SQUID-detected magnetic resonance imaging in zero static magnetic field

Conventional magnetic resonance imaging (MRI) is performed in a static homogenous magnetic field B$_{0}$ in the presence of applied field gradients that generate a magnetic field change $\Delta $B $<<$ B$_{0}$ across the sample. In this case, the concomitant gradients can be ignored and the applied gradients are unidirectional. When $\Delta $B $\sim $ B$_{0}$, this approximation breaks down and the concomitant gradients distort the image. In the limit B$_{0}\to $ 0 these distortions can be eliminated by means of a pulse sequence consisting of a train of short, spatially uniform magnetic field pulses. Between the pulses, the spins evolve in a pure gradient field (with zero spatial average). The effect of the pulse train is to average out the concomitant terms to leave an effectively unidirectional gradient field (Meriles \textit{et al}., \textit{PNAS} 102, 1840 (2005)). We acquire magnetic signals with a superconducting gradiometer coupled to the input loop of a low-transition temperature superconducting quantum interference device. Using this pulse sequence we have acquired undistorted two-dimensional images of methanol phantoms in a residual static field $<$ 1 $\mu $T. Supported by USDOE..