Magnetic Resonance Angiography: Principles and Applications

In most cases, MRA has replaced conventional angiography and computed tomographic angiography CTA in the evaluation of vascular disease. One exciting facet of MRA is the ability to obtain time-resolved imaging allowing for evaluation of the hemodynamic effects of various vascular lesions. MRA also allows for high-resolution angiographic images with and without the use of intravenous contrast, thus allowing for improved patient safety, especially among those with chronic renal disease.

In this review, we discuss some commonly used and state-of-the-art MRA techniques and their exciting applications in the diagnosis and treatment of vascular disease. To guarantee accurate bolus timing, our practice generally employs automated detection of the contrast material and real-time depiction of bolus arrival for neurologic applications, although we sometimes use the test bolus technique for body applications.

CE-MRA images are performed with 3D gradient echo sequences using short repetition times and echo times. The slab volume can be acquired in any plane of orientation.

Fat saturation pulses or water-fat separation techniques can be helpful to suppress high signal intensity of surrounding fat. Multiplanar reformats are used as problem-solving tools because they are easily performed with commercially available software. A second approach to 3D MRA is to perform time-resolved imaging with temporal interpolation. Temporal interpolation is a process in which the low spatial frequency data within the center of k-space is sampled more frequently than the higher spatial frequency data located more peripherally. The main advantage of this technique is high temporal resolution and image quality, even in the setting of motion and poor breath holds.

Disadvantages of TOF include a long acquisition time and signal loss of in-plane flow resulting in overestimation of the degree of stenosis in some cases. PC-MRA provides excellent contrast and spatial resolution. Limitations to PC-MRA include loss of signal in vessels with turbulent flow and the susceptibility to motion artifacts.

Fast spin echo MRA produces bright blood MRA images by taking advantage of the flow void effect of fast systolic arterial flow. Motion artifact is a major limitation of this technique. Steady-state free precession SSFP angiography is another effective non-contrast imaging technique.

This technique has been applied in the aorta, specifically with ECG gating in the thoracic aorta as well as in children with congenital heart disease where the technique can be used with a respiratory navigator to provide high-resolution images of the coronary arteries. Limitations of SSFP MRA include the fact that there is usually little flow dependence of these sequences, as well as susceptibility to field heterogeneities.

These sequences can be used to follow aneurysm size, for example, but may underestimate or even miss an intravascular thrombus. This technique uses ECG-triggered, fat-suppressed two-dimensional 2D SSFP sequences with a quiescent interval to allow for enhancement of inflowing blood. Velocity-encoded cine VEC magnetic resonance imaging MRI is a time-resolved imaging modality that can be performed using phase-contrast sequences.

In general, phase-contrast sequences can be 2D or 3D. The VENC determines the range of blood flow velocity that can be measured. Velocities greater than the VENC result in aliasing artifact, which is difficult to correct. Sensitivity decreases with higher VENC.

It is possible to acquire one position within a breath hold with a temporal resolution of about 30 msec depending on the heart rate and other imaging parameters. A higher temporal resolution of about 10 msec can be attained with free breathing or breath-gated acquisitions. Two-dimensional VEC MRI has been demonstrated to provide accurate blood flow measurements in pediatric and adult pathologies as well as within endovascular stent grafts. MRI contrast agents affect the relaxation times of nearby protons.

Gadolinium is by far the most commonly used MRI contrast agent available. A wide variety of gadolinium agents are in use today. Gadolinium agents are classified as ionic-linear ligand, non-ionic-linear ligand, ionic-cyclic ligand, and non-ionic-cyclic ligand.

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The most stable gadolinium contrast agents are the macrocyclics and the least stable are the non-ionic-linear chelates such as gadodiamide and gadoversetamide. Gadolinium contrast agents are given by bolus injection and have pharmacokinetics similar to those of iodine-based radiographic contrast agents. Gadolinium contrast agents are generally excreted by passive glomerular filtration without secretion or reabsorption. Elimination is typically complete in 24—48 hours.

