MR angiographic findings in patients with aortic endoprostheses

Imaging Challenges in Chronic Dissection

Chronic aortic dissection comprises a heterogeneous group of unrepaired and repaired disease requiring lifelong clinical and imaging surveillance. CT and MRI are the main imaging modalities for longitudinal surveillance, with growing interest in emerging imaging techniques for prognostic potential. Imaging difficulties span technical and diagnostic challenges, some of which are unique to the repaired aorta, with specific complications depending on the type of repair. This review describes existing and emerging imaging techniques, outlines the technical and diagnostic challenges encountered at CT and MRI, and highlights the diagnostic pitfalls of chronic aortic dissection.

Thoracic Endovascular Aortic Repair for Chronic Type B Aortic Dissection: Pre- and Postprocedural Imaging

Aortic dissection is a chronic disease that requires lifelong clinical and imaging surveillance, long after the acute event. Imaging has an important role in prognosis, timing of repair, device sizing, and monitoring for complications, especially in the endovascular therapy era. Important anatomic features at preprocedural imaging include the location of the primary intimal tear and aortic zonal and branch vessel involvement, which influence the treatment strategy. Challenges of repair in the chronic phase include a small true lumen in conjunction with a stiff intimal flap, complex anatomy, and retrograde perfusion from distal reentry tears. The role of thoracic endovascular aortic repair (TEVAR) remains controversial for treatment of chronic aortic dissection. Standard TEVAR is aimed at excluding the primary intimal tear to decrease false lumen perfusion, induce false lumen thrombosis, promote aortic remodeling, and prevent aortic growth. In addition to covering the primary intimal tear with an endograft, several adjunctive techniques have been developed to mitigate retrograde false lumen perfusion. These techniques are broadly categorized into false lumen obliteration and landing zone optimization strategies, such as the provisional extension to induce complete attachment (PETTICOAT), false lumen embolization, cheese-wire fenestration, and knickerbocker techniques. Familiarity with these techniques is important to recognize expected changes and complications at postintervention imaging. The authors detail imaging options, provide examples of simple and complex endovascular repairs of aortic dissections, and highlight complications that can be associated with various techniques. Online supplemental material is available for this article. ^©RSNA, 2022.

Surveillance Imaging following Endovascular Aneurysm Repair: State of the Art

Endovascular aneurysmal repair (EVAR) has become a prominent modality for the treatment of abdominal aortic aneurysm. Surveillance imaging is important for the detection of device-related complications, which include endoleak, structural abnormalities, and infection. Currently used modalities include ultrasound, X-ray, computed tomography, magnetic resonance imaging, and angiography. Understanding the advantages and drawbacks of each modality, as well available guidelines, can guide selection of the appropriate technique for individual patients. We review complications following EVAR and advances in surveillance imaging modalities.

MRI assessment of thoracic stent grafts after emergency implantation in multi trauma patients: a feasibility study

PURPOSE

To evaluate the feasibility of MRI for static and dynamic assessment of the deployment of thoracic aortic stent grafts after emergency implantation in trauma patients.

METHODS

Twenty patients initially presenting with a rupture of the thoracic aorta were enrolled in this study. All patients underwent thoracic endovascular aortic repair (TEVAR). The deployment of the implanted stent graft was assessed by CTA and MRI, comprising the assessment of the aortic arch with and without contrast agent, and the assessment of the motion of the stent graft over the cardiac cycle.

RESULTS

The stent graft geometry and motion over the cardiac cycle were assessable by MRI in all patients. Flow-mediated signal variations in areas of flow acceleration could be well visualised. No statistically significant differences in stent-graft diameters were observed between CT and MRI measurements.

CONCLUSION

MRI appears to be a valuable tool for the assessment of thoracic stent grafts. It shows similar performance in the accurate assessment of stent-graft dimensions to the current gold standard CTA. Its capability of providing additional functional information and the lack of ionising radiation and nephrotoxic contrast agents may make MRI a valuable tool for monitoring patients after TEVAR.

