Asymmetric appearance of intracranial vessels on routine spin-echo MR images: a pulse sequence-dependent phenomenon.

PURPOSE To determine the cause of right to left signal intensity differences arising from intracranial vessels during routine spin-echo axial MR imaging of the head. METHODS AND RESULTS Using a normal imaging sequence in which the default directions of the frequency and phase axes were horizontal and vertical, respectively, differences in signal intensity arising from the vertebral arteries were observed in a healthy subject. With the exchange of the frequency and phase axes relative to the normal sequence, no signal intensity differences between the vertebral arteries were recognized. Other pulse sequence modifications, ie, the use of motion-compensating gradients and the reversed polarity of the frequency-encoding gradient, also resulted in variable appearances of the vertebral arteries, indicating that the right-to-left signal asymmetry of the vertebral arteries observed on the normal spin-echo image results from a pulse sequence dependent phenomenon. CONCLUSIONS Frequency-encoding and slice-selection gradients both produce motion-induced phase shifts. These phase shifts depend on the angle between the direction of flow and that of the effective vector sum of these gradients. The asymmetric appearance of the vertebral arteries during normal spin-echo imaging was found to result from the angle dependence of motion-induced phase shifts. Awareness of this artifactual phenomenon is important to avoid confusing it with conditions such as stenosis/occlusion, dissection, or slow flow.