| Non-contrast enhanced magnetic resonance angiography(MRA)is more and more popular for clinical applications because it is noninvasive,radiationless and free of contrast agent.These properties make angiography safe for people,especially for those whose renal function is impaired.On the other hand,with the development of the imaging technology,more and more small vessel diseases are found,which promote the demand of imaging methods with higher sensitivity to small vessels.Some non-contrast enhanced MRA methods have been used routinely in the diagnosis process,such as time-of-flight(TOF)and phase contrast angiography(PCA),however,their performance is limited when imaging the small vessels.Theoretically,the contrast between vessels and background tissues comes from the flow-in,flow-out or flow-dephasing effect,but the lumen diameter of small vessels is small,and the blood flow is slow,so it is difficult to acquire the vessel-background contrast using routine non-contrast enhanced MRA methods.Additionally,high spatial resolution is needed because of the small lumen diameter,otherwise the signal of vessels and background tissues will be mixed together.However,high spatial resolution will decrease the images’ signal-to-noise ratio(SNR)and prolong the scan time,which could make it not practical for clinical applications.In this thesis,some advanced non-contrast enhanced MRA technologies were investigated for imaging lenticulostriate arteries(LSAs)and vessels in both liver and kidney.Hybrid of opposite-contrast(HOP)and flow-sensitive black blood(FSBB)sequences were used for LSAs imaging,and velocity selective MRA(VSMRA)sequence was used for liver and kidney vessels imaging.In this work,the imaging sequences were developed,the designs of the sequences were improved and the imaging parameters were optimized on a 3T Philips clinical MR scanner,which lead to better visualizations of small vessels and make the methods more practical for clinical applications. |
