309 research outputs found
Double Cross Magnetic Wall Decoupling for Quadrature Transceiver RF Array Coils using Common-Mode Differential-mode Resonators
In contrast to linearly polarized RF coil arrays, quadrature transceiver coil
arrays are capable of improving signal-to-noise ratio (SNR), spatial resolution
and parallel imaging performance. Owing to a reduced excitation power, low
specific absorption rate can be also obtained using quadrature RF coils.
However, due to the complex nature of their structure and their electromagnetic
proprieties, it is challenging to achieve sufficient electromagnetic decoupling
while designing multichannel quadrature RF coil arrays, particularly at
ultrahigh fields. In this work, we proposed a double cross magnetic wall
decoupling for quadrature transceiver RF arrays and implemented the decoupling
method on common-mode differential mode quadrature (CMDM) quadrature
transceiver arrays at ultrahigh field of 7T. The proposed magnetic decoupling
wall comprised of two intrinsic decoupled loops is used to reduce the mutual
coupling between all the multi-mode current present in the quadrature CMDM
array. The decoupling network has no physical connection with the CMDMs' coils
giving leverage over size adjustable RF arrays. In order to validate the
feasibility of the proposed cross magnetic decoupling wall, systematic studies
on the decoupling performance based on the impedance of two intrinsic loops are
numerically performed. A pair of quadrature transceiver CMDMs is constructed
along with the proposed decoupling network and their scattering matrix is
characterized using a network analyzer. The measured results show all the
current modes coupling are concurrently suppressed using the proposed cross
magnetic wall. Moreover, field distribution, and SNR intensity are numerically
obtained for a well-decoupled 8-channel quadrature knee-coil array.Comment: 9 pages, 10 Figure
Electric Field and SAR Reduction in High Impedance RF Arrays by Using High Permittivity Materials for 7T MR Imaging
Higher frequencies and shorter wavelengths present significant design issues
at ultra-high fields, making multi-channel array setup a critical component for
ultra-high field MR imaging. The requirement for multi-channel arrays, as well
as ongoing efforts to increase the number of channels in an array, are always
limited by the major issue known as inter-element coupling. This coupling
affects the current and field distribution, noise correlation between channels,
and frequency of array elements, lowering imaging quality and performance. To
realize the full potential of UHF MRI, we must ensure that the coupling between
array elements is kept to a minimum. High-impedance coils allow array systems
to completely realize their potential by providing optimal isolation while
requiring minimal design modifications. These minor design changes, which
demand the use of low capacitance on the conventional loop to induce elevated
impedance, result in a significant safety hazard that cannot be overlooked.
High electric fields are formed across these low capacitance lumped elements,
which may result in higher SAR values in the imaging subject, depositing more
power and, ultimately, providing a greater risk of tissue heating-related
injury to the human sample. We propose an innovative method of utilizing
high-dielectric material to effectively reduce electric fields and SAR values
in the imaging sample while preserving the B1 efficiency and inter-element
decoupling between the array elements to address this important safety concern
with minimal changes to the existing array design comprising high-impedance
coils.Comment: 12 pages, 18 figures, 2 table
Dual-tuned Coaxial-transmission-line RF coils with Independent Tuning Capabilities for X-nuclear Metabolic MRS Imaging at Ultrahigh Magnetic Fields
Information on the metabolism of tissues in both healthy and diseased states
has potential for detecting tumors, neurodegeneration diseases, diabetes, and
many metabolic disorders in biomedical studies. Hyperpolarized carbon-13
magnetic resonance imaging (13C-HPMRI) and deuterium metabolic imaging (2H-DMI)
are two emerging X-nuclei used as practical imaging tools to investigate tissue
metabolism. However due to their low gyromagnetic ratios ( = 10.7
MHz/T; = 6.5 MHz/T) and natural abundance, such method required
the use of a sophisticated dual-tuned radio frequency (RF) coil where the
X-nucleus signal is associated with the proton signal used for anatomical
reference. Here, we report a dual-tuned coaxial transmission line (CTL) RF coil
agile for metabolite information operating at 7T with independent tuning
capability. Analysis based on full-wave simulation has demonstrated how both
resonant frequencies can be individually controlled by simply varying the
constituent of the design parameters. A broadband tuning range capability is
obtained, covering most of the X-nucleus signal, especially the 13C and 2H
