Rapid methods for concurrent measurement of the RF-pulse flip angle and the longitudinal relaxation time.

Measuring both the flip angle (FA) and the longitudinal relaxation time T(1) is essential in quantitative and longitudinal studies because the signal amplitude is dependent on these quantities. Conventional methods can only measure one of them at a time and require long scan times. In this work, two mutually consistent methods are developed; each can acquire multislice data for determining both the FA and T(1) in a scan time about half the time needed for a conventional FA measurement. On the basis of a recent development of longitudinal-relaxation measurement (Hsu and Lowe, J Magn Reson 2004;169:270-278; Hsu and Glover, J Magn Reson 2006;181:98-106), one of the methods uses RF pulse trains of two FAs whereas the other uses pulse trains of different pulse spacing. When only the FA or T(1) is needed, the present methods can still be faster than conventional methods for the needed quantity. In benchmarking with a uniform-density sample, both methods generate precise T(1) values independent of the FA chosen (except at and near 90 degrees ). In the demonstration with three normal volunteers at 3 T, the T(1) values of frontal and occipital white matter, putamen, and caudate are compared; the T(1) values are in agreement with literature values and the intrasubject deviation is 0.2%-2.8%.