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. Learning Objectives: 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.
ASJC Scopus subject areas
- Radiology Nuclear Medicine and imaging