Contrast-Enhanced MR Angiography at 1.5 T After Implantation of Platinum Stents: In Vitro and In Vivo Comparison with Conventional Stent Designs

Cardiac Magnetic Resonance Imaging to Determine Single Ventricle Function in a Pediatric Population is Feasible in a Large Trial Setting: Experience from the Single Ventricle Reconstruction Trial Longitudinal Follow up

The Single Ventricle Reconstruction (SVR) Trial was a randomized prospective trial designed to determine survival advantage of the modified Blalock-Taussig-Thomas shunt (BTTS) vs the right ventricle to pulmonary artery conduit (RVPAS) for patients with hypoplastic left heart syndrome. The primary aim of the long-term follow-up (SVRIII) was to determine the impact of shunt type on RV function. In this work, we describe the use of CMR in a large cohort follow up from the SVR Trial as a focused study of single ventricle function. The SVRIII protocol included short axis steady-state free precession imaging to assess single ventricle systolic function and flow quantification. There were 313 eligible SVRIII participants and 237 enrolled, ages ranging from 10 to 12.5 years. 177/237 (75%) participants underwent CMR. The most common reasons for not undergoing CMR exam were requirement for anesthesia (n = 14) or ICD/pacemaker (n = 11). A total of 168/177 (94%) CMR studies were diagnostic for RVEF. Median exam time was 54 [IQR 40-74] minutes, cine function exam time 20 [IQR 14-27] minutes, and flow quantification time 18 [IQR 12-25] minutes. There were 69/177 (39%) studies noted to have intra-thoracic artifacts, most common being susceptibility artifact from intra-thoracic metal. Not all artifacts resulted in non-diagnostic exams. These data describe the use and limitations of CMR for the assessment of cardiac function in a prospective trial setting in a grade-school-aged pediatric population with congenital heart disease. Many of the limitations are expected to decrease with the continued advancement of CMR technology.

The implications of magnetic resonance angiography artifacts caused by different types of intracranial flow diverters

BACKGROUND

Serial cerebral angiographic imaging is necessary to ensure cerebral aneurysm occlusion after flow diverter placement. Time-of-flight (TOF)-magnetic resonance angiography (MRA) is used for this purpose due to its lack of radiation, contrast media and complications. The comparative diagnostic yield of TOF-MRA for different flow diverters has not been previously analyzed.

PURPOSE

To evaluate the diagnostic accuracy of TOF-MRA in cerebral aneurysms treated w divertersith different flow diverters.

MATERIALS AND METHODS

Flow-diverted patients whose cerebral follow-up MRA and digital subtraction angiograms (DSA) were obtained within 6 weeks were retrospectively identified. The DSA (as gold standard) and MRA images of these patients were compared by two readers (blinded to both patient data and endovascular procedure data) for residual aneurysms and the status of the parent artery for each type of flow diverter. In a second group of patients, magnetic susceptibility artifacts were manually measured and compared for different FDs.

RESULTS

Seventy-six patients (85 aneurysms) were included in group one, and 86 patients (95 aneurysms) were included in group 2. TOF-MRA and DSA showed almost perfect agreement for residual aneurysms (κ = 0.88, p < 0.001) (positive predictive value (PPV) = 1.00, specificity = 1.00, negative predictive value (NPV) = 0.89, sensitivity = 0.89). Intermodality agreement (κ = 0.97 vs. κ = 0.74, p < 0.005) and sensitivity (0.97 vs. 0.77, NPV: 0.96 vs. 0.77) were highest with nitinol stents. MRA and DSA showed no agreement for occluded or stenotic parent vessels (κ = 0.13, p = 0.015, specificity = 0.44, NPV = 1.00, sensitivity = 1.00). Specificity was lower in chromium-cobalt based FDs than in nitinol devices (specificity = 0.08 vs. 0.60). Chromium-cobalt stents generated the largest artifacts (p < 0.005). The size of the device-related artifact, in millimeters, increased in respective order, for the Silk, Derivo, Pipeline and Surpass devices.

CONCLUSION

Unlike DSA, TOF-MRA is susceptible to dissimilarities between flow diverters. MRA is not well-suited for research studies comparing different flow diverters. Nitinol FDs appear to be advantageous for TOF-MRA follow-up so as not to miss small aneurysm remnants or clinically relevant parent artery stenosis.

