AFI (actual flip angle imaging) represents an interesting approach to map the B 1 transmit fields by measuring the spatial variations of the effective flip angle. However, the accuracy of the technique relies on the adequate spoiling of transverse magnetization. In the present work configuration theory was employed to develop a proper RF and gradient spoiling scheme for the AFI technique, making the sequence robust against off-resonance without the need of large spoiling gradients. In MRI applications, spatially varying sensitivity profiles of the RF transmit coils and local variations of the conductive and dielectric properties in the tissue may result in a spatially inhomogeneous B 1 transmit field. Therefore, accurate B 1 mapping is required for a variety of applications such as quantitative image processing (1) and multielement transmit applications (2,3) like RF-shimming (4), supported by appropriate high-field MRI systems. Most existing B 1 mapping approaches employ multiple acquisitions with different flip angles (5-9) or stimulated echoes in multipulse sequences (10 -13). Despite recent advances (1,8,9,13), many of these techniques are still too complicated and time-consuming for routine use. This is especially the case for RF shimming applications based on multiple RF transmit coil elements, where B 1 maps have to be acquired for each particular coil element.Recently, a fast steady-state B 1 mapping approach dubbed AFI (actual flip angle imaging) was introduced (14,15), consisting of two interleaved spoiled gradient echo acquisitions with different repetition times. In this approach, the flip angle is estimated by a robust and simple approximation, allowing an automatic derivation of the B 1 maps for a broad range of T 1 values. Therefore, this technique potentially enables fast in vivo B 1 mapping, making, e.g., in vivo B 1 shimming of multiple transmit coil elements possible.However, this approach is based on the assumption that the transverse magnetization is completely spoiled prior to each RF pulse. In previous studies RF spoiling has been proposed for this purpose (14 -16), but a thorough validation of this concept has not yet been presented for the AFI sequence. Moreover, large spoiling gradients were applied to overcome the off-resonance sensitivity of the sequence (14).In the present study a comprehensive analysis of the steady-state properties of the AFI approach was performed both qualitatively and quantitatively in the framework of configuration theory (17,18). Thus, a proper description of the steady-state conditions is developed, resulting in a novel RF and gradient spoiling scheme for the AFI sequence. Furthermore, numerical simulations are conducted, predicting the accuracy of the method dependent on the sequence and tissue parameters. Finally, experimental results on phantoms and volunteers will be presented for validation.
THEORY
AFI SequenceThe AFI approach employs a dual TR steady-state sequence. It may be regarded as a conventional spoiled gradient echo sequence, where an additional...