Department of Medical Physics and Biomedical Engineering, University College London, London, United Kingdom.
Physiol Meas. 2018 Aug 20;39(8):085003. doi: 10.1088/1361-6579/aad5f4.
Electrical impedance tomography (EIT) can be used to image impedance changes associated with epileptiform activity and so holds therapeutic potential for improving presurgical localisation of the ictal onset zone in patients with treatment-resistant epilepsy. There are two principal impedance changes which occur during seizures that may be imaged with EIT: (a) a fast, transient impedance decrease over milliseconds due to hypersynchronous neuronal depolarisation in individual ictal discharges; and (b) a larger, slow impedance increase caused by cell swelling over the course of the seizure. The magnitude of these signals is highly dependent on the carrier frequency of applied current used for obtaining impedance measurements. The purpose of this work was to characterise the frequency response of the fast and slow impedance changes during epileptiform activity.
Seizures were induced in anaesthetised rats by electrically stimulating the cerebral cortex. During each seizure, impedance measurements were obtained by delivering 50 µA, through two electrodes on an epicortical array, at one of 20 frequencies in the 1-10 kHz range. Recordings were demodulated to determine the magnitude of fast and slow impedance responses at each frequency.
The fast impedance change during averaged ictal discharges reached a maximal amplitude and signal-to-noise ratio (SNR) of -0.36% ± 0.05% and 50.2 ± 11.3, respectively, at 1355 Hz. At this frequency, the slow impedance change had an amplitude of 4.61% ± 1.32% and an SNR of 545 ± 125, which did not significantly change across frequency (p > 0.01).
We conclude that the optimal frequency for imaging epileptiform activity is 1355 Hz, which maximises the SNR of fast neural changes whilst enabling simultaneous measurement of slow changes. These findings will inform future investigations aimed at imaging epilepsy in subcortical brain structures, where SNR is considerably reduced, and those using parallel, multi-frequency EIT.
电阻抗断层成像（EIT）可用于对与癫痫样活动相关的阻抗变化进行成像，因此在治疗耐药性癫痫患者中具有改善发作起始区术前定位的治疗潜力。在癫痫发作过程中可能会出现两种主要的阻抗变化，可以用 EIT 进行成像：（a）由于单个发作放电中的神经元去极化过度同步，在毫秒级内发生快速、短暂的阻抗降低；（b）在癫痫发作过程中细胞肿胀引起的较大、缓慢的阻抗增加。这些信号的幅度高度依赖于用于获得阻抗测量的施加电流的载波频率。这项工作的目的是描述癫痫样活动过程中快速和缓慢阻抗变化的频率响应。
通过对大脑皮层进行电刺激，在麻醉大鼠中诱发癫痫发作。在每次癫痫发作过程中，通过在皮层表面电极阵列上的两个电极施加 50µA 的电流，在 1-10kHz 范围内的 20 个频率之一下获得阻抗测量值。对记录进行解调，以确定每个频率下快速和缓慢阻抗响应的幅度。
在平均发作放电期间，快速阻抗变化达到最大幅度和信噪比（SNR）分别为-0.36%±0.05%和 50.2±11.3，频率为 1355Hz。在该频率下，缓慢阻抗变化的幅度为 4.61%±1.32%，SNR 为 545±125，在整个频率范围内没有显著变化（p>0.01）。
我们得出结论，用于成像癫痫样活动的最佳频率为 1355Hz，该频率可最大化快速神经变化的 SNR，同时能够同时测量缓慢变化。这些发现将为未来旨在成像皮质下脑结构中的癫痫以及使用并行、多频 EIT 的研究提供信息，在这些结构中 SNR 会大大降低。