MR Imaging of Thoracic Veins

MR imaging of thoracic veins is performed to evaluate the heart and thoracic vasculature. The protocol can be customized to the clinical question. In the embryo, systemic and pulmonary vein development is closely related to heart development. Congenital anomalies of the thoracic veins are strongly associated with other cardiac and situs abnormalities. Acquired venous abnormalities are often iatrogenic, or secondary to malignancy. This article discusses development and anatomy of the thoracic venous systems, clinical MR imaging methods for their evaluation, and illustrates the MR imaging appearance of congenital and acquired abnormalities of systemic thoracic veins, coronary sinus, and pulmonary veins.

Neurosonology and noninvasive imaging of the carotid arteries

In this chapter, we review imaging of the extracranial carotid arteries and the indications for noninvasive carotid artery evaluation, measuring the degree of arterial stenosis and plaque morphology. We also analyze the types of noninvasive imaging, including carotid duplex ultrasound, transcranial Doppler, magnetic resonance angiography, and computer tomography angiography. We look at each of these modalities, briefly discussing techniques, benefits, limitations, and sources of error. Furthermore, we discuss the apparent accuracy and the need for multimodality imaging. Finally, an imaging algorithm for the evaluation of the extracranial carotid arteries is proposed, which is in routine use at our hospital.

Non-enhanced magnetic resonance angiography can evaluate restenosis after carotid artery stenting with the Carotid Wallstent

BACKGROUND

Carotid artery stenting (CAS) requires follow-up imaging to assess in-stent restenosis (ISR). This study aimed to determine whether non-enhanced magnetic resonance angiography (NE-MRA) is useful for evaluating ISR.

METHOD

Between 2009 and 2013, we performed 118 consecutive CAS procedures using the Precise stent (n = 78) and the Carotid Wallstent (n = 40). We reviewed 1.5 T NE-MRA and examined visualization of the stent lumen and the degree of ISR if present. Other imaging modalities were used as references.

RESULTS

NE-MRA performed just after CAS was not able to visualize the stent lumen in all patients because of metal artifacts. In the Carotid Wallstent group, follow-up NE-MRA was available in 22 patients. The stent lumen was visible more than three months after CAS in all patients. Among them, >40 % ISR was observed by other modalities in eight lesions. The degree of restenosis measured by NE-MRA (y%) had a linear relationship with that measured by conventional angiography (x%) (y = 0.97x-0.4, r = 0.79, P = 0.021). In one case among 17 without ISR (6 %), NE-MRA showed false ISR. In the Precise stent group, NE-MRA did not visualize the stent lumen in the follow-up period.

CONCLUSIONS

NE-MRA can visualize the stent lumen in the Carotid Wallstent more than three months after CAS, but not in the Precise stent at follow-up. This delayed visualization might depend on endothelialization of the stent lumen. The degree of ISR measured by NE-MRA is comparable to that by conventional angiography. NE-MRA can evaluate ISR after CAS with the Carotid Wallstent (100 % sensitivity and 94 % specificity).

Pediatric cardiovascular interventional devices: effect on CMR images at 1.5 and 3 Tesla

BACKGROUND

To predict the type and extent of CMR artifacts caused by commonly used pediatric trans-catheter devices at 1.5 T and 3 T as an aid to clinical planning and patient screening.

METHODS

Eleven commonly used interventional, catheter-based devices including stents, septal occluders, vascular plugs and embolization coils made from either stainless steel or nitinol were evaluated ex-vivo at both 1.5T and 3T. Pulse sequences and protocols commonly used for cardiovascular magnetic resonance (CMR) were evaluated, including 3D high-resolution MR angiography (MRA), time-resolved MRA, 2D balanced-SSFP cine and 2D phase-contrast gradient echo imaging (GRE). We defined the signal void amplification factor (F) as the ratio of signal void dimension to true device dimension. F1 and F2 were measured in the long axis and short axes respectively of the device. We defined F3 as the maximum extent of the off-resonance dark band artifact on SSFP measured in the B0direction. The effects of field strength, sequence type, orientation, flip angle and phase encode direction were tested. Clinical CMR images in 3 patients with various indwelling devices were reviewed for correlation with the in-vitro findings.

