Time-resolved magnetic resonance angiography as a noninvasive method to characterize endoleaks: initial results compared with conventional angiography☆
Article Outline
Abstract
Purpose
Several types of endoleaks have been described, each with different methods of treatment. Conventional arteriography is widely regarded as the gold standard for the classification of endoleaks. Recently, faster magnetic resonance gradients have allowed for rapid data acquisition and review of vascular studies as a real-time continuous angiogram (time resolved magnetic resonance angiography [TR-MRA]). This study was performed to compare the findings of TR-MRA with conventional angiography for the characterization of endoleaks.
Methods
Between June 2002 and June 2003, 12 patients with documented endoleaks following endovascular repair of aortic aneurysms (10 abdominal and two thoracic) underwent TR-MRA to identify and characterize the endoleak. All patients had nitinol-based aortic stent grafts. MRA was performed on a 1.5-Tesla magnet (Sonata class; Siemens Medical Systems, Iselin, NJ). The TR-MRA studies were reviewed under continuous observation as a “cine MR angiogram.” These MRA data sets were used to classify the endoleaks into types 1 through 3. The patients underwent conventional angiography following the MRA to confirm the findings and to plan treatment. The MRA findings were compared with the findings made at conventional arteriography.
Results
TR-MRA identified seven patients with type 1 leaks, including four proximal and three distal. Four patients had type 2 leaks, including two arising from the inferior mesenteric artery and two from an iliolumbar artery. One patient had a type 3 leak. Conventional angiography confirmed the type of endoleak in all 12 patients.
Conclusion: These initial results demonstrate TR-MRA to be an effective noninvasive method for classifying endoleaks. This technique may allow for screening of patients with endoleaks to identify those requiring urgent repair.
Since being first described over a decade ago, the endovascular repair of aortic aneurysms has emerged as an attractive alternative to traditional open repair.1 The current enthusiasm for the endovascular technique has been tempered by the relatively high incidence of complications following the procedure.2 The most common complication reported following endovascular aneurysm repair has been endoleak, or persistent perfusion of the aneurysm sac.3 Four different types of endoleaks have been previously described. Each of these types has distinct implications for the aneurysm repair, with different influence on the growth of the aneurysm sac as well as risk of aneurysm rupture.4, 5, 6 Recently, several authors have reported a preference for urgent treatment of attachment site (type 1) leaks and fabric tear or modular component separation (type 3) leaks and a conservative observation of retrograde side branch (type 2) leaks.7, 8, 9 As a result of this difference in treatment for different types of endoleaks, it has become crucial to identify the type of endoleak occurring in each patient during follow-up.
At present, the overwhelming majority of patients who have undergone endovascular aneurysm repair are followed up with computed tomographic angiography (CTA). Although numerous reports have described the high sensitivity and specificity of CTA in identifying endoleaks, no study has addressed its accuracy in characterizing an endoleak as a specific type.10 At present, conventional digital subtraction angiography is regarded as the gold standard for characterizing endoleaks.11 Magnetic resonance angiography (MRA) has been previously reported as an acceptable imaging modality for the follow-up of patients who have undergone endovascular aneurysm repair, with excellent sensitivity and specificity in the diagnosis of endoleaks when compared with CTA.12, 13
Contrast-enhanced time-resolved MRA (TR-MRA) is a technique first described in 1996 as a method to observe changes in direction of vascular flow over time during a MR imaging examination.14, 15, 16, 17 With the use of faster gradients, TR-MRA can currently be performed with subsecond acquisitions.18 The resulting dynamic MRA, consisting of dozens of acquisitions obtained in a single breath-hold, can be viewed as a continuous loop similar to a conventional flush aortogram. To date, no report has studied the ability of TR-MRA to characterize endoleaks.
The purpose of this study is to describe the initial use of TR-MRA in the characterization of endoleaks and to compare those results with conventional digital subtraction angiography.
Material and methods
The TR-MRA technique, which was developed and evaluated with a protocol approved by the Mount Sinai Medical Center institutional review board, was subsequently incorporated into routine clinical practice.
All patients in this study received treatment as part of a single-institution sponsor-investigator investigational device exemption study for the endovascular repair of aortic aneurysms in patients at high risk for open surgical repair.
The institutional review board approved the protocol for endovascular repair of patients with aortic aneurysms, including procedural technique and follow-up. All patients gave informed consent.
