Characterizing longitudinal and transverse relaxation rates of ferrofluids in microtesla magnetic fields

Shortening spin-lattice relaxation rates (1/T ₁) or spin-spin relaxation rates (1/T ₂) is the purpose of magnetic resonance imaging contrast agents. In this work, an ultralow field nuclear magnetic resonance spectrometer and imager are set up to characterize the spin relaxation rates of Fe ₃O ₄ superparamagnetic iron oxide (SPIO) for image contrast. It was found that both 1/T ₁ and 1/T ₂ increase linearly when the magnetic susceptibility of SPIO increases by increasing the concentration of SPIO dispersed in water. In an applied field, magnetic moments of SPIO generate microscopic field gradients that weaken the field homogeneity, in turn de-phasing the proton's nuclear spin and enhancing the relaxation rates. A T ₁-contrast image is demonstrated, using SPIO as the contrast agent and high-T _c superconducting quantum interference devices as the detector. T ₁-contrast imaging in microtesla fields might provide a potential modality for discriminating cancer.