On flow effects in balanced steady‐state free precession imaging: Pictorial description, parameter dependence, and clinical implications

Abstract

To present a pictorial description of the origin of flow effects in balanced steady-state free precession (SSFP) imaging that can result in considerable frequency offset-dependent signal changes originating from outflow of spins that can still contribute to the total SSFP signal (out-of-slice contributions), to analyze the parameter dependence and slice broadening associated with outflow effects, and to illustrate clinical implications such as frequency offset-dependent flow artifacts and spatial misencoding. Computer simulations were used to create a pictorial description of flow effects that illustrates the origin and parameter dependence of the observed signal changes and links their frequency dependency to the phase distribution of spins flowing in and out of the imaging slice. Slice broadening associated with out-of-slice contributions and flow-related signal enhancement was characterized by an effective slice thickness, which depends on flow rate, the T2* decay of signal magnitude from spins that have left the imaging slice, and the ratio of the net out-of-slice magnetization relative to the thickness of the imaging slice. Both simulated SSFP signal intensities and effective slice broadening vary with flip angle and demonstrate a nonlinear dependence on inflow. Simulations with bloodlike T1 and T2 show that the effective slice thickness can be as large as 15 times the prescribed slice thickness. These effects can have significant clinical implications such as large frequency offset-dependent signal enhancement, pulsatile flow artifacts, and spatial misrepresentation of blood signal that has already left the imaging slice. Flow-related signal changes in SSFP imaging exhibit highly complex parameter dependence, which predominantly has to be associated with frequency offset-dependent outflow effects and a resulting broadening of the slice thickness.