Effects of RF inhomogeneity at 3.0T on ramped RF excitation: Application to 3D time‐of‐flight MR angiography of the intracranial arteries

Abstract

Purpose:To demonstrate the effects of inherent RF inhomogeneity on ramped RF excitation at 3.0T, and to introduce a simple correction for improving visualization of distal intracranial arteries in three-dimensional time-of-flight MR angiography (3D-TOF-MRA). Materials and Methods:At 3.0T, the effects of RF inhomogeneity arising from RF interference were demonstrated for ramped RF excitation in intracranial 3D-TOF-MRA. Computer simulations and experiments on phantoms and eight normal volunteers were performed. Four different ramp shapes were tested as a possible means of countering the reduced RF field that affects the distal intracranial arteries.Results: RF destructive interference alters the ramp pulse shape, which is problematic for vessels that proceed from the center to the edge of the brain. Increasing the ramp pulse slope was shown to be an effective yet simple correction to counter the falling-off of the RF field toward the periphery of the head. With this approach, circle-of-Willis 3D-TOF-MRA studies had improved distal visibility. Conclusion:Ramped RF excitation is severely affected by RF interference at 3.0T, which makes the ramp profile suboptimal for distal intracranial blood vessels. A simple correction of the ramp slope can make a marked improvement.

Imaging and analysis of lenticulostriate arteries using 7.0‐Tesla magnetic resonance angiography

“…What is needed to be explored with the SAR issue is the development of new techniques, such as adiabatic RF pulse sequence and parallel imaging techniques, whereby the SAR effect can be minimized (25, 26). There are also other technical problems that remain, such as B 1 ‐field inhomogeneity (27, 28).…”

Evaluation of MR angiography at 7.0 Tesla MRI using birdcage radio frequency coils with end caps

“…A report indicated that B1 inhomogeneity would severely affect the ramped excitation, which was usually used for 3D TOF MRA (16) and that the adjustment of the ramp slope could resolve this problem. However, this method could reduce the relative visibility of blood vessels in the bottom region of the imaging slab.…”

“…Furthermore, there was also increased signal intensity at the superior sagittal sinus on 2D TOF images and greater resolution of blood vessels in the S–I direction on 3D TOF images. However, there might be less improvement in 3D TOF MRA than in 2D, because of the ramped excitation distorted by B1 inhomogeneity at 7.0T MRI (16).…”

“…For the best MR angiography (MRA) performance, dedicated head coils have been designed for conventional 1.5T MRI (15). Adjustment of the ramp–pulse slope was also introduced as a method that compensates for the ramped radio frequency (RF) excitation profile being severely affected by B1 inhomogeneity in three‐dimensional (3D) TOF MRA applications (16). At ultra–high field strength, especially at 7.0T, several BC head coils and multiple phase‐arrayed coils have been investigated for high‐resolution anatomical imaging (17–19).…”

Simultaneous time‐of‐flight MR angiography and quantitative susceptibility mapping with key time‐of‐flight features

“…First, to suppress static background signals, TOF-MRA requires a higher flip angle than that used in SWI/QSM. 34 Second, the RF excitation in 3D TOF is typically a ramped RF pulse to maximize vessel contrast, particularly for slower flowing vessels, 35,36 as opposed to a flat RF profile used in SWI/QSM. Third, to further improve inflow, 3D TOF is typically performed with MOTSA, 37 while SWI/QSM methods tend to use one large slab.…”