Optimal Flip Angle Set in Spoiled Gradient-Echo MR Imaging Technique

Introduction: In dynmiac contrast enchanted-MRI (DCE-MRI), T1-Weghited spoiled Gradient-Echo (SPGR) sequences are utilized to monitor the distribution of the paramagnetically labeled CAs (contrast agent) in cerebral tumors due to high temporal and spatial resolution [1]. The tissue concentration of CAs measured by CAs effects on the longitudinal relaxation rate of water protons R1 is used by tracer kinetic modeling [2]. In spoiled gradient echo sequences, the T1-weighting of image contrast is strongly affected by a nonlinear interaction of two sequence parameters, repetition time (TR) and flip angle (α) [3]. The optimal TR and α are chosen to produce the field-dependent contrast behavior in MRI imaging, because the longitudinal relaxation time T1 is field dependent [4]. However, the variation of signal to noise ratio (SNR) directly affects on the accuracy of the various quantitative methods and consequently introduces bias in estimation of MR and physiological parameters. Therefore, a pulse sequence with a set of flip angels which provides a best signal to noise ratio would be useful in various quantitative methods. In this study, a set of optimal flip angles that are necessary to provide better tissue contrast at different magnetic field strengths (3T, 7T) is determined. Materials and Methods: The SPGRE signal based on the Bloch equation for T1 relaxation time is given by [3] equation-1. Where S0 denotes the equilibrium longitudinal magnetization. Equation-1 can be rewritten as a linear form to compute the longitudinal relaxation time (T1) for given TR and α as [4].This shows that the signal contrast is a function of both TR and flip angle. A set of optimal flip angels