Many heart valve prostheses and annuloplasty rings have been evaluated for MR issues, especially with regard to the presence of magnetic field interactions associated with exposure to MR systems operating at field strengths of as high as 4.7-Tesla. Of these, the majority displayed measurable yet relatively minor magnetic field interactions. That is, because the actual attractive forces exerted on the heart valve prostheses and annuloplasty rings were minimal compared to the force exerted by the beating heart (i.e., approximately 7.2-N), an MR procedure is not considered to be hazardous for a patient that has any heart valve prosthesis or annuloplasty ring tested relative to the field strength of the magnet (i.e., MR system) used for the evaluation.
Importantly, this recommendation includes the Starr-Edwards Model Pre-6000 heart valve prosthesis previously suggested to be a potential risk for a patient undergoing an MR examination.
With respect to clinical MR procedures, there has been no report of a patient incident or injury related to the presence of a heart valve prosthesis or annuloplasty ring. However, it should be noted that not all heart valve prostheses have been evaluated and at least one prototype exists that has magnetic components.
Heart Valve Prostheses and the Lenz's Related Forces. Condon and Hadley (2000) reported the theoretical possibility of a previously unconsidered electromagnetic interaction with prosthetic heart valves that contain metallic disks or leaflets. Basically, any metal (i.e. not just ferromagnetic material) moving through a magnetic field will develop another magnetic field that opposes the primary magnetic field. This phenomenon is referred to as the “Lenz Effect" or Lenz's Law Related Forces. In theory, “resistive pressure” may develop in heart valve prostheses that have metallic disks or leaflets (note, at the present time, mostly tissue valves are used instead of those with metallic leaflets) with the potential to inhibit both the opening and closing aspects of the valve. The Lenz Effect is proportional to the strength of the static magnetic field. Accordingly, it has been suggested that there may be problems for patients with heart valves that have metallic discs or leaflets in association with MRI procedures, especially those performed at very high static magnetic fields.
Edwards, et al. (2015) conducted an in vitro study of the occurrence of Lenz-related forces on various heart valve prostheses at 1.5-Tesla and assessed the risk of the impedance of valve function. The findings provided evidence of the Lenz Effect on certain cardiac valve prostheses exposed to the static magnetic field, which resulted in functional valve impedance and a potentially increased risk of valve regurgitation. While further evaluation of this phenomenon may be warranted, to date, the Lenz Effect has not been observed in association with clinical MR examinations nor has it posed additional risks for patients with certain heart valve prostheses (i.e., those with metallic leaflets or disks) undergoing MRI.
MRI at 3-Tesla and Heart Valve Prostheses and Annuloplasty Rings.Findings obtained at 3-Tesla for various heart valve prostheses and annuloplasty rings that underwent testing indicated that certain implants exhibit relatively minor magnetic field interactions. Similar to heart valve prostheses and annuloplasty rings tested at 1.5-Tesla, because the actual attractive forces exerted on these implants are deemed minimal compared to the force exerted by the beating heart, MR procedures at 3-Tesla are not considered to be hazardous for patients or individuals that have these devices. To date, for the heart valves that have been tested, MRI-related heating has not been shown to reach substantial levels.
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