Development of a 7 T RF coil system for breast imaging

In ultrahigh‐field MRI, such as 7 T, the signal‐to‐noise ratio (SNR) increases while transmit (Tx) field (B₁⁺) can be degraded due to inhomogeneity and elevated specific absorption rate (SAR). By applying new array coil concepts to both Tx and receive (Rx) coils, the B₁⁺ homogeneity and SNR can be improved. In this study, we developed and tested in vivo a new RF coil system for 7 T breast MRI. An RF coil system composed of an eight‐channel Tx‐only array based on a tic‐tac‐toe design (can be combined to operate in single‐Tx mode) in conjunction with an eight‐channel Rx‐only insert was developed. Characterizations of the B₁⁺ field and associated SAR generated by the developed RF coil system were numerically calculated and empirically measured using an anatomically detailed breast model, phantom and human breasts. In vivo comparisons between 3 T (using standard commercial solutions) and 7 T (using the newly developed coil system) breast imaging were made. At 7 T, about 20% B₁⁺ inhomogeneity (standard deviation over the mean) was measured within the breast tissue for both the RF simulations and 7 T experiments. The addition of the Rx‐only array enhances the SNR by a factor of about three. High‐quality MR images of human breast were acquired in vivo at 7 T. For the in vivo comparisons between 3 T and 7 T, an approximately fourfold increase of SNR was measured with 7 T imaging. The B₁⁺ field distributions in the breast model, phantom and in vivo were in reasonable agreement. High‐quality 7 T in vivo breast MRI was successfully acquired at 0.6 mm isotropic resolution using the newly developed RF coil system.     

Comparison of RF body coils for MRI at 3 T: a simulation study using parallel transmission on various anatomical targets

The performance of multichannel transmit coil layouts and parallel transmission (pTx) RF pulse design was evaluated with respect to transmit B₁ (B₁ ⁺) homogeneity and specific absorption rate (SAR) at 3 T for a whole body coil. Five specific coils were modeled and compared: a 32‐rung birdcage body coil (driven either in a fixed quadrature mode or a two‐channel transmit mode), two single‐ring stripline arrays (with either 8 or 16 elements), and two multi‐ring stripline arrays (with two or three identical rings, stacked in the z axis and each comprising eight azimuthally distributed elements). Three anatomical targets were considered, each defined by a 3D volume representative of a meaningful region of interest (ROI) in routine clinical applications. For a given anatomical target, global or local SAR controlled pTx pulses were designed to homogenize RF excitation within the ROI. At the B₁ ⁺ homogeneity achieved by the quadrature driven birdcage design, pTx pulses with multichannel transmit coils achieved up to about eightfold reduction in local and global SAR. When used for imaging head and cervical spine or imaging thoracic spine, the double‐ring array outperformed all coils, including the single‐ring arrays. While the advantage of the double‐ring array became much less pronounced for pelvic imaging, with a substantially larger ROI, the pTx approach still provided significant gains over the quadrature birdcage coil. For all design scenarios, using the three‐ring array did not necessarily improve the RF performance. Our results suggest that pTx pulses with multichannel transmit coils can reduce local and global SAR substantially for body coils while attaining improved B₁ ⁺ homogeneity, particularly for a “z‐stacked” double‐ring design with coil elements arranged on two transaxial rings. Copyright © 2015 John Wiley & Sons, Ltd.     

