Birdcage Coil Research Articles

A novel switched frequency 3He/1H high‐pass birdcage coil for imaging at 1.5 tesla

ABSTRACTThe ability to produce hyperpolarized noble gases 3He and 129Xe has opened up exciting possibilities for pulmonary magnetic resonance imaging (MRI). We have recently built a hyperpolarizer with the goal of using hyperpolarized 3He gas for MRI in neonatal lungs in a dedicated small foot‐print 1.5 T MR scanner developed at our institution and sited in our Neonatal Intensive Care Unit. Although hyperpolarized gas imaging can provide unique insights into lung ventilation, acinar microstructure, and gas‐exchange dynamics, there is an undiminished need for 1H MRI of the lung to provide anatomic references, B1 and B0 maps, and 1H images of lung parenchyma. To address this need, we designed, built and tested a novel radiofrequency body coil that provides a high‐pass birdcage coil that can be used for both 3He and 1H frequencies (48.65 and 63.86 MHz, respectively, at 1.5 T). To switch between frequencies, the birdcage coil has a large mechanical actuator that simultaneously changes the capacitance between every rung of the birdcage. Advantages of this coil design include: 1) quadrature excitation and reception at the 3He and 1H frequencies, 2) identical B1 field maps for 3He and 1H imaging, 3) excellent signal‐to‐noise ratio and B1 homogeneity at both frequencies, and 4) rapid (10–20 s) switching times between 3He and 1H operation. This report provides details of the coil's design and fabrication. Images of hyperpolarized 3He and 1H in phantoms and ex vivo rabbit lungs demonstrate the image quality obtained with the coil. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 45B: 174–182, 2015

Design of Crisscrossed Double-Layer Birdcage Coil for Improving B1+Field Homogeneity for Small-Animal Magnetic Resonance Imaging at 300 MHz

We design a crisscrossed double-layer birdcage (DLBC) coil by modifying the coil geometry of a standard single-layer BC (SLBC) coil to enhance the homogeneity of transmitting magnetic flux density (B₁+) along the main magnetic field (B 0 )-direction for small-animal magnetic resonance imaging (MRI) at 300 MHz. The performance assessment of the crisscrossed DLBC coil is conducted by computational analysis with the finitedifference time domain method (FDTD) and compared with SLBC coil in terms of the B₁ and the B₁+ distribution. As per the computational calculation studies, the mean value in the two-dimensional B₁+ map obtained at the mid-axial slice with the proposed DLBC coil is slightly lower than that obtained with the SLBC coil, but the B₁ + value of the DLBC coil in the outermost plane (40 mm away from the central plane) shows improvements of 19.3% and 24.8% over the SLBC coil B₁+ value when simulating a spherical phantom and realistic mouse body modeling. These simulation results indicate that, the B₁ + homogeneity along the z-direction was improved by using DLBC configuration. Our approach enables B₁ + homogeneity improvement along the zdirection, and it can also be applied to ultra-high field (UHF) MRI systems.

Design and simulation of a dual-tuned 1H/23Na birdcage coil for MRS studies in human calf

Sodium magnetic resonance is a very promising tool for achieving biochemical information on tissue viability, cell integrity and function in quantitative and noninvasive manner. Although it has already been applied in vivo in most human tissues, the low detectable sodium signal gives rise to technological limitations in terms of data quality when using clinical scanners. The design of dedicated coils capable of providing large field of view with high signal-to-noise ratio data is a requirement for quantifying tissutal sodium. This work describes design, simulation, construction and test of a dual-tuned 1H/23Na birdcage coil for magnetic resonance (MR) studies in human calf performed with a 3-T MR scanner. Coil simulation was performed using an electromagnetic solver based on finite-difference time-domain (FDTD) method, while the design included matching, tuning and trap circuits’ realization for 1H/23Na decoupling. Successively, a prototype of the coil was built and tested at workbench, for quality factors, Q ratio measurements and 1H/23Na channels’ decoupling evaluation. Finally, the coil was employed in a 3-Tesla scanner for acquiring MR data. The results are presented as signal profiles for both coil channels extracted from the phantom chemical shift image and with in vivo imaging performed on human calfs. The designed dual-tuned coil provided good decoupling between H and Na channels, by permitting to maximize the homogeneity of the magnetic field at both the frequencies of interest. Moreover, the simulations accuracy was demonstrated by good agreement between the theoretical and experimental coil signal profiles.

