Signal intensity in fast NMR imaging with short repetition times

Abstract In fast magnetic resonance imaging with short repetition times and balanced phase-encoding gradients, the steady-state transverse magnetization that develops depends on the local relaxation times in a way that differs substantially from more conventional imaging. To optimize these imaging methods for different applications requires an accurate mathematical description of the dependence of the signal intensity on the repetition time, the RF pulse flip angle, and the local relaxation times. Several previously proposed equations for the signal intensity are reviewed, and an expression appropriate to the usual practical application of pulse sequences with constant but unbalanced imaging gradients is derived. Phantom experiments, using water doped with MnCl 2 and NiCl 2 to alter the relaxation times, were used to test the accuracy of these different mathematical expressions for the signal intensity. The new expression provides a good fit to data measured over a large range of TR and flip angle, but implies a transverse relaxation time shorter than that measured by varying the echo time in a spin-echo experiment. Possible sources of this effect are discussed.