fig 2.

A and B, Signals (normalized to value at full relaxation) for FLAIR (A) and STIR (B) sequences plotted against RF flip angle (degrees) of the initial “inversion” pulse. Sequence parameters have been set at ∞/90 (TR/TE) and TI = 2100 for the FLAIR sequence and ∞/30, TI = 130 for the STIR sequence. 1.0-T values for tissue T1/T2 (ms) were used as follows: brain (mixed gray and white matter) = 700/80, CSF = 4500/4500, fat = 185/80, and muscle = 500/40. The desired performance (which is an effective flip angle of 180°) is shown at the extreme right-hand end of the graphs. For brain and CSF with the FLAIR sequence, there is a gradual rise of CSF signal as the flip angle is decreased (ie, shifts to the left). Equal signals are seen at 112°. With the STIR sequence, fat increases its signal as the flip angle is reduced. Muscle shows a reduced signal magnitude and is nulled at a flip angle of 108°. It has a higher signal than fat at flip angles less than 75° (B)