Magnetic resonance angiography: vascular and flow imaging

Abstract

Magnetic resonance angiography (MRA) techniques are increasingly being used in addition to or in place of conventional x-ray angiography (CA) methods for studies of blood flow and blood vessel morphology. MRA has evolved from magnetic resonance imaging (MRI) techniques for noninvasive visualization of blood flow and vasculature. MRI, aided by computers, generates images (angiograms) created by the contrast of flowing blood and the surrounding tissues; the magnetically depolarized flowing blood contrasts with the magnetically saturated stationary tissues, resulting in a differential high-signal intensity. MRA signals are MRI signals encoded with spatial data achieved by Fourier and echoplanar imaging, projection reconstruction, and spiral scanning. MRA's vasculature "flow map" incorporates both anatomic and physiologic information. This publication explores the history, principles, techniques (including time-of-flight and phase-contrast imaging), clinical applications, and indications and contraindications of MRA use. In addition, data on particular areas of study such as the head and neck and cerebral, thoracic, abdominal, and peripheral vasculature are provided. Two-and three-dimensional MRA methods are discussed, and comparisons are made between MRA and CA. Recommendations from several health service agencies and institutions are provided. Drawbacks, e.g., generally poorer resolution compared with CA, and restrictions of MRA use are discussed. Corollary studies with standard MRI or CA methods are sometimes advisable, and MRA alone is not always sufficient for comprehensive analysis of blood flow and blood vessel vasculature. MRA techniques have been developing in response to the hazards and limitations of CA; MRA involves no exposure to ionizing radiation and generally has a shorter information accrual time compared with conventional scans, in addition to being noninvasive and circumventing the systemic reactions sometimes caused by contrast agents in CA. Another advantage to MRA use is that it can be conducted in an outpatient setting, and many restrictions that apply to CA do not apply to MRA techniques. MRA is useful in detecting aneurysms, occlusions, and stenoses and is especially important in cases in which the use of contrast agents presents high risk. MRA is a promising technology for accurate and noninvasive evaluation of blood flow and blood vessel morphology. Though it cannot at present be considered a standard technique, it is gaining wider acceptance, especially in diagnoses of patients whose condition contraindicates standard angiography.