7 - MR angiography of the head and neck: basic principles and clinical applications

Summary

Since the first publication of a clinical magnetic resonance angiogram (MRA) in 1985, there has been extensive growth in the vascular applications of magnetic resonance. Although for many years a purely investigational tool, the technology and validation studies have progressed to the point that MRA has largely supplanted X-ray angiography (XRA) for evaluation of the extracranial carotid arteries, and is often an alternative to XRA for evaluation of the vertebral arteries and circle of Willis.

In this chapter, we will first review the basic principles of MRA, including time of flight and phase contrast techniques, and introduce the use of paramagnetic contrast agents for MRA. Advantages and pitfalls of MRA as compared with duplex sonography (DUS) and XRA will be addressed, and we will consider future directions for this rapidly advancing technology.

Basic principles of MRA

Unlike computed tomography (CT), which relies solely on the attenuation of X-ray photons to generate an image, MR uses a combination of magnetic fields and radiofrequency energy in order to produce images. The appearance of blood in an MR image depends on the intrinsic magnetic relaxation properties (T₁ and T₂), the oxygenation status and physical state of the blood (e.g. venous vs. arterial vs. hematoma), the direction, rate, and pulsatility of flow, as well as the presence of exogenously administered contrast agents.