Forced Current Excitation in Selectable Field of View Coils for 7T MRI and MRS

High field magnetic resonance imaging (MRI) provides improved signal-to-noise ratio (SNR) which can be translated to higher image resolution or reduced scan time. 7 Tesla (T) breast imaging and 7 T spine imaging are of clinical value, but they are challenging for several reasons: A bilateral breast coil requires the use of closely-spaced elements that are subject to severe mutual coupling which leads to uncontrollable current distribution and non-uniform field pattern; A spine coil at 7T requires a large field of view (FOV) in the z direction and good RF penetration into the human body. Additionally, the ability to switch FOV without the use of expensive high power RF amplifiers is desired in both applications. This capability would allow reconfigurable power distribution and avoid unnecessary heat deposition into human body. Forced-Current Excitation (FCE) is a transmission line-based method that maintains equal current distribution across an array, alleviating mutual coupling effects and allowing current/field replication across a large FOV. At the same time, the nature of this method enables selectable FOV with the inclusion of PIN diodes and a controller. In this doctoral work, the theory of FCE is explained in detail, along with its benefits and drawbacks. Electromagnetic simulation considerations of FCE-driven coils are also discussed. Two FCE-driven coils were designed and implemented: a switchable bilateral/unilateral 7T breast coil, and a segmented dipole for spine imaging at 7T with reconfigurable length. For the breast coil, shielded loop elements were used to form a volume coil, whereas for the spine coil, a segmented dipole was chosen as the final design due to improved RF penetration. Electromagnetic simulations were performed to assist the design of the two coils as well as to predict the SAR (specific absorption rate) generated in the phantom. The coils were evaluated on bench and through MRI experiments in different configurations to validate the design. The switchable breast coil provides uniform excitation in both unilateral and bilateral mode. In unilateral mode, the signal in the contralateral breast is successfully suppressed and higher power is concentrated into the breast of interest; The segmented dipole was compared to a regular dipole with the same length used for 7T spine imaging. The segmented dipole shows a large FOV in the long mode. In the short mode, the residual signal from other part of the dipole is successfully suppressed. The ability to switch FOV and reconfigure the power distribution improves the B1 generated with unit specific absorption rate towards the edge of the dipole, compared to the regular dipole

Forced Current Excitation in Selectable Field of View Coils for 7T MRI and MRS

High field magnetic resonance imaging (MRI) provides improved signal-to-noise ratio (SNR) which can be translated to higher image resolution or reduced scan time. 7 Tesla (T) breast imaging and 7 T spine imaging are of clinical value, but they are challenging for several reasons: A bilateral breast coil requires the use of closely-spaced elements that are subject to severe mutual coupling which leads to uncontrollable current distribution and non-uniform field pattern; A spine coil at 7T requires a large field of view (FOV) in the z direction and good RF penetration into the human body. Additionally, the ability to switch FOV without the use of expensive high power RF amplifiers is desired in both applications. This capability would allow reconfigurable power distribution and avoid unnecessary heat deposition into human body. Forced-Current Excitation (FCE) is a transmission line-based method that maintains equal current distribution across an array, alleviating mutual coupling effects and allowing current/field replication across a large FOV. At the same time, the nature of this method enables selectable FOV with the inclusion of PIN diodes and a controller. In this doctoral work, the theory of FCE is explained in detail, along with its benefits and drawbacks. Electromagnetic simulation considerations of FCE-driven coils are also discussed. Two FCE-driven coils were designed and implemented: a switchable bilateral/unilateral 7T breast coil, and a segmented dipole for spine imaging at 7T with reconfigurable length. For the breast coil, shielded loop elements were used to form a volume coil, whereas for the spine coil, a segmented dipole was chosen as the final design due to improved RF penetration. Electromagnetic simulations were performed to assist the design of the two coils as well as to predict the SAR (specific absorption rate) generated in the phantom. The coils were evaluated on bench and through MRI experiments in different configurations to validate the design. The switchable breast coil provides uniform excitation in both unilateral and bilateral mode. In unilateral mode, the signal in the contralateral breast is successfully suppressed and higher power is concentrated into the breast of interest; The segmented dipole was compared to a regular dipole with the same length used for 7T spine imaging. The segmented dipole shows a large FOV in the long mode. In the short mode, the residual signal from other part of the dipole is successfully suppressed. The ability to switch FOV and reconfigure the power distribution improves the B1 generated with unit specific absorption rate towards the edge of the dipole, compared to the regular dipole

