Search

Crystal Structure Prediction (CSP) is a fundamental computational problem in materials science. Basin-hopping is a prominent CSP method that combines global Monte Carlo sampling to search over candidate trial structures with local energy minimisation of these candidates. The sampling uses a stochastic policy to randomly choose which action (such as a swap of atoms) will be used to transform the current structure into the next. Typically hand-tuned for a specific system before the run starts, such a policy is simply a fixed discrete probability distribution of possible actions, which does not depend on the current structure and does not adapt during a CSP run. We show that reinforcement learning (RL) can generate a dynamic policy that both depends on the current structure and improves on the fly during the CSP run. We demonstrate the efficacy of our approach on two CSP codes, FUSE and MC-EMMA. Specifically, we show that, when applied to the autonomous exploration of a phase field to identify the accessible crystal structures, RL can save up to 46% of the computation time.

PURPOSE: Demonstrate ability to produce reasonable simulations of temperature using numerical models of the human body with a limited number of tissues. METHODS: For both a male and female human body model, numerical simulations were used to calculate temperature distributions in three different models of the same human body: the original model with 35 tissues for the male model and 76 tissues for the female model, a simplified model having only three tissues (muscle, fat, and lung), and a simplified model having six tissues (muscle, fat, lung, bone, brain, and skin). RESULTS: Although a three-tissue model gave reasonable specific absorption rate estimates in comparison to an original with many more tissues, because of tissue-specific thermal and physiological properties that do not affect specific absorption rate, such as rate of perfusion by blood, the three-tissue model did not provide temperature distributions similar to those of the original model. Inclusion of a few additional tissues, as in the six-tissue model, produced results in much better agreement with those from the original model. CONCLUSION: Reasonable estimates of temperature can be simulated with a limited number of tissues, although this number is higher than the number of tissues required to produce reasonable simulations of specific absorption rate. For exposures primarily in the head and thorax, six tissues may be adequate for reasonable estimates of temperature.

Magnetic resonance imaging and spectroscopy (MRI/MRS) at 7T represents an exciting advance in MR technology, with intriguing possibilities to enhance image spatial, spectral, and contrast resolution. To ensure the safe use of this technology while still harnessing its potential, clinical staff and researchers need to be cognizant of some safety concerns arising from the increased magnetic field strength and higher Larmor frequency. The higher static magnetic fields give rise to enhanced transient bioeffects and an increased risk of adverse incidents related to electrically conductive implants. Many technical challenges remain and the continuing rapid pace of development of 7T MRI/MRS is likely to present further challenges to ensuring safety of this technology in the years ahead. The recent regulatory clearance for clinical diagnostic imaging at 7T will likely increase the installed base of 7T systems, particularly in hospital environments with little prior ultrahigh-field MR experience. Informed risk/benefit analyses will be required, particularly where implant manufacturer-published 7T safety guidelines for implants are unavailable. On behalf of the International Society for Magnetic Resonance in Medicine, the aim of this article is to provide a reference document to assist institutions developing local institutional policies and procedures that are specific to the safe operation of 7T MRI/MRS. Details of current 7T technology and the physics underpinning its functionality are reviewed, with the aim of supporting efforts to expand the use of 7T MRI/MRS in both research and clinical environments. Current gaps in knowledge are also identified, where additional research and development are required. Level of Evidence 5 Technical Efficacy 2 J. MAGN. RESON. IMAGING 2021;53:333-346.

In medical imaging, phantoms with known geometries and material compositions are utilized to reproducibly and precisely characterize ability of a medical imaging system to safely produce accurate images. Today, a variety of commercial phantoms are available with a range of shapes, materials, and uses. However, these phantoms are typically costly and often feature simplified geometry. Three-dimensional (3D) printing, with its ability to rapidly and accurately produce new designs with the reproduction of complex, anatomical shapes based on patient-specific imaging data, promises new methods and possibilities for the production of medical imaging phantoms. In this chapter, a brief discussion of material properties pertinent to each imaging modality is described, and a variety of examples and methods to create 3D printed phantoms are included.

