Download or read book Design of Radiofrequency Pulses for Parallel Transmit Systems at High field MRI written by Alessandro Sbrizzi and published by . This book was released on 2009 with total page 114 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Design of Multidimensional Large Tip Angle Radiofrequency Pulses for Parallel Transmission in Magnetic Resonance Imaging written by and published by . This book was released on 2007 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Non selective Refocusing Pulse Design in Parallel Transmission for Magnetic Resonance Imaging of the Human Brain at Ultra High Field written by Aurélien Massire and published by . This book was released on 2014 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: In Magnetic Resonance Imaging (MRI), the increase of the static magnetic field strength is used to provide in theory a higher signal-to-noise ratio, thereby improving the overall image quality. The purpose of ultra-high-field MRI is to achieve a spatial image resolution sufficiently high to be able to distinguish structures so fine that they are currently impossible to view in a non-invasive manner. However, at such static magnetic fields strengths, the wavelength of the electromagnetic waves sent to flip the water proton spins is of the same order of magnitude than the scanned object. Interference wave phenomena are then observed, which are caused by the radiofrequency (RF) field inhomogeneity within the object. These generate signal and/or contrast artifacts in MR images, making their exploitation difficult, if not impossible, in certain areas of the body. It is therefore crucial to provide solutions to mitigate the non-uniformity of the spins excitation. Failing this, these imaging systems with very high fields will not reach their full potential.For relevant high field clinical diagnosis, it is therefore necessary to create RF pulses homogenizing the excitation of all spins (here of the human brain), and optimized for each individual to be imaged. For this, an 8-channel parallel transmission system (pTX) was installed in our 7 Tesla scanner. While most clinical MRI systems only use a single transmission channel, the pTX extension allows to simultaneously playing various forms of RF pulses on all channels. The resulting sum of the interference must be optimized in order to reduce the non-uniformity typically seen.The objective of this thesis is to synthesize this type of tailored RF pulses, using parallel transmission. These pulses will have as an additional constraint the compliance with the international exposure limits for radiofrequency exposure, which induces a temperature rise in the tissue. In this sense, many electromagnetic and temperature simulations were carried out as an introduction of this thesis, in order to assess the relationship between the recommended RF exposure limits and the temperature rise actually predicted in tissues.This thesis focuses specifically on the design of all RF refocusing pulses used in non-selective MRI sequences based on the spin-echo. Initially, only one RF pulse was generated for a simple application: the reversal of spin dephasing in the transverse plane, as part of a classic spin echo sequence. In a second time, sequences with very long refocusing echo train applied to in vivo imaging are considered. In all cases, the mathematical operator acting on the magnetization, and not its final state as is done conventionally, is optimized. The gain in high field imaging is clearly visible, as the necessary mathematical operations (that is to say, the rotation of the spins) are performed with a much greater fidelity than with the methods of the state of the art. For this, the generation of RF pulses is combining a k-space-based spin excitation method, the kT-points, and an optimization algorithm, called Gradient Ascent Pulse Engineering (GRAPE), using optimal control.This design is relatively fast thanks to analytical calculations rather than finite difference methods. The inclusion of a large number of parameters requires the use of GPUs (Graphics Processing Units) to achieve computation times compatible with clinical examinations. This method of designing RF pulses has been experimentally validated successfully on the NeuroSpin 7 Tesla scanner, with a cohort of healthy volunteers. An imaging protocol was developed to assess the image quality improvement using these RF pulses compared to typically used non-optimized RF pulses. All methodological developments made during this thesis have contributed to improve the performance of ultra-high-field MRI in NeuroSpin, while increasing the number of MRI sequences compatible with parallel transmission.

