Need for a Saturation RF Band to Mitigate Flow Artifact in 3D-MPRAGE On a Compact 3T Scanner
M In*, N Campeau, J Huston, D Kang, L Bardwell Speltz, N Meyer, J Trzasko, Y Shu, M Bernstein, Mayo Clinic, Rochester, MN
Presentations
TH-IePD-TRACK 2-6 (Thursday, 7/29/2021) 3:00 PM - 3:30 PM [Eastern Time (GMT-4)]
Purpose: Due to the smaller dimensions (inner diameter 37 cm, length 40 cm) of the RF-transmit coil on compact 3T (C3T) scanner [1], the B1 field falls off rapidly outside the head. This offers great potential for safe brain imaging of patients with implanted devices without sacrificing any imaging capabilities [2]. For a whole-body scanner, during a fast gradient-echo scan, the arterial blood usually experiences repeated RF pulses before flowing into the brain, so the magnetization becomes saturated. However, because of the relatively shorter RF coverage of the C3T, inflowing blood saturates less than with a conventional, whole-body transmit RF coil does (Fig. 1). This results in strong hyperintense intravascular signal and intense flow-related ghost artifacts. In this study, we demonstrate that these specific 3D-MPRAGE flow artifacts can be mitigated by applying an additional inferior spatial pre-saturation.
Methods: An RF spatially selective saturation band module was added to the standard 3D-MPRAGE sequence to saturate incoming arterial flow from the heart (Fig. 2) and consequently suppress the intensity of intracranial vasculature. 3D-MPRAGE imaging was performed on the C3T both without and with use of the inferior RF saturation band in two human subjects under an IRB IRB-approved protocol. Image assessment was performed by two board board-certified neuroradiologists.
Results: With the additional saturation RF pulses, the acquisition time was increased 20 seconds compared to the standard acquisition (5:02 vs. 5:22). While prominent ghost artifacts were observed along the in-plane phase-encoding (i.e., inferior-to-posterior) direction in the standard sagittal scan, the pulsation effects were substantially reduced in the proposed scan due to RF pre-saturation of the incoming arterial signal (Fig. 3).
Conclusion: 3D-MPRAGE imaging with an additional RF saturation band is an effective method to suppress vascular ghost artifacts caused by the arterial blood flow inherent to smaller bore MRI system such as C3T.
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