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 lin early increasing flip angles (ramp pulses) in the main direction of fl ow. We developed a model for calculating the signal distribution of pr oton flow within the excitation volume (slab) for different ramp slope s and compared the results with the measured distribution for the lowe r-leg arteries. The ramp pulses were generated using the Fourier trans formation of the desired excitation profiles. With a bandwidth of 6 kH z and a pulse length of 2.56 ms satisfactory ramps with variable slope s were generated and applied in a standard flow-compensated 3D FISP se quence. The effects on the signal distribution in the resulting angiog rams of the lower limbs revealed a considerable reduction of saturatio n losses in agreement with the calculations. Calculated optimal ramp s lopes are provided for flow Velocities ranging from 5 to 50 cm/s and e xcitation Volumes ranging from 5 to 25 cm.