spectra at 7T. Numerical results has demonstrated the effectiveness of the
magnetic field produced by the proposed dual-tuned 1H/13C and 1H/2H CTLs RF
coils. Furthermore, in order to validate the feasibility of the proposed
design, both dual-tuned CTLs prototypes are designed and fabricated using a
semi-flexible RG-405 .086" coaxial cable and bench test results (scattering
parameters and magnetic field efficiency/distributions) are successfully
obtained.Comment: 9 pages, 7 figure
The UTE and ZTE Sequences at Ultra-High Magnetic Field Strengths: A Survey
UTE (Ultrashort Echo Time) and ZTE (Zero Echo Time) sequences have been
developed to detect short T2 relaxation signals coming from regions that are
unable to be detected by conventional MRI methods. Due to the high
dipole-dipole interactions in solid and semi-solid tissues, the echo time
generated is simply not enough to produce a signal using conventional imaging
method, often leading to void signal coming from the discussed areas. By the
application of these techniques, solid and semi-solid areas can be imaged which
can have a profound impact in clinical imaging. High and Ultra-high field
strength (UHF) provides a vital advantage in providing better sensitivity and
specificity of MR imaging. When coupled with the UTE and ZTE sequences, the
image can recover void signals as well as a much-improved signal quality. To
further this strategy, secondary data from various research tools was obtained
to further validate the research while addressing the drawbacks to this
approach. It was found that UTE and ZTE sequences coupled with some techniques
such as qualitative imaging and new trajectories are very crucial for accurate
image depiction of the areas of the musculoskeletal system, neural system, lung
imaging and dental imaging
On the subjective acceptance during cardiovascular magnetic resonance imaging at 7.0 Tesla
PURPOSE: This study examines the subjective acceptance during UHF-CMR in a cohort of healthy volunteers who underwent a cardiac MR examination at 7.0T. METHODS: Within a period of two-and-a-half years (January 2012 to June 2014) a total of 165 healthy volunteers (41 female, 124 male) without any known history of cardiac disease underwent UHF-CMR. For the assessment of the subjective acceptance a questionnaire was used to examine the participants experience prior, during and after the UHF-CMR examination. For this purpose, subjects were asked to respond to the questionnaire in an exit interview held immediately after the completion of the UHF-CMR examination under supervision of a study nurse to ensure accurate understanding of the questions. All questions were answered with "yes" or "no" including space for additional comments. RESULTS: Transient muscular contraction was documented in 12.7% of the questionnaires. Muscular contraction was reported to occur only during periods of scanning with the magnetic field gradients being rapidly switched. Dizziness during the study was reported by 12.7% of the subjects. Taste of metal was reported by 10.1% of the study population. Light flashes were reported by 3.6% of the entire cohort. 13% of the subjects reported side effects/observations which were not explicitly listed in the questionnaire but covered by the question about other side effects. No severe side effects as vomiting or syncope after scanning occurred. No increase in heart rate was observed during the UHF-CMR exam versus the baseline clinical examination. CONCLUSIONS: This study adds to the literature by detailing the subjective acceptance of cardiovascular magnetic resonance imaging examinations at a magnetic field strength of 7.0T. Cardiac MR examinations at 7.0T are well tolerated by healthy subjects. Broader observational and multi-center studies including patient cohorts with cardiac diseases are required to gain further insights into the subjective acceptance of UHF-CMR examinations
Ultrahigh Field Functional Magnetic Resonance Electrical Impedance Tomography (fMREIT) in Neural Activity Imaging
abstract: A direct Magnetic Resonance (MR)-based neural activity mapping technique with high spatial and temporal resolution may accelerate studies of brain functional organization.
The most widely used technique for brain functional imaging is functional Magnetic Resonance Image (fMRI). The spatial resolution of fMRI is high. However, fMRI signals are highly influenced by the vasculature in each voxel and can be affected by capillary orientation and vessel size. Functional MRI analysis may, therefore, produce misleading results when voxels are nearby large vessels. Another problem in fMRI is that hemodynamic responses are slower than the neuronal activity. Therefore, temporal resolution is limited in fMRI. Furthermore, the correlation between neural activity and the hemodynamic response is not fully understood. fMRI can only be considered an indirect method of functional brain imaging.
