fig 1.

Operation of signal-nulling IR sequences. Normalized signal plotted against inversion time.

A, The longitudinal magnetization of the target tissue to be nulled must pass through zero when the 90° pulse is applied (curve I). Tissues with other T1 values (eg, curve II) produce signals at this time. For example, at 100 ms (when curve I passes through zero), curve II yields a signal of −0.63.

B, An incorrect flip angle of the inverting pulse produces only partial inversion of the longitudinal magnetization and now results in a recovery curve that has already passed through zero when the 90° pulse is applied (curve I) so that the target tissue is no longer nulled. The signal from the other tissue at this time is changed from −0.63 to −0.43.