Imaging Techniques for Detection and Management of Endoleaks after Endovascular Aortic Aneurysm Repair1

Endovascular aortic aneurysm repair (EVAR) is evolving into a viable alternative to open surgical repair for many patients with abdominal and thoracic aortic aneurysms. Endoleak development is a complication of EVAR and represents one of the limitations of this procedure. Endoleaks represent blood flow outside the stent-graft lumen but within the aneurysm sac. Lifelong imaging surveillance of patients after EVAR is critical to detect endoleaks for the patient's benefit and to determine the long-term performance of the stent-graft. Although computed tomographic angiography is the most commonly used examination for imaging surveillance, magnetic resonance angiography, ultrasonography, and digital subtraction angiography all have a role in endoleak detection and management. This review will focus on imaging techniques used for endoleak detection and the role imaging surveillance plays in the overall care of the post-EVAR patient.

MRI virtual biopsies: analysis of an explanted endovascular device and perspectives for the future

Information that can be obtained by magnetic resonance imaging (MRI) of explanted endovascular devices must be validated as this method is non-destructive. Histology of such a device together with its encroached tissues can be elegantly performed after polymethymethacrylate (PMMA) embedding, but this approach requires destruction of the specimen. The issue is therefore to determine if the MRI is sufficient to fully validate an explanted device based upon the characterization of an explanted specimen. An AneuRx device deployed percutaneously 25 months earlier in a 75-year-old patient was removed en bloc at autopsy together with the surrounding aneurysmal sac and segments of the upstream and downstream arteries. Macroscopic pictures were taken and a slice of the cross-section was processed for histology after polymethylmethacrylate (PMMA) embedding. For the magnetic resonance imaging investigation, the device was inserted in a Biospec 4.7 T MRI system with a 20 mm diameter birdcage resonator used for both emission and reception. A Spin-Echo (SE) was used to acquire both T1 proton density (PD) and T2 weighted images. A gradient-echo (GE) sampling of a free induction decay (GESFID) was used to generate multiple GE images using a single excitation pulse so that four images at different TE were obtained in the same acquisition. The selected explanted device was outstandingly well-healed compared to most devices harvested from humans. No inflammatory process was observed in contact or at distance of the materials. In MRI T1 images display no specific contrast and were homogeneous in the different tissues. The contrast was improved on proton density weighed images. On the T2 weighed images, the different areas were well identified. The diffusion images displayed in the surrounding B region had the greatest diffusion coefficient and the greatest anisotropy. The MRI analysis of the explanted AneuRx device illustrates the possibilities of this technique to characterize the interaction of the endovascular graft with the surrounding tissues. MRI is a breakthrough to investigate explanted medical devices but it also can be advantageously used in vivo to obtain virtual biopsies, because real biopsies to determine the 3 Bs (biocompatibility, biofunctionality and bioresilience) cannot be carried out as they could obviously initiate infection and degradation of the foreign materials.

Endovascular Repair of Thoracic and Abdominal Aortic Aneurysms: Pre- and Postprocedural Imaging

Endovascular repair of thoracic and abdominal aortic aneurysms is a safe alternative to conventional open surgical repair. Clinical success, however, is highly dependent on patient selection. Diagnostic vascular imaging has an essential role for this selection process. Following endovascular aneurysm repair (EVAR), patients require long-term surveillance and again vascular imaging serves an integral function. This article reviews EVAR selection criteria and post-EVAR assessment and then discusses the imaging modalities used to evaluate these patients, namely multi-detector-row computed tomographic angiography, magnetic resonance imaging/angiography, duplex ultrasonography, and catheter angiography.

