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[Display omitted] • Cost-Effective Enhancement: Proposes a simple and low-cost wireless resonator to improve TMJ MRI. • Innovative Coil Design: Utilizes a local receive array as the primary coil for enhanced imaging quality. • Distortion-Free Imaging: Introduces a receive-only wireless coil, ensuring no transmit field distortion. • Clinical Adaptability: Facilitates seamless integration into clinical applications without the need for coil parameter modifications. • Parallel Imaging capability: Preserves parallel imaging capability in the primary receive array. MRI is essential for evaluating and diagnosing various conditions affecting the temporomandibular joint (TMJ) and surrounding structures, as it provides highly detailed images that enable healthcare professionals to assess the joints and surroundings in great detail. While commercial MRI scanners typically come equipped with basic receive coils, such as the head receive array, RF coils tailored for specialized applications like TMJ MRI must be obtained separately. Consequently, TMJ MRI scans are often conducted using the head receive array, yet this configuration proves suboptimal due to the lack of specialized coils. In this study, we introduce a simple, low-cost, and easy-to-reproduce wireless resonator insert to enhance the quality of TMJ MRI at 1.5 T. The wireless resonator shows a significant improvement in signal-to-noise ratio (SNR) and noticeably better imaging quality than the head array alone configuration in both phantom and in vivo images. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • High spatial resolution MRI can be performed using RF phase gradients only. • A solenoid can be twisted to generate a novel RF phase gradient coil. • The twisted solenoid retains the geometric and efficiency advantages of solenoids. • Power balancing between geometrically decoupled array elements is not necessary. Abstract TRASE is an MRI k -space encoding method that uses radio-frequency (RF or B 1 ) transmit phase gradient fields to achieve millimeter-level spatial resolution. Image quality is critically dependent upon the efficient generation of B 1 fields with uniform magnitude and strong phase gradients. We present the design of a new family of phase gradient transmit coil based upon a solenoid twisted about a transverse axis. This design has many attractive geometric, electrical and magnetic characteristics, including the capability to spatially encode in the direction of the main static B 0 field without obstructing access to the bore. Analytical, numerical simulation and experimental results are presented, including demonstration of 1-dimensional TRASE encoding without the use of PIN diode switches. Twisted solenoid coils significantly expand the capabilities of TRASE MRI. [ABSTRACT FROM AUTHOR]

[Display omitted] • A passive LC resonant coil is added to the surface of the RF coil to improve the signal-to-noise ratio of the Unilateral Magnetic Resonance sensor. • This work introduces a method for quickly calculating the AC resistance of an RF coil. • According to experiments, the LC resonant coil boosts the UMR's signal-to-noise ratio by three times. Single-sided or unilateral magnetic resonance (UMR) technology has various benefits, such as an open structure, low cost, portability, and nondestructive measurement, in contrast to the conventional closed magnet structure. UMR is widely used in material analysis, well logging, and biomedicine. However, its development is constrained by its poor signal-to-noise ratio (SNR). To enhance the SNR of UMR sensor, a surface coil of LC resonator is added on the Radio Frequency (RF) coil. First, a method of calculating the current in the RF coil including LC resonator is derived. Next, the equivalent AC resistance of the coil is calculated using the partial-element equivalent-circuit (PEEC) method. Finally, the SNR of a UMR sensor incorporating LC resonator is analyzed, and its sensitivity map is provided. Experimental comparisons are made between the UMR sensor with and without a LC resonator. Results show that the SNR of the UMR can be enhanced by up to three times after the LC resonator is loaded. The SNR improves within 30 mm of the coil surface, and this beneficial effect steadily diminishes as the distance increases. This study offers a useful method for improving the signal of UMR sensors. [ABSTRACT FROM AUTHOR]

