Implementations and examples of systems, apparatus and techniques for using magnetic resonance imaging to measure spin perturbations. In one implementation, a sample containing nuclear spins is magnetized using a principle magnetic field generated external to the sample. A periodic pulse sequence is applied to the sample. The pulse sequence includes multiple radio frequency (rf) pulses and multiple recovery times between the rf pulses. The pulse sequence is configured to generate, in the presence of a magnetic field perturbation, a sequence of multiple different steady states of magnetization in the sample during each period of the pulse sequence. A magnetic resonance signal acquired from the sample is processed to identify characteristics of a magnetic field perturbation in the sample. In some implementations, processing the signal to identify characteristics of a magnetic field perturbation in the sample includes processing the signal to identify characteristics of an electric current in the sample.