Download or read book Parallel Radiofrequency Transmission for 3 Tesla and 7 Tesla Magnetic Resonance Imaging written by Filiz Yetişir and published by . This book was released on 2017 with total page 156 pages. Available in PDF, EPUB and Kindle. Book excerpt: Magnetic resonance imaging (MRI) is a noninvasive imaging technique with high soft tissue contrast. MR scanners are characterized by their main magnetic field strength. Commercially available clinical MR scanners commonly have main field strengths of 1.5 and 3 Tesla. Researchers increasingly explore clinical benefits of higher field strength scanners as they provide higher signal to noise ratio and higher resolution images. On the other hand, higher field strength imaging comes with increased image shading leading to non-uniform image contrast. Moreover, the tissue heating rate due to radiofrequency (RF) energy deposition (also called specific absorption rate or SAR) increases, limiting the imaging speed. Parallel RF transmission (pTx) was proposed to address both of these challenges by optimization of RF pulses transmitted from multiple independent channels simultaneously. However, both the RF pulse design and RF safety management become more complicated with pTx. In this work, a framework to apply pTx to 3T fetal and 7T brain imaging is developed to address the image shading and high SAR issues. Fetal imaging where a large pregnant torso is imaged rapidly to avoid fetal motion artifacts, suffers from similar levels of image shading and imaging limitations by SAR to 7T brain MRI. Hence the same techniques benefit both application domains. First, a SAR constrained pTx RF pulse design technique is developed for slice selective high flip angle imaging which is clinically the most common imaging technique. Next, the performance of the developed technique in reducing SAR and the image contrast non-uniformity is demonstrated through simulations and in phantom experiments for 7T brain imaging. Then, a comprehensive RF safety workflow for an 8 channel pTx system at 7T is developed. Finally, the potential of pTx for fetal imaging at 3T is demonstrated with simulation studies and a protected fetus mode of pTx was created using additional constraints in the RF pulse design. By addressing the two main RF transmission challenges associated with high and ultrahigh field MRI, this work aims to help bring the benefits of 7T brain imaging into routine clinical use and significantly improve the clinical experience for 3T fetal imaging.

Download or read book Applications of Parallel Radiofrequency Transmission to Ultra high field Magnetic Resonance Imaging written by Yan Tong and published by . This book was released on 2021 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Modular Scalable Techniques for Advanced Magnetic Resonance Systems and Radio frequency Instrumentation written by Pascal Pawel Stang and published by . This book was released on 2012 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Magnetic resonance imaging (MRI) is unique among imaging modalities in providing exceptional versatility for examining a wide spectrum of anatomy and physiological functions. Modern MRI systems are moving toward higher magnetic field strengths for increased signal, and depend on rising receiver and transmitter channel counts to enhance speed, image quality, specificity, and safety. This has left researchers in a predicament. Commercial MRI scanners are becoming ever more capable and complex, yet by their nature inevitably lag the leading edge of science, failing to anticipate new research directions. Moreover, their closed proprietary hardware, software, and interfaces frustrate any expansion or adaptation to new techniques and experiments. Developed to address these challenges, the Medusa MRI console is a modular scalable open platform for advanced MRI research and development. Medusa enables MR methods and applications that would have been challenging or impossible using only commercial hardware, and does so with an efficient flexible architecture designed to grow to meet future needs. The Medusa MRI console delivers the complete set of functionality required for MRI including multi-channel radio-frequency (RF) excitation and reception, gradient waveform generation, coil and amplifier gating, and an open software platform for pulse sequencing and experimental development. Key features of Medusa include the use of direct-conversion digital RF components, distributed processing and memory for modularity and flexibility, and USB 2.0 High-Speed (480Mb/s) interfaces to the host PC. Medusa was first tested with the novel Stanford University Pre-Polarized MRI scanner, and is now used regularly for experiments on a diverse range of commercial and custom-built imaging systems. A significant application of Medusa has been the investigation and development of MRI Parallel Transmit techniques (PTx), which hold promise for improving MR safety and high-field image quality. Yet PTx methods demand high-fidelity delivery of RF pulses at kilowatt power levels across multiple channels into an unknown load. Vector Iterative Predistortion (VIP) was developed to address this challenge, using Medusa to encode the non-ideal response from RF amplifiers using sensors and iteratively pre-distort the input to achieve desired output. Even when used with cost-efficient low-fidelity RF power amps, VIP drives errors to below 0.1~dB and +/-1~degree phase, and is capable of correcting time-dynamic memory effects where traditional look-up table methods fail. Accurate RF sensing is essential in any RF control system, and on-coil and in-line vector RF sensing approaches were developed to enable real-time current, power, and impedance measurements. Notably, this monitoring permits not only the hardware characterization and control needed for VIP, but can also detect patient motion, cardiac and respiratory rhythms, and has the potential to assess and improve MR safety for patients with implanted devices.

