Here are some websites which my teacher gave us about MRI.The first one is pretty good (and cool-looking too!)
MRI animation on how it works
http://www.simplyphysics.com/IntroToMRI.html
http://www.technologyreview.com/articles/visualize0701.asp
http://www.cis.rit.edu/htbooks/mri/inside.htm
http://www.cis.rit.edu/htbooks/mri/
Images for functional MRI
http://www.fmrib.ox.ac.uk/image_gallery/
MRI and acupuncture points
http://www.reflexology-research.com/mri.htm
As for my interpretation... what the heck, it can only help me to explain it to someone else:
The Identify dot points: fairly self-explanatory. You don't really need more detail than what's already written
Describe how net spin is obtained:
every nuclear particle spins on own axis. Nuclear particles can be either spin up or spin down.
The combination of the spin of protons and neutrons gives net spin. there are rules for whether the nucleus will have 0,integer or fractional spin depending on the nymber of protons and neurons present but they're not important. The hydrogen atom has a spin of +/- 1/2
Explain that the behaviour of nuclei with a net spin, esp hydrogen is related to the magnetic field they produce:
A moving charge creates a magnetics field(M+G) NB:In a diagram the direction of the magnetic field of the nucleus is related to the direction of spin by the solenoid rule
descirbe the changes that occur in the orientation of the magnetic axis of nuclei before and after the application of B
Normally the orientation of the nuclei is random. When a magnetic field is applied they line up either in the same direction as the magnetic field(parallel) or opposite(anti-parallel). When the external field is removed they slowly reurn to random orientation
define precessing
this is when the nucleus spins not only on it's own axis but around a central axis
and relate the freq of precessing to the composition of the nuclei and the strength of the external field
the freq is directly proportional to the external field strength and is defined by the Larmour equation. The freq is also determined by the composition of the nucleus because hydrogen is greatly affected due to having only one proton and no neutrons.
discuss the effect of subjecting precessing nuclei to pulses of radio waves
parallel and anti-parallel do not have the same energy. Parallel nuclei are at a slightly lower enrgy state so a few more per billion are parallel than anti-parallel. The larmour freq for hydrogen atoms happens to be the same freq as radio waves. When the nuclei are subjected to radio waves it gives some parallel nuclei enough energy to flip to anti-parallel. after the radio pulse is over the nuclei will return to parallel, emitting the extra energy which we detect to form images
explain that the amp of the signal given out is related to the no of nuclei present
seems fairly self-exlanatory to me... the more nuclei the more EM waves emitted, the waves super-position to create a signal with higher amp. As Antdude said the time taken for the intensity to drop to 63% is used to measure proton density and is known as T1 and lasts about a second
explain the large differences occurring in relaxation time between various tissues
the hydrogen acts differently depending on how free it is to respond to the RF signal. In water its only loosely bound, in fat and stuff its more tightly bound. The relaxation time is much faster in muscle than fat and in fat than it is in water
Some miscellaneous info not fitting into above points:
*Hydrogen is targeted because it has greatest response and because it's so common everywhere.
*Slice of the body are taken by using a gradient magnetic field and a horizontal one, which means that the Larmour freq will differ slightly depending on which part of the body you want to image
*images look different depending on whether they are weighted towards T1 or T2 relaxation. T2 is the process by which the protons go out of phase with eachother and lasts about 50ms