Investigation of Parallel Radiofrequency Transmission for the Reduction of Heating in Long Conductive Leads in 3 Tesla Magnetic Resonance Imaging.

Deep Brain Stimulation (DBS) is increasingly used to treat a variety of brain diseases by sending electrical impulses to deep brain nuclei through long, electrically conductive leads. Magnetic resonance imaging (MRI) of patients pre- and post-implantation is desirable to target and position the implant, to evaluate possible side-effects and to examine DBS patients who have other health conditions. Although MRI is the preferred modality for pre-operative planning, MRI post-implantation is limited due to the risk of high local power deposition, and therefore tissue heating, at the tip of the lead. The localized power deposition arises from currents induced in the leads caused by coupling with the radiofrequency (RF) transmission field during imaging. In the present work, parallel RF transmission (pTx) is used to tailor the RF electric field to suppress coupling effects. Electromagnetic simulations were performed for three pTx coil configurations with 2, 4, and 8-elements, respectively. Optimal input voltages to minimize coupling, while maintaining RF magnetic field homogeneity, were determined for all configurations using a Nelder-Mead optimization algorithm. Resulting electric and magnetic fields were compared to that of a 16-rung birdcage coil. Experimental validation was performed with a custom-built 4-element pTx coil. In simulation, 95-99% reduction of the electric field at the tip of the lead was observed between the various pTx coil configurations and the birdcage coil. Maximal reduction in E-field was obtained with the 8-element pTx coil. Magnetic field homogeneity was comparable to the birdcage coil for the 4- and 8-element pTx configurations. In experiment, a temperature increase of 2±0.15°C was observed at the tip of the wire using the birdcage coil, whereas negligible increase (0.2±0.15°C) was observed with the optimized pTx system. Although further research is required, these initial results suggest that the concept of optimizing pTx to reduce DBS heating effects holds considerable promise.

Transmit‐only/receive‐only radiofrequency coil configuration for hyperpolarized 129Xe MRI of rat lungs

AbstractWe report a novel radiofrequency (RF) transmit‐only/receive‐only (TO/RO) coil configuration providing excellent transmit B1+ field uniformity as well as high sensitivity for hyperpolarized 129Xe MR lung imaging of rats at 3T (35.34 MHz). The TO/RO coil configuration consisted of two separate components: (i) a high‐pass birdcage transmit coil which produces a homogeneous B1+ magnetic field, (ii) a saddle‐shaped single‐turn receive‐only surface coil that couples closely to the rat lung. On transmit, the receive‐only coil is decoupled from the transmit coil using a detuning circuit. On receive, the bird‐cage coil is deactivated through the use of PIN diodes. The sensitivity and uniformity of the saddle‐shaped receive coil were optimized solving the Biot‐Savart equation using 3D finite element modeling. The electrical performance of the new TO/RO configuration in transmit/receive (T/R) mode was compared with a commercial T/R birdcage coil of similar diameter, which was considering to be the gold standard for conventional T/R mode imaging. Experimental results in phantoms confirm that our novel TO/RO coil configuration provides a factor of three increase in SNR without compromising B1 transmit uniformity compared with the commercial T/R birdcage coil configuration. The novel TO/RO coil was successfully tested for in vivo rat lung imaging. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 45B: 115–124, 2015

Investigation of the B1 field distribution and RF power deposition in a birdcage coil as functions of the number of coil legs at 4.7 T, 7.0 T, and 11.7 T