Download or read book Excitation and Readout Designs for High Field Spectroscopic Imaging written by Joonsung Lee and published by . This book was released on 2011 with total page 90 pages. Available in PDF, EPUB and Kindle. Book excerpt: In this thesis we state and demonstrate solutions to three engineering problems that arise in magnetic resonance imaging RF excitation with parallel transmission (pTx) and magnetic resonance spectroscopic imaging (MRSI). Recent work in parallel RF excitation in MRI has been demonstrated to offer dramatically improved flexibility for manipulation of magnetization preparation for imaging than is feasible with conventional single-channel transmission. We address two design problems that need to be solved before this emerging technology can be deployed in the clinical and research domain of human imaging at high field. First, we demonstrate a method for rapid and robust acquisition of the non-uniform fields of RF excitation due to arrays that are commonly used in pTx at high field. Our method achieves high-fidelity single-slice excitation and reception field mapping in 20 seconds, and we propose ways to extend this to multi-slice mapping in two minutes for twenty slices. A fundamental constraint to the application of pTx is the management of the deposition of power in human tissue, quantified by the specific absorption rate (SAR). The complex behavior of the spatial distribution of SAR in transmission arrays poses problems not encountered in conventional single-channel systems, and we propose a pTx design method to incorporate local SAR constraints within computation times that accommodate pTx pulse design during MRI acquisition of human subjects. Our approach builds on recent work to capture local SAR distribution with much lower computational complexity than a brute-force evaluation, and we demonstrate that this approach can reduce peak local SAR by 20~40% for commonly applied pTx design targets. This thesis focuses on the design of excitation methods for high field system (7T parallel transmit (pTx) system) and fast readout and post-processing methods to reduce the lipid contamination to the brain. The contributions include fast B1+ mapping and pTx RF pulse design with the local SAR constraints for excitation. Regarding the readout method we developed a real time filter design, variable density spiral trajectory, and iterative non-linear reconstruction technique that reduce the lipid contamination. The proposed excitation methods were demonstrated using a 7T pTx system and the readout methods were implemented in a 3T system. Our third contribution addresses a recurring problem in MRSI of the brain, namely strong contaminating artifacts in low signal-to-noise ratio brain metabolite maps due to subcutaneous, high-concentration lipid sources. We demonstrate two methods to address this problems, one during the acquisition stage where a spatial filter is designed based on spatial priors acquired from the subject being scanned, and the second is a post-processing method that applies the brain and lipid source prior for further artifact minimization. These methods are demonstrated to achieve 20~4OdB enhancement of lipid suppression in brain MRSI of human subjects.

Download or read book Ultra High Field Magnetic Resonance Imaging written by Pierre-Marie Robitaille and published by Springer Science & Business Media. This book was released on 2007-12-31 with total page 487 pages. Available in PDF, EPUB and Kindle. Book excerpt: The foundation for understanding the function and dynamics of biological systems is not only knowledge of their structure, but the new methodologies and applications used to determine that structure. This volume in Biological Magnetic Resonance emphasizes the methods that involve Ultra High Field Magnetic Resonance Imaging. It will interest researchers working in the field of imaging.

