment noise by up to 84% in average power and 91% in inter-channel power variability. Subject safety was assessed and confirmed via numerical simulations of RF power distribution and temperature measurements on a phantom model, building on the limited existing literature at ultra-high field. MRI data degradation effects due to the EEG system were characterized via B0 and B1(+) field mapping on a human volunteer, demonstrating important, although not prohibitive, B1 disruption effects. With the optimized setup, simultaneous EEG-fMRI acquisitions were performed on 5 healthy volunteers undergoing two visual paradigms: an eyes-open/eyes-closed task, and a visual evoked potential (VEP) paradigm using reversing-checkerboard stimulation. EEG data exhibited clear occipital alpha modulation and average VEPs, respectively, with concomitant BOLD signal changes. On a single-trial level, alpha power variations could be observed with relative confidence on all trials; VEP detection was more limited, although statistically significant responses could be detected in more than 50% of trials for every subject. Overall, we conclude that the proposed setup is well suited for simultaneous EEG-fMRI at 7 T.

A double-quadrature radiofrequency coil design for proton-decoupled carbon-13 magnetic resonance spectroscopy in humans at 7T.

PURPOSE: Carbon-13 magnetic resonance spectroscopy ((13) C-MRS) is challenging because of the inherent low sensitivity of (13) C detection and the need for radiofrequency transmission at the (1) H frequency while receiving the (13) C signal, the latter requiring electrical decoupling of the (13) C and (1) H radiofrequency channels. In this study, we added traps to the (13) C coil to construct a quadrature-(13) C/quadrature-(1) H surface coil, with sufficient isolation between channels to allow simultaneous operation at both frequencies without compromise in coil performance. METHODS: Isolation between channels was evaluated on the bench by measuring all coupling parameters. The quadrature mode of the quadrature-(13) C coil was assessed using in vitro (23) Na gradient echo images. The signal-to-noise ratio (SNR) was measured on the glycogen and glucose resonances by (13) C-MRS in vitro, compared with that obtained with a linear-(13) C/quadrature-(1) H coil, and validated by (13) C-MRS in vivo in the human calf at 7T. RESULTS: Isolation between channels was better than -30 dB. The (23) Na gradient echo images indicate a region where the field is strongly circularly polarized. The quadrature coil provided an SNR enhancement over a linear coil of 1.4, in vitro and in vivo. CONCLUSION: It is feasible to construct a double-quadrature (13) C-(1) H surface coil for proton decoupled sensitivity enhanced (13) C-NMR spectroscopy in humans at 7T.

Intersubject local SAR variation for 7T prostate MR imaging with an eight-channel single-side adapted dipole antenna array.

PURPOSE: Surface transmit arrays used in ultra-high field body MRI require local specific absorption rate (SAR) assessment. As local SAR cannot be measured directly, local SAR is determined by simulations using dielectric patient models. In this study, the inter-patient local SAR variation is investigated for 7T prostate imaging with the single-side adapted dipole antenna array. METHOD: Four-dedicated dielectric models were created by segmenting Dixon water-fat separated images that were obtained from four subjects with a 1.5T scanner and the surface array in place. Electromagnetic simulations were performed to calculate the SAR distribution for each model. Radio frequency (RF) exposure variations were determined by analyzing the SAR(10g) distributions (1) with one element active, (2) using a Q-matrix eigenvalue/eigenvector approach, (3) with the maximum potential SAR in each voxel, and (4) for a phase shimmed prostate measurement. RESULTS: Maximum potential local SAR levels for 1 W time-averaged accepted power per transmit channel range from 4.1 to 7.1 W/kg. CONCLUSION: These variations show that one model is not sufficient to determine safe scan settings. For the operation of the surface array conservative power settings were derived based on a worst-case SAR evaluation and the most SAR-sensitive body model.

Specific absorption rate intersubject variability in 7T parallel transmit MRI of the head.

Patient-specific radiofrequency shimming in high-field MRI strengthens the need for online, patient-specific specific absorption rate (SAR) monitoring. Numerical simulation is currently most effective for this purpose but may require a patient-specific dielectric model. To investigate whether a generic model may be combined with a safety factor to account for variation within the population, generic SAR behavior is studied for 7T MRI of the head. For six detailed head models, radiofrequency fields were simulated for an eight-channel parallel transmit array. SAR behavior is studied through comparison of the eigenvalues/eigenvectors of the local Q-matrices. Furthermore, numerical radiofrequency shimming experiments without and with SAR constraints were performed where SAR during optimization was evaluated on a generic model. In both cases, the ability of different generic models to predict actual SAR levels was evaluated. The largest eigenvalue distribution is comparable between models. Radiofrequency shimming without constraints improves the |B +1| homogeneity while the SAR increases substantially. Imposing constraints on SAR during optimization, estimating SAR on a generic model, was effective. A safety factor of 1.4 was found to be sufficient. Generic SAR behavior makes a generic head model a practical alternative to patient-specific models and allows effective |B +1| shimming with SAR constraints.

Compositional and electric field dependence of the dissociation of charge transfer excitons in alternating polyfluorene copolymer/fullerene blends.

The electro-optical properties of thin films of electron donor-acceptor blends of a fluorene copolymer (PF10TBT) and a fullerene derivative (PCBM) were studied. Transmission electron microscopy shows that in these films nanocrystalline PCBM clusters are formed at high PCBM content. For all concentrations, a charge transfer (CT) transition is observed with absorption spectroscopy, photoluminescence, and electroluminescence. The CT emission is used as a probe to investigate the dissociation of CT excited states at the donor-acceptor interface in photovoltaic devices, as a function of an applied external electric field and PCBM concentration. We find that the maximum of the CT emission shifts to lower energy and decreases in intensity with higher PCBM content. We explain the red shift of the emission and the lowering of the open-circuit voltage (V(OC)) of photovoltaic devices prepared from these blends with the higher relative permittivity of PCBM (epsilon(r) = 4.0) compared to that of the polymer (epsilon(r) = 3.4), stabilizing the energy (E(CT)) of CT states and of the free charge carriers in blends with higher PCBM concentration. We show that the CT state has a short decay time (tau = ca. 4 ns) that is reduced by the application of an external electric field or with increasing PCBM content. The field-induced quenching can be explained quantitatively with the Onsager-Braun model for the dissociation of the CT states when including a high electron mobility in nanocrystalline PCBM clusters. Furthermore, photoinduced absorption spectroscopy shows that increasing the PCBM concentration reduces the yield of neutral triplet excitons forming via electron-hole recombination, and increases the lifetime of radical cations. The presence of nanocrystalline domains with high local carrier mobility of at least one of the two components in an organic heterojunction may explain efficient dissociation of CT states into free charge carriers.