Artifacts of the Craniocervical Arterial System on MRI

Abstract

Three-dimensional time-of-flight (3DTOF) MRA utilizes the concept of flow-related enhancement (FRE) to produce bright signal on a gradient echo acquisition, where fresh spins in blood flowing into (and preferentially perpendicular to) the slice are unsaturated and exhibit strong signal prior to being saturated. Arteries do not always follow a perpendicular orientation, however. Particular examples include tortuous internal carotid arteries (ICAs) in the cervical region, vertebral arteries (VAs) near their origin, or the normal horizontal course of the petrous ICAs. When travelling horizontally within a slice, the phenomenon of in-plane saturation may reduce FRE and thus decrease the visualized flow of vasculature. This phenomenon is overcome on contrast-enhanced MRA (CEMRA), as the T1-bright gadolinium usually overpowers the saturation effect. Note that the FRE effect is also useful with 2D or 3DTOF MR venography, most commonly in the transverse sinuses with an axial acquisition, or within the superior sagittal sinus with a sagittal acquisition. It is also important to point out that 3D acquisitions can be susceptible to saturation if a larger volume is obtained, or if the technique of multiple overlapping thin slab acquisition (MOTSA) is not used properly. Hence, most facilities utilize overlapping slabs to decrease in-plane saturation for noncontrast cranial MRA, but saturation may still occur if flow occurs within the same plane as the plane of acquisition.