Design and use of variable flip angle schedules in transient balanced SSFP subtractive imaging

In subtractive imaging modalities, the differential longitudinal magnetization decays with time, necessitating signal-efficient scanning methods. Balanced steady-state free precession pulse sequences offer greater signal strength than conventional spoiled gradient echo sequences, even during the transient approach to steady state. Although traditional balanced steady-state free precession requires that each excitation pulse use the same flip angle, operating in the transient regimen permits the application of variable flip angle schedules that can be tailored to optimize certain signal characteristics. A computationally efficient technique is presented to generate variable flip angle schedules efficiently for any optimization metric. The validity of the technique is shown using two phantoms, and its potential is demonstrated in vivo with a variable angle schedule to increase the signal-to-noise ratio (SNR) in myocardial tissue. Using variable flip angles, the mean SNR improvement in subtractive imaging of myocardial tissue was 18.2% compared to conventional, constant flip angle, balanced steady-state free precession (P = 0.0078).