Neuroradiologic Applications of Dynamic MR Angiography at 3 T

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Four-dimensional time-resolved MR angiography (4D-MRA) using keyhole imaging techniques is a new method of performing contrastenhanced vascular imaging. Combining parallel imaging and keyhole imaging techniques, it is possible to obtain dynamic MRA scans up to 60 times faster, thereby achieving subsecond sampling of the contrast hemodynamics. Furthermore, imaging at 3 T gives higher signal, thus affording higher spatial resolution and allowing dynamic 3D MRA to approach the diagnostic performance of conventional digital subtraction angiography. This article presents the authors' clinical experience using 4D-MRA to evaluate various vascular abnormalities in the brain, spine, orbits, and neck at 3 T, demonstrates the imaging findings of this novel technique, and discusses its advantages and use in current neuroradiology practice.

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Techniques

For the clinical examples presented in this article, contrast-enhanced MRA was performed on a 16-channel 3.0-T system equipped with a commercially available eight-channel SENSE-capable head coil. The dual gradient system allows gradient amplitude of 80 mT/m at 100 mT/m/ms or 40 mT/m at 200 mT/m/ms slew rate. Patients were positioned with a 20-gauge intravenous catheter inserted into the antecubital vein. An automated power injector was used for a biphasic injection protocol comprised of 20 cm2

Arteriovenous Malformations and Arteriovenous Fistula

Using 4D-MRA at 3 T, it is possible to identify correctly normal vasculature, enlarged arterial pedicles, lesion nidus, and the venous drainage pattern of an AVM and to resolve arterial and venous structures separately (Fig. 4). Similarly, in patients who have AVF (Fig. 5A, B), medium- and large-sized arterial pedicles are visualized readily, and synchronous opacification of the diseased sinus or vein indicates an arteriovenous shunt. Sometimes in AVF the direct arterial feeders or the fistula

Vascular Malformations

As in intracranial AVM, dynamic contrast-enhanced MRA is helpful in identifying correctly normal vasculature, enlarged arterial pedicles, lesion nidus, and the venous drainage pattern. MRI has the added advantage of visualizing the rest of the soft tissues around the AVM. The authors increasingly are using dynamic MRA for follow-up posttreatment head and neck AVM (Fig. 7A and B), reserving DSA for cases in which there is an abnormality of the dynamic MRA.

Carotid Body Tumor

Diseases involving the carotid space can include vascular tumors such as carotid body tumors, lymphoma, and nerve sheath tumors. In such patients visualization of contrast enhancement within the tumor in the early arterial phase is important for a correct diagnosis, because other tumors do not show such early enhancement (Fig. 8). Although, after contrast enhancement, routine MRI can show this lesion, the authors believe that the MR imaging sometimes may not show the early arterial phase as

Discussion

The clinical usefulness of 4D-MRA for dynamic MRA with complete hemodynamic information of the arterial and venous system in the head and neck is shown here. Dynamic information obtained from this technique is useful in evaluating complex vascular abnormalities, such as AVF and AVM. Furthermore, arterial and venous phases can be separated either by simple subtraction (Fig. 3) or by more sophisticated postprocessing methods based on correlation14 or on contrast arrival time maps.15

It was

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