  Purpose: To design a low-cost, portable permanent magnet-based MRI system
capable of obtaining in vivo MR images within a reasonable scan time.
  Methods: A discretized Halbach permanent magnet array with a clear bore
diameter of 27 cm was designed for operation at 50 mT. Custom built gradient
coils, radiofrequency coil, gradient amplifiers and radiofrequency amplifier
were integrated and tested on both phantoms and in vivo.
  Results: Phantom results showed that the gradient non-linearity in the y- and
z-directions was less than 5% over a 15 cm field-of-view and did not need
correcting. For the x-direction, it was significantly greater, but could be
partially corrected in post-processing. Three dimensional In vivo scans of the
brain of a healthy volunteer using a turbo-spin echo sequence were acquired at
a spatial resolution of 4x4x4 mm in a time of ~2 mins. T1-weighted and
T2-weighted scans showed a good degree of tissue contrast. In addition, in vivo
scans of the knee of a healthy volunteer were acquired at a spatial resolution
of ~3x2x2 mm within a twelve minutes to show the applicability of the system to
extremity imaging.
  Conclusion: This work has shown that it is possible to construct a low-field
MRI unit with hardware components costing less than 10000 euros, which is able
to acquire human images in vivo within a reasonable data acquisition time. The
system has a high degree of portability with magnet weight ~75 kg, gradient and
RF amplifiers each 15 kg, gradient coils 10 kg and spectrometer 5 kg.