Between June 2002 and June 2003, 12 patients with endoleaks documented on previous CT angiograms following endovascular repair with nitinol based devices (Talent; Medtronic, Minneapolis, Minn), including 10 patients with aortoiliac devices and two patients with thoracic devices, were referred for MRA before conventional angiography in an attempt to characterize the endoleak into types 1 through 4. All 12 patients were scheduled for conventional angiography because of enlarging aneurysm diameter during follow-up.
MRA
All studies were performed with a 1.5-Tesla MR system (Magnetom Sonata; Siemens Medical Systems, Iselin, NJ). Initial-survey MR images of the entire area of interest were acquired in all patients with one-heartbeat non–breath-hold true fast imaging with steady-state precession (FISP) imaging (3.2/1.6 repetition time [millisecond/echo time millisecond]) in the sagittal, coronal, and axial planes. This was performed to localize the stent graft.
Localization was followed by a timing bolus of 3 cc of gadopentate dimeglumine, which was observed at the level of the celiac axis. This was followed by conventional three-dimensional (3D) MRA of the anatomic area of interest. Conventional 3D MRA was performed by injecting 30 mL of gadopentate dimeglumine at a rate of 5 cc/sec. This was immediately followed by the injection of 20 mL of saline at 5 cc/sec. Breath holding and the start of the data acquisition was timed to coincide with the arrival of the contrast-material bolus. The final sequence for the conventional 3D MRA was an axial fat saturation T1 weighted sequence (FLASH).
The conventional 3D MRA was then reviewed to diagnose the extent of the endoleak cavity, including location within the aneurysm sac as well as the involvement of any branch vessels. During the review, a protocol for the TR-MRA was formed to include the entire stent graft and the endoleak cavity, including involved side branches.
TR-MRA
The technique for this imaging has been previously described.19 Our institutional technique is summarized in Table I. 20 The protocol included the injection of 20 cc gadopentate dimeglumine at a rate of 5 cc/sec followed by 20 cc saline at a rate of 5 cc/sec. Patients were instructed to hold their breath, and imaging acquisition in the coronal plane was started simultaneously with the injection of contrast material. Measurements were repeated at intervals of 0.6-2 sec for a total of 40 sec. The TR data set was then reconstructed using 3D online digital subtraction techniques. This 3D data set was used to create a “cine loop,” which was reviewed as a contrast-enhanced 3D MR flush aortogram to characterize the direction of flow in the endoleak channel and aortic side branches if patent.
Table I. Mount Sinai magnetic resonance angiography (MRA) technique for endoleak characterization
| Parameter | TruFisp | Time-resolved MRA | Conventional three-dimensional MRA | FLASH |
|---|---|---|---|---|
| Repetition time (msec) | 3.17 | 2.1 | 2.96 | 188 |
| Echo time (msec) | 1.59 | 0.91 | 1.13 | 2.48 |
| Flip angle (degrees) | 19 | 25 | ||
| Partition thickness (mm) | 16 | 2.0 | ||
| Matrix size | 320 × 207 | 256 × 230 | ||
| Pixel size (mm2) | 1.5 × 1.3 × 16 | 1.4 × 1.3 × 2.0 | ||
| Bandwidth (Hz/pixel) | 1420 | 500 | ||
| Imaging time for each data set (sec) | 0.6–2.0 | 15 |
Digital subtraction angiography
Following MRA examination all patients were referred for conventional digital subtraction angiography to classify the endoleak. The time interval from MRA to conventional angiography ranged from 1 to 58 days, with a mean of 16 and a median of 8 days. All conventional angiography was performed in a dedicated peripheral angiography suite using a Phillips INTEGRIS system. (Phillips Medical Systems, Andover, MA).
Angiography was performed using standard Seldinger technique from a femoral approach. Flush aortography was performed through 5-French calibrated marker catheters to identify the source of the endoleak. The catheter was initially placed at the level of the renal arteries, and aortography was performed in the anteroposterior and lateral projections. All flush injections were performed with ioversol (Mallinckrodt Inc, St. Louis, MO), 320 mg iodine per milliliter at a rate of 15 cc/sec for 2 sec. The catheter was then retracted into the stent graft device, and repeat flush angiography was performed. If no endoleak was seen, a selective superior mesenteric artery injection was performed as well as a selective hypogastric artery injection to exclude a type 2 leak.
In the patients with the thoracic device, a 5-French calibrated marker catheter was placed in the ascending aortic arch, and a flush arch aortogram was performed in left anterior oblique and lateral projections. The catheter was then retracted into the device, and flush aortography was performed of the descending thoracic aorta in anteroposterior and lateral projections.
