Xinqiang Yan

7T Transmit/Receive Arrays Using ICE Decoupling for Human Head MR Imaging

In designing large-sized volume type phased array coils for human head imaging at ultrahigh fields, e.g., 7T, minimizing electromagnetic coupling among array elements is technically challenging. A new decoupling method based on induced current compensation or elimination (ICE) for a microstrip line planar array has recently been proposed. In this study, an eight-channel transmit/receive volume array with ICE-decoupled loop elements was built and investigated to demonstrate its feasibility and robustness for human head imaging at 7T. Isolation between adjacent loop elements was better than - 25 dB with a human head load. The worst-case of the isolation between all of the elements was about - 17.5 dB. All of the MRI experiments were performed on a 7T whole-body human MR scanner. Images of the phantom and human head were acquired and g-factor maps were measured and calculated to evaluate the performance of the coil array. Compared with the conventional capacitively decoupled array, the ICE-decoupled array demonstrated improved parallel imaging ability and had a higher SNR. The experimental results indicate that the transceiver array design with ICE decoupling technique might be a promising solution to designing high performance transmit/receive coil arrays for human head imaging at ultrahigh fields.

Magnetic wall decoupling method for monopole coil array in ultrahigh field MRI: a feasibility test.

Ultrahigh field (UHF) MR imaging of deeply located target in high dielectric biological samples faces challenges due to the reduced penetration depth at the corresponding high frequencies. Radiative coils, e.g., dipole and monopole coils, have recently been applied for UHF MRI applications to obtain better signal-noise-ratio (SNR) in the area deep inside the human head and body. However, due to the unique structure of radiative coil elements, electromagnetic (EM) coupling between elements in radiative coil arrays cannot be readily addressed by using traditional decoupling methods such as element overlapping and L/C decoupling network. A new decoupling method based on induced current elimination (ICE) or magnetic wall technique has recently been proposed and has demonstrated feasibility in designing microstrip transmission line (MTL) arrays and L/C loop arrays. In this study, an array of two monopole elements decoupled using magnetic wall decoupling technique was designed, constructed and analyzed numerically and experimentally to investigate the feasibility of the decoupling technique in radiative coil array designs for MR imaging at 7 T. An L-shaped capacitive network was employed as the matching circuit and the reflection coefficients (S11) of the monopole element achieved -30 dB or better. Isolation between the two monopole elements was improved from about -10 dB (without decoupling treatment) to better than -30 dB with the ICE/magnetic wall decoupling method. B1 maps and MR images of the phantom were acquired and SNR maps were measured and calculated to evaluate the performance of the ICE/magnetic wall decoupling method. Compared with the monopole elements without decoupling methods, the ICE-decoupled array demonstrated more independent image profiles from each element and had a higher SNR in the peripheral area of the imaging subject. The experimental and simulation results indicate that the ICE/magnetic wall decoupling technique might be a promising solution to reducing the EM coupling of monopole arrays for UHF MRI.

Multichannel Double-Row Transmission Line Array for Human MR Imaging at Ultrahigh Fields

Objective: In microstrip transmission line (MTL) transmit/receive (transceive) arrays used for ultrahigh field MRI, the array length is often constrained by the required resonant frequency, limiting the image coverage. The purpose of this study is to increase the imaging coverage and also improve its parallel imaging capability by utilizing a double-row design. Methods: A 16-channel double-row MTL transceive array was designed, constructed, and tested for human head imaging at 7 T. Array elements between two rows were decoupled by using the induced current elimination or magnetic wall decoupling technique. In vivo human head images were acquired, and g-factor results were calculated to evaluate the performance of this double-row array. Results: Testing results showed that all coil elements were well decoupled with a better than -18 dB transmission coefficient between any two elements. The double-row array improves the imaging quality of the lower portion of the human head, and has low g-factors even at high acceleration rates. Conclusion: Compared with a regular single-row MTL array, the double-row array demonstrated a larger imaging coverage along the z-direction with improved parallel imaging capability. Significance: The proposed technique is particularly suitable for the design of large-sized transceive arrays with large channel counts, which ultimately benefits the imaging performance in human MRI.

Array-compressed parallel transmit pulse design.

To design array‐compressed parallel transmit radiofrequency (RF) pulses and compare them to pulses designed with existing transmit array compression strategies.

