The two major reasons for the interest in the efficacious and noninvasive imaging of the extracranial carotid vascular system are the relatively high incidence of atherosclerotic disease in the US population and the ease with which this disease can be treated [1-5]. Strokes and stroke-related disability remain an important public health problem in the United States. Of the number of alternatives for vascular imaging of the extracranial cerebral vasculature, the following are the most common available modalities:
Computed tomographic angiography (CTA)
Digital subtraction angiography (DSA)
Magnetic resonance angiography (MRA) (three-dimensional [3D] or two-dimensional [2D] time-of-flight [TOP], phase-contrast, or gadolinium-enhanced first-pass)
Radionuclide angiography
Ultrasound (US) (Doppler, B-mode, duplex, or color-flow)
It is possible to further divide these imaging modalities into "flow-dependent" and "filling" techniques. "Filling" techniques depend on filling the lumen of the vascular structures with a contrast media. The available imaging modalities that use "filling" techniques include radionuclide angiography (used for brain death evaluations in some locales), DSA or conventional angiography, CTA, and gadolinium-enhanced MRA (typically using a first-pass technique). These modalities require the passage of a bolus of contrast media through the imaged volume to allow depiction of the vessel lumen. When cross-sectional imaging techniques are used, not only is the lumen defined, but the surrounding arterial wall and other soft tissues also are depicted.
Modalities that use "flow-dependent" techniques include US (Doppler, duplex, or color-flow) and MRA (TOP). For US and MRA to provide data for generation of images, blood must flow into the volume of interest. Again, information about the vascular anatomy and surrounding tissues can be obtained with US and MRA. Although "flow-dependent" and "filling" imaging techniques are distinctly different, some overlap between the two techniques exists.
Each of these techniques has strengths and weaknesses, and any single modality will likely have specific deficiencies or pitfalls in certain applications. The most common pitfalls or problems related to the available noninvasive vascular imaging modalities include a flow-velocity threshold for evaluation of flow-dependent techniques; contraindications to MRA; costs and complications associated with the use of invasive techniques as screening studies; contraindications to iodinated contrast media; and avoidance of ionizing radiation, claustrophobia, and operator dependence for some of the techniques, especially US [6-8].