Arjen Lindenholz

53 The Use and Pitfalls of Intracranial Vessel Wall Imaging 3 with the isotropic spatial resolution, does not require additional planning in oblique angles. B 0 -shimming and dielectric pads are additionally used to decrease inhomogeneity of the main magnetic and radiofrequency transmit field, which otherwise can result in considerable signal loss in both temporal lobes (including the M2-M3 trajectory). 49 Key parameters of our 7T protocol are shown in Table 2 . Use of contrast agents The use of contrast material-enhanced MR imaging is considered a necessary component of vessel wall MRI protocols.1,2 Sensitivity for contrast enhancement depends on optimal timing of contrast agent injection. Currently, no head-to-head comparisons between vessel wall MRI sequences at different time points after contrast injection have been performed. For timing of contrast enhancement on standard brain MRI sequences, results from studies in brain tumors are generally used. 50 As a general rule, optimal timing of sequence acquisition is between approximately 5-10 minutes after contrast agent injection; contrast enhancement may be weak within the first 5 minutes after injection, and evidence of the added value of a late-phase acquisition ( > 10 minutes) is lacking. Because many of the current vessel wall MRI sequences have an acquisition duration between 5-7 minutes, the ideal start time would be 5 minutes after contrast injection. At our institution, we either perform one anatomic sequence after contrast agent injection – such as an axial T 2 -weighted TSE or diffusion-weighted imaging (DWI) sequence – or implement a 5-minute break before starting the vessel wall MRI sequence. The choice of whether or not to obtain both pre- and postcontrast images depends on both the specific setting and clinical question. At 7T, we perform the vessel wall MRI sequence before and after contrast material administration, because we mainly examine patients in a research setting or challenging cases and therefore have time (45-60 minute time slot) to acquire both pre- and postcontrast images in addition to the standard anatomical brain images. With this setup, contrast- enhancing lesions can be distinguished from non-enhancing lesions with a high signal intensity by comparing pre- and postcontrast images. However, within the context of fixed, shorter MRI time slots and a maximum acquisition time based on patient comfort, in clinical practice it is often necessary to limit the time for intracranial vessel wall MRI at 3T. An option that reduces total protocol time, and which we currently use in our institution, is to only acquire postcontrast images. In our experience, most vessel wall lesions – even when not enhancing – can be detected on the postcontrast vessel wall MRI sequence and it saves half the acquisition time relative to the combination of a pre- and postcontrast sequence. Still, studies are needed to compare this alternative approach (postcontrast-only) to the classic approach (pre- and postcontrast). One of the major limitations of using a postcontrast vessel wall MRI sequence only is that findings such as intraplaque hemorrhage and intracranial arterial dissections may be missed. 51