Image analysis
The TR-MRA data sets were reviewed before conventional angiography by both an interventional radiologist and a vascular surgeon. These results were prospectively recorded and were compared with the results of the conventional angiogram. An independent interventional radiologist and vascular surgeon reviewed the angiogram during the examination to categorize the endoleak into type 1, 2, 3, or 4.
Definitions
The classification of endoleaks for this study was in accordance with the standards of the Ad Hoc Committee for Standardized Reporting Practices in Vascular Surgery of The Society for Vascular Surgery/American Association for Vascular Surgery.21
Results
All patients underwent successful MRA as well as conventional angiography. There were no complications related to either procedure. Of the 10 patients with abdominal endografts, nine were bifurcated devices, and one was an aortouniiliac device. The aortic lumen as well as the aneurysm sac could be visualized in the nine patients with bifurcated devices. The patient with the aortouniiliac device had significant metallic artifact over the caudal aspect of the aneurysm and the distal attachment site as a result of the stainless steel embolization coils used to occlude the contralateral common iliac artery. This artifact did not obscure the visualization of the proximal attachment site and the proximal type 1 leak in this patient. In all 10 patients the endoleak channel could be visualized and characterized with TR-MRA.
In the two patients with the thoracic devices, there was excellent visualization of the aneurysm sac, endograft, and endoleak channel. In all 12 patients, the postprocedure CT angiogram had characterized the endoleak as type 2, thus placing all patients in follow-up for monitoring for aneurysm enlargement. This characterization was incorrect in 8 of the 12 patients in this series. Our series demonstrated 7 type 1 leaks, 4 type 2 leaks, and 1 type 3 leak. There was 100% concordance between the results obtained from the TR-MRA and those obtained from the conventional angiogram (Fig. 1). The results of the endoleak characterization are summarized in Table II.
Fig 1.
A, Multidetector computed tomographic angiogram of a patient with a bifurcated aortic stent graft and a large endoleak seen to the left side of the graft. Shaded surface display and maximum intensity projection demonstrate the endoleak to be in continuity with the inferior mesenteric artery. The leak is in proximity to the proximal attachment site of the device. B, Contrast-enhanced magnetic resonance angiography in anteroposterior and lateral projections demonstrate the endoleak channel. C, Time-resolved magnetic resonance angiography imaged in the coronal plane demonstrates that this patient with a bifurcated abdominal endograft has a proximal type 1 endoleak with outflow via the inferior mesenteric artery. D, Conventional digital subtraction angiogram confirms the diagnosis of the proximal type.
Table II. Results of time-resolved magnetic resonance angiography versus conventional angiography
| Patient | Computed tomographic angiography | Time-resolved magnetic resonance angiography | Conventional angiogram |
|---|---|---|---|
| Abdominal | |||
| 1 | Type 2 | Type 1-Proximal | Type 1-Proximal |
| 2 | Type 2 | Type 1-Distal | Type 1-Distal |
| 3 | Type 2 | Type 2-IMA | Type 2-IMA |
| 4 | Type 2 | Type 1-Proximal | Type 1-Proximal |
| 5 | Type 2 | Type 2-Lumbar | Type 2 Lumbar |
| 6 | Type 2 | Type 1-Proximal | Type 1-Proximal |
| 7 | Type 2 | Type 3 | Type 3 |
| 8 | Type 2 | Type 1-Distal | Type 1-Distal |
| 9 | Type 2 | Type 2-IMA | Type 2-IMA |
| 10 | Type 2 | Type 2-Lumbar | Type 2-Lumbar |
| Thoracic | |||
| 1 | Type 2 | Type 1-Distal | Type 1-Distal |
| 2 | Type 2 | Type 1-Proximal | Type 1-Proximal |
Discussion
This initial feasibility study demonstrates the strength of TR-MRA for the follow-up of patients following endovascular aneurysm repair as compared with CTA. The inherent problem with CTA for follow-up is that the endoleak is characterized on the basis of a static temporal image from the CT angiogram. Some imaging characteristics are specific for certain types of endoleaks, but many are not specific. In particular, enhancement of aortic side branches can be an imaging characteristic of all types of endoleaks, depending on the direction of flow in the vessel. Types 1, 3, and 4 will have antegrade flow in the aortic side branches, whereas type 2 leaks will have retrograde flow in at least one aortic side branch. Many centers are currently using 3D reconstructions of CT datasets from commercial workstations or programs. Although this technology is extremely useful for detecting endoleaks and aneurysm expansion in the follow-up of patients, it is a static temporal data set that cannot visualize direction of flow. This limitation in CT angiography technology resulted in several patients at our institution with type 1 or type 3 leaks that were misdiagnosed as type 2 leaks. Our data demonstrate this limitation, in that all of the patients studied in this series were characterized as having type 2 leaks and were placed into follow-up. Several patients in fact had type 1 or 3 leaks. These patients had aneurysms that were not excluded and were at significant risk for expansion and rupture.