The proper design of birdcage (BC) coils plays a very important role in the acquisition of highresolution magnetic resonance imaging (MRI) of small animals such as rodents. In this context, we investigate multiple-leg (8-, 16-, 32-, 64-, and 128-leg) BC coils operating at ultra-high fields (UHF) of 7.0 T and 11.7 T and a high-field (HF) of 4.7 T for rodent magnetic resonance imaging (MRI). Primarily, Our study comparatively examines the parameters of the radiofrequency (RF) transmission (|B1 +|)-field, the magnetic flux (|B1|)-field, and RF power deposition (RF-PD) as functions of the number of BC-coil legs via finite-difference time-domain (FDTD) calculations under realistic loading conditions with a biological phantom. In particular, the specific ratio |E/B1 +| is defined for predicting RF-PD values in different coil structures. Our results indicate that the optimal number of legs of the BC coil can be chosen for different resonance frequencies of 200 MHz, 300 MHz, and 500 MHz and that this choice can be lead to superior |B1 +|-field intensity and |B1|-field homogeneity and decreased RF-PD. We believe that our approach to determining the optimal number of legs for a BC coil can contribute to rodent MR imaging.

SU-D-303-06: Evaluations of Quantified Magnetic Moments From Different MRI Hardware

Purpose: Our quick study here is to validate whether the quantified magnetic moments of the same object are the same from different MRI machines and rf head coils. Methods: We constructed a gel phantom consisting of an empty straw with a diameter of 2.6-mm, a 3-mm diameter glass bead, and a 5-mm diameter glass bead. We imaged the phantom with a 4 echo gradient echo sequence on a 1.5-T Siemens Sonata and a 3-T Verio. On the 1.5-T, we used a quadrature single-channel birdcage coil and an 8-channel coil. On the 3-T, we used a single channel circular-polarized (CP) receiving coil and a 12-channel coil, which outputs only 4 channels. After removing the background phase with the SHARP method [1], we quantified magnetic moments of the three objects at each echo time for each coil, using the CISSCO method described in [2, 3]. Results: The quantified magnetic moments for each object are consistent among 4 echo times and different hardware. In addition, the quantified values agree with the expected values within uncertainties. Conclusion: For each of the three objects, we have demonstrated self consistencies of quantified magnetic moments between four different setups at four different echo times, using the CISSCO method. These results support that, with the appropriate post processing procedures and methods applied on images acquired from any MRI hardware or imaging parameters, we should obtain the same magnetic moment of the same object of interest. However, phase images improperly post processed may or may not lead to the correct quantified magnetic moment of the object. Post processing methods such as quantitative susceptibility mapping or combination of phase images from individual rf channels require further studies. Ref:[1] Schweser et. al, NeuroImage, 2011, pp. 2789–2807. [2] Cheng et. al, MRI, 2015, in press. [3] Cheng et. al, PMB, 2009, pp. 7025–7044. DOD/USAMRAA W81XWH-12-1-0522

Iterative multi-channel radio frequency pulse calibration with improving B1 field uniformity in high field MRI.

BackgroundIn high field MRI capable of multi-channel radio frequency (RF) transmission, B1 shimming is a time-consuming job because conventional B1 shimming techniques require B1 mapping for each channel. After acquiring the complex-numbered B1 field maps, the optimal amplitude and phase of the driving RF pulse are determined for each channel to maximize the B1 field uniformity in conventional B1 shimming. However, time-consuming B1 shimming procedures at the pre-scan may not be tolerated in the clinical imaging in which patient throughput is one of the important factors.MethodsTo avoid the time-consuming B1 mapping, the first spin echo and the stimulated echo were repeatedly acquired in the slice-selective stimulated echo sequence without imaging gradients. A cost function of the amplitudes and phases of the driving RF pulse for every channel was defined in a way that the ratio between the spin echo and stimulated echo amplitudes rapidly converged to √ 2. The amplitude and phase of the driving RF pulse were iteratively modified over the repeating RF pulse sequence so that the cost function was minimized.ResultsFrom the finite-difference-time-domain (FDTD) electromagnetic field simulations with a human body model placed in a birdcage coil operating at 3 T, it was observed that the RF pulse calibration with iterative cost function minimization can give improvement of B1 field uniformity as well as flip-angle calibration. The experiments at 3 T also showed improvement of RF field uniformity in the phantom imaging studies.ConclusionsSince the proposed RF pulse calibration is not based on B1 mapping, the RF pulse calibration time could be much shorter than the B1-mapping based methods. The proposed method is expected to be a practical substitute for the B1-mapping-based B1 shimming methods when long pre-scan time is not tolerable.