[Display omitted] • The novel Balun is miniature and flexible. • It exhibits excellent rejection ability for common-mode current. • It has negligible insertion loss for the differential-mode signals. • It demonstrates robust performance. • Validation was conducted through EM simulation, bench tests, and MRI experiments. Flexible coils offer improved patient comfort and better imaging quality. However, rigid and bulky baluns in RF coils limit flexibility and manufacturing. A miniaturized and flexible balun design was proposed to address this issue. It replaced rigid components with an ultra-flexible rubber tube and a flexible coaxial capacitor. Simulations validated the concept, and bench tests confirmed its performance, including a measured common-mode rejection ratio of −15.8 dB. The flexible balun was integrated into a 4-channel coil array, evaluating impedance changes caused by the "hand effect." Compared to coils without the balun, the flexible coil with the proposed balun showed improved robustness in impedance matching and inter-element couplings. Transmit efficiency of the flexible coil with the balun was compared to coils without a balun and with a rigid, shielded cable trap. Results demonstrated that the proposed balun circuit maintained high transmit efficiency. Overall, the flexible balun design offers a promising solution for improving the flexibility and performance of RF coil arrays in MRI applications. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • A flexible transceiver array based on TLRs is fabricated for cardiac MRI at 7 T. • Array elements are decoupled using a decoupling-ring based technique. • Acceleration factors up to 3 in bent configuration are demonstrated. • The fabricated array is compatible with parallel transmission techniques. • The array enables geometrical conformity to bodies within a large range of size. Abstract A flexible transceiver array based on transmission line resonators (TLRs) combining the advantages of coil arrays with the possibility of form-fitting targeting cardiac MRI at 7 T is presented. The design contains 12 elements which are fabricated on a flexible substrate with rigid PCBs attached on the center of each element to place the interface components, i.e. transmit/receive (T/R) switch, power splitter, pre-amplifier and capacitive tuning/matching circuitry. The mutual coupling between elements is cancelled using a decoupling ring-based technique. The performance of the developed array is evaluated by 3D electromagnetic simulations, bench tests, and MR measurements using phantoms. Efficient inter-element decoupling is demonstrated in flat configuration on a box-shaped phantom (S ij < −19 dB), and bent on a human torso phantom (S ij < −16 dB). Acceleration factors up to 3 can be employed in bent configuration with reasonable g-factors (<1.7) in an ROI at the position of the heart. The array enables geometrical conformity to bodies within a large range of size and shape and is compatible with parallel imaging and parallel transmission techniques. [ABSTRACT FROM AUTHOR]

• A tight-fitting helmet-shape receive array with a single transmit loop for anesthetized macaque brain MRI at 7T was fabricated with four openings constructed to accommodate multimodal neuroimaging and neurostimulation devices. • The coil provides satisfactory transmit efficiency / homogeneity, signal coverage and SNR over the macaque brain. • Promising results obtained by combining fMRI with infrared neural stimulation, focused ultrasound stimulation, or transcranial direct current stimulation. • This pilot study reveals the opportunity for brain investigation at multiple spatiotemporal scales, which may advance our understanding in dynamic brain networks. Blood-oxygen-level-dependent functional MRI allows to investigte neural activities and connectivity. While the non-human primate plays an essential role in neuroscience research, multimodal methods combining functional MRI with other neuroimaging and neuromodulation enable us to understand the brain network at multiple scales. In this study, a tight-fitting helmet-shape receive array with a single transmit loop for anesthetized macaque brain MRI at 7T was fabricated with four openings constructed in the coil housing to accommodate multimodal devices, and the coil performance was quantitatively evaluated and compared to a commercial knee coil. In addition, experiments over three macaques with infrared neural stimulation (INS), focused ultrasound stimulation (FUS), and transcranial direct current stimulation (tDCS) were conducted. The RF coil showed higher transmit efficiency, comparable homogeneity, improved SNR and enlarged signal coverage over the macaque brain. Infrared neural stimulation was applied to the amygdala in deep brain region, and activations in stimulation sites and connected sites were detected, with the connectivity consistent with anatomical information. Focused ultrasound stimulation was applied to the left visual cortex, and activations were acquired along the ultrasound traveling path, with all time course curves consistent with pre-designed paradigms. The existence of transcranial direct current stimulation electrodes brought no interference to the RF system, as evidenced through high-resolution MPRAGE structure images. This pilot study reveals the feasibility for brain investigation at multiple spatiotemporal scales, which may advance our understanding in dynamic brain networks. [ABSTRACT FROM AUTHOR]