Another MR-based method of functional brain mapping is neuronal current magnetic resonance imaging (ncMRI), which has been studied over several years. However, the amplitude of these neuronal current signals is an order of magnitude smaller than the physiological noise. Works on ncMRI include simulation, phantom experiments, and studies in tissue including isolated ganglia, optic nerves, and human brains. However, ncMRI development has been hampered due to the extremely small signal amplitude, as well as the presence of confounding signals from hemodynamic changes and other physiological noise.
Magnetic Resonance Electrical Impedance Tomography (MREIT) methods could have the potential for the detection of neuronal activity. In this technique, small external currents are applied to a body during MR scans. This current flow produces a magnetic field as well as an electric field. The altered magnetic flux density along the main magnetic field direction caused by this current flow can be obtained from phase images. When there is neural activity, the conductivity of the neural cell membrane changes and the current paths around the neurons change consequently. Neural spiking activity during external current injection, therefore, causes differential phase accumulation in MR data. Statistical analysis methods can be used to identify neuronal-current-induced magnetic field changes.Dissertation/ThesisDoctoral Dissertation Biomedical Engineering 201
Detailing radio frequency heating induced by coronary stents: a 7.0 tesla magnetic resonance study
The sensitivity gain of ultrahigh field Magnetic Resonance (UHF-MR) holds the promise to enhance spatial and temporal resolution. Such improvements could be beneficial for cardiovascular MR. However, intracoronary stents used for treatment of coronary artery disease are currently considered to be contra-indications for UHF-MR. The antenna effect induced by a stent together with RF wavelength shortening could increase local radiofrequency (RF) power deposition at 7.0 T and bears the potential to induce local heating, which might cause tissue damage. Realizing these constraints, this work examines RF heating effects of stents using electro-magnetic field (EMF) simulations and phantoms with properties that mimic myocardium. For this purpose, RF power deposition that exceeds the clinical limits was induced by a dedicated birdcage coil. Fiber optic probes and MR thermometry were applied for temperature monitoring using agarose phantoms containing copper tubes or coronary stents. The results demonstrate an agreement between RF heating induced temperature changes derived from EMF simulations versus MR thermometry. The birdcage coil tailored for RF heating was capable of irradiating power exceeding the specific-absorption rate (SAR) limits defined by the IEC guidelines by a factor of three. This setup afforded RF induced temperature changes up to +27 K in a reference phantom. The maximum extra temperature increase, induced by a copper tube or a coronary stent was less than 3 K. The coronary stents examined showed an RF heating behavior similar to a copper tube. Our results suggest that, if IEC guidelines for local/global SAR are followed, the extra RF heating induced in myocardial tissue by stents may not be significant versus the baseline heating induced by the energy deposited by a tailored cardiac transmit RF coil at 7.0 T, and may be smaller if not insignificant than the extra RF heating observed under the circumstances used in this study
Accelerated fast spin-echo magnetic resonance imaging of the heart using a self-calibrated split-echo approach
PURPOSE: Design, validation and application of an accelerated fast spin-echo (FSE) variant that uses a split-echo approach for self-calibrated parallel imaging. METHODS: For self-calibrated, split-echo FSE (SCSE-FSE), extra displacement gradients were incorporated into FSE to decompose odd and even echo groups which were independently phase encoded to derive coil sensitivity maps, and to generate undersampled data (reduction factor up to R = 3). Reference and undersampled data were acquired simultaneously. SENSE reconstruction was employed. RESULTS: The feasibility of SCSE-FSE was demonstrated in phantom studies. Point spread function performance of SCSE-FSE was found to be competitive with traditional FSE variants. The immunity of SCSE-FSE for motion induced mis-registration between reference and undersampled data was shown using a dynamic left ventricular model and cardiac imaging. The applicability of black blood prepared SCSE-FSE for cardiac imaging was demonstrated in healthy volunteers including accelerated multi-slice per breath-hold imaging and accelerated high spatial resolution imaging. CONCLUSION: SCSE-FSE obviates the need of external reference scans for SENSE reconstructed parallel imaging with FSE. SCSE-FSE reduces the risk for mis-registration between reference scans and accelerated acquisitions. SCSE-FSE is feasible for imaging of the heart and of large cardiac vessels but also meets the needs of brain, abdominal and liver imaging
