Project Summary In this project we propose a novel magnetic resonance imaging (MRI) concept based on our innovative fast- switching-field magnet design. We believe that this MRI magnet will make possible new contrast mechanisms and ratiometric molecular imaging for biomedical research. We will develop and demonstrate a liquid-helium (LHe)-free superconducting magnet that can change the field very quickly in time between significantly different field strengths: a high (3T) field of modest-homogeneity adequate for relaxometry and prepolarization; and a low (0.5T) field of high-homogeneity for spectroscopy and imaging. New contrast mechanisms enabled by this magnet include level-crossing between spin 1/2 and quadrupolar nuclei, accelerated spin-lattice relaxation, and adiabatic demagnetization-remagnetization. By measuring relaxation parameters at different fields, relaxation contrast sources that have different relative relaxation rates at the fields could be distinguished. This will help quantify imaging agent concentrations against background or differentiate states of individual agents that exhibit environmentally-dependent changes in relaxation behavior. The fast field change in a superconducting magnet can be achieved by adopting most advanced low-AC-loss high-temperature superconductors (HTS) having a sufficiently high temperature margin to avoid quench. The field switching time between two fields will be rapid enough (<1 s) to retain polarization and relaxometry yet safe for animal and human. Other applications of our switching-field magnet might include interventional MR and intraoperative surgery. At 0.5 T susceptibility artifacts from slightly magnetic devices and implants, and RF heating from conductive devices are much less. We believe the images can be comparable to 1.5 T with progressing MR spectroscopy and imaging technologies. We are convinced that our proposed switching-field MR magnet can not only promise novel contrast mechanisms but also serve as a stepping stone for preclinical animal and even human MRI competitive and cost effective against conventional MRI magnets. The specific aims are to develop a LHe-free fast-switching-field MRI magnet prototype by combining two HTS magnets—a 2.5-T switched-field magnet and a 0.5-T MRI magnet—and demonstrate the MRI field homogeneity and stability during fast field-switching (up to 3 T/s). We will also perform a conceptual design of a human-size fast-switching-field MRI magnet with optimum design fields. The unique features of our magnet design are very low-loss and stable in a fast-switching field, impossible with a conventional low-temperature superconductor (LTS), and configuration of two separately operating HTS magnets—a steady low field of high-homogeneity and a fast-cycling high field of modest-field-quality—to achieve fast-switching feature efficiently in performance and cost. Our solid-nitrogen-cooled cryogenic system does not rely on helium, which has become a scarce and unrelia...