T2 in Nuclear Magnetic Resonance & MRI

[riezer]

I want to understand MRI and what exactly is T2. I have read the November issue of Sci-am and I'm still confused what exactly is T2 as it has different descriptions. The article "The Incredible Shrinking Scanner" by Bernhard Blumich:


"The system can also monitor the precessing spins as they fall randomly out of sync (T2 graph)"

"The synchronous precession of magnetic spins induces an oscillating voltages in the coil that decays with a characteristic T2 time constant for each spin type as the spins fall out of synchrony."

Can you give in your own worlds what you think is T2 for MRI in particular and NMR in general?

Also I guess in MRI, they only deal with Hydrogen atoms in water while NMR handle the rest of the elements beside Hydrogen?

What are the best references or web sites about MRI out there (I have read the wiki but it is not so good).

Thank you.

 

[ZapperZ]

T2 is known as the spin-spin relaxation time.

When the material in question is put in an external magnetic field, the nuclear spins will be aligned parallel, or antiparallel to this field. How, if you apply an RF field 90 degrees to this direction, you will cause the spins to change, causing the net bulk magnetization to flip 90 degrees. After this short pulse, the material will try to get back to its original configuration (it is still in this static, external field).

There are two characteristic times that are relevant here, T1 (spin-lattice relaxation time), and T2 (spin-spin relaxation time). The name itself tells you the relevant dynamics involved in these times. These all measures the time it takes for the system to go back to where it was, but each one measure different dynamics. T2 measure the "dephasing" of the spins due to the surrounding spins. Right after the 90 degree pulse, the spins are aligned in the plane perpendicular to the z-direction, i.e. the direction of the static external field. Once the 90-degree pulse is over, the spin component in this plane will start to dephase and will go back to being random so that the net spin this this plane is zero (all the spin components in the z-direction will go back to being parallel or antiparallel to the external field). The dephasing rate depends on the inhomogeneity of the local spin field. So it is the interaction strength of the each spin with other spins around it that will determine how fast it will dephase. So that's why it is called the spin-spin relaxation time. It gives a measure of the spin inhomogeneity within the sample.

Zz.