RESULTS

F1 and F2 were higher (p<0.05) at 3T than at 1.5T for all sequences except 3D-MRA. Stainless steel devices produced greater off-resonance artifact on SSFP compared to nitinol devices (p<0.05). Artifacts were most severe with the stainless steel Flipper detachable embolization coil (Cook Medical, Bloomington, IN), with F1 and F2 10 times greater than with stainless steel stents. The orientation of stents changed the size of off-resonance artifacts by up to two fold. Sequence type did influence the size of signal void or off-resonance artifact (p<0.05). Varying the flip angle and phase encode direction did not affect image artifact.

CONCLUSION

Stainless steel embolization coils render large zones of anatomy uninterpretable, consistent with predictions based on ex-vivo testing. Most other commonly used devices produce only mild artifact ex-vivo and are compatible with diagnostic quality in-vivo studies. Knowledge of ex-vivo device behavior can help predict the technical success or failure of CMR scans and may preempt the performance of costly, futile studies.

Artificial luminal narrowing on contrast-enhanced magnetic resonance angiograms on an occasion of stent-assisted coiling of intracranial aneurysm: in vitro comparison using two different stents with variable imaging parameters

Objective

Intracranial stenting for stent-assisted coiling of aneurysms requires adequate follow-up imaging. The aim of this in vitro study was to compare in-stent artificial luminal narrowing on contrast-enhanced MR angiograms (CE-MRA) when applying Neuroform® and Enterprise® stents for stent-assisted coiling.

Materials and Methods

Two intracranial nitinol stents (Enterprise® and Neuroform®) were placed in silicon tubes and then imaged at 3 T and 1.5 T by the use of a T1-weighted three-dimensional spoiled gradient-echo sequence with minimal TR and TE. CE-MRAs were obtained by using different imaging planes, voxel sizes, and bandwidths, and with or without parallel imaging. Artificial lumen narrowing (ALN) was calculated and the results were compared.

Results

Lower magnetic field strength, axial plane perpendicular to axis of stent, and wider bandwidth resulted in a lower ALN on CE-MRA for both stents. Larger voxel size resulted in lower ALN for Neuroform® stent. The parallel imaging acceleration factor did not affect ALN. The mean ALN was lower for Neuroform®, but it was not significant by a paired t test.

Conclusion

CE-MRA of the stented lumen of vascular phantom was partially impaired with ALN. Consequently, image plane orientation, magnetic field strength, bandwidth, and voxel size should be adjusted appropriately to reduce ALN.

MRI may be sufficient for noninvasive assessment of great vessel stents: an in vitro comparison of MRI, CT, and conventional angiography

OBJECTIVE

The purpose of this study was to compare the diagnostic performance of MRI and CT assessment of great vessel stents in an in vitro model.

MATERIALS AND METHODS

Three contemporary great vessel stent materials (nitinol, platinum-iridium, and stainless steel) were assessed with three luminal conditions: no stenosis, internal stenosis, and external stenosis. Stents of the same material were implanted into an aorta model that was attached to an animal bypass pump with pulsatile flow. Each stent was imaged with conventional angiography as reference standard, 10 different MRI sequences, and CT. The sensitivity and specificity for the identification of stent stenosis was determined and stent lumen measurements compared.

RESULTS

Of the investigated MRI sequences, three had the highest overall sensitivity and specificity for the identification of stent stenosis in all studied materials: through-plane gradientrecalled echo (GRE) with 75° flip angle (100% and 95%, respectively), in- and through-plane steady-state free precession (SSFP) (99% and 90%) and MR angiography (MRA) with 75° flip angle (93% and 85%). Comparable sensitivity and specificity were achieved with CT (98% and 93%). GRE, SSFP, and MRA sequences tended to underestimate stent lumen diameter in externally nonstenosed stents and overestimate diameter in internally stenosed stents (p < 0.05). CT slightly underestimated external stenoses in all stent types (p < 0.05).

CONCLUSION

Defined MRI sequences are feasible to assess nitinol, platinum-iridium, and stainless steel great vessel stents with diagnostic performance comparable with CT.

