Method for High Spatial Resolution of Proximal Stations in 3D Time-Resolved Fluoroscopically-Triggered Bolus Chase MRA

Introduction: Bolus chase 3D contrast-enhanced MRA of the peripheral vasculature requires both rapid k-space sampling and precise timing of table motion to achieve high spatial resolution throughout an extended field-of-view while avoiding venous contamination at the lower extremities. Bolus chase methods typically guide table motion based on an estimate of the time for the contrast bolus to reach the most distal station, which is usually determined using a separate test injection of contrast. A potentially more accurate approach is to use MR fluoroscopy to trigger table motion in real time [1]. MR fluoroscopy has traditionally used low resolution 2D images with frame times as short as 1.0 sec to provide precise triggering [2]. However, given the time constraints in bolus chase MRA, it is desirable that the fluoroscopic images themselves be used for diagnostic purposes. This requires generation of 3D images with both high (1.0 mm isotropic) spatial resolution and short frame times. However, achieving frame times even as short as 2.5 sec is challenging. To date this has only been achieved using a high degree of view sharing with an extended temporal footprint of 20 sec or more, which is often longer than the time available to image a proximal station. As a result, there is often loss in lateral sharpness of vessels due to incomplete sampling of the kY-kZ phase encoding plane. In this work a modified CAPR-based fluoroscopic acquisition is described which maintains the 2.5 sec frame time while significantly reducing the temporal footprint, thereby improving the lateral sharpness of vessels in proximal stations by allowing more rapid sampling of the phase encoding plane. In vivo calf arteriograms from calf-foot studies demonstrate the high quality that can be obtained using the technique while rapidly generating time frames within a limited acquisition time window.