Arjen Lindenholz

95 Comparison of 3T Intracranial Vessel Wall MRI Sequences 4 of the postcontrast vessel walls is contrast absorption into the vessel wall itself. The flowartifacts seen in variant 7may be a potential problemand a DANTE prepulse might be a viable addition for more effective blood suppression. A third option to improve the contrast between the vessel wall and surrounding tissue is using inversion recovery. 30 This technique is mainly applied in 2D acquisitions; however, also reports have been published with 3D acquisitions on a 7T MR-system. 31 Generally, a lower minimum refocusing angle results in a better flow suppression but also in a lower SNR. Because of the higher minimum refocusing angles in the TSE-train of vessel wall variant 2, 4, 5 and 7 they may yield more SNRs but probably at the cost of blurring due to a worse point spread function (PSF). 16,32 This may also partly explain the ‘smoother’ appearance in variant 4 and 7 ( Figure 2 ). However, these sequences do show the vessel wall clearly, and in our opinion, radiologists often prefer blurring compared with images with a very narrow point spread function because this results in more noisy images which are more difficult to assess, even though the true resolution is higher. A disadvantage of variant 3, 4 and 7 might be the anisotropic resolution in the slice direction, which makes it less beneficial for multi-planar assessment ( Figure 2 and 3 ). 1,19 In specific clinical situations, multiplanar reconstructions may be required, for example, when assessing lesions in the middle cerebral artery. However, in daily clinical practice, image interpretation can regularly be performed in the axial or oblique plane alone, and in these situations, a nearly isotropic voxel-size may be adequate for interpretation of vessel wall images. 23,24,33 In addition, the scan variants with anisotropic voxel-size have a larger overall voxel-size, which results in higher SNR per voxel, but they also can offer a shorter effective scan time. On the other hand, using an anisotropic voxel-size potentially increases partial volume effects compared with the isotropic-derived variants. 34 Consequently, vessel wall thickness might be overestimated or small lesions might be missed. To reduce the scan duration further, we applied an elliptical k -space shutter in 5 variants. Theoretically this might also reduce the image quality, because by enabling the elliptical k -space shutter, the outer part of the k -space is cut off, leading to a reduction in the effective resolution. However, in practice, the effect may be limited, particularly because the longer scan durations needed for a full acquisition make the images more prone to motion artifacts. Another way to reduce scan duration is the use of 2D methods as an alternative to 3D imaging methods. These 2D methods have anisotropic voxels as the slice thickness is normally much larger than the in-plane voxel-size. When these slices are planned perpendicular to the vessels, this is not a problem. However, these 2D methods have the drawback of a small coverage and need to be focused on a limited vessel wall region. The optimal region of interest may be difficult to determine beforehand. 26,35