How to get an electron to tell secrets

Image via Wikipedia

About 5 years ago I was sitting in an introductory biomedical engineering course when the professor* suddenly got very excited and started to spin around wildly, demonstrating what he called “The Electron Dance”. Professor Matthew O’Donnell’s frenzied dancing promptly shocked all of us out of our mid-day stupor as he yelled out what each part of the dance signified. Within moments, he had all of us out of our seats doing the dance with him. Prof. O’Donnell was trying to teach us how magnetic resonance imaging (MRI) works, and his hands-on lesson has certainly stuck with me.

As a class, the spin of our bodies are we swung in increasingly sloppy circles represented the spin of hydrogen atoms and the angle of our arms in relation to our body represented the orbit of our lone electron. Each time Professor O’Donnell would yell “BANG!” we were to drop our arms and slowly raise them back up as we continued to spin around, representing how the orbit of a hydrogen atom would be uniformly perturbed by the massive magnetic pulses inside of an MRI machine and then recover. The data, and hence the pictures, lie in how different densities of tissue provide different electromagnetic contexts for their hydrogen atoms. The different densities will show up differently on an MRI scan because their hydrogen atoms will respond to a strong magnetic pulse differently than other tissues around them.

When someone is loaded into an MRI machine, they are also being placed in the middle of a very strong, uniform magnetic field. As they enter the field, all the hydrogen atoms in their body start to spin in the same direction**, providing a smooth reference. When the imaging itself starts, another incredibly powerful magnet is fired in a brief burst at an angle to the reference field, and all the hydrogens’ electrons fall over. The rate at which the electrons get back up varies by the density of the surrounding tissue and because the electrons continue to orbit even as they’re climbing back up they give off a measurable oscillatory signal that we can detect. We can then compile all of those signals into one composite picture through the use of a Fourier transform algorithm that allows exquisite detail of the internal body to be revealed.

MRI, however, is static. Patients have to lie very still or else the images are ruined because the machine peers into the body a tiny slice at a time and even miniscule movement would jostle one slice out of alignment with another. To visualize real time changes going on inside the body, doctors and researchers can use fMRI instead. fMRI is just like regular MRI, but it adds a wrinkle to the process that allows functional data to be gathered, particularly from the brain. fMRI relies upon a special magnetic characteristic of blood: oxygenated blood is diamagnetic, which means that it pushes against an external magnetic field, while deoxygenated blood is paramagnetic, which means that it pulls towards an external magnetic field. This means that fMRI is able to map real-time changes in blood oxygenation by monitoring changes in push or pull against its magnetic field in blood vessels.

The human brain consumes an enormous volume of energy and nutrients relative to its size, and this means that its proper function is dependent upon large volumes of oxygenated blood. By measuring the places in the brain at which oxygenated blood becomes deoxygenated in an MRI machine, doctors and researchers can determine which parts of the brain are active in response to a given stimulus. This, in turn, allows for finely detailed, almost real-time(1-5s delay, usually), representations of functional brain activity. This has been applied to a wide range of cognitive behaviors, from reading processes in the study cited this Monday to the neural circuitry of lying vs. truth telling and has yielded powerful results and insights into the mechanics of our human consciousness. fMRI as a tool has allowed for unprecedented accuracy and detail to be obtained in neuroscience research, and the benefits of its application thereto will continue to evolve in the coming years.

*This was at the University of Michigan – Ann Arbor, before Professor O’Donnell became Dean of Engineering at the University of Washington.
**Luckily this process doesn’t hurt.