MR imaging in patients with intracranial aneurysm clips

Four patients with intracranial aneurysm clips made from a variety of alloys were studied without incidence by MR imaging at field strengths ranging from 0.35 to 0.6 T. Knowledge of the type of alloy used in the manufacturing of an aneurysm clip is important in determining whether the clip will or will not deflect in a magnetic field. Ferromagnetic clips show deflection and torque in a magnetic field and have the potential to dislodge from the aneurysm. Nonferromagnetic or weakly ferromagnetic aneurysm clips such as the Sugita (Elgiloy), Yasargil (316 LVM stainless steel), Heifetz (Elgiloy), Yasargil (Phynox), and Vari-Angle McFadden (MP35N) do not deflect or deflect weakly in the magnetic field and therefore would not be expected to dislodge during MR. The option of imaging many patients with intracranial aneurysm clips with MR extends the usefulness of the technique to a previously excluded population.     

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.     

Ex vivo evaluation of ferromagnetism and artifacts of cardiac occluders exposed to a 1.5-T MR system

Magnetic resonance (MR) procedures are contraindicated for patients with certain ferromagnetic biomedical implants, primarily owing to the risk of movement or dislodgment of the implants by the static magnetic field. An additional concern is the amount of artifact that the implant produces, which can affect image quality and interpretation of the examination. Therefore, an ex vivo assessment of ferromagnetism and artifact was conducted for 12 different occluders used to treat patients with patent ductus arteriosus, atrial septal defects, and ventricular septal defects, in a 1.5-T MR system. Seven of the occluders, made of 304 stainless steel, were ferromagnetic and displayed deflection forces of 248–299 dynes. Five of the implants, made of MP35n, were nonferromagnetic. Artifacts were variable and depended primarily on the type and amount of metal used to construct the implant. The authors conclude that patients with ferromagnetic cardiac occluders may undergo MR procedures approximately 6 weeks after placement of these devices, to allow tissue growth to provide additional retentive force. After this time, it is highly unlikely that the magnetic fields associated with a 1.5-T MR system are capable of moving or dislodging any of these cardiac occluders.     

High-field-strength MR imaging and metallic biomedical implants: an ex vivo evaluation of deflection forces

Ferromagnetic biomedical implants are considered a contraindication for MR imaging primarily because of the potential hazards associated with their movement or dislodgment. Many metallic biomedical implants are composed of nonferromagnetic materials and do not present a danger to patients during MR imaging. Therefore, to evaluate the ferromagnetic qualities of 36 different metallic biomedical implants (four aneurysm clips, six hemostatic clips, four dental implants, seven prosthetic heart valves, eight orthopedic prostheses, one artificial urinary sphincter, three contraceptive diaphragms, and three cerebral ventricular shunt tube connectors) not previously evaluated with a high-field-strength MR system, we measured deflection forces at the portal of the magnet of a 1.5-T MR system. Fourteen of the 36 metallic biomedical implants were determined to be ferromagnetic as indicated by their deflection in the static magnetic field. However, only the four aneurysm clips (Drake, Mayfield, McFadden, and Sundt-Kees) had sufficient ferromagnetism to warrant exclusion of patients with these implants from imaging with a 1.5-T MR system because of the possibility of movement or displacement. The calculated deflection forces for these aneurysm clips were comparable with previously reported values of certain aneurysm clips that have been designated to present a risk for patients undergoing MR imaging. Patients with 32 of 36 metallic biomedical implants tested can be safely imaged with high-field-strength MR systems.     