Comparing different MR angiography strategies of carotid stents in a vascular flow model: toward stent-specific recommendations in MR follow-up

INTRODUCTION

Carotid artery stenting (CAS) requires adequate follow-up imaging to assess complications such as in-stent stenosis or occlusion. Options include digital subtraction angiography, CT angiography, ultrasound, and MR angiography (MRA), which may offer a non-invasive option for CAS follow-up imaging. The aim of this study was to assess contrast-enhanced MRA (CE-MRA) and three-dimensional time-of-flight MRA (3D-TOF) for visualization of the in-stent lumen in different carotid stents.

METHODS

In this study, we compared CE-MRA and 3D-TOF of five different carotid stents (Guidant Acculink®, Cordis Precise®, Boston Wallstent®, Abbot Vascular Xact®, Cook Zilver®) in three diameters (4, 6, and 8 mm) using a vascular flow model at 3.0 T with the help of a recently developed carotid surface coil. Stent-related artifacts were objectively assessed by calculating artificial lumen narrowing (ALN) and relative in-stent signal (RIS).

RESULTS

RIS and ALN depended heavily on stent type, stent diameter, and the employed MR sequence. ALN and RIS were relatively favorable for Acculink®, Precise®, and Zilver® stents with both CE-MRA and 3D-TOF. CE-MRA provided better results for the Wallstent, while the Xact stent was difficult to visualize with both MRA protocols.

CONCLUSION

Both CE-MRA and 3D-TOF are viable options for depicting the in-stent lumen in carotid stents. For specific stents, 3D-TOF provided image quality comparable to CE-MRA and may thus be suitable for in vivo assessment. Development of stent-specific pathways for follow-up imaging seems advisable to address stent-related differences in image quality.

Contrast-enhanced magnetic resonance angiography following subintimal recanalization

PURPOSE

To describe the appearance of lower extremity runoff vessels following subintimal recanalization (SIR) on contrast-enhanced magnetic resonance angiography (ceMRA) and compare 2 different ceMRA techniques.

METHODS

A total of 6 patients underwent stepping table 3-dimensional (3D) ceMRA and time-resolved 2-dimensional (2D) MRA within 1 to 3 days (mean 1.83 days) following SIR. The 2 techniques were compared with intra-arterial digital subtraction angiography (DSA).

RESULTS

A total of 15 arteries were recanalized in 6 patients. Three-dimensional ceMRA allowed evaluation of patency in all segments above the knee. Postprocedural hyperemia impaired the assessment of the trifurcation vessels on 3D ceMRA. Due to its higher temporal resolution 2D MRA was not affected by venous contamination and allowed reliable confirmation of patency of the recanalized vessels.

CONCLUSIONS

Diagnostic MRA studies of the lower extremity runoff vessels following SIR is possible, but a hybrid technique using a stepping table MR DSA and a time-resolved sequence like 2D MRA of the calf station is necessary for runoff assessment.

A platinum-chromium steel for cardiovascular stents

The desire to reduce the strut thickness of cardiovascular stents has driven the development of a new high strength radiopaque alloy, based on additions of platinum to a chromium-rich iron based matrix. This paper reports on initial development of the alloy and the rationale for selection of the composition. Data is presented for tensile and microstructural characterization, surface oxide analysis, corrosion resistance and endothelial cell response of the alloy. The results demonstrate the solid solution strengthening effect of the platinum, with an average yield strength of 480 MPa achieved. The material surface consists of primarily chromium oxide which contributes to the high corrosion resistance observed. The cell assay result suggests that surfaces of this Pt-enhanced alloy endothelialize in a manner comparable to stainless steel.

The evolution of cardiovascular stent materials and surfaces in response to clinical drivers: a review

This review examines cardiovascular stent materials from the perspective of a range of clinical drivers and the materials that have been developed in response to these drivers. The review is generally chronological and outlines how stent materials have evolved from initial basic stainless steel devices all the way through to the novel biodegradable devices currently being explored. Where appropriate, pre-clinical or clinical data that influenced decisions and selections along the way is referenced. Opinions are given as to the merit and direction of various ongoing and future developments.