Download or read book RF Current Source Drive and Decoupling Technique for Parallel Transmit Arrays in Magnetic Resonance Imaging written by Won-Je Lee and published by . This book was released on 2008 with total page 130 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Local and Global SAR Constrained Large Tip Angle 3D KT points Parallel Transmit Pulse Design at 7 T written by Filiz Yetişir and published by . This book was released on 2014 with total page 66 pages. Available in PDF, EPUB and Kindle. Book excerpt: In the last decade, there has been a push towards higher main field strengths in magnetic resonance imaging (MRI) to achieve better contrast and higher signal to noise ratio (SNR). A drawback to current imaging systems at high field is transmit magnetic field inhomogeneity, which degrades both SNR and contrast. This inhomogeneity can be mitigated through radio frequency (RF) pulse design with conventional single-channel RF systems, but at the cost of impractically long pulses. Parallel transmit (pTx) systems with multiple independent and simultaneous RF transmit channels achieve transmit field inhomogeneity mitigation with practical pulse duration, but with potentially higher deposition of RF energy as measured by the specific absorption rate (SAR). In addition, RF pulse design becomes a more challenging task with pTx systems. The electric fields of independent channels can constructively interfere (linearly add up) at points in the body and create local hot spots, whose effect is quantified by local SAR. The peak local SAR inside the body needs to be kept under specified safety limits, hence a large number of constraints are forced upon the RF pulse during the design process. In order to avoid a large number of constraints which complicate and slow down the pulse design, it is possible to limit local SAR indirectly by limiting the RF pulse power. Although this approach results in a simplified and faster RF pulse design process, it is not optimal because pulse power and local SAR are not linearly related. At small tip angles (low RF energy deposition) with spokes trajectory, it has been demonstrated that the control of pulse power does not explicitly control local SAR and simultaneous constraints of pulse power and local SAR are required to yield optimal pulses satisfying system and regulatory limits. At large tip angles (high RF energy deposition) the local SAR constraint is more restricting for RF pulse performance relative to the small tip angle case. Therefore, the ability to design optimal RF pulses for the same constraints becomes more crucial at large tip angle regime. To our best knowledge, none of the existing large tip angle pTx pulse design methods are capable of controlling local SAR directly. In this thesis the small tip angle approach is extended to large tip angle and local SAR, global SAR and pulse power constraints are explicitly incorporated in the design of 3D kT-point large tip angle RF excitation pulses at 7 Tesla. It is demonstrated that the excitation fidelity of the large tip angle pulses improves significantly when local SAR constraint is imposed directly instead of indirectly via control of the peak RF power.

Download or read book An Introduction to Variational Autoencoders written by Diederik P. Kingma and published by . This book was released on 2019-11-12 with total page 102 pages. Available in PDF, EPUB and Kindle. Book excerpt: An Introduction to Variational Autoencoders provides a quick summary for the of a topic that has become an important tool in modern-day deep learning techniques.

Download or read book Transmit Field Pattern Control for High Field Magnetic Resonance Imaging with Integrated RF Current Sources written by Krishna Nagaraj Kurpad and published by . This book was released on 2005 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: The primary design criterion for RF transmit coils for MRI is uniform transverse magnetic (B1) field. Currently, most high frequency transmit coils are designed as periodic, symmetric structures that are resonant at the imaging frequency, as determined by the static magnetic (B0) field strength. These coils are excited by one or more voltage sources. The distribution of currents on the coil elements or rungs is determined by the symmetry of the coil structure. At field strengths of 3T and above, electric properties such as the dielectric constant and conductivity of the load lead to 11 field inhomogeneity due to wavelength effects and perturbation of the coil current distribution from the ideal. The B1 field homogeneity under such conditions may be optimized by adjusting the amplitudes and phases of the currents on the rungs. However, such adjustments require independent control of current amplitudes and phases on each rung of the resonant coil. Due to both the strong coupling among the rungs of a resonant coil and the sensitivity to loading, such independent control would not be possible and B1 homogeneity optimization would involve a time consuming and impractical iterative procedure in the absence of exact knowledge of interactions among coil elements and between the coil and load. This dissertation is based on the work done towards the design and development of a RF current source that drives high amplitude RF current through an integrated array element. The arrangement is referred to as a current element. Independent control of current amplitude and phase on the current elements is demonstrated. A non-resonant coil structure consisting of current elements is implemented and B1 field pattern control is demonstrated. It is therefore demonstrated that this technology would enable effective B1 field optimization in the presence of lossy dielectric loads at high field strengths.