Aneurysm clips: evaluation of magnetic field interactions and translational attraction by use of "long-bore" and "short-bore" 3.0-T MR imaging systems

BACKGROUND AND PURPOSE: The use of 3.0-T MR systems is increasing worldwide. We evaluated magnetic field interactions and translational attraction for 32 aneurysm clips in association with exposure to "long-bore" and "short-bore" 3.0-T MR imaging systems. METHODS: Thirty-two different aneurysm clips were evaluated in this investigation. Each aneurysm clip was qualitatively evaluated for magnetic field interactions and quantitatively assessed for translational attraction by using the deflection angle test. The deflection angle tests were performed at the points of the highest spatial gradients for long-bore and short-bore 3.0-T MR imaging systems. RESULTS: Seventeen of the 32 aneurysm clips showed positive magnetic field interactions. Deflection angles for the aneurysm clips were significantly (P <.001) higher on the short-bore (range, 0-18 degrees) compared with those recorded on the long-bore (range, 0-16 degrees) 3.0-T MR imaging system. Aneurysm clips made from commercially pure titanium and titanium alloy displayed no translational attraction (n = 15), whereas those made from stainless steel alloy, Phynox, and Elgiloy displayed positive deflection angles (n = 17). CONCLUSION: The 32 different aneurysm clips passed (angle <45 degrees) the deflection angle test by using the long- and short-bore 3.0-T MR imaging systems, indicating that they are safe for patients and other persons in these MR environments (ie, immediate area of MR imaging systems). However, only clips made from the titanium and titanium alloy are entirely safe for patients undergoing MR imaging procedures because of the total lack of magnetic field interactions. The remaining clips require characterization of magnetic field-induced torque. Because of possible differences in the points of the highest spatial gradients for different 3.0-T MR imaging systems, the results are specific to the imaging units and bore designs used in this investigation.     

MR imaging of metallic implants and materials: a compilation of the literature

Ferromagnetic metallic implants and materials are regarded as contraindications for MR imaging because of the potential risks associated with their movement or displacement. To date, 14 published articles have evaluated the ferromagnetic qualities of 127 different metallic implants and other materials, including aneurysm and hemostatic clips (32); dental implants and materials (five); intravascular coils, filters, and stents (13); ear implants (14); prosthetic heart valves (29); orthopedic implants and materials (eight); penile implants (nine); and miscellaneous metallic implants and materials (17). All of these materials were evaluated by measuring the deflection forces induced by static magnetic fields at strengths ranging from 0.147 to 4.7 T. This article is a compilation of the results of these studies; it lists all 127 of the materials tested, indicates whether they were found to be deflected by the static magnetic fields, and gives the highest static magnetic field strength at which they were evaluated. Of the metallic implants tested, 66 were nonferromagnetic, and 29 exhibited only minimal deflection relative to their in vivo applications (i.e., the deflection forces were thought to be insufficient to move or dislodge the implant or material in situ). The authors of these studies concluded that patients with these particular metallic implants or materials (95/127, 75%) can be examined safely by MR imaging with scanners having static magnetic field strengths up to and including those used for the specific evaluations. Patients with other ferromagnetic materials or implants may also undergo MR imaging safely; however, both careful consideration of the factors that influence the deflection of metallic implants and prudent clinical judgment are required before patients who have these objects are examined via MR imaging.     

MR imaging and metallic implants for anterior cruciate ligament reconstruction: assessment of ferromagnetism and artifact.

Magnetic resonance (MR) imaging is contraindicated for patients with certain ferromagnetic implants, primarily because of potential risks related to movement or dislodgment of the devices. An additional problem with metallic implants is the potential image distortion that may affect the interpretation of the MR study. Since MR imaging is frequently useful for the evaluation of postoperative anterior cruciate ligament (ACL) reconstruction, the ferromagnetic qualities and artifacts associated with MR imaging were determined for five metallic orthopedic implants commonly used for this surgery. Only the Perfix interference screw displayed a substantial deflection force and caused extensive signal loss. Images of the knee of one patient with two Perfix screws in place were not interpretable because of the image distortion caused by these implants. Therefore, alternative nonferromagnetic implants should be considered for reconstruction of the ACL.     

