The authors developed ABI alloy, which mechanically resembles stainless
      steel 316. The main elements of ABI alloy are palladium and silver.
      Magnetic resonance (MR) images and radiographs of ABI alloy and stainless
      steel 316 stent models and of nitinol, tantalum, and Elgiloy stents were
      compared. ABI alloy showed the least MR imaging artifacts and was more
      radiopaque than stainless steel 316. ABI alloy has the potential to
      replace stainless steel 316 for construction of balloon-expandable MR
      imaging-compatible stents

Dijk, B.P. (Bastiaan) van

Dijk, L.C. (Lukas) van

Holten, J. van

Matheijssen, N.A.

Pattynama, P.M.T. (Peter)

Dijk, Lucas

Holten, Jacqueline

van Dijk, BP

Mattheijssen, NAA

Pattynama, Peter

EUR Research Repository

Lukas C. van Dijk, MDJacqueline van Holten, MDBastiaan P. van Dijk, BScNiels A. A. Matheijssen, PhDPeter M. T. Pattynama, MDIndex terms:Magnetic resonance (MR), artifact,9p.129412, 9p.129422Magnetic resonance (MR), vascularstudies, 9p.129412, 9p.12942Stents and prostheses, 9p.1268Radiology 2001; 219:284–2871 From the Department of Radiology,Erasmus University Medical Center Rot-terdam, “Dijkzigt,” Dr Molewaterplein40, 3015 GD Rotterdam, the Nether-lands (L.C.v.D., J.v.H., N.A.A.M., P.M.T.P.),and Elephant Dental, Hoorn, the Neth-erlands (B.P.v.D.). Received March 7,2000; revision requested April 26; revi-sion received August 1; accepted Sep-tember 6. Address correspondence toL.C.v.D. (e-mail: lcvandijk@rond.azr.nl).B.P.v.D. is a research and develop-ment employer for Elephant Dental.2 9p. Vascular system, location unspeci-fied© RSNA, 2001Author contributions:Guarantors of integrity of entire study,L.C.v.D., P.M.T.P.; study concepts, allauthors; study design, L.C.v.D., J.v.H.,N.A.A.M.; literature research, L.C.v.D.,J.v.H.; experimental studies, L.C.v.D.,J.v.H., B.P.v.D., N.A.A.M.; data acquisi-tion, L.C.v.D., J.v.H.; data analysis, allauthors; manuscript preparation, L.C.v.D.,J.v.H.; manuscript editing, B.P.v.D.,N.A.A.M., P.M.T.P.; manuscript review,N.A.A.M., P.M.T.P; manuscript final ver-sion approval, all authors.A Precious Metal Alloyfor Construction of MRImaging–compatible Balloon-expandable Vascular Stents1The authors developed ABI alloy,which mechanically resembles stain-less steel 316. The main elements ofABI alloy are palladium and silver.Magnetic resonance (MR) imagesand radiographs of ABI alloy andstainless steel 316 stent models andof nitinol, tantalum, and Elgiloy stentswere compared. ABI alloy showedthe least MR imaging artifacts andwas more radiopaque than stainlesssteel 316. ABI alloy has the potentialto replace stainless steel 316 for con-struction of balloon-expandable MRimaging–compatible stents.Magnetic resonance (MR) angiography isgaining importance as a vascular imagingmodality (1) and has the potential to re-place digital subtraction angiography forthe detection of recurrent stenosis afterstent placement in peripheral and coro-nary arteries. Metal stents implanted inthe body, especially those made of stain-less steel 316, cause major MR imagingartifacts, which are known as black-holeartifacts (2). Therefore, MR angiographyprovides no information about vesselsegments that have stents or vessel areasthat are close to the metal stent. Almostall stents placed in the coronary systemand about half of those placed in theperipheral system are balloon-expand-able devices made of stainless steel 316.Thus, to improve the reliability of MRimaging after balloon-expandable stentplacement, a stent material with betterMR imaging compatibility than stainlesssteel 316 is needed.The MR imaging compatibility of ametal alloy depends on the type andquantity of elements used. Iron-contain-ing alloys in particular and, to a lesserextent, alloys that contain cobalt, nickel,or chromium are known to cause MR im-aging artifacts (3) and therefore degradethe MR imaging characteristics of vascu-lar stents. We developed ABI alloy, ametal alloy composed of precious metalelements. The purpose of this initialstudy was to test the MR imaging com-patibility of stents made of ABI alloy, es-pecially as compared with those made ofstainless steel 316. Because it is knownthat some MR compatibility is achievedwith the use of other metal alloys, such asnitinol (4–6) and tantalum (7), we alsocompared the MR imaging of ABI alloywith that of commercially availablestents made of nitinol, tantalum, and El-giloy (Elgiloy, Elgin, Ill).Materials and MethodsABI alloy is an iron- and nickel-freeprecious metal alloy. The two main ele-ments of this alloy are palladium andsilver. The mechanical properties of ABIalloy have been tested by using industrialexaminations. The elasticity was 1.1 3105 N/mm2; the elongation, 30%; andthe hardness, 220 Hv (International Stan-dards Organization no. 6892). These val-ues are in the same range as those forstainless steel 316. The biocompatibilityof this precious metal alloy group hasbeen proved in industrial tests conductedby North American Science Associates,including tests for genotoxicity, cytotox-icity, local effects after implantation, irri-tation and sensitization, and hemolysis(International Standards Organizationnos. 