Heart Valve Prostheses and Annuloplasty Rings

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.

A study by Edwards et al. examined the effects of aging on cardiac tissue strength and its ability to withstand forces associated with a 4.7-T magnet. The goal of this research was to determine the forces required to cause partial or total detachment of a heart valve prosthesis in patients with age-related degenerative diseases exposed to MRI. Edwards et al. concluded that specific age-related degenerative cardiac diseases stiffen and strengthen tissue, resulting in significant forces required to pull a suture through valve annulus tissue. Importantly, these forces are substantially greater than those magnetically induced at 4.7-T. Accordingly, patients with degenerative valvular diseases are unlikely to be at risk during exposure to static magnetic fields less than or equal to 4.7-T.

Heart Valve Prostheses and the Lenz Effect. Condon and Hadley reported the theoretical possibility of a previously unconsidered electromagnetic interaction with 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”. In theory, “resistive pressure” may develop with the potential to inhibit both the opening and closing aspects of the mechanical heart valve prosthesis. The Lenz effect is proportional to the strength of the static magnetic field. Accordingly, there may be problems for patients with heart valves that have metal leaflets undergoing MR procedures using MR systems operating at static magnetic fields greater than 1.5-Tesla. However, this phenomenon has not been observed in association with MR procedures.

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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