Hybrid of opposite-contrast MR angiography (HOP-MRA) combining time-of-flight and flow-sensitive black-blood contrasts.

For the purpose of visualizing low-flow as well as high-flow blood vessels without using contrast agents, we propose a new technique called a hybrid of opposite-contrast MR angiography (HOP-MRA). HOP-MRA is a combination of standard time-of-flight (TOF) using a full first-order velocity-compensation for white-blood (WB) and flow-sensitive black-blood (FSBB) techniques, which use motion-probing gradients to introduce intravoxel flow dephasing. A dual-echo three-dimensional gradient echo sequence was used to reduce both imaging time and misregistration. HOP-MRA images were obtained using a simple-weighted subtraction (SWS) or a frequency-weighted subtraction (FWS) applying different spatial filtering for WB and BB images. We then assessed the relationships among the contrast-to-noise ratios (CNR) of the blood-to-background signals for those three images. In both volunteer and clinical brain studies, low-flow vessels were well visualized and the background signal was well suppressed by HOP-MRA compared with standard TOF- or BB-MRA. The FWS was better than the SWS when whole-maximum intensity projection was performed on a larger volume including with different types of tissue. The proposed HOP-MRA was proven to visualize low-flow to high-flow vessels and, therefore, demonstrates excellent potential to become a clinically useful technique, especially for visualizing collateral vessels which is difficult with standard TOF-MRA.