Magnetic resonance angiography (MRA) techniques continue to evolve and improve, including the more recent use of noncontrast-only imaging in patients with renal insufficiency. Two-dimensional time of flight, three-dimensional imaging, contrast enhancement with gadolinium, subtraction, cardiac gating, bolus chase, parallel imaging, optimized K-space filling, 3T magnet strength, and improved coil technology have led to improved temporal resolution, spatial resolution, and signal to noise in MRA. Its sensitivity and specificity for detection of stenoses >50% are now in the 90% to 100% range, which is much better than catheter angiography provides. As a result, MRA is now a first-line technique in many centers for the imaging of peripheral vascular disease. Dedicated time resolved imaging of the calves and pedal arteries provides accurate identification of infrageniculate arteries and pedal arteries as potential touch-down sites for bypass surgeries.
The majority of magnetic resonance (MR) approaches use noncontrast sequences followed by the intravenous administration of a gadolinium-based agent. In comparison to color duplex US, contrast-enhanced MRA is more accurate for detecting significant stenoses and for preoperative planning. Both MRA and computed tomography angiography (CTA) are more cost-effective than duplex US, and MRA is more cost-effective and safer than DSA. For postoperative and postangioplasty surveillance, small studies have shown MRA to be helpful in detecting recurrent disease, but improved outcomes for such surveillance have not been documented. Regarding comparative studies, MRA typically has not supplanted DSA as a reference standard. However, technology, experience, and protocol optimization have enhanced the use of contrast-enhanced MRA as a replacement for DSA in the initial evaluation. These improvements include 3-Tesla field strength, whole-body angiography, reduced gadolinium doses, and contrast agents with improved relaxivity and vascular retention characteristics.
Recent advancements in noncontrast MRA techniques for imaging peripheral artery disease have expanded the sequence options from time-of-flight and phase-contrast imaging to include electrocardiography (ECG)-gated fresh blood partial Fourier fast spin echo, balanced steady-state free precession, and spin labeling. Two alternative approaches using balanced steady state for peripheral MRA applications include flow-sensitive dephasing and quiescent-interval single shot. When compared to bolus-chase and time-resolved gadolinium-enhanced MRA, initial studies of fresh blood imaging of the calf and pedal arteries have provided accurate imaging when technically successful. Overall, these methods are being increasingly adopted for patients with renal insufficiency.
Some technical problems limit the utility of MRA for imaging peripheral vascular disease. Challenges may include image quality related to low signal/noise ratio, limited spatial resolution, motion artifacts, long acquisition times, unreliable visualization of lesions with high flow and turbulence (excessive signal loss at regions of high-grade stenoses), nonvisualization of patent vessel segments with reversed blood flow, the need to exclude patients with pacemakers or other metallic implants, and loss of signal in arterial segments within metal stents or adjacent to metallic clips or prosthetic joints. Some of these problems have been addressed successfully with the use of newer imaging sequences and the addition of MR contrast agents. With the newer noncontrast techniques, cardiac arrhythmia can impair image quality, limiting evaluation of the distal calf and pedal arteries. Although useful tools to improve image quality have been suggested, larger-scale trials are required for evaluation of small-vessel peripheral artery disease with noncontrast MRA.
MRA has not yet replaced catheter angiography as the gold standard in comparative studies, but it has largely replaced angiography in some institutions for preintervention planning. This is due to improvements in imaging sequences as well as experience among radiologists. In addition, contrast agents are considered safe in patients with normal renal function. In these patients, MRA is likely to entirely supplant catheter angiography as a pure diagnostic tool.
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