Gadofosveset is eliminated through both renal and hepatobiliary pathways. In general, gadolinium contrast agents are considered to be safer than iodinated contrast agents due to the rarity of life-threatening contrast reaction and the relatively decreased nephrotoxicity of gadolinium compared with iodinated contrast agents. However, gadolinium contrast agents are not without their risks. Patients with reduced renal function are at risk of nephrogenic systemic fibrosis. Nephrogenic systemic fibrosis is a sometimes fatal fibrosing disease which involves the skin and subcutaneous tissues but can also affect the lungs, esophagus, heart, and skeletal muscles.

One recently published study demonstrated that patients receiving multiple gadolinium boluses have higher T1 signal intensity on pre-gadolinium images in their dentate nucleus and basal ganglia, presumably secondary to gadolinium deposition.

1. Introduction

The clinical significance of these findings has not been established. In evaluation of thoracic aortic aneurysms, ECG-gated gradient-echo 3D MRA is the preferred method due to high isotropic spatial resolution. Further, there is less motion artifact affecting frequently pulsatile structures, such as the aortic root, with 3D ECG-gated techniques. Aneurysms of the thoracic aorta have been shown to be associated with complex abnormal helical flow patterns that are thought to play a role in the progression of aneurysms.

Flow eccentricity can be quantified and has been shown to correlate with focally elevated wall shear stress and aortic dilatation, factors associated with aneurysm development. Advanced MRA techniques are essential in evaluation of aneurysms of the descending aorta as well. MRA is often used to select patients who undergo thoracic endovascular aortic repair. Challenges faced by endovascular surgeons in assessing aortic aneurysms include proper evaluation of aortic arch angulation, determining the size of proximal and distal landing zones, evaluation of true and false lumen in type B aortic dissections, and evaluation of branch vessels.

ECG-gated MRA has a number of advantages in that it allows the surgeon to appreciate the changes in size of the anatomical areas of interest during the cardiac cycle with high spatial resolution. However, this high spatial resolution comes at the expense of long imaging and post-processing times. MRA has very high accuracy in evaluation of aortic dissection. CE-MRA is the gold standard MRA technique used in evaluation of aortic dissection due to its high spatial and contrast resolution and its accuracy in detection of branch vessel involvement. CE-MRA allows for accurate assessment of the anatomy of aortic dissection with high signal-to-noise ratios.

However, bolus timing is necessary for accurate assessment. A number of non-contrast MRA techniques are helpful in evaluation of aortic dissection. Black blood sequences covering the aorta allow for adequate contrast between the aortic lumen and the vessel wall layers. With these imaging sequences, the dissection flap appears as a dark linear thin intraluminal structure. Figure 1 Intramural hemorrhage. A Right coronary angiogram demonstrating a dissection extending to the aortic root. B Black blood sequence demonstrates a large subintimal dissection with hyperintense hemorrhage consistent with aortic dissection red arrow.

Functional imaging with dynamic sequences allows for improved evaluation of how the dissection affects aortic hemodynamics and possible aortic regurgitation, and evaluation of clot burden. Phase-contrast imaging can be used for evaluation of inflow and outflow patterns through both the true and false lumen. The patent false lumen of chronic dissections has been shown to have slower helical or laminar flow than the true lumen.

Helical flow has been shown to be associated with progression of disease. In patients with intramural hematoma, MRI can be used to assess the acuity of the hematoma due to changes in the signal characteristics of hemoglobin degradation products. In the hyperacute phase, intramural hematoma may be isointense on T1-weighted images and hyperintense on T2-weighted images. Black blood images are also used in evaluation of penetrating atheromatous ulcer. Imaging reveals intimal disruption with extension of the ulcer into the thickened tunica media.

MRA is a valuable tool in evaluating the presence of atheromatous plaque that could be the underlying cause of cryptogenic stroke. Because the descending aorta is distal to the left subclavian artery and retrograde embolization is thought to be unlikely, these plaques have not usually been considered a potential source of stroke.

While transesophageal echocardiography is considered the historical gold standard for evaluation of aortic arch plaque, MRI and MRA have emerged as superior imaging modalities. Four-dimensional PC-MRI has been shown to be an effective tool for the evaluation of cryptogenic stroke. Diastolic retrograde flow in the descending aorta may represent an overlooked mechanism of retrograde embolization in stroke patients. The underlying physiology is related to the increased aortic stiffness due to aortic atherosclerosis. Increased pulse wave velocity and earlier wave reflection at the periphery can then result in marked diastolic retrograde descending aortic flow, even in the absence of aortic valve insufficiency.