Numerical methods and software tools for simulation, design, and resonant mode analysis of radio frequency birdcage coils used in MRI

ABSTRACTDesign of magnetic resonance imaging (MRI) radiofrequency (RF) coils using lumped circuit modeling based techniques begins to fail at high frequencies, and therefore more accurate models based on the electromagnetic field calculations must be used. Field calculations are also necessary to understand the interactions between the RF field and the subject inside the coil. Furthermore, observing the resonance behavior of the coil and the fields at the resonance frequencies have importance for design and analysis. In this study, finite element method (FEM) based methods have been proposed for accurate time‐harmonic electromagnetic simulations, estimation of the tuning capacitors on the rungs or end rings, and the resonant mode analysis of the birdcage coils. Capacitance estimation was achieved by maximizing the magnitude of the port impedance at the desired frequency while simultaneously minimizing the variance of RF magnetic field in the region of interest. In order for the proposed methods to be conveniently applicable, two software tools, resonant mode and frequency domain analyzer (RM‐FDA) and Optimum Capacitance Finder (OptiCF), were developed. Simulation results for the validation and verification of the software tools are provided for different cases including human head simulations. Additionally, two handmade birdcage coils (low‐pass and high‐pass) were built and resonance mode measurements were made. Results of the software tools are compared with the measurement results as well as with the results of the lumped circuit modeling based method. It has been shown that the proposed software tools can be used for accurate simulation and design of birdcage coils. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 45B: 13–32, 2015

Cardiac Metabolism in a Pig Model of Ischemia–Reperfusion by Cardiac Magnetic Resonance with Hyperpolarized 13C-Pyruvate

BackgroundMagnetic resonance (MRI) with hyperpolarized 13C-pyruvate is a new technique for the assessment of myocardial metabolism. AimsThe aim of this study is to assess the effectiveness of MRI with hyperpolarized 13C-pyruvate to detect cardiac metabolic changes in a model of ischemia/reperfusion. MethodsA pneumatic occluder was placed around the left anterior descending artery in 7 pigs. A 3T scanner with a 13C quadrature birdcage coil was used. Hyperpolarized 13C-pyruvate was injected intravenously at rest, during coronary occlusion and 5min after reperfusion. Metabolic images were acquired using a 3D-IDEAL spiral CSI during the injection of 13C-pyruvate and 3D-parametric maps of 13C-pyruvate, 13C-lactate and 13C-bicarbonate were generated. Metabolic Activity Mismatch (MAM) was defined as the relative change between a) resting state and coronary occlusion or b) resting and reperfusion in all the myocardial segments. ResultsDuring occlusion, a decrease in 13C-lactate (−21±26% vs baseline 3±16%, P<0.0001) and 13C-bicarbonate (−29±34% vs 33±52%, P<0.0001) was found in myocardial segments at risk, as compared with remote segments. In ischemic segments, the 13C-lactate signal increased during reperfusion (20±42% vs −7±22%, P=0.0007), while 13C-bicarbonate was persistently reduced (−38±27% vs 36±51%, p<0.0001). Conclusions13C-pyruvate MRI is able to detect transient changes in regional metabolism in an in-vivo model of myocardial ischemia–reperfusion.

Magnetic resonance imaging of the inner ear by using a hybrid radiofrequency coil at 7 T

