Methods, systems, and devices are disclosed for implementing magnetoencephalography (MEG) source imaging. In one aspect, a method includes determining a covariance matrix based on sensor signal data in the time domain or frequency domain, the sensor signal data representing magnetic-field signals emitted by a brain of a subject and detected by MEG sensors in a sensor array surrounding the brain, defining a source grid containing source locations within the brain that generate magnetic signals, the source locations having a particular resolution, in which a number of source locations is greater than a number of sensors in the sensor array, and generating a source value of signal power for each location in the source grid by fitting the selected sensor covariance matrix, in which the covariance matrix is time-independent based on time or frequency information of the sensor signal data.