Concepts in Magnetic Resonance Part B-Magnetic Resonance Engineering最新文献

A novel radio frequency (RF) coil for ultra-high-field MRI in the form of a slotted waveguide array (SWGA) filled with a low-loss high-permittivity dielectric is proposed, evaluated, and demonstrated. A comprehensive computational electromagnetics study, along with basic RF measurements, to characterize the SWGA RF coil at 7T is presented. Slotted waveguides are robust structures capable of handling high powers. They are inherently narrow-band and have well defined linear polarization. When arranged in an array, they effectively generate high-quality field with strong and extremely low and components. With added dielectric lenses, the observed transmit efficiencies exceed in the human head model phantom, which is much higher than all results reported in literature. Moreover, we show that the proposed exciter, as an array with well-decoupled elements (measured isolation between elements is 33 dB or higher), can effectively be used for RF shimming. The novel coil generates RF magnetic field with excellent circular polarization, good uniformity, and negligible axial z-component, and it provides arbitrarily large or small field of view and excellent transmit efficiency, with and without dielectric lenses. It features well-defined narrowband operation, excellent isolation between ports/channels, and inherent possibilities for field optimizations via RF shimming and parallel imaging.

The morphological investigation of insects is usually performed using histologic serial sections and subsequent reconstruction of the structures from these sections. The achievement of cross sections for microtomy is time-consuming and the risk of damaging sections is inevitable. Recently, X-ray computed tomography (micro-CT) was used to provide adequate spatial resolution without destroying the specimens. Micro-CT is limited by the low x-ray contrast of the insect soft tissues and image quality is relatively poor. Magnetic resonance imaging (MRI) allows the study of morphologic classification of the insects with sufficient spatial resolution and provides a noninvasive mean to determine disease abnormalities and progression in vivo and longitudinally. The morphologic classification of the insects with sufficient spatial resolution analyzed the potential of MR imaging. However, a stag beetle has a particularly hard exoskeleton protecting internal organs and nerves. It is challenge to obtain high spatial resolution images using MRI. The aim of this study was to characterize optimal MRI protocols for the investigation of stag beetles and to evaluate the morphologic characterization of the stag beetles by a 9.4 T MRI scanner. In this study, MR imaging provided the spatial resolution necessary for the examination of morphologic structures of the insects on our hardware-software platform. This study plays a significant role in providing the high spatial resolution, ideally required for routine application to the study of internal morphology of insects, arachnids and crustaceans whose organs, nerves and muscles are protected by the hard exoskeleton.

Some MRI applications require the generation of a series high power RF pulses in which the spatial transmit B1 field pattern over the sample is modified between one pulse and the next. This requirement may be realized by a RF transmit array with the capability to enable and disable individual elements to switch between field patterns with switching times of the order 10 μs. Our application is for a TRASE (“Transmit Array Spatial Encoding”) array for which short high power pulses are necessary to achieve high resolution (mm-level) spatial encoding. We present designs for coil array, coil switching circuits, and a high power PIN diode driver together capable of robust and rapid switching of short (~120 μs) high power pulses for a 24 cm TRASE phase gradient coil suitable for imaging extremities at 8 MHz. We describe in detail the selection of suitable coil components and switch circuit designs to satisfy a specific requirement for maximum B1 field strength, and provide all circuit designs.

This article presents a dual-tuned (DT) radiofrequency (RF) coil for signal acquisition of 2 nuclei, namely, hydrogen (¹H) and sodium (²³Na), in the ultra-high magnetic field of a 7-T magnetic resonance imaging (MRI) system. The double-layered dual-tuned (DLDT) coil comprises a 2-loop coil configuration per single-pair geometry, with the ¹H and ²³Na coils being located on the outside and inside, respectively. The ¹H and ²³Na single-pair elements are tuned to resonance frequencies of 297.20 and 78.61 MHz, respectively. The single-pair geometry of the DLDT coil is extended to an 8-pair configuration to cover the human head, and the operation mode is transmission/reception (Tx/Rx). The 8-pair DLDT Tx/Rx coil array is designed with a non-overlapped single pair between the ¹H coil elements for geometric decoupling, and capacitive decoupling is implemented to minimize the mutual inductance coupling. The 2 resonance frequencies are fed through a single RF port to a common matching board, and each frequency is selected using the voltages at both ends of a PIN diode. Through use of the PIN diode in the DLDT coil configuration, with a voltage drop at both ends, different resonance frequencies can be selected for each coil element in accordance with the diode ON/OFF state. The experiments conducted showed that the proposed DLDT coil is effective in acquiring signals of ¹H and ²³Na in the MRI system.

The magic angle coil spinning (MACS) technique has provided a breakthrough in enhancing sensitivity in magic angle spinning (MAS) NMR. However, efforts in improving the MACS detector for higher spinning speeds have been lacking. One published MACS construction technique is to solder a handwound solenoidal coil to a commercial non-magnetic capacitor and subsequently centering the detector inside the MAS rotor. An alternative method to realize these detectors is by using MEMS fabrication at the wafer scale, potentially capable of achieving reproducible MACS detectors. However, it is also important that the performance of the sensors does not deteriorate as a result of microfabrication constraints. The footprint of the detectors is a limiting factor in achieving higher spinning speeds. One of the key elements of a micro-resonator is its tuning capacitor, whose geometry has a significant influence on its electrical and mechanical performance. The quality factor of the capacitor, along with the induced eddy currents, are the key performance parameters considered. The article addresses these concerns by presenting a study of microfabricated on-chip capacitors for magic angle coil spinning (MACS) detectors. The capacitors are juxtaposed with commercially available capacitors and the most suitable fit to be integrated with a micro-coil is established.

The simplest design of surface coils for magnetic resonance imaging (MRI) applications is circular and square loops, both producing a magnetic field perpendicular to the coil plane in the central region-of-interest (ROI), with an amplitude that decreases along the coil axis. However, a surface coil constituted by a loop with different geometry could be necessary when particular field-of-views (FOVs) are desired, especially for performing imaging in an elongated region. This can be achieved by using an elliptical loop, which can guarantee a wide longitudinal FOV and a good penetration in deep sample regions. This work proposes the application of a method for elliptical coil Signal-to-Noise Ratio (SNR) estimation previously developed for circular and square loop design, in which coil inductance and resistance are analytically calculated and the magnetic field pattern is estimated using the magnetostatic approach, while the sample-induced resistance is calculated with the vector potential calculation method. In the second part of the paper, we propose the simulation and the design of a transmit/receive elliptical coil for MRI in mice with a 3T clinical scanner. We also evaluated the coil performance in a preliminary magnetic resonance spectroscopy (MRS) study in phantom.

In MRI systems, cable-free receive arrays would simplify setup while reducing the bulk and weight of coil arrays and improve patient comfort and throughput. Since battery power would limit scan time, wireless power transfer (WPT) is a viable option to continuously supply several watts of power to on-coil electronics. To minimize added noise and decouple the wireless power system from MRI coils, restrictions are placed on the coil geometry of the wireless power system, which are shown to limit its efficiency. Continuous power harvesting can also cause a large increase in the background noise of the image due to diode rectifier up-conversion of noise around the frequency of the transmitted power. However, by RF gating the transmitted power off during the MRI receive time while continuing to supply power from a storage capacitor, WPT is demonstrated to have minimal impact on image quality at received power levels up to 11 W. The integration of WPT with a 1.5T scanner is demonstrated.