Magnetic dressing for optical atomic magnetometer and ultra-low-field MRI

MRI machines use large superconducting magnets to produce powerful magnetic fields necessary for imaging. However, these superconducting magnets only operate at cryogenic temperatures, making MRI cumbersome and expensive. Scientists are looking into the possibility of detecting MRI signals in an ultra-low field regime (ULFR) – without the need for superconducting magnets.

The ULFR, however, requires the use of highly sensitive non-inductive detectors, such as SQUIDs, but these devices still rely on cryogenics. Optical atomic magnetometers, which run at room temperature, are seen as an alternative.

OAMs operate on the basis of paramagnetic atoms in which resonant light as a polarization tool induces atomic magnetic resonance (AMR). However, since an OAM’s sensitivity relies on AMR’s narrow spectral width, and the magnetic field gradients in MRI would severely broaden that width, OAM would lose its sensitivity when used to detect the MRI signal in situ.

Bevilacqua et al. demonstrated a way to compensate for this line broadening using an inhomogeneous magnetic dressing technique. It involves applying a strong time-dependent magnetic field, which slows down the atomic precession. The dressing field is stronger where the MRI magnetic field gradient induces faster precession, thus counteracting the atomic line broadening and restoring OAM sensitivity.

To test the method, the researchers built a dual-sensor OAM device, with each sensor coupled to the source of a dressing field. A differential technique was used, which enabled the operation in an unshielded environment and demonstrated the possibility of arranging multi-sensor setups.

The device was able to detect MRI signal from their sample – water protons contained in disk-like compartments inside a polymeric cartridge. Expanding to a multi-sensor architecture could help to improve the sensitivity and accelerate the measurements.

Source: “Sub-millimetric ultra-low-field MRI detected in situ by a dressed atomic magnetometer,” by Giuseppe Bevilacqua, Valerio Biancalana, Yordanka Dancheva, and Antonio Vigilante, Applied Physics Letters (2019). The article can be accessed at https://doi.org/10.1063/1.5123653 .