Cath Lab Digest - December 2007 - (Page 5) 5 Table 1. Common implanted devices and their MR labeling Coronary and peripheral artery stents Non cardiovascular stents Aortic stent grafts (see specific type) Prosthetic heart valves Annuloplasty rings Sternal wires Cardiac closure and occluder devices Vena cava filters Embolization coils Loop recorders Temporary pacemakers Cardiac defibrillators MR safe MR conditional. See specific details for device. MR safe, conditional, unsafe. MR safe MR safe MR conditional. See specific details for device. MR safe and/or conditional. See specific details. MR safe MR safe MR conditional. See specific details for device. MR unsafe MR unsafe Tissue healing after device implantation provides additional anchoring, and thus it is safest to wait at least six weeks before MR imaging of many of these devices if the true MR implant status is unknown. spatial magnetic gradient, or radiofrequency energy and specific radiofrequency absorption rates. Additional conditions may be described for an implant to carry the label MR conditional. An MR unsafe item is that which poses a hazard in all MR environments, including any magnetic items such as a pair of ferromagnetic scissors, clips, or other such tools. Let’s look at some examples. A nonferromagnetic passive implant carries no concern for MR-related risk of heating, injury or movement. When patients have a weakly ferromagnetic implant, MR examinations generally produce only a weak attractive force to move or dislodge coils or stents which are firmly implanted in tissues, and are unaffected by the attractive magnetic forces of an MR examination. Tissue healing after device implantation provides additional anchoring, and thus it is safest to wait at least six weeks before MR imaging of many of these devices if the true MR implant status is unknown. Table 1 shows common implanted devices and their MR labeling. ■ released by the realigning fields of the cells is measured in time and space, hence the MRI image. Some of the RF energy is absorbed and can cause heating of a metallic implant. Certain metallic devices such as pacemaker leads can act as antenna and concentrate radiofrequency energy at one point. Excessive local heating at the tip of these implants has been reported. Fractured pacemaker leads may pose a particular high risk of thermal injury. The concentration of radiofrequency energy is device-dependent and varies with field strength. Is Your Device MR Safe? The American Society for Testing of Materials (ASTM) International has a new set of terms to describe implants subject to MRI. These are: 1) MR safe, 2) MR conditional, and 3) MR unsafe. These names are now applied (but not mandated) to many of implants and devices overseen by the Food and Drug Administration. MR safe means an item poses no known hazards in all MR environments. These include nonconducting, nonmetallic, nonmagnetic items such as plastic forceps, dish or cup. MR conditional means an item has been demonstrated to pose no hazard in a specific MR environment under specified conditions. An MR environment is then defined for the implant. The conditions would include static magnetic field strength, References 1. Shellock FG. Reference Manual for Magnetic Resonance Safety, Implants, and Devices: 2006 Edition. Los Angeles: Biomedical Research Publishing Group; 2006. 2. Kanal E, Borgstede JP, Barkovich AJ, et al. American College of Radiology White Paper on MR, Safety: 2004 update and revisions. AJR Am J Roentgenol 2004 May; 182(5):1111-1114. LETTER TO THE EDITOR To the Editor: Your September 2007 Editor’s Corner “Is Crossing the Stenotic Aortic Valve Obsolete?” relates the new recommendations from the ACC/AHA discouraging routine attempts to cross the aortic valve due to the risk of stroke. You also cite the study by Omran in Lancet which is oft quoted by fellows to prove that the valve should not be crossed (Omran H, Schmidt H, Hackenbroch M, et al. Silent and apparent cerebral embolism after retrograde catheterisation of the aortic valve in valvular stenosis: A prospective, randomised study. Lancet 2003;361:1241–1246.). I have not heard anyone question the technique used in the Omran study. They used a Terumo wire in crossing the valve. This hydrophilic wire is known to have a tendency to go through and under plaques and can easily cause dissections. I believe it is reasonable to suggest that a hydrophilic tip wire would be more likely to dislodge calcified debris than a more conventional one. It may be the wire that was the problem. I do not think the Omran study can be used to suggest that all attempts at crossing a stenotic aortic valve should be abandoned. I would say that the study showed that the use of a hydrophilic wire to cross the stenotic valve has an unacceptably high incidence of silent and apparent stroke, and should not be used. The study should be repeated with a more conventional wire. Sincerely, Marshall H. Crenshaw, MD, FACC Assistant Professor of Medicine Vanderbilt Heart and Vascular Institute Nashville, TN marshall.h.crenshaw@vanderbilt.edu Dr. Kern’s response: Dr. Crenshaw identifies an important fact little known to most of us. I agree that the Terumo wire carries more risk than non-coated wires with the potential to lift plaques. I have never used this technique for aortic stenosis and agree with Dr. Crenshaw that this study should be repeated with alternate wire technique. Thanks for an astute observation. — MK Letters to the Editor should be addressed to Rebecca Kapur, Managing Editor, at rkapur@hmpcommunications.com, or via regular mail at 7427 Winding Way, Brecksville, OH 44141.
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