Estimation of torque on mechanical heart valves due to magnetic resonance imaging including an estimation of the significance of the Lenz effect using a computational model.
Mitral and aortic valve replacement is a procedure which is common in cardiac surgery. Some of these replacement valves are mechanical and contain moving metal parts. Should the patient in whom such a valve has been implanted be involved in magnetic resonance imaging, there is a possible dangerous interaction between the moving metal parts and the static magnetic field due to the Lenz effect. Mathematical models of two relatively common forms of single-leaflet valves have been derived and the magnitude of the torque which opposes the motion of the valve leaflet has been calculated for a valve disc of solid metal. In addition, a differential model of a ring-strengthener valve type has been considered to determine the likely significance of the Lenz effect in the context of the human heart. For common magnetic field strengths at present, i.e. 1 to 2 T, the effect is not particularly significant. However, there is a marked increase in back pressure as static magnetic field strength increases. There are concerns that, since field strengths in the range 3 to 4 T are increasingly being used, the Lenz effect could become significant. At 5 to 10 T the malfunction of the mechanical heart valve could cause the heart to behave as though it is diseased. For unhealthy or old patients this could possibly prove fatal.
Robertson
N M
NM
neil.robertson@physics.org
Diaz-Gomez
M
M
Condon
B
B
D001021:Aortic Valve / Q000502:physiology; D003198:Computer Simulation; D006350:Heart Valve Prosthesis; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000009:adverse effects* / Q000379:methods*; D009682:Magnetic Resonance Spectroscopy / Q000009:adverse effects / Q000379:methods; D008280:Magnetics; D008943:Mitral Valve / Q000502:physiology; D008962:Models, Theoretical; D013997:Time Factors; D019415:Torque