# MRI Signal Sources

INCORRECT. This is the T2 time constant. T2 decay only describes effects which cause a loss of phase coherence (dephasing) which are *irreversible*. This includes pure spin-spin interactions, in which spins affect each other by their individual oscillating magnetic fields. This changes their frequency of precession (according to the Larmor equation), and they move out of phase.

Try again.

INCORRECT. Spin-"lattice" interactions are a T1 recovery process, and do not contribute to a loss of signal in the transverse plane (though spin-spin interaction can accompany spin-lattice interaction).

Dephasing of spins causes M

_{xy}to decay away. The rotating frame is shown

CORRECT. T2* is like T2, but with an extra dephasing effects included.

T2 includes only *irreversible* causes of a loss of phase coherence (dephasing) of the net magnetisation vectors which add up to make M_{xy}. These spin-spin interactions are at the atomic and molecular levels, in which spins affect each other by their individual oscillating magnetic fields. This changes their frequency of precession (according to the Larmor equation), and they move out of phase.

T2* includes dephasing caused by *magnetic field inhomogeneities and susceptibility effects* as well. These also cause variations in the magnetic field experienced by nuclear spins. This changes their frequency of precession even more, and they move out of phase much faster—a faster loss of signal.

The FID decays with a T2* envelope in gradient echo imaging.

Dephasing effects included in T2* only (not in T2) are reversible in some circumstances, which is where this distinction is useful. Principally, T2* effects are reversed in one type of MRI imaging method: the spin echo pulse sequence. T2* describes a quicker loss of signal than T2; T2* is always smaller than T2.

T2 and T2* are related by the following equation:

1/T2* = 1/T2 + ½γΔB

where γ is the gyromagnetic ratio, and ΔB is the variation in magnetic field.

Thus, a magnet with good field homogeneity will allow T2* values to be closer to the true T2 values of tissues. A magnet with poor field homogeneity will cause T2* to be much shorter than T2, causing faster-decaying signal intensity and affecting image contrast.

Further reading on this topic:

Books: Q&A in MRI p36, MRI The Basics p47, MRI From Picture to Proton p38

Online: The Basics of MRI, e-MRI