Microsoft Word - ISMRM2009-000726.DOC

Introduction Due to its high imaging speed and SNR efficiency, spoiled gradient echo imaging plays an important role in many quantitative MR methods. For example, a variable flip angle (VFA) method [1] is now widely used for in vivo T1 mapping due to its time efficiency and large 3D anatomical coverage. Recently, an actual flipangle imaging (AFI) technique [2], which utilizes two interleaved spoiled gradient echo acquisitions with different TRs, has been proposed for rapid mapping of transmitted radiofrequency field B1. B1 mapping has become increasingly important due to the growing availability of high-field clinical and research scanners operating at 3T and higher. Critical to the accuracy of these quantitative methods based on spoiled gradient echo imaging is the complete spoiling of the transverse magnetization at the end of each TR. However, it was recently recognized that conventional RF spoiling yields non-ideal steady state signal intensities, particularly at larger flip angles and T1/TR ratios, leading to significant quantification errors [3-4]. The purpose of this work is to propose an alternative spoiling scheme based on random gradient moment and RF phase, and to compare the performance of the proposed spoiling scheme with conventional RF spoiling for T1 mapping and flip angle correction. Theory In conventional RF spoiling, the RF phase is often set to be a quadratic function of TR number. A constant gradient moment is also applied at the end of each TR period to create a range of resonance offset angles within each imaging voxel. This scheme leads to a steady state that is generally different from the ideal spoiling condition (Fig. 1a). In the proposed random spoiling scheme, both the RF phase and the gradient moments at the end of each TR are randomized (with intra-voxel phase dispersion in the range of [20π, 40π]). Although this leads to slight TR-to-TR variations of the signal in each voxel, the average voxel signal intensity matches the ideally spoiled condition at a wide range of T1/TR ratios and flip-angles (Fig. 1b). As the signal variations can cause ghosting artifacts in conventional Cartesian imaging, radial acquisition could be utilized to suppress these artifacts, taking advantage of the averaging effect of oversampling in central k-space. It was found that using either random RF phase or random gradient moment alone results in higher deviation of the average signal from ideal levels.