Abstract:
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Purpose: To simulate and measure the transient response with and without RF
phase-modulation (RFPM) between 3D FIESTA-C volume acquisitions.
Methods and Materials: Phase-cycled Steady State Free Precession (SSFP) imaging
techniques are useful for high resolution imaging of structures such as the
internal auditory canal or cartilage. For example, 3D FIESTA-C acquires
multiple image volumes, each with its own steady-state (SS) phase-cycling
scheme, and combines the resulting volumes to obtain an image with reduced SSFP
banding artifact. Image volumes are acquired sequentially in order to maintain
the SS condition, increasing the sensitivity of the combined images to motion
artifacts. A technique has been introduced using reverse elliptic centric view
ordering for the first volume, followed by an elliptic centric acquisition for
the second volume. Thus, the centers of k-space are acquired in close temporal
proximity, reducing gross misregistration between volumes. However, dummy
repetitions (disdaqs) are required between the volumes to reduce transient
signals as the second volume approaches SS. Linear RFPM during some of the
disdaqs may reduce the severity of the transient signals. In this work, the
transient response with and without RFPM was analyzed using numerical
simulations and phantom experiments. Both on- and off-resonance spins were
analyzed for oscillatory behavior as well as transverse signal magnitude. MR
signal magnitude of the water phantom was measured using the FIESTA-C pulse
sequence with phase- and slice-encoding gradients disabled. The center
frequency was adjusted to obtain on- and off-resonance spins.
Results: In the simulations, off-resonance spins exhibited no oscillatory
behavior. Without RFPM, other spins exhibited oscillatory behavior for about
3*T1 seconds between volumes. Spins with RFPM showed negligible oscillatory
behavior, even with as few as 30 disdaqs. However, phase-modulated spins still
relax to their SS positions over ~3*T1 seconds. Phantom data confirmed the
observations. However, non-RFPM phantom data showed a shorter duration of
oscillatory behavior than simulation. This may reflect experimental
imperfections such as B0 inhomogeneity or varying flip angles within the
slice-select slab.
Conclusion: A minimal number of disdaqs with RFPM (0.5 sec) vastly reduces the
oscillatory behavior of the transient SSFP response. Increasing the number of
disdaqs with RFPM does not appear to provide additional benefit for oscillatory
behavior and actually slows the approach to SS signal levels. (H.W., J.P. are
employees of GE Medical Systems.)
Ward PhD, H,
Effect of RF Phase Modulation on Transient Response between Acquisitions in 3D FIESTA-C. Radiological Society of North America 2003 Scientific Assembly and Annual Meeting, November 30 - December 5, 2003 ,Chicago IL.
http://archive.rsna.org/2003/3107536.html
