Gradient-Free Low-Field MRI using the Bloch-Siegert Shift for RF Spatial Encoding

Abstract

Magnetic resonance imaging (MRI) at low magnetic field strengths enables low-cost, portable point-of-care imaging and new MR imaging applications. However, images can be corrupted due to the lack of electromagnetic shielding, while conventional gradient systems for spatial encoding constrain scanner design and drives up cost. To address the former problem we developed the ``EDITER'' electromagnetic interference (EMI) inteference removal method which replaces expensive shielded rooms with external sensors to detect EMI and remove it retrospectively from images collected at 47.5 milliTesla. The method was then leveraged to enable the development of novel gradient-free radiofrequency (RF) gradient-based spatial encoding methods using the Bloch-Siegert (BS) shift. We first developed power-efficient square-root solenoid RF gradient encoding and saddle imaging coil systems for BS phase encoding of phantoms and the human wrist, along with a BS phase encoding pulse sequence and Fourier image reconstruction. We then developed a novel transformer system that enables simultaneous transmission of an BS encoding pulse during signal reception for BS frequency encoding. That system was leveraged to acquire the first ever BS frequency encoded images.