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Radio-frequency Transmit Coils and Imaging Techniques for TRASE MRI

  • Author / Creator
    Sun, Hongwei
  • Magnetic Resonance Imaging (MRI) is a powerful diagnostic technique capable of revealing the internal structure and function of the human body. However, access to commercial MRI systems is restricted due to very high purchase, installation and running costs. These MRI scanners are generally confined to hospital suites, thus precluding mobile operation under many circumstances. Therefore, there is a growing interest in new MRI alternatives to mainstream MRI systems for affordable use by under-served patient populations, including those in smaller and remote centers, and in portable scenarios such as surgical intervention, triage, primary care suites, and even in space. An alternative MRI encoding principle known as Transmit Array Spatial Encoding (TRASE), which functions entirely without using magnetic field gradients was proposed in 2010. This is highly advantageous because much of the cost and complexity of MRI systems is associated with the generation of the switched magnetic field gradients needed for image encoding. TRASE is also particularly suitable for use in low-field magnets, enabling further cost savings. Compared with other novel MRI approaches, this method has the potential to generate sub-mm clinical image resolution, using the novel radio-frequency (RF) technology. TRASE has been very well received, most notably being featured in "Research Highlights" in Nature in 2013. Up to the point where I started my research, 2D TRASE encoded in vivo MRI images of human wrist have been obtained, showing promise for its clinical use. However, the obtained in vivo image resolution was limited due to an inefficient RF system design, rendering a long scan duration vulnerable to tissue MR signal losses. Therefore, the goal of this project is to 1) design and optimize of a new RF transmit coil geometry, which allows an efficient MRI encoding using TRASE principles; 2) based on the new coil design, develop necessary RF and imaging techniques to combine multiple RF transmit coils, achieving a multi-dimensional TRASE encoding even for short T2 samples. A new twisted solenoid TRASE RF coil was first designed, optimized and constructed. This new type of coil is ideal for TRASE application due to its high efficiency, uniformity, and large usable imaging volume relative to its aperture. By rotating the coil former, this twisted solenoid is capable of encoding in any transverse direction, so a pair of such coils were combined with regular solenoids attached to accomplish geometric coil decoupling, obtaining high-resolution MRI images. To achieve a 2D TRASE encoding, the geometrically-decoupled twisted solenoid pair was combined with a saddle coil, forming a 2D TRASE three-coil set. One main challenge for this combination is the effective enable/disable function of each coil, otherwise the strong interactions (coupling) among coils may, while transmitting with the primary coil, lead to cross-talk among other coil(s), causing severe MRI image distortions. Such image artifacts were indeed observed in MRI experiments with the geometrically-decoupled three-coil set, indicating that 2D TRASE is extremely sensitive to coil coupling, and to obtain a clean 2D TRASE image, additional coil decoupling capacity is required. With such in-depth understanding, a low-field, inexpensive parallel transmit system was developed upon a waveform generator chip AD9106 and a controller Cyclone V System-on- Chip. In bench measurements such system can generate multiple independent RF drive signals with user-defined amplitudes and phases, being implemented as an active decoupling approach. This new RF transmit system is expected to be combined with the previous geometric decoupling techniques to significantly minimize interactions among the three-coil set, making a successful 2D TRASE MRI encoding.

  • Subjects / Keywords
  • Graduation date
    Fall 2020
  • Type of Item
  • Degree
    Doctor of Philosophy
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