[Display omitted] • In contrast to linearly polarized RF coil arrays, quadrature transceiver coil arrays are capable of improving signal-to-noise ratio (SNR), spatial resolution and parallel imaging performance. • Electrodynamic interaction between resonant elements of an array is a major source of degradation in the B1 field strength and the parallel imaging performance. • Magnetic wall decoupling technique or induced current elimination (ICE) is a non-overlapping decoupling method. • Double cross magnetic wall (i.e. Induced Current Elimination or ICE) decoupling technique is proposed to reduce undesired field coupling for multichannel quadrature RF coil arrays at 7 Tesla. In contrast to linearly polarized RF coil arrays, quadrature transceiver coil arrays are capable of improving signal-to-noise ratio (SNR), spatial resolution, and parallel imaging performance. Owing to a reduced excitation power, a low specific absorption rate can also be obtained using quadrature RF coils. However, due to the complex nature of their structure and their electromagnetic properties, it is challenging to achieve sufficient electromagnetic decoupling while designing multichannel quadrature RF coil arrays, particularly in ultra-high fields. In this work, we proposed a double-cross magnetic wall decoupling for quadrature transceiver RF arrays and implemented the decoupling method on common-mode differential mode quadrature (CMDM) quadrature transceiver arrays at an ultrahigh field of 7 T. The proposed magnetic decoupling wall, comprised of two intrinsically decoupled loops, is used to reduce the mutual coupling between all the multi-mode currents present in the quadrature CMDM array. The decoupling network has no physical connection with the CMDMs' resonators, which provides less design constraint over size-adjustable RF arrays. To validate the feasibility of the proposed cross-magnetic decoupling wall, systematic studies on the decoupling performance based on the impedance of two intrinsic loops are numerically performed. A pair of quadrature transceiver CMDMs is constructed along with the proposed decoupling network, and their scattering matrix is characterized using a network analyzer. The measured results indicate that all the current modes from coupling are simultaneously suppressed using the proposed cross-magnetic wall. Moreover, field distribution and local specific absorption rate (SAR) are numerically obtained for a well-decoupled 8-channel quadrature knee-coil array. [ABSTRACT FROM AUTHOR]

The common marmoset ( Callithrix jacchus ) is an increasingly popular animal model for translational neuroscience studies, during which anatomical and functional MRI can be useful investigative tools. To attain the requisite SNR for high-resolution acquisitions, the radiofrequency coil must be optimized for the marmoset; however, relatively few custom coils have been developed that maximize SNR and are compatible with accelerated acquisitions. For the study of large populations of animals, the heterogeneity in animal size reduces the effectiveness of a “one size fits all” approach to coil sizing and makes coils tailored to individual animals cost and time prohibitive. The approach taken in this study was to create an 8-channel phased-array receive coil that was adjustable to the width of the marmoset head, thereby negating the need for tailored coils while still maintaining high SNR. Two marmosets of different size were imaged on a 9.4-T small-animal scanner. Consistent SNR was achieved in the periphery of the brain between head sizes. When compared to a 15-channel, “one size fits all” receive coil, the adjustable coil achieved 57% higher SNR in the superior frontal and parietal cortices and 29% higher SNR in the centre of the brain. The mean geometry factor of the adjustable coil was less than 1.2 for a 2-fold reduction factor in the left-right and anterior-posterior directions. Geometry factors were compared to the 15-channel coil to guide future designs. The adjustable coil was shown to be a practical means for anatomical and echo-planar imaging of marmoset cohorts. [ABSTRACT FROM AUTHOR]

MR velocimetry of liquid flow in opaque porous filters may play an important role in better understanding the mechanisms of deep bed filtration. With this knowledge, the efficiency of separating the suspended solid particles from the vertically flowing liquid can be improved, and thus a wide range of industrial applications such as wastewater treatment and desalination can be optimized. However, MR velocimetry is challenging for such studies due to the low velocities, the severe B 0 inhomogeneity in porous structures, and the demand for high spatial resolution and an appropriate total measurement time during which the particle deposition will change velocities only marginally. In this work, a modified RARE-based MR velocimetry method is proposed to address these issues for velocity mapping on a deep bed filtration cell. A dedicated RF coil with a high filling factor is constructed considering the limited space available for the vertical cell in a horizontal MR magnet. Several means are applied to optimize the phase contrast RARE MRI pulse sequence for accurately measuring the phase contrast in a long echo train, even in the case of a low B 1 homogeneity. Two means are of particular importance. One uses data acquired with zero flow to correct the phase contrast offsets from gradient imperfections, and the other combines the phase contrast from signals of both odd and even echoes. Results obtained on a 7T preclinical MR scanner indicate that the low velocities in the heterogeneous system can be correctly quantified with high spatial resolution and an adequate total measurement time, enabling future studies on flow during the filtration process. [ABSTRACT FROM AUTHOR]

[Display omitted] • Compact solenoidal TRASE RF phase gradient coils with 80% diameter imaging volume. • Artifact-free 1D TRASE profiles under inhomogeneous B 0 field by 2DFT. • TRASE in 3 MHz regime with light-weight Halbach array. Transmit array spatial encoding (TRASE) is an MR imaging technique that achieves k-space encoding through the use of phase gradients in the RF transmit field. Without requiring B 0 gradient fields, TRASE MRI can be performed using significantly cheaper bi-planar permanent magnets or Halbach arrays. For TRASE encoding with these magnets, the twisted solenoid has been demonstrated as the most efficient RF transmit coil; however, this specific geometry results in a long coil with a relatively short imaging volume. We introduce a new truncated design to increase the usable imaging volume relative to the coil length. Based on simulations of optimal parameters, a 200 mm long, 100 mm inner diameter coil pair was constructed with an imaging volume 100 mm in length and 80 mm in diameter. The coil pair was tested using an un-shimmed 2.84 MHz Halbach array. Results indicate the truncated design can create a similar imaging volume and quality to the untruncated version whilst significantly reducing the length of the coil by as much as a half. [ABSTRACT FROM AUTHOR]

