Can Magnetic Resonance Imaging Be the Key Technique to Visualize and Investigate Endovascular Biomaterials?

OBJECTIVE

Magnetic resonance imaging (MRI) is an established modality in clinical use but may be potentially underutilized to visualize and investigate biomaterials. As its use is totally contraindicated only for ferromagnetic devices, it was employed to visualize deployment, biofonctionality, healing, and biodurability of a commercially available endovascular device, namely the Medtronic-AVE AneuRx. The quality of the observations coupled with the absence of ionizing radiations are likely to make this technique an attractive imaging modality in the future.

METHOD

The potential benefits of the MRI technique were investigated in a GE Vectra-MR 0.5T MRI for the Medtronic-AVE AneuRx endovascular prosthesis, under different conditions: undeployed i.e., inserted in the delivery cartridge as received from the manufacturer (step 1), deployed in a mock glass-aneurysm tube (step 2), and as a pathological explant harvested at the autopsy of a patient (step 3). The device was submitted to X-rays for examination in addition to MRI. At step 3, the device was further investigated with light microscopy and scanning electron microscopy (SEM) together with X-ray diffraction.

RESULTS

The device which was inserted and pleated in the delivery cartridge did not demonstrate any significant observation either in MRI or in X-rays. When it was deployed in the mock aneurysmal glass tube, light artefacts were associated with the T2 weighed FSE images around the Nitinol whereas X-rays gave images of indisputable interest. Similar results were noted using the explanted device. Very high contrasts were obtained with T1 whereas T2 images were almost defect free. The X-rays allowed to accurate imaging of the Nitinol skeleton but were poor to discriminate between the different tissues. Pathology observations using light microscopy were not really challenged, as the magnetic resonance imaging was performed using a 0.5T machine.

DISCUSSION

The benefits of magnetic resonance imaging as a quality control technique to examine an endovascular device within its cartridge remains ill defined. Similarly, the role of conventional X-rays is unknown. The observation of devices fully deployed in a mock aneurysmal glass-tube under MRI are potentially useful but X-rays images allowed better definition. The MRI examination of the explanted device does permit observations related to the healing of the device that might be obtained in vivo and, thus offers new avenues for the follow-up of implanted devices. The pathological investigations brought additional informations about the tissues and the corrosion of the Nitinol. However, it is unlikely that MRI will permit detailed analysis of the biomaterials and in particular the corrosion process of the stents.

CONCLUSION

These early observations of the follow-up of devices using MRI warrant further investigation. The absence of ionizing radiation with MRI makes this technique particularly attractive. As there is no emission of ionizing radiation associated with magnetic resonance, it is recommended that further investigation using this environment friendly technique for the follow-up of devices made of biomaterials that are MRI compatible.

Polymethylmethacrylate (PMMA) as an embedding medium preserving tissues and foreign materials encroaching in endovascular devices

Problems of displacement, poor healing, degradation of the polymers and corrosion of the metallic frame in endovascular devices still require in-depth investigations. As the tissues and the foreign materials are in close contact, it is of paramount importance to efficiently investigate the interfaces between them. Inclusion in polymethymethacrylate (PMMA) permits us to obtain thin slides and preserve the capacity to perform the appropriate stainings. An AneuRx prosthesis was harvested in bloc with the surrounding tissues at the autopsy of a patient 25 months post deployment in a 5.7 cm diameter AAA and sectioned in the direction of the blood flow in two halves. A cross-section of the encapsulated distal segment together with the surrounding aneuryshmal sac was embedded in polymethylmethacrylate (PMMA). Further to complete polymerization, slices of the specimen were cut on a precision banding saw under coolant. They were affixed onto methacrylate slides with a UV cured adhesive. Binding and polishing were done on a numeric grinder and slices 25 to 30 microm in thickness were stained with toluidine blue prior to observation in light microscopy. Additional slices were prepared for scanning electron microscopy and X-ray energy dispersive spectrometry for determination of the elemental composition of the Nitinol stent. The aortic wall did not demonstrate complete integrity along with its circumference. Some areas of rupture were noted. The content of the sac was heavily shrunk and was mostly acellular. The walls of the device were very well encapsulated. The PMMA embedding permitted the polyester wall, the Nitinol wire and the collagen to keep in close contact. Scanning electron microscopy involved backscattered electrons and confirmed the corrosion the Nitinol wire at the boundary with living tissues. Based upon the results obtained, we believe that PMMA embedding is the most appropriate method to process endovascular devices for histological and material investigation. Needless to say, that paraffin embedding would have not been feasible for such a big size specimen involving different materials.

