Magnetic resonance imaging with RF encoding on curved natural slices.

While the idea of using spatial encoding fields (SEM) for image formation has been proven, conventional wisdom still holds that a magnetic resonance imaging (MRI) system begins with a highly uniform magnetic field. In particular, radio frequency (RF) encoding MRIs designed and tested to date have largely used uniform magnetic fields. Here we demonstrate magnetic resonance imaging in a magnetic field with a built-in gradient that gives non-planar slices - curved surfaces - when the nuclear spins are excited with narrow band RF pulses. Image encoding on these naturally occurring non-planar slices was accomplished with RF encoding using a non-linear spatially varying B1 phase gradient. The imaging methods were demonstrated on a small prototype MRI instrument. The MRI has no switched magnetic field gradients - it is "gradient-free". A low field gradient-free MRI with low mass permanent magnets and simple, low power, RF encoding hardware is ideal for deployment on the International Space Station for the study of astronaut muscle and bone mass loss. Gradient-free natural slice encoding MRI designs would also be portable enough for application in remote terrestrial locations, in emergency rooms and in operating rooms where they can be used with minimally invasive and robotic surgery.