MR imaging and biomedical implants, materials, and devices: an updated review

Certain ferromagnetic metallic implants, materials, and devices are regarded as contraindications for magnetic resonance imaging, primarily because of the risks associated with their movement or dislodgment. More than 40 publications have reported the ferromagnetic qualities of 261 different metallic objects (aneurysm and hemostatic clips, 32; carotid artery vascular clamps, five; dental devices or materials, 16; heart valve prostheses, 29; intravascular coils, filters, and stents, 14; ocular implants, 12; orthopedic implants, materials, and devices, 15; otologic implants, 56; pellets and bullets, 23; penile implants, nine; vascular access ports, 33; and miscellaneous, 17) on the basis of measurements of deflection forces or attraction during exposure to static magnetic fields at strengths of 0.147-4.7 T. The results of these studies are listed with respect to the specific object tested, the material used to construct the object (if known), whether or not the object was deflected or moved during exposure to the static magnetic field, and the highest static magnetic field strength used for testing the object.     

Metallic otologic implants: in vitro assessment of ferromagnetism at 1.5 T

MR imaging is contraindicated for patients with certain ferromagnetic implants because of potential risks related to movement or dislodgement. This is especially true for metallic implants located in sensitive areas of the body, such as those placed in and around the ear. Therefore, the ferromagnetic qualities of 35 different metallic otologic implants were assessed by placing them individually on a millimeter scale in a plastic petri dish that was slowly moved into the center of a 1.5-T MR imaging system. None of the metallic otologic implants moved during this procedure. The results demonstrate that each of these implants are made from nonferromagnetic materials and do not pose a risk to patients undergoing high-field-strength MR imaging. These data effectively expand the list of metallic implants that appear to be safe for MR imaging.     

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.     

MR-guided percutaneous drainage of abdominal fluid collections in combination with X-ray fluoroscopy: initial clinical experience

The aim of this study was to examine the feasibility of a hybrid interventional MR system, which combines a closed bore magnet with a C-arm fluoroscopy unit for percutaneous drainage of abdominal fluid collections. During the past 2 years, we have performed four drainage procedures in four patients (mean age 47 years). Three patients had abscesses (psoas muscle, kidney, subphrenic location) and the fourth patient had a recurrent splenic cyst. All procedures were performed on an interventional MR system consisting of a 1.5-T ACS-NT scanner combined with a specially shielded C-arm. The drainages were guided by T1-weighted fast gradient-echo images, T2-weighted single-shot turbo spin-echo images or both. A standard 18 G (1.2 mm) nonferromagnetic stainless steel needle with a Teflon sheath was used for the punctures following which a 0.89 mm nitinol guidewire was inserted into the fluid collection. Thereafter, the patient was positioned in the immediate adjacent fluoroscopy unit and a drainage catheter was placed under fluoroscopic control. All drainage catheters were successfully placed into the fluid collections, as proven by fluid aspiration and resolution of the collection. The mean time needed for the entire drainage procedure (MR and fluoroscopy) was 110 min. No procedure-related complications occurred. It is feasible to perform drainage procedures on a closed-bore MR scanner. The multiplanar imaging capabilities of MR are particularly helpful for fluid collections in the subphrenic location. Received: 3 April 2000 Revised: 30 August 2000 Accepted: 31 August 2000     