Surveillance for endoleaks: How to detect all of them

Endovascular aneurysm repair has proven to be a valuable alternative to open repair in selected patients. This less invasive procedure, however, requires long-term surveillance for its own set of potential complications, including perigraft leakage, or endoleak. This article focuses on the detection of these leaks, first defining and classifying endoleaks and then describing various means of detecting them, including computed tomographic angiography, magnetic resonance angiography, color-flow duplex ultrasonography, and conventional angiography.

MRA is useful as a follow-up technique after endovascular repair of aortic aneurysms with nitinol endoprostheses

PURPOSE

To evaluate whether MR angiography (MRA) is a useful tool for the follow-up of aortic aneurysms treated with nitinol endoluminal grafts.

MATERIALS AND METHODS

We examined 28 patients treated with nitinol endovascular stents to repair an aortic aneurysm with CT angiography (CTA) and MRA. Eleven patients (group 1) underwent an MRA after a positive CTA for endoleak was observed. Afterwards, 17 patients (group 2) were scheduled for both follow-up examinations. The kind of endoleak that occurred and the maximum aortic diameter were compared. The sensitivity of CTA relative to MRA for detecting endoleaks in group 2 was calculated. Signal-to-noise ratios (SNRs) were measured in the aortoiliac lumen at the arterial phase in, above, and below the endoprostheses. Student's t-test was used to compare aneurysm dimensions and SNR measurements.

RESULTS

Three type III leaks were correctly assessed at both examinations; however, CTA was less sensitive (50%) than MRA in depicting type II or unclassified leaks. No differences in aneurismal size were observed between the two examinations or between arterial SNRs observed in or out of the devices.

CONCLUSION

MRA can provide all relevant information necessary for the follow-up of patients treated with nitinol endoprostheses, and performs better than CTA in detecting endoleaks.

Suitability of 7 Aortic Stent-Graft Models for MRI-Based Surveillance

PURPOSE

To evaluate the magnetic resonance imaging (MRI) characteristics of commercially available stent-grafts used for abdominal aortic aneurysm repair.

METHODS

Seven endovascular grafts (AneuRx, Lifepath, Talent, Excluder, Zenith, Quantum LP, and Ancure) were suspended in a water bath containing gadolinium and scanned using a 1.5-T clinical MRI scanner. Two different scan techniques (T(1)-weighted spoiled gradient echo and spin echo) based upon a clinical MRI endograft surveillance protocol were used for each stent-graft. The scans were evaluated for susceptibility artifacts and radiofrequency (RF) shielding and caging artifacts.

RESULTS

For most endografts, the lumen and structures surrounding the endograft were well visualized. However, the ferromagnetic properties of the Zenith and Lifepath devices resulted in large susceptibly artifacts that obliterated the endograft lumen as well as adjacent structures. All fully supported grafts showed some amount of signal loss from the graft lumen caused by RF caging. For the Ancure graft, evaluation around the attachment sites might be problematic.

CONCLUSIONS

An MRI-based surveillance protocol appears to be a viable option for the AneuRx, Talent, Excluder, and Quantum LP stent-grafts.

3.0 Tesla Magnetic Resonance Angiography of Endovascular Aortic Stent Grafts

RATIONALE AND OBJECTIVES

This study evaluated different stent grafts by 3 T magnetic resonance angiography (MRA) with respect to lumen visibility, susceptibility-induced signal loss, and type of stent artifacts compared with 1.5 T MRA in a phantom model.

METHODS

Six different stent-grafts (tube: n = 3, bifurcated: n = 3) were evaluated by 3 T and 1.5 T MRA using a tube phantom. MRA was performed using T1-weighted sequences at both systems with comparable parameters (3T: TR 5.4/TE 2.0/FA 30 degrees, 1.5 T: TR 6.2/TE 2.2/FA 30 degrees). A blind study of the image quality, including artifacts, was performed by 3 radiologists. Furthermore, signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) values were calculated. Statistical analysis was performed with Student's t test (P < 0.05).

