Error assessment due to coronary stents in flow-encoded phase contrast MR angiography: a phantom study.

Phase contrast magnetic resonance imaging (PC MRI) is a promising method for assessing coronary flow. MR angiography images in the presence of coronary stents display artifacts because of the metal present in the stent. Using a flow phantom, the goal of this in vitro study was to assess quantitatively the effects of flow dephasing caused by magnetic susceptibility in velocity measurements in a region where the artifact is not visualized in a magnitude image. The results showed that for high velocities, significant errors in measurements exist around the stent, outside the susceptibility artifact visible on a magnitude image. J. Magn. Reson. Imaging 1999;10:899-902.     

Ability of MR cholangiography to reveal stent position and luminal diameter in patients with biliary endoprostheses: in vitro measurements and in vivo results in 30 patients

OBJECTIVE: Our goal was to evaluate the ability of MR cholangiography to show stent position and luminal diameter in patients with biliary endoprostheses. MATERIALS AND METHODS: Susceptibility artifacts were evaluated in vitro in three different stent systems (cobalt alloy-based, nitinol-based, and polyethylene) using two breath-hold sequences (rapid acquisition with relaxation enhancement, half-Fourier acquisition single-shot turbo spin echo) on a 1.5-T MR imaging system. The size of the stent-related artifact was measured, and the relative stent lumen was calculated. In vivo stent position and patency were determined in 30 patients (10 cobalt alloy-based stents, five nitinol-based stents, and 15 polyethylene stents). RESULTS: In vitro, the susceptibility artifact of the cobalt stent caused complete obliteration of the stent lumen. The relative stent lumens of the nitinol-based and polyethylene stents were 38-50% and 67-100%, respectively. In vivo, all stents were patent at the time of imaging. The position of the cobalt alloy-based stent could be determined in nine of 10 patients, but stent patency could not be evaluated. Stent position of nitinol stents could not be adequately evaluated in any of the five patients, and internal stent diameter could be visualized in only one patient. In nine of 15 patients, the fluid column within the implanted polyethylene stent was seen on MR cholangiography. CONCLUSION: The internal stent lumen could be visualized in most patients with an indwelling polyethylene stent, but not in patients with cobalt alloy- or nitinol-based stents.     

Contrast-enhanced MR angiography at 1.5 T after implantation of platinum stents: in vitro and in vivo comparison with conventional stent designs

OBJECTIVE: The objective of our study was to evaluate the in vitro and in vivo 3D contrast-enhanced MR angiography characteristics of a new platinum-based balloon-expandable stent system and compare this system with a variety of competing metallic stents. MATERIALS AND METHODS: All experiments were performed on 1.5-T scanners. In vitro experiments were performed using 10 stents implanted into a custom-built phantom. Different orientations of the stents along the magnetic field and multiple flip angles were examined. In addition, 19 patients underwent contrast-enhanced MR angiography after the implantation of 36 stents, including four patients with six platinum stents. Angiographic correlation was available for all 19 patients, and luminal patency and stent-induced artifacts were assessed quantitatively. RESULTS: Of the tested balloon-expandable stents, only the platinum-based stents created artifact causing luminal narrowing of 30% or less. All other balloon-expandable stents induced larger artifacts that resulted in higher degrees of narrowing. Thus, if patent, the platinum-based stents allow significant in-stent stenosis to be ruled out reliably. Selected nitinol- or tantalum-based self-expandable stents also are suitable in this regard. CONCLUSION: Of the tested devices, platinum-based stents are the only type of currently available balloon-expandable stent that creates 30% or less artifact-induced apparent stenosis and thus are suitable for MR angiography.     

