Three-dimensional MRI in a homogenous 27 cm diameter bore Halbach array magnet

Modern clinical MRI systems utilise very high magnetic fields strengths to produce high resolution
images of the human body. The high up-front and maintenance cost of these systems means that much
of the world lacks access to this technology. In this paper we propose a low cost, head-only, homogenous
Halbach magnet array with the potential for paediatric neuroimaging in low-resource settings. The
homogeneity of the Halbach array is improved by allowing the diameter of the Halbach array to vary
along its length, and also adding smaller internal shim magnets. The constructed magnet has a bore diameter
of 27 cm, mean B0 field strength of 50.4 mT and a homogeneity of 2400 ppm over a 20 cm diameter
spherical volume. The level of homogeneity of the system means that coil-based gradients can be used for
spatial encoding which greatly increases the flexibility in image acquisition. 3D images of a ‘‘brain phantom”
were acquired over a 22 22 22 cm field of view with a 3.5 mm isotropic resolution using a spinecho
sequence. Future development of a low-cost gradient amplifier and an open-source spectrometer
has the potential of offering a fully open-source, low-cost MRI system for paediatric neuroimaging in
low-resource settings.