Multiphoton parallel transmission (MP-pTx): Pulse design methods and numerical validation.

In ultra-high field (UHF) imaging environments, it has been demonstrated that multiple-mouse magnetic resonance imaging (MM-MRI) is dependent on key factors such as the radiofrequency (RF) coil hardware, imaging protocol, and experimental setup for obtaining high-resolution MR images. A key aspect is the RF coil, and a number of MM-MRI studies have investigated the application of single-channel RF transmit (Tx)/receive (Rx) coils or multi-channel phased array (PA) coil configurations under a single gradient coil set. However, despite applying a variety of RF coils, Tx (|B1+ |)-field inhomogeneity still remains a major problem due to the relative shortening of the effective RF wavelength in the UHF environment. To address this issue, we propose a relatively smaller size of individual Tx-only coils in a multiple birdcage (MBC) coil for MM-MRI to image up to three mice. We use electromagnetic (EM) simulations in the finite-difference time-domain (FDTD) environment to obtain the |B1 |-field distribution. Our results clearly show that the single birdcage (SBC) high-pass filter (HPF) configuration, which is referred to as the SBCHPF , under the absence of an RF shield exhibits a high |B1 |-field intensity in comparison with other coil configurations such as the low-pass filter (LPF) and band-pass filter (BPF) configurations. In a 7-T MRI experiment, the signal-to-noise ratio (SNR) map of the SBCHPF configuration shows the highest coil performance compared to other coil configurations. The MBCHPF coil, which is comprised of a triple-SBCHPF configuration combined with additional decoupling techniques, is developed for simultaneous image acquisition of three mice. SCANNING 38:747-756, 2016. © 2016 Wiley Periodicals, Inc.

Novel and facile criterion to assess the accuracy of WSS estimation by 4D flow MRI.

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.

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.

This work describes the study of coils for Magnetic Resonance Imaging (MRI) applications. The principal objective is the design of a birdcage Radio Frequency (RF) coil to use in a 7 Tesla (7T) scanner. Higher strength field generates a better SNR and increased chemical shift effect, improving spectral fat suppression and spectroscopy. Moreover, a better SNR increases the spatial resolution or reduces the imaging time. This research work presented recent developments based on high field 7T design using CST studio. The birdcage coil achieves circular polarization and generates a high homogeneous radio frequency magnetic field under many conditions. Design of a Birdcage coil for a 7T to obtain the images from s mall animals (i.e. mouse). It opens the door to design and construct a Birdcage coil for a 7T to obtain human brain images. Firstly we design a birdcage coil then the results are obtained with simulator CST Wave Studio, creating a 3D model and generating a simulation. Finally the parameters are re adjusted to obtain our desired Larmor frequency 298.2 MHz for a correct operation in 7T. This research work demonstrates the theoretical results from our design and shows the designed antenna behavior.

PurposeSimultaneous multi‐slice techniques are reliant on multiband RF pulses, for which conventional design strategies result in long pulse durations, lengthening echo‐times so lowering SNR for spin‐echo imaging, and lengthening repetition times for gradient echo sequences. Pulse durations can be reduced with advanced RF pulse design methods that use time‐variable selection gradients. However, the ability of gradient systems to reproduce fast switching pulses is often limited and can lead to image artifacts when ignored. We propose a time‐efficient pulse design method that inherently produces gradient waveforms with lower temporal bandwidth.MethodsEfficient multiband RF pulses with time‐variable gradients were designed using time‐optimal VERSE. Using VERSE directly on multiband pulses leads to gradient waveforms with high temporal bandwidth, whereas VERSE applied first to singleband RF pulses and then modulated to make them multiband, significantly reduces this. The relative performance of these approaches was compared using simulation and experimental measurements.ResultsApplying VERSE before multiband modulation was successful at removing out‐of‐band slice distortion. This effectively removes the need for high frequency modulation in the gradient waveform while preserving the benefit of time‐efficiency inherited from VERSE.ConclusionWe propose a time‐efficient RF pulse design that produces gradient pulses with lower temporal bandwidth, reducing image artifacts associated with finite temporal bandwidth of gradient systems.