Non-proton MRI has recently garnered gathering interest with the increased availability of ultra high-field MRI system. Assuming the availability of a broadband RF amplifier, performing multinuclear MR experiments essentially requires additional hardware, such as an RF resonator and a T/R switch for each nucleus. A double- or triple-resonant RF probe is typically constructed using traps or PIN-diode circuits, but this approach degrades the signal-to-noise ratio (SNR) and image quality compared to a single-resonant coil and this is a limiting factor. In this work, we have designed the required hardware for multinuclear MR imaging experiments employing six single-resonant coil sets and a purpose-built animal bed; these have been implemented into a home-integrated 9.4 T preclinical MRI scanner. System capabilities are demonstrated by distinguishing concentration differences and sensitivity of X-nuclei imaging and spectroscopy without SNR penalty for any nuclei, no subject interruption and no degradation of the static shim conditions. [ABSTRACT FROM AUTHOR]

A novel design scheme for monolithic transmission line resonators (TLRs) is presented - the multi-turn multi-gap TLR (MTMG-TLR) design. The MTMG-TLR design enables the construction of TLRs with multiple turns and multiple gaps. This presents an additional degree of freedom in tuning self-resonant TLRs, as their resonance frequency is fully determined by the coil geometry (e.g. diameter, number of turns, conductor width, etc.). The novel design is evaluated at 4.7 T and 7 T by simulations and experiments, where it is demonstrated that MTMG-TLRs can be used for MRI, and that the B 1 distribution of MTMG-TLRs strongly depends on the number and distribution of turns. A comparison to conventional loop coils revealed that the B 1 performance of MTMG-TLRs is comparable to a loop coil with the same mean diameter; however, lower 10g SAR values were found for MTMG-TLRs. The MTMG-TLR design is expected to bring most benefits at high static field, where it allows for independent size and frequency selection, which cannot be achieved with standard TLR design. However, it also enables more accurate geometric optimization at low static field. Thereby, the MTMG-TLR design preserves the intrinsic advantages of TLRs, i.e. mechanical flexibility, high SAR efficiency, mass production, and coil miniaturization. [ABSTRACT FROM AUTHOR]

Developing a radiofrequency (RF) coil system that produces a uniform B 1 + field (circularly polarized component of the transverse magnetic field responsible for excitation) and low specific absorption rate (SAR) is critical for high performance ultrahigh field human imaging. In this study, we provide the design of a new eight channel radiofrequency (RF) transmit (Tx) array for breast MRI at 7 T. A numerical analysis utilizing an in-house finite difference time domain (FDTD) package was carried out in (1) four breast models, (2) homogeneous spherical model and (3) full body model to calculate the B 1 + intensity (μT) and homogeneity represented by coefficient of variation (CoV = standard deviation/mean) in the proposed RF array design. The numerical results were compared with that measured in breast phantom (Bphantom) and homogeneous spherical phantom at 7 T MRI and showed very good agreement. Average and peak SARs were also calculated in the four breast models and the temperature rises due to the operation of the RF array were also measured in the Bphantom. The proposed RF array; which can operate in a single or multi transmit modes, demonstrates homogeneous RF field excitation with acceptable local/average SAR levels for breast MRI at 7 T. [ABSTRACT FROM AUTHOR]

The purpose of this work was to develop a novel integrated radiofrequency and shim (RF/shim) coil array that can perform parallel reception and localized B 0 shimming in the human brain with the same coils, thereby maximizing both the signal-to-noise ratio and shimming efficiency. A 32-channel receive-only head coil array was modified to enable both RF currents (for signal reception) and direct currents (for B 0 shimming) to flow in individual coil elements. Its in vivo performance was assessed in the frontal brain region, which is affected by large susceptibility-induced B 0 inhomogeneities. The coil modifications did not reduce their quality factor or signal-to-noise ratio. Axial B 0 maps and echo-planar images acquired in vivo with direct currents optimized to shim specific slices showed substantially reduced B 0 inhomogeneities and image distortions in the frontal brain region. The B 0 root-mean-square error in the anterior half of the brain was reduced by 60.3% as compared to that obtained with second-order spherical harmonic shimming. These results demonstrate that the integrated RF/shim coil array can perform parallel reception and localized B 0 shimming in the human brain and provide a much more effective shimming than conventional spherical harmonic shimming alone, without taking up additional space in the magnet bore and without compromising the signal-to-noise ratio or shimming performance. [ABSTRACT FROM AUTHOR]

