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

A design for a permanent magnet system is proposed that generates spatially homogeneous, constant magnetic field gradients, thus creating conditions suitable for MRI without gradient coils and amplifiers. This is achieved by superimposing a weak Halbach quadrupole on a strong Halbach dipole. Rotation of either the quadrupole or the entire magnet assembly can be used to generate two-dimensional images via filtered backprojection. Additionally, the mutual rotation of two quadrupoles can be used to scale the resulting gradient. If both gradients have identical strength the gradient can even be made to vanish. The concept is demonstrated by analytical considerations and FEM simulations. However, a demonstration on a working prototype is still pending. © 2016 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 46B: 41–48, 2016

In high field MRI, the spatial distribution of the radiofrequency magnetic () field is usually affected by the presence of the sample. For hardware design and to aid interpretation of experimental results, it is important both to anticipate and to accurately simulate the behavior of these fields. Fields generated by a radiofrequency surface coil were simulated using dyadic Green's functions, or experimentally measured over a range of frequencies inside an object whose electrical properties were varied to illustrate a variety of transmit () and receive () field patterns. In this work, we examine how changes in polarization of the field and interference of propagating waves in an object can affect the spatial distribution. Results are explained conceptually using Maxwell's equations and intuitive illustrations. We demonstrate that the electrical conductivity alters the spatial distribution of distinct polarized components of the field, causing “twisted” transmit and receive field patterns, and asymmetries between and . Additionally, interference patterns due to wavelength effects are observed at high field in samples with high relative permittivity and near-zero conductivity, but are not present in lossy samples due to the attenuation of propagating EM fields. This work provides a conceptual framework for understanding spatial distributions for surface coils and can provide guidance for RF engineers. © 2016 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 46B: 25–40, 2016

This work describes the measurement of complex permittivity (or dielectric constant) of powders (water insoluble), solutions, and suspensions using a parallel-plate capacitor cell. An impedance analyzer measures the cell's impedance at radio frequencies used in nuclear magnetic resonance and magnetic resonance imaging (10–300 MHz) through an appropriate test fixture. The cell's impedance is fitted to an equivalent circuit using a Matlab script or other software, and the permittivity of the material is extracted after calibration with known materials. The permittivity of the solid material is obtained from that of the powder suspension using known mixing rules. Measurements for most materials tested are in agreement with those obtained using standard coaxial probes. Some discrepancies are observed for loose powders because of the difficulty of controlling the amount of packing. For high-permittivity materials or conductive solutions an enhanced equivalent circuit has been found that characterizes the cell's behavior over the full frequency range. © 2016 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 46B: 19–24, 2016

Electromagnetic field simulations are increasingly used to assure RF safety of patients during MRI exams. In practice, however, tissue property distribution of the patient being imaged is not known, but may be represented with a pre-existing model. Repeatedly, agreement in transmit magnetic () field distributions between two geometries has been used to suggest agreement in heating distributions. Here we examine relative effects of anatomical differences on distribution, specific absorption rate (SAR), and temperature change (ΔT). Numerical simulations were performed for a single surface coil positioned adjacent a homogeneous phantom and bovine phantom, each with slight geometric variations, and adjacent two different human body models. Experimental demonstration was performed on a bovine phantom using MR thermometry and mapping. Simulations and experiments demonstrate that distributions in different samples can be well correlated, while notable difference in maximum SAR and ΔT occur. This work illustrates challenges associated with utilizing simulations or experiments for RF safety assurance purposes. Reliance on distributions alone for validation of simulations and/or experiments with a sample or subject for assurance of safety in another should be performed with caution. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 46B: 8–18, 2016

