  Access to and availability of MRI scanners is typically limited by their
cost, siting and infrastructure requirements. This precludes MRI diagnostics,
the reference standard for neurological assessment, in patients who cannot be
transported to specialized scanner suites. This includes patients who are
critically ill and unstable, and patients located in low-resource settings. The
scanner design presented here aims to extend the reach of MRI by substantially
reducing these limitations. Our goal is to shift the cost-benefit calculation
for MRI toward more frequent and varied use, including improved accessibility
worldwide and point of care operation. Here, we describe a portable brain MRI
scanner using a compact, lightweight permanent magnet, with a built-in readout
field gradient. Our low-field (80 mT) Halbach cylinder design of rare-earth
permanent magnets results in a 122 kg magnet with minimal stray-field,
requiring neither cryogenics nor external power. The built-in magnetic field
gradient reduces reliance on high-power gradient drivers, which not only lowers
overall system power and cooling requirements, but also reduces acoustic noise.
Imperfections in the encoding fields are mitigated with a generalized iterative
image reconstruction technique, that uses prior characterization of the field
patterns. Our system was validated using T1, T2 and proton density weighted in
vivo brain images with a spatial resolution of 2.2 x 1.3 x 6.8 mm$^3$.
