Improvements in gradient performance and novel image acquisition schemes such as parallel imaging have greatly accelerated MR image acquisition. Until recently, these advancements have been used almost entirely for improving the spatial resolution of MR angiography (MRA). For most clinical applications, the goal of optimization of parameters for highest possible spatial resolution is advantageous for MRA, especially for contrast-enhanced (CE) MRA. High spatial resolution is often necessary for the confident visualization of smaller vessels, grading of arterial stenoses and proper identification of more subtle vascular pathologies such as penetrating ulcers.
However, faster imaging speed can also be used to improve the temporal resolution of CE MRA. The dynamic assessment of contrast media bolus progression through a vascular territory, or vascular lesion has long been an essential diagnostic tool for conventional x-ray angiography, especially x-ray digital subtraction angiography (DSA). The development of faster MR scanners and innovative software has enabled the performance of analogous dynamic viewing of Gd-chelate contrast media using fast two-dimensional (2D) and/or three-dimensional (3D) MRA acquisitions. Time resolved MRA is rapidly emerging as a valuable MRA tool in select diagnostic situations.
Until recently, time resolved imaging has primarily been available using two-dimensional (2D) pulse sequences for limited planar or "projection-al" viewing. However, with the advent of ultrafast gradients (slew rates of faster than 100 T/m/s for "sub-second" 3D CE MRA ) and alternative k-
space schemes (e.g. Time Resolved Imaging with Contrast Kinetics or TRICKS ), time-resolved imaging can now be achieved using three-dimensional (3D) MRA, for "4D" or time-resolved volumetric dynamic evaluations.
This chapter will explore some of the available techniques for time-resolved MRA and will highlight clinical scenarios in which high temporal resolution imaging can improve the diagnostic value for an MRA exam. Time-resolved imaging is a particularly helpful tool for multi-station MRA exams, identification of flow patterns associated with certain vascular pathologies and arterial-venous segmentation of complex vascular malformations or arrangements.
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