New insights into the mechanisms of signal formation in RF-spoiled gradient echo sequences.
RF spoiling is a well established method to produce T(1)-weighted images with short repetition-time gradient-echo sequences, by eliminating coherent transverse magnetization with appropriate RF phase modulation. This paper presents 2 novel approaches to describe signal formation in such sequences. Both methods rely on the formulation of RF spoiling as a linear increase of the precession angle between RF pulses, which is an alternative to the commonly used quadratic pulse phase scheme. The first technique demonstrates that a steady state signal can be obtained by integrating over all precession angles within the voxel, in spite of the lack of a genuine steady-state for separate isochromats. This clear mathematical framework allows a straightforward incorporation of off-resonance effects and detector phase settings. Moreover, it naturally introduces the need for a large net gradient area per repetition interval. In the second step, a modified partition method including RF spoiling is developed to obtain explicit expressions for all signal components. This provides a physical interpretation of the deviations from ideal spoiling behavior in FLASH and echo-shifted sequences. The results of the partition method in the small flip angle regime are compared with numerical simulations based on a Fourier decomposition of magnetization states. Measurements performed with in vitro solutions were in good agreement with numerical simulations at short relaxation times (T(1)/TR = 32 and T(2)/TR = 4); larger deviations occurred at long relaxation times (T(1)/TR = 114 and T(2)/TR = 82).
Denolin
Vincent
V
Unité de Résonance Magnétique, Imagerie Médicale, Hôpital Erasme, Université Libre de Bruxelles, Belgium. vdenolin@ulb.ac.be
Azizieh
Céline
C
Metens
Thierry
T
D000465:Algorithms; D001921:Brain / Q000033:anatomy & histology*; D003198:Computer Simulation; D006801:Humans; D007089:Image Enhancement / Q000379:methods*; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D021621:Imaging, Three-Dimensional / Q000379:methods; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008954:Models, Biological; D019047:Phantoms, Imaging; D011846:Radio Waves; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D012815:Signal Processing, Computer-Assisted