T2 weighted image
T2 weighted image (T2WI) is one of the basic pulse sequences in MRI. The sequence weighting highlights differences in the T2 relaxation time of tissues.
Summary
- repetition time (TR): long
- echo time (TE): long
- flip angle: less important than with T1 weighting
- fat: intermediate-bright
- fluid: bright
Physics
A T2WI relies upon the transverse relaxation (also known as "spin-spin" relaxation) of the net magnetization vector (NMV). T2 weighting tends to require long TE and TR times.
One way to think about T2 relaxation is as follows:
- after an RF excitation pulse, there is relaxation of the spins from the transverse plane toward the main longitudinal magnetic vector (B0) - this is T1 weighting
- at the same time, spins are decaying from their aligned precession in the transverse plane - differences in this decay are captured in T2 weighting
The amount of T2 decay a tissue experiences depends on multiple factors. Each tissue has an inherent T2 value, but external factors (such as magnetic field inhomogeneity) can decrease the T2 relaxation time. This additional effect is captured in T2*. The refocusing pulse in spin-echo sequences helps to mitigate these extraneous influences on the T2 relaxation time, trying to keep the image T2 weighted rather than T2* weighted.
Paramagnetic contrast agents (e.g. gadolinium-containing compounds) do not cause the same bright tissue contrast as they do on T1WI. Gadolinium shortens T2 relaxation time and actually results in hypointense signal.
If the TE is extended out over a very long time, only tissues with a very long T2 relaxation time will retain signal. This is the basis of MRCP.
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