Low SAR 31P (multi‐echo) spectroscopic imaging using an integrated whole‐body transmit coil at 7T

Phosphorus (³¹P) MRSI provides opportunities to monitor potential biomarkers. However, current applications of ³¹P MRS are generally restricted to relatively small volumes as small coils are used. Conventional surface coils require high energy adiabatic RF pulses to achieve flip angle homogeneity, leading to high specific absorption rates (SARs), and occupy space within the MRI bore. A birdcage coil behind the bore cover can potentially reduce the SAR constraints massively by use of conventional amplitude modulated pulses without sacrificing patient space. Here, we demonstrate that the integrated ³¹P birdcage coil setup with a high power RF amplifier at 7 T allows for low flip angle excitations with short repetition time (T_R) for fast 3D chemical shift imaging (CSI) and 3D T₁‐weighted CSI as well as high flip angle multi‐refocusing pulses, enabling multi‐echo CSI that can measure metabolite T₂, over a large field of view in the body. B₁⁺ calibration showed a variation of only 30% in maximum B₁ in four volunteers. High signal‐to‐noise ratio (SNR) MRSI was obtained in the gluteal muscle using two fast in vivo 3D spectroscopic imaging protocols, with low and high flip angles, and with multi‐echo MRSI without exceeding SAR levels. In addition, full liver MRSI was achieved within SAR constraints. The integrated ³¹P body coil allowed for fast spectroscopic imaging and successful implementation of the multi‐echo method in the body at 7 T. Moreover, no additional enclosing hardware was needed for ³¹P excitation, paving the way to include larger subjects and more space for receiver arrays. The increase in possible number of RF excitations per scan time, due to the improved B₁⁺ homogeneity and low SAR, allows SNR to be exchanged for spatial resolution in CSI and/or T₁ weighting by simply manipulating T_R and/or flip angle to detect and quantify ratios from different molecular species.     

Ultra‐high‐field RF coil development for evaluating upper extremity imaging applications

The purpose of this study is to develop and evaluate a custom‐designed 7  T MRI coil and explore its use for upper extremity applications. An RF system composed of a transverse electromagnetic transmit coil and an eight‐channel receive‐only array was developed for 7  T upper extremity applications. The RF system was characterized and evaluated using scattering parameters and B₁⁺ mapping. Finite difference time domain simulations were performed to evaluate the B₁⁺ field distribution and specific absorption rate for the forearm region of the upper extremity. High‐resolution 7  T images were acquired and compared with those at 3 T. The simulation and experimental results show very good B₁⁺ field homogeneity across the forearm. High‐resolution images of musculotendinous, osseocartilaginous, and neurovascular structures in the upper extremity are presented with T₁ volumetric interpolated breath‐hold examination, T₂ double‐echo steady state, T₂* susceptibility weighted imaging (SWI), diffusion tensor imaging, and time‐of‐flight sequences. Comparison between 3  T and 7  T is shown. Intricate contextual anatomy can be delineated in synovial, fibrocartilaginous, interosseous, and intraosseous trabecular structures of the forearm, as well as palmar and digital vascular anatomy (including microvascular detail in SWI). Ultra‐high‐field 7  T imaging holds great potential in improving the sensitivity and specificity of upper extremity imaging, especially in wrist and hand pathology secondary to bone, ligament, nerve, vascular, and other soft or hard tissue etiology.     

Evaluation of transmit efficiency and SAR for a tight fit transceiver human head phased array at 9.4 T

Ultra‐high field (UHF, ≥7 T) tight fit transceiver phased arrays improve transmit (Tx) efficiency (B₁⁺/√P) in comparison with Tx‐only arrays, which are usually larger to fit receive (Rx)‐only arrays inside. One of the major problems limiting applications of tight fit arrays at UHFs is the anticipated increase of local tissue heating, which is commonly evaluated by the local specific absorption rate (SAR). To investigate the tradeoff between Tx efficiency and SAR when a tight fit UHF human head transceiver phased array is used instead of a Tx‐only/Rx‐only RF system, a single‐row eight‐element prototype of a 400 MHz transceiver head phased array was constructed. The Tx efficiency and SAR of the array were evaluated and compared with that of a larger Tx‐only array, which could also be used in combination with an 18‐channel Rx‐only array. Data were acquired on the Siemens Magnetom whole body 9.4 T human MRI system. Depending on the head size, positioning and the RF shim strategy, the smaller array provides from 11 to 23% higher Tx efficiency. In general, the Tx performance, evaluated as B₁⁺/√SAR, i.e. the safety excitation efficiency (SEE), is also not compromised. The two arrays provide very similar SEEs evaluated over 1000 random RF shim sets. We demonstrated that, in general, the tight fit transceiver array improves Tx performance without compromising SEE. However, in specific cases, the SEE value may vary, favoring one of the arrays, and therefore must be carefully evaluated.     

