Arjen Lindenholz

94 CHAPTER 4 Discussion The purpose of this study was to evaluate different variants of MRI sequences for intracranial vessel wall imaging in order to reduce scan duration, provided that an acceptable image quality could be maintained. In the pre-contrast comparison, the fastest variant 7 (4 minutes 39 seconds), had a significantly higher CNR between the vessel wall and blood, compared with variant 3, while it was about 30% faster. Also, the CNR between the vessel wall and CSF was better for variant 7 compared with variant 3. The favorable SNRs and CNRs of variant 7 also remained in the second comparison in which both precontrast and postcontrast images were acquired and were higher compared with our clinically used variant 3 ( Table 2 ). Especially in the elderly population the superior contrast between the vessel wall and CSF can be beneficial. In the aging brain, tissue atrophy increases and consequently the intracranial vessels are more richly surrounded by CSF. The image quality of an MR image is a balance between resolution, SNR/CNR and scan duration. A gain in one aspect, results in a sacrifice of another. The ideal balance is difficult to determine and depends on the specific application and the clinical question to be answered. Several pulse sequence optimizations are developed to address these technical challenges. 17-19,21,26 For a highCNRof the vessel wall, both blood and CSF need to be simultaneously suppressed. 1 3D variable flip- angle refocusing pulse sequences are used because of their high intrinsic black- blood properties and the ability to generate multiplanar reformatted images, but have less effect on the slowly flowing CSF. 17,19-22,27 For better CSF suppression, Proton Density-weighted imaging with radial ordering modulation of the k -space (short echo-time), 19 Anti-Driven Equilibrium (ADE), 18,23,28 or prepulses such as Delay Alternating with Nutation for Tailored Excitation (DANTE) 17,21,29 and inversion- recovery 30 have been proposed. In 4 of 7 variants, the anti-driven equilibrium technique was used. This technique drives the magnetization back to the negative M z -axis by further radiofrequency excitation and results in more adequate CSF suppression. 18,24,27 A second possibility to improve image contrast between vessel wall and CSF is the use of a DANTE prepulse. 17,21,29 When one applies DANTE pulse trains, the longitudinal magnetization of static tissue largely remains but flowing spins are suppressed due to a spoiling effect. 29 This suppression is less for slow flowing fluid, especially below 0.1cm/s; therefore, DANTE will be less useful for CSF suppression around the distal cerebral arteries. The DANTE prepulse also improves the suppression of blood, notably for slow and turbulent flow. Slow-flow is a well-known pitfall in the assessment of vessel wall imaging. 2 In the current study, the postcontrast vessel wall showed a higher SNR/CNR compared with the precontrast vessel wall, which may suggest slow-slow. Another explanation that may have led to higher SNRs