MR of ballistic materials: imaging artifacts and potential hazards

The most common ballistic materials available in the urban setting were studied for their MR effects on deflection force, rotation, heating, and imaging artifacts at 1.5 T to determine the potential efficacy and safety for imaging patients with ballistic injuries. The 28 missiles tested covered the range of bullet types and materials suggested by the Cleveland Police Department. The deflection force was measured by the New method. Rotation was evaluated 30 min after bullets had been placed in a 10% (weight per weight) ballistic gelatin designed to simulate brain tissue, with the long axis of the bullet placed parallel and perpendicular to the Z axis of the magnet. Heating was measured with alcohol thermometers by imaging for 1 hr alternatively with gradient-echo and spin-echo sequences (RF absorption = 0.033 and 0.326 w/kg respectively). Image artifacts on routine sequences were evaluated. All the steel-containing bullets except for the Winchester armor-piercing 38 caliber exhibited deflection. A nonsteel 7.38-mm Mauser also deflected. Deflection range was 514 to 15,504 dynes. Rotation occurred when the bullets were not parallel to the Z axis. Temperature changes were not significant. Deflecting projectiles resulted in obliteration of the image. The artifacts from other projectiles were small but varied by content. The artifact of the Winchester armor-piercing 38-caliber bullet was similar to those without steel. Bullets that contain steel or ferromagnetic contaminates such as nickel can be rotated within the MR unit.(ABSTRACT TRUNCATED AT 250 WORDS)     

Implantable spinal fusion stimulator: assessment of MR safety and artifacts

The objective of this investigation was to perform magnetic resonance (MR) imaging safety and artifact testing of an implantable spinal fusion stimulator. Magnetic field interactions, artifacts, and operational aspects of an implantable spinal fusion stimulator were evaluated in association with a 1.5 T MR system. Magnetic field-related translational attraction was measured using the deflection angle test. A special test apparatus was used to determine torque at 4.7 T. Artifacts were characterized using fast multiplanar spoiled gradient-echo, T1-weighted spin-echo, and T1-weighted fast spin-echo sequences. Operational aspects of the implantable spinal fusion stimulator before and after exposure to MR imaging at 1.5 T were assessed. In addition, nine patients (six lumbar spine and three cervical spine) with implantable spinal fusion stimulators underwent MR imaging. The findings indicated that magnetic field interactions were relatively minor, artifacts were well characterized and should not create diagnostic problems, and there were no changes in the operation of the spinal fusion stimulator. The nine patients underwent MR procedures without substantial adverse events or complaints. Based on the results of this investigation and in consideration of the findings from previous studies of MR imaging safety for the implantable spinal fusion stimulator, MR imaging may be performed safely in patients using MR systems operating at 1.5 T or less following specific recommendations and precautions.     

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.     

Low-artifact intravascular devices: MR imaging evaluation

Flow-phantom magnetic resonance (MR) imaging, with use of both spin-echo (SE) and gradient-echo (GRE) techniques at 1.5 T, was performed on the percutaneous Greenfield (beta-III titanium alloy [TMA wire]), Amplatz (MP32-N alloy), and Simon nitinol filters and TMA wire facsimiles of the bird's nest, Gunther, new retrievable, and Amplatz vena caval filters. SE imaging allowed detection of thrombi as small as 5 X 5 mm trapped within the percutaneous Greenfield, Simon nitinol, and TMA-wire facsimile filters; with the MP32-N Amplatz filter, a larger volume of thrombus (10 X 20-mm clots) was necessary for clot detection. GRE imaging allowed detection of intraluminal tilting of the percutaneous Greenfield and facsimile Amplatz (TMA-wire) filters. GRE imaging was useful for demonstrating postfilter turbulence due to clots, which was greatest for the Amplatz filter. Imaging of facsimile vascular devices made of tantalum or TMA wire did not cause the severe "black-hole" MR artifacts typical of the stainless-steel devices. SE and GRE imaging were very useful for determining caval patency in two patients with previously placed Mobin-Uddin filters. Noninvasive MR evaluation of blood vessels in the presence of a variety of low-artifact intravascular devices appears feasible.     

Feasibility of stent placement in carotid arteries with real-time MR imaging guidance in pigs

All examinations were performed with approval from the institutional animal care and use committee of Columbia University. To assess the feasibility of real-time magnetic resonance (MR) imaging-guided neurovascular intervention in a swine model, the authors placed stents in the carotid arteries of five domestic pigs. Seven-French vascular sheaths were placed in the target carotid arteries via femoral access by using active MR tracking. Ten nitinol stents (8-10 x 20-40 mm) were successfully deployed in the target segments of carotid arteries bilaterally. MR imaging and necropsy findings confirmed stent position. Necropsy revealed no gross vascular injury. Study results demonstrated the feasibility of performing real-time MR imaging-guided neurovascular intervention by using an active-tracking technique in an animal model.     