The inherent limitation of CT angiography has led many investigators to examine the role of duplex ultrasound for the follow-up of these patients. Previous reports have demonstrated excellent preliminary results in the ability of ultrasound to identify and characterize endoleaks.22, 23 No study to date has prospectively compared the results of duplex ultrasound with conventional angiography in the characterization of endoleaks.
Magnetic resonance angiography has been shown to be an excellent alternative to CTA for the follow-up of these patients. Several reports have shown MRA to be as sensitive as CTA for the detection of endoleaks.12 One report has shown MRA to be more sensitive than CTA for the detection of type 2 leaks.13 We have found similar results at our institution and have begun using MRA liberally to follow up patients with renal insufficiency.
One limitation of MRA is its somewhat limited applicability to only those patients with nitinol-based supported or unsupported devices. Patients with stainless steel devices are unable to be followed with MRA because of the significant metallic artifact caused by the stainless steel components. Similarly, patients who have undergone embolization procedures with stainless steel coils are unable to be followed with MRA because of the artifact from the coils.24 This artifact was present for the visualization of the iliac arteries for one of the patients in this study. Platinum embolization coils, in contrast, are nearly invisible for MRI and create no artifact. At present, at our institution we are only placing nitinol-based supported devices and have switched to using platinum coils for all embolization procedures. This allows any patient treated at our institution to undergo MRA examination if necessary.
TR-MRA has been described previously as an effective imaging technique to assess congenital cardiac anomalies, pulmonary perfusion abnormalities including congenital arteriovenous fistulae, and the circulation to the peripheral vasculature.13, 14, 15 To our knowledge, this study is the first description of this MRA technique in the characterization of endoleaks and the first study to compare this technique to conventional angiography.
Previously, at our institution patients had a flush aortogram performed in the operating suite at the completion of the stent-graft implantation. If no type 1 or type 3 endoleak was seen, the procedure was completed. The patient was then scheduled for follow-up CT angiography within the first month and then at 6 months, 12 months, and annually. Any endoleak detected at follow-up was characterized by the CT angiogram. All type 1 and type 3 endoleaks were scheduled for conventional angiography. Any patient with a type 2 leak was observed unless the aneurysm sac was shown to have expanded by 5 mm or more in diameter.
The results of this study brought about a change in the way that our endovascular aneurysm repair patients are followed (Fig. 2). At present, the intraoperative evaluation is unchanged. The patient is then referred for CTA during the first month following the procedure. If no endoleak is present, the patient returns at 6 months and then at 12 months to monitor the aneurysm diameter. If an endoleak is present, the patient returns at 3 months to see whether the endoleak is still present. If it is still present, the CT is reviewed by an interventional radiologist as well as a vascular surgeon to attempt to characterize the endoleak. Type 1 and type 3 leaks are referred for conventional angiography to plan treatment. Type 2 leaks are monitored with CTA every 3 months to monitor for aneurysm expansion. Any patient with an endoleak that cannot be uniformly classified on the basis of CTA is sent for TR-MRA examination to characterize the endoleak and treat the patient accordingly. Because both the interventional radiology department and the vascular surgery department believe in the usefulness of this diagnostic modality, there exists a low threshold to refer a patient with an endoleak for a TR-MRA examination. This allows our patients to avoid an invasive angiogram simply to diagnose the more common type 2 endoleak that is currently treated conservatively at our institution.
Fig 2.
Algorithm for diagnosing and treating endoleaks.
As endovascular technology is used more and more frequently to treat patients with aneurysms, there will be an increasing number of patients who have endoleak as a complication. This initial study demonstrates that TR-MRA can visualize the direction of flow in an endoleak cavity. TR-MRA allows for a noninvasive method to characterize the endoleak and prescribe the appropriate treatment. Further studies will be required to further define the exact role of TR-MRA for all patients with aortic endografts.
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☆ Competition of interest: none.
PII: S0741-5214(03)01411-3
doi:10.1016/j.jvs.2003.09.035
© 2004 The Society for Vascular Surgery and The American Association for Vascular Surgery. Published by Elsevier Inc. All rights reserved.
