(Radiology. 1999;212:371-377.)
© RSNA, 1999
Vascular and Interventional Radiology |
Iliofemoral Arterial Occlusive Disease: Contrast-enhanced MR Angiography for Preinterventional Evaluation and Follow-up after Stent Placement1
Johann Link, MD,
Johann C. Steffens, MD,
Joachim Brossmann, MD,
Joachim Graessner, PhD,
Stephanie Hackethal, MD and
Martin Heller, MD
1 From the Department of Radiology, Christian-Albrechts-University of Kiel, Germany (J.L., J.C.S., J.B., S.H., M.H.); and Siemens Medical Systems, Hamburg, Germany (J.G.). Received December 1, 1997; revision requested February 23, 1998; final revision received November 16; accepted February 22, 1999. Address reprint requests to J.L., Institut für Röntgendiagnostik, Universitätsklinikum Regensburg, Regensburg 93042, Germany (e-mail: johann.link@klinik.uni-regensburg.de).
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Abstract
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PURPOSE: To evaluate the efficacy of contrast material–enhanced magnetic resonance (MR) angiography for the diagnosis of peripheral arterial occlusion and follow-up after stent placement.
MATERIALS AND METHODS: Sixty-seven patients (21 women, 46 men; mean age, 64.6 years) were examined. Digital subtraction angiography and contrast-enhanced MR angiography were performed in 28 patients for preinterventional evaluation of iliofemoral arterial occlusion and in 39 patients for follow-up after stent placement in the iliac or femoral arteries, which had been performed several months before.
RESULTS: All 24 occlusions were correctly diagnosed with contrast-enhanced MR angiography. Of the 59 stenoses, 36 were greater than 50% and 23 were 50% or less. Sensitivity and specificity for the detection of stenoses greater than 50% were 100% and 83%, respectively. Patency of the different stents was determined correctly with contrast-enhanced MR angiography. Some stents caused signal intensity dropout, which made MR evaluation of stents difficult. Generally, these signal intensity artifacts were most severe in stainless steel stents and mild in some nitinol stents.
CONCLUSION: Contrast-enhanced MR angiography is comparable to digital subtraction angiography for the detection of stenosis greater than 50% and occlusion in the iliofemoral arteries. Stent patency can be determined, but contrast-enhanced MR angiography is not suitable for stent evaluation owing to signal intensity dropout; however, it provides information about the vascular anatomic areas proximal and distal to the stent.
Index terms: Arteries, grafts and prostheses, 922.1268, 984.1268, 986.1268 • Arteries, stenosis or obstruction, 922.721, 984.721, 986.721 • Magnetic resonance (MR), comparative studies • Magnetic resonance (MR), vascular studies, 922.12942, 922.12943, 984.12942, 984.12943, 986.12942, 986.12943 • Stents and prostheses, 922.1268, 984.1268, 986.1268
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Introduction
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Magnetic resonance (MR) angiography is an alternative to intraarterial digital subtraction angiography (DSA) for the diagnosis of peripheral arterial occlusive disease. The limitations of MR angiography have been long imaging times and artifacts caused by elongated vessels, superimposed venous structures, and slow and pulsatile flow (1–3).
With the advent of contrast material–enhanced MR angiography, a technique is available that makes use of the shortened T1 relaxation times of blood after the administration of gadopentetate dimeglumine and thus achieves high signal intensity from blood with the use of three-dimensional gradient-echo sequences and short relaxation times (4–13).
The goals of this prospective study were to determine the role of contrast-enhanced MR angiography in the evaluation of iliofemoral arterial occlusive disease and to evaluate its accuracy in assessing the patency of iliofemoral arterial stents by using DSA as the reference standard.
