Arjen Lindenholz

96 CHAPTER 4 Besides several advanced pulse sequence optimization techniques, better hardware may also improve image quality. In the current study a relatively low number of receiver channels (8 channels) was used. Currently, up to 32 channels are commonly used to investigate vessel wall sequences. 16,20,27 Also, when one uses a higher number of channels in de receiver coil, more possibilities, such as parallel imaging, might be available to reduce scan duration without severely compromising image quality. Moreover, parallel imaging, like SENSE, also performs better at higher field strengths. 36 Another novel acceleration technique to reduce the scan duration even further may be compressed sensing. 37 This technique has shown already promising results for extracranial carotid vessel wall imaging. 38 This study has some limitations. First, the true image quality of an MR image is the result of amultidimensional trade-off between numerous parameters. Many of the parameters are also interrelated and may have a combined effect on resolution, SNR/CNR or scan duration and it is difficult to compare these parameters one-to- one on true image quality. Furthermore, in the comparison of all 7 variants, the same order of sequences was used, which may have led to more motion artifacts in the later scheduled sequences. However, the extent of movement, measured in ∆ Rotation and ∆ Translation between subsequent sequences, did not increase during acquisition time ( Supplemental Figure 2 ). Also, in the second comparison (variant 3 versus variant 7), contrast enhancement may have been stronger due to a longer absorption time in the latest scheduled variant (in our study mostly variant 7). However, statistical comparison (variant 3 and 7 versus variant 7 and 3) did not show any significant differences (data not shown). This study lacked a systematical comparison between the different parameters to investigate the effect on image quality. Vessel wall variants were created to reduce scan duration with the focus on development of a clinical usable sequence that can be incorporated in existing scan protocols. The second limitation is the manually drawn ROIs for measuring the SNR. Although we tried to be consistent in the location, it was not always possible to use the exact same slice number for all variants for the SNR measurement of the vessel wall and CSF, mainly due to intra-subject differences such as patient movement and inter-subject differences in planning and anatomy. Furthermore, due to flow effects of CSF in the basal cisterns, a better delineation can be made of the proximal intracranial arteries compared with the more distal intracranial arteries where the CSF flow is lower. In general, the SNR of the vessel wall may be underestimated because of partial volume effects and difficulties of manually drawing a ROI around the circumferential of the vessel wall. This effect is even more obvious in the thinner basilar artery vessel wall, where the measured SNRs are consistently lower compared with the intracranial internal carotid artery vessel wall with surrounding tissue/CSF. Also, the CSF was often more suppressed