Four-dimensional noncontrast-enhanced MR angiography at ultrahigh field
© Metzger et al; licensee BioMed Central Ltd. 2013
Published: 30 January 2013
Arterial spin labeled (ASL) 4D MRA has been shown to be a valuable method for simultaneously assessing anatomic structure and dynamic filling of cerebral arteries. Beyond improved SNR and parallel imaging performance, performing these studies at ultrahigh field will benefit from longer T1s resulting in more persistent labeling allowing for improved visualization of distal vessels, especially in conditions of reduced flow.
To address the challenges of B1+ homogeneity and efficiency at 7T, B1+ shimming was performed based on a small flip angle, calibration scan. Two B1+ shimming strategies were compared; 1) a static approach which used a CP-like mode over the entire brain for all RF pulses in the sequence, and 2) a dynamic approach which used a separate B1+ shim optimized for efficiency for the IR labeling pulse (Shim 1) and the CP-like mode for excitation and background suppression (Shim 2) (Fig. 1).
A dynamically applied transmit B1 (B1+) shimming strategy was employed to accommodate limitations on peak B1+, B1+ homogeneity and SAR for four-dimensional non-contrast enhanced angiography at 7T. Initial results showing signal persistence in distal arteries is made possible by the increased labeling efficiency of the dynamic B1+ shimming approach as well as the increased T1s of blood at ultrahigh field.
Funding Provided by P41 EB015894, S10 RR26783 and WM KECK Foundation.
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