11/22/2023 0 Comments Basha mri near meHowever, other mechanisms can generate adequate blood suppression with 3D sequences. For example, spatial presaturation requires that spins in the imaging volume are replaced with inflowing suppressed spins, but this is unlikely to occur for the entirety of a large 3D imaging slab. These sequences minimize this blurring by varying the flip angle over the length of the echo train to maintain relatively stable signal.īlood suppression techniques used with 2D sequences are generally less effective with 3D sequences. These sequences have very long echo trains, a potential source of imaging blurring from decay of transverse magnetization between the start and end of the readout. The most commonly used 3D sequences for intracranial VW-MR imaging are the variable flip angle refocusing pulse, fast spin-echo sequences 7 with brand names such as VISTA (volume isotropic turbo spin-echo acquisition Philips Healthcare, Best, the Netherlands), SPACE (sampling perfection with application-optimized contrasts by using different flip angle evolutions Siemens, Erlangen, Germany), and Cube (GE Healthcare, Milwaukee, Wisconsin). Fat suppression is necessary for VW-MR imaging of the external carotid artery branches in the scalp (eg, in patients with suspected temporal arteritis) but is generally not needed for intracranial VW-MR imaging. A T2-weighted VW-MR imaging sequence is often helpful. The disadvantages of proton-density weighting are that contrast enhancement may be less conspicuous and CSF signal intensity can approach vessel wall intensity. It is possible to use a proton-density–weighted sequence instead of a T1-weighted sequence because the former provides higher SNR. Most examinations require a T1-weighted vessel wall sequence before and after intravenous gadolinium contrast. Low-velocity flow can cause intravascular signal loss on time-of-flight MRA, so in patients who have pronounced luminal narrowing or dilation, it is helpful to add a gadolinium-bolus MRA to accurately define the contour of the lumen (ie, the boundary between the lumen and wall). Time-of-flight MRA is mainly used to characterize luminal abnormality and act as a localizer for subsequent vessel wall sequences. Ongoing advances in MR imaging technology, including higher magnetic field strength, 2 may enable further increases in spatial resolution and image quality. Most experienced centers are using isotropic voxel dimensions in the 0.4- to 0.7-mm range for 3D acquisitions. At 3T with a 3D sequence, a voxel size of 0.5 mm isotropic is a reasonable starting point ( Fig 1 B), and it is possible to cover the circle of Willis and second-/third-order branches in 7–10 minutes. At 3T with a 2D sequence, a voxel size of 2.0 × 0.4 × 0.4 mm provides a reasonable balance between spatial resolution and signal-to-noise ratio, with a scan duration of approximately 5–7 minutes for a 2- to 4-cm-thick section of tissue ( Fig 1 A). The higher signal-to-noise ratio at 3T than at 1.5T is advantageous for intracranial VW-MR imaging and, in many cases, necessary. In addition, vessel wall disease often results in wall thickening, which increases its conspicuity. However, it is possible to image the intracranial arterial wall because the wall generates detectable MR imaging signal and one can suppress the MR imaging signal arising from neighboring blood and CSF within the voxel. The normal middle cerebral artery and basilar artery wall thickness is 0.2–0.3 mm, which is approximately one-tenth of the luminal diameter 1 and smaller than the VW-MR imaging voxel dimensions currently achievable. The principal technical requirements for intracranial VW-MR imaging are the following: 1) high spatial resolution, 2) multiplanar 2D acquisitions or 3D acquisitions, 3) multiple tissue weightings, and 4) suppression of signal in luminal blood and CSF. The technical sections that follow provide general recommendations on the development of an intracranial VW-MR imaging protocol, and we are also launching a dynamic document (via the American Society of Neuroradiology Web site) through which experienced centers can describe their MR imaging systems and the specific pulse sequences and scan parameters that they have found useful. Selection of sequences and scan parameters for VW-MR imaging is highly dependent on the particular scanner hardware and software available at a center. Until such sequences are widely available, it is possible to adjust the scan parameters of existing sequences and obtain vessel wall images of sufficient quality for clinical use. The American Society of Neuroradiology Vessel Wall Imaging Study Group is working with MR imaging vendors to promote the development and dissemination of commercial pulse sequences that are optimized for intracranial VW-MR imaging.
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