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MATERIALS AND METHODS
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Sixty-seven patients (21 women, 46 men; age range, 29–87 years; mean age ± SD, 64.6 years ± 11) who were referred to our hospital for angiography and underwent contrast-enhanced MR angiography of the iliac and femoral arteries in addition to intraarterial DSA were included in this study. MR angiography was performed in all patients the day before intraarterial DSA. Informed consent was obtained from all patients, and institutional approval to perform contrast-enhanced MR angiographic studies for comparison with DSA was obtained. The contrast-enhanced MR angiographic studies were performed by using a three-dimensional gradient-echo sequence (repetition time msec/echo time msec, 7.8/2.1; flip angle, 30°; effective section thickness, 1.82 mm; slab thickness, 116 mm; field of view, 438 x 500 mm; matrix, 224 x 512) with a 1.5-T MR imaging unit (Magnetom Vision; Siemens Medical Systems, Erlangen, Germany). The circular polarized body coil served as a transmit-receive coil. The data acquisition time was 1 minute 54 seconds. Owing to the field of view limitation of 500 mm, contrast-enhanced MR angiography enabled the assessment of only the iliofemoral region; popliteal and infrapopliteal arteries were not evaluated with this imaging modality.
Seven targeted views were reconstructed with the maximum intensity projection technique in intervals of 0° and 180° (left lateral–to–right lateral view) and in steps of 30°. In all patients, the maximum intensity projection images and raw data were analyzed together. Thirty milliliters of gadodiamide (Omniscan; Nycomed, Oslo, Norway) was injected with an MR-compatible injector (Spectris; Medrad, Volkach, Germany) at a flow rate of 0.6 mL/sec and an injection delay of 10 seconds.
Indications for DSA were preinterventional evaluation of iliofemoral arterial occlusive disease in 28 (42%) symptomatic patients and follow-up after stent placement in 39 (58%) patients in whom 41 stents had been implanted several months before. In each patient who underwent DSA, for either preinterventional evaluation of iliofemoral occlusive disease or follow-up after stent placement, additional contrast-enhanced MR angiography was performed. Intraarterial DSA was performed in five steps, from the iliac region to the lower limb, in an anteroposterior view. Occasionally, in cases of uncertainty, multiple views were obtained. A 4-F pigtail catheter was inserted above the aortic bifurcation. In each step, 30 mL of iomeprol (300 mg/mL) (Imeron 300; Byk Gulden, Konstanz, Germany) was injected at a flow rate of 14 mL/sec.
With DSA only anteroposterior views were available; however, contrast-enhanced MR angiography provided anteroposterior views and additional views of 30°, 60°, and 90° projections on each side. In both contrast-enhanced MR angiography and DSA, narrowing of the vessels was measured with calipers on the film hard-copy images. The most severe narrowing was related to the normal diameter of the artery above and below the lesion.
Twenty-four occlusions and 59 stenoses were observed in all 67 patients. The grade of stenoses was 50% or less in 23 (39%) of 59 cases and greater than 50% in 36 (61%) of 59 cases. Five occlusions, 14 stenoses greater than 50%, and 16 stenoses of 50% or less were observed in the iliac region. The other 19 occlusions, 22 stenoses greater than 50%, and seven stenoses of 50% or less were located in the superficial femoral arteries.
In the 39 patients with 41 stents, follow-up angiography was performed. The 24 stents implanted in the iliac arteries were as follows: eight Cragg and two Cragg Endopro stents (Boston Scientific, Watertown, Mass), seven Memotherm stents (Bard-Angiomed, Karlsruhe, Germany), and seven Wallstents (Schneider, Buelach, Switzerland). The 17 stents placed in the femoral arteries were Cragg Endopro (n = 12) and Palmaz (Johnson & Johnson Interventional Systems, Warren, NJ) (n = 5) stents. The sizes of these stents are shown in the Table.
The evaluation criteria for the comparative study were the quality of contrast-enhanced MR versus intraarterial digital subtraction angiograms with regard to venous superimposition, and the accuracy of contrast-enhanced MR angiography versus that of DSA in depicting occlusions and stenoses of 50% or less and greater than 50% in the iliac and femoral region and in assessing patency and stenosis after stent placement at 6-month follow-up.
All digital subtraction angiograms and MR angiograms were evaluated separately by two experienced cardiovascular radiologists (J.L., J.C.S.) in a blinded manner. The entire vascular tree, from the distal aorta to the femoral arteries, was assessed. Discrepancies were resolved by consensus. Statistical analyses were performed by calculating sensitivity and specificity values. Overestimations of stenosis were considered to be false-positive findings, and underestimations were considered to be false-negative findings. To determine interobserver variability, the Spearman-rank correlation test was used.
