| As a non-invasive diagnostic tool, magnetic resonance imaging (MRI) can not only detect the structure change of the organization in the body, but also display the change of physiological information. It has been widely applied in the field of scientific researches and clinical diagnosis. The superconducting MR scanner has not been popularly used in the domestric hospitals due to its high cost and the low research and development level in China, especially the hospitals with a small or medium size in non-coastal and non-developed regions. In the current notional conditions, MRI system with low field is favored by the enterprise and hospitals due to the lower price. But the qulity of the images acquired in low-filed scanner are poor because of its natural low signal-to-noise-ratio (SNR). Then pulse sequence design will play an important role in the MR imaging in low-filed scanner.Magnetic resonance angiography (MRA) has recently been one of the conventional technologies for the diagnosis of vascular disease. Time-of-flight (TOF) is the most widely used clinical MRA method nowadays. TOF MRA, which relies on the flow-related enhancement (FRE) effect, is actually one kind of gradient echo (GRE) imaging methods. In TOF sequence, the first-order-gradient-moment nulling is often choosed to reduce flow-related artifacts. In the case of complex-flow, such as the most cerebral vessels, the high-order flow can cause spin dephasing that would lead to signal loss. Invisibility of some true vessels in angiography resulting from signal loss hinders the wide clinical application of TOF-MRA. In addition, Because three dimension (3D) TOF acquisitions are phase-encoded in two orthogonal directions, a typical whole brain scan could be about10minutes which is relatively long for patients, especially for elders and children. In this study, the optimization and improvement of original3D TOF sequence was completed on our0.35T MRI system. The main contents of this thesis include:1. The partial k-space acquisition was studied to reduce TE and the magnitude images were reconstructed by the projection onto convex sets (POCS) algorithm. A3D Fermi filter in k-space was demonstrated to increase the SNR. 2. The Shinnar-Le Roux (SLR) algorithm was studied to design the minimum phase RF pulse for3D TOF. The RF pulse can be applied to obtain a good slice profile and shorten TE.3. The TE shortening was studied to recude the signal loss resulting from spin dephasing effectively in TOF MRA. Three TE optimization methods were studied at low-field MRI. First, the maximal amplitude and minimal ramp time allowable were used to get the minimal duration for the flow-compensated gradients. Second, the partial-echo acquisitions were applied to reduce the time from the starting of acquisition to the echo center. Third, the short asymmetric radio frequency (RF) pulse was used to decrease the time from the peak where the spins effectively start dephasing to the end.4. A dual-echo3D TOF pulse sequence was developed. Comparing to single-echo acquisition, only half of the scan time is taken, but the MRA with a little lower CNRs is obtained in dual-echo3D TOF acquisition. When the same scan time is used, the dual-echo acquisition is demonstrated to improve the signal-to-noise-ratio (SNR) or the spatial resolution of MRA... |
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