OBJECTIVE: High-permittivity pads have shown promising results in enhancing SNR and transmit efficiency when used for MRI of the brain, but fewer studies have been conducted to examine the performance of high-permittivity pads in other parts of the patient. In this work, we evaluate the impact on SNR and transmit efficiency distributions when high-permittivity pads with different thickness are positioned near the chest of the patient in combination with a transmit/receive array coil. METHODS: The performance of the pads is evaluated through numerical simulations, and both the SNR distribution and the transmit efficiency maps are compared with those obtained when the pads are not present and the distance between the coils and the patient is minimal. The average improvement of SNR and transmit efficiency in the heart is also evaluated for different values of the permittivity of the pads. RESULTS: In the scenario examined, high-permittivity pads can increase SNR and transmit efficiency in the heart volume by as much as 16% and 65%, respectively.

Across dialects, Spanish uses the third person forms usted and ustedes to refer to the addressee. In this squib, we propose an imposter analysis of these forms in the framework of Collins and Postal (2012. Imposters. MIT Press, Cambridge.).

MRI scanners with higher B0 fields typically provide a stronger Signal to Noise Ratio (SNR) which results in MRI images with their resolution or shorter acquisition time [1] . Stronger B0 fields require higher operating frequencies of the B1 field, and the shorter wavelengths cause in homogeneities in the B1 field distribution due to scattering and interference in the tissues. At higher B0 RF fields are limited to not cause excessive absorption of power in the tissues. Specifically, electric fields induce currents in conductive tissues causing heat dissipation which raise local temperature potentially leading to patient discomfort and even tissue damage. Hence, local Specific Absorprtion Rate (SAR) and temperature are limited.

High-permittivity pads have been used to enhance MRI performed in high-field static magnets (1-4). Here we perform an evaluation of fields and displacement currents associated with resonances in a thin pad or plate of high-permittivity material (HPM) in order to formulate basic design principles.

PURPOSE: To demonstrate that strategic use of materials with high electric permittivity along with integrated head-sized coil arrays can improve SNR in the entire brain. METHODS: Numerical simulations were used to design a high-permittivity material (HPM) helmet for enhancing SNR throughout the brain in receive arrays of 8 and 28 channels. Then, two 30-channel head coils of identical geometry were constructed: one fitted with a prototype helmet-shaped ceramic HPM helmet, and the second with a helmet-shaped low-permittivity shell, each 8-mm thick. An eight-channel dipole array was used for excitation. In vivo maps of excitation flip angle and SNR were acquired. RESULTS: Simulation results showed improvement in transmit efficiency by up to 65% and in receive-side SNR by up to 47% on average through the head with use of an HPM helmet. Experimental results showed that experimental transmit efficiency was improved by approximately 56% at the center of brain, and experimental receive-side SNR (SNR normalized to flip angle) was improved by approximately 21% on average through orthogonal planes through the cerebrum, including at the center of the brain, with the HPM. CONCLUSION: Although HPM is used increasingly to improve transmit efficiency locally in situations in which the transmit coil and imaging volume are much larger than the HPM, here we demonstrate that HPM can also be used to improve transmit efficiency and receive-side SNR throughout the brain by improving performance of a head-sized receive array. This includes the center of the brain, where it is difficult to improve SNR by other means.

High electric permittivity materials (HPM) have been used to improve transmit efficiency and receive sensitivity in targeted regions. In this work we demonstrate improvements in the entire b rain by designing and implementing an integrated HPM shell for a head coil with 8-channel transmit and 30-channel receive arryas. The coil with HPM insert provides gains of approximately55% in transmit efficiency and 20% in SNR compared to an identical coil withou t the insert.

MR-guided Focused Ultrasound (MRgFUS) of the head at 3 Tesla suffers from low image Signal-to-Noise ration in exactly the region of interest in the brain due to severe dielectric effects from the water bolus surrounding the head to facilitate cooling of the scalp and transmission of ultrasonic energy. We show that by changing the medium of the bath and adding slots to the ground plane it is possible to go from low SNR to high SNR I the region of interest.

Purpose Evaluate the possibility to reduce specific energy absorption rate (SAR)-induced maximum temperature and thermal dose by rearranging the order and spacing of sequences without increasing duration of the MRI examination. Methods Using numerical simulations based on an actual SAR-intensive MRI examination, optimizations to reduce either maximum temperature or thermal dose were performed. For each permutation of groups of sequences having the same patient table position, temperature and thermal dose were computed very rapidly using recently published methods. Disposition of sequences was further adjusted by optimizing the spacing between each sequence without exceeding the original exam duration. Results The maximum simulated temperature in the original exam was 42.38°C, and the maximum thermal dose was 3.23 cumulative effective minutes at 43°C (CEM43). After optimization to reduce maximum temperature, it was 41.77°C, and after optimization to minimize the thermal dose, it was 1.42 CEM43. Conclusion It is possible to reduce maximum temperature and thermal dose in the exam by changing the arrangement and spacing of the sequences without increasing the duration of the exam (by increasing TR or adding delays) or compromising image quality (by reducing flip angles).

