Arjen Lindenholz

46 CHAPTER 3 The imaging protocol - What do we need? Vessel wall MRI sequence prerequisites For intracranial vessel wall MRI, both a high contrast-to-noise ratio (CNR) and a high spatial resolution are needed to visualize the thin arterial vessel wall and to characterize vessel wall lesions. 1,15-19 CNR in vessel wall imaging comprises signal contrast of the vessel wall relative to its direct surroundings, that is, blood and cerebrospinal fluid (CSF). It is dependent on both sequence parameters and magnetic field strength, where CNR generally increases with increasing field strength. The achievable spatial resolution is also dependent on the field strength, as well as on acquisition time. Finally, the different image contrast weightings need to be considered when developing a vessel wall MRI protocol. 1 Vessel wall CNR The first step in achieving a high vessel wall CNR is to suppress the luminal blood. Most intracranial vessel wall MRI sequences rely on the intrinsic “black blood” properties of three-dimensional (3D) turbo spin-echo pulse sequences (TSE) with variable flip-angle refocusing pulses. 17,19-23 In these sequences, black blood is achieved by intravoxel dephasing of flowing blood, which is most effective when low flip-angle refocusing pulses are used. 17,24 Alternatively, preparation pulses can be used to obtain blood suppression, such as double inversion recovery, 25 motion- sensitizing preparation pulses, 26 or delay alternating with nutation for tailored excitation (DANTE) preparation pulses. 20 Adding preparation pulses generally increases acquisition time. Currently, there is a need for a thorough analysis, both theoretically and experimentally, assessing the performance of the various blood suppression techniques in the presence of slow flow (directly next to the vessel wall). For optimal CNR, not only suppression of blood signal to depict the inner boundary of the vessel wall, but also suppression of CSF signal to depict the outer boundary is essential, especially when outward vascular remodeling is present. 27-30 At 3T, the CNR between vessel wall and lumen is superior to CNR between vessel wall and CSF on most pulse sequences. Because an inversion recovery pulse as a CSF suppressing technique is both time-consuming and detrimental to the vessel wall signal-to-noise ratio (SNR), these vessel wall MRI sequences rely implicitly (without usingCSF suppressionpreparationpulses) or explicitly (usingDelay Alternatingwith Nutation for Tailored Excitation; DANTE) on CSF flow for CSF suppression. 19-21,31,32 Consequently, suppression of CSF will be less in compartments where there is a slower flow of CSF, for example around the vessel wall and at locations where there is little CSF surrounding the vessels. However, a recent promising development for improving CSF suppression at 3T is the incorporation of an anti-driven-equilibrium (ADE) pulse, which relies on T 1 and T 2 relaxation properties and is independent