Abstract
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.
Highlights
Over the past few decades, magnetic resonance imaging (MRI) has become an essential imaging modality in modern bioimaging research and clinical applications
All the lumped elements were replaced with excitation ports, and the full port S-parameters were generated via EM simulation and exported to MATLAB (The MathWorks, Natick, MA, USA) to calculate the optimal capacitor by minimizing the cost function for the desired mode at the resonant frequency
Using the radio frequency (RF) coil circuit co-simulation approach, the optimal end-ring capacitors were calculated such that the reflection coefficients (Sii ) at each port and the isolation coefficients (Sij ) between adjacent ports were at least −15 dB before including the matching circuit
Summary
Over the past few decades, magnetic resonance imaging (MRI) has become an essential imaging modality in modern bioimaging research and clinical applications. In MRI, radio frequency (RF) coils are crucial for transmitting and receiving RF signals in the target object, where a highly homogenous RF magnetic (B1 ) field, a large field-of-view (FOV), and a high signal-to-noise ratio (SNR) are required. The birdcage RF coil introduced over four decades ago [1] remains as the preferred option for transmitting RF coils in clinical MRI systems (1.5 T, 3.0 T, and 7.0 T) owing to its capability in generating a uniform B1 -field over a large portion of the imaging volume. The birdcage RF coil driven in quadrature mode generates a circularly polarized B1 -field, which reduces the RF power requirement by a factor of 0.5 to 0.7 depending on the shape and the size of the subject; it √. Many different variants of birdcage RF coils were designed, such as circular, elliptical [4], and asymmetric shapes [5,6], to closely match the RF coil outline to the subject outline as well as the space constraint for improved
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