Measurement of Magnetically Induced Torque on Lightweight Medical Devices in the Magnetic Resonance Environment for ASTM F2213

Use of medical devices in the magnetic resonance environment is regulated by standards that include the ASTM-F2213 magnetically induced torque. This standard prescribes five tests. However, none can be directly applied to measure very low torques of slender lightweight devices such as needles. Methods: We present a variant of an ASTM torsional spring method that makes a “spring” of 2 strings that suspend the needle by its ends. The magnetically induced torque on the needle causes it to rotate. The strings tilt and lift the needle. At equilibrium, the magnetically induced potential energy is balanced by the gravitational potential energy of the lift. Static equilibrium allows calculating the torque from the needle rotation angle, which is measured. Moreover, a maximum rotation angle corresponds to the maximum acceptable magnetically induced torque, under the most conservative ASTM acceptability criterion. A simple apparatus using the 2-string method is shown, it can be 3D printed, and the design files are shared. Results: The analytical methods were tested against a numeric dynamic model, showing perfect concordance. The method was then tested experimentally in 1.5T and 3T MRI with commercial biopsy needles. Numeric test errors were immeasurably small. Torques between 0.0001Nm and 0.0018Nm were measured in MRI with 7.7% maximum difference between tests. The cost to make the apparatus is 58USD and design files are shared. Conclusion: The apparatus is simple and inexpensive and provides good accuracy as well. Significance: The 2-string method provides a solution to measure very low torques in the MRI.

磁共振环境中医疗器械的使用受包括ASTM-F2213磁感应扭矩（magnetically induced torque）在内的标准规范。该标准规定了五项测试，但均无法直接应用于测量针类细长轻质器械的极低扭矩。

方法：我们呈现了ASTM扭转弹簧法（ASTM torsional spring method）的一种变体，该变体采用两根弦线构成“弹簧”，通过两端悬挂针。针上的磁感应扭矩使其旋转，弦线随之倾斜并抬升针体。平衡状态下，磁感应势能与抬升产生的重力势能相互抵消。静平衡状态下，可通过测得的针旋转角度计算扭矩。此外，在最保守的ASTM可接受性标准下，最大旋转角度对应最大可接受磁感应扭矩。本文展示了采用双弦线法的简易装置，该装置可3D打印，且设计文件已共享。

结果：分析方法通过数值动力学模型验证，显示完全一致。随后，该方法在1.5T和3T磁共振成像（MRI）中使用商用活检针进行实验测试。数值测试误差小到无法测量。在磁共振成像中测得的扭矩范围为0.0001Nm至0.0018Nm，测试间最大差异为7.7%。该装置制造成本为58美元，且设计文件已共享。

结论：该装置简单、低成本且具有良好的准确性。

意义：双弦线法为磁共振环境中极低扭矩的测量提供了一种解决方案。