In vivo T$_{1}$ maps at fields from 0.001 to 150 mT with SQUID based MRI

High field magnetic resonance imaging (MRI) uses differences in the longitudinal relaxation times (T$_{1})$ of protons to differentiate tissue types. Using phantoms and \textit{ex vivo} human tissue, we have demonstrated that T$_{1}$ contrast at low fields can be much greater than that at high fields, suggesting that one can, for example, distinguish tumors from healthy tissue without the use of contrast agents. Our MRI system polarizes protons at fields up to 150 mT using a water-cooled electromagnet. Subsequently, we ramp down the polarizing field and measure the proton nuclear magnetic resonance at typically 0.132 mT in the presence of three-dimensional field gradients using a SQUID coupled to an untuned, second-derivative gradiometer. Our system is capable of imaging the human arm \textit{in vivo} with a resolution of 2x2x10 mm$^{3}$. By switching the field to an intermediate field value for a variable time, we can obtain T$_{1}$ maps at fields between 0.001 and 150 mT. We present \textit{in vivo} T$_{1}$ dispersion curves in the same range of magnetic fields for several types of tissue in the human arm. Work supported by USDOE.