Magnetic resonance imaging is the most popular method for scanning the brain both for research and for clinical investigations. I’ve just found a wonderfully written article that gives a great introduction to the physics of how MRI scanners work.
It is both clearly written for the non-specialist and fantastically complete. This is a rare and valuable combination.
There are some other guides to MRI physics which are also wonderfully written but most lack the sufficient detail that would bring you up to ‘entry level’ in the field.
For example, How Stuff Work’s guide to MRI is a great place to start, but it won’t tell you about why and how T1 and T2 imaging are different, or any of the other things you need to know to understand the fundamentals of MRI technology.
You don’t need to know maths to understand the article (the downfall of most ‘introductory’ guides to MRI) and the author uses wonderfully clear analogies throughout.
The article is written by radiologist Robert Pooley, who should give himself a pat on the back for such a great job. It was published as an open-access paper in the journal RadioGraphics. Perfection!
Link to article ‘Fundamental Physics of MR Imaging’.
Mind Hacks 
I’d also add Sharma, Brain Imaging in Schizophrenia: Insights and Applications Remedica Publishing (January 2001). A bit dated, but still has a great intro to MRI.