The effect of frequency (64-498 MHz) on specific absorption rate adjacent to metallic orthopedic screws in MRI: A numerical simulation study.
The effect of frequency (64-498 MHz) on specific absorption rate adjacent to metallic orthopedic screws in MRI: A numerical simulation study.
复制标题
频率 (64-498 MHz) 对 MRI 中金属骨科螺钉附近特定吸收率的影响：数值模拟研究。
DOI：
10.1002/mp.16902
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发表时间：
2024
期刊：
影响因子：
3.8
通讯作者：
Fagan,AndrewJ
中科院分区：
文献类型：
--
作者：
Jacobs,Paul;Fagan,AndrewJ
背景通过超高场磁共振成像，植入导电设备的患者的成像受到与由于入射电磁场的能量耦合到植入物中而在植入物附近诱导组织加热的潜力相关的不确定性的阻碍。现有文献中比较组织加热场强与其替代物--特定吸收率(SAR)的文献缺乏且相互矛盾，导致对使用此类设备的成像患者的安全性的进一步怀疑。目的通过全波电磁模拟研究不同长度、频率从到498兆赫兹的骨科螺钉附近的射频感应SAR，以提供不同MRI场强的准确比较。在没有螺丝存在的情况下，天线的输入功率被限制为实现以下目标：(I)E_i=1100nV/m，(Ii)B1+=2SART，以及(Iii)全球平均-μ_s=33.2nW/kg。在螺丝周围体积的空间分辨率为0.2 mm的情况下进行模拟，得到76-137个MCcell，注意到每种情况下的最大平均SAR值为1微克。结果当限制偶极天线的输入功率以使螺丝位置的背景组织中的电场保持恒定时，出现在共振螺丝长度处的峰值SAR随着频率的降低而显著增加。当限制输入功率以获得恒定的B1+和全局平均SAR时，也观察到类似的模式。植入螺丝钉的组织的介电特性决定了297 MHz和128 MHz之间的SAR比较。结论该研究设计允许直接比较不同频率和种植体长度的SAR，而不受可变入射电场的干扰。对于接近共振长度的植入物，较低的频率会产生相当大的SAR值，因为最糟糕的情况下，均匀的入射电场会沿着螺丝长度。这些数据可能会为在7亿特斯拉的新临床磁场强度下进行整形外科植入物成像患者的风险-收益评估提供信息。
BackgroundThe imaging of patients with implanted electrically‐conductive devices via magnetic resonance imaging at ultra‐high fields is hampered by uncertainties relating to the potential for inducing tissue heating adjacent to the implant due to coupling of energy from the incident electromagnetic field into the implant. Existing data in the peer‐reviewed literature of comparisons across field strengths of tissue heating and its surrogate, the specific absorption rate (SAR), is scarce and contradictory, leading to further doubts pertaining to the safety of imaging patients with such devices.PurposeThe radiofrequency‐induced SAR adjacent to orthopedic screws of varying length and at frequencies of 64 to 498 MHz was investigated via full‐wave electromagnetic simulations, to provide an accurate comparison of SAR across MRI field strengths.MethodsDipole antennas were used for RF transmission to achieve a uniform electric field tangential to the screws located 120 mm above the antenna midpoints, embedded in a bone‐mimicking material. The input power to the antennas was constrained to achieve the following targets without the screw present: (i) E = 100 V/m, (ii) B1+= 2 μT, and (iii) global‐average‐SAR = 3.2 W/kg. Simulations were performed with a spatial resolution of 0.2 mm in the volume surrounding the screws, resulting in 76–137 MCells, noting the maximum 1 g‐averaged SAR value in each case. Simulations were repeated at 128 and 297 MHz for screws embedded in muscle tissue.ResultsThe peak SAR, occurring at the resonant screw length, substantially increased as the frequency decreased when the input power to the dipole antenna was constrained to achieve constant electric field in background tissue at the screws’ locations. A similar pattern was observed when constraining input power to achieve constant B1+and global‐average‐SAR. The dielectric properties of the tissue in which the screws were embedded dominated the SAR comparisons between 297 and 128 MHz.ConclusionsThe study design allowed for a direct comparison to be performed of SAR across frequencies and implant lengths without the confounding effect of variable incident electric field. Lower frequencies produced substantially larger SAR values for implants approaching the resonant length for the worst‐case uniform incident electric field along the screws’ length. The data may inform risk‐benefit assessments for imaging patients with orthopedic implants at the new clinical field strength of 7 Tesla.