252. Prediction of subject-specific SAR distribution in MSK MR exam at 7 T

Purpose we predict SAR during MRI exam using a 7 T 1H 298 MHz eight-channel degenerate birdcage coil1 combining SAR simulations with subject-specific measured (RF) maps. Materials and Methods We simulated the coil1 in CST MW Suite, loaded by a model of human knee (Fig. 1, top). was calculated in an axial slice crossing the patella. The maximum local SAR for an Axial “Zero” Time-of-Echo (ZTE) sequence “SILENT”2 was calculated. We acquired maps of an adult (female) knee with a Bloch-Siegert sequence on 7 axial slices, centered on the same slice of the simulation, on a GE MR950 7T human system. For each slice a coefficient C, proportional to avg, was used to scale the SAR simulated3. Results Fig. 1 shows: bottom left, simulated magnitude; bottom center, local SAR for an input of 1 W per channel; bottom right, simulated magnitude for a FA = 90° (length = 3.2 ms) sinc-pulse in the slice previously chosen. Fig. 2 shows the subject-specific measured for a FA = 90° sinc-pulse. The predicted SAR obtained with scaled maps are 0.50 W/kg (global) and 3.68 W/kg (maximum). Conclusions we obtained a good agreement between simulated and measured in vivo maps, and we were able to calculate the distribution of SAR exposure, a safety MRI parameter not available in current exams, where only global SAR is provided, combining simulations and subject-specific measurements. Limits on global and local SAR (20 W/kg) were met for this sequence [1], [2], [3]

Musculoskeletal MRI at 7 T: do we need more or is it more than enough?

Ultra-high field magnetic resonance imaging (UHF-MRI) provides important diagnostic improvements in musculoskeletal imaging. The higher signal-to-noise ratio leads to higher spatial and temporal resolution which results in improved anatomic detail and higher diagnostic confidence. Several methods, such as T2, T2*, T1rho mapping, delayed gadolinium-enhanced, diffusion, chemical exchange saturation transfer, and magnetisation transfer techniques, permit a better tissue characterisation. Furthermore, UHF-MRI enables in vivo measurements by low-γ nuclei (23Na, 31P, 13C, and 39K) and the evaluation of different tissue metabolic pathways. European Union and Food and Drug Administration approvals for clinical imaging at UHF have been the first step towards a more routinely use of this technology, but some drawbacks are still present limiting its widespread clinical application. This review aims to provide a clinically oriented overview about the application of UHF-MRI in the different anatomical districts and tissues of musculoskeletal system and its pros and cons. Further studies are needed to consolidate the added value of the use of UHF-MRI in the routine clinical practice and promising efforts in technology development are already in progress

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

Magnetic resonance spectroscopic imaging using parallel transmission at 7T

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2011.Cataloged from PDF version of thesis.Includes bibliographical references (p. 123-130).Conventional magnetic resonance spectroscopic imaging (MRSI), also known as phase-encoded (PE) chemical shift imaging (CSI), suffers from both low signal-to-noise ratio (SNR) of the brain metabolites, as well as inflexible tradeoffs between acquisition time and spatial resolution. In addition, although CSI at higher main field strengths, e.g. 7 Tesla (T), offers improved SNR over clinical 1.5T or 3.OT scanners, the realization of these benefits is limited by severe inhomogeneities of the radio frequency (RF) excitation magnetic field (B,+), which is responsible for significant signal variation within the volume of interest (VOI) resulting in spatially dependent SNR losses. The work presented in this dissertation aims to provide the necessary means for using spectroscopic imaging for reliable and robust whole brain metabolite detection and quantification at high main field strengths. It addresses the challenges mentioned above by improving both the excitation and the readout components of the CSI acquisition. The long acquisition times of the PE CSI are significantly shortened (at least 20 fold) by implementing the time-efficient spiral CSI algorithm, while the B1 non-uniformities are corrected for using RF pulses designed for new RF excitation hardware at 7T, so-called parallel transmission (pTx). The B1 homogeneity of the pTx excitations improved at least by a factor of 4 (measured by the normalized spatial standard deviations) compared to conventional single channel transmit systems. The first contribution of this thesis describes the implementation of spiral CSI algorithm for online gradient waveform design and spectroscopic image reconstruction with standard clinical excitation protocols and applied in studies of Late-Onset Tay- Sachs (LOTS), adrenoleukodystrophy (ALD) and brain tumors. A major contribution of this thesis is pTx excitation design for CSI to provide spectral-spatial mitigation of the B1+ inhomogeneities at 7T. Novel pTx RF designs are proposed and demonstrated to yield excellent flip angle mitigation of the brain metabolites, and also enable improved suppression of the undesired water and lipid signals. A major obstacle to the deployment of 7T pTx applications for clinical imaging is the monitoring and management of local specific absorption rate (SAR). This thesis also proposes a pTx SAR monitoring system with real-time RF monitoring and shut-off capabilities.by Borjan Aleksandar Gagoski.Ph.D