Radiofrequency Coil Design for Magnetic Resonance Imaging and Spectroscopy of the Human Breast and Models of Duchenne Muscular Dystrophy

The known value of magnetic resonance imaging and spectroscopy (MRI/MRS) to detect and monitor disease with high sensitivity has driven researchers and clinicians to continually push boundaries beyond clinically-standard ^1H MRI. This has led to many advancements including high field MRI and MRS and non^1H MRI and MRS. MRI of the breast is commonly used as a supplemental tool to mammography throughout various stages of diseases. Specifically, benefits of high field MRI and the use of array coils have enabled studies such as dynamic contrast enhanced MRI (DCEMRI) and diffusion weighted imaging (DWI) with high spatial and/or temporal resolution. These studies provide additional information about morphology and kinetics of breast lesions to distinguish between malignant and benign tumors with improved diagnostic accuracy. MR imaging and spectroscopy have been used to study progressive muscular degenerative disorders, such as Duchenne muscular dystrophy (DMD). ^1H MRI has been used to assess skeletal muscle composition, such as muscle fat-fraction. Additionally, ^23Na imaging and ^31P spectroscopy have provided supplementary information pertaining to tissue viability and metabolic function to evaluate disease even before any measurable change in muscle composition has occurred. This dissertation covers the construction and characterization of radiofrequency (RF) coils and corresponding hardware to enable studies pertaining to breast cancer and DMD. A 32-channel breast array and modified forced-current excitation (FCE) volume coil was constructed for bilateral breast imaging at 7T. Ultimately, coil performance was evaluated based on improvements in SNR and feasibility of accelerated, high spatial resolution imaging. Two double-tuned birdcage coils (^1H/^23Na and ^1H/^31P) were constructed for imaging and spectroscopy at 4.7T of rectus femoris muscles excised from genetically-homologous animal models of DMD, or golden retriever muscular dystrophy (GRMD). Initially, coil performance was evaluated on various phantoms for homogeneity and ability to distinguish between various biological concentrations of sodium and phosphorus. The coils were then used to collect data from a variety of GRMD tissue samples in order to characterize biomarkers corresponding to age/disease progression. Overall, this work has contributed hardware advancements to enable MRI/MRS studies beyond clinically-standard ^1H MRI to assess disease

ADVANCES IN ULTRA-HIGH FIELD 7 TESLA (T) HUMAN MRI: DESIGN AND METHODOLOGY OF TRANSMIT (TX) AND RECEIVE (RX) RADIO FREQUENCY (RF) COILS

