There is described an MRI system in which the detection of the NMR signal is performed by a resonant input at a median field range above 100 gauss and where the main field is cycled to a low field of below 50% of the resonant frequency after the excitation of the NMR signal for a period sufficient to develop differences in magnetisation (T1). The advantage of this system is that images can be generated at much lower field intensities than prior art systems and is able to detect abnormalities in tissue such as cancerous tissues in a patient.