Wednesday, February 9, 2011

Physics for understanding fMRI artifacts: Part Two

We continue our review of the key principles of NMR with another video courtesy of Paul Callaghan. In it, Prof Callaghan introduces the idea of bulk magnetization; the thing that you induce in your subject's brain (which is ~80% water) when you slide the subject into the magnet, and which you then manipulate to produce images.

In the video shown previously (see Part One), Prof Callaghan introduced the phenomenon of resonance and demonstrated it with a spinning wheel. In MRI the resonance frequency is governed by a simple proportionality, as given in the Larmor equation. We will use this equation later on to establish different frequencies across an object, thereby encoding spatial information and yielding, ultimately, an image of that object. We will also see how the Larmor equation is used in the k-space formalism, so make sure you have a good understanding of this deceptively simple yet intuitively valuable equation.