Time-resolved MRA from head to toe – nice toy or helpful clinical tool?

Time-resolved MR angiography (MRA) has been proposed as a non-invasive and nonionizing imaging method. Compared with monophasic CTA and MRA it provides better arteriovenous differentiation and functional dynamic information that so far has been gained only with DSA. When contrast-enhanced MRA was first introduced in 1993 acquisition time was in the range of several minutes. In addition the image acquisition had to be synchronized with the bolus arrival of the contrast agent. Several techniques have been developed since then in order to align the image acquisition with the bolus arrival including test bolus, automated bolus detection and fluoroscopic triggering. Methods with a centric k-space ordering were introduced that sample the central phase encoding views entirely during the arterial phase when triggered at the arrival of contrast material in the volume of interest. It is critical to initiate the centric k-space sampling so that the first views correspond to the peak of the arterial contrast opacification to avoid the presence of marked edge enhancement artifacts with little contrast in the center of the arteries. Despite of all technical attempts to improve the bolus timing and to predict the optimal time delay contrast-enhanced MRA still requires expertise and trained personnel. Time-resolved MRA is a new concept that allows for fast dynamic acquisitions with acceptable spatial resolution without the need of distinctive timing methods by combining techniques to accelerate the MR acquisition. Parallel imaging (PI) has proved to allow for substantial improvement in temporal resolution as well as in spatial resolution in contrast-enhanced MRA. Although PI acceleration factors are limited at 1.5T because of signal-to-noise