Fig. 1. The basic principle of RF-OPMs. In the laboratory coordinate system, a z-axis pump light polarizes the atoms under a bias field B₀ applied in the same direction. When the frequency of the transverse RF field B_rf to be measured coincides with the Larmor frequency, magnetic resonances occur. This phenomenon can be easily understood in the rotating coordinate system.

Fig. 2. (a) Imaging principles for conductive materials and magnetically permeable materials. The primary coil applies the primary magnetic field B_p to the test sample. The primary field induces eddy currents on the surface of materials with high conductivity, resulting in a secondary field opposite to the primary field. For materials with high magnetic permeability, the primary field induces local magnetization on the surface, leading to a secondary magnetic field in the same direction as the primary field. (b) Imaging of a square 90 mm stainless steel plate with a circular recess (diameter 25 mm) using an RF-OPM, capturing changes in amplitude and phase of the signal of the RF-OPM. Adapted from Bevington et al., 2020^[46].

Fig. 3. (a) Typical configuration of ULF-MRI with a flux transformer. Phantom is the test sample for ULF-MRI. (b) Comparison of ULF-MRI of the human head with traditional MRI. Adapted from Zotev et al., 2018^[67].

Fig. 4. (a) Spatial encoding in MPI. After applying a modulation field to SPIONs, the magnetic field at the center (field-free point, FFP) of the inhomogeneous selection field becomes zero. The magnetization of SPIONs undergoes substantial oscillations due to the modulation field. At the edges of the selection field, where the magnetic field intensity is higher, the magnetization of SPIONs tends to saturate, resulting in a reduction in oscillation amplitude. The adjustment of the FFP is accomplished through the use of a focus field, facilitating spatial encoding. (b) In vivo overlays of MPI images (red) onto anatomical MRI images (gray) of mouse hearts and cardiovascular systems obtained using tracer materials. Adapted from Knopp et al., 2012^[83].