不同场强影响介入性磁共振穿刺针成像因素的初步探讨

目的　探讨 0 .3T、1.5T场强下影响介入性磁共振穿刺针成像的因素。以便提供正确的穿刺方法及成像技术参数 ,确保介入性磁共振操作的安全性及准确性。方法　将MR相容性介入穿刺针置于琼脂模型内 ,在 0 .3T、1.5T场强下 ,采用自旋回波序列、快速自旋回波序列、梯度回波序列 ,将穿刺针平行、垂直于主磁场 ,频率编码方向平行或垂直于穿刺针的方向进行扫描 ,在工作站测量图像上穿刺针的宽度及针尖的位置 ,比较图像上和实际针尖位置的差异 ,并比较不同场强下影响穿刺针伪影的因素。结果　各序列中梯度回波序列产生的伪影较大 ,快速自旋回波序列产生的伪影较小 ,自选回波序列产生的伪影最小 ,但快速自旋回波序列与自旋回波序列产生的伪影无明显差异。快速自旋回波序列与自旋回波序列中 ,当频率编码轴垂直于穿刺针长轴时伪影较大 ;当穿刺针方向逐渐平行于主磁场方向时 ,伪影逐渐减小。所有图像上针尖位置与实际针尖位置的差异在 1cm内。结论　 1.5T场强下伪影宽度大于 0 .3T场强。在 0 .3T、1.5T场强下改变频率编码方向、脉冲序列、成像参数时 ,针尖位置的变化在 1cm内 ,穿刺针平行于主磁场方向时 ,在不同场强下明显降低伪影的宽度     

Aneurysm clips: evaluation of magnetic field interactions with an 8.0 T MR system

This study was conducted to evaluate magnetic field interactions for aneurysm clips exposed to an 8.0 T magnetic resonance (MR) system. Twenty-six different aneurysm clips were tested for magnetic field translational attraction (deflection angle test) and torque (qualitative assessment method) using previously described techniques. Six of the specific aneurysm clips (i.e. type, model, blade length) made from stainless steel alloy (Perneczky) and Phynox (Yasargil, models FE 748 and FE 750) displayed deflection angles above 45 degrees and torque measurements of +4, indicating that these aneurysm clips maybe unsafe for patients or individuals in an 8.0 T MR environment. The specific aneurysm clips (i.e. type, model, blade length) made from commercially pure titanium (Spetzler), Elgiloy (Sugita), titanium alloy (Yasargil, model FE 750T), and MP35N (Sundt) displayed deflection angles less than 45 degrees and torque that ranged from + 1 to +4. Accordingly, these aneurysm clips are likely to be safe for patients or individuals exposed to an 8.0 T MR system. Depending on the actual dimensions and mass, an aneurysm clip made from Elgiloy may or may not be acceptable for a patient or individual in the 8.0 T MR environment.     

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.     

Head and neck lesions: MR-guided aspiration biopsy

Aspiration biopsy guided with computed tomography (CT) has long been a valuable tool in the evaluation of head and neck disease. The ability to obtain diagnoses without the need for surgery has had a significant effect on patient treatment. Magnetic resonance (MR) imaging is now rapidly replacing CT as the primary imaging study for many head and neck diseases. The standard stainless steel needles used for CT-guided biopsy are unsuitable for MR-guided biopsy because significant ferromagnetic artifacts obscure the underlying anatomy. A new needle has recently been designed specifically for use with MR imaging. This needle has far less magnetic susceptibility and therefore does not cause significant image distortion. The authors describe the use of this needle in MR-guided aspiration biopsy of a variety of lesions in the head and neck.