10993-3, 10993-5, 10993-6, 10993-10,and MG074-100, respectively; NorthAmerican Science Associates, Northwood,Ohio).We performed three MR imaging ex-aminations. In the first examination, aself-made stent model made of stainlesssteel 316 was compared with an identicalstent model made of ABI alloy. Bothmodels were composed of two 0.4-mm-284thick wires made into a helix—one clock-wise and the other counterclockwise. Thetwo wires were bound together by usingsilk at each wire crossing to form a basicexpanded stent model with a length of40 mm, diameter of 8 mm, and 12 loops.In the second examination, we com-pared the self-made ABI alloy stent modelwith the following commercially avail-able devices: Memotherm nitinol stent(Bard Interventional Products, Billerica,Mass), Strecker tantalum stent (BostonScientific/Vascular, Watertown, Mass), andWallstent elgiloy stent (Schneider, Min-neapolis, Minn), all of which have a di-ameter of 8 mm.In the third examination, the ABI alloystent model was compared with the niti-nol stent by using a three-dimensionalvolume gradient sequence, because it isknown that nitinol stents have the bestMR imaging compatibility.The MR imaging experiments were car-ried out with a 1.5-T magnet (ACS-NT;Philips Medical Systems, Best, the Neth-erlands) and a surface coil. In the first twoexaminations, images were obtained witha gradient-echo technique (10.0/6.2 [rep-etition time msec/echo time msec], 30°flip angle, 205 3 256 matrix, 2-mm sec-tion thickness). In the third examina-tion, a three-dimensional gradient-echotechnique was used (27.0/7.3, 30° flip an-gle, 512 3 512 matrix, 0.8-mm sectionthickness). The devices were placed inplastic tubing that was taped to the bot-tom of a plastic container. The tube andthe container were filled with a coppersulfate in water solution (1 g/L).The presence and degree of artifacts onthe resulting MR images were quantifiedaccording to the method of Teitelbaumet al (8): no artifact, mild artifact, mod-erate artifact, severe black-hole artifact,and severe black-hole artifact with markedimage distortion (Table). A mild artifactwas judged to be smaller than the devicecausing the artifact (8). A moderate arti-fact was approximately the same size asthe device. A severe black-hole artifact waslarger than the device. Distortion was de-fined as bending, warping, or obliterationof the image contours normally observed.The virtual lumen reduction caused bythe MR artifacts was estimated, in steps of10%, after measuring the luminal diam-eter of the stents and of the tubes sur-rounding the stents on the MR images(Figs 1, 2).In addition to MR imaging, we used adifferential method to determine the ra-diopacity of stainless steel 316 and ABIalloy (9). Simultaneously to imaging thestent models, a radiograph of an alumi-num wedge was obtained at 60 kV andwith 25-mm aluminum filtering. A com-parison was made on the radiograph be-tween the optical density (ie, blackening)produced by the wedge and the points ofinterest of the two stent models. The alu-minum thickness values of the wedge,which produced the same optical densityas the stent models on the radiograph,were read. The optical densities of thestent model materials were expressed inmillimeters of aluminum equivalent.ResultsThe measurements of the MR imagingartifacts caused by the different materialsSemiquantitative Descriptionof Stent ArtifactsDevice ArtifactTest 1Self-made ABI-alloy stentmodel, 0.4 mm-wire 1Self-made stainless-steelstent model, 0.4-mmwire 1111Test 2Elgiloy stent 111Nitinol stent 11Tantalum stent 11Self-made ABI-alloy stentmodel, 0.4-mm wire 1Note. —1 5 mild artifact, 11 5 moderateartifact, 111 5 severe black-hole artifact,1111 5 severe black-hole artifact withmarked image distortion.Figure 1. Transverse gradient-echo MR image (10.0/6.2) of stainlesssteel 316 (left) and ABI alloy (right) stent models. Right: The stentstruts (gray area), stent lumen (white area), and plastic tube (blackouter circle with gap at the top, which corresponds to a hole thatallows air bubbles to escape) can be differentiated. The stainless steelstent, stent lumen, and