OBJECT: To use high permittivity materials (HPM) positioned near radiofrequency (RF) surface coils to manipulate transmit/receive field patterns. MATERIALS AND METHODS: A large HPM pad was placed below the RF coil to extend the field-of-view (FOV). The resulting signal-to-noise (SNR) was compared to the performance of other coil configurations covering the same FOV in simulations and experiments at 7T. Transmit/receive efficiency was evaluated when HPM discs with or without a partial shield were positioned at a distance from the coil. Finally, we evaluated the improvement in transmit homogeneity for a four-channel array with HPM discs interposed between adjacent coil elements. RESULTS: Various configurations of HPM improved SNR, transmit/receive efficiency, extended the excitation/reception sensitivity overlap and FOV when positioned near a surface coil. For a four channel array driven in quadrature, shielded HPM discs enhanced the field below the discs as well as at the center of the sample as compared to other configurations with or without unshielded HPM. CONCLUSION: Strategically positioning HPM at a distance from a surface coil or array can increase the overlap between excitation/reception sensitivities, and extend the FOV of a single coil for reducing the number of channels in an array while minimally affecting the SNR.

Purpose To describe a new bench measurement based on quality (Q) factors to estimate the coil noise relative to the sample noise of dipole antennas at 7 T. Methods Placing a dipole antenna close to a highly conductive sample surrogate (HCSS) greatly reduces radiation loss, and using QHCSS gives a more accurate estimate of coil resistance than Qunloaded. Instead of using the ratio of unloaded and sample-loaded Q factors, the ratio of HCSS-loaded and sample-loaded Q factors should be used at ultra-high fields. A series of simulations were carried out to analyze the power budget of sample-loaded or HCSS-loaded dipole antennas. Two prototype dipole antennas were also constructed for bench measurements to validate the simulations. Results Simulations showed that radiation loss was suppressed when the dipole antenna was HCSS-loaded, and coil loss was largely the same as when the dipole was loaded by the sample. Bench measurements also showed good alignment with simulations. Conclusions Using the ratio QHCSS/Qloaded gives a good estimate of the coil loss for dipole antennas at 7 T, and provides a convenient bench measurement to predict the body noise dominance of dipole antenna designs. The new approach also applies to conventional surface loop coils at ultra-high fields. Magn Reson Med 2017. © 2017 International Society for Magnetic Resonance in Medicine.

Purpose To examine the possibility that MR-induced RF power deposition (SAR) and the resulting effects on temperature-dependent metabolic rates or perfusion rates might affect observed 18FDG signal in PET/MR. Methods Using numerical simulations of the SAR, consequent temperature increase, effect on rates of metabolism or perfusion, and [18FDG] throughout the body, we simulated the potential effect of maximum-allowable whole-body SAR for the entire duration of an hour-long PET/MR scan on observed PET signal for two different 18FDG injection times: one hour before onset of imaging and concurrent with the beginning of imaging. This was all repeated three times with the head, the heart, and the abdomen (kidneys) at the center of the RF coil. Results Qualitatively, little effect of MR-induced heating is observed on simulated PET images. Maximum relative increases in PET signal (26% and 31% increase, respectively, for the uptake models based on metabolism and the perfusion) occur in regions of low baseline metabolic rate (also associated with low perfusion and, thus, greater potential temperature increase due to high local SAR), such that PET signal in these areas remains comparatively low. Maximum relative increases in regions of high metabolic rate (and also high perfusion: heart, thyroid, brain, etc.) are affected mostly by the relatively small increase in core body temperature and thus are not affected greatly (10% maximum increase). Conclusions Even for worst-case heating, little effect of MR-induced heating is expected on 18FDG PET images during PET/MR for many clinically relevant applications. For quantitative, dynamic MR/PET studies requiring high SAR for extended periods, it is hoped that methods like those introduced here can help account for such potential effects in design of a given study, including selection of reference locations that should not experience notable increase in temperature.