Magnetic Resonance Imaging (MRI) is known as non-invasive imaging modality that provides superb anatomical soft tissue details. Over a decade, studies in ultrahigh field (UHF) MRI have been widely carried out in order to further improve the technology as well as understand the diseases. RF coil design and performance studies have been widely conducted to fully utilize the advantages that are given by UHF MRI. The aim of the design is typically producing uniform electromagnetic (EM) field distribution within the volume of interest while lowering the specific absorption rate (SAR) for Tx array and preserving/enhancing high signal to noise ratio (SNR) with Rx array. Clinically available MRI such as 1.5 Tesla (T) and 3T, uses a whole body RF coil that is embedded in the scanner as the field exciter/transmitter, and separate receivers for different parts of the body. This body coil is not available in UHF MRI due to shorter wavelength resulting in inhomogeneous EM field distributions and the large size of the body coil challenges to tune and match the coil at resonant frequency of the UHF MRI, ~300MHz as well as the size of the UHF MRI market is too small that the vendors are not actively investing in the body coil development. Due to the inexistence of the body coil at UHF MRI, it is critical to develop a RF transmit coil systems that produce a uniform B1+ field (clockwise rotating circularly polarized transverse magnetic field that is responsible for excitation) and low specific absorption rate (SAR). Aim of this thesis is to develop and evaluate new design of transmit (Tx) arrays and receive (Rx) arrays for breast and extremity (knee and ankle/foot) MR imaging at 7T. This thesis consists of several articles associated with breast and extremity MR imaging, namely: 1) Experimental and numerical analysis of B1+ field and SAR with a new transmit array design for 7T breast MRI (published as a first author), 2) Development of a 7T RF coil system for breast imaging (published as a first author), 3) Two way - split RF array development for knee MRI at 7T (under preparation as a main author), 4) A new RF Transmit Coil for Foot and Ankle Imaging at 7T MRI (published as a second author), and Overall, the work of this thesis contributes toward the understanding RF coil design and evaluation for UHF MRI human imaging

Engineering Parallel Transmit/Receive Radio-Frequency Coil Arrays for Human Brain MRI at 7 Tesla

Magnetic resonance imaging is widely used in medical diagnosis to obtain anatomical details of the human body in a non-invasive way. Clinical MR scanners typically operate at a static magnetic field strength (B0) of 1.5T or 3T. However, going to higher field is of great interest since the signal-to-noise ratio is proportional to B0. Therefore, higher image resolution and better contrast between the human tissues could be achieved. Nevertheless, new challenges arise when increasing B0. The wavelength associated with the radio-frequency field B1+ has smaller dimensions - approx. 12 cm for human brain tissues - than the human brain itself (20 cm in length), the organ of interest in this thesis. The main consequence is that the transmit field distribution pattern (B1+) is altered and the final MR images present bright and dark signal spots. These effects prevent the ultra-high field MR scanners (>= 7T) to be used for routine clinical diagnosis. Parallel-transmit is one approach to address these new challenges. Instead of using an RF coil connected to a single power input as it is commonly done at lower magnetic fields, multiple RF coils are used with independent power inputs. The subsequent distinct RF signals can be manipulated separately, which provides an additional degree of freedom to generate homogeneous B1+-field distributions over large or specific regions in the human body. A transmit/receive RF coil array optimized for whole-brain MR imaging was developed and is described in this thesis. Dipoles antennas were used since they could provide a large longitudinal (vertical axis-head to neck) coverage and high transmit field efficiency. Results demonstrated a complete coverage of the human brain, and particularly high homogeneity over the cerebellum. However, since the receive sensitivity over large field-of-views is related to the number of channels available to detect the NMR signal, the next work was to add a 32-channel receive loop coil array to the transmit coil array. The complete coverage of the human brain was assessed with a substantial increase in signal-to-noise compared to the transmit/receive dipole coil array alone. Moreover, acquisition time was shortened since higher acceleration factors could be used. To optimize the individual RF fields and generate an homogeneous B1+-field, a method was developed making use of the particle-swarm algorithm. A user-friendly graphical interface was implemented. Good homogeneity could be achieved over the whole-brain after optimization with the coil array built in this study. Moreover, the optimization was shown to be robust across multiple subjects. The last project was focused on the single transmit system. Local volume coils (single transmit) present pronounced transmit field inhomogeneities in specific regions of the human brain such as the temporal lobes. A widely used approach to address locally these challenges is to add dielectric pads inside the volume coils to enhance the local transmit field efficiency. It was shown in this thesis that constructing dedicated surface coils is a valuable alternative to the dielectric pads in terms of transmit field efficiency and MR spectroscopy results. Two RF coil setups were developed for the temporal and frontal lobes of the human brain, respectively. This thesis provides extensive insight on MR engineering of RF coils at ultra-high field and the potential of parallel-transmit to address the future needs in clinical applications