Kuethe D O, Small K W, Blinder R A
Duke University Medical Center, Durham, North Carolina.
Invest Radiol. 1991 Jan;26(1):1-7. doi: 10.1097/00004424-199101000-00001.
Should patients with cobalt alloy (ASTM F563) retinal tacks (Grieshaber cat. #611.95) in their eyes be subjected to the magnetic fields used in magnetic resonance imaging? Although the tacks are not ferromagnetic, they will experience a retarding torque when they are moved at the high angular velocities of human eye motion. Because retinal tacks are small (2.85 mm x 0.9 mm), the torque is difficult to measure. Rather, we measured the torque on a model 25.4 times larger and used a scaling law derived from Maxwell's equations to calculate the force on the tack. The scaling law states that the torque varies with the cube of the object's length. To mimic the motion, models of retinal tacks were attached to Plexiglas rods and the assemblies were swung as pendulums. The pendulums were oriented in the magnetic field of a 1.5 T imager to experience the greatest retardation. Retarding torques were estimated from the rate of decrease of the pendulum amplitude, both inside and outside the magnet. Even if the retinal tacks were as conductive as 6061T6 aluminum alloy (25 MS/m) and the velocity of the surface of the eye were 24 cm/s (angular vel. of 1130 deg/s), the retarding torque would be only 1.6 times the weight of the tack acting with a lever arm as long as the distance from its tip to its center of gravity. The maximum retarding torque on an implanted retinal tack in a 1.5 T magnet is similar to the torque produced by gravity alone acting on the tack and is a tolerable risk.(ABSTRACT TRUNCATED AT 250 WORDS)
眼睛中植入钴合金（ASTM F563）视网膜钉（Grieshaber产品编号611.95）的患者是否应接受磁共振成像中使用的磁场？尽管这些钉子不是铁磁性的，但当它们以人眼运动的高角速度移动时，会受到一个减速扭矩。由于视网膜钉很小（2.85毫米×0.9毫米），该扭矩很难测量。相反，我们测量了一个尺寸大25.4倍的模型上的扭矩，并使用从麦克斯韦方程推导出来的比例定律来计算钉子上的力。该比例定律表明，扭矩随物体长度的立方而变化。为了模拟这种运动，将视网膜钉模型连接到有机玻璃棒上，然后将组件像钟摆一样摆动。钟摆在1.5T成像仪的磁场中定向，以经历最大的减速。根据钟摆在磁体内部和外部摆动幅度的减小速率来估计减速扭矩。即使视网膜钉的导电性与6061T6铝合金（25 MS/m）一样，并且眼表面的速度为24厘米/秒（角速度为1130度/秒），减速扭矩也仅为钉子重量的1.6倍，其力臂与从钉尖到其重心的距离一样长。在1.5T磁体中植入的视网膜钉上的最大减速扭矩类似于仅由重力作用在钉子上产生的扭矩，是一个可接受的风险。（摘要截短于250字）
