T2 decay curves (Mxy(t) = Mxy(0)exp(-t/T2)).
CORRECT. The answer—waiting for signal decay—refers to changing the echo time (TE). The TE enhances or minimises the difference in signal due to differences in the T2 of tissues.
Recall that we cannot measure an MRI signal unless the net magnetisation vector is pushed away from alignment with the external magnetic field. Then it precesses, generating a signal perpendicular to the external magnetic field.
So, if we rotate the net magnetisation vector by 90°, the entire magnetisation is tipped into the xy-plane. The amplitude of the signal then decays away exponentially with a time constant T2.
T2-weighted spine image. Note the cerebrospinal fluid is bright.
If we wait only a short amount of time before measuring the signal (a short TE), then not much difference between the signal amplitudes of tissues will have developed. However, if a longer time passes (a longer TE), a larger difference between the signal amplitudes of tissues will have developed.
Thus, a longer TE enhances differences in signal arising from differences in the T2 of tissues (even though a shorter TE would provide a higher signal from both tissues). Using a longer TE achieves T2-weighting.
The TE is an operator-selectable parameter on the MRI scanner. Contrast differences due to T2 in an MRI image can be manipulated.
INCORRECT. This answer refers to changing the repetition time (TR). The TR enhances or minimises the differences in signal due to differences in the T1 of tissues, not the T2.
INCORRECT. The native T2 relaxation time of a tissue cannot be changed (for a specific magnetic field strength).