MO-D-213CD-01: Cartesian Methods for Rapid Time-Resolved MR Angiography

The physics of the MR image formation fundamentally trades off spatial resolution with temporal resolution. Time spent in acquiring data for the second image of a time series can alternatively be spent in sampling higher spatial frequencies for the first image to improve its spatial resolution. Historically this tradeoff has been addressed by making the k-space sampling rate high, such as with very short repetition times, and with methods such as view sharing in which only a portion of k-space is updated from one image to the next in a time series. Over a decade ago the method of parallel acquisition was proposed in which the signals detected by the individual elements comprising a multi-element receiver coil are used to provide further spatial discrimination and reduce acquisition time. These approaches include those based in image space (SENSE) or in k- space (SMASH, GRAPPA). In the last decade these methods have been integrated in contrast-enhanced MRA (CE-MRA) to provide a radical improvement in performance. CE-MRA is an application particularly well suited to these methods. The general desire for MRA images to be three-dimensional allows the use of 2D implementation of parallel acquisition, generally much more robust than 1D implementation. Also, the SNR loss associated with parallel acquisition is tempered in CE-MRA because high, arterial-phase signal is sampled throughout the data acquisition. Cartesian MR data acquisition, performed along a rectilinear sampling pattern in k-space, offers specific advantages in relative ease of implementation of 2D parallel acquisition and in "freezing" the status of the time-varying object at a specific timepoint by use of centric view ordering. This presentation will provide a review of these methods and how they have been effectively developed and integrated within the last decade for improved time-resolved MRA. Cartesian k-space sampling patterns can now be quickly selected on a patient- and anatomy-specific basis for optimum acceleration. Receiver coil arrays have been adapted to allow up to 20× reduction in the number of k-space points sampled for a given spatial resolution. Reconstruction hardware now allows generation of 3D images within only hundreds of msec after data acquisition, permitting real-time generation of diagnostic quality images and their use in interactively guiding other processes.

1. Understand recently developed physics techniques which have allowed a 20x improvement in the speed of data acquisition for MR angiography 2. Understand how Cartesian sampling of k-space facilitates the practical and effective implementation of these techniques 3. Show how contemporary implementation of these physics techniques has provided a significant improvement in MRA image quality over the last decade.