Röschmann P
Philips GmbH Forschungslaboratorium Hamburg, Federal Republic of Germany.
Magn Reson Med. 1991 Oct;21(2):197-215. doi: 10.1002/mrm.1910210205.
The threshold conditions for an auditory perception of pulsed radiofrequency (RF) energy absorption in the human head have been studied on six volunteers with RF coils for magnetic resonance (MR) imaging. For homogeneous RF exposure with MR head coils in the 2.4- to 170-MHz range and pulse widths 3 microseconds less than or equal to Tp less than 100 microseconds, the auditory thresholds were observed at 16 +/- 4 mJ pulse energy. Localized RF exposure with optimized surface coils positioned flush with the ear lowers the auditory threshold to only 3 +/- 0.6 mJ. The hearing threshold of RF pulses with Tp greater than 200 microseconds occurs at more or less constant peak power levels of typically 150 +/- 50 W for head coils and as low as 20 W for surface coils. The results from this study confirm theoretical predictions from a thermoelastic expansion model and compare well with reported thresholds from near field antenna measurements at 425 to 3000 MHz. Details of the threshold dependence on RF pulse length reveal primary sites of RF to acoustic energy conversion at the mastoid and temporal bone region and the outer layer of the brain from where thermoelastically generated pressure transients excite audible pressure waves at the resonance modes of the skull around 1.7 kHz and of the brain around 11 kHz. If not masked by usually dominating noise from switched gradients, the conditions for hearing RF pulses, as applied to head coils in MR studies with flip angle alpha at main field B0, is given by Tp/ms less than or equal to 0.4 (alpha/pi)B0/[T]. At peak power levels up to 15 kW presently available in clinical MR systems, there is no evidence known for detrimental health effects arising from the RF auditory phenomenon which is a secondary cause associated with primary RF to thermal energy conversion in body tissues. To avoid the RF-evoked sound pressure levels in the head rising above the discomfort threshold at 110 dB SPL, an upper limit of 30 kW applied peak pulse power is suggested for head coils and 6 kW for surface coils.
利用用于磁共振（MR）成像的射频线圈，在6名志愿者身上研究了人体头部对脉冲射频（RF）能量吸收产生听觉感知的阈值条件。对于在2.4至170 MHz范围内使用MR头部线圈进行的均匀RF暴露，以及脉冲宽度3微秒≤Tp＜100微秒的情况，观察到听觉阈值为16±4 mJ脉冲能量。使用与耳朵齐平放置的优化表面线圈进行局部RF暴露，可将听觉阈值降低至仅3±0.6 mJ。对于Tp大于200微秒的RF脉冲，头部线圈的听觉阈值出现在大致恒定的峰值功率水平，通常为150±50 W，而表面线圈则低至20 W。本研究结果证实了热弹性膨胀模型的理论预测，并与425至3000 MHz近场天线测量报告的阈值进行了很好的比较。阈值对RF脉冲长度的依赖性细节揭示了RF到声能转换的主要部位在乳突和颞骨区域以及大脑外层，热弹性产生的压力瞬变从这些部位在颅骨约1.7 kHz和大脑约11 kHz的共振模式下激发可听压力波。如果没有被通常占主导的开关梯度噪声掩盖，在主磁场B0下使用翻转角α的MR研究中，应用于头部线圈时听到RF脉冲的条件为Tp/ms≤0.4(α/π)B0/[T]。在临床MR系统目前可用的高达15 kW的峰值功率水平下，没有已知证据表明RF听觉现象会产生有害健康影响，RF听觉现象是与身体组织中RF到热能转换相关的次要原因。为避免头部的RF诱发声压水平超过110 dB SPL的不适阈值，建议头部线圈的施加峰值脉冲功率上限为30 kW，表面线圈为6 kW。