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RESULTS
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All of the arteries could be evaluated. All of the nondiseased arteries were assessed correctly, and no stenosis was overestimated as an occlusion. Veins were enhanced in eight examinations; however, there was no superimposition on the arteries.
The mean length of the stenoses and occlusions (± SD) were 2.7 cm (± 1.7) and 3.5 (± 2.3), respectively. The results of analyses of all the lesions, including the 59 stenoses and 24 occlusions, with contrast-enhanced MR angiography and intraarterial DSA agreed in 79 (95%) of the 83 cases. In the evaluation of the 36 high-grade (greater than 50%) stenoses and 24 occlusions, there was 100% agreement between contrast-enhanced MR angiography and intraarterial DSA; both the sensitivity and specificity of MR angiography were 100%.
Stenoses of 50% of less were overestimated with MR angiography—that is, interpreted as stenoses greater than 50%—in four (17%) of 23 cases. Thus, the sensitivity of MR angiography for the detection of stenoses greater than 50% was 100%, and the specificity was 83% (19 of 23 cases).
The overall agreement between contrast-enhanced MR angiography and intraarterial DSA was 93% in the iliac region (Fig 1); the sensitivity and specificity of MR angiography for the detection of stenosis greater than 50% in this region were 100% and 88%, respectively. There was 93% agreement between contrast-enhanced MR angiography and intraarterial DSA in depicting stenosis greater than 50% in the femoral region, with a sensitivity of 100% and specificity of 71%. The interobserver variability for identifying and grading all lesions was 0.93 (P < .001).
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Figure 1a. (a, b) Anteroposterior digital subtraction angiograms obtained in a 72-year-old man show high-grade stenosis of the right external iliac artery (arrow in a) and occlusion of the right (curved arrow in b) and left (straight arrow in b) superficial femoral arteries. (c) Anteroposterior contrast-enhanced MR angiogram (7.8/2.1, 30° flip angle) obtained in the same patient shows high-grade stenosis of the right external iliac artery (short arrow) to the same extent as that in a. The occlusions of both superficial femoral arteries (long arrows), which were seen in b, are depicted equally well at contrast-enhanced MR angiography.
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Figure 1b. (a, b) Anteroposterior digital subtraction angiograms obtained in a 72-year-old man show high-grade stenosis of the right external iliac artery (arrow in a) and occlusion of the right (curved arrow in b) and left (straight arrow in b) superficial femoral arteries. (c) Anteroposterior contrast-enhanced MR angiogram (7.8/2.1, 30° flip angle) obtained in the same patient shows high-grade stenosis of the right external iliac artery (short arrow) to the same extent as that in a. The occlusions of both superficial femoral arteries (long arrows), which were seen in b, are depicted equally well at contrast-enhanced MR angiography.
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Figure 1c. (a, b) Anteroposterior digital subtraction angiograms obtained in a 72-year-old man show high-grade stenosis of the right external iliac artery (arrow in a) and occlusion of the right (curved arrow in b) and left (straight arrow in b) superficial femoral arteries. (c) Anteroposterior contrast-enhanced MR angiogram (7.8/2.1, 30° flip angle) obtained in the same patient shows high-grade stenosis of the right external iliac artery (short arrow) to the same extent as that in a. The occlusions of both superficial femoral arteries (long arrows), which were seen in b, are depicted equally well at contrast-enhanced MR angiography.
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Four femoral stents were incompletely visualized with contrast-enhanced MR angiography because of their location at the distal end of the field of view. The depiction of the remaining different stents on contrast-enhanced MR angiograms was heterogeneous. The Wallstent caused a reduction in signal intensity in four of seven cases (Fig 2) and a complete loss of signal intensity in three of seven cases (Fig 3). The Palmaz stent caused a total loss of signal intensity in the area in which it was placed in all five cases (Fig 3). The Memotherm stent caused a loss of signal intensity in four cases (Fig 4) and reduced signal intensity in three cases. The Cragg and Cragg Endopro stents caused discrete reductions in signal intensity along the stents and circumscribed signal intensity voids at the ends of these prostheses (Fig 3).