Purpose To illustrate the potential for high permittivity materials to be used in decreasing peak local SAR associated with implants when the imaging region is far from the implant. Methods We performed numerical simulations of a human subject with a pacemaker in a body-sized birdcage coil driven at 128 MHz with and without a thin (5 mm) shell of material of high electric permittivity around the head. Results For a shell with relative permittivity of 600, the maximum specific energy absorption rate averaged over any 1 g of tissue near the pacemaker was reduced by 73.5% for a given B1 field strength at the center of the brain. Conclusion Although further study is required, initial simulations indicate that strategic use of high permittivity materials may broaden the conditions under which patients with certain implants can be imaged safely. Magn Reson Med, 2016. © 2016 International Society for Magnetic Resonance in Medicine.

In high field MRI, the spatial distribution of the radiofrequency magnetic ( ) field is usually affected by the presence of the sample. For hardware design and to aid interpretation of experimental results, it is important both to anticipate and to accurately simulate the behavior of these fields. Fields generated by a radiofrequency surface coil were simulated using dyadic Green's functions, or experimentally measured over a range of frequencies inside an object whose electrical properties were varied to illustrate a variety of transmit ( ) and receive ( ) field patterns. In this work, we examine how changes in polarization of the field and interference of propagating waves in an object can affect the spatial distribution. Results are explained conceptually using Maxwell's equations and intuitive illustrations. We demonstrate that the electrical conductivity alters the spatial distribution of distinct polarized components of the field, causing “twisted” transmit and receive field patterns, and asymmetries between and . Additionally, interference patterns due to wavelength effects are observed at high field in samples with high relative permittivity and near-zero conductivity, but are not present in lossy samples due to the attenuation of propagating EM fields. This work provides a conceptual framework for understanding spatial distributions for surface coils and can provide guidance for RF engineers. © 2016 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 46B: 25–40, 2016

One of the defining characteristics of the B cell receptor (BCR) is the extensive diversity in the repertoire of immunoglobulin genes that make up the BCR, resulting in broad range of specificity. Gammaherpesviruses are B lymphotropic viruses that establish life-long infection in B cells, and although the B cell receptor plays a central role in B cell biology, very little is known about the immunoglobulin repertoire of gammaherpesvirus infected cells. To begin to characterize the Ig genes expressed by murine gammaherpesvirus 68 (MHV68) infected cells, we utilized single cell sorting to sequence and clone the Ig variable regions of infected germinal center (GC) B cells and plasma cells. We show that MHV68 infection is biased towards cells that express the Igλ light chain along with a single heavy chain variable gene, IGHV10-1*01. This population arises through clonal expansion but is not viral antigen specific. Furthermore, we show that class-switching in MHV68 infected cells differs from that of uninfected cells. Fewer infected GC B cells are class-switched compared to uninfected GC B cells, while more infected plasma cells are class-switched compared to uninfected plasma cells. Additionally, although they are germinal center derived, the majority of class switched plasma cells display no somatic hypermutation regardless of infection status. Taken together, these data indicate that selection of infected B cells with a specific BCR, as well as virus mediated manipulation of class switching and somatic hypermutation, are critical aspects in establishing life-long gammaherpesvirus infection., Author summary Murine gammaherpesvirus 68 is a rodent pathogen that is closely related to the human gammaherpesviruses Epstein-Barr virus and Kaposi’s sarcoma-associated virus. All know gammaherpesviruses are associated with the development of lymphomas, as well as other cancers, in a small subset of infected individuals–particularly those with underlying defects in their immune system (i.e., transplant recipients and HIV infected patients). Because there are very limited small animal models for the human gammaherpesviruses, studies on murine gammaherepsviruses 68 can provide important insights into critical aspects of gammaherpesvirus infections and the association of these viruses with disease development. Another feature of all gammaherpesviruses is their ability to establish a chronic infection of their host–where the virus is maintained for the lifetime of the infected individual. The major target cell harboring chronic gammaherepsvirus infection are B lymphocytes–the cells in the immune system that produce antibodies in response to infections. Here we provide a detailed characterization of the populations of B lymphocytes that become infected by murine gammaherpesvirus 68. This has led to the identification of a specific population of B lymphocytes that is preferentially infected by the virus. This supports a model in which murine gammaherpesvirus infection of B lymphocytes is not random. However, it remains unclear why the virus targets this specific population of B cells for infection.

In this paper we evaluate through simulations the SNR improvement in the head when a high-permittivity helmet former is used with a 28 channel receive array coil. The impact on the SNR of parallel imaging applications is also considered. An overall improvement in the brain is observed when the high-permittivity helmet is used with an average SNR increase of about 51%, and a maximum improvement of 220% in a location on the side of the head. During parallel imaging applications a significant improvement can be still observed in presence of the high-permittivity helmet, but some SNR reduction is observed in the head when high acceleration factors are performed.