Visualization of the membranous structures of the inner ear has been limited to the detection of the normal fluid signal intensity within the bony labyrinth by using magnetic resonance imaging (MRI) equipped with a 1.5 Tesla (T) magnet. High-field (HF) MRI has been available for more than a decade, and numerous studies have documented its significant advantages over conventional MRI with regards to its use in basic scientific research and routine clinical assessments. No previous studies of the inner ear by using HF MRI have been reported, in part because high-quality resolution of mastoid pneumatization is challenging due to artifacts generated in the HF environment and insufficient performance of radiofrequency (RF) coils. Therefore, a hybrid RF coil with integrated circuitry was developed at 7 T and was targeted for anatomical imaging to achieve a high resolution image of the structure of the human inner ear, excluding the bony portion. The inner-ear’s structure is composed of soft tissues containing hydrogen ions and includes the membranous labyrinth, endolymphatic space, perilymphatic space, and cochlear-vestibular nerves. Visualization of the inner-ear’s anatomy was performed in-vivo with a custom-designed hybrid RF coil and a specific imaging protocol based on an interpolated breath-held examination sequence. The comparative signal intensity value at 30-mm away from the phantom side was 88% higher for the hybrid RF coil and 24% higher for the 8-channel transmit/receive (Tx/Rx) coil than for the commercial birdcage coil. The optimized MRI protocol employed a hybrid RF coil because it enabled high-resolution imaging of the inner-ear’s anatomy and accurate mapping of structures including the cochlea and the semicircular canals. These results indicate that 7 T MRI achieves high spatial resolution visualization of the inner-ear’s anatomy. Therefore, MRI imaging using a hybrid RF coil at 7 T could provide a powerful tool for clinical investigations of petrous pathologies of the inner ear.

Algebraic method to synthesize specified modal currents in ladder resonators: Application to noncircular birdcage coils.

Detectors such as birdcage coils often consist of networks of coupled resonant circuits that must produce specified magnetic field distributions. In many cases, such as quadrature asymmetric insert body coils, calculating the capacitance values required to achieve specified currents and frequencies simultaneously is a challenging task that previously had only approximate or computationally inefficient solutions. A general algebraic method was developed that is applicable to linear networks having planar representations such as birdcage coils, transverse electromagnetic (TEM) coils, and numerous variants of ladder networks. Unlike previous iterative or approximate methods, the algebraic method is computationally efficient and determines current distribution and resonant frequency using a single matrix inversion. The method was demonstrated by specifying irregular current distributions on a highly elliptical birdcage coil at 3 Tesla. Measurements of the modal frequency spectrum and transmit field distribution of the two specified modes agrees with the theory. Accuracy is limited in practice only by how accurately the matrix of self and mutual inductances of the network is known. The algebraic method overcomes the inability of the existing inductance equalization method to account for all elements of the inductance matrix and the inability to accommodate modal currents that are not (co)sinusoidal.

Cylindrical rf network antennas for coupled plasma sources Copper legs delayed in time system stability analysis

— In this article, Very Crucial subject discussed cylindrical (closed) RF network antennas for coupled plasma sources copper legs delayed in time system stability analysis. Resonant RF network antennas are important to plasma sources with many applications. The cylindrical resonant RF network antennas run as large volume plasma sources and have stability switching due to system's copper legs parasitic effects. The cylindrical RF network antenna structure is 16-leg cylindrical (Birdcage) RF antenna which has electrical circuit and opposite points of RF feeding and grounding. The vacuum vessel is a glass cylinder closed at the top and bottom by grounding metal plates. Generally there are two popular different resonant RF network assemblies: a cylindrical and a planar RF antenna. The cylindrical RF antenna is built as a high-pass Birdcage coil. The antenna is mounted outside a glass tube. The RF antenna consists of 16 copper legs equally spaced interconnected with capacitors. Due to RF antenna copper leg parasitic effect we get copper leg's current and current derivative with delay τ1-k and τ2-k (k is leg number index, k=1,…,16). The uncooled antenna is fed at the midpoint and operated with opposite grounded. Alternatively, it can be fed by another transmitter unit. Due to cylindrical antenna parasitic delayed in time, there is a stability issue by analyzing its operation. We consider for simplicity that all copper leg's current parasitic time delayed are equal (τ1-1= τ1-2=… =τ1-16) and current derivative parasitic time delayed are equal (τ2-1= τ2-2=… =τ2-16). The cylindrical RF network antennas delayed in time equivalent circuit can represent as a delayed differential equations which depend on variable parameters and delays. The investigation of our cylindrical network antenna with copper leg system, a differential equation is based on bifurcation theory [1], a study of possible changes in the structure of the orbits of a delayed differential equation depending on variable parameters. Cylindrical RF network antenna analysis is done under two series of different time delays respect to antenna's copper legs current and current derivative. All of that for optimization of a cylindrical RF network antenna circuit parameter analysis to get the best performance. The cylindrical network antenna with copper leg system can be represented as delayed differential equations which, depending on variable parameters and delays. There is a practical guideline that combines graphical information with analytical work to effectively study the local stability of models involving delay dependent parameters. The stability of a given steady state is determined by the graphs of some function of τi-1, ..,τi-16 (i=1, 2) [2] [3] [4].

