# Full Recovery Time

INCORRECT. The T2* relaxation time describes the decay of M_{xy}, not the recovery of M_{z}.

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Example recovery curve of M

_{z}(time constant T1).

CORRECT. After a 90° rotation of M_{0}, we must wait 5·T1. (T1 is the longitudinal relaxation time of the tissue in question.)

Magnitude of M_{z} is given by

M_{z}(t) = M_{0}(1-exp^{(-t/T1)})

At t=5·T1, this equals

M_{z}(t) = M_{0}(1-exp^{(-5T1/T1)})

and so

M_{z}(t) = 0.993·M_{0}

i.e. 99.3% recovery has occurred. Even after a 180° RF pulse, 98.7% recovery has occurred at t=5·T1.

Note that longitudinal recovery follows an exponential shape and thus is asymptotic to the equilibrium value (M_{0}). So in effect you could say that the (classically described) spin system never returns to equilibrium!

INCORRECT. Although ten seconds is long enough for even the slowest recovering tissues in MRI (longest T1), this is not the rule.

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