Search Results (11)

Background: Low-field Magnetic Resonance Imaging (MRI) (fields below 0.5 T) has received increasing attention since the images produced have been shown to be diagnostically equivalent to high-field MR images for specific applications, such as musculoskeletal studies. In recent years, low-field MRI has made great strides in clinical relevance due to advances in high-performance gradients, magnet technology, and the development of organ-specific radiofrequency (RF) coils, as well as advances in acquisition sequence design. For achieving optimized image homogeneity and signal-to-noise Ratio (SNR), the design and simulation of dedicated RF coils is a constraint both in clinical and in many research studies. Methods: This paper describes the application of a numerical full-wave method based on the finite-difference time-domain (FDTD) algorithm for the simulation and the design of birdcage coils for musculoskeletal low-field MRI. In particular, the magnetic field pattern in loaded and unloaded conditions was investigated. Moreover, the magnetic field homogeneity variations and the coil detuning after an RF shield insertion were evaluated. Finally, the coil inductance and the sample-induced resistance were estimated. Results: The accuracy of the results was verified by data acquired from two lowpass birdcage prototypes designed for musculoskeletal experiments on a 0.18 T open MR clinical scanner. Conclusions: This work describes the capability of numerical simulations to design RF coils for various scenarios, including the presence of electromagnetic shields and different load conditions. Full article

This study presents the feasibility of a dual-mode high-pass birdcage RF coil to acquire MR images at both ¹H and ²³Na frequencies at ultra-high-field MR scanner, 7 T. A dual-mode circuit (DMC) in the dual-mode birdcage (DMBC) RF coil operates at two frequencies, addressing the limitations of sensitivity reduction and isolation between two frequencies as in traditional dual-tuned RF coil. Finite-difference time-domain (FDTD) based electromagnetic (EM) simulations were performed to verify the RF coil at each frequency on the three-dimensional human head model. The DMBC RF coil resonated at proton (¹H) and sodium (²³Na) frequencies, and also single-tuned high-pass birdcage RF coils were constructed for both ¹H and ²³Na frequencies. The bench test performance of the RF coils was evaluated using network analysis parameters, including the measurement of scattering parameters (S-parameters) and quality factors (Q-factors). Q-factor of the DMBC coil at ¹H port was 10.2% lower than that of ¹H single-tuned birdcage (STBC) coil, with a modest SNR reduction of 6.5%. Similarly, the Q-factor for the DMBC coil at ²³Na port was 12.3% less than that of ²³Na STBC coil, and the SNR showed a minimal reduction of 5.4%. Utilizing the DMBC coil, promising ¹H and ²³Na MR images were acquired compared to those by using STBC coils. In conclusion, deploying a DMBC ¹H/²³Na coil has been demonstrated to overcome traditional constraints associated with dual-tuned RF coils, achieving this with only nominal signal attenuation across both nuclei operational frequencies. Full article

The aim of the present study is the optimization, construction, and workbench validation of a double-tuned ¹H- ²³Na volume radio frequency (RF) coil suitable for human head imaging at 7 T, based on the birdcage geometry. The birdcage-like design which is considered is the four-ring model, in which two standard birdcage-like structures with the same diameters are nested along the longitudinal axis. Simulations based on Maxwell’s equations are performed to evaluate the RF magnetic field homogeneity and the RF coil efficiency varying the coil geometrical parameters. The RF magnetic field homogeneity is evaluated both on the transverse (z = 0) and longitudinal (y = 0) planes without performing the impedance matching procedure, so that the RF coil symmetry is not perturbed by the matching network. The RF coil efficiency is instead dependent on the effective coil input RF power, and it is evaluated after matching the coil, so that the reflected power is minimized, assuming that the stimulation power is totally delivered to the RF coil. Considering the simulation results and the target application, the useful RF coil geometrical parameters are fixed. The four-ring model, which showed the best performances, has been built and tested on a workbench, using a cylindrical phantom filled with a 0.05 M saline solution as load. This provides the first example of a four-ring realization intended ¹H- ²³Na for human head imaging at 7 T. Full article

