Reconstruction Strategies for MRI with Simultaneous Excitation and Acquisition

Introduction Imaging of samples with very short T2 requires data acquisition to start immediately after excitation. In the radial FID method [1] this is accomplished by excitation and acquisition during a permanently applied gradient. However, for high bandwidths the FID approach must use very short RF pulses, limiting the possible flip angle according to the given maximum B1 amplitude. On the other hand, under identical conditions pulses with a frequency sweep and longer duration provide considerably larger flip angles [2]. With such an excitation, signals from different locations are shifted in time and dephased, and can be reconstructed after rephasing [3]. The SWIFT technique [4] combines the two described concepts by playing out a gapped frequency-swept pulse while concurrently acquiring data during the gaps. Thus samples with short T2 can be imaged also under B1 restrictions. Furthermore, the method incorporates the agreeable features of being fast and silent [1] and creating a reduced dynamic signal range [3]. With SWIFT a new, general concept of simultaneous excitation and acquisition (SEA) has been introduced. Data obtained in such a manner experience a hybrid net encoding that requires specific image reconstruction. For the SWIFT technique a deconvolution approach and different correction schemes have been proposed [5, 6]. Here, a more general reconstruction procedure is presented, derived from a description of the signal evolution for SEA. It is highly flexible and enables taking into account not only the pulse effects but also prolonged acquisition, gapped acquisition, and oversampling.