Abstract: Originally proposed as a method to increase sensitivity by extending the locally high-sensitivity of small surface coil elements to larger areas via reception, the term parallel imaging now includes the use of array coils to perform image encoding. This methodology has impacted clinical imaging to the point where many examinations are performed with an array comprising multiple smaller surface coil elements as the detector of the MR signal. This article reviews the theoretical and experimental basis for the trend towards higher channel counts relying on insights gained from modeling and experimental studies as well as the theoretical analysis of the so-called “ultimate” SNR and g-factor. We also review the methods for optimally combining array data and changes in RF methodology needed to construct massively parallel MRI detector arrays and show some examples of state-of-the-art for highly accelerated imaging with the resulting highly parallel arrays. [Copyright &y& Elsevier]

Abstract: A Helmholtz-pair local transmit RF coil with an integrated four-element receive array RF coil and foot immobilization platform was designed and constructed for imaging the distal tibia in a whole-body 7T MRI scanner. Simulations and measurements of the B 1 field distribution of the transmit coil are described, along with SAR considerations for operation at 7T. Results of imaging the trabecular bone of three volunteers at 1.5T, 3T and 7T are presented, using identical 1.5T and 3T versions of the 7T four-element receive array. The spatially registered images reveal improved visibility for individual trabeculae and show average gains in SNR of 2.8× and 4.9× for imaging at 7T compared to 3T and 1.5T, respectively. The results thus display an approximately linear dependence of SNR with field strength and enable the practical utility of 7T scanners for micro-MRI of trabecular bone. [Copyright &y& Elsevier]

Abstract: This paper reports on design and construction of a double coil high-homogeneity ensuring Nuclear Magnetic Resonance Probe for off-resonance relaxation time measurements. NMR off-resonance experiments pose unique technical problems. Long irradiation can overheat the sample, dephase the spins because of B 1 field inhomogeneity and degrade the signal received by requiring the receiver bandwidth to be broader than that needed for normal experiment. The probe proposed solves these problems by introducing a separate off-resonance irradiation coil which is larger than the receiver coil and is wound up on the dewar tube that separates it from the receiver coil thus also thermally protects the sample from overheating. Large size of the irradiation coil also improves the field homogeneity because as a ratio of the sample diameter to the magnet (coil) diameter increases, the field inhomogeneity also increases (Blümich et al., 2008) . The small receiver coil offers maximization of the filling factor and a high signal to the noise ratio. [ABSTRACT FROM AUTHOR]

Abstract: Morphological and functional cardiac MRI can potentially benefit greatly from the recent advent of commercial high-field (7tesla and above) MRI systems. However, conventional hardware configurations at lower field using a body-coil for homogeneous transmission are not available at these field strengths. Sophisticated multiple-transmit-channel systems have been shown to be able to image the human heart at 7tesla but such systems are currently not widely available. In this paper, we empirically optimize the design of a simple quadrature coil for cardiac imaging at 7tesla. The size, geometry, and position have been chosen to produce a B1 field with no tissue-induced signal voids within the heart. Standard navigator echoes for gating were adapted for operation at the heart/lung interface, directly along the head–foot direction. Using this setup, conventional and high-resolution cine functional imaging have been successfully performed, as has morphological imaging of the right coronary artery. [Copyright &y& Elsevier]

Abstract: A method for enhancing the signal to noise ratio (SNR) in NMR volume coils is described. By introducing inserts made of low-loss, high dielectric constant material into specific locations in the coil, the SNR can often be enhanced by up to 20%, while homogeneity is hardly affected. A model for predicting the limit of the SNR improvement is also presented. The model accurately predicts the SNR gain obtained in both numerical simulations and experiment. An experiment was conducted on a mini-MRI system. Experimental results are in very good agreement with the simulations in regard to both SNR improvement and enhancement in transmission. Inserts made of ultra high dielectric constant materials can be as thin as few millimeters, thus, conveniently fitting into existing coil-sample gaps in volume coils. [Copyright &y& Elsevier]