Numerical comparison of local transceiver arrays of fractionated dipoles and microstrip antennas for body imaging at 7 T

Longitudinally orientated dipoles and microstrip antennas have both demonstrated superior results as RF transmit elements for body imaging at 7 T MRI, and are as of today the most commonly used transmit elements. In this study, the performances of the two antenna concepts were compared for use in local RF antenna arrays by numerical simulations. Antenna elements investigated are the fractionated dipole and the microstrip line with meander structures. Phantom simulations with a single antenna element were performed and evaluated with regard to specific absorption rate (SAR) efficiency in the center of the subject. Simulations of array configurations with 8 and 16 elements were performed with anatomical body models. Both antenna elements were combined with a loop coil to compare hybrid configurations. Singular value decomposition of the B₁⁺ fields, RF shimming, and calculation of the voxel-wise power and SAR efficiencies were performed in regions of interest with varying sizes to evaluate the transmit performance. The signal-to-noise ratio (SNR) was evaluated to estimate the receive performance. Simulated data show similar transmit profiles for the two antenna types in the center of the phantom (penetration depth > 20 mm). For body imaging, no considerable differences were determined for the different antenna configurations with regard to the transmit performance. Results show the advantage of 16 transmit channels compared with today's commonly used 8-channel systems (minimum RF shimming excitation error of 4.7% (4.3%) versus 2.7% (2.8%) for the 8-channel and 16-channel configurations with the microstrip antennas in a (5 cm)³ cube in the center of a male (female) body model). Highest SNR is achieved for the 16-channel configuration with fractionated dipoles. The combination of either fractionated dipoles or microstrip antennas with loop coils is more favorable with regard to the transmit performance compared with only increasing the number of elements.     

Homogeneous B1+ for bilateral breast imaging at 7 T using a five dipole transmit array merged with a high density receive loop array

To explore the use of five meandering dipole antennas in a multi‐transmit setup, combined with a high density receive array for breast imaging at 7 T for improved penetration depth and more homogeneous B₁ field. Five meandering dipole antennas and 30 receiver loops were positioned on two cups around the breasts. Finite difference time domain simulations were performed to evaluate RF safety limits of the transmit setup. Scattering parameters of the transmit setup and coupling between the antennas and the detuned loops were measured. In vivo parallel imaging performance was investigated for various acceleration factors. After RF shimming, a B₁ map, a T₁‐weighted image, and a T₂‐weighted image were acquired to assess B₁ efficiency, uniformity in contrast weighting, and imaging performance in clinical applications. The maximum achievable local SAR_10g value was 7.0 W/kg for 5 × 1 W accepted power. The dipoles were tuned and matched to a maximum reflection of ?11.8 dB, and a maximum inter‐element coupling of ?14.2 dB. The maximum coupling between the antennas and the receive loops was ?18.2 dB and the mean noise correlation for the 30 receive loops 7.83 ± 8.69%. In vivo measurements showed an increased field of view, which reached to the axilla, and a high transmit efficiency. This coil enabled the acquisition of T₁‐weighted images with a high spatial resolution of 0.7 mm³ isotropic and T₂‐weighted spin echo images with uniformly weighted contrast.     