A monopole/loop dual-tuned RF coil for ultrahigh field MRI

Proton and heteronuclear MRI/MRS using dual-tuned (DT) coils could provide both anatomical and metabolic images without repositioning the subject. However, it is technologically challenging to attain sufficiently electromagnetic (EM) decoupling between the heteronuclear channel and proton channel, and keep the imaging areas and profiles of two nuclear channels highly matched. In this study, a hybrid monopole/loop technique was proposed for DT coil design and this technique was validated by implementing and testing a DT (1)H/(23)Na coil for MR imaging at 7T. The RF fields of the monopole ((1)H channel) and regular L/C loop ((23)Na channel) were orthogonal and intrinsically EM decoupled. Bench measurement results demonstrated the isolation between the two nuclear channels was better than -28 dB at both nuclear frequencies. Compared with the conventional DT coil using trap circuits, the monopole/loop DT coil had higher MR sensitivity for sodium imaging. The experimental results indicated that the monopole/loop technique might be a simple and efficient design for multinuclear imaging at ultrahigh fields. Additionally, the proposed DT coils based on the monopole/loop technique can be used as building blocks in designing multichannel DT coil arrays.

Experimental implementation of array‐compressed parallel transmission at 7 tesla

To implement and validate a hardware‐based array‐compressed parallel transmission (acpTx) system.

A hybrid sodium/proton double-resonant transceiver array for 9.4T MRI

High field MRI makes it possible to obtain images of non-proton (X) nuclei due to its increased sensitivity. A double-resonant coil can provide both images of X nucleus and proton without the need to reposition the patient. Most double-resonant coils use lossy trap circuits to minimum interactions between X nucleus and H nucleus. In this study, we propose a hybrid sodium/proton double-tuned transceiver array for 9.4T MRI. The interactions were reduced by hybrid structure without using a lossy circuit. Compared with birdcage or TEM coils, the sensitivity of phased-array was better at the peripheral region of the phantom. For proton imaging at 9.4T, the microstrip transmission line (MTL) array structure was used as coil element for its good performance at ultra-high field. The RF circuit and 3-D EM co-simulation method was used to evaluate the performance of this hybrid double-resonant coil. A practical double-resonant coil with same parameters was built and its feasibility was validated through bench test.

Optimizing the ICE decoupling element distance to improve monopole antenna arrays for 7 Tesla MRI

The induced current elimination (ICE) method has been previously applied to decouple monopole coil arrays in ultrahigh field MRI. However, the method creates low B1+ spots near the decoupling elements. In this study, we aim to improve the performance of ICE-decoupled monopole array in human head imaging at 7 Tesla. Eight-channel ICE-decoupled monopole arrays were optimized by varying the position of the decoupling elements. A series of numerical studies were performed using the co-simulation method. In simulation, decoupling performance, quality (Q-) values and transmit field (B1+) were comparatively investigated. In addition, we constructed an optimized ICE-decoupled monopole array and compared its performance with the unoptimized array. The simulation results showed that a good trade-off between decoupling and B1+ loss can be obtained when decoupling elements were moved 2.5-cm away from coil elements. This was validated by in-vivo MR imaging using the constructed array. Compared with the unoptimized ICE decoupled monopole array, the optimized array had a more homogeneous transmit field and no dark spots or signal cancellations in the MR images.

Optimized MTL array with serial capacitors for 7T MRI

Transmit/receive MTL (Microstrip Transmission Line) array has been widely used for ultra-high-field MRI because of its low mutual coupling characteristics. However, several design issues such as limit to the length of a single coil element still remains. For traditional MTL coils, two capacitors (one fixed, one variable) were put at both ends of the coil element for tuning. Based on our experimental observations, the length of the traditional MTL element is limited at 7T MRI because of increased non-uniform distribution of B1 field along the Z direction. In this study, an optimized MTL array with six capacitors equally distributed along the strip line was investigated. The optimized MTL array can generate a more uniform B1 field along the Z direction compared with the traditional MTL array. The numerical investigation was performed using the RF circuit and 3-D EM co-simulation.

Self-decoupled radiofrequency coils for magnetic resonance imaging

Arrays of radiofrequency coils are widely used in magnetic resonance imaging to achieve high signal-to-noise ratios and flexible volume coverage, to accelerate scans using parallel reception, and to mitigate field non-uniformity using parallel transmission. However, conventional coil arrays require complex decoupling technologies to reduce electromagnetic coupling between coil elements, which would otherwise amplify noise and limit transmitted power. Here we report a novel self-decoupled RF coil design with a simple structure that requires only an intentional redistribution of electrical impedances around the length of the coil loop. We show that self-decoupled coils achieve high inter-coil isolation between adjacent and non-adjacent elements of loop arrays and mixed arrays of loops and dipoles. Self-decoupled coils are also robust to coil separation, making them attractive for size-adjustable and flexible coil arrays.Conventional coil arrays require complex decoupling technologies to reduce electromagnetic coupling between coil elements. Here, the authors report a self-decoupled RF coil design that achieves high inter-coil isolation between adjacent and non-adjacent elements and mixed arrays of loops and dipoles