[Display omitted] • A modified coil with a high homogeneous radio frequency field based on the loopgap coil was proposed. • Simulations and experiments demonstrated that the homogeneity of T coil was far better than classical coils such as solenoids. • The high homogeneity of the radio frequency field reduced the distortion of the relaxation signals from large sample with multiple components greatly. • In-vivo experiments demonstrated the effectiveness of the proposed method applied to human tissues. Noninvasive NMR measurement of human tissues, such as fingers, to achieve early detection for metabolic diseases is of important significance. The NMR relaxation measurements have a wide application prospect due to simplicity, portability, and low cost, as the static magnetic field is not required to be highly homogeneous. However, the inhomogeneous radiofrequency (RF) magnetic field (B 1) results in errors in the magnetic resonance relaxation times. This is inevitable in in-vivo localized human tissue measurements with a portable MR scanner, as signals from tissues close to the edge of RF coil are excited with a different B 1 field amplitude. A novel RF coil termed T coil with high B 1 field homogeneity is presented. Numerical simulation and phantom measurements were implemented. The novel RF coil was compared with a regular solenoid coil and a variable width coil. In-vivo experiments were performed. The T coil has a better B 1 field homogeneity than the regular solenoid coil and the variable width coil, producing more accurate magnetic resonance relaxation times. Improved detection accuracy has been achieved with the T coil. This work may promote the development of noninvasive human tissue diagnosis based on NMR relaxation methods. [ABSTRACT FROM AUTHOR]

Abstract: The presence and diagnosis of radiation damping could have major implications in NMR experiments with hyperpolarised gases, where accurate knowledge of the flip angle is imperative. In this work radiation damping was observed and investigated in a low-pass birdcage resonator (Q =250) with samples of hyperpolarised 3He at 1.5T. With an initially highly polarised (P =38%) sample of 3He in a spherical cell, the observed FID had a distorted line shape with a spectral line width that was three times that of the same sample in a virtually depolarised state (1Hz line width for P <1%). Moreover a linear relation between the sample’s magnetisation (M 0) and the line width of the spectrum was observed which is indicative of radiation damping. With highly polarised samples, significant radiation damping was observed and the effect was a lower than expected rate of depletion of M 0 in RF flip angle calibration experiments, which led to significant underestimate of the RF flip angle. To our knowledge this is the first report of radiation damping in a birdcage resonator with samples hyperpolarised or otherwise. Experimental observation of radiation damping could be used as means of measuring coil efficiency as an alternative to the geometrical filling factor (η) the definition of which is open to question for a birdcage resonator. Estimates of the birdcage filling factor from the measured damping time constants (η RD =0.4%) are compared to those derived from electromagnetic energy ratios (η E =1.6%) and metallic sphere frequency shift methods (η fs =1.4%). These figures are much lower than the simple volume geometrical upper limit of η v =3.7% derived from the ratio of cell volume to total coil volume (shield included). The physical explanation for this shortfall is that the bulk of the magnetic energy stored in the birdcage is spatially distributed predominantly between the rungs and the shield, and not in the coil centre where the sample is placed and where the field has its highest spatial homogeneity. [Copyright &y& Elsevier]

Abstract: A new portable NMR sensor with a novel one-sided access magnet design, termed NMR-MOLE (MObile Lateral Explorer), has been characterised in terms of sensitivity and depth penetration. The magnet has been designed to be portable and create a volume with a relatively homogeneous magnetic field, 15,000ppm over a region from 4 to 16mm away from the probe, with maximum sensitivity at a depth of 10mm. The proton NMR frequency is 3.3MHz. We have demonstrated that with this approach a highly sensitive, portable, unilateral NMR sensor can be built. Such a design is especially suited for the characterisation of liquids in situations where unilateral or portable access is required. [Copyright &y& Elsevier]

Abstract: By independent control of the phases and amplitudes of its elements, the microstrip transmission-line array can mitigate sample-induced RF non-uniformities, and has been widely used as the transceiver in parallel imaging applications. One major challenge in implementing the microstrip array is the reduction of mutual coupling among individual elements. The low-input impedance preamplifier is commonly used for the decoupling purpose. However, it is impractical in the transceiver array design. Although interconnecting capacitors can be utilized to reduce the mutual coupling, they only efficiently work for the neighbor elements. In addition, this approach is impractical at fields higher than 300MHz, in which the required decoupling capacitance is commonly less than 0.5pF. We propose a novel decoupling approach by using decoupling inductors in this study. Due to the fact that the decoupling inductance is independent of the resonant frequency, the microstrip arrays can be well decoupled at ultra-high fields. To verify the proposed approach, an eight-channel microstrip array is fabricated and tested at 9.4T. For this prototype, couplings between elements are significantly reduced by using the interconnecting inductors. The phantom experiment shows that the inductively decoupled microstrip array has good parallel imaging performance. [Copyright &y& Elsevier]