Whole‐body radiofrequency coil for 31P MRSI at 7 T

Widespread use of ultrahigh‐field ³¹P MRSI in clinical studies is hindered by the limited field of view and non‐uniform radiofrequency (RF) field obtained from surface transceivers. The non‐uniform RF field necessitates the use of high specific absorption rate (SAR)‐demanding adiabatic RF pulses, limiting the signal‐to‐noise ratio (SNR) per unit of time. Here, we demonstrate the feasibility of using a body‐sized volume RF coil at 7 T, which enables uniform excitation and ultrafast power calibration by pick‐up probes. The performance of the body coil is examined by bench tests, and phantom and in vivo measurements in a 7‐T MRI scanner. The accuracy of power calibration with pick‐up probes is analyzed at a clinical 3‐T MR system with a close to identical ¹H body coil integrated at the MR system. Finally, we demonstrate high‐quality three‐dimensional ³¹P MRSI of the human body at 7 T within 5 min of data acquisition that includes RF power calibration. Copyright © 2016 John Wiley & Sons, Ltd.     

Extended RF shimming: Sequence‐level parallel transmission optimization applied to steady‐state free precession MRI of the heart

Cardiac magnetic resonance imaging (MRI) at high field presents challenges because of the high specific absorption rate and significant transmit field (B₁⁺) inhomogeneities. Parallel transmission MRI offers the ability to correct for both issues at the level of individual radiofrequency (RF) pulses, but must operate within strict hardware and safety constraints. The constraints are themselves affected by sequence parameters, such as the RF pulse duration and TR, meaning that an overall optimal operating point exists for a given sequence. This work seeks to obtain optimal performance by performing a ‘sequence‐level’ optimization in which pulse sequence parameters are included as part of an RF shimming calculation. The method is applied to balanced steady‐state free precession cardiac MRI with the objective of minimizing TR, hence reducing the imaging duration. Results are demonstrated using an eight‐channel parallel transmit system operating at 3 T, with an in vivo study carried out on seven male subjects of varying body mass index (BMI). Compared with single‐channel operation, a mean‐squared‐error shimming approach leads to reduced imaging durations of 32 ± 3% with simultaneous improvement in flip angle homogeneity of 32 ± 8% within the myocardium.     

Wideband Self‐Grounded Bow‐Tie Antenna for Thermal MR

The objective of this study was the design, implementation, evaluation and application of a compact wideband self‐grounded bow‐tie (SGBT) radiofrequency (RF) antenna building block that supports anatomical proton (¹H) MRI, fluorine (¹⁹F) MRI, MR thermometry and broadband thermal intervention integrated in a whole‐body 7.0 T system. Design considerations and optimizations were conducted with numerical electromagnetic field (EMF) simulations to facilitate a broadband thermal intervention frequency of the RF antenna building block. RF transmission (B₁⁺) field efficiency and specific absorption rate (SAR) were obtained in a phantom, and the thigh of human voxel models (Ella, Duke) for ¹H and ¹⁹F MRI at 7.0 T. B₁⁺ efficiency simulations were validated with actual flip‐angle imaging measurements. The feasibility of thermal intervention was examined by temperature simulations (f = 300, 400 and 500 MHz) in a phantom. The RF heating intervention (P_in = 100 W, t = 120 seconds) was validated experimentally using the proton resonance shift method and fiberoptic probes for temperature monitoring. The applicability of the SGBT RF antenna building block for in vivo ¹H and ¹⁹F MRI was demonstrated for the thigh and forearm of a healthy volunteer. The SGBT RF antenna building block facilitated ¹⁹F and ¹H MRI at 7.0 T as well as broadband thermal intervention (234‐561 MHz). For the thigh of the human voxel models, a B₁⁺ efficiency ≥11.8 μT/√kW was achieved at a depth of 50 mm. Temperature simulations and heating experiments in a phantom demonstrated a temperature increase ΔT >7 K at a depth of 10 mm. The compact SGBT antenna building block provides technology for the design of integrated high‐density RF applicators and for the study of the role of temperature in (patho‐) physiological processes by adding a thermal intervention dimension to an MRI device (Thermal MR).     

