Experimental study of various factors affecting signal intensities in MR angiography

Time of flight effects generated by flowing protons in the blood are influenced by various factors including velocity, direction and type of the flow, relaxation time of blood (T1), pulse sequence and its parameters (TR, TE, flip angle), saturation of the flowing blood, and geometories of excitations and detections. We analyzed the influence of each factor on steadily flowing water, using a 3 dimentional flow-phantom and a stenotic tube flow phantom. The factors, which affect contrast ratios most, were flip angles and flow velocities. Optimum flip angle were between 15-25 degrees. Flip angle should be larger to evaluate faster flow, but smaller to demonstrate slower flow. When TRs and flip angles were constant with changing flow velocities, contrast ratio showed the peak value at 40 cm/s in the tube of 7 mm in diameter. When 2D FLASH and 3D FISP were compared at the same flow velocities, decease of signal intensity due to saturation was less remarkable in 2D FLASH. Axial slab section less influenced upon decrease of signal intensity due to saturation of protons. Contrast ratio decreased at flow velocity bigger than 20-25 cm/s in the tube of 10 mm in inner diameter and contrast ratio around stenotic region decreased in more severe stenoses. (author)