Simulation and comparison of coils for Hyperpolarized 13C MRS cardiac metabolism studies in pigs

Hyperpolarized 13C Magnetic Resonance represents a promising modality for in vivo spectroscopy since it provides a unique opportunity for the non-invasive assessment of regional cardiac metabolism. Although it represents a powerful tool for the study of the heart physiology in pig models, by permitting metabolic activity mapping, a number of technological problems still limit this technology and need innovative solutions such as the design of suitable radiofrequency (RF) coils, capable to provide a large sensitivity region.This work describes the simulation and the comparison of different 13C coil configurations, constituted by various arrangement of circular, butterfly and birdcage coils designed for hyperpolarized studies of pig heart with a clinical 3T scanner. The coils characterization is performed by developing a Signal-to-Noise Ratio (SNR) model, previously validated with experimental results, for coils performance evaluation in terms of coil resistance, sample-induced resistance and magnetic field pattern.In particular, coil resistances were calculated from Ohm’s law, while magnetic field patterns and sample induced resistances were calculated using a numerical Finite-Difference Time-Domain (FDTD) algorithm. Theoretical SNR-vs-depth profiles were calculated for each coil configuration.We believe the paper could be interesting for graduate students and researchers in the field of magnetic resonance coil design and development, especially for 13C studies.

An improved RF and gradient coil system for high resolution in vivo guinea pig cochlea imaging on a 3T clinical magnet

ABSTRACTPurposeThe purpose of this work was to develop and evaluate a system consisting of an insertable gradient, RF coils, and animal positioning hardware to enable imaging of the internal structure of the cochlea and other inner ear structures of a guinea pig model at 3T.MethodsTransmit and receive RF coils, and an animal positioning system, were developed and integrated with an insertable gradient and animal monitoring system for improved guinea pig cochlea imaging when compared to a standard clinical 3T MRI system. A resolution phantom with 120 μm line separations was used to assess the contrast improvements with the insertable gradient. A homogeneous phantom was used to assess the SNR and homogeneity of the RF coils. FLASH imaging was used to observe the temporal passage of gadolinium contrast through the guinea pig cochlea.ResultsThe insertable gradient enabled increased resolution imaging with nearly twice the contrast‐to‐noise ratio. The small animal array had better SNR for imaging the guinea pig cochlea (about 14 mm deep) than the commercial Siemens wrist coil (2.2×) and the small birdcage coil (1.6×). The positioning device kept the animal secure and stationary. Injected gadolinium contrast allowed visualization of the internal cochlear structures with FLASH image acquisition, which achieved 100 μm isotropic resolution images in 33 min.ConclusionThe specialized RF coils and animal holding equipment designed to operate within the composite gradient enabled higher resolution images than could be obtained from the available RF coils operating in the conventional gradients on the clinical MRI system. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 44B: 89–101, 2015

Quadrature birdcage coil with distributed capacitors for 7.0 T magnetic resonance data acquisition of small animals

ABSTRACTHigh static magnetic field magnetic resonance imaging (MRI) is commonly used for preclinical studies in rodents. In this context, minimization of coil losses is mandatory to scan samples that are small compared to the radiofrequency wavelength in the medium. In this study we construct a radiofrequency (RF) birdcage probe with distributed capacitors, operating in quadrature, tailored for 7.0T 1H MRI of small animals. The design eliminates the need for extra electrical components on the probe structure and affords a high SNR, a uniform field (homogeneity of 93% in the axial plain of the phantom) and a coil sensitivity of 9.8 . Feasibility experiments of mouse imaging are conducted and the competitive capability of a 7.0 T human system equipped with the proposed coil is demonstrated in both body and brain preclinical imaging. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 44B: 83–88, 2015

A form-fitted three channel (31) P, two channel (1) H transceiver coil array for calf muscle studies at 7 T.