In ultrahigh-field (UHF) magnetic resonance imaging (MRI) system, the RF power required to excite the nuclei of the target object increases. As the strength of the main magnetic field (${\mathrm{B}}_0$ field) increases, the improvement of the RF transmit field (${\mathrm{B}}_1^{+}$ field) efficiency and receive field (${\mathrm{B}}_1^{-}$ field) sensitivity of radio-frequency (RF) coils is essential to reduce their specific absorption rate and power deposition in UHF MRI. To address these problems, we previously proposed a method to simultaneously improve the ${\mathrm{B}}_1^{+}$ field efficiency and ${\mathrm{B}}_1^{-}$ field sensitivity of 16-leg bandpass birdcage RF coils (BP-BC RF coils) by combining a multichannel wireless RF element (MCWE) and segmented cylindrical high-permittivity material (scHPM) comprising 16 elements in 7.0 T MRI. In this work, we further improved the performance of transmit/receive RF coils. A new combination of RF coil with wireless element and HPM was proposed by comparing the BP-BC RF coil with the MCWE and the scHPM proposed in the previous study and the multichannel RF coils with a birdcage RF coil-type wireless element (BCWE) and the scHPM proposed in this study. The proposed 16-ch RF coils with the BCWE and scHPM provided excellent ${\mathrm{B}}_1^{+}$ field efficiency and ${\mathrm{B}}_1^{-}$ field sensitivity improvement. Full article

Improvements in transmission and reception sensitivities of radiofrequency (RF) coils used in ultra-high field (UHF) magnetic resonance imaging (MRI) are needed to reduce specific absorption rates (SAR) and RF power deposition, albeit without applying high-power RF. Here, we propose a method to simultaneously improve transmission efficiency and reception sensitivity of a band-pass birdcage RF coil (BP-BC RF coil) by combining a multi-channel wireless RF element (MCWE) with a high permittivity material (HPM) in a 7.0 T MRI. Electromagnetic field (EM-field) simulations, performed using two types of phantoms, viz., a cylindrical phantom filled with oil and a human head model, were used to compare the effects of MCWE and HPM on BP-BC RF coils. EM-fields were calculated using the finite difference time-domain (FDTD) method and analyzed using Matlab software. Next, to improve RF transmission efficiency, we compared two HPM structures, namely, a hollow cylinder shape HPM (hcHPM) and segmented cylinder shape HPM (scHPM). The scHPM and MCWE model comprised 16 elements (16-rad BP-BC RF coil) and this coil configuration demonstrated superior RF transmission efficiency and reception sensitivity along with an acceptable SAR. We expect wider clinical application of this combination in 7.0 T MRIs, which were recently approved by the United States Food and Drug Administration. Full article

Magnetic resonance imaging (MRI) systems must undergo quantitative evaluation through daily and periodic performance assessments. In general, the reference or standard radiofrequency (RF) coils for these performance assessments of 1.5 to 7.0 T MRI systems have been low-pass-type birdcage (LP-BC) RF coils. However, LP-BC RF coils are inappropriate for use as reference RF coils because of their relatively lower magnetic field (B₁-field) sensitivity than other types of BC RF coils, especially in ultrahigh-field (UHF) MRI systems above 3.0 T. Herein, we propose a hybrid-type BC (Hybrid-BC) RF coil as a reference RF coil with improved B₁-field sensitivity in UHF MRI system and applied it to an 11.7 T MRI system. An electromagnetic field (EM-field) analysis on the Hybrid-BC RF coil was performed to provide the proper dimensions for its use as a reference RF coil. Commercial finite difference time-domain program was used in EM-field simulation, and home-made analysis programs were used in analysis. The optimal specifications of the proposed Hybrid-BC RF coils for them to qualify as reference RF coils are proposed based on their B₁⁺-field sensitivity under unnormalized conditions, as well as by considering their B₁⁺-field uniformity and RF safety under normalized conditions. Full article

A proton-frequency-transparent (PFT) birdcage RF coil that contains carbon-proton switching circuits (CPSCs) is presented to acquire ¹³C MR signals, which, in turn, enable ¹H imaging with existing ¹H RF coils without being affected by a transparent ¹³C birdcage RF coil. CPSCs were installed in the PFT ¹³C birdcage RF coil to cut the RF coil circuits during ¹H MR imaging. Finite-difference time-domain (FDTD) electromagnetic (EM) simulations were performed to verify the performance of the proposed CPSCs. The performance of the PFT ¹³C birdcage RF coil with CPSCs was verified via phantom and in vivo MR studies. In the phantom MR studies, ¹H MR images and ¹³C MR spectra were acquired and compared with each other using the ¹³C birdcage RF coil with and without the CPSCs. For the in vivo MR studies, hyperpolarized ¹³C cardiac MRS and MRSI of swine were performed. The proposed PFT ¹³C birdcage RF coil with CPSCs led to a percent image uniformity (PIU) reduction of 1.53% in the proton MR images when compared with the case without it. FDTD EM simulations revealed PIU reduction of 0.06% under the same conditions as the phantom MR studies. Furthermore, an SNR reduction of 5.5% was observed at ¹³C MR spectra of corn-oil phantom using the PFT ¹³C birdcage RF coil with CPSCs compared with that of the ¹³C birdcage RF coil without CPSCs. Utilizing the PFT ¹³C birdcage RF coil, ¹³C-enriched compounds were successfully acquired via in vivo hyperpolarized ¹³C MRS/MRSI experiments. In conclusion, the applicability and utility of the proposed 16-leg low-pass PFT ¹³C birdcage RF coil with CPSCs were verified via ¹H MR imaging and hyperpolarized ¹³C MRS/MRSI studies using a 3.0 T MRI system. Full article