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Figure 2a. (a) Follow-up anteroposterior digital subtraction angiogram obtained in a 61-year-old man 6 months after placement of a Wallstent (solid straight arrows) that extends from the distal common iliac artery to the external iliac artery. Although the Wallstent is crossing the origin of the left internal iliac artery (open arrow), DSA shows patency of the left internal and external (curved arrow) iliac arteries. (b) Follow- up anteroposterior contrast-enhanced MR angiogram (7.8/2.1, 30° flip angle) obtained in the same patient shows signal intensity voids at the proximal and distal ends of the left common iliac arterial segment treated with a Wallstent. There is complete signal intensity dropout in the center of the Wallstent (open arrow). The origin of the left internal iliac artery (solid straight arrow) is not visible owing to the signal intensity loss caused by the stent placement. Patency of the Wallstent is defined by the high signal intensity of the external iliac artery (curved arrow) without the presence of collateral vessels.
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Figure 2b. (a) Follow-up anteroposterior digital subtraction angiogram obtained in a 61-year-old man 6 months after placement of a Wallstent (solid straight arrows) that extends from the distal common iliac artery to the external iliac artery. Although the Wallstent is crossing the origin of the left internal iliac artery (open arrow), DSA shows patency of the left internal and external (curved arrow) iliac arteries. (b) Follow- up anteroposterior contrast-enhanced MR angiogram (7.8/2.1, 30° flip angle) obtained in the same patient shows signal intensity voids at the proximal and distal ends of the left common iliac arterial segment treated with a Wallstent. There is complete signal intensity dropout in the center of the Wallstent (open arrow). The origin of the left internal iliac artery (solid straight arrow) is not visible owing to the signal intensity loss caused by the stent placement. Patency of the Wallstent is defined by the high signal intensity of the external iliac artery (curved arrow) without the presence of collateral vessels.
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Figure 3a. (a) Follow-up anteroposterior digital subtraction angiograms obtained in a 64-year-old man after placement of a Wallstent (thick arrow) in the left common iliac artery, Palmaz stent (open arrow) in the left superficial femoral artery, and Cragg Endopro stent (curved arrow) in the right superficial femoral artery. The angiograms show patency of the stents in the left iliac artery (thin arrow) and in both superficial femoral arteries (arrowheads). (b) On the follow-up anteroposterior contrast-enhanced MR angiogram (7.8/2.1, 30° flip angle) obtained in the same patient, complete signal intensity loss is seen in the left common iliac artery segment treated with a Wallstent (thick arrow) and in the left superficial femoral artery treated with a Palmaz stent (curved arrow). There are circumscribed signal intensity voids (thin arrows) at the ends of the Cragg Endopro stent in the right superficial femoral artery.
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Figure 3b. (a) Follow-up anteroposterior digital subtraction angiograms obtained in a 64-year-old man after placement of a Wallstent (thick arrow) in the left common iliac artery, Palmaz stent (open arrow) in the left superficial femoral artery, and Cragg Endopro stent (curved arrow) in the right superficial femoral artery. The angiograms show patency of the stents in the left iliac artery (thin arrow) and in both superficial femoral arteries (arrowheads). (b) On the follow-up anteroposterior contrast-enhanced MR angiogram (7.8/2.1, 30° flip angle) obtained in the same patient, complete signal intensity loss is seen in the left common iliac artery segment treated with a Wallstent (thick arrow) and in the left superficial femoral artery treated with a Palmaz stent (curved arrow). There are circumscribed signal intensity voids (thin arrows) at the ends of the Cragg Endopro stent in the right superficial femoral artery.
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Figure 4a. (a, b) Follow-up anteroposterior digital subtraction angiograms obtained in a 69-year-old man 6 months after placement of a Memotherm stent (thin arrows in a) show patency of the left common iliac artery and an elongated right common iliac artery with stenosis greater than 50% (thick arrow in a). There is an aneurysm (open arrow in a) in the distal common femoral artery. The left superficial femoral artery (curved arrow in b) is occluded. (c) Follow-up left anterior oblique (60°)(left) and anteroposterior (right) contrast-enhanced MR angiograms (7.8/2.1, 30° flip angle) obtained in the same patient show complete signal intensity loss in the left common iliac artery (large arrow) due to the placement of the Memotherm stent. The high-signal-intensity vessel distal to the stent makes stent patency evident. There is an aneurysm (open arrow) at the origin of the occluded left superficial femoral artery. High signal intensity can be seen distally in the superficial femoral artery (curved arrow). Elongation of the right common iliac artery with stenosis greater than 50% (small arrow) also is seen.