In this paper we evaluate an 8-channel electric dipole array placed around a high permittivity material helmet former in simulation. Transmit efficiency and coil coupling are evaluated, with the intent to build an 8-channel transmit array around a tight fitting dense receive array and a dielectric helmet for head imaging at 7 Tesla.

PURPOSE: To investigate how high-permittivity materials (HPMs) can improve signal-to-noise ratio (SNR) when placed between magnetic resonance detectors and the imaged body. METHODS: We used a simulation framework based on dyadic Green’s functions to calculate the electromagnetic field inside a uniform dielectric sphere at 7 Tesla, with and without a surrounding layer of HPM. SNR-optimizing (“ideal”) current patterns were expressed as the sum of signal-optimizing (“signal-only”) current patterns and dark mode current patterns that minimize sample noise while contributing nothing to signal. We investigated how HPM affects the shape and amplitude of these current patterns, sample noise, and array SNR. RESULTS: Ideal and signal-only current patterns were identical for a central voxel. HPMs introduced a phase shift into these patterns, compensating for signal propagation delay in the HPMs. For an intermediate location within the sphere, dark mode current patterns were present and illustrated the mechanisms by which HPMs can reduce sample noise. High-amplitude signal-only current patterns were observed for HPM configurations that shield the electromagnetic field from the sample. For coil-arrays, these configurations corresponded to poor SNR in deep regions, but resulted in large SNR gains near the surface, due to enhanced fields in the vicinity of the HPM. For very high relative permittivity values, HPM thickness corresponding to even multiples of λ/4 resulted in coil SNR gains throughout the sample. CONCLUSION: HPMs affect both signal sensitivity and sample noise. Lower-amplitude signal-only optimal currents corresponded to higher array SNR performance, and could guide the design of coils integrated with HPM.

Electromagnetic field simulations are increasingly used to assure RF safety of patients during MRI exams. In practice, however, tissue property distribution of the patient being imaged is not known, but may be represented with a pre-existing model. Repeatedly, agreement in transmit magnetic (B1+) field distributions between two geometries has been used to suggest agreement in heating distributions. Here we examine relative effects of anatomical differences on B1+ distribution, Specific Absorption Rate (SAR) and temperature change (ΔT). Numerical simulations were performed for a single surface coil positioned adjacent a homogeneous phantom and bovine phantom, each with slight geometric variations, and adjacent two different human body models. Experimental demonstration was performed on a bovine phantom using MR thermometry and B1+ mapping. Simulations and experiments demonstrate that B1+ distributions in different samples can be well correlated, while notable difference in maximum SAR and ΔT occur. This work illustrates challenges associated with utilizing simulations or experiments for RF safety assurance purposes. Reliance on B1+ distributions alone for validation of simulations and/or experiments with a sample or subject for assurance of safety in another should be performed with caution.

Purpose Present a novel method for rapid prediction of temperature in vivo for a series of pulse sequences with differing levels and distributions of specific energy absorption rate (SAR). Theory and Methods After the temperature response to a brief period of heating is characterized, a rapid estimate of temperature during a series of periods at different heating levels is made using a linear heat equation and impulse-response (IR) concepts. Here the initial characterization and long-term prediction for a complete spine exam are made with the Pennes' bioheat equation where, at first, core body temperature is allowed to increase and local perfusion is not. Then corrections through time allowing variation in local perfusion are introduced. Results The fast IR-based method predicted maximum temperature increase within 1% of that with a full finite difference simulation, but required less than 3.5% of the computation time. Even higher accelerations are possible depending on the time step size chosen, with loss in temporal resolution. Correction for temperature-dependent perfusion requires negligible additional time and can be adjusted to be more or less conservative than the corresponding finite difference simulation. Conclusion With appropriate methods, it is possible to rapidly predict temperature increase throughout the body for actual MR examinations. Magn Reson Med, 2015. © 2015 Wiley Periodicals, Inc.

