Arjen Lindenholz

70 CHAPTER 3 Future prospects Technical developments As discussed in the previous paragraphs, the difficulty in intracranial vessel wall MRI is that ideally, one strives for a maximum spatial resolution to detect lesions of the small intracranial arterial vessel walls, and maximum suppression of both blood signal within and CSF signal surrounding the arteries. Both will significantly increase acquisition time. For clinical implementation, acquisition time reduction is necessary to perform both pre- and postcontrast vessel wall MRI within a clinical MRI time slot (approximately 25-35 minutes), without a substantial decrease in SNR and CNR of the arterial vessel wall. Innovative methods for reducing acquisition time (e.g., compressed sensing 63,64 ), as well as different CSF suppression techniques (DANTE, 19,20,65 ADE 32,33 ), in combination with various 2D- and 3D acquisition methods with different coverage, need to be compared in the upcoming years. Spatial coverage At 3T, most sequences are limited to a region that covers either the circle of Willis or the (known) stenotic vessel wall lesion, with a few surrounding centimeters. An option for ‘increasing coverage’ is to acquire the vessel wall images in a more angulated coronal plane and include the proximal vertebral arteries, as we generally do with our 3T sequence. ‘Real’ whole-brain coverage – i.e. increasing the FOV – benefits from non-selective pulses (shorter echo spacing in TSE trains), 2D SENSE and no need for oversampling. 48 However, because flow suppression is often based on dephasing during the echo train, with too short echo train lengths flow suppression will inevitably decrease. Next to improvements in pulse sequence design, hardware improvements may also show promise in vessel wall MRI. Recently, an advanced coil system for joint intracranial and extracranial vessel wall imaging has been developed. 66 This coil system provides the opportunity to image both intracranial and extracranial arteries at once for an optimal assessment of the association of carotid and intracranial atherosclerotic plaques and ischemic stroke within a reasonable scan time (5 minutes 54 seconds to 7 minutes 36 seconds). Acquisition time Efficient k -space sampling trajectories (view-ordering) in combination with parallel imaging techniques have been described to reduce acquisition time in 3D turbo spin-echo sequences, while the trajectories can also be optimized for reduced T 2 -weighting. 17,23 Compressed sensing allows image reconstruction from fewer k -space data and, thus, shorter scan times. Compressed sensing needs yet to be investigated for intracranial vessel wall MRI but might be a way to further reduce the acquisition time. 63,64