Birdcage coils have been widely used to generate radiofrequency (RF) pulses for magnetic resonance imaging (MRI) because of their capability to produce a highly homogeneous B/sub 1/-field over a large volume within the coil. This is true even if the coil is loaded with a human body or a human head provided that the frequency of the B/sub 1/-field is low (less than 50 MHz). However, when birdcage coils are employed for high frequency MRI, the interaction between the electromagnetic (EM) field and the biological object to be imaged deteriorates the B/sub 1/-field homogeneity. Such a deterioration reduces the quality of the MR images and causes difficulties for advanced MRI studies such as functional MRI and MR spectroscopy. Moreover, the EM interaction at high frequencies also leads to an increased electric field, thus, an increased specific energy absorption rate (SAR) in the biological object, causing concern about the possible adverse effects of MRI on human health. Jin et al. (see Phys. Med. Biol., vol.41, p.2719-38, 1996) proposed a method that was suitable for modelling open MRI coils; however, it has difficulties in modelling coils with an RF shield, which is often used to reduce the unwanted interactions between the EM fields and the exterior structures. We employ the finite-difference time-domain (FDTD) method to study this problem. It is found that the FDTD method can effectively model the complex anatomical structure of the human head, the currents in the MRI coil, and the RF shield.

Static field (B0) inhomogeneities present a major challenge in high-field MRI. Multicoil shimming using independent, local, direct-current (DC) shim coils has emerged as a powerful and flexible technique to address this issue. However, many-turn DC coils can lead to significant mutual coupling with radiofrequency (RF) coils, causing transmit field (B1 +) distortions and signal-to-noise ratio degradation. We introduce an innovative RF-transparent DC coil that performs B0 shimming while minimizing RF performance impact. The design incorporates float traps to maintain high RF impedance, allowing flexible placement relative to the RF coil without compromising signal-to-noise ratio or affecting B1 +. We fabricated square-shaped DC coils with float traps for 3T MRI and compared them with conventional DC coils. To demonstrate high ΔB0/Amp efficiency, we conducted a B0 shimming experiment around a metal hip implant. Bench tests and MRI experimental results demonstrated that the RF-transparent DC coil effectively minimized RF interference, preserved signal-to-noise ratio, and maintained B1 +, even when placed near the RF receive coil. Additionally, the DC coil significantly improved B0 homogeneity near metal implants and substantially reduced image distortion. The RF-transparent DC coil offers a flexible, effective solution for managing B0 inhomogeneities, paving the way for integrating multiturn DC coils in clinical MRI settings without extensive hardware modifications.

Motivation: Safe postoperative MRI imaging is essential for children with DBS devices, as conventional MRI poses risks of tissue heating. This study addresses the challenge of balancing B1+ homogeneity with SAR safety in pediatric DBS patients. Goal(s): To evaluate the performance of double-row pTx coil compared to conventional birdcage coils in pediatric DBS patients. Approach: We compared a 16-channel double-row pTx coil and a birdcage coil on four child models and one adult model using FDTD simulation, applying SAR-regularized-MLS optimisation to balance B1+ homogeneity and SAR1g. Results: The double-row coil demonstrated superior B1+ homogeneity and reduced SAR1g, enhancing MRI safety for pediatric DBS patients. Impact: The double-row pTx coil enhances MRI safety for paediatric DBS patients by balancing B1+ homogeneity and reducing SAR, minimizing tissue heating risks and enabling safer, more reliable imaging protocols with consistent performance across models, potentially improving clinical outcomes.

The use of detached endcaps for 3 T birdcage coils was investigated both theoretically and experimentally. Finite difference time domain analysis, along with workbench and MRI techniques, were used to map the radiofrequency (RF) B(1) distribution along the coil axis with and without an endcap. Without an endcap the measured B(1) value at the service end of the birdcage was only 45% of the value at the coil's center. This was improved to 85% with a detached endcap of maximum achievable diameter (375 mm), positioned 4 mm from the RF shield. The B(1) field distribution on the patient side of the coil was unaffected by the presence of the endcap. The dependence of the B(1) distribution as a function of endcap diameter was also investigated. Surprisingly, simulations and experiments show that there is an optimum ratio of endcap-to-birdcage coil diameter (approximately 1.08) that gives the best B(1) homogeneity. In the human head the optimized endcap, positioned 16 mm from the RF shield, improves the MRI signal amplitude from 55% to 85% of maximum toward the service end. This novel endcap design is easy to implement with existing birdcage coils, and could prove useful when flexibility in access to the RF coil is required.

