T2 decay

• Decay of NMV in the transverse plane in an exponential manner
• Occurs due to spin-spin interactions
• The intrinsic magnetic fields of nuclei causes loss of coherence, resulting in dephasing
• It is intrinsic to tissue
• Time taken for 63% of the transverse magnetization to be lost due to dephasing
• This dephasing occurs during TE – the time between the RF pulse and MR signal
• Depends on strength of external field. Higher the strength, longer the T2 decay

Fat

• The more closely packed the molecules in a tissue, the more efficient the T2 decay
• Fat has closely packed molecules
• As a result spin-spin interaction is very efficient
• So the T2 decay time is very short

Water

• The molecules are spaced apart
• Spin-spin interaction is poor
• T2 decay time is long

Controlling T2 decay using TE

• A short TE is insufficient to permit dephasing in fat or water. As a result tissue contrast is less
• Long TE demonstrates good tissue contrast thanks to ample time for full dephasing of fat and water