Resonant inductive decoupling (RID) for transceiver arrays to compensate for both reactive and resistive components of the mutual impedance

Transceiver surface coil arrays improve transmit performance (B₁/√kW) and B₁ homogeneity for head imaging up to 9.4 T. To further improve reception performance and parallel imaging, the number of array elements must be increased with a corresponding decrease in their size. With a large number of small interacting antennas, decoupling is one of the most challenging aspects in the design and construction of transceiver arrays. Previously described decoupling techniques using geometric overlap, inductive or capacitive decoupling have focused on the elimination of the reactance of the mutual impedance only, which can limit the obtainable decoupling to –10 dB as a result of residual mutual resistance. A novel resonant inductive decoupling (RID) method, which allows compensation for both reactive and resistive components of the mutual impedance between the adjacent surface coils, has been developed and verified experimentally. This method provides an easy way to adjust the decoupling remotely by changing the resonance frequency of the RID circuit through the adjustment of a variable capacitor. As an example, a single‐row (1 × 16) 7‐T transceiver head array of n = 16 small overlapped surface coils using RID decoupling between adjacent coils was built. In combination with overlapped coils, the RID technique achieved better than –24 dB of decoupling for all adjacent coils. Copyright © 2013 John Wiley & Sons, Ltd.     

Combination of surface and ‘vertical’ loop elements improves receive performance of a human head transceiver array at 9.4 T

Ultra‐high‐field (UHF, ≥7 T) human magnetic resonance imaging (MRI) provides undisputed advantages over low‐field MRI (≤3 T), but its development remains challenging because of numerous technical issues, including the low efficiency of transmit (Tx) radiofrequency (RF) coils caused by the increase in tissue power deposition with frequency. Tight‐fit human head transceiver (TxRx) arrays improve Tx efficiency in comparison with Tx‐only arrays, which are larger in order to fit multi‐channel receive (Rx)‐only arrays inside. A drawback of the TxRx design is that the number of elements in an array is limited by the number of available high‐power RF Tx channels (commonly 8 or 16), which is not sufficient for optimal Rx performance. In this work, as a proof of concept, we developed a method for increasing the number of Rx elements in a human head TxRx surface loop array without the need to move the loops away from a sample, which compromises the array Tx performance. We designed and constructed a prototype 16‐channel tight‐fit array, which consists of eight TxRx surface loops placed on a cylindrical holder circumscribing a head, and eight Rx‐only vertical loops positioned along the central axis (parallel to the magnetic field B₀) of each TxRx loop, perpendicular to its surface. We demonstrated both experimentally and numerically that the addition of the vertical loops has no measurable effect on the Tx efficiency of the array. An increase in the maximum local specific absorption rate (SAR), evaluated using two human head voxel models (Duke and Ella), measured 3.4% or less. At the same time, the 16‐element array provided 30% improvement of central signal‐to‐noise ratio (SNR) in vivo relative to a surface loop eight‐element array. The novel array design also demonstrated an improvement in the parallel Rx performance in the transversal plane. Thus, using this method, both the Rx and Tx performance of the human head array can be optimized simultaneously.     

Small‐animal,whole‐body imaging with metamaterial‐inspired RF coil

Particular applications in preclinical magnetic resonance imaging require the entire body of an animal to be imaged with sufficient quality. This is usually performed by combining regions scanned with small coils with high sensitivity or long scans using large coils with low sensitivity. Here, a metamaterial‐inspired design employing a parallel array of wires operating on the principle of eigenmode hybridization was used to produce a small‐animal imaging coil. The coil field distribution responsible for the coil field of view and sensitivity was simulated in an electromagnetic simulation package and the coil geometrical parameters were optimized for whole‐body imaging. A prototype coil was then manufactured and assembled using brass telescopic tubes with copper plates as distributed capacitance. Its field distribution was measured experimentally using the B₁⁺ mapping technique and was found to be in close correspondence with the simulated results. The coil field distribution was found to be suitable for large field of view small‐animal imaging and the coil image quality was compared with a commercially available coil by whole‐body scanning of living mice. Signal‐to‐noise measurements in living mice showed higher values than those of a commercially available coil with large receptive fields, and rivalled the performance of small receptive field and high‐sensitivity coils. The coil was deemed to be suitable for some whole‐body, small‐animal preclinical applications.     

