| This dissertation describes a novel imaging strategy for Magnetic Resonance Angiography (MRA) that provides significant reductions of several types of flow-related artifacts. Vascular images must be generated with sufficient diagnostic accuracy, but for some MRA methods, such as conventional two-dimensional time-of-flight (2D TOF), visualization of complex flow is limited by artifacts (signal loss and distortion) that arise from the signal generation (excitation) and data acquisition schemes. Accordingly, the new imaging strategy incorporates alternative excitation and acquisition schemes to minimize the interval between excitation and readout (TE) and permit consistent depictions of arterial structures with fewer disruptive artifacts due to motion, pulsatility, displacement, and dephasing.;Because this work addresses issues in MRA, a tutorial overview is provided, describing underlying principles of MR, components of an MR experiment, and common MRA techniques. The remainder of the dissertation focuses on acquisition and excitation schemes for improved vascular imaging. In particular, because the order and pattern (trajectory) of the 2D data collection strongly influences the appearance of certain artifacts, a radial-line-based acquisition pattern (e.g., the pattern formed by the spokes of a wheel) is examined as an alternative to the conventional left-right top-bottom raster pattern (2DFT). The intrinsic averaging of low spatial frequencies due to oversampling in radial patterns permits a reduction in pulsatile flow and vessel motion artifacts, while a carefully designed trajectory offers potential scan time reductions. The twisting-radial line (TwiRL) acquisition trajectory is offered as a representative sampling pattern. Conventional excitations, on the other hand, are commonly associated with flow dephasing and displacement artifacts, which usually occur in regions of fast or irregular flow and result in an overestimation of stenoses, clearly unacceptable for guiding surgical decisions. To minimize these artifacts, a "half-pulse" excitation scheme is proposed. This excitation scheme provides a significant reduction of spin dephasing and displacement by eliminating the need for gradient moment nulling. A half-pulse excitation/radial-line acquisition combination, therefore, maximizes the benefits for flow imaging and permits ultra-short echo times, limited by system constraints only. Application to peripheral and carotid arteriography demonstrates the improvements afforded by the new imaging method when compared with conventional 2D TOF MRA. |
