The effects of linearly increasing flip angles on 3D inflow MR angiography.
As recently demonstrated, spin saturation effects in 3D time-of-flight (TOF) MR angiography (MRA) can be reduced by using RF pulses with linearly increasing flip angles (ramp pulses) in the main direction of flow. We developed a model for calculating the signal distribution of proton flow within the excitation volume (slab) for different ramp slopes and compared the results with the measured distribution for the lower-leg arteries. The ramp pulses were generated using the Fourier transformation of the desired excitation profiles. With a bandwidth of 6 kHz and a pulse length of 2.56 ms satisfactory ramps with variable slopes were generated and applied in a standard flow-compensated 3D FISP sequence. The effects on the signal distribution in the resulting angiograms of the lower limbs revealed a considerable reduction of saturation losses in agreement with the calculations. Calculated optimal ramp slopes are provided for flow velocities ranging from 5 to 50 cm/s and excitation volumes ranging from 5 to 25 cm.
Department of Neuroradiology, University of Tübingen, Germany.
T
T
Nägele
U
U
Klose
W
W
Grodd
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D
Petersen
J
J
Tintera
D000465:Algorithms; D001158:Arteries / Q000502:physiology; D016477:Artifacts; D001775:Blood Circulation / Q000502:physiology; D001783:Blood Flow Velocity / Q000502:physiology; D001808:Blood Vessels / Q000033:anatomy & histology / Q000502:physiology*; D005360:Fibula / Q000098:blood supply; D005583:Fourier Analysis; D006801:Humans; D007089:Image Enhancement / Q000379:methods*; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008955:Models, Cardiovascular; D011150:Popliteal Artery / Q000502:physiology; D012039:Regional Blood Flow / Q000502:physiology; D015203:Reproducibility of Results; D016909:Tibial Arteries / Q000502:physiology