CT angiography (CTA) is a highly sensitive and specific modality for large and medium vessel pathologies of the head and neck. Vessel wall thickening is often identified on CTA from a subintimal or intramural hematoma and frequently corresponds with “methemoglobin crescent sign” on MRA . Dissecting aneurysms may be readily identified as focal outpouchings of the enhancing arterial lumen (Fig. 84.1), with or without associated thrombus, and often in an orientation parallel to the vessel. CTA is especially attractive in imaging patients with blunt or penetrating trauma, where imaging with MRI is difficult or sometimes contraindicated. Streak and beam hardening artifact from a patient’s dental hardware may limit evaluation of the mid-cervical internal carotid artery (ICA), a commonly involved site for dissections.
MRI/MRA remains the initial screening modality of choice to evaluate patients with suspected non-traumatic cervical dissections. Fat-saturated axial T1- and T2-weighted sequences provide a sensitive technique to identify subtle dissections where no significant luminal narrowing may be appreciated on digital subtraction angiography (DSA). The periluminal (intramural) hematoma can be uniquely identified on MR imaging in the subacute stage (3–14 days). The classic finding is crescentic hyperintense signal peripheral to the flow void of an irregularly narrowed internal carotid artery. However, depending on the stage and composition of the hemorrhagic products, the signal on T1- and T2-weighted sequences may vary (Figs. 84.2, 84.3). In our experience, the presence of restricted diffusion on diffusion-weighted imaging (DWI) of the neck may also provide increased sensitivity for detection of intramural hematomas and detection of cervical dissections. Additionally, since both head and neck imaging are routinely performed, screening for ischemic intracranial complications can be simultaneously performed using DWI. 3D contrast-enhanced MRA of the neck has surpassed traditional time-of-flight (TOF) techniques, with higher spatial resolution and elimination of flow artifacts such as in-plane dephasing at arterial bends and signal loss due to slow or turbulent flow in carotid dissections .