Abstract: We present an open volume, high isolation, RF system suitable for pulsed NMR and EPR spectrometers with reduced dead time. It comprises a set of three RF surface coils disposed with mutually parallel RF fields and a double-channel receiver (RX). Theoretical and experimental results obtained with a prototype operating at about 100MHz are reported. Each surface RF coil (diameter 5.5cm) was tuned to f0=100.00±0.01MHz when isolated. Because of the mutual coupling and the geometry of the RF coils, only two resonances at f1=97.94MHz and f2=101.85MHz were observed. We show they are associated with two different RF field spatial distributions. In continuous mode (CW) operation the isolation between the TX coil and one of the RX coils (single-channel) was about −10dB. By setting the double-channel RF assembly in subtraction mode the isolation values at f1 or f2 could be optimised to about −75dB. Following a TX RF pulse (5μs duration) an exponential decay with time constant of about 600ns was observed. The isolation with single-channel RX coil was about −11dB and it increased to about −47dB with the double-channel RX in subtraction mode. Similar results were obtained with the RF pulse frequency selected to f2 and also with shorter (500ns) RF pulses. The above geometrical parameters and operating frequency of the RF assembly were selected as a model for potential applications in solid state NMR and in free radical EPR spectroscopy and imaging. [Copyright &y& Elsevier]

A high-frequency RF volume coil based on the use of microstrip transmission line (MTL) has been developed for in vivo 1H MR applications on the human head at 4 T. This coil is characterized by major advantages: (i) completely distributed coil circuit, (ii) high-quality factor (Q), (iii) simple coil structure, and (iv) better sensitivity and less signal-intensity variation in the MR image of the human head compared with an RF shielded birdcage coil of similar coil size. The proposed MTL volume coil does not require additional RF shielding for preventing Q degradation from radiation losses due to the unique MTL structure; thus, it provides a maximal useable space inside the volume coil when compared with most volume coils available at high fields with the same overall coil size. The intrinsic B1 distribution of the MTL volume coil effectively compensates for the dielectric resonance effect at 4 T and improves the signal homogeneity in human head MR images in the transaxial planes. The results of this study demonstrate that the MTL volume coil design provides an efficient and simple solution to RF volume coil design for human MR studies at high fields. [Copyright &y& Elsevier]

Proper design of a birdcage coil plays a very important role in obtaining high-resolution small animal magnetic resonance imaging. The RF field homogeneity and the coil filling factor directly affect the signal-to-noise ratio (S/N) and therefore limit the resolution. It has been shown that a conductive end-cap placed on one side of the coil can improve the RF field inhomogeneity near this area. This also contributes to an increase in the S/N by reducing the length of the RF coil. While this is true near the end-cap, the distal half of the coil still suffers from poor homogeneity and S/N. Consequently, such a shortfall may hinder small animal whole body imaging. In order to improve the coil performance for a larger imaging volume, we designed a new small animal birdcage RF coil by adding a detachable second end-cap to the open end. The performance of single end and double end RF coils was compared experimentally. The results indicate that the double end-cap can provide superior uniformity along the long axis of the coil. Furthermore, if one wishes to obtain the same homogeneity within a given volume, a double end-cap would have less than half of the length of the single end-cap coil leading to a superior S/N performance. [Copyright &y& Elsevier]

[Display omitted] • Meander-line, or zig-zag, surface coils have poorer field uniformity than predicted. • Invalid approximations in existing coil theory revealed and new theory presented. • Simple new coil design gives order-of-magnitude field uniformity improvement. Meander-line, or zig-zag, MRI surface coils theoretically promise spatially uniform fields with optimal field localization close to the coil. In reality, they suffer poorer than expected field localizations and acquired images are often highly inhomogeneous, plagued by repeating stripe-like signal-loss artifacts. We show that both these detrimental effects arise from coil design based on the same invalid approximation in the underlying theory. Here, the conventional approximation is corrected, yielding a modified coil design that validates the new theory by rectifying the above problems. Specifically, an easily implementable coil correction, which amounts to the addition of a single extra turn of wire, is introduced and shown to increase signal uniformity by an order of magnitude, eliminate image artifacts, and reduce unwanted signal interference from deeper within the sample by tightening the coil field localization to close to the coil, as intended for zig-zag designs. With independent optimization of coil size and imaging depth possible, such corrected meander-lines surface coils may be well suited for large area, near-surface imaging and spectroscopy applications. [ABSTRACT FROM AUTHOR]