With the increasing demand for higher data rates and more reliable service capabilities for wireless devices, wireless service providers are facing an unprecedented challenge to overcome a global bandwidth shortage. Early global activities on beyond fourth-generation (B4G) and fifth-generation (5G) wireless communication systems suggest that millimeter-wave (mmWave) frequencies are very promising for future wireless communication networks due to the massive amount of raw bandwidth and potential multigigabit-per-second (Gb/s) data rates [1]?[3]. Both industry and academia have begun the exploration of the untapped mmWave frequency spectrum for future broadband mobile communication networks. In April 2014, the Brooklyn 5G Summit [4], sponsored by Nokia and the New York University (NYU) WIRELESS research center, drew global attention to mmWave communications and channel modeling. In July 2014, the IEEE 802.11 next-generation 60-GHz study group was formed to increase the data rates to over 20 Gb/s in the unlicensed 60-GHz frequency band while maintaining backward compatibility with the emerging IEEE 802.11ad wireless local area network (WLAN) standard [5].

BACKGROUND There is growing interest in detecting cerebro-cerebellar circuits, which requires adequate blood oxygenation level dependent contrast and signal-to-noise ratio (SNR) throughout the brain. Although 7T scanners offer increased SNR, coverage of commercial head coils is currently limited to the cerebrum. PURPOSE To improve cerebellar functional MRI (fMRI) at 7T with high permittivity material (HPM) pads extending the sensitivity of a commercial coil. STUDY TYPE Simulations were used to determine HPM pad configuration and assess radiofrequency (RF) safety. In vivo experiments were performed to evaluate RF field distributions and SNR and assess improvements of cerebellar fMRI. SUBJECTS Eight healthy volunteers enrolled in a prospective motor fMRI study with and without HPM. FIELD STRENGTH/SEQUENCE Gradient echo (GRE) echo planar imaging for fMRI, turbo FLASH for flip angle mapping, GRE sequence for SNR maps, and T1 -weighted MPRAGE were acquired with and without HPM pads at 7T. ASSESSMENT Field maps, SNR maps, and anatomical images were evaluated for coverage. Simulation results were used to assess SAR levels of the experiment. Activation data from fMRI experiments were compared with and without HPM pads. STATISTICAL TESTS: fMRI data were analyzed using FEAT FSL for each subject followed by group level analysis using paired t-test of acquisitions with and without HPM. RESULTS Simulations showed 52% improvement in transmit efficiency in cerebellum with HPM and SAR levels well below recommended limits. Experiments showed 27% improvement in SNR in cerebellum and improvement in coverage on T1 -weighted images. fMRI showed greater cerebellar activation in individual subjects with the HPM pad present (Z > = 4), especially in inferior slices of cerebellum, with 59% average increase in number of activated voxels in the cerebellum. Group-level analysis showed improved functional activation (Z > = 2.3) in cerebellar regions with HPM pads without loss of measured activation elsewhere. DATA CONCLUSION HPM pads can improve cerebellar fMRI at 7T with a commonly-used head coil without compromising RF safety. LEVEL OF EVIDENCE 2 Technical Efficacy: Stage 1 J. MAGN. RESON. IMAGING 2018;48:431-440.

As we rely increasingly on numerical simulations to evaluate safety of RF fields in MRI, there lies a danger in treating the simulations like a ”black box” and trusting results that may not be correct. An intuitive understanding of what to expect in general can help avoid such a situation. Here we examine what to expect in terms of Specific energy Absorption Rate (SAR) distributions for different types of surface coils and for volume coils both in the quasi-static regime and when wavelength effects become significant.

In this paper we evaluate the safety (maximum local SAR and temperature) of three different algorithms to drive the currents of a transmit array: 1) the birdcage coil algorithm; 2) the phase-only optimization algorithm; 3) the phase and amplitude optimization algorithm. The last two algorithms are evaluated when optimizing the transmit efficiency of RF fields in two different locations, one close to the center, and one close to the extremities of a human body subject. All the fields were normalized so that the whole-body average SAR is equal to 2W/kg, and temperature simulations were computed for a continuous exposure 1 hour long.

In this work we present a systematic evaluation of the potential of combining dielectric materials with an array of electric dipoles for MRI. Design parameters include the permittivity and length of a dielectric sleeve, as well as the dipole length and position of the tuning inductors. Results show that the combined approach improves transmit efficiency and SNR by ∼10 to 15 % compared to an optimized dipole array without dielectric sleeve. The resulting induced current densities reflect an improved correspondence with the ideal current pattern, which explains the gains in performance.

A 29-channel receive-only dense dipole head array for 7T MRI was built, methods for isolating the coil elements from each other and detuning each coil element during transmit was explored, and its performance was evaluated and compared with a commercial 24ch head array (Nova Medical. Wilmington, MA).