Also, surely the presence of beta waves (12-30Hz) needs to be detected by an electroencephalogram (EEG), not fMRI? I thought fMRI has a much lower temporal resolution (~1Hz).
I think the image was mischaracterised. The fMRI will show changes in activity and if the image is actually from an fMRI post meditation then the authour may have just extrapolated from 'reduction in beta waves' to 'less activity on this colour gradient image'.
Perhaps the authour isn't really that familiar with the material?
I can't find a good article atm, but afaik there's an established correlation between fMRI activity (indeed about 0.5/1 slices per second depending on settings) and beta waves.
Btw beta waves can also be recorded using a single electrode: normally these get used to record spikes from a single neuron, but they also pick up the lower frequency bands of all surrounding neurons. The nice thing is this provides a more local view (iirc everything that is closer than a couple of mm around the electrode), whereas an EEG gets you 'everything' so it's more an indication about general brain state without giving information about which part is active.
Perhaps the authour isn't really that familiar with the material?