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Figure 4b. (a, b) Follow-up anteroposterior digital subtraction angiograms obtained in a 69-year-old man 6 months after placement of a Memotherm stent (thin arrows in a) show patency of the left common iliac artery and an elongated right common iliac artery with stenosis greater than 50% (thick arrow in a). There is an aneurysm (open arrow in a) in the distal common femoral artery. The left superficial femoral artery (curved arrow in b) is occluded. (c) Follow-up left anterior oblique (60°)(left) and anteroposterior (right) contrast-enhanced MR angiograms (7.8/2.1, 30° flip angle) obtained in the same patient show complete signal intensity loss in the left common iliac artery (large arrow) due to the placement of the Memotherm stent. The high-signal-intensity vessel distal to the stent makes stent patency evident. There is an aneurysm (open arrow) at the origin of the occluded left superficial femoral artery. High signal intensity can be seen distally in the superficial femoral artery (curved arrow). Elongation of the right common iliac artery with stenosis greater than 50% (small arrow) also is seen.
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Figure 4c. (a, b) Follow-up anteroposterior digital subtraction angiograms obtained in a 69-year-old man 6 months after placement of a Memotherm stent (thin arrows in a) show patency of the left common iliac artery and an elongated right common iliac artery with stenosis greater than 50% (thick arrow in a). There is an aneurysm (open arrow in a) in the distal common femoral artery. The left superficial femoral artery (curved arrow in b) is occluded. (c) Follow-up left anterior oblique (60°)(left) and anteroposterior (right) contrast-enhanced MR angiograms (7.8/2.1, 30° flip angle) obtained in the same patient show complete signal intensity loss in the left common iliac artery (large arrow) due to the placement of the Memotherm stent. The high-signal-intensity vessel distal to the stent makes stent patency evident. There is an aneurysm (open arrow) at the origin of the occluded left superficial femoral artery. High signal intensity can be seen distally in the superficial femoral artery (curved arrow). Elongation of the right common iliac artery with stenosis greater than 50% (small arrow) also is seen.
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Four occlusions that were visible after the placement of Cragg Endopro stents in the femoral arteries were detected with both contrast-enhanced MR angiography and DSA. Furthermore, eight restenoses of greater than 70% were observed at intraarterial DSA. These occurred at the ends of the Cragg Endopro stent in four cases and inside the Cragg stent in two cases, Palmaz stent in one case, and Wallstent in one case. Only one of the four restenoses that occurred at the ends of the Cragg Endopro stent was detected with contrast-enhanced MR angiography (Fig 5).
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Figure 5a. (a, b) Anteroposterior digital subtraction angiograms obtained in a 58-year-old man after placement of two Cragg Endopro stents in the right superficial femoral artery show high-grade stenosis (open arrow in a) at the upper end of the proximal Cragg Endopro stent. The platinum markers at the overlap of the two stents (straight arrow in b) are indicated. There is restenosis (curved arrow) at the lower end of the distal Cragg Endopro stent. (c) Right anterior oblique (30°) contrast-enhanced MR angiogram (7.8/2.1, 30° flip angle) obtained in the same patient shows circumscribed signal intensity loss (open arrow) at the upper end of the proximal Cragg Endopro stent; high-grade restenosis in this area was presumed. The decreased signal intensity at the overlap of the two stents (solid arrow) was caused by the platinum markers. An occlusion (curved arrow) of the left superficial femoral artery also is seen. The lower end of the distal Cragg Endopro stent was not depicted at contrast-enhanced MR angiography.
