ultralowfield

Ultralow-field (ULF) refers to a regime of magnetic resonance conducted at extremely weak static magnetic fields, typically in the microtesla to tens of microtesla range, in contrast to conventional high-field MRI or NMR that use tesla-scale fields. In ULF, the Zeeman interaction is comparable to or smaller than spin-spin couplings, so spectra reflect J-couplings and other interactions rather than chemical shifts alone. ULF techniques include ultralow-field NMR and ultralow-field MRI, often using prepolarization in a strong field followed by transport to a shielded region where detection occurs at near-zero field.

Detection is usually performed with highly sensitive quantum sensors such as superconducting quantum interference devices (SQUIDs) or atomic magnetometers based on optically pumped alkali vapors; spin-exchange relaxation-free (SERF) magnetometers are particularly effective at suppressing spin relaxation at high atomic density and low fields.

Historically, ULF emerged from interest in portable, low-cost magnetic resonance and from advances in quantum sensing. In ULF NMR, the sample is polarized in a higher field, then moved to a shielded region where signals at microtesla fields reveal J-coupling patterns and, in some cases, information similar to low-field NMR spectra but with different resolution. In ULF MRI, imaging is performed with array sensors and low prepolarizing fields, enabling potentially compact systems.

Advantages of ultralow-field approaches include reduced equipment complexity, avoidance of cryogenics, and potential for safer, portable devices. Challenges include low inherent signal strength, the need for magnetic shielding or active field control, and longer acquisition or processing requirements.

Research continues to expand sensitivity, multi-nuclear capabilities (1H, 13C, 19F), and imaging speeds, with potential applications in chemical analysis, materials science, and point-of-care diagnostics.