SQUID-detected in vivo MRI at microtesla magnetic fields

We use a low transition temperature (T_c) Super-conducting Quantum Interference Device (SQUID) to perform in vivo magnetic resonance imaging (MRI) at magnetic fields around 100 microtesla, corresponding to proton Larmor frequencies of about 5 kHz. In such low fields, broadening of the nuclear magnetic resonance lines due to inhomogeneous magnetic fields and susceptibility variations of the sample are minimized, enabling us to obtain high quality images. To reduce environmental noise the signal is detected by a second-order gradiometer, coupled to the SQUID, and the experiment is surrounded by a 3-mm thick Al shield. To increase the signal-to-noise ratio (SNR), we prepolarize the samples in a field up to 100 mT. Three-dimensional images are acquired in less than 6 minutes with a standard spin-echo phase-encoding sequence. Using encoding gradients of ∼100 μT/m we obtain three-dimensional images of bell peppers with a resolution of 2×2×8 mm³. Our system is ideally suited to acquiring images of small, peripheral parts of the human body such as hands and arms. In vivo images of an arm, acquired at 132 μT, show 24-mm sections of the forearm with a resolution of 3×3 mm² and a SNR of 10. We discuss possible applications of MRI at these low magnetic fields.