A pictorial description of steady-states in rapid magnetic resonance imaging

Magnetic resonance imaging in biochemical and clinical research requires rapid imaging sequences. Time-resolved imaging of heart movement and the acquisition of a three-dimensional image block within the circulation time of a contrast agent bolus are two typical examples. Rapid imaging sequences are characterized by a very fast train of radiofrequency (rf) and gradient pulses. Between these rf pulses, the excited magnetization is unable to return to its thermal equilibrium. As a consequence, further rf pulses will influence both the remaining transversal and the remaining equilibrium state. The steady-state magnetization of a multi-rf pulse and gradient pulse experiment is thus a mixture or superposition of different transversal and longitudinal states and the acquired image amplitude becomes a complex function of the investigated tissue's relaxation properties. Based on the works of Woessner, Kaiser, and Hennig, this article intends to give a pictorial description of rapid multipulse imaging experiments. It also provides an extension of this theory applied to modern imaging sequences such as TRUE FISP and rf-spoiled techniques