For anyone who has tried explaining MRI physics to the layman, the expression on his face follows a very particular progression: First the eyes are narrowed and attentive, the brow slightly furrowed, as you speak of water and hydrogen, blood and oxygen, tissue and bone. These are tangible, they are real; the man can feel them on his own body, or he has an easy enough time picturing them. His look of concentration wavers a bit once you talk about spin, and how it’s an intrinsic property of all atoms, how it’s both like and unlike the spin he experienced as a child on the merry-go-round. And he can be forgiven for looking puzzled when you describe spins as either aligning with a magnetic field or aligning in the opposite direction, partly due to the field but also partly due to chance, and that these spins, in and of themselves, are either up or down; they do not pass through some intermediate stage. And that although we have many figures and paintings of spinning electrons, we evidently draw from memory as we do of a distant loved one; as the electron is a very shy lady indeed, and no one has ever taken her picture or seen her in the flesh.
But that shadow of doubt is gone in an instant, his demeanor ready for more, once you begin talking about magnets. Magnets! Everyone has played with them; everyone understands intuitively the nature of the poles, attracting their opposites, repelling their identical twins. Everyone has observed them acting through solid matter: Tables, books, hands; none of these stop the magnet from pulling on filaments, metal, other magnets. An invisible force, whose effects are plain as day. It is only when you begin talking about gyromagnetic ratios and resonance that his mind begins to falter. Yes, the atoms spin at an incredible rate; yes, we can push them periodically just as we would a child in a swing, tilting the atoms on their side. And then this potential energy is released, and the signal is picked up by sensitive recording devices inside the scanner. So far, so good.
But magnetic gradients? K-space? At this point our listener’s inner eye becomes clouded over. There is something about Fourier transforms, and how each point in k-space corresponds to the magnitude of the image - or was that the contrast? In any case, he will attempt to understand it the next day, or the day after that; but it invariably comes to pass that our thinker finds himself frustrated, and, not seeing any purpose to continue wasting his time trying to understand - unless he is a very eager student indeed - he quits.
Nobody would claim that a video would clear up all of his confusion, but it might go a long way toward making MRI physics more accessible. The video above contains an impressive illustration of how MRI machines work, a brief but effective description of MRI physics, and an animation of how images are reconstructed from k-space. I recommend this to any student who has found himself bewildered by the topic, and I hope that it helps everyone appreciate just how complex and wonderful these machines are.
Now if we could just get a picture of that electron.