Postoperative Surveillance

MRA has great usefulness in the post procedure monitoring of patients who have undergone en-dovascular repair of abdominal aortic aneurysm

Eat Stop Eat
Fig. 3a, b. Normal post-operative MRA in two patients with different endovascu-lar stent grafts for AAA repair
Stent Graft Endoleak Postoperativ

Fig. 4a, b. Post-operative CTA (a) and MRA (b) of same patient shows an endoleak that is only seen on MRA (arrow)

Aaa Stent Graft Cta

Fig. 4a, b. Post-operative CTA (a) and MRA (b) of same patient shows an endoleak that is only seen on MRA (arrow)

(Fig. 3). The success of the treatment is dependant on continued exclusion of blood flow into the aneurysm sac. Persistent flow into the sac or endoleak is a sign of treatment failure. Because MRA relies on general arterial opacification it is quite sensitive at detecting perigraft flow.

MRA 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 [4]. One report has shown MRA to be more sensitive than CTA for the detection of type 2 leaks [5]. We have found similar results at our institution and have begun using MRA liberally to follow up patients with renal insufficiency (Fig. 4).

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. 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 em-bolization procedures. This allows any patient treated at our institution to undergo MRA examination if necessary.

Time resolved MRA (TR-MRA) has been described previously as an effective imaging technique to assess congenital cardiac anomalies, pulmonary perfusion abnormalities including con

Arc Riolan
Fig. 5a-f. Time resolved MRA dataset shows an endoleak (arrowin c) fed from the arc of Riolan (arrowheadin c) and the inferior mesen-teric artery

genital arteriovenous fistulae, and the circulation to the peripheral vasculature. Our institution recently published the first experience with the use of TR-MRA for the characterization of endoleaks. This study was 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. At present, the intra-

operative 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 en-doleak and treat the patient accordingly. This allows our patients to avoid an invasive angiogram simply to diagnose the more common type 2 en-doleak that is currently treated conservatively at our institution (Fig. 5) [6].

The current classification for endoleaks has been described by the American Association for Vascular Surgery (AAVS) and is as follows:

• a type 1 endoleak is relatively uncommon and is defined as the presence of an incomplete seal at the proximal or distal attachment site. A type 1 endoleak can also occur due to an incomplete seal at the site of an iliac occluder device.

• a type 2 endoleak is the most common type of leak and is defined as retrograde filling of the aneurysm sac from a patent branch vessel. The most common sources of a type 2 endoleak are the inferior mesenteric artery and the iliolum-bar artery or a lumbar artery arising off the sac.

• a type 3 endoleak is rare and is due to a tear in the fabric of the device from shear stress or from a fracture of one of the metallic struts of the framework.

• a type 4 endoleak is also very rare and is due to flow through the fabric of the graft due to increased porosity. This is most common in Dacron grafts and usually is a transient phenomenon that occurs immediately after device implantation and resolves after a few days [7].

3D reconstruction of MRA data sets is extremely helpful for the postoperative evaluation of these patients. The 3D image processing allows excellent visualization of aneurysm and endograft deformation. Endograft deformation can lead to catastrophic consequences including endoleak, which are at great risk for aneurysm rupture. 3D reconstruction can also aid in the characterization of endoleak by delineating the inflow or outflow sources.

As mentioned above, aneurysm volume measurements can be an effective method of following up these patients. Recent articles have demonstrated that patients that do not have a decreasing aneurysm volume may be at risk for endoleak formation. Similarly decreasing aneurysm volume correlates with successful treatment [8].

Typical recommended schedules of follow up studies include 1 month post procedure, then 6 months post procedure, and then yearly after the procedure. In patients in whom an endoleak is identified, the interval decreases to 3 months in order to monitor the stability of the leak and possibly plan future intervention.

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