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.

The adhesive properties of endothelial cells on endovascular stent coated by substrates of poly-l-lysine and fibronectin

Optimizing endothelial cell growth and adhesion on the surface of metallic stents implanted in the vascular system is a fundamental issue in understanding and improving their long-term biocompatibility. The ability of the endothelial cell to attach and adhere to the luminal stent surface as well as the capacity to withstand the significant shear stress associated with blood flow are important determinants. The adhesive characteristics of human umbilical vein endothelial cellsectin (HUVEC) on stent surfaces coated with either Poly-L-Lysine (PLL) or fibron (FN) were compared with uncoated controls. Increasing concentrations of PLL and FN were measured using a micropipette aspiration system. The adhesivenamic properties of HUVECs under static flow conditions were compared to a dy environment on endovascular stents using a parallel-plate-flow chamber. A scanning electron microscope picture was used to measure the number and the adhesive cell ratio as well as the percentage of surface coverage of stent by endothelial cells. The adhesive forces of HUVECs on foreign surfaces coated with PLL and FN were higher compared to uncoated surfaces, and were dependent on incr ing concentrations. These coatings resulted in significant increase of the adhesive force of HUVECs. The influence of substrates on the adhesion of the endothelial cell monolayer under static or dynamic flow conditions was highly significant compared with controls (p<0.01). No significant differences were observed between PLL and FN substrates. Both PLL and FN coated surfaces can significantly increase the adhesion and growth of HUVECs on metallic stent surfaces.

Solid-state 1H and 27Al NMR studies of amorphous aluminum hydroxides

Two kinds of amorphous aluminum hydroxides, a sample precipitated from admixing AlCl3 and NaOH aqueous solutions and the commercial product, were measured by 27Al and 1H solid-state NMR spectroscopy. Pentahedral and tetrahedral coordinations, as well as octahedral coordination of oxygen atoms for aluminum, are observed in 27Al magic angle spinning (MAS) spectra of both amorphous samples. In contrast, octahedral coordination is only observed in gibbsite, bayerite, and boehmite. According to 1H MAS-NMR spectra under conditions of high spinning rate (35 kHz) and high field (14.09 T), free waters and OH groups coupled with aluminum for amorphous samples are observed at approximately 5 and approximately 4.5 ppm, respectively, the latter peak being broader. This is consistent with the differential spectra between spin echo and transfer of populations in double resonance. We conclude that the subunits of AlO4, AlO5, and AlO6 in amorphous aluminum hydroxides are bound through hydrogen bonds with a wide distribution of bonding strength.

Nuclear magnetic resonance spectroscopy of bone substitutes

Calcium phosphate bone replacement biomaterials are widely used in different applications. Structure, composition, and organization are, before implantation, analyzed with different methods. Among them, X-ray diffraction is a recognized test. As bioresorption produces more amorphous material, the process is observed and quantified via scanning electron microscopy. Comparatively high-resolution 31P solid-state nuclear magnetic resonance spectroscopy is able to analyze raw ceramics composition and to estimate osteoformation.

A New Straightforward Approach to Generate Si-H Groups on Silica

In this paper it is demonstrated unequivocally by solid-state nuclear magnetic resonance and diffuse reflectance infrared Fourier transform spectroscopies that, at high temperature, hydrogen reacts directly with pyrogenic silica to form stable hydride Si-H groups. Such modified silicas should exhibit specific properties far from those of silicas having only surface silanol groups. The interest for hydride-modified silicas arises from the possibility of using their Si-H groups as reducing agents or as active sites for the synthesis of new chromatographic phases. Copyright 1999 Academic Press.