plastic tube surrounding the stent cannot bedifferentiated.Figure 2. Coronal gradient-echo MR image (10.0/6.2) of, from topto bottom, the Elgiloy, nitinol, and tantalum stents and the ABI alloystent model. The plastic tubes in which the stents are mounted showno signal intensity. The artifacts of the stents within the tubes causeda virtual lumen reduction in the Elgiloy (30%), nitinol (20%), andtantalum (20%) stents. The struts of the ABI alloy stent model arevisible as black dots.Volume 219 z Number 1 Metal Alloy for Construction of MR-compatible Vascular Stents z 285are summarized in the Table. In test 1,the stainless steel 316 model caused severeblack-hole artifacts with marked distor-tion of the MR images. The ABI alloymodel caused mild artifacts that weresmaller than the stent model device (Fig 1).In test 2, the Elgiloy stent caused asevere black-hole artifact, the nitinol andtantalum stents caused moderate artifacts,and the ABI alloy stent model caused amild artifact (Fig 2).In test 3, the ABI alloy stent modelcaused fewer artifacts than did the niti-nol stent (Fig 3). On the MR images of theABI alloy stent model, delineation of theplastic tube from the stent material waspossible, but this was not the case withthe nitinol stent. Both the nitinol stentand the ABI alloy stent model causedsome signal intensity decrease within thelumen.The virtual lumen reduction caused bythe MR artifacts was 0% (lumen, 10 mm;tube, 10 mm) for the ABI alloy model,100% (lumen, 0 mm; tube, 10 mm) forthe stainless steel 316 model, 20% (lu-men, 8 mm; tube, 10 mm) for the nitinolstent, 20% (lumen, 8 mm; tube, 10 mm)for the tantalum stent, and 30% (lumen,8 mm; tube, 11 mm) for the Elgiloy stent(Figs 1, 2).With use of the differential method todetermine the radiopacity of both metalalloys, stainless steel 316 was equal to a5.4-mm aluminum equivalent (60.5 mm)and ABI alloy was equal to a 12.8-mmaluminum equivalent (60.5 mm). ABI al-loy was therefore approximately 2.4 timesmore radiopaque than stainless steel 316(Fig 4).DiscussionThe results of this in vitro study showthat ABI alloy causes substantially fewerartifacts at MR imaging than does stain-less steel 316.Balloon-expandable stents are preferredfor the coronary arteries by almost allinterventional cardiologists and for sev-eral peripheral vessels by many interven-tional radiologists because of their accu-racy and ease of placement comparedwith those of self-expandable stents.Stainless steel 316 has the mechanicalproperties necessary for construction ofballoon-expandable stents—that is, plas-tic deformability to allow crimping on alow-profile balloon and expansion to thevessel diameter, combined with goodhoop strength at full expansion and re-sistance to breakage. Because the me-chanical properties of ABI alloy are simi-lar to those of stainless steel 316, ourmain goal was to compare this materialwith the standard materials used to con-struct balloon-expandable stents.Artifacts in stents on MR images aredependent on the material and design ofthe stents. In the first test, we used self-made stent models with an identical de-sign, which allowed a comparison of the(isolated) effect of the different materials.In the second test, the stent materialsand stent designs were different. There-fore, the different MR artifacts observed withthe ABI alloy stent model and the nitinol,tantalum, and Elgiloy stents were causedby a combination of material and designinfluences. Because stent design is dictatedby many factors, including the mechani-cal properties of the material and patentissues, to our knowledge clinically testedstents of identical design but differentmaterial are not yet available for in vitroand in vivo comparisons of MR artifacts.MR artifacts are dependent on the MRsequence. In the described experiments,we used gradient-echo sequences withrelatively long echo times to exaggeratethe artifact sizes, because longer echotimes are associated with larger artifacts.Modern MR systems allow fast imagingsequences for MR angiography and coro-nary MR angiography with shorter repe-tition and echo times. It is reasonable toassume that the severity of artifacts willbe reduced if these sequences are used,but the differences between the materialswill still exist.MR evaluation of the stent lumen is, tosome extent, possible with nitinol andtantalum stents (5,10), but only stenosesof 50% or more can be correctly assessed,because smaller stenoses are overesti-mated owing to the artifacts (11). TheABI alloy stent model allowed clear delin-eation of the stent from the plastic tubeFigure 4. Radiograph (60 kV, 25-mm aluminum filtering) of the aluminum wedge (bottom) andthe ABI alloy (left) and stainless steel 316 (right) stent models. ABI alloy was approximately 2.4times more radiopaque than stainless steel 316.Figure 3. Sagittal gradient-echo MR image (27.0/7.3) of the ABI alloy stent model (top) and thenitinol stent (bottom). The plastic tubes show no signal intensity. The ABI alloy stent modelshows no virtual lumen reduction; the stent material can be recognized by the black dots. Thenitinol stent caused moderate artifacts that resulted in a virtual lumen reduction on the image.286 z Radiology z April 2001 van Dijk et alsurrounding the stent. This suggests thatwith ABI alloy stents, not only MR imag-ing evaluation of stent patency but alsoMR analysis of the vessel wall surround-ing the stent is possible. The ability toimage close to the stent material couldmean that MR angiographic evaluationof even mild within-stent recurrent ste-nosis is possible after ABI alloy stentplacement. In the lumina of both the ABIalloy stent model and nitinol stent, someMR signal intensity decrease was notice-able, probably owing to the Faraday cageeffect.The radiograph showed that the ra-diopacity of ABI alloy is approximately2.4 times higher than that of stainlesssteel 316. This implies that fluoroscopicimaging for positioning of stents made ofABI alloy should be at least as good asthat for positioning of stents made ofstainless steel 316 when stents of identi-cal design are compared. In conclusion,initial examination results showed thatABI alloy causes fewer artifacts at MR im-aging than does stainless steel 316.References1. Duerinckx AJ. Coronary MR angiography.Radiol Clin North Am 1999; 37:273–318.2. Bernardino ME, Steinberg HV, PearsonTC, Gedaudas-McClees RK, Torres WE,Henderson JM. Shunts for portal hyper-tension: MR and angiography for deter-mination of patency. Radiology 1986;158:57–61.3. Bellon EM, Haacke EM, Coleman PE,Sacco DC, Steiger DA, Gangarosa RE. MRartifacts: a review. AJR Am J Roentgenol1986; 147:1271–1281.4. Nitatori T, Hanaoka H, Hachiya J,Yokoyama K. MRI artifacts of metallicstents derived from imaging sequencingand the ferromagnetic nature of materi-als. Radiat Med 1999; 17:329–334.5. Hilfiker PR, Quick HH, Debatin JF. Plainand covered stent-grafts: in vitro evalua-tion of characteristics at three-dimen-sional MR angiography. Radiology 1999;211:693–697.6. Schurmann K, Vorwerk D, Bucker A, et al.Perigraft inflammation due to Dacron-covered stent-grafts in sheep iliac arteries:correlation of MR imaging and histopatho-logic findings. Radiology 1997; 204:757–763.7. Amano Y, Gemma K, Kawamata H, Kuma-zaki T. Intraluminal signal intensity of il-iac artery stents investigated by contrast-enhanced three-dimensional MR angiog-raphy. Comput Med Imaging Graph1998; 22:9–12.8. Teitelbaum GP, Bradley WG, Klein BD.MR imaging artifacts, ferromagnetism,and magnetic torque of intravascular fil-ters, stents, and coils. Radiology 1988; 166:657–664.9. Balz G, Birkner R. Die bestimmung desaluminiumschachungsgleichwertes vonknochengewebe beim lebenden. Strahl-entherapie 1956; 99:221–226.10. Matsumoto AH, Teitelbaum GP, CarvlinMJ, Barth KH, Savin MA, Strecker EP.Gadolinium enhanced MR imaging ofvascular stents. J Comput Assist Tomogr1990; 14:357–361.11. Link J, Steffens JC, Brossmann J, Graess-ner J, Hackethal S, Heller M. Iliofemoralarterial occlusive disease: contrast-en-hanced MR angiography for preinterven-tional evaluation and follow-up after stentplacement. Radiology 1999; 212:371–377.Volume 219 z Number 1 Metal Alloy for Construction of MR-compatible Vascular Stents z 287

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A precious metal alloy for construction of MR Imaging - compatible balloon- expandable vascular stents

The authors developed ABI alloy, which mechanically resembles stainless steel 316. The main elements of ABI alloy are palladium and silver. Magnetic resonance (MR) images and radiographs of ABI alloy and stainless steel 316 stent models and of nitinol, tantalum, and Elgiloy stents were compared. ABI alloy showed the least MR imaging artifacts and was more radiopaque than stainless steel 316. ABI alloy has the potential to replace stainless steel 316 for construction of balloon-expandable MR imaging-compatible stents

Van Dijk, L. C.

Van Holten, J.

Van Dijk, B. P.

Matheijssen, N. A.A.

Pattynama, P. M.T.

NARCIS 

A precious metal alloy for construction of MR imaging-compatible balloon-expandable vascular stents

Erasmus University Digital Repository

English

A precious metal alloy for construction of MR imaging-compatible balloon-expandable vascular stents

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