The human body's high hydrogen concentration, due to being ~70% water and having significant fat, makes it detectable by MRI.

“because we know that our bodies composed of roughly 70% water there's a lot of hydrogen on fat's so can we see that frequency difference we can see it on a bench top but what about in people so we have more hydrogen in us if we were just going to count up the atoms and as hydrogen wins all day long because as you said it for 70% of water that's two to one hydrogen over oxygen there and then all the fat that's in us is all the hydrogen - the carbon there and there's basically hydrogen in protein as well”