Molecular Conformation and Mobility of Polyamines Adsorbed on Silica Studied by Spin Labeling

Poly(ethyleneimine) and poly(4-vinylpyridine) chains adsorbed on Nucleosil silica have been randomly labeled with nitroxide free radicals. The EPR signal is very sensitive to the molecular Brownian motion of the segments and shows generally at least two different environments: trains adsorbed on the surface with a restricted mobility, and loops and tails protruding into the solution with a fast motion. The rotational correlation times and the relative fraction of each population are given as functions of the coverage achieved and of the specific surface area of the silica for the samples in contact with methanol and sometimes water. The overall picture is consistent with a diffuse layer for the PEI and a more compact one with some heterogeneities for the P4VP. Copyright 1999 Academic Press.

Water Adsorption on Pyrogenic Silica Followed by 1H MAS NMR

On the surface of two commercial pyrogenic silicas (Degussa and Cabot), five resonances were identified on the basis of the chemical shift, homonuclear coupling (T2), and spin-lattice relaxation behavior (T1). In accordance with previous studies we observed three different types of silanol groups: (i) weakly coupled (long T2), water inaccessible, isolated "internal" silanols at 1.8 ppm; (ii) weakly coupled, external "free" silanols revealed upon dehydration at 2.5 ppm; and (iii) strongly coupled external hydrogen bound silanols with an unresolved broad resonance between 3 and 7 ppm. The resonance of water, whose position between 2.6 and 4.6 ppm depended on water content, corresponded to two unresolved species of slightly different T1. By equating this resonance to the weighted average of two distinct populations of water, we were able to distinguish the first layer of strongly hydrogen bound water at 2.7 ppm from liquid-like water at 5 ppm. The first layer is complete for water relative humidity as low as 3.6% and corresponds to a surface coverage of 4.75 H2O/nm2. If we assumed a cristobalite-based surface structure, this meant a 1:1 ratio between surface hydroxyls and the first layer of physisorbed water. This ratio was the same for the two silicas regardless of surface area. Copyright 1997 Academic Press. Copyright 1997Academic Press

High-resolution 29Si solid-state NMR study of silicon functionality distribution on the surface of silicas

29Si solid-state NMR allows us to distinguish silicon atoms involved in siloxane bridges (Q4), single silanol (Q3) and geminal silanol groups (Q2) on the surface of silica. The relative proportions of each species can be obtained by Magic Angle Spinning (MAS) associated with 1H-->29Si crosspolarization (CP/MAS). This preliminary study deals with the silicon functionality of MCM-41, a purely siliceous, amorphous and mesoporous silica. From the 29Si spectra the relative proportions are estimated to be 3.7% Q2 27.3% Q3 and 69.0% Q4, while the ratio Q2/Q2 + Q3) is 0.12. These results are found to be in perfect agreement with a binomial probability distribution of sites.

Hydroxyl groups in bone mineral

The question of whether the apatite crystals of bone contain hydroxyl groups was explored using magic angle spinning, proton nuclear magnetic resonance spectroscopy, and resolution enhanced Fourier transform infrared spectroscopy. The powdered bone samples were heated at 300 degrees C in air, in CO2 at 4 bar atmosphere, and at 300 degrees C in air and subsequently at 300 degrees C in CO2, to eliminate the effects of water tightly bound to the crystals and to prevent the degradation of carbonate groups and the elimination of potentially present OH groups. Results confirm earlier findings that bone apatite crystals do not contain detectable amounts of hydroxyl ions.

Solid-state 31P NMR spectroscopy of bone and bone substitutes

Using magic angle sample spinning 31P NMR spectroscopy, we studied NMR parameters of hydroxyapatite, calcium-deficient hydroxyapatite, and beta-tricalcium phosphate. These data were compared to rabbit bone parameters. Rabbit bone spectrum is similar to deficient hydroxyapatite spectrum but high-power proton decoupling did not modify its linewidth. Moreover, both bone T1 and deficient hydroxyapatite T1 were much longer than pure hydroxyapatite T1 and beta-tricalcium phosphate T1. We used these results to develop quantitative analysis of newly deposited bone inside beta-tricalcium phosphate implants in rabbit. This method made it possible to demonstrate the effect of porosity on osteoinduction of biomaterials.