RF Encoding for Gradient-Free MRI

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Project Summary The goal of this project is to translate RF encoding methods developed in an R21 project to human imaging, by implementing them on a very low field human MRI scanner. Its successful completion will enable silent, low-cost and more portable MRI systems, leading to a substantial reduction in the cost of imaging and improved patient compliance and comfort. In conventional MRI, a received signal is localized to its spatial location of origin based on its temporal frequency, which is controlled using magnetic fields that are parallel to the main (B0) field of the scanner and vary linearly across space. There are many problems with these B0 gradient fields: they are loud and induce peripheral nerve stimulation, compromising patient comfort; they have relatively long switching times due to the high inductance of the coils; they require bulky cooling systems and customized amplifiers; and they are expen- sive, representing 25-30% of the cost of a clinical scanner. B0 gradient encoding also suffers from spatial errors due to concomitant terms, which increase with decreasing B0 field strength and will limit the performance of emerging portable and low-cost MRI systems. A potential solution to these problems is to replace B0 gradients with RF gradients, which are silent and low-cost. Unfortunately, in spite of its potential RF gradient encoding has not yet become a clinical or commercial success. This is largely due to the fact that no existing RF gradient encoding method offers the orthogonality between contrast development and spatial encoding that is enjoyed by B0 gradients, or a straightforward path to convert existing B0 gradient-based MRI scans to use RF encoding. The methods developed in this project are the first to meet these requirements, and will thus be the first truly viable RF gradient-based imaging methods. The central innovation of this project is to use the Bloch-Siegert (BS) shift to spatially encode the MRI signal. As with B0 gradients, this encoding mechanism is based on the application of phase shifts to magnetization directly in the transverse plane, and therefore does not modulate the magnitude of the transverse magnetization, leaving image contrast unaffected by spatial encoding. The first Aim of the project is to develop array and solenoid RF gradient coils and associated RF hardware to enable 2D and 3D Cartesian brain imaging on a human 0.0475 Tesla MRI scanner, including strategies for simultaneous RF transmission and reception to enable frequency encoding by BS shift. The second Aim is to develop and implement RF-encoded pulse sequences for brain imaging based on the BS shift, leveraging key developments from the R21 phase of the project including swept RF pulses for phase encoding, a theoretical basis and pulse sequence for BS frequency encoding, and RF pulses for RF gradient-based slice-selective excitation and slice-encoding. The third Aim is to develop image reconstructions and evaluate the encoding methods in human brain imaging. Successful completion of these Aims will establish the first viable fully RF-encoded human imaging system and pave the way for commercialization and clinical use.