A new boundary element method (BEM) based method is described for the design of coils for magnetic resonance imaging (MRI) systems. BEM is an effective approach for solving the electromagnetic forward problem and has been used in the design of MRI gradient coils. However, BEM-based gradient coil design faces an ill-posed mathematical problem, which is conventionally handled by means of a Lagrange multiplication method. This work attempts to improve the BEM method for MRI coil designs by applying the Tikhonov regularization scheme to solve the ill-posed matrix system formulated by the BEM forward model. The objective of the study is not to design some specific gradient or radio-frequency (RF) coils for MRI system, but to discuss the design scheme with practical regularization and constraints. The proposed approach was explained in the design of MRI coils including biplanar transverse gradient coils and RF phased array coils. With the consideration of the practical engineering requirements, physical constraints such as wire intervals are transformed into mathematical constraints and formulated into BEM equations. The examples demonstrate that the proposed method is efficient and flexible for the design of MRI coils with arbitrary geometries and engineering constraints.

The modular (twin) gradient coil—high resolution, high contrast, diffusion weighted EPI at 1.0 Tesla

This paper proposes a new method for joint design of radiofrequency (RF) and gradient waveforms in Magnetic Resonance Imaging (MRI), and applies it to the design of 3D spatially tailored saturation and inversion pulses. The joint design of both waveforms is characterized by the ODE Bloch equations, to which there is no known direct solution. Existing approaches therefore typically rely on simplified problem formulations based on, e.g., the small-tip approximation or constraining the gradient waveforms to particular shapes, and often apply only to specific objective functions for a narrow set of design goals (e.g., ignoring hardware constraints). This paper develops and exploits an auto-differentiable Bloch simulator to directly compute Jacobians of the (Bloch-simulated) excitation pattern with respect to RF and gradient waveforms. This approach is compatible with arbitrary sub-differentiable loss functions, and optimizes the RF and gradients directly without restricting the waveform shapes. For computational efficiency, we derive and implement explicit Bloch simulator Jacobians (approximately halving computation time and memory usage). To enforce hardware limits (peak RF, gradient, and slew rate), we use a change of variables that makes the 3D pulse design problem effectively unconstrained; we then optimize the resulting problem directly using the proposed auto-differentiation framework. We demonstrate our approach with two kinds of 3D excitation pulses that cannot be easily designed with conventional approaches: Outer-volume saturation (90° flip angle), and inner-volume inversion.

The never ending quest for higher magnetic field strengths in MRI and MRS has led to small and medium bore scanners at 9.4 T and above for both human and animal use; however, these bore diameters restrict the size of object that can be accommodated when using a conventional gradient coil. By replacing a cylindrical gradient-coil insert with a single-sided gradient coil, the scanner's functionality can be extended to include localized imaging of wider samples. As a prototype, a three-axis, cradle-shaped gradient coil was designed, fabricated and implemented in a 9.4 T animal MRI scanner. Since gradient fields are required only to be monotonic over the desired field of view, the cradle gradient coil was designed to produce high gradient efficiencies (up to 2.25 mT m−1 A−1 over a 5 cm imaging region) at the expense of gradient linearity. A dedicated three-dimensional algorithm was developed to correct the resultant image distortion. Preliminary images of a grid phantom and a mouse demonstrated the fidelity of the algorithm in correcting image distortion of greater than 200%. Eddy currents were measured along each gradient axis. A large 65.2 (Hz mT−1 m) B0 eddy current was produced by the y-axis, suggesting potential limitations of single-sided gradient coils.

Multimodal surface coils for low field MR imaging