Shoujun Xu

Magnetic resonance imaging (MRI) is a noninvasive technique employed in fields as diverse as materials science, chemistry, and medicine. Conventional MRI is conducted in a strong magnetic field provided by a superconducting magnet. The cost and constrained accessibility limit the applicability of MRI in many circumstances. Furthermore, high fields are not favorable for certain measurements, including those involving heterogeneous samples with large magnetic-susceptibility gradients. Low-field MRI will eliminate these problems. However, conventional inductive detection is not suitable for low-field because its sensitivity is proportional to the strength of the static magnetic field according to Faraday’s Law. Alternative detection techniques are therefore preferred for low-field MRI.

Laser detection is based on nonlinear magneto-optical resonance of a polarized light induced by an alkali vapor. In the presence of an external magnetic field, the ground state of the alkali atoms undergoes Zeeman splitting. A polarized laser beam with appropriate wavelength selectively excites a certain combination of the Zeeman sublevels, generating a coherent ground state of the alkali atoms. The resulting atomic polarization precesses in the external magnetic field and consequently rotates the polarization of the incident beam. If the laser is modulated, in frequency or amplitude, a resonance in optical rotation occurs when the modulation of the laser is synchronized with the precession. The magnitude of the magnetic field can therefore be deduced from the corresponding modulation frequency of the laser.

Laser-detected MRI in low-field is a noninvasive, inexpensive, and portable imaging modality with extensive potential applications. Low-field imaging opens up avenues of investigations of magnetically inhomogenous samples, such as porous material with paramagnetic impurities and human with metal implants. It is also useful for samples that are too big to be introduced into the bore of a superconducting magnet. This technique requires no cryogenics, making it applicable for no-cryogenics situations and dramatically reducing the cost, size and maintenance of the apparatus.

Current projects include: a) METHODOLOGY: sensitivity improvement and new imaging schemes; b) APPLICATIONS: general low-field imaging, microfluidics, porous materials optimization, and direct magnetic field detection; c) OTHER: new alternative detection techniques for magnetic resonance.