Breast cancer is the most frequently diagnosed cancer in women. High field studies have shown the diagnostic value of breast MRI, but the examination costs greatly exceed those of competing conventional mammography. Low field MRI offers typical MRI contrast at substantially lower cost, but has suffered from lower spatial resolution. Specificity of breast MRI can potentially be increased by acquiring MR imaging with higher spatial or temporal resolution, but the signal-to-noise ratio (SNR) achievable in a given imaging time becomes limiting. SNR for the particular pulse sequence and magnet field strength is strongly influenced by the characteristics of the radio-frequency coil. An optimal breast coil should yield excellent SNR but also generate a homogeneous B₁ field, while allowing imaging of the both breasts simultaneously and maintaining patient comfort. RF receiver coil design is a key determinant of image quality, thus to address this we have designed and constructed a low field breast imaging coil. The coil was tested with a 4-post 0.2T MRI providing high quality breast images. Designed and constructed saddle rf coil allows to obtain good quality image of the breast using low 0.2 T MRI system within 2 minutes. The coil provides patient comfort as breast compression is not required and minimizes artefacts caused by respiration or motion. A high contrast, low-cost and pain-free breast examination using optimized low field MRI system has the potential to serve a large patient population for whom current technologies have deficiencies. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 46B: 3–7, 2016

In this work, we developed and tested a multi-channel radio frequency (RF) transmission system with compact metal-oxide semiconductor field effect transistor (MOSFET) amplifiers for parallel excitation in 7 T animal MRI scanner. The system is composed of a multi-channel RF controller and four independent RF power amplifiers. Each power amplifier contains two amplification stages. The design was validated by simulation and bench test. The power gain for the amplifier is 18.7 dB at 300 MHz, demonstrating the sufficient amplification capability of the transmission system for small animal parallel excitation applications at 7 T. This compact RF power amplifier can be potentially used for on-coil amplification in multichannel RF array system. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 45B: 191–200, 2015

Radiative antenna techniques, e.g., dipole and monopole, have been proposed for radiofrequency (RF) coil array designs in ultrahigh field MRI to obtain stronger B₁ field and higher signal-to-noise ratio (SNR) gain in the areas deep inside human head or body. It is known that element decoupling performance is crucial to SNR and parallel imaging ability of array coil and has been a challenging issue in radiative antenna array designs for MR imaging. Magnetic wall or induced current elimination (ICE) technique has proven to be a simple and effective way of achieving sufficient decoupling for radiative array coils experimentally. In this study, this decoupling technique for radiative coil array was analyzed theoretically and verified by a simulation study. The decoupling conditions were derived and obtained from the theory. By applying the predicated decoupling conditions, the isolation of two radiative elements could be improved from about −8 dB to better than −35 dB. The decoupling performance has also been validated by current distribution along the radiative elements and magnetic field profiles in a water phantom. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 45B: 183–190, 2015

The ability to produce hyperpolarized noble gases ³He and ¹²⁹Xe has opened up exciting possibilities for pulmonary magnetic resonance imaging (MRI). We have recently built a hyperpolarizer with the goal of using hyperpolarized ³He gas for MRI in neonatal lungs in a dedicated small foot-print 1.5 T MR scanner developed at our institution and sited in our Neonatal Intensive Care Unit. Although hyperpolarized gas imaging can provide unique insights into lung ventilation, acinar microstructure, and gas-exchange dynamics, there is an undiminished need for ¹H MRI of the lung to provide anatomic references, B₁ and B₀ maps, and ¹H images of lung parenchyma. To address this need, we designed, built and tested a novel radiofrequency body coil that provides a high-pass birdcage coil that can be used for both ³He and ¹H frequencies (48.65 and 63.86 MHz, respectively, at 1.5 T). To switch between frequencies, the birdcage coil has a large mechanical actuator that simultaneously changes the capacitance between every rung of the birdcage. Advantages of this coil design include: 1) quadrature excitation and reception at the ³He and ¹H frequencies, 2) identical B₁ field maps for ³He and ¹H imaging, 3) excellent signal-to-noise ratio and B₁ homogeneity at both frequencies, and 4) rapid (10–20 s) switching times between ³He and ¹H operation. This report provides details of the coil's design and fabrication. Images of hyperpolarized ³He and ¹H in phantoms and ex vivo rabbit lungs demonstrate the image quality obtained with the coil. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 45B: 174–182, 2015