The feasibility and the development of a four-port elliptical birdcage radio frequency (RF) coil for generating a homogenous RF magnetic (B₁) field is presented for a space-constrained narrow-bore magnetic resonance imaging (MRI) system. Optimization was performed for the elliptical birdcage RF coil by adjusting the position and the structure of the legs to maximize the B₁⁺-field uniformity. Electromagnetic (EM) simulations based on RF coil circuit co-simulations were performed on a cylindrical uniform phantom and a three-dimensional human model to evaluate the B₁⁺-field uniformity, the transmission efficiency, and the specific absorption rate (SAR) deposition. An elliptical birdcage RF coil was constructed, and its performance was evaluated through network analysis measurements such as S-parameters and Q-factor. Quadrature transmit and receive MRI experiments were conducted using both phantom and in vivo human for validation. The EM simulation results indicate reasonable B₁⁺-field uniformity and transmission efficiency for the proposed elliptical birdcage RF coil. The signal-to-noise ratio and the flip angle maps of the uniform phantom and the in vivo human MR images acquired using an elliptical birdcage (62 cm × 58 cm) were similar to those of a commercial circular birdcage (diameter, 58 cm), thereby indicating acceptable performance. In conclusion, the proposed split-type asymmetric elliptical birdcage RF coil is useful for whole-body MRI applications and can be used for imaging larger human subjects comfortably in a spacious imaging space. Full article

This paper proposes the use of a triple-line microstrip array for transmitting a magnetic field (|B₁⁺|) into the whole body for magnetic resonance applications at ultra-high field strength, such as 7 T. We explored some technologies that can potentially be applied for whole-body 7 T magnetic resonance imaging, as there is ongoing research on this topic. The triple-line microstrip transmission line (t-MTL) array consists of 32 channels. Each channel has a t-MTL, comprising a main conductor line and two adjacent coupled lines. The adjacent lines are not connected directly to the source. This configuration resulted in increased intensity and a centered |B₁⁺|-field. We compared the proposed structure and some reference radiofrequency (RF) transmitters, such as a patch antenna, using a magnet bore as a waveguide and a whole-body birdcage coil. We evaluated the performance of the t-MTL using cylindrical phantoms. We computed the |B₁⁺|-field from each RF transmitter inside a 3D human model containing more than 200 tissues. We compared their uniformity and field intensity and proposed a t-MTL array that yielded better performance. The proposed design also showed a lower specific absorption rate compared with a patch antenna. Full article

The radio frequency (RF) coil is one of the key components of the magnetic resonance imaging (MRI) system. It has a significant impact on the performance of the nuclear magnetic resonance (NMR) detection. Among numerous practical designs of RF coils for NMR imaging, the birdcage RF coil is the most popular choice from low field to ultra-high field MRI systems. In the transmission mode, it can establish a strong and homogeneous transverse magnetic field B1 for any element at its Larmor frequency. Similarly, in the reception mode, it exhibits extremely high sensitivity for the detection of even faint NMR signals from the volume of interest. Despite the sophisticated 3D structure of the birdcage coil, the developments in the design, analysis, and implementation technologies during the past decade have rendered the development of the birdcage coils quite reasonable. This article provides a detailed review of the recent progress in the birdcage RF coil technology for the MRI system. Full article

A novel analytical solution for the designing of the birdcage RF coil has been demonstrated in this paper. A new concept of dominant resonance path has been introduced in this paper which is used to identify the specific closed current loop in the birdcage RF coil which is responsible for the dominant resonance frequency mode. This concept is used to determine the precise numerical values of the lumped capacitance deployed in the legs and/or end-rings of the birdcage RF coil for its proper operation at the desired resonance frequency. The analytical solution presented in this paper has been established by performing the two-port network based equivalent circuit modeling of the birdcage RF coil. The proposed analytical solution uses T-matrix theory and develops a relationship between the input impedance of the birdcage coil and the impedances of its leg and end-ring segments. The proposed analytical solution provides the information about the resonance frequency spectrum of the birdcage RF coil and solves the issue of its interfacing with external circuits without affecting its resonance characteristics. Based upon the proposed analysis and designing strategy presented in this paper, the low pass, high pass and band pass configurations of the birdcage RF coil were successfully implemented with FPCB (Flexible Printed Circuit board) technique for small volume NMR imaging applications at 1.5 T and 3.0 T MRI system. The results obtained for the implemented birdcage coils using the proposed analysis and designing technique are in closed agreement with already established methods. Full article