To enhance sensitivity and coverage for calf muscle studies, a novel, form-fitted, three-channel phosphorus-31 ((31) P), two-channel proton ((1) H) transceiver coil array for 7 T MR imaging and spectroscopy is presented. Electromagnetic simulations employing individually generated voxel models were performed to design a coil array for studying nonpathological muscle metabolism. Static phase combinations of the coil elements' transmit fields were optimized based on homogeneity and efficiency for several voxel models. The best-performing design was built and tested both on phantoms and in vivo. Simulations revealed that a shared conductor array for (31) P provides more robust interelement decoupling and better homogeneity than an overlap array in this configuration. A static B1 (+) shim setting that suited various calf anatomies was identified and implemented. Simulations showed that the (31) P array provides signal-to-noise ratio (SNR) benefits over a single loop and a birdcage coil of equal radius by factors of 3.2 and 2.6 in the gastrocnemius and by 2.5 and 2.0 in the soleus muscle. The performance of the coil in terms of B1 (+) and achievable SNR allows for spatially localized dynamic (31) P spectroscopy studies in the human calf. The associated higher specificity with respect to nonlocalized measurements permits distinguishing the functional responses of different muscles.

Birdcage coils: Equivalent capacitance and equivalent inductance

ABSTRACTBirdcage coil is extensively used in MR systems thanks to its possibility to provide high signal‐to‐ noise ratio and high radiofrequency magnetic field homogeneity that guarantee a large field of view. This work describes how to schematize the birdcage coil in terms of an equivalent inductance and an equivalent capacitance, whose knowledge can be useful for coil design and characterization. In particular, the knowledge of equivalent capacitance and equivalent inductance permits to estimate theoretically coil resonant frequency, quality factors and matching circuit capacitor values in a quick way, while workbench tests permit to estimate coil resistance and sample‐induced resistance. The presented theory is validated for both lowpass and highpass birdcage coils.by using literature data. © 2014 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 44B: 32–38, 2014

Design and numerical evaluation of a volume coil array for parallel MR imaging at ultrahigh fields.

In this work, we propose and investigate a volume coil array design method using different types of birdcage coils for MR imaging. Unlike the conventional radiofrequency (RF) coil arrays of which the array elements are surface coils, the proposed volume coil array consists of a set of independent volume coils including a conventional birdcage coil, a transverse birdcage coil, and a helix birdcage coil. The magnetic fluxes of these three birdcage coils are intrinsically cancelled, yielding a highly decoupled volume coil array. In contrast to conventional non-array type volume coils, the volume coil array would be beneficial in improving MR signal-to-noise ratio (SNR) and also gain the capability of implementing parallel imaging. The volume coil array is evaluated at the ultrahigh field of 7T using FDTD numerical simulations, and the g-factor map at different acceleration rates was also calculated to investigate its parallel imaging performance.

Design and Analysis of Microstrip-Based RF Birdcage Coil for 1.5 T Magnetic Resonance Imaging

Magnetic resonance imaging (MRI) technique is widely used to capture the images of the liquid items inside the human body. The radio-frequency (RF) coil is one of the important modules present inside an MRI system, which plays a major role in image quality. In this work, a microstrip-based high-pass RF birdcage coil is proposed for 1.5 T MRI. The cylindrical-shaped birdcage coil consists of 12 microstrip radiating elements and tuning capacitors to achieve a resonance at 63.85 MHz. The coil is made up of 10 mm polytetrafluoroethylene substrate coated by a conducting transmission line of desired length and width. A finite difference time domain simulation is carried out to analyze the return loss (S11), magnetic field homogeneity and Specific Absorption Rate (SAR) parameters of the RF coil. The SAR values of the proposed microstrip-based 1.5 T birdcage coil was compared with 3 T RF birdcage coil. The simulation results indicate the proposed birdcage coil structure gives optimal values of S11, magnetic field homogeneity and SAR.