A method and system are disclosed for gathering information about an object including single domain particles which have a diameter in the range of about 5 to 80 nm. In one aspect, a method includes generating a static magnetic field of less than about 0.1 Tesla on the object and generating an RF energy, pulsed or continuous wave, so as to generate electron paramagnetic resonance of the single domain particles. The method also includes detecting the electron paramagnetic resonance of the single domain particles in the form of an image of the object. The single domain particles may have a predetermined diameter and a predetermined saturation magnetization and the applied magnetic field may be such that the single domain particles reach a magnetization being at least about 10% of the saturation magnetization. The method may be used for detecting tags in an object and for activating tags.