Magnetic Resonance Angiography

In recent years, magnetic resonance imaging (MRI) and magnetic resonance arteriography (MRA) have been employed with increased frequency as diagnostic tools in the management of peripheral arterial disease (18).

Basic Principles

MRI is based on the reactions of various tissues to a magnetic field followed by a radiofrequency radiation pulse. MRA imaging is generated by taking advantage of blood flow-related effects relative to the stationary surrounding soft tissues. Two techniques are currently used for MRA:

Time of flight (TOF) angiography: TOF MRA relies on signal difference between stationary protons in the vessel wall and the surrounding soft tissues, compared to moving protons (flowing blood).

Phase contrast (PC) angiography: Protons undergo a change in the phase of their rotation as they move through a magnetic field. PC MRA uses gadolinium that shortens the T1 relaxation of blood protons, thereby increasing the intravascular signal.

Background can be eliminated by subtraction protocols and its suppression is greater for PC than for TOF MRA. Because of inherent limitations, TOF MRA has never become a real alternative to contrast angiography except in very few highly experienced centers. Currently, PC MRA is considered the best alternative to contrast angiography.

Clinical Applications

MRA has evolved as a noninvasive, sensitive accurate, and cost-effective method of imaging of the peripheral arterial circulation. Although contrast angiography is still considered the gold standard, it carries an overall (major and minor) complication rate of approximately 8%. Local complications related to the arterial puncture (bleeding, hematoma, infection, thrombosis, stenosis, pseudoaneurysm, etc.) and systemic complications (contrast-induced allergic reactions or renal insufficiency) are not uncommon. MRA is an alternative noninvasive imaging method for the peripheral vessels that avoids the risk of these complications. Its sensitivity in and specificity in detecting patent segments, hemodynamically significant stenoses and/or occlusions approaches 100%. In a recent review of the existing literature, TOF MRA was found to have sensitivity and specificity of 82% and 84%, respectively, whereas PC MRA had a 96% sensitivity and specificity compared to conventional angiography (18). The latter was also reported to be more sensitive in detecting patent distal run-off vessels in patients with severely compromised distal circulation. This advantage is inherent to the mechanism of image formation in MRA, which requires only the presence of local flow with velocity as low as 2 cm/s. Presently, MRA is considered the imaging modality of choice for the diagnosis of vascular arterio-venous malformations and popliteal entrapment syndrome. In several centers, MRA is used as the sole pre-operative imaging technique for distal revascularization procedures with great success. It is currently considered to be more cost-effective compared to contrast angiography.

Nevertheless, not all patients are suitable for MRI/MRA exams. Individuals with implanted metal devices (pacemakers, cerebral vascular clips, etc.) should not be considered for such imaging. Claustrophobia remains a relative contraindication. The exam may also be limited by respiratory movements, but with contrast-enhanced MRA the acquisition time has significantly decreased (approximately 20 seconds for a complete limb exam), making breath-hold imagespossible.

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