In vivo diffusion-weighted magnetic resonance imaging is limited in signal-to-noise-ratio (SNR) and acquisition time, which constrains spatial resolution to the macroscale regime. Ex vivo imaging, which allows for arbitrarily long scan times, is critical for exploring human brain structure in the mesoscale regime without loss of SNR. Standard head array coils designed for patients are sub-optimal for imaging ex vivo whole brain specimens. The goal of this work was to design and construct a 48-channel ex vivo whole brain array coil for high-resolution and high b -value diffusion-weighted imaging on a 3T Connectome scanner. The coil was validated with bench measurements and characterized by imaging metrics on an agar brain phantom and an ex vivo human brain sample. The two-segment coil former was constructed for a close fit to a whole human brain, with small receive elements distributed over the entire brain. Imaging tests including SNR and G-factor maps were compared to a 64-channel head coil designed for in vivo use. There was a 2.9-fold increase in SNR in the peripheral cortex and a 1.3-fold gain in the center when compared to the 64-channel head coil. The 48-channel ex vivo whole brain coil also decreases noise amplification in highly parallel imaging, allowing acceleration factors of approximately one unit higher for a given noise amplification level. The acquired diffusion-weighted images in a whole ex vivo brain specimen demonstrate the applicability and advantage of the developed coil for high-resolution and high b -value diffusion-weighted ex vivo brain MRI studies. [ABSTRACT FROM AUTHOR]

• A quality assurance (QA) protocol for 13C RF coils is proposed. • The protocol allows SNR comparisons between volume and surface coils. • Seven different 13C coils are evaluated across platforms and MR sites. • The results show the importance of careful QA for 13C coils at 3T. • All data and source code of the proposed protocol are made publicly available. A quality assurance protocol for RF coils is proposed, which can be used for volume (Tx/Rx) and surface (Rx) coils. Following this protocol, a benchmarking of seven coils (from three different MR sites) dedicated to 13C MRI at 3T is reported. Coil performance is particularly important for 3T MRI at the 13C frequency, since the coil-to-sample noise ratio is typically high. The coils are evaluated experimentally using the proposed protocol based on MR spectroscopic imaging performed with two different phantoms: one head-shaped, and one with cylindrical shape and nearly twice the volume of the first one. To achieve an unbiased SNR comparison of volume and array coils, coil combination was done using sensitivity profiles extracted from the data. SNR, noise correlation matrices and example g-factor maps are reported. For globally calibrated, equal excitation angles, the measured SNR shows large differences for the volume coils of up to 115% at the phantom center for a head phantom. The arrays show lower differences in superficial SNR. The sample surface depth at which the volume coils outperform the arrays is estimated to 7 cm, and SNR furthest away from the coil surface is 28% lower for the best array compared to the best volume coil. A broad set of coils for 13C at 3T have been benchmarked. The results reported, and the method used to benchmark them, should guide the 13C community to choose the most suitable coil for a given experiment. [ABSTRACT FROM AUTHOR]

Functional magnetic resonance imaging (fMRI) in monkeys is important for bridging the gap between invasive animal brain studies and non-invasive human brain studies. To resolve the finer functional structure of the monkey brain, ultra-high-field (UHF) MR is essential, and high-performance, close-fitting RF receive coils are typically desired to fully leverage the intrinsic gains provided by UHF MRI. Moreover, static field (B0) inhomogeneity arising from the tissue susceptibility interface is more severe at UHF, presenting an obstacle to achieving high-resolution fMRI. B0 shim of the monkey head is challenging due to its smaller size and more complex sources of B0 offsets in multi-modal imaging tasks. In the present work, we have customized an array coil for lightly-anesthetized monkey fMRI in the 7T human scanner that combines RF and multi-coil (MC) B0 shim functionality (also referred to as AC/DC coils) to provide high imaging SNR and high-spatial-order, rapidly switchable B0-shim capability. Additional space was retained on the coil to render it compatible with monkey multi-modal imaging studies. Both MC global (whole-volume) and dynamic (slice-optimized) shim methods were tested and evaluated, and the benefits of MC shim for fMRI experiments was also studied. A minor reduction in RF coil performance was found after introducing additional B0 shim circuitry. However, the proposed RF coil provided higher image SNR and more uniform contrast compared to a commercially available coil for human knee imaging. Compared with static 2nd-order shim, the B0 inhomogeneity was reduced by 56.8%, and 95-percentile B0 offset was reduced to within 28.2 Hz through MC shim, versus 68.7 Hz with 2nd-order static shim. As a result, functional image quality could be improved, and brain activation can be better detected using the proposed AC/DC monkey coil. • A monkey coil array in 7T human scanner was designed for multi-modal neuroimaging. • The coil provided high SNR and high-order, rapidly switchable B0-shim capability. • B0 inhomogeneity was reduced by 56.8% compared with scanner 2nd-order static shim. [ABSTRACT FROM AUTHOR]