Download or read book Radio frequency Coil Design for High Field Magnetic Resonance Imaging written by Gene Bogdanov and published by . This book was released on 2002 with total page 404 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Modeling Design and Safety of RF Transmitters for High Field MRI written by Jan Paška and published by . This book was released on 2014 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book A 64 Channel Transmit System for Single Echo Acquisition MRI written by Ke Feng and published by . This book was released on 2012 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Magnetic Resonance Imaging (MRI) is considered as a slow imaging technique. Various approaches to accelerate MRI imaging have been explored by researchers in the past decades. Earlier gradient based methods have reached the safety limit. Parallel receiving techniques achieve accelerations by reducing phase encoding steps. Among these methods, SEA Imaging achieved the highest possible acceleration by completely eliminating phase encoding. However, SEA imaging is limited to thin planar slices above the array due to the correction needed for the inherent phase cancellation caused by voxel-sized coils. A phase compensation gradient pulse is used for this correction in SEA imaging. This phase compensation is dependent on slice position and thickness as well as the orientation of the array elements, placing stringent restrictions on SEA imaging, limiting its applications. Converting the SEA system into Transmit / Receive (T/R) mode, which is the main purpose of this study, eliminates the requirement for phase compensation gradient because phase departed during transmit is refocused during receiving. Independent amplitude and phase control of RF pulse for each coil of a SEA array is achieved using a low cost scalable parallel transmit system design. The first 64-channel parallel transmitter for MRI in the world is constructed and tested. Software is also developed to control the phase and amplitude of all the 64 channels of RF excitation pulses independently through National Instruments DAQ system. The system consists of vector modulators controlled by digital controlled potentiometers, two-stage amplifiers and T/R switches on the transmit side. All these are combined with newly designed and constructed preamplifiers and the existing 64-channel parallel receivers on the receive side, leading to the only 64-channel parallel T/R system available for MRI. As a bonus, the system can be easily updated to full Transmit SENSE capability. Furthermore, simulations and images are done to synthesize transmit patterns thanks to the large channel count. Testing results show that the system is capable of 100W per channel simultaneous transmission. Using this system, transmit field can be synthesized by varying the phase and amplitude across channel without traditionally required complicated pulse sequences involving simultaneous RF and gradient fields. Curved slice excitation has conventionally been considered a difficult task for MRI, achievable only through complicated pulses sequences. Using this system and flexible array wrapped around the subject to be imaged, the system is able to excite curved slice using one shot. TR images indicate that the system is capable of high speed surface imaging at 200 frames per second following the surface of a flexible SEA array coil which has not been achieved using other methods in MRI.

Download or read book Documents concernant le film Ma vache et moi 1926 written by and published by . This book was released on with total page 6 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Design of Multi channel Radio frequency Front end for 200mhz Parallel Magnetic Resonance Imaging written by Xiaoqun Liu and published by . This book was released on 2010 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: The increasing demands for improving magnetic resonance imaging (MRI) quality, especially reducing the imaging time have been driving the channel number of parallel magnetic resonance imaging (Parallel MRI) to increase. When the channel number increases to 64 or even 128, the traditional method of stacking the same number of radio-frequency (RF) receivers with very low level of integration becomes expensive and cumbersome. However, the cost, size, power consumption of the Parallel MRI receivers can be dramatically reduced by designing a whole receiver front-end even multiple receiver front-ends on a single chip using CMOS technology, and multiplexing the output signal of each receiver front-end into one channel so that as much hardware resource can be shared by as many channels as possible, especially the digitizer. The main object of this research is focused on the analysis and design of fully integrated multi-channel RF receiver and multiplexing technology. First, different architectures of RF receiver and different multiplexing method are analyzed. After comparing the advantages and the disadvantages of these architectures, an architecture of receiver front-end which is most suitable for fully on-chip multi-channel design is proposed and a multiplexing method is selected. According to this proposed architecture, a four-channel receiver front-end was designed and fabricated using TSMC 0.18Bm technology on a single chip and methods of testing in the MRI system using parallel planar coil array and phase coil array respectively as target coils were presented. Each channel of the receiver front-end includes an ultra low noise amplifier (LNA), a quadrature image rejection down-converter, a buffer, and a low-pass filter (LPF) which also acts as a variable gain amplifier (VGA). The quadrature image rejection downconverter consists of a quadrature generator, a passive mixer with a transimpedance amplifier which converts the output current signal of the passive mixer into voltage signal while acts as a LPF, and a polyphase filter after the TIA. The receiver has an over NF of 0.935dB, variable gain from about 80dB to 90dB, power consumption of 30.8mW, and chip area of 6mm2. Next, a prototype of 4-channel RF receiver with Time Domain Multiplexing (TDM) on a single printed circuit board (PCB) was designed and bench-tested. Then Parallel MRI experiment was carried out and images were acquired using this prototype. The testing results verify the proposed concepts.