Wireless coils based on resonant and nonresonant coupled‐wire structure for small animal multinuclear imaging

Earlier work on RF metasurfaces for preclinical MRI has targeted applications such as whole‐body imaging and dual‐frequency coils. In these studies, a nonresonant loop was used to induce currents into a metasurface that was operated as a passive inductively powered resonator. However, as we show in this study, the strategy of using a resonant metasurface reduces the impact of the loop on the global performance of the assembled coil. To mitigate this deficiency, we developed a new approach that relies on the combination of a commercial surface coil and a coupled‐wire structure operated away from its resonance. This strategy enables the extension of the sensitive volume of the surface coil while maintaining its local high sensitivity without any hardware modification. A wireless coil based on a two parallel coupled‐wire structure was designed and electromagnetic field simulations were carried out with different levels of matching and coupling between both components of the coil. For experimental characterization, a prototype was built and tested at two frequencies, 300 MHz for ¹H and 282.6 MHz for ¹⁹F at 7 T. Phantom and in vivo MRI experiments were conducted in different configurations to study signal and noise figures of the structure. The results showed that the proposed strategy improves the overall sensitive volume while simultaneously maintaining a high signal‐to‐noise ratio (SNR). Metasurfaces based on coupled wires are therefore shown here as promising and versatile elements in the MRI RF chain, as they allow customized adjustment of the sensitive volume as a function of SNR yield. In addition, they can be easily adapted to different Larmor frequencies without loss of performance.     

Optimized 31P MRS in the human brain at 7 T with a dedicated RF coil setup

The design and construction of a dedicated RF coil setup for human brain imaging (¹H) and spectroscopy (³¹P) at ultra‐high magnetic field strength (7 T) is presented. The setup is optimized for signal handling at the resonance frequencies for ¹H (297.2 MHz) and ³¹P (120.3 MHz). It consists of an eight‐channel ¹H transmit–receive head coil with multi‐transmit capabilities, and an insertable, actively detunable ³¹P birdcage (transmit–receive and transmit only), which can be combined with a seven‐channel receive‐only ³¹P array. The setup enables anatomical imaging and ³¹P studies without removal of the coil or the patient. By separating transmit and receive channels and by optimized addition of array signals with whitened singular value decomposition we can obtain a sevenfold increase in SNR of ³¹P signals in the occipital lobe of the human brain compared with the birdcage alone. These signals can be further enhanced by 30 ± 9% using the nuclear Overhauser effect by B₁‐shimmed low‐power irradiation of water protons. Together, these features enable acquisition of ³¹P MRSI at high spatial resolutions (3.0 cm³ voxel) in the occipital lobe of the human brain in clinically acceptable scan times (~15 min). © 2015 The Authors. NMR in Biomedicine published by John Wiley & Sons Ltd.     

