  Prepolarized Magnetic Resonance Imaging (PMRI) is a long-established
technique conceived to counteract the loss in signal-to-noise ratio (SNR)
inherent to low-field MRI systems. When it comes to hard biological tissues and
solid-state matter, PMRI is severely restricted by their ultra-short
characteristic relaxation times. Here we demonstrate that efficient hard tissue
prepolarization is within reach with a special-purpose 0.26 T scanner designed
for dental MRI and equipped with suitable high-power electronics. We have
characterized the performance of a 0.5 T prepolarizer module which can be
switched on and off in just 200 us. To that end, we have used resin, dental and
bone samples, all with T1 times in the order of 20 ms at our field strength.
The measured SNR enhancement is in good agreement with a simple theoretical
model, and small deviations in extreme regimes can be attributed to mechanical
vibrations due to the magnetic interaction between the prepolarization and main
magnets. Finally, we argue that these results can be applied to clinical dental
imaging, opening the door to replacing hazardous X-ray systems with low-field
PMRI scanners.
