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The effects of linearly increasing flip angles on 3D inflow MR angiography
Author(s) -
Nägele Thomas,
Klose Uwe,
Grodd Wolfgang,
Petersen Dirk,
Tintera Jaroslav
Publication year - 1994
Publication title -
magnetic resonance in medicine
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.696
H-Index - 225
eISSN - 1522-2594
pISSN - 0740-3194
DOI - 10.1002/mrm.1910310515
Subject(s) - flip angle , excitation , nuclear magnetic resonance , saturation (graph theory) , materials science , signal (programming language) , optics , physics , magnetic resonance imaging , mathematics , radiology , medicine , quantum mechanics , combinatorics , computer science , programming language
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.