A number of studies in patients with cryptogenic stroke have shown that a high proportion of these patients demonstrate retrograde embolization of aortic plaque that could reach all aortic branch vessels. A number of sequences can and should be used in the preoperative evaluation of aortic coarctation. Morphologic study can be obtained with dark blood T1-weighted sequences. Cine imaging with SSFP sequences allow for assessment of morphologic features as well as flow perturbances. Three-dimensional flow patterns can be identified that correlate with post-repair complications, including aneurysm and rupture.

Four-dimensional flow MRI has provided further insights into the pathophysiology of aortic coarctation as well. These altered flow patterns are thought to result in endothelial dysfunction and arterial remodeling downstream to stenosis. Elevated wall shear stress in the setting of aortic coarctation is thought to predispose these patients to development of ascending aortic aneurysms.

The primary pathologies encountered in imaging of the mesenteric arteries include chronic atherosclerosis, acute embolism, and aneurysm. In the past, conventional angiography was by far the best method for evaluating disease of the mesenteric vasculature. Non-contrast TOF MRA has been shown to be effective in imaging the proximal portions of the celiac, superior mesenteric, and inferior mesenteric arteries.

However, this technique is limited due to long scan times and flow-dependent signal loss from in-plane and turbulent flow. CE-MRA benefits from a high signal-to-noise ratio and short image times. Phase-contrast MRA is also an important tool in evaluation of the splanchnic vasculature. PC-MRA can be used in evaluation of splanchnic perfusion.

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For example, one study found that patients with chronic mesenteric ischemia have impairment in the normal post-prandial increase in superior mesenteric artery flow when compared with normal controls. CE-MRA has been shown to be equally as accurate in assessment of vascular stenoses within the lower extremity vasculature. With this technique, contrast is injected and images are obtained in the arterial first-pass phase. Blood pool gadolinium contrast agents with prolonged intravascular stay can be used for first-pass and steady-state MRA.

These are valuable in detecting collateral flow pathways, assessing asymmetric flow states and in visualizing arterial to venous shunting. Time-resolved imaging of contrast kinetics TRICKS sequences has been shown to provide optimal spatial and temporal resolution.

Figure 2 Cartesian acquisition with projection-like reconstruction lower extremity run-off. Time-resolved magnetic resonance angiography of the lower extremities with high spatial and temporal performed using Cartesian acquisition with projection-like reconstruction.

Magnetic resonance angiography: physical principles and applications.

One disadvantage of CE-MRA is that many patients with peripheral arterial disease suffer from concomitant chronic renal failure. Large contrast boluses are required to evaluate the pedal arteries as well. Further, at high field strengths, high-resolution QISS allows for improved contrast between arteries and surrounding soft tissues. MRA has the potential to provide more physiological and pathophysiological data over the disease in addition to the anatomical information.

This book is divided into three sections. The first section discusses the basics of MRI angiography. It starts with focus on the contrast agents that are mainly used in MR angiography with detailed discussi It starts with focus on the contrast agents that are mainly used in MR angiography with detailed discussion of advantage and limitations of different types of contrast. The second chapter is oriented more towards the technical consideration that contribute to good quality examination, both the non contrast and contrast based sequences from black to bright blood imaging , contrast enhanced MRA, review of clinical application of MRA in different body systems and MR venography.

The second section reviews the clinical application of MRI mainly in the head and neck and brain ischemia imaging. The new high resolution intracranial plaque imaging of the branch athermanous disease, to the hemodynamic of intracranial atherosclerotic stroke and quantitative MRA imaging in neurovascular imaging, are the topics in this section. Also this section covers the future prospective and the new frontiers MRI angiography is exploring.

By Amit Mehndiratta, Michael V. Knopp and Fredrik L. This is made possible by the EU reverse charge method. Edited by Theophanides Theophile. Edited by Chia-Hung Hsieh. Edited by Oleg Minin. Edited by Peter Bright. Edited by Bernhard Schaller.