High‐resolution MRI encoding using radiofrequency phase gradients

Although MRI offers highly diagnostic medical imagery, patient access to this modality worldwide is very limited when compared with X‐ray or ultrasound. One reason for this is the expense and complexity of the equipment used to generate the switched magnetic fields necessary for MRI encoding. These field gradients are also responsible for intense acoustic noise and have the potential to induce nerve stimulation. We present results with a new MRI encoding principle which operates entirely without the use of conventional B₀ field gradients. This new approach – ‘Transmit Array Spatial Encoding’ (TRASE) – uses only the resonant radiofrequency (RF) field to produce Fourier spatial encoding equivalent to conventional MRI. k‐space traversal (image encoding) is achieved by spin refocusing with phase gradient transmit fields in spin echo trains. A transmit coil array, driven by just a single transmitter channel, was constructed to produce four phase gradient fields, which allows the encoding of two orthogonal spatial axes. High‐resolution two‐dimensional‐encoded in vivo MR images of hand and wrist were obtained at 0.2 T. TRASE exploits RF field phase gradients, and offers the possibility of very low‐cost diagnostics and novel experiments exploiting unique capabilities, such as imaging without disturbance of the main B₀ magnetic field. Lower field imaging (<1 T) and micro‐imaging are favorable application domains as, in both cases, it is technically easier to achieve the short RF pulses desirable for long echo trains, and also to limit RF power deposition. As TRASE is simply an alternative mechanism (and technology) of moving through k space, there are many close analogies between it and conventional B₀‐encoded techniques. TRASE is compatible with both B₀ gradient encoding and parallel imaging, and so hybrid sequences containing all three spatial encoding approaches are possible. Copyright © 2013 John Wiley & Sons, Ltd.     

Design of a forward view antenna for prostate imaging at 7 T

Purpose

To design a forward view antenna for prostate imaging at 7 T, which is placed between the legs of the subject in addition to a dipole array.

Materials and methods

The forward view antenna is realized by placing a cross‐dipole antenna at the end of a small rectangular waveguide. Quadrature drive of the cross‐dipole can excite a circularly polarized wave propagating along the axial direction to and from the prostate region. Functioning of the forward view antenna is validated by comparing measurements and simulations. Antenna performance is evaluated by numerical simulations and measurements at 7 T.

Results

Simulations of B₁⁺ on a phantom are in good correspondence with measurements. Simulations on a human model indicate that the signal‐to‐noise ratio (SNR), specific absorption rate (SAR) efficiency and SAR increase when adding the forward view antenna to a previously published dipole array. The SNR increases by up to 18% when adding the forward view antenna as a receive antenna to an eight‐channel dipole array in vivo.

Conclusions

A design for a forward view antenna is presented and evaluated. SNR improvements up to 18% are demonstrated when adding the forward view antenna to a dipole array.     

Simultaneous bilateral hip joint imaging at 7 Tesla using fast transmit B(1) shimming methods and multichannel transmission - a feasibility study

The objective of this study was to demonstrate the feasibility of simultaneous bilateral hip imaging at 7 Tesla. Hip joint MRI becomes clinically critical since recent advances have made hip arthroscopy an efficacious approach to treat a variety of early hip diseases. The success of these treatments requires a reliable and accurate diagnosis of intraarticular abnormalities at an early stage. Articular cartilage assessment is especially important to guide surgical decisions but is difficult to achieve with current MR methods. Because of gains in tissue contrast and spatial resolution reported at ultra high magnetic fields, there are strong expectations that imaging the hip joint at 7 Tesla will improve diagnostic accuracy. Furthermore, there is growing evidence that the majority of these hip abnormalities occur bilaterally, emphasizing the need for bilateral imaging. However, obtaining high quality images in the human torso, in particular of both hips simultaneously, must overcome a major challenge arising from the damped traveling wave behaviour of RF waves at 7 Tesla that leads to severe inhomogeneities in transmit B1 (B₁⁺) phase and magnitude, typically resulting in areas of low signal and contrast, and consequently impairing use for clinical applications. To overcome this problem, a 16‐channel stripline transceiver RF coil was used, together with a B1 shimming algorithm aiming at maximizing B₁⁺ in six regions of interest over the hips that were identified on axial scout images. Our successful results demonstrate that this approach effectively reduces inhomogeneities observed before B1 shimming and provides high joint tissue contrast in both hips while reducing the required RF power. Critical to this success was a fast small flip angle B₁⁺ calibration scan that permitted the computation of subject‐specific B1 shimming solutions, a necessary step to account for large spatial variations in B₁⁺ phase observed in different subjects. Copyright © 2012 John Wiley & Sons, Ltd.     