RF Coil Design, Imaging Methods and Measurement of Ventilation with 19F C3F8 MRI

This thesis attempts to address the challenge of low signal in fluorinated gas ventilation imaging and optimize imaging methods considering the particular MR parameters of C3F8 by the following approaches: (i) Exploration of coil designs capable of imaging both proton (1H – 63.8 MHz at 1.5T) and fluorine (19F – 60.1 MHz at 1.5T) nuclei involved: 1. The novel use of microelectromechanical systems to switch a single transceive vest coil between the two nuclei was compared to hard-wired or PIN diode switching. 2. The design of an 8 element transceive array with an additional 6 receive only coils for 19F imaging. MEMs was utilized for broadband transmit-receive switching. 3. The amalgamation of a ladder resonator coil with a 6-element transceive array to reduce SAR and improve transmit homogeneity when compared to standard vest coil designs. (ii) Development of imaging methods involved: 1. The optimization and comparison of steady-state free precession and spoiled gradient 19F imaging with C3F8 at 1.5T and 3T. Simulation of the optimal SNR was verified through comprehensive phantom and in-vivo imaging experiments. 2. The investigation of compressed sensing via incoherent sparse k-space sampling to maximize the resolution in 19F ventilation imaging under the constraint of low SNR. Retrospective simulation with hyperpolarized gas images were corroborated by prospective 19F imaging of a 3D printed lung phantom and in-vivo measurements of the lungs. (iii) In-vivo ventilation metrics obtained by 19F ventilation imaging were explored by: 1. The in-vivo mapping of T1 at 1.5T and 3T and mapping of FV and T2* at 3 T. The apparent diffusion coefficient (1.5T) and the evaluation of ventilated volume (1.5T and 3T) was also compared to imaging performed with 129Xe (1.5T). 2. The optimization of imaging for the evaluation of percent ventilated volume with 19F at 3T with a commercial birdcage coil

Brain and Human Body Modeling

This open access book describes modern applications of computational human modeling with specific emphasis in the areas of neurology and neuroelectromagnetics, depression and cancer treatments, radio-frequency studies and wireless communications. Special consideration is also given to the use of human modeling to the computational assessment of relevant regulatory and safety requirements. Readers working on applications that may expose human subjects to electromagnetic radiation will benefit from this book’s coverage of the latest developments in computational modelling and human phantom development to assess a given technology’s safety and efficacy in a timely manner. Describes construction and application of computational human models including anatomically detailed and subject specific models; Explains new practices in computational human modeling for neuroelectromagnetics, electromagnetic safety, and exposure evaluations; Includes a survey of modern applications for which computational human models are critical; Describes cellular-level interactions between the human body and electromagnetic fields

Brain and Human Body Modeling

This open access book describes modern applications of computational human modeling with specific emphasis in the areas of neurology and neuroelectromagnetics, depression and cancer treatments, radio-frequency studies and wireless communications. Special consideration is also given to the use of human modeling to the computational assessment of relevant regulatory and safety requirements. Readers working on applications that may expose human subjects to electromagnetic radiation will benefit from this book’s coverage of the latest developments in computational modelling and human phantom development to assess a given technology’s safety and efficacy in a timely manner. Describes construction and application of computational human models including anatomically detailed and subject specific models; Explains new practices in computational human modeling for neuroelectromagnetics, electromagnetic safety, and exposure evaluations; Includes a survey of modern applications for which computational human models are critical; Describes cellular-level interactions between the human body and electromagnetic fields

Robust Magnetic Resonance Imaging of Short T2 Tissues

Tissues with short transverse relaxation times are defined as ‘short T2 tissues’, and short T2 tissues often appear dark on images generated by conventional magnetic resonance imaging techniques. Common short T2 tissues include tendons, meniscus, and cortical bone. Ultrashort Echo Time (UTE) pulse sequences can provide morphologic contrasts and quantitative maps for short T2 tissues by reducing time-of-echo to the system minimum (e.g., less than 100 us). Therefore, UTE sequences have become a powerful imaging tool for visualizing and quantifying short T2 tissues in many applications. In this work, we developed a new Flexible Ultra Short time Echo (FUSE) pulse sequence employing a total of thirteen acquisition features with adjustable parameters, including optimized radiofrequency pulses, trajectories, choice of two or three dimensions, and multiple long-T2 suppression techniques. Together with the FUSE sequence, an improved analytical density correction and an auto-deblurring algorithm were incorporated as part of a novel reconstruction pipeline for reducing imaging artifacts. Firstly, we evaluated the FUSE sequence using a phantom containing short T2 components. The results demonstrated that differing UTE acquisition methods, improving the density correction functions and improving the deblurring algorithm could reduce the various artifacts, improve the overall signal, and enhance short T2 contrast. Secondly, we applied the FUSE sequence in bovine stifle joints (similar to the human knee) for morphologic imaging and quantitative assessment. The results showed that it was feasible to use the FUSE sequence to create morphologic images that isolate signals from the various knee joint tissues and carry out comprehensive quantitative assessments, using the meniscus as a model, including the mappings of longitudinal relaxation (T1) times, quantitative magnetization transfer parameters, and effective transverse relaxation (T2*) times. Lastly, we utilized the FUSE sequence to image the human skull for evaluating its feasibility in synthetic computed tomography (CT) generation and radiation treatment planning. The results demonstrated that the radiation treatment plans created using the FUSE-based synthetic CT and traditional CT data were able to present comparable dose calculations with the dose difference of mean less than a percent. In summary, this thesis clearly demonstrated the need for the FUSE sequence and its potential for robustly imaging short T2 tissues in various applications

Subject-specific knee SAR prediction using a degenerate birdcage at 7T

We performed a study to predict global and local subject-specific Specific Absorption Rate (SAR) exposure in 7T Musculoskeletal (MSK) acquisition sequences. Such prediction, that is not available in current Magnetic Resonance (MR) exams, is possible combining sequences SAR exposure simulated on the generic anatomical model with subject-specific measured B+1 maps. The procedure has been implemented with a degenerate birdcage coil specifically designed for knee exams. Results show a good agreement between simulated B+1 and B+1 magnitude measure in-vivo, while values of global and local SAR calculated are below the recommended limit, respectively 8 W/kg and 20 W/kg