Frequency-encoded magnetic resonance imaging with dynamic radio frequency field gradients

Magnetic Resonance Imaging is the gold standard for soft tissue diagnostic imaging but relies on expensive, failure-prone, and loud magnetic field gradients for spatial encoding. Radiofrequency field gradients are a promising alternative, yet they have never achieved frequency encoding, the technique that allows an image to be acquired continuously in milliseconds and without which scan times would increase by orders of magnitude. Here we show that the Bloch-Siegert shift can apply a spatial frequency gradient simultaneously with signal recording, without displacing magnetization from the plane where it generates signal. Combined with an injection transformer based simultaneous transmit-and-receive system that prevents the strong encoding field from overwhelming the weak tissue signal, this approach produces images on a 47.5 mT low-field scanner equivalent in quality to those from conventional gradient encoding. This work enables smaller, quieter, and less expensive scanners without compromising scan time or diagnostic image quality, with particular promise for expanding access to imaging in resource-limited settings.