Skin sodium measured with 23Na MRI at 7.0 T

Skin sodium (Na⁺) storage, as a physiologically important regulatory mechanism for blood pressure, volume regulation and, indeed, survival, has recently been rediscovered. This has prompted the development of MRI methods to assess Na⁺ storage in humans (²³Na MRI) at 3.0 T. This work examines the feasibility of high in‐plane spatial resolution ²³Na MRI in skin at 7.0 T. A two‐channel transceiver radiofrequency (RF) coil array tailored for skin MRI at 7.0 T (f = 78.5 MHz) is proposed. Specific absorption rate (SAR) simulations and a thorough assessment of RF power deposition were performed to meet the safety requirements. Human skin was examined in an in vivo feasibility study using two‐dimensional gradient echo imaging. Normal male adult volunteers (n = 17; mean ± standard deviation, 46 ± 18 years; range, 20–79 years) were investigated. Transverse slices of the calf were imaged with ²³Na MRI using a high in‐plane resolution of 0.9 × 0.9 mm². Skin Na⁺ content was determined using external agarose standards covering a physiological range of Na⁺ concentrations. To assess the intra‐subject reproducibility, each volunteer was examined three to five times with each session including a 5‐min walk and repositioning/preparation of the subject. The age dependence of skin Na⁺ content was investigated. The ²³Na RF coil provides improved sensitivity within a range of 1 cm from its surface versus a volume RF coil which facilitates high in‐plane spatial resolution imaging of human skin. Intra‐subject variability of human skin Na⁺ content in the volunteer population was <10.3%. An age‐dependent increase in skin Na⁺ content was observed (r = 0.78). The assignment of Na⁺ stores with ²³Na MRI techniques could be improved at 7.0 T compared with current 3.0 T technology. The benefits of such improvements may have the potential to aid basic research and clinical applications designed to unlock questions regarding the Na⁺ balance and Na⁺ storage function of skin. Copyright © 2014 John Wiley & Sons, Ltd.     

Sodium MRI of the human heart at 7.0 T: preliminary results

The objective of this work was to examine the feasibility of three‐dimensional (3D) and whole heart coverage ²³Na cardiac MRI at 7.0 T including single‐cardiac‐phase and cinematic (cine) regimes. A four‐channel transceiver RF coil array tailored for ²³Na MRI of the heart at 7.0 T (f = 78.5 MHz) is proposed. An integrated bow‐tie antenna building block is used for ¹H MR to support shimming, localization and planning in a clinical workflow. Signal absorption rate simulations and assessment of RF power deposition were performed to meet the RF safety requirements. ²³Na cardiac MR was conducted in an in vivo feasibility study. 3D gradient echo (GRE) imaging in conjunction with Cartesian phase encoding (total acquisition time T_AQ = 6 min 16 s) and whole heart coverage imaging employing a density‐adapted 3D radial acquisition technique (T_AQ = 18 min 20 s) were used. For 3D GRE‐based ²³Na MRI, acquisition of standard views of the heart using a nominal in‐plane resolution of (5.0 × 5.0) mm² and a slice thickness of 15 mm were feasible. For whole heart coverage 3D density‐adapted radial ²³Na acquisitions a nominal isotropic spatial resolution of 6 mm was accomplished. This improvement versus 3D conventional GRE acquisitions reduced partial volume effects along the slice direction and enabled retrospective image reconstruction of standard or arbitrary views of the heart. Sodium cine imaging capabilities were achieved with the proposed RF coil configuration in conjunction with 3D radial acquisitions and cardiac gating. Cardiac‐gated reconstruction provided an enhancement in blood–myocardium contrast of 20% versus the same data reconstructed without cardiac gating. The proposed transceiver array enables ²³Na MR of the human heart at 7.0 T within clinical acceptable scan times. This capability is in positive alignment with the needs of explorations that are designed to examine the potential of ²³Na MRI for the assessment of cardiovascular and metabolic diseases. Copyright © 2015 John Wiley & Sons, Ltd.