A General Trajectory Tester

Introduction While the vast majority of clinical MRI sequences utilize Cartesian sampling strategies, in a growing number of applications nonCartesian schemes are being employed for a variety of technical reasons. For example, our own motivation is diffusion tensor imaging (DTI) wherein we use spiral trajectories [1], in part because they facilitate efficient self-navigation schemes for artifact mitigation. Non-Cartesian sampling schemes require some method of non-uniform Fourier reconstruction in order to produce an image. Moreover, although most non-Cartesian methods to date have been 2D, there is growing interest in true 3D trajectories. Aside from the theoretical issue of how to design 3D trajectories, there is a significant practical problem of how to practically evaluate such trajectories, which requires two essential elements: 1) A precisely known image (a “gold standard”) by which to compare the relative performance of different trajectories, and 2) A general reconstruction method for arbitrary trajectories (and interleaves). A major roadblock to studying the trajectory problem is the lack of a platform within which it is possible to explore the multitude of issues in a practical manner. To address this issue, we have developed a software platform called the General Trajectory Tester (GTT) that allows users to input arbitrary 3D k-space trajectories, in an arbitrary number of interleaves, which are then used to sample and reconstruct the well known 3D analytical Shepp-Logan phantom [2]. Reconstructions can then be quantitatively compared with a true image. At the heart of the GTT is our extension to 3D of the 2D gridded reconstruction method of Inati and Greengard [3] used in our previous work [1], which allows for the Fourier reconstruction of arbitrarily spaced data. The GTT can also simulate diffusion weighting, including arbitrary diffusion angular encoding schemes for DTI, multiple b-values, eddy current and motion induced artifacts and self-navigation, and so is a natural platform to test efficient DTI acquisition and self-navigation schemes.