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### QUESTIONS» Image Creation

How is the MRI image created? This section explores slice selection, spatial encoding and the FFT.

# Full Recovery Time

## How long should we allow for "full" recovery of Mz after a 90° RF excitation pulse?

A time equal to the T2* relaxation time.

A time approximately equal to 5·T1.

Ten seconds.

INCORRECT. The T2* relaxation time describes the decay of Mxy, not the recovery of Mz.

Try again.

Example recovery curve of Mz (time constant T1).

CORRECT. After a 90° rotation of M0, we must wait 5·T1. (T1 is the longitudinal relaxation time of the tissue in question.)

Magnitude of Mz is given by

Mz(t) = M0(1-exp(-t/T1))

At t=5·T1, this equals

Mz(t) = M0(1-exp(-5T1/T1))

and so

Mz(t) = 0.993·M0

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 (M0). 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.

Try again.