Radio Frequency Antenna Designs and Methodologies for Human Brain Computer Interface and Ultrahigh Field Magnetic Resonance Imaging
Brain Computer Interface (BCI) and Magnetic Resonance Imaging (MRI) are two powerful medical diagnostic techniques used for human brain studies. However, wired power connection is a huge impediment for the clinical application of BCI, and most current BCIs have only been designed for immobile users in a carefully controlled environment. For the ultrahigh field (≥7T) MRI, limitations such as inhomogeneous distribution of the transmit field (B1+) and potential high power deposition inside the human tissues have not yet been fully combated by existing methods and are central in making ultrahigh field MRI practical for clinical use. In this dissertation, radio frequency (RF) methods are applied and RF antennas/coils are designed and optimized in order to overcome these barriers. These methods include: 1) designing implanted miniature antennas to transmit power wirelessly for implanted BCIs; 2) optimizing a new 20-channel transmit array design for 7 Tesla MRI neuroimaging applications; and 3) developing and implementing a dual-optimization method to design the RF shielding for fast MRI imaging methods.
First, three miniaturized implanted antennas are designed and results obtained using finite difference time domain (FDTD) simulations demonstrate that a maximum RF power of up to 1.8 miliwatts can be received at 2 GHz when the antennas are implanted at the dura, without violating the government safety regulations. Second, Eigenmode arrangement of the 20-channel transmit coil allows control of RF excitation not only at the XY plane but also along the Z direction. The presented results show the optimized eigenmode could generate 3D uniform transmit B1+ excitations. The optimization results have been verified by in-vivo experiments, and they are applied with different protocol sequences on a Siemens 7 Tesla MRI human whole body scanner equipped with 8 parallel transmit channels. Third, echo planar imaging (EPI), B1+ maps and S matrix measurements are used to verify that the proposed RF shielding can suppress the eddy currents while maintaining the RF characteristics of the transmit coil.
The contributions presented here will provide a long-term and safer power transmission path compared to the wire-connected implanted BCIs and will bring ultrahigh field MRI technology closer to clinical applications
WHOLE BODY AND UPPER EXTREMITY ULTRA-HIGH FIELD MAGNETIC RESONANCE IMAGING: COIL DEVELOPMENT AND CLINICAL IMPLEMENTATION
Since Magnetic Resonance Imaging (MRI)’s introduction into the clinical imaging application arena, MRI has become one of the most promising non-invasive methods for evaluating and identifying body organs in normal and diseased conditions. In the last two decades, a few research groups have been working on addressing the challenges to Ultra-High Field (UHF) imaging (≥ 7 Tesla), such as magnetic field inhomogeneities and elevated Radiofrequency (RF) power absorption through technological developments. In recent years, imaging at 7 Tesla has shown an inherent ability to improve scan time and anatomical resolution.
To address the current challenges associated with UHF imaging, this thesis presents the development of innovative whole body and extremity RF coil systems for 7 Tesla imaging. For body imaging, the transmit (Tx) coil is based on the innovative Tic-Tac-Toe (TTT) design, which possesses a load insensitive characteristic in terms of magnetic and electric field distributions. 7 Tesla homogenous whole-body in-vivo imaging with and without a receive (Rx) only insert array is demonstrated showing excellent anatomical detail.
As a part of upper extremity imaging, we have developed a transverse electromagnetic (TEM) coil as a transmitter in conjunction with an eight channel receive only insert for 7 Tesla hand/forearm imaging. We have acquired a wide variety of different sequences and used post-processing methods to extract specific anatomy from high resolution scans (i.e. nerve and vessels), which in turn has helped in exploring new clinical applications, such as arm transplantation, and has added knowledge to existing ones.
The developed RF coil systems and methodologies not only enhance the fundamental scientific knowledge of RF coil design approaches at high frequencies but they also add to the realm of clinical applications of UHF human imaging
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