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Figure 5b. (a, b) Anteroposterior digital subtraction angiograms obtained in a 58-year-old man after placement of two Cragg Endopro stents in the right superficial femoral artery show high-grade stenosis (open arrow in a) at the upper end of the proximal Cragg Endopro stent. The platinum markers at the overlap of the two stents (straight arrow in b) are indicated. There is restenosis (curved arrow) at the lower end of the distal Cragg Endopro stent. (c) Right anterior oblique (30°) contrast-enhanced MR angiogram (7.8/2.1, 30° flip angle) obtained in the same patient shows circumscribed signal intensity loss (open arrow) at the upper end of the proximal Cragg Endopro stent; high-grade restenosis in this area was presumed. The decreased signal intensity at the overlap of the two stents (solid arrow) was caused by the platinum markers. An occlusion (curved arrow) of the left superficial femoral artery also is seen. The lower end of the distal Cragg Endopro stent was not depicted at contrast-enhanced MR angiography.
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Figure 5c. (a, b) Anteroposterior digital subtraction angiograms obtained in a 58-year-old man after placement of two Cragg Endopro stents in the right superficial femoral artery show high-grade stenosis (open arrow in a) at the upper end of the proximal Cragg Endopro stent. The platinum markers at the overlap of the two stents (straight arrow in b) are indicated. There is restenosis (curved arrow) at the lower end of the distal Cragg Endopro stent. (c) Right anterior oblique (30°) contrast-enhanced MR angiogram (7.8/2.1, 30° flip angle) obtained in the same patient shows circumscribed signal intensity loss (open arrow) at the upper end of the proximal Cragg Endopro stent; high-grade restenosis in this area was presumed. The decreased signal intensity at the overlap of the two stents (solid arrow) was caused by the platinum markers. An occlusion (curved arrow) of the left superficial femoral artery also is seen. The lower end of the distal Cragg Endopro stent was not depicted at contrast-enhanced MR angiography.
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DISCUSSION
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Contrast-enhanced MR angiography is a promising technique that may have the potential to replace angiography in the assessment of peripheral arterial occlusive disease (13). Usually, only stenoses greater than 50% require surgical or interventional treatment; however, some 50% stenoses may be hemodynamically relevant, as proved by pressure measurements obtained before and after the administration of vasodilators. In the present study, the maximum intensity projection reconstruction of targeted parts of the vessels allowed an unobstructed view of the arteries in all cases. Veins were visible on eight contrast-enhanced MR angiographic studies, but the diagnostic value of these images was not reduced. No false-positive overestimations of normal arteries as stenosed or of high-grade stenoses as occluded occurred in this study. However, four stenoses of 50% or less were given false-positive evaluations as stenoses greater than 50%. These four overestimated stenoses were interpreted as 40%–45% stenoses on the DSA studies.
Compared with contrast-enhanced MR angiography, the time-of-flight MR angiographic technique that we used previously often had disadvantages in imaging of the iliac arteries; the transversal orientation of vessel branches was hardly visualized owing to inplane saturation effects on the original axial sections. This effect causes normal arteries to be misinterpreted as stenosed or occluded (3). Another cause of problems with time-of-flight MR angiography is turbulent blood flow in the poststenotic region, which causes an overestimation of stenosis. Good agreement between conventional angiography and time-of-flight MR angiography in the detection of 50%–90% stenoses has been reported in the literature (3,14–16).
In previous studies (2,3), use of thin transaxial sections in combination with segmented and triggered acquisition in two-dimensional time-of-flight MR angiography caused a reduction of saturation effects. This technique, however, requires a long acquisition time for the examination of iliac and femoral vessels (2,3). Nevertheless, there are reports that two-dimensional time-of-flight MR angiography may depict stenoses and occlusions in elongated iliac vessels when there is slow blood flow (1,3).
The contrast mechanism of contrast-enhanced MR angiography circumvents all of the obstacles mentioned above. With the use of very short repetition and echo times at moderate (25°–40°) flip angles, the shortening of the T1 relaxation time of blood, from normally about 1,200 msec at 1.5 T to less than 50 msec, results in high signal intensity in the entire vessel lumen (4–13). Because of its independence from inflow effects, contrast-enhanced MR angiography enables the acquisition of data in the coronal plane. This allows a long vessel segment to be depicted with one short sequence (17). The measurement time was less than 2 minutes in our study. Two-dimensional segmented and triggered time-of-flight MR angiography, as well as thick-slab triggered two-dimensional phase-contrast MR angiography, requires 10–30 minutes to cover the same range of anatomy (2,3). The three-dimensional phase-contrast MR angiographic technique is generally no longer used because it involves extremely long acquisition times in imaging the peripheral arteries. Two-dimensional phase-contrast MR angiography has yielded good results in the depiction of peripheral arterial occlusive disease in the coronal plane, but it also has caused overestimations of stenoses in the region of the vessel branches owing to turbulence (2).
