MRI Basics: What is MRI?

A typical MRI scanner

In which I explain the very basics of MRI...

Nuclear Magnetic Resonance (NMR) allows us to detect a signal due to a fundamental quantum property of atoms - spin (which I will explain in an upcoming post). Magnetic Resonance Imaging (MRI) uses these same principles to obtain images of (usually) hydrogen atoms in a substance/tissue.

(N)MRI is a non-invasive, non-ionising, imaging modality and is widely used in hospitals worldwide.[1] What does all that mean?


MRI should really be referred to as nuclear magnetic resonance imaging (NMRI) but since it was being developed during the 1970s, when 'nuclear' had bad associations, the nuclear was dropped.[2]

The word nuclear in this sense means 'of or relating to the atomic nucleus' and is not short for fission. This distinction is often missed in the media and so it's probably best not to use the word nuclear in a medical technique.


An invasive method involves breaking the skin and sampling a tissue or fluid. An example is a biopsy to work out if a lump is a tumour. A small part of the tissue is cut out of the body and the cells are analysed to see if they are cancerous.

A non-invasive method, conversely, is any method which does not involve breaking the skin. Examples include:

  • the majority of imaging methods
  • EEGs etc.
  • lung function test (spirometry)


Ionising radiation is any form of radiation which upon interacting with an atom/molecule adds or removes an electron changing it from a neutral atom/molecule to a charged ion. When this happens in the body it can cause significant damage to cells leading to cell death and mutation.

MRI uses radio-waves to excite the hydrogen atoms which are non-ionising. X-rays etc. are ionising and can cause damage to cells.


Atoms in your body are weakly magnetic (more on this when we discuss spin!). MRI exploits this to get a usable signal and this requires a very powerful magnet:


Resonance is when something oscillates with a larger amplitude at certain frequencies (it's what makes ultrasound break glass). If you use a radio-wave (Radiofrequency pulse) at the resonating frequency of a hydrogen atom you can make it give off another radio-wave which is what an MRI scanner detects.[3]

"I have not yet lost a feeling of wonder, and of delight, that this delicate motion should reside in all the things around us, revealing itself only to him who looks for it. I remember, in the winter of our first experiments, just seven years ago, looking on snow with new eyes. There the snow lay around my doorstep - great heaps of protons quietly precessing in the earth's magnetic field. To see the world for a moment as something rich and strange is the private reward of many a discovery." ~ Edward Mills Purcell

This post very briefly touches on some important topics which I'll explain more in depth in upcoming posts :)

Tom Out!


[1] OECD (2003). Health at a glance 2003: OECD indicators, Paris: OECD Publishing.

[2] Joseph P. Hornak (2010). The Basics of MRI. Online Book: < > [11 January 2013].

[3] Matt A. Bernstein, Kevin Franklin King, and Xiaohong Joe Zhou (2004). Handbook of MRI pulse sequences, Oxford: Academic Press.