Modelling of axonal fields in the optic nerve for direct MR detection studies
Modelling of axonal fields in the optic nerve for direct MR detection studies
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DOI：
10.1016/j.imavis.2008.05.001
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发表时间：
2009-03-03
影响因子：
4.7
通讯作者：
Cook, Greg G.
中科院分区：
文献类型：
--
作者：
Paley, Martyn N. J.;Chow, Li Sze;Cook, Greg G.
许多研究已经表明，通过核磁共振成像直接检测神经元放电是可能的。视神经将所有视觉信息从眼睛传递到大脑，是这些测量的一个特别有希望的目标。然而，假设轴突放电的影响可能无法检测到，因为单个放电事件产生约1ms持续时间的双极波形，其影响应该抵消油mr。已经开发了一种模拟磁调制的方法，该方法可以由视神经在较长时间内以不同的放电速率产生。计算了模型神经节细胞轴突阵列的霍奇金-赫胥黎方程，假设轴突阵列作为电压-脉冲频率转换器。研究了调制波形与相对动作电位触发时间的关系。虽然模拟波形在开始时是双极性的，但在MR采集期间，不同的频率结合在一起产生了一个很大的正波形。模拟只包括对比度亮度变化，不包括颜色、空间相关性或视网膜中其他更复杂的处理。利用梯度回波序列在3t下对经实际模拟轴突波形调制的幻像进行重建交流电流估计（GRACE）方法分析。在3岁的成年志愿者中，视神经也用同样的方法在频闪灯的视觉刺激下成像