Characterization of an NbTaWZr alloy designed for magnetic resonance angiography compatible stents

The majority of stent materials are not fully compatible with magnetic resonance imaging due to their ferromagnetic or paramagnetic compositions. This leads to image artifact which can obscure clinical data in the vicinity of the stent. An Nb-28Ta-3.5W-1.3Zr alloy has been developed specifically to provide reduced magnetic susceptibility and therefore reduce image artifact. This study reports on initial surface characterization, corrosion behaviour, endothelial cell response and MR image performance. Surface analysis confirms the presence of a niobium oxide with some tantalum oxide also present. Electrochemical corrosion testing demonstrates the oxide to be stable with no evidence of film breakdown. Leaching of metallic ions during a 60-day immersion test shows low levels of release, comparable to cobalt-chromium L605. A short term endothelial cell adhesion study shows that the Nb-28Ta-3.5W-1.3Zr may be similar to stainless steel for supporting cell growth. The MR artifact assessment shows that the material has significantly reduced artifact compared to stainless steel. In summary, results from this initial study show that the Nb-28Ta-3.5W-1.3Zr meets many on the criteria expected of a stent material and that improved MR imaging behaviour is also obtained.

Development of a new niobium-based alloy for vascular stent applications

This study was performed in order to develop a new stent material that would provide reduced MR image artifact compared to current stent materials. Alloy design rationale is initially presented and following this the development of a Nb-28 Ta-3.5 W-1.3 Zr alloy is described, including the manufacture of stent tubing. Tensile testing of this new alloy showed that it had approximately twice the yield strength of current Nb-1 Zr material with a 25% higher elastic modulus. The new alloy was also confirmed to have suitably low magnetic susceptibility. Mechanical testing of demonstration coronary stents made from the new alloy were shown to have acceptable compression strength and elastic recoil performance. It is concluded that this new Nb-28 Ta-3.5 W-1.3 Zr alloy is a practical candidate stent material for both coronary applications and peripheral uses such as carotid or intracranial stenting, where reduced MR image artifact would be beneficial.

The role of CT and MRI in the assessment of peripheral vascular disease

Atherosclerosis is a serious health problem in developed countries and the main cause of morbidity and mortality. Treatment strategies such as surgical procedures, percutaneous catheter-based interventions or pharmacologic options depend on the accurate classification of atherosclerotic disease with respect to location, extent, and severity of arterial involvement. For this purpose, several imaging techniques are in clinical use. Due to recent improvements in scanner technology, noninvasive imaging modalities are now widely available for evaluating atherosclerotic lesions throughout the body. This article focuses on the clinical role of these imaging modalities in the assessment of peripheral vascular disease, including the aorta and extracranial arteries.

CTA and MRA in Mesenteric Ischemia: Part 2, Normal Findings and Complications After Surgical and Endovascular Treatment

OBJECTIVE

A number of surgical and endovascular options exist for the treatment of acute and chronic mesenteric ischemia. Both surgical and endovascular treatments necessitate close clinical and imaging follow-up because the consequences of acute occlusions can be catastrophic. MDCT angiography (CTA) and contrast-enhanced MR angiography (MRA) are the preferred imaging techniques in this setting.

CONCLUSION

We review the appearance of the normal and complicated surgical and endovascular treatment on CTA and MRA.

In vitroevaluation of stent patency and in-stent stenoses in 10 metallic stents using MR angiography

In vitro study to investigate the suitability of contrast enhanced magnetic resonance angiography (CEMRA) for determination of stent patency and grading of in-stent stenoses in 10 metallic stents. The Acculink carotid, DynaLink, Easy Wallstent, JostentSelfX XF, Luminexx, Omnilink, sinus-SuperFlex, SMART, Symphony and ZA stent were separately placed in a vascular phantom. Dedicated stenoses inside the stents generated a concentric lumen narrowing of 50%. CEMRA was performed for each stent. Signal loss inside the stents and artificial lumen narrowing were assessed objectively using the evaluation software of the MR imager. Moreover, three blinded observers determined visibility of stent patency and in-stent stenoses subjectively on a 3-point scale and graded in-stent stenoses. Loss of signal intensity within the stent lumen ranged between 90% (Wallstent) and 5% (ZA), artificial lumen narrowing between 56% (Symphony) and 22% (ZA). For the Symphony and Wallstent, visibility of patency and in-stent stenoses was impaired and the observers' grading exaggerated the degree of stenoses (by 23% and 33%, respectively). For the remainder of stents, patency and stenoses were visible and stenoses were graded accurately (less than 10% discrepancy from reference standard). In this in vitro study, eight of 10 stents presented with MRI characteristics which enabled determination of stent patency and accurate grading of clinically relevant in-stent stenoses.