Arjen Lindenholz

32 CHAPTER 2 from aneurysm vessel wall enhancement, regional differences in wall thickness of unruptured aneurysms have been demonstrated on 7T MRI, and thinner regions have been associated with regions of higher wall shear stress determined with phase-contrast MRI. 83,84 Future studies still need to address if variations in wall thickness are associated with risk of aneurysm rupture. Arteriovenous malformations and cavernomas Although DSA is still considered the gold standard for assessment of brain arteriovenous malformations (AVMs), arterial TOF-MRA has become a non-invasive alternative for detection and follow-up of AVMs. 76,80 In a recent comparative study with 1.5T arterial TOF-MRA and DSA, non-contrast-enhanced 7T MPRAGE as well as 7T arterial TOF-MRA have been found superior to 1.5T arterial TOF-MRA in the delineation of the nidus, feeder(s), draining vein(s), and in the relationship between AVMs and adjacent vessels (non-feeding vessels relevant for surgical treatment). 85 Compared to non-enhanced arterial TOF-MRA, MPRAGE offers a very high-quality of adjacent brain structures, but is less performant in the visualization of draining veins, probably due to saturation effects of slow flowing arterialized venous blood. 85 Although overall image quality at 7T equals DSA, all described MRA techniques at 7T still miss dynamic information about the blood flow patterns within the AVM. 85 This may be overcome with phase contrast 4D flow imaging, which is a currently rapidly developing field and also possible at 7T with higher SNR than at 3T. 86 Future studies will still need to assess the diagnostic potential for treatment follow-up of brain AVMs as hemorrhagic changes or artefacts due to embolic agents or clips could be misinterpreted as residual flow. 85 Also, the role of high-resolution vessel-wall MRI for brain AVMs still needs to be established. Finally, 7T MRI significantly improves the visualization of cavernomas and associated developmental venous anomalies (DVA) due to a higher spatial resolution and susceptibility sensitivity. 87-89 Future directions Different experimental and functional techniques are currently being developed and tested to assess cerebrovascular function with 7T MRI on a microvascular level. 72 Amongst others, these include 3D phase contrast (PC)-MRA and blood oxygenation level dependent (BOLD) functional MRI (fMRI). 7T PC-MRA is a promising technique to visualize and measure blood flow and pulsatility, for example of the CoW and lenticulostriate arteries . 90,91 Functional assessment with 3D PC-MRI may better discriminate between healthy and diseased arteries/ arterioles compared to anatomical imaging alone, and may as such be applied to assess the relationship between white matter hyperintensities and lacunes on the