The advantage of contrast-enhanced MR angiography compared with two-dimensional triggered phase-contrast MR angiography is its capability to enable one to view multiple reconstructed maximum intensity projection images from the three-dimensional data without venous superimposition in most cases. Two-dimensional phase-contrast MR angiography tends to cause underestimations of stenoses owing to superimposed venous structures (2,18). Another advantage of the described contrast-enhanced MR angiographic technique is the use of a 512 matrix, which provides better resolution and improved suppression of stent artifacts because of the smaller voxels and results in decreased dephasing effects.
Four reocclusions of femoral arteries treated with Cragg Endopro stents were diagnosed owing to a lack of enhancement within the stent and distinct collateralization around the narrowed region. On the other hand, the patency of Palmaz stents was evaluated despite the complete loss of signal intensity in the stent. When there is signal intensity loss in the area of stent placement, the patency of the prosthesis can be determined when there is a high-signal-intensity vessel distal to the stent with no collateral vessels present.
The composition and orientation of the stent influence the imaging artifacts caused by stents at contrast-enhanced MR angiography. Contrast-enhanced MR angiography revealed signal intensity loss in all cases in which stainless steel Palmaz stents were placed. However, in all cases of nitinol Cragg and Cragg Endopro stent placement, there was slightly diminished signal intensity along the stent at contrast-enhanced MR angiography but circumscribed signal intensity voids at the ends owing to the platinum markers. Olree et al (19) observed a variety of susceptibility artifacts with different stent designs. These artifacts were most severe with stainless steel stents and mild with nitinol stents. Similar to Schuermann et al (20), we observed different signal intensity behaviors in the nitinol stents from different manufacturers. This was probably due to the different compositions of the nickel-titanium alloy in these devices.
The signal intensity characteristics of the nitinol Memotherm stent and of the metallic cobalt-chromium-nickel alloy in the Wallstent were nonuniform at contrast-enhanced MR angiography. The different signal intensity behaviors of stents made of the same alloy are due to the orientation of the stents to the main magnetic field. Artifacts increase when a stent is angulated more perpendicularly to the main magnetic field (19). There is higher potential for artifacts when high field strengths and gradient-echo sequences are used (21,22).
A limitation of the current contrast-enhanced MR angiographic technique is the 500-mm field of view for the examination of the lower limb arteries. At present, this technique allows demonstration of only the iliac and femoral region. In four patients, the stent in the femoropopliteal region was not completely depicted because of the height of the patient. The missing demonstration of the runoff vessels is a further disadvantage of contrast-enhanced MR angiography in treatment planning. Information about the runoff vessels is less important for lesions in the iliac arteries, but it is essential for long segmented femoropopliteal lesions. The distal runoff is an indication of the success of a bypass or interventional procedure. When information about the runoff vessels is missing, only conservative treatment or amputation is considered. In the future, this shortcoming can be overcome by carrying out a second contrast-enhanced MR angiographic study in which the popliteal and lower limb arteries are imaged, with a slight overlap in the femoropopliteal area. The advantages of contrast-enhanced MR angiography compared with the older nonenhanced MR angiographic techniques are dramatically reduced susceptibility artifacts and extremely short acquisition times.
In conclusion, contrast-enhanced MR angiography can enable highly accurate diagnosis of occlusion and stenosis greater than 50% in the iliac and femoral arteries; the patency of stents can be determined, but restenoses within stents may not be detected.
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Footnotes
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Abbreviation: DSA = digital subtraction angiography
Author contributions: Guarantor of integrity of entire study, J.L.; study concepts, J.L.; study design, J.L., J.C.S.; definition of intellectual content, J.L., M.H.; literature research, S.H.; clinical studies, J.L., J.C.S., J.B.; data acquisition, J.C.S., J.B.; data analysis, J.L., J.C.S.; statistical analysis, S.H.; manuscript preparation, J.L., J.G.; manuscript editing, J.B.; manuscript review, M.H.
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Abstract
Introduction
