Röschmann P
Philips GmbH Forschungslaboratorium Hamburg, Federal Republic of Germany.
Med Phys. 1987 Nov-Dec;14(6):922-31. doi: 10.1118/1.595995.
This study presents experimental results about the effective depth of penetration and about the radiofrequency (rf) power absorption in humans as a function of frequency. The frequency range investigated covers 10 up to 220 MHz. For the main part, the results were derived from bench measurements of the quality factor Q, and of the resonance frequency shift due to the loading of the coil. Different types of head-, body-, and surface coils were investigated loaded with volunteers or metallic phantoms. For spin-echo imaging at 2 T (85 MHz), the local specific absorption rate (SAR) was found to be approximately equal to 0.05 W/kg using a pi pulse of 1-ms duration and pulse repetition time TR = 1 s. Measurements of the quality factor Q as a function of frequency show that the SAR depends upon the frequency f according to approximately f2.15. The effective depth of rf penetration as derived drops from 17 cm at 85 MHz to 7 cm at 220 MHz. Head imaging with B1 penetrating from practically all sides into the object should be possible up to 220 MHz (5 T) with SAR values staying within the local limit of 2 W/kg as set by the FDA. Whole-body imaging of large subjects as well as surface coil imaging is depth limited above 100-MHz frequency. Perturbation methods are applied in order to separate the total rf power deposition in the patient into dielectric and magnetic contributions. The observed effects due to interactions of rf magnetic fields with biological tissue contradict predictions based on homogeneous tissue models. A refined tissue model with regions of high electrical conductivity, subdivided by quasi-insulating adipose layers, provides a rationale for a better understanding of the underlying processes. At frequencies below 100 MHz, the rf power deposition in patients is apparently more evenly distributed over the exposed body volume than currently assumed.
本研究给出了关于人体中射频（rf）穿透有效深度以及作为频率函数的射频功率吸收的实验结果。所研究的频率范围涵盖10至220MHz。在主要部分，结果源自对品质因数Q以及由于线圈加载导致的共振频率偏移的实验台测量。研究了不同类型的头部、身体和表面线圈，这些线圈加载有志愿者或金属模型。对于2T（85MHz）下的自旋回波成像，使用持续时间为1ms的π脉冲且脉冲重复时间TR = 1s时，发现局部比吸收率（SAR）约等于0.05W/kg。品质因数Q作为频率函数的测量表明，SAR大约根据f2.15依赖于频率f。推导得出的射频穿透有效深度从85MHz时的17cm降至220MHz时的7cm。对于B1从几乎所有方向穿透进入物体的头部成像，在220MHz（5T）时应该是可行的，且SAR值保持在美国食品药品监督管理局（FDA）设定的2W/kg局部限值内。对于大型受试者的全身成像以及表面线圈成像，在频率高于100MHz时深度受到限制。应用微扰方法以便将患者体内的总射频功率沉积分离为介电和磁贡献。观察到的由于射频磁场与生物组织相互作用产生的效应与基于均匀组织模型的预测相矛盾。一种具有高电导率区域且由准绝缘脂肪层细分的精细组织模型为更好地理解潜在过程提供了理论依据。在频率低于100MHz时，患者体内的射频功率沉积显然比目前所认为的在暴露身体体积上分布得更均匀。
