Advances in Portable TRASE MRI
Creators: Purchase, Aaron R.
Publisher: name: University of Alberta Library
Year: 2022

<p>Magnetic resonance imaging (MRI) is a valuable medical diagnostic technique due to its exceptional soft tissue contrast. Unfortunately, standard clinical MRI systems have limited accessibility worldwide, especially in developing countries, due to high costs, high weight, large physical dimensions, and maintenance and operating complexity. Standard MRI systems are currently inaccessible to over half the world&#39;s population. In MRI accessible regions, they are the bottlenecks of the clinical workflow due to high patient volume and low scanner numbers. Portable, low-cost, and clinically relevant MRI systems have many applications worldwide, such as alleviating accessibility issues in developing countries, rural areas, emergency rooms and medical clinics for point-of-care diagnostic imaging. In addition, mobile MRI systems may find application in ground and air ambulance services, in the military, and even beyond Earth on long-duration spaceflight.</p>

<p>Standard MRI systems use gradients of the main magnetic field for spatial encoding, requiring expensive and bulky hardware. A novel MRI encoding method, called Transmit Array Spatial Encoding (TRASE), uses the phase of the radiofrequency (RF) field for spatial encoding signal rather than applying a switching B0 gradient. TRASE removes the need for the entire standard B0 gradient system and relaxes the main magnet&#39;s homogeneity requirement, leading to compact, lower cost, and portable MRI systems. However, the first in vivo TRASE MR wrist images, obtained in 2013, were acquired using an immobile 0.2 T magnet and with low spatial resolution due to RF hardware limitations. The objectives of this research were to (a) design and construct a new RF amplifier to improve TRASE spatial resolution; (b) design and construct a new portable magnet for TRASE to reduce the overall weight, size and cost of the system; (c) investigate magnetic field modifications allowing in vivo TRASE imaging on the constructed portable magnet; and (d) design and construct an accurate magnet rotation system that would simplify the 2D TRASE MRI hardware and acquisition technique.</p>

<p>An RF power amplifier is an essential component in all MRI systems. Unfortunately, no commercial amplifier exists to fulfil the needs of the TRASE MRI technique, requiring a high duty cycle, high RF output power and independently controlled multi-channel capability. Therefore, we designed and constructed an RF power amplifier and tested it on the bench. In addition, the amplifier performance was tested using a 0.22 T MRI magnet with a twisted solenoid and saddle RF coil combination capable of single-axis TRASE. We showed that the amplifier is capable of sequential, dual-channel operation up to 50% duty cycle, 1 kW peak output per channel and highly stable 100 us RF pulse trains. Furthermore, high spatial resolution one-dimensional TRASE was obtained with the power amplifier to demonstrate its capability.</p>

<p>Although TRASE resolution was improved with the new RF amplifier, the main magnet prevents portability and has high associated costs. Recently designed Halbach magnets, made of permanent magnet blocks distributed around a cylinder, used for portable MRI systems are much lighter and more compact than standard biplanar permanent or superconductive magnets. However, improved designs and manufacturing techniques aiming at a lower weight and smaller external size are of continuing interest, especially for space flight applications. In this work, we designed and constructed a 67 mT Halbach magnet with a very low aspect-ratio (length per inner diameter ~ 1:1) that produces almost identical homogeneity (11,152 ppm) as simulations (11,451 ppm) within a 12.7 cm diameter, 1 cm long cylinder region of interest (ROI). The magnet support structure was 3D printed ring-by-ring and assembled coaxially. The final magnet weight is only 25 kilograms and may be disassembled for transportation.</p>

<p>Although the constructed Halbach magnet is compact and portable, the remaining field inhomogeneity is not well suited to slice selection using the twisted-solenoid TRASE RF coils. Therefore, the bare Halbach magnet&#39;s field requires adjustments for in-vivo TRASE MRI. As a first approach to field adjustments, a simulation study was completed to determine the feasibility and performance of various permanent magnet block configurations used as a shim array to achieve a desired target field in the ROI. Although the presented shim arrays would be inexpensive and straightforward to manufacture, excitation volumes are always present outside the ROI, requiring further field optimization or development of a new receive system to allow in-vivo TRASE.</p>

<p>Despite the magnet&#39;s field inhomogeneity, magnet rotation allows a 2D TRASE image acquisition using two RF transmit coils rather than three, significantly reducing challenges with RF coil decoupling and reducing costs due to one less required RF amplifier channel. Accurate and high-resolution angular rotation of the Halbach magnet was achieved using an inexpensive stepper motor and driver. The proof-of-concept was verified by obtaining a set of 1D TRASE projections and using this data in a 2D TRASE reconstruction technique.</p>