Magnetic Resonance Imaging (MRI) is a non-invasive medical imaging technique widely used in hospitals and clinics, which provides high spatial and temporal resolution. Most of the clinical MRI use superconducting magnets to create high magnetic field (1.5-3 tesla (T), with a cost of $1M/Tesla) in order to have higher sensitivity, but those massive magnets usually require strict infrastructures and conditions (e.g. cooling system, Faraday cage, no magnetic material around).
The authors present an ultra-low field (ULF) MRI at 6.5mT using electromagnets without any cryogenic system. Indeed, the data acquisition through a single channel is shortened using 3D balanced steady state free precession sequences (b-SSFP), which refocus the spin magnetization following measurements, then eliminating the delays of T2 decay and T1 recovery. Moreover, to accelerate image processing, they sparsely sample the k-space using a variable Gaussian pattern (focusing in the center with the most information and skipping randomly near the edges) and obtain relatively high Signal to Noise Ratio (SNR). In vivo brain ULF MRI and high field MRI (T1 and T2) are obtained from healthy volunteers, where the results are shown in the figure. Although the SNR and the contrast are not as good, but most anatomic features can be seen like in high field MRI.
Then, further improvements can be achieved with higher gradient strength, decreasing the total imaging time while maintaining SNR. Moreover, an optimization of SNR by reducing electronic noise floor (sum of all noise signals from electronics), can drastically reduce the acquisition and image processing time, combining with multiple channel acquisition and parallelized computing. Thus, ULF MRI scanners could be considered as a complement of traditional MRI and can also be portable for a larger use and at lower cost in the future.
Figure. Comparison of single channel ULF MRI to 32-channel high magnetic field scans (axial position). (a) b-SSFP at 6.5 mT at a spatial resolution of (2.5 × 3.5 × 8.5) mm3. (b) T2 and (c) T1 weighted contrast at 3 T at a spatial resolution of (1 × 1 × 1) mm3.
Reference :
Sarracanie, M., LaPierre, C., Salameh, N. et al. Low-Cost High-Performance MRI. Sci Rep 5, 15177 (2015). https://doi.org/10.1038/srep15177