Stent appearance at contrast-enhanced MR angiography: in vitro examination with 14 stents

PURPOSE: To evaluate signal intensity changes influencing assessment of stent patency at contrast material-enhanced magnetic resonance (MR) angiography. MATERIALS AND METHODS: By using an in vitro model, 14 stents-nine nitinol, one tantalum, two stainless steel, and two cobalt alloy-were investigated regarding their appearance at MR imaging. A vascular phantom consisting of tubes filled with 2.00 mmol/L gadopentetate dimeglumine in saline solution was studied in different orientations within the magnetic field. Imaging was performed with a fast three-dimensional gradient-echo sequence (4. 70/1.89 [repetition time msec/echo time msec]). Relative signal intensity reduction within the stents and the degree of artificial narrowing of the stent lumen were calculated. RESULTS: The stent lumen was visible within 13 stents. A total signal void inside the stent lumen appeared in only one cobalt alloy stent. Artificial narrowing of the diameter was less than 33% in 10 of 14 stents. The tantalum stent and four nitinol stents seemed best suited for contrast-enhanced MR angiography. A bandlike artifact occurred at the ends of the stents when positioned along the readout direction. CONCLUSION: To differentiate between artifacts and stenoses, knowledge of the degree of signal intensity reduction and artificial lumen narrowing within vascular stents is essential. Stent geometry, relative orientation to the magnetic field, and alloy composition influence signal intensity alteration within the stent lumen.     

Ferromagnetism and magnetic resonance artifacts of platinum embolization microcoils

We performedin vitro tests of ferromagnetism of platinum microcoils of two manufacturers. In addition, we performedin vitro magnetic resonance imaging (MRI) on these two devices to determine their levels of magnetic susceptibility artifact production. We found that both devices were nonferromagnetic and produced a very low level of artifact during MRI.In vivo MRI of a dog and 2 patients having previous placement of platinum microcoils was performed without coil mignation or significant artifact production. Platinum microcoils are highly MR-compatible.     

Metallic ballistic fragments: MR imaging safety and artifacts

The ferromagnetism of various bullets and shotgun pellets was tested in vitro. Magnetic deflection showed that four of 21 metallic specimens tested (all bullets) demonstrated marked ferromagnetism. Three of these four were made outside the United States; two of the four were known to contain steel, and the other two were reportedly either copper or copper-nickel-jacketed lead bullets (indicating that the ferromagnetism was due to impurities in the bullet jackets or cores). Ferromagnetic bullets readily rotated within a gelatin phantom in response to magnetic torque. Nonferromagnetic bullets and pellets demonstrated only mild to moderate metal artifact during spin-echo and gradient-echo magnetic resonance (MR) imaging. However, all four of the ferromagnetic bullets produced severe MR artifacts and image distortion. MR studies of seven patients with retained bullets, pellets, or shrapnel were reviewed. In six of the seven, only mild MR artifacts were seen. Only intracranial shrapnel (presumably steel) in one patient created significant artifact. All seven patients with retained bullets and shotgun pellets were imaged safely with MR. However, caution should be exercised with MR imaging in the presence of metallic foreign bodies, particularly if they are located near vital neural, vascular, or soft-tissue structures.     

MR imaging of vascular stents: effects of susceptibility, flow, and radiofrequency eddy currents

PURPOSE: The purpose of this in vitro study was to examine the various sources of artifacts in magnetic resonance (MR) imaging and angiography of vascular stents. MATERIALS AND METHODS: Five low-artifact stents-Wallstent (cobalt alloy), Memotherm (nitinol), Perflex (stainless steel), Passager (tantalum), and Smart (nitinol)-were imaged in a vascular flow phantom, consisting of a thin-walled cellulose vessel model connected to a pump system. The echo time and the angulation of the stents with respect to the direction of the main magnetic field were varied. Spin echo and gradient echo images as well as three-dimensional MR angiograms were obtained to study the effects of flow, magnetic susceptibility, and radiofrequency-induced eddy currents. RESULTS: Susceptibility artifacts were restricted to the stents' direct environment and were mildest at short echo times and with the stents aligned with the main magnetic field. Nitinol stents showed less artifacts than steel stents did. Radiofrequency artifacts obscuring the stent lumen and flow-related lumen displacement were seen in all stents. The extent to which these occurred depended on strut geometry and orientation. CONCLUSIONS: For low-artifact stents, the material the stent is made of is not the only important factor in the process of artifact formation. Susceptibility artifacts, radiofrequency eddy currents and flow-related artifacts all contribute to the image distortion, and are dependent on the geometry and orientation of the struts and on the orientation of the stent in the main magnetic field.     

Spiral CT in evaluation of head and neck lesions: work in progress.

Spiral computed tomography (CT) was used in the evaluation of 21 patients with head and neck lesions. Scanning time ranged from 24 to 36 seconds, and high-quality diagnostic scans with excellent anatomic resolution and minimal motion artifact were produced. Vascular opacification was optimized with substantially less contrast medium than used in conventional studies. These preliminary results show spiral CT to be at least comparable with conventional CT in the evaluation of the head and neck.     

Three-dimensional MR angiography of a nitinol-based abdominal aortic stent graft: assessment of heating and imaging characteristics

Objective: To assess heating- and 3D MRA imaging characteristics of a commonly used aortic stent graft in a 1.5T MR-environment. Materials and methods: A bifurcated stent graft (Vanguard; Boston Scientific, Oakland, N. J.) was evaluated in vitro regarding localized heating effects as well as imaging appearance using fast 3D GRE sequences. To quantitate stent related artifacts, stent wall thickness and luminal diameters were measured. Subsequently eight patients were imaged three months following placement of an aortic stentgraft with 3D MRA. Images were assessed for the presence of stent leaks, luminal patency, and stent configuration. Results: There were no temperature changes associated with the stent during scanning. Wall thickness measurements overestimated true stent thickness, resulting in minimal underestimation of luminal diameters on 3D MRA images. In vivo imaging confirmed these results. Stent patency was confirmed in all 8 patients. Conclusion: Contrast-enhanced 3D MRA appears well suited for the evaluation of the abdominal and pelvic vasculature following aortic implantation of a Vanguard stent. Received: 24 August 1998; Revised: 10 December 1998; Accepted: 2 April 1999     

Metallic artifacts of coronary and iliac arteries stents in MR angiography and contrast-enhanced CT.

Metallic artifacts of intravascular stents were assessed with MR angiography and contrast-enhanced spiral CT. Stainless steel showed less metal artifact than tantalum stent in CT. Metallic artifact in coronary and iliac arteries treated with tantalum stent was not remarkable in MR angiography. Contrast-enhanced CT might be preferable to assess patency of arteries treated with stainless steel stent. while MR angiography was useful in depicting intraluminal signal in tantalum stent.     

An unusual cause of susceptibility artifact in magnetic resonance imaging

Magnetic susceptibility artifact has been documented with various implants and devices, which require appropriate identification with screening of patients prior to subjecting them to magnetic resonance examination. We performed cardiovascular magnetic resonance (CMR) examination of the aorta in a 24-year-old woman in the setting of repaired aortic coarctation, and found magnetic susceptibility artifact arising from the stomach in the absence of known susceptibility artifact-producing material in this region. Further history revealed that she had ingested a prenatal vitamin prior to imaging, prompting experimental analyses that led us to conclude that iron-containing vitamins may be a source of magnetic susceptibility artifact.     

Magnetic resonance characteristics of the LGM vena cava filter: Technical note

The LGM (Vena Tech) IVC filter is a recently introduced device for caval interruption. The magnetic resonance imaging safety and imaging characteristics of this filter were evaluated. The filter was proven to lack ferromagnetic properties. It was imaged with minimal artifact and no detectable motion in the magnetic field.     

Magnetic resonance angiography of intravascular endoprostheses: Investigation of three devices

Purpose To assess the value of magnetic resonance angiography (MRA) in the evaluation of vascular patency after intravascular endoprosthesis placement.Methods Three different metallic stents (Wallstent, Strecker, Palmaz) were studied in vitro, and in vivo in six patients with spin-echo (SE) and gradient-echo (GRE) MR imaging. Time-of-flight, two-dimensional (2D) gadolinium-enhanced MRA was performed with GRE and flow-compensation technique, and reconstructed with a maximum-intensity projection (MIP) algorithm. MRA was compared to digital angiograms.Results In vitro studies demonstrated that the signal intensity (SI) within the stent differed according to the device employed, the lowest SI being observed within the Palmaz stent (p = .001). There was no difference in SI or apparent diameter of the stent according to the sequence (SE vs GRE) or length of echo time (TE). In patients, the endoprostheses recorded as a well-defined area of signal void or drop-out (p = 0.004), whereas vessels above and below the stent displayed high signal intensities.Conclusion MRA does not seem as yet to be well suited for evaluating vascular patency after endoprosthesis placement, even if the Strecker and Wallstent endoprostheses provide fewer artifacts than the Palmaz stent.     

The V-shaped artifact of the gallbladder wall

Among 20,000 patients who had undergone abdominal or gallbladder sonographic examinations, 36 had a short V-shaped artifact originating from an hyperechoic focus in the gallbladder wall. Associated symptoms were varied and nonspecific. The 10 gallbladders excised because of associated cholelithiasis showed a thickened wall, and seven patients had diverticulosis of the wall with impacted stones. In vitro sonographic examination of five gallbladders reproduced the "V" artifact and showed that it originated from a small intramural cholesterol stone. The V artifact should be distinguished from the larger comet-tail and ring-down artifacts, which arise from metal or gas.     

MR imaging artifacts, ferromagnetism, and magnetic torque of intravascular filters, stents, and coils

Experiments were conducted in which various intravascular filters, stents, and coils were imaged using magnetic resonance (MR) spin-echo technique at 0.35 T. These devices were also evaluated for ferromagnetism (at 0.35, 1.5, and 4.7 T), magnetic torque (at 0.35 and 1.5 T), and magnetically induced migration within a plastic tube (at 0.35 and 1.5 T for the Greenfield filter [GF]). The stainless-steel GF was evaluated in vitro for its propensity to perforate canine inferior venae cavae (IVC). Magnetic force and torque at 1.5 T did not dislodge the GF or result in perforation of canine IVC by the GF. Beta-3 titanium alloy (used in a new percutaneous version of the GF) is apparently one of the best-suited metals for use with MR imaging because of its lack of ferromagnetism (up to 4.7 T) and absence of MR imaging artifacts (at 0.35 T). Devices composed of Elgiloy (Mobin-Uddin filter), nitinol, and MP32-N (Amplatz filter) alloys all created mild artifacts. Devices fashioned from 304 and 316L (GF and Palmaz stent) stainless-steel alloys created severe "black-hole" artifacts, with the 304 alloy devices also showing marked image distortion. Generally, the greater the ferromagnetism of a device, the greater its magnetic susceptibility artifact.     

冠状动脉支架内腔的可视度:64层螺旋CT与宝石CT的比较

目的评价宝石CT(HDCT)显示冠状动脉支架的能力,并与64层螺旋CT进行比较。方法 52例患者(共放置85个支架)进行64层螺旋CT和HDCT扫描,分别测量支架内CT值和相邻未放置支架节段的CT值,同时测量支架内径,并对两者进行比较。结果在HDCT上,支架内腔的显示率明显高于64层螺旋CT(分别为69.0%和42.9%,P<0.01)。支架内CT值的上升值分别为15.2 HU和54.2 HU(P<0.01)。结论由于空间分辨率的提高以及有效抑制线束硬化伪影,HDCT与64层螺旋CT比较,提高了支架内腔的可视度,能更准确测量支架的内径。     

Shunts for portal hypertension: MR and angiography for determination of patency

Twenty-eight patients with selective and nonselective shunts for portal hypertension were evaluated using magnetic resonance (MR) imaging. Angiographic correlation was obtained in 25 patients. MR imaging enabled the detection of a patent shunt by visualizing the "flow void" phenomenon in 21 patients. Two patients had thrombosed shunts. In these 23 patients, there was no discrepancy between the findings from MR imaging and those from angiography. In the remaining five patients, there was an area of artifact in which no signal was noted, and the shunt could not be evaluated. In all five patients who had this artifact, steel coils were noted in the area of the phenomenon. Thus, MR imaging seems to be an accurate method for detecting shunt patency in all patients with shunts except those who have had prior embolization with steel coils.     

In vitro evaluation of platinum Guglielmi detachable coils at 3 T with a porcine model: safety issues and artifacts

PURPOSE: To evaluate safety-related issues and imaging artifacts of Guglielmi detachable coils in vitro with 3-T magnetic resonance (MR) imaging. MATERIALS AND METHODS: Two aneurysm models were constructed: one from porcine carotid artery and the other from a pharmaceutical capsule. Both were filled with Guglielmi detachable coils. The models were tested with a 3-T MR imager for heating, deflection, and imaging artifact. Testing for heating and deflection was performed (a) at static points both inside and outside the bore, (b) during movement into the imager, and (c) during clinical imaging sequences. RESULTS: No change in temperature was measured during movement into the imager bore or at different points within the bore. No differences in heating from radio-frequency energy were found between aneurysm models and controls. Similarly, no evidence of deflection of the coil mass (capsule model) was found. Minor susceptibility artifacts were found in the readout direction during gradient-echo sequences. Magnetic field mapping showed no induced field inhomogeneity. CONCLUSION: MR imaging at field strengths of 3 T in patients with aneurysms treated with Guglielmi detachable coils is safe. Imaging artifacts are likely to be minimal.     

Magnetic resonance angiography of stents

PURPOSE: To understand the behavior of stents during contrast-enhanced magnetic resonance angiography (MRA). MATERIALS AND METHODS: Seven different types of stents are explored in vitro with spin echo (SE) T2 sequence and MRA sequences using classification of the intra-prothesis signal. Six patients with 11 different proximal stents are studied by Gadolinium enhanced MRA with the same classification. RESULTS: Stent material is not the only determinant of behavior. In this way, one stainless steel stent produced less intra-luminal hyposignal than a nitinol device. Nevertheless, only a nitinol stent did not alter the intra-prothesis signal and allowed an appreciation of patency, despite a decrease of diameter and presence of edge artifacts. CONCLUSION: Even if stent compatibility is well known with MRI, even with Gadolinium, cannot be used alone to depict stenosis or diameter reduction. Even patency based on the presence of a constant signal, cannot be confirmed routinely with confidence: indeed, only a single stent does not modify intra-arterial signal.     

Intracranial aneurysm stenting: follow-up with MR angiography

Intracranial stenting is increasingly being used to treat intracranial aneurysms and stenoses. We wanted to assess the utility of magnetic resonance angiography (MRA) in the follow-up of patients treated with various types of intracranial stents and to assess the utility of performing gadolinium-enhanced MRA. A total of 19 patients having undergone intracranial stenting for aneurysms were imaged by MRI at 1.5T. A total of 20 stents were placed in 19 patients. In addition to conventional T2- and diffusion-weighted MRI, 3D time-of-flight MRA was performed before and after contrast administration. In the case of metallic INX stents (N = 7), there was a signal drop at the level of the vessel. which did not allow to evaluating the parent vessel, whereas this was visible in Nitinol stents (N = 8). Additionally a stent with a wire had a small artifact (N = 3). Contrast administration also improved vessel lumen visualization. In the case of Nitinol stents, MRA can be used to reliably demonstrate the vessel lumen after intracranial stenting. The use of postcontrast 3D time-of-flight imaging helps improve the intraluminal definition.