DOI: 10.1148/rg.23si035515
(Radiographics. 2003;23:S59-S78.)
© RSNA, 2003
From the RSNA Refresher Courses
MR Imaging of Aortic and Peripheral Vascular Disease1
Servet Tatli, MD,
Martin J. Lipton, MD,
Brian D. Davison, MD,
Ronald B. Skorstad, RT(R), MR and
E. Kent Yucel, MD
1 From the Department of Radiology, Brigham and Women’s Hospital, Harvard Medical School, 75 Francis St, Boston, MA 02115 (S.T., M.J.L., B.D.D., R.B.S., E.K.Y.); and the Department of Radiology, Beth Israel Deaconess Hospital, Harvard Medical School, Boston (M.J.L.). Presented as a refresher course at the 2002 RSNA scientific assembly. Received March 6, 2003; revision requested May 7 and received June 17; accepted July 11. Address correspondence to S.T. (e-mail: statli@partners.org).
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Abstract
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Acquired diseases of the aorta and peripheral arteries are common. Owing to technical advances, magnetic resonance (MR) angiography has become the primary imaging modality for assessment of aortic and peripheral arterial disease. Contrast material–enhanced MR angiography is a rapid and robust technique that has emerged as the principal MR angiographic technique for evaluation of vascular disease. Two-dimensional time-of-flight MR angiography still has some well-validated applications, especially in distal peripheral vascular disease. Phase-contrast flow imaging is an important technique for quantification of blood flow. Black-blood imaging is a valuable tool for evaluation of the vessel wall. Understanding the principles of the main MR angiographic techniques is essential for consistent acquisition of diagnostic images. In addition, tailoring the acquisition parameters and the imaging protocol to the vessel being imaged and the clinical question is mandatory for optimal results. Future technical developments that will lead to faster image acquisition and better contrast agents promise to further improve image quality.
© RSNA, 2003
Index Terms: Aneurysm, aortic, 56.73, 94.73 • Aorta, dissection, 56.74, 94.74 • Aorta, stenosis or obstruction, 56.75, 94.72 • Arteries, extremities, 92.12942, 98.12942 • Arteries, grafts and prostheses, 92.452, 98.452 • Arteries, peripheral, 92.12942, 98.12942 • Arteries, stenosis or obstruction, 92.72, 98.72 • Magnetic resonance (MR), vascular studies
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LEARNING OBJECTIVES FOR TEST 3
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After reading this article and taking the test, the reader will be able to:
- Describe current MR angiography techniques for imaging diseases of the aorta and peripheral arteries.
- Discuss the advantages and shortcomings of current MR angiography techniques.
- Identify MR angiographic findings of common diseases of the aorta and peripheral arteries in adults.
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Introduction
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The age-adjusted prevalence of peripheral arterial disease is approximately 12% (1). Acquired disease of the thoracic and abdominal aorta is also widespread and in the past required invasive angiography to depict structural abnormalities. Remarkable advances in noninvasive imaging methods, notably computed tomography (CT) and magnetic resonance (MR) imaging, have replaced many invasive angiographic procedures, lowering the cost and morbidity of diagnosis (2). Several different MR angiography techniques are used to image the arteries. These include black-blood imaging (3), phase-contrast imaging (4), time-of-flight (TOF) imaging (5), and contrast material–enhanced MR angiography (6). Contrast-enhanced MR angiography is the most widely used method because it is rapid and robust.
In this article, present methods for studying the aorta and lower limb vasculature are reviewed and illustrated. Specific topics discussed are MR angiography techniques, clinical applications, and new developments. Although advanced MR imaging technology is required for optimum performance, the protocols described are intended to be as generic and widely applicable as possible.
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MR Angiography Techniques
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Black-Blood Vascular Imaging
In conventional spin-echo and fast spin-echo MR imaging, blood flow tends to be low in signal intensity due to effects related to movement of spins between 90° and 180° radio-frequency pulses as well as dephasing of turbulent flow. However, for better depiction of intraluminal or mural abnormality (intimal flaps, atherosclerotic plaque, wall thickening, intramural hematoma), the dedicated black-blood technique is optimal. This is a fast spin-echo, sequential-section imaging sequence that uses two magnetization-preparation inversion pulses that suppress the signal in the vascular lumen (Fig 1). First, a nonselective inversion pulse is applied, followed by a section-selective inversion pulse that restores the signal in the imaged section. The time delay (TI) between the second inversion pulse and the imaging sequence is chosen to null the signal from protons within blood. During this TI, the presumption is that the protons within the imaged section that received the second restorative inversion pulse will be replaced by the nulled protons from outside the imaged section.
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Figure 1. Black-blood imaging of the aorta. Axial image shows the ascending (black arrowhead) and descending (white arrowhead) aorta at the level of the right pulmonary artery (RPA). Note the excellent suppression of the luminal blood signal and demonstration of the vessel wall. The image was obtained in 13 seconds with cardiac gating and breath holding.
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The imaging sequence following the TI is a fast spin-echo sequence with an echo train length of eight to 16. We use cardiac gating with a repetition time equal to one R-R interval and an echo time of 20–30 msec, producing T1-weighted imaging and, in cooperative patients, breath holding. The black-blood effect is maximized by imaging in a plane perpendicular to the vessel of interest. (The axial plane is generally optimal for the aorta, although this may be supplemented by coronal imaging of the arch.) Although black-blood MR angiography provides better suppression of the signal from flowing blood, it is substantially less efficient in terms of imaging time than the fast spin-echo technique.
Phase-Contrast Imaging
Phase-contrast imaging is particularly valuable due to its ability to quantify flow and can be used in many clinical applications to evaluate physiologic properties of blood flow. In phase-contrast imaging, the phase shift of the moving spins in the blood is compared with that in the surrounding stationary tissue by using a bipolar gradient, allowing detection of flow velocity. Two acquisitions are performed: a flow-sensitive acquisition and a flow-compensated reference acquisition. These two acquisitions are automatically subtracted to eliminate phase shift induced by other sequence parameters. The information from phase-contrast measurements is processed into two sets of images: magnitude (bright-blood and anatomic) images (Fig 2a) and phase-contrast (velocity) images (Fig 2b).
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Figure 2a. Phase-contrast imaging of the aorta. Magnitude (a) and phase (b) axial images show the ascending (white arrowhead) and descending (black arrowhead) aorta at the level of the main pulmonary artery (MPA in a). Flow encoding was superior to inferior. On the phase image (b), the ascending aorta and main pulmonary artery appear black and the descending aorta appears white due to the opposite directions of flow in these arteries.
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Figure 2b. Phase-contrast imaging of the aorta. Magnitude (a) and phase (b) axial images show the ascending (white arrowhead) and descending (black arrowhead) aorta at the level of the main pulmonary artery (MPA in a). Flow encoding was superior to inferior. On the phase image (b), the ascending aorta and main pulmonary artery appear black and the descending aorta appears white due to the opposite directions of flow in these arteries.
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In phase-contrast images, the gray value of each pixel represents velocity information in that pixel, with higher flow velocities represented by higher signal intensities. Blood flowing in the direction opposite to the flow-encoded gradient will return no signal and thus appear dark. With knowledge of the cross-sectional area of a vessel, blood flow can be calculated quantitatively including volume as well as velocity. The technique requires the operator to actively choose a velocity-encoding factor to display the necessary vasculature. The phase-contrast technique can also be used for MR angiography (7). However, this approach to MR angiography has not found widespread acceptance due to problems with flow voids in areas of turbulence, requirements for gating pulsatile vessels, and the flow-velocity dependence of signal intensity.
TOF MR Angiography
Although TOF MR angiography has been replaced by contrast-enhanced MR angiography for many applications, some well-validated and important clinical applications remain, primarily in the distal runoff vessels (8,9). TOF MR angiography is based on the phenomenon of flow-related enhancement of spins entering into a partially saturated imaging section. These unsaturated spins give more signal than surrounding stationary spins. With the two-dimensional (2D) technique, multiple thin, sequential sections are acquired by using a flow-compensated gradient-echo sequence (Fig 3a). The acquired sections are either viewed individually or reformatted with the MIP technique to obtain a three-dimensional (3D) image (Fig 3b). Below the diaphragm, selective arterial imaging is performed by prescribing a presaturation pulse inferior to and tracking with the imaged sections to eliminate venous flow signal.
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Figure 3a. TOF MR angiography of the right calf. Axial source (a) and coronal maximum intensity projection (MIP) (b) images show pulsation artifacts (arrows) along the phase-encoding direction.
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Figure 3b. TOF MR angiography of the right calf. Axial source (a) and coronal maximum intensity projection (MIP) (b) images show pulsation artifacts (arrows) along the phase-encoding direction.
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TOF imaging of the extremity is time-consuming. Each location of approximately 100 sections requires several minutes of imaging time in addition to extra time repositioning extremity coils, if used. Limitations of TOF MR angiography can be summarized as follows (6,10): (a) complex flow producing loss of signal due to intravoxel dephasing may simulate disease; (b) vessels not truly perpendicular to the acquired section plane are more likely to show saturation effect, which causes low signal intensity; (c) triphasic pulsatile flow in vessels with relatively preserved inflow produces ghost artifacts unless cardiac gating is used, which further prolongs imaging time; and (d) retrograde flow in collateral and reconstituted vessels may also be saturated, obscuring the true level of obstruction and simulating an occluded segment that is longer than the actual length. The lengthy imaging times and these artifacts associated with TOF MR angiography have led to its rapid replacement with contrast-enhanced MR angiography for most applications. Specific uses with lower-extremity MR angiography will be discussed later.
In nonpulsatile vessels, as is typical of reconstituted arteries beyond an occlusion, conventional TOF imaging performs well. However, in the presence of normal, triphasic arterial flow, images can be degraded by ghost artifact in the phase-encoding direction (Fig 3), which can be alleviated by use of either electrocardiographic or finger plethysmography gating. In general, an imaging window of about 400 msec is effective at minimizing ghost artifact without an inordinate increase in imaging time. A saturation pulse is used to suppress the signal from veins. A spatial presaturation slab is placed 5 mm inferior to the section and programmed to move together with each section that is being acquired. More space between acquired sections and the saturation band may cause venous contamination from slow venous flow, which is commonly seen in patients with peripheral arterial disease.
Dynamic Contrast-enhanced MR Angiography
With contrast-enhanced MR angiography, high spatial resolution images can be obtained in much less time and with fewer artifacts than with other MR angiography techniques. This technique was described for imaging of aortoiliac disease in 1993 (6). In this early study, long acquisition and injection times were used. Subsequently, technical developments that resulted in new imaging units with shorter repetition times allowed acquisition times within a single breath hold. This permitted imaging of the arterial first pass of a rapidly injected bolus of gadolinium contrast material.
Basics of Contrast-enhanced MR Angiography.—
Contrast-enhanced MR angiography uses the T1-shortening effects of a gadolinium-based contrast agent. Signal enhancement and the overall image quality of contrast-enhanced MR angiography depend on the intraarterial contrast agent concentration. Coordination of image acquisition and the arrival of the contrast agent bolus in the region of interest is crucial to achieve high image quality. Collection of the central lines of k space during the plateau phase of arterial enhancement is essential for optimal contrast-enhanced MR angiography (11). Since image contrast depends mainly on central k-space data, filling the central portion of k space during peak arterial transit results in selective arterial enhancement. If the central portion of k space is filled prior to or during the upslope of contrast agent arrival, severe ringing artifacts limit the diagnostic usefulness of the image.
Specific timing methods will be discussed later, but routine use of a second acquisition is useful to help recover diagnostic information in case of an early acquisition or to detect late-enhancing vascular structures. Images acquired too long after peak arterial contrast agent arrival are frequently obscured by enhancement of veins and soft tissues, especially on MIP projections, and reliance on original coronal sections and reformatted images is required. The use of heavily T1-weighted sequences introduces the problem of background signal from fat. Therefore, a precontrast mask image is acquired and used to subtract the fat signal. Both the mask and contrast-enhanced images (Fig 4a, 4b) are obtained with breath holding in the thorax and abdomen, and the patient should not move between acquisition of the mask image and the contrast-enhanced image.
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Figure 4a. Contrast-enhanced MR angiography of the aorta. Source (a), subtracted (b), and MIP (c) coronal images show the aortic arch. The subtracted image (b) was obtained by subtracting the mask image from the source image (a) and demonstrates better suppression of the background signal from the body in comparison with the source image. On the MIP image (c), note the significant focal stenosis of the left subclavian artery (arrowhead). Also note the focal signal loss in the midportion of the left subclavian artery (arrows in c), which was due to concentrated gadolinium contrast material in the adjacent vein.
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Figure 4b. Contrast-enhanced MR angiography of the aorta. Source (a), subtracted (b), and MIP (c) coronal images show the aortic arch. The subtracted image (b) was obtained by subtracting the mask image from the source image (a) and demonstrates better suppression of the background signal from the body in comparison with the source image. On the MIP image (c), note the significant focal stenosis of the left subclavian artery (arrowhead). Also note the focal signal loss in the midportion of the left subclavian artery (arrows in c), which was due to concentrated gadolinium contrast material in the adjacent vein.
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Figure 4c. Contrast-enhanced MR angiography of the aorta. Source (a), subtracted (b), and MIP (c) coronal images show the aortic arch. The subtracted image (b) was obtained by subtracting the mask image from the source image (a) and demonstrates better suppression of the background signal from the body in comparison with the source image. On the MIP image (c), note the significant focal stenosis of the left subclavian artery (arrowhead). Also note the focal signal loss in the midportion of the left subclavian artery (arrows in c), which was due to concentrated gadolinium contrast material in the adjacent vein.
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Imaging Parameters.—
Contrast-enhanced MR angiography is performed by using a 3D T1-weighted gradient-echo sequence. T1 weighting is achieved by using gradient spoiling, a minimum repetition time and echo time, and a relatively high flip angle. (The optimal flip angle is repetition time dependent; however, with a repetition time of <10 msec, flip angles in the 25°–40° range work well.) Other parameters are as follows: A bandwidth of 62.5 kHz is used to minimize repetition time, partial field of view or partial Fourier acquisition helps minimize imaging time, and use of 384–512 frequency-encoding steps maximizes spatial resolution. The number of y (in-plane) and z (section-selection) phase-encoding steps is determined by the coverage and imaging duration required; interpolation in both the y and z phase-encoding directions improves image quality. To minimize imaging time, the imaging plane should be optimized to the anatomy being studied (sagittal oblique for the thoracic aorta, coronal for most other anatomic regions). In addition to routine MIP reformation (Fig 4c), axial reformation is helpful for imaging of the aorta and is routinely performed. The acquisition parameters used in our department for contrast-enhanced MR angiography of the aorta are summarized in the Table.
Contrast Material Timing Methods.—
A high concentration of contrast material should be maintained in the artery of interest during the entire acquisition time. Therefore, contrast material volume and delivery rate, delay time, as well as "chasing" saline solution volume and delivery rate are important parameters to optimize image quality. We use 40–50 mL of contrast material delivered at 2–2.5 mL/sec followed by 20 mL of saline solution delivered at the same rate. However, contrast material dosage and delivery rate should be adjusted with special attention to the acquisition time in individual patients. The contrast material injection site should be in the right antecubital fossa for thoracic aortic imaging to avoid artifact arising from undiluted contrast material in the left brachiocephalic vein (Fig 4c). Several methods are used to determine the optimal delay between the start of intravenous contrast material injection and the start of image acquisition.
Empirical timing is the easiest timing method and is performed by using a fixed imaging delay with a "best guess" value. Delay times of 25 seconds for the thoracic aorta and 30 seconds for the abdominal aorta are generally reliable (12). However, timing errors can occur, especially in patients with aortic aneurysms or low cardiac output.
A more precise method of estimating contrast material travel time is to perform a timing bolus acquisition (13) by using a small amount of contrast material (2 mL) followed by 20 mL of normal saline solution. The injection rate should be the same as in the planned contrast-enhanced MR angiography study. A timing bolus acquisition is performed with a fast 2D gradient-echo axial sequence with superior and inferior presaturation for the aorta at the level of interest. The acquisition is repeated at a rate of one image every 2 seconds. Graphic analysis of the data yields the time to peak enhancement, to which 4–5 seconds is added for the actual imaging delay to account for the effect of the longer bolus administration.
It is possible for MR imaging units to automatically detect the arrival of the contrast material bolus and trigger performance of the MR angiography sequence (14). In this method, a small tracker volume is placed in the aorta at the desired level. The signal level of the tracker volume is repeatedly sampled during the contrast material injection, and the acquisition is started automatically when the signal level rises above a predetermined threshold. Typically, a delay of 8–10 seconds between bolus detection and the start of the acquisition is added to allow the contrast material concentration to achieve a plateau and to permit the initiation of breath holding. When automated timing is used, the patient should breathe quietly and avoid taking large breaths or making substantial body motions, which may cause false triggering.
Another method of timing the contrast material bolus is triggering of image acquisition in real time with the MR fluoroscopic technique (15). In this technique, fast 2D real-time images are obtained during administration of the full volume of contrast material. The acquisition is initiated, and 2D images of the same anatomic location are obtained and displayed on the imaging unit monitor at near real-time rates. The operator manually triggers the imaging unit to perform 3D contrast-enhanced MR angiography when enhancement from the arrival of contrast material is seen.
Breath Holding.—
Breath holding is especially important for thoracic and abdominal aortic imaging and significantly improves image quality; however, it may not be possible in every case. Supplemental oxygen and hyperventilation can also help improve breath-holding capacity to 25 seconds in most patients (16). In ill or uncooperative patients who cannot breath hold for more than a few seconds, normal shallow breathing may work well and provide diagnostic images. Breath holding needs to be incorporated between the start of contrast material injection and the start of contrast-enhanced imaging. New operators need training and practice to perform this task reproducibly.
Optimization of k-Space Traversal.—
Choices for k-space traversal include the sequential, centric, and elliptic centric approaches. The peripheral lines of k space contain data encoding primarily spatial resolution, whereas the central lines of k space contain data related to image contrast. In a standard technique, k space is filled sequentially by the phase-encoding gradient in a linear method from bottom to top (or top to bottom) with filling of the central lines in the middle of the acquisition time. In a centric order, filling starts from the center lines and progresses to the periphery. Therefore, image data in the center of k space, which determines image contrast, are collected earlier in the acquisition. Centric encoding optimizes only one out of two phase-encoding directions. However, the elliptic centric approach provides true 3D centric k-space ordering in both phase-encoding directions.
Collection of the central lines of k space during the plateau phase of arterial enhancement is essential for optimal contrast-enhanced MR angiography, since image contrast depends mainly on central k-space data. If the central portion of k space is filled prior to or during the upslope of contrast material arrival, severe ringing artifacts limit the diagnostic usefulness of the image. On the other hand, images acquired too long after peak arterial contrast material arrival are frequently obscured by enhancement of veins and soft tissues. The elliptic centric approach provides high-quality arterial angiograms with intrinsic venous suppression by collecting the central k-space phase-encoding lines during peak arterial contrast material passage while permitting data acquisition to continue until filling of the peripheral phase-encoding lines to obtain images that also have higher spatial resolution (17). Coordination of image acquisition and the arrival of the contrast agent bolus in the region of interest is crucial to achieve high image quality. In general, the elliptic centric approach is reserved for applications in which venous contamination is a problem (the aortic arch, calf and thigh stations of runoff).
MR Angiography of Peripheral Arteries
Evaluation of peripheral arterial disease with MR angiography requires demonstration of long segments of vascular anatomy from the abdomen to the ankle. The standard TOF technique is clinically impractical for global evaluation due to flow artifacts in the aortoiliac region and long imaging times. Fast 3D contrast-enhanced MR angiography acquisitions in conjunction with rapid table movement permit the contrast agent bolus to be followed distally. Evaluation of peripheral arterial disease with MR angiography requires routine imaging of at least three stations: aortoiliac, thigh, and calf. In patients with limb-threatening ischemia (rest pain or tissue loss), high-resolution, dedicated evaluation of the symptomatic foot is also required to demonstrate the pedal runoff from any reconstituted tibial vessels. Although a technique of multiple, separate injections of contrast material with imaging of three individual stations has been described (18), the recent introduction of moving-table technology has allowed us to "chase" the bolus distally. Bolus chase technology, while an advance in MR angiography of peripheral arterial disease, is still evolving.
Three-Station, Moving-Table Contrast-enhanced MR Angiography.—
Coronal images from multiple stations can be acquired by using 3D contrast-enhanced MR angiography during injection of contrast material. The patient is placed in the magnet feet first, and the distal lower extremities are elevated to the anterior-posterior level of the more proximal arteries. Scout and then mask images are acquired at up to three locations, typically the pelvis, thigh, and calf. In addition to routine scout images, a stack of low-resolution TOF images is also obtained for planning. These help ensure that the 3D volumes for contrast-enhanced MR angiography are matched as closely as possible to the actual course of the vessels. The aortoiliac region is imaged first following injection of contrast material. Then, the table is moved and the thigh station is imaged. Finally, the table is moved again and the calf station is imaged.
Hybrid Methods.—
Contrast-enhanced MR angiography performed with the bolus chasing technique at three stations may give unsatisfactory results in the region of the tibial arteries due to venous contamination. Although this technique produces reliably excellent diagnostic images at the first two stations (aortoiliac and thigh), nondiagnostic calf images are a problem because current technology is not fast enough to chase contrast material from the aorta to the calf in all cases before venous enhancement occurs. The frequency of nondiagnostic calf images is higher in patients with limb-threatening ischemia, in whom the status of the tibial vessels is critically important. Hybrid methods have been implemented to overcome this problem.
In this technique, two sets of contrast-enhanced MR angiograms are obtained with separate contrast material injections. First, single-station, dedicated contrast-enhanced MR angiography of the calf is performed with 20 mL of contrast material; then, two-station contrast-enhanced MR angiography of the aortoiliac and thigh regions is performed with 40 mL of contrast material. Both injections should be followed by 20 mL of saline solution delivered at 1.5 mL/sec. The acquisition parameters for these studies are shown in the Table. Elliptic centric k-space filling is used for the calf and thigh stations, whereas sequential encoding is used for the aortoiliac station. For determination of the delay time, we use timing bolus detection. Since mask images are obtained before evaluation of each station, contrast material contamination from the earlier calf contrast agent injection is not an issue. Mask images are subtracted from injection images, and MIP images are obtained for each station (Fig 5). In addition, having the patient ambulate (if the patient is able to walk) for 10–15 minutes following the calf injection helps prevent venous contamination in the pelvic and thigh stations. We reconstruct contrast-enhanced MR angiograms of each calf separately to prevent superimposition on the lateral projection (Fig 5c, 5d). It is important to have source images for review, since the technologist may inadvertently exclude vascular structures of interest from the imaged volume or the degree of stenosis or artifacts may be better evaluated on source images.
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Figure 5a. Contrast-enhanced MR angiography of the lower extremity performed with the hybrid method. Imaging of the calf was performed first with 20 mL of gadolinium contrast material, and MIP images of each side were produced separately to prevent superimposition on the lateral projections. After the calf study, imaging of the pelvis and thigh was performed with 40 mL of gadolinium contrast material and the moving-table technique (from the pelvis to the thigh). Coronal MIP images show the arteries of the pelvis (a), thigh (b), right calf (c), and left calf (d). Note the short segmental occlusion of the right superficial femoral artery with reconstitution (arrows in b) and the high origin of the right posterior tibial artery (arrowhead in c).
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Figure 5b. Contrast-enhanced MR angiography of the lower extremity performed with the hybrid method. Imaging of the calf was performed first with 20 mL of gadolinium contrast material, and MIP images of each side were produced separately to prevent superimposition on the lateral projections. After the calf study, imaging of the pelvis and thigh was performed with 40 mL of gadolinium contrast material and the moving-table technique (from the pelvis to the thigh). Coronal MIP images show the arteries of the pelvis (a), thigh (b), right calf (c), and left calf (d). Note the short segmental occlusion of the right superficial femoral artery with reconstitution (arrows in b) and the high origin of the right posterior tibial artery (arrowhead in c).
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Figure 5c. Contrast-enhanced MR angiography of the lower extremity performed with the hybrid method. Imaging of the calf was performed first with 20 mL of gadolinium contrast material, and MIP images of each side were produced separately to prevent superimposition on the lateral projections. After the calf study, imaging of the pelvis and thigh was performed with 40 mL of gadolinium contrast material and the moving-table technique (from the pelvis to the thigh). Coronal MIP images show the arteries of the pelvis (a), thigh (b), right calf (c), and left calf (d). Note the short segmental occlusion of the right superficial femoral artery with reconstitution (arrows in b) and the high origin of the right posterior tibial artery (arrowhead in c).
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Figure 5d. Contrast-enhanced MR angiography of the lower extremity performed with the hybrid method. Imaging of the calf was performed first with 20 mL of gadolinium contrast material, and MIP images of each side were produced separately to prevent superimposition on the lateral projections. After the calf study, imaging of the pelvis and thigh was performed with 40 mL of gadolinium contrast material and the moving-table technique (from the pelvis to the thigh). Coronal MIP images show the arteries of the pelvis (a), thigh (b), right calf (c), and left calf (d). Note the short segmental occlusion of the right superficial femoral artery with reconstitution (arrows in b) and the high origin of the right posterior tibial artery (arrowhead in c).
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Because of the importance of demonstrating the pedal vasculature in these patients, we supplement the contrast-enhanced study with TOF MR angiography of the foot and ankle, a well-validated test for this application (2,8,19,20). In a routine runoff protocol, we first acquire TOF images of the symptomatic foot and ankle by using the extremity coil, followed by contrast-enhanced MR angiography. Our typical acquisition parameters are shown in the Table.
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Clinical Applications
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Aortic Disease
With the advent of contrast-enhanced MR angiography, it has become possible to achieve imaging times for the entire thoracic or abdominal aorta short enough for a breath hold, allowing routine production of high-quality images and reducing motion artifacts caused by respiration. This has been a breakthrough for MR angiography of the chest and abdomen, which has almost replaced conventional angiography for most clinical applications. MR angiography of the aorta is one of the most common indications for MR angiography. We combine contrast-enhanced MR angiography with T1-weighted black-blood imaging to visualize both intraluminal features as well as the surrounding soft tissues (Fig 1). Postcontrast T1-weighted images with fat suppression may be helpful in some conditions of the aorta such as arteritis, mycotic aneurysm, or graft infection. Aneurysms, dissection, occlusive disease due to atherosclerosis, and congenital malformations are the most common indications for MR angiography of the aorta.
Aortic Aneurysms.—
Most thoracic and abdominal aortic aneurysms are secondary to atherosclerosis. Aortic valvular disease can cause ascending aortic aneurysm. Infection, inflammation, syphilis, and cystic medial necrosis are other causes of aortic aneurysms. In Marfan syndrome, aneurysms most commonly occur in the proximal portion of the ascending aorta, involving the aortic root (Fig 6). Atherosclerotic aneurysms are typically fusiform, although focal eccentric aneurysms due to atherosclerosis are occasionally encountered (Fig 7). Important imaging features of aortic aneurysms are the maximum diameter, the length, and involvement of major branch vessels. All of these features can be identified and characterized with MR imaging.
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Figure 6a. Ascending aortic aneurysm in a 34-year-old man. (a) Sagittal oblique contrast-enhanced MR angiogram shows marked dilatation of the aortic root (arrow) and the proximal portion of the ascending aorta. This type of aneurysm is typical of Marfan syndrome, but the patient did not have other features of that disease. (b) Axial oblique steady-state free precession cine image of the left ventricular outflow tract shows a markedly dilated aortic root (large arrow) with regurgitant flow (small arrow).
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Figure 6b. Ascending aortic aneurysm in a 34-year-old man. (a) Sagittal oblique contrast-enhanced MR angiogram shows marked dilatation of the aortic root (arrow) and the proximal portion of the ascending aorta. This type of aneurysm is typical of Marfan syndrome, but the patient did not have other features of that disease. (b) Axial oblique steady-state free precession cine image of the left ventricular outflow tract shows a markedly dilated aortic root (large arrow) with regurgitant flow (small arrow).
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Figure 7. Saccular atherosclerotic aneurysm in a 75-year-old man. Sagittal oblique MIP image from contrast-enhanced MR angiography shows a saccular aneurysm of the aortic arch (arrow).
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The infrarenal abdominal aorta is the most common site of atherosclerotic aneurysms. Contrast-enhanced MR angiography allows assessment of involvement of the iliac and renal arteries. In these conditions, although the resolution of contrast-enhanced MR imaging remains lower than that of multidetector CT, 3D volumes and reformatted images from contrast-enhanced MR imaging can be used to make the measurements required to assess the patient’s suitability for a stent-graft. This may be especially useful in patients with contraindications to iodinated contrast material. Mycotic aneurysms result from weakening of the vessel wall by a bacterial infection, causing saccular outpouching, most commonly involving the suprarenal portion of the aorta. Contrast-enhanced MR angiography can demonstrate the aneurysm itself, whereas postcontrast T1-weighted imaging may demonstrate enhancement in and around the vessel wall.
Aortic Dissection.—
Aortic dissection occurs when blood dissects into the media of the aortic wall through an intimal tear. It is generally secondary to hypertension. In young patients with aortic dissection, an underlying process such as Marfan syndrome should be investigated. The proximal ascending aorta and the descending aorta just distal to the left subclavian artery are two common sites for initiation of the dissection. Aortic dissections involving the ascending aorta (Stanford type A) are associated with higher mortality, requiring surgical repair (Fig 8). Cine imaging of the left ventricular outflow tract may be added to the imaging protocol for suspected associated aortic valvular insufficiency. Axial reformatted images from contrast-enhanced MR angiography or axial black-blood images extending from the arch to the base of the neck are helpful in assessing extension of the dissection into the great vessels.
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Figure 8a. Stanford type A aortic dissection in a 60-year-old man. (a) Axial T1-weighted black-blood image shows nearly circumferential compression of the true aortic lumen by a false lumen (arrow). High signal intensity in the false lumen makes it difficult to differentiate thrombosis from flowing blood. (b) Axial reformatted image from contrast-enhanced MR angiography shows an intimal flap (black arrow) with flow in the false lumen (white arrow).
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Figure 8b. Stanford type A aortic dissection in a 60-year-old man. (a) Axial T1-weighted black-blood image shows nearly circumferential compression of the true aortic lumen by a false lumen (arrow). High signal intensity in the false lumen makes it difficult to differentiate thrombosis from flowing blood. (b) Axial reformatted image from contrast-enhanced MR angiography shows an intimal flap (black arrow) with flow in the false lumen (white arrow).
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Dissections arising distal to the left subclavian artery (Stanford type B) are usually treated medically. An MR angiography study must evaluate the extent of the dissection, the sizes of the true and false lumina, the patency of the false lumen, and branch vessel involvement. Imaging should extend from the arch to the aortic bifurcation. A section thickness of 8–10 mm with gaps may be necessary for black-blood imaging to achieve this coverage in a reasonable time. Standard T1-weighted imaging may be used as a more efficient alternative to black-blood imaging, especially if breath holding is problematic. Axial T1-weighted images should preferentially be obtained with cardiac gating and breath holding to prevent linear artifacts mimicking the intimal flap. MIP images may not show the intimal flap; however, axial reformatted images from contrast-enhanced MR angiography are helpful for visualization of the intimal tear and reentry sites (Fig 8b).
Intramural hematoma is an atypical form of dissection without flow in the false lumen or a discrete intraluminal flap (21). It can be identified as crescentic thickening of the aortic wall with increased intramural signal intensity on T1-weighted images (Fig 9). Precontrast fat saturation images can be helpful in differentiating intramural hematoma from surrounding mediastinal fat. Intramural hematoma most frequently involves the ascending aorta. The prognostic impact of the location of the intramural hematoma and its standard treatment are considered to be similar to those of classic aortic dissection (22).
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Figure 9. Intramural hematoma in an 87-year-old man with chest pain. Axial T1-weighted black-blood image shows eccentric circumferential thickening of the wall of the descending aorta with increased signal intensity (arrow), findings consistent with intramural hematoma.
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Penetrating atherosclerotic ulcer is characterized by ulceration of an atherosclerotic plaque that penetrates through the intima into the media of the aortic wall (23). It typically affects elderly individuals with hypertension and extensive aortic atherosclerosis. It is seen as an outpouching extending beyond the contour of the aortic lumen and can become quite large (Fig 10). A penetrating atherosclerotic ulcer is typically located in the descending aorta and can be associated with a variable degree of hematoma within the aortic wall (21). Diagnosis can be difficult when the presentation overlaps with that of atypical aortic dissection. Placement of an endovascular stent-graft is becoming a popular method of treating this entity, given that the disease tends to occur in elderly patients with comorbid conditions that put them at high surgical risk (21).
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Figure 10. Penetrating atherosclerotic ulcer in a 61-year-old man with high blood pressure. Sagittal oblique contrast-enhanced MR angiogram shows a diffusely aneurysmal descending aorta with a large outpouching in the medial anterior region (arrow), an appearance consistent with penetrating atherosclerotic ulcer. Prior studies showed development of a focal aneurysm 2 years earlier.
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Occlusive Disease of Aorta and Branches.—
The vast majority of stenotic and occlusive disease of the aorta in the western world results from atherosclerosis. Radiation-induced, Takayasu, and giant cell arteritis are other relatively frequent causes of aortic diseases. In Takayasu disease, so-called pulseless syndrome, the aortic arch vessels are primarily affected, but the thoracic and abdominal aorta and the pulmonary arteries may also be involved (24) (Fig 11). Postcontrast T1-weighted images are important in evaluating arteritis, which may exhibit enhancing wall thickening in its early stages (24) (Fig 12). Although atherosclerosis may cause occlusion of the distal abdominal aorta (as in Leriche syndrome), most commonly major branch involvement causes significant clinical consequences (Fig 13).
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Figure 11. Takayasu arteritis in a 41-year-old woman. Coronal MIP image from contrast-enhanced MR angiography shows occlusive disease involving the major arteries of the aortic arch. The left subclavian artery is occluded just beyond its origin (large white arrow). The left vertebral artery originates from the aortic arch and is stenotic at its origin (small white arrow). The right common carotid artery has a long stenotic segment (small black arrows). The left common carotid artery is stenotic in its proximal segment (large black arrow).
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Figure 12. Aortic wall thickening in a 67-year-old man with clinically suspected vasculitis. Axial gadolinium-enhanced T1-weighted black-blood image obtained with fat suppression shows significant enhancement of the wall of the aorta at the level of the aortic arch (arrows). Contrast-enhanced MR angiography showed occlusive disease of the distal right subclavian artery.
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Figure 13. Severe atherosclerotic disease of the aorta in a 60-year-old man. Coronal MIP image from contrast-enhanced MR angiography shows an occluded infrarenal aorta with marked atherosclerotic contour irregularities (large arrows). Note the high-grade focal stenosis of the right renal artery (small arrows).
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Contrast-enhanced MR angiography is most helpful for evaluating narrowing of the aorta and branch vessels. Atherosclerotic lesions are responsible for over 90% of all renovascular disease and are more typical in the older age group. The stenosis generally involves the origin and proximal portion of the renal artery (Fig 13). Fibromuscular dysplasia is a less frequent cause of renovascular hypertension and is more frequently seen in women at a younger age; it manifests as a string-of-beads appearance (Fig 14). Saccular aneurysms of the renal arteries are generally multiple and affect both kidneys (Fig 15). Intestinal ischemia is caused by atherosclerotic narrowing or obstruction of the major splanchnic arteries. Most stenoses involve the proximal segment of the superior mesenteric artery (SMA) near its origin and can be easily identified with contrast-enhanced MR angiography. Isolated dissection of the SMA may rarely cause mesenteric ischemia (Fig 16).
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Figure 14. Fibromuscular dysplasia in a 61-year-old woman with high blood pressure. Coronal MIP image from contrast-enhanced MR angiography shows that the right renal artery has a string-of-beads appearance (arrows), which is typical of fibromuscular dysplasia. The diagnosis was confirmed with conventional angiography and pressure measurements. The patient was treated with angioplasty.
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Figure 15. Renal artery aneurysm in a 38-year-old woman with hypertension who was previously treated for an aneurysm of the right renal artery. Coronal contrast-enhanced MR angiogram shows a saccular aneurysm of the left renal artery (arrows).
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Figure 16a. Dissection of the SMA in a 50-year-old man with acute onset of abdominal pain during weight lifting. (a) Coronal contrast-enhanced MR angiogram shows an intimal flap in the proximal segment of the SMA (arrow). (b) Sagittal MIP image shows a dilated SMA (white arrows). The intimal flap is not well seen; however, occlusion of the SMA after the origin of the ileocolic branch is clearly demonstrated (black arrow).
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Figure 16b. Dissection of the SMA in a 50-year-old man with acute onset of abdominal pain during weight lifting. (a) Coronal contrast-enhanced MR angiogram shows an intimal flap in the proximal segment of the SMA (arrow). (b) Sagittal MIP image shows a dilated SMA (white arrows). The intimal flap is not well seen; however, occlusion of the SMA after the origin of the ileocolic branch is clearly demonstrated (black arrow).
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Peripheral Arterial Disease
Atherosclerosis is the leading cause of occlusive arterial disease in patients older than 40 years, and the prevalence increases with age (25). Symptomatic atherosclerotic occlusive disease of the lower limbs affects 7% of the older population (1). Patients with peripheral arterial disease are also at risk of myocardial infarction and ischemic stroke (26). Conventional runoff angiography is invasive, has an increased frequency of adverse events with age, and can fail to demonstrate distal vessels suitably for reconstructive surgery (2,27). MR angiography is a safer technique, especially in the elderly, and has great potential as a screening and diagnostic tool to guide interventional procedures. With current MR angiography techniques, the location, severity, and extent of atherosclerotic disease can be demonstrated. Contrast-enhanced MR angiography can demonstrate atherosclerotic disease involving the iliac and femoral arteries and provide guidance for endovascular therapy (Fig 17). In addition to identifying occluded or stenotic inflow vessels, current MR angiography techniques can identify reconstituted target vessels for bypass grafting, in some cases even better than traditional angiography can (Figs 18, 19).
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Figure 17. Atherosclerotic disease of the iliac arteries in a 66-year-old woman with left-sided claudication. Coronal MIP image from contrast-enhanced MR angiography of the first station runoff shows mild atherosclerotic disease of the distal aorta with aneurysmal dilatation (large arrow). The right common iliac artery is moderately narrowed with a weblike stenosis at the iliac bifurcation (arrowheads). The left common iliac artery is severely stenosed at its origin (small arrow).
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Figure 18a. Reconstitution of the calf arteries in a 55-year-old woman with left-sided claudication. (a) Coronal MIP image from contrast-enhanced MR angiography performed with dedicated calf injection shows focal occlusion of the left popliteal artery just above the trifurcation (large arrow) with reconstitution of the calf arteries. Note the collateral vessel feeding the anterior tibial artery (small arrows). (b) Sagittal MIP image from high-resolution TOF angiography shows a patent posterior tibial artery feeding the plantar arteries (large arrows). The patent anterior tibial artery continues as the dorsalis pedis artery (small arrows).
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Figure 18b. Reconstitution of the calf arteries in a 55-year-old woman with left-sided claudication. (a) Coronal MIP image from contrast-enhanced MR angiography performed with dedicated calf injection shows focal occlusion of the left popliteal artery just above the trifurcation (large arrow) with reconstitution of the calf arteries. Note the collateral vessel feeding the anterior tibial artery (small arrows). (b) Sagittal MIP image from high-resolution TOF angiography shows a patent posterior tibial artery feeding the plantar arteries (large arrows). The patent anterior tibial artery continues as the dorsalis pedis artery (small arrows).
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Figure 19a. Peripheral arterial disease in a 65-year-old man with ischemic symptoms in the left leg. (a) Coronal MIP image from contrast-enhanced MR angiography of the thigh station shows diffuse disease of both superficial femoral arteries with a totally occluded left popliteal artery (arrow). (b) Coronal image from contrast-enhanced MR angiography performed with dedicated calf injection shows the occluded popliteal artery (large arrow) with a reconstituted posterior tibial artery (small arrows) and significantly diseased peroneal and anterior tibial arteries. (c) Coronal MIP image from TOF imaging of the same calf shows that the distal posterior tibial artery is widely patent (large arrows) and the anterior tibial artery is diminutive (small arrows). (d) Coronal TOF image of the left foot shows a patent plantar arch (small arrows) supplied by the posterior tibial artery (large arrow). The dorsalis pedis artery is absent.
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Figure 19b. Peripheral arterial disease in a 65-year-old man with ischemic symptoms in the left leg. (a) Coronal MIP image from contrast-enhanced MR angiography of the thigh station shows diffuse disease of both superficial femoral arteries with a totally occluded left popliteal artery (arrow). (b) Coronal image from contrast-enhanced MR angiography performed with dedicated calf injection shows the occluded popliteal artery (large arrow) with a reconstituted posterior tibial artery (small arrows) and significantly diseased peroneal and anterior tibial arteries. (c) Coronal MIP image from TOF imaging of the same calf shows that the distal posterior tibial artery is widely patent (large arrows) and the anterior tibial artery is diminutive (small arrows). (d) Coronal TOF image of the left foot shows a patent plantar arch (small arrows) supplied by the posterior tibial artery (large arrow). The dorsalis pedis artery is absent.
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Figure 19c. Peripheral arterial disease in a 65-year-old man with ischemic symptoms in the left leg. (a) Coronal MIP image from contrast-enhanced MR angiography of the thigh station shows diffuse disease of both superficial femoral arteries with a totally occluded left popliteal artery (arrow). (b) Coronal image from contrast-enhanced MR angiography performed with dedicated calf injection shows the occluded popliteal artery (large arrow) with a reconstituted posterior tibial artery (small arrows) and significantly diseased peroneal and anterior tibial arteries. (c) Coronal MIP image from TOF imaging of the same calf shows that the distal posterior tibial artery is widely patent (large arrows) and the anterior tibial artery is diminutive (small arrows). (d) Coronal TOF image of the left foot shows a patent plantar arch (small arrows) supplied by the posterior tibial artery (large arrow). The dorsalis pedis artery is absent.
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Figure 19d. Peripheral arterial disease in a 65-year-old man with ischemic symptoms in the left leg. (a) Coronal MIP image from contrast-enhanced MR angiography of the thigh station shows diffuse disease of both superficial femoral arteries with a totally occluded left popliteal artery (arrow). (b) Coronal image from contrast-enhanced MR angiography performed with dedicated calf injection shows the occluded popliteal artery (large arrow) with a reconstituted posterior tibial artery (small arrows) and significantly diseased peroneal and anterior tibial arteries. (c) Coronal MIP image from TOF imaging of the same calf shows that the distal posterior tibial artery is widely patent (large arrows) and the anterior tibial artery is diminutive (small arrows). (d) Coronal TOF image of the left foot shows a patent plantar arch (small arrows) supplied by the posterior tibial artery (large arrow). The dorsalis pedis artery is absent.
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Pseudoaneurysms following surgery or arterial catheterization are commonly seen in the femoral artery (Fig 20). Although they are well seen with ultrasonography (US), MR angiography can also demonstrate the abnormality. The popliteal artery is the most common site of lower-extremity aneurysm, and such aneurysms are characterized by an increase in the diameter of the artery of more than 50% compared with the adjacent normal segment (Fig 21). Unlike other aneurysms, rupture of popliteal aneurysms is rare; they are usually complicated by thrombosis or peripheral embolization. Therefore, tibial imaging is an important advantage of MR angiography over US in popliteal aneurysms.
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Figure 20. Pseudoaneurysm after aortobifemoral bypass in a 75-year-old man. Coronal MIP image shows a patent aortobifemoral bypass graft (small arrows) with a contrast material-filled outpouching in the region of the left femoral artery (large arrow), a finding consistent with a postsurgical pseudoaneurysm. The left renal artery is absent due to prior nephrectomy.
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Figure 21a. Lower-extremity aneurysms in a 60-year-old man with deep vein thrombosis. (a, b) Coronal MIP images from contrast-enhanced MR angiography of the aortoiliac (a) and thigh (b) stations show aneurysms of both common iliac arteries (arrows in a), the left common femoral artery, and both popliteal arteries (arrows in b). (c) Coronal contrast-enhanced MR angiogram of the thigh shows disappearance of the aneurysms after treatment with bilateral bypass grafts. Note the clip artifacts in the grafts (arrows).
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Figure 21b. Lower-extremity aneurysms in a 60-year-old man with deep vein thrombosis. (a, b) Coronal MIP images from contrast-enhanced MR angiography of the aortoiliac (a) and thigh (b) stations show aneurysms of both common iliac arteries (arrows in a), the left common femoral artery, and both popliteal arteries (arrows in b). (c) Coronal contrast-enhanced MR angiogram of the thigh shows disappearance of the aneurysms after treatment with bilateral bypass grafts. Note the clip artifacts in the grafts (arrows).
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Figure 21c. Lower-extremity aneurysms in a 60-year-old man with deep vein thrombosis. (a, b) Coronal MIP images from contrast-enhanced MR angiography of the aortoiliac (a) and thigh (b) stations show aneurysms of both common iliac arteries (arrows in a), the left common femoral artery, and both popliteal arteries (arrows in b). (c) Coronal contrast-enhanced MR angiogram of the thigh shows disappearance of the aneurysms after treatment with bilateral bypass grafts. Note the clip artifacts in the grafts (arrows).
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Infrapopliteal Arterial Disease.—
It may be difficult to visualize the circulation in the calf and foot with conventional angiography due to dilution of contrast material, slow flow, and nonselective injection (27,28). Unger et al (28) showed that 2D TOF MR angiography produces images of the arterial anatomy of the foot and ankle that are of higher quality than conventional angiograms or provide better information. It has been shown in other studies that MR angiography demonstrates patent vessel segments not visualized at conventional angiography (8,27,29). Pedal evaluation is important in patients with peripheral arterial disease, especially those with limb-threatening ischemia who require tibial bypass. The distal end of the bypass graft may be anastomosed to a pedal vessel (Fig 22). Therefore, identification of the major runoff into the foot to the level of the plantar arch is necessary for surgical planning (30).
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Figure 22. Graft stenosis in a 75-year-old man with diabetes and peripheral vascular disease. He presented with gangrenous changes in the left toes after placement of a bypass graft. Coronal contrast-enhanced MR angiogram obtained with calf injection shows significant stenosis at the origin of a popliteal-to-distal artery bypass graft (arrow). The distal portion of the graft is widely patent.
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Severe tibial disease is especially common in diabetic patients, who may also have renal impairment, making MR angiography an attractive alternative to x-ray angiography with its risk of nephropathy. For this purpose, dedicated calf and foot imaging should be added to the imaging protocol. It is difficult to perform contrast-enhanced MR angiography of the foot given a standard three-station MR angiography runoff protocol. Therefore, we rely on TOF imaging routinely and bring patients back, if needed, for dedicated contrast-enhanced MR angiography of the foot (Fig 23). Occasionally, in patients in whom inflow is not the problem, we will perform dedicated contrast-enhanced MR angiography of the foot at the initial examination.
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Figure 23. Contrast-enhanced MR angiography of the foot in a 33-year-old man with numbness and coldness of the left foot. Sagittal contrast-enhanced MR angiogram obtained with foot injection shows the main arterial anatomy of the foot. The anterior tibial artery feeds the dorsalis pedis artery (large arrow), which constitutes the plantar arch (small arrows) along with the lateral plantar artery from the posterior tibial artery (medium-sized arrow).
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MR Angiography of Grafts.—
Inflow grafts (aorto–bi-iliac, aortobifemoral, axillobifemoral) are occasionally imaged in conjunction with an evaluation for peripheral arterial disease (Fig 20). Imaging with contrast-enhanced MR angiography is usually straightforward, other than the possibility of mistaking a metallic clip for a stenosis (Fig 21c). On the other hand, outflow grafts present special problems. These most commonly originate from the common femoral artery, although takeoff from the superficial femoral artery or even the popliteal artery is possible (Fig 21). The distal anastomosis may be at the above- or below-knee popliteal level, at one of the tibial arteries in the calf, or even at a pedal artery (Fig 22). Long-term (>1 year) graft failure is usually due to progression of native disease in the outflow vessels. Graft failure in the first year is often due to intimal hyperplasia within the graft, progressing to graft thrombosis.
For this reason, periodic graft surveillance within the first year with duplex US has become routine. MR angiography may be requested for further evaluation prior to graft revision. In these cases, special care must be taken not to mistake metallic clips adjacent to the graft for graft stenosis (Fig 22). Careful review of source images may reveal that the source of signal loss actually lies adjacent to, rather than within, the graft lumen. Metal artifacts are associated with foci of high signal intensity immediately adjacent to the area of signal void. Correlation with US findings is also helpful in assessing the significance of areas of potential stenosis versus clip artifact on MR angiograms. However, MR angiography is superior to US in runoff evaluation beyond the graft.
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New Developments
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There has been a trade-off between spatial and temporal resolution in MR imaging. The main goal of today’s researchers is to develop an ideal acquisition technique that acquires images with the highest spatial resolution in the shortest possible time.
Time-resolved acquisition techniques, such as time-resolved imaging of contrast kinetics (TRICKS), are a recently developed method for obtaining highly temporally resolved images (31). This imaging technique enables rapid updating of 3D data sets in several seconds, eliminating the need for timing acquisitions or triggering methods and separate acquisition of a precontrast mask image (32). There is some reduction in spatial resolution in exchange for temporal resolution when compared with standard 3D acquisitions. In this technique, low-spatial-frequency data are sampled more often than high-spatial-frequency data, which are shared among adjacent frames to fill k space for each frame (31). Parallel imaging (simultaneous acquisition of spatial harmonics [SMASH] [33], sensitivity encoding [SENSE] [34]) is a novel method of acquiring high-resolution images in a short acquisition time by using multiple coils to fill multiple lines of k space simultaneously. The signal-to-noise ratio is inversely proportional to the square root of the number of coils used. However, in contrast-enhanced MR angiography, much of this signal-to-noise ratio loss can be overcome by a faster rate of contrast material injection. Recently, projection reconstruction techniques have been described as a way of filling k space more efficiently in contrast-enhanced MR angiography (35–38).
In addition to conventional contrast agents, several newer contrast agents are in various stages of clinical development. These include agents similar to current extracellular agents but with slightly higher relaxivity (gadobenate dimeglumine) or in higher concentration (gadobutrol) (39–41). A new family of blood pool agents has several members, including MS-325 (albumin-binding gadolinium chelate), gadomer-17 (polymeric gadolinium chelate), and ultrasmall iron particles (42–44). Given the dependence of current contrast-enhanced MR angiography on imaging during the arterial first pass, the ideal blood pool agent should have the ability to be administered as an intravenous bolus, good T1 signal at high concentration during the arterial first pass, stable blood signal for at least 1 hour, fairly rapid excretion thereafter, and safety comparable with that of the current generation of extracellular agents. The prolonged enhancement of the blood pool may permit more detailed, high-resolution imaging of the vessels during the equilibrium phase with the potential to perform more quantitative assessment of the degree of stenosis and the amount of plaque on cross-sectional images of the blood vessel. However, since arteries and veins are equally enhanced during the blood pool phase, more advanced visualization techniques than the current standard MIP projection will be necessary to display the arteries separately from the veins.
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Conclusions
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MR angiography is a powerful tool for imaging the aorta and peripheral arteries. Although contrast-enhanced MR angiography is becoming the primary method for evaluation of vascular diseases and may become the only method in the future, 2D TOF imaging still has some remaining, well-validated applications. Understanding the essentials of the main MR angiography techniques is important to be able to acquire diagnostic images consistently. In addition, tailoring the acquisition parameters as well as the imaging protocol according to the vessel to be imaged and the clinical question is mandatory for optimal results. Future technical developments leading to faster image acquisition as well as better contrast agents promise to further improve image quality.
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Footnotes
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Abbreviations: MIP = maximum intensity projection,
SMA = superior mesenteric artery,
TOF = time of flight,
2D = two-dimensional,
3D = three-dimensional
Editor’s Note.—In keeping with the highest standards of professional integrity and ethics, RSNA requires that authors of continuing medical education articles fully disclose to their readers that they will discuss the unlabeled use of a commercial product, device, or pharmaceutical that has not been approved for such purpose by the FDA. All of the authors have submitted such a disclosure statement for this article.
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Abstract
LEARNING OBJECTIVES FOR TEST...