RESULTS

One Elgiloy stent graft showed almost a complete intraluminal signal loss at 1.5 and 3 T. All other models could be evaluated by both systems by MRA, resulting in a favorable lumen visibility (score: 1) for prostheses made of nitinol. Scores for overall image quality and artifacts were the same for both MR systems. SNR and CNR values of the stented part of the vessel phantom increased from 320 +/- 33 to 618 +/- 40 and from 306 +/- 34 to 596 +/- 40 at 3 T when compared with 1.5 T, resulting in a significant signal gain of 93% at the higher field strength.

CONCLUSIONS

3 Tesla MRA of aortic stent grafts in a phantom model demonstrates an increase in SNR and CNR when compared with 1.5 T. However, the magnitude of imaging artifacts as well as coherent intraluminal signal loss within the stent does not increase equally in both MR systems.

Magnetic resonance-guided placement of aortic stents grafts: feasibility with real-time magnetic resonance fluoroscopy

PURPOSE

The artifact behavior of different aortic stent-grafts was evaluated with real-time magnetic resonance (MR) imaging, and the feasibility of real-time MR-guided stent-graft placement in the abdominal aorta was tested.

MATERIALS AND METHODS

Seven different stent-grafts and their deployment systems were analyzed in an in vitro setting regarding their artifacts on different real-time MR images with cartesian, spiral, and radial k-space filling. The device and the real-time sequence with the fewest artifacts were used for an in vivo study in a swine model. In four animals, a straight aortic stent-graft was placed in the infrarenal abdominal aorta under real-time MR guidance. Results were controlled with MR angiography, multislice spiral computed tomography, and digital subtraction angiography.

RESULTS

The in vitro study proved only one of the seven stent grafts (including deployment systems) to be suitable for real-time MR-guided intervention. MR-guided placement of the stent-grafts was possible in all animals. In one animal, the stent-graft dislocated during withdrawal of the deployment system as a result of a mismatch between stent-graft diameter and size of the infrarenal aorta.

CONCLUSION

Real-time MR-guided placement of stent-grafts in the abdominal aorta with use of commercially available standard instruments is feasible. However, for clinical use, dedicated devices and deployment systems producing less artifacts are required.

Midterm outcomes of thoracic aortic stent-grafts: complications and imaging techniques

PURPOSE

To evaluate the midterm outcomes of thoracic aortic stent-grafting and the performance of computed tomographic angiography (CTA), radiography, and magnetic resonance angiography (MRA) in endograft surveillance.

METHODS

Forty-seven patients with traumatic thoracic aortic ruptures (n=16), aneurysms (n=14), false aneurysms (n=3), penetrating ulcers (n=3), and dissections (n=11) treated with stent-grafts were monitored in follow-up using chest radiography and CTA in all patients and MRA in 23 patients. Two perpendicular maximal aortic diameters, the sum of these diameters, and the elliptical cross-sectional area were determined and compared to baseline for the entire group and in subgroup analyses according to lesion type. CTA, MRA, and radiography were compared for their ability to detect endoleak, monitor stent-graft configuration, and measure aortic diameters.

RESULTS

The mortality rate was 8.5%. Severe complications were observed in 14.8% (6% neurological complications); 12 (25.5%) patients had primary endoleaks. Over a mean 11-month follow-up (range 0.25-46 months), the aortic diameters decreased for all patients without endoleak (p<0.001). In the diameter/area subgroup analyses, only the traumatic rupture cohort demonstrated significant decreases in all 4 measurements. CTA and MRA measurements correlated well, but chest radiography was superior to both for visualizing stent-graft shape. In terms of endoleak detection, MRA missed only 1 (12.5%) endoleak (type II) seen on CTA; there were no false positive results with MRA.

CONCLUSIONS

Morbidity and mortality observed after thoracic stent-grafting are acceptable. Radiography is better for monitoring stent-graft conformation, while CTA provides the best overall morphological information. The performance of MRA in endoleak detection is encouraging.