Magnetic Resonance Angiography and Magnetic Resonance Venography
Because an MRI signal requires detection of a brief period of excitation and decay, moving tissue often passes out of the plane of assessment before the return signal can be detected, resulting in the black “flow void” that characterizes MRI scans of vascular channels with flow. However, protons that are excited in one slice and then move to another may be specifically imaged using the 3-dimensional assessment technique that underlies MRA and MRV. MRA signals may also be obtained from gadolinium-enhanced images. This technique is particularly useful for imaging the proximal large vessels of the chest and neck. MRA with gadolinium contrast has a very short acquisition time, reducing the potential for patient movement–related artifacts. MRA provides excellent noninvasive information about large- and medium-size vessels (Fig 2-12). Because MRA depends on flow physiology, it tends to overestimate vascular stenosis, and the reduced image resolution limits the ability to visualize smaller vessels or vasculitis. MRV may be helpful in excluding thrombosis within the dural venous sinuses (Fig 2-13), a condition that may cause papilledema (see Chapter 14).
Figure 2-11 A contrast angiogram, lateral view, demonstrates an aneurysm of the posterior communicating artery (arrow). ACA = anterior cerebral artery; ICA = internal carotid artery; MCA = middle cerebral artery.
(Courtesy of Rod Foroozan, MD.)
Figure 2-12 Magnetic resonance angiography (MRA) provides excellent noninvasive information about large- and medium-size vessels. A, An MRA source image shows an abnormality at the junction of the posterior communicating artery and internal carotid artery (arrow). A volume-rendered 3-dimensional image (B) and a maximal intensity projection image (C) clearly show a posterior communicating artery aneurysm (arrows).
(Courtesy of M. Tariq Bhatti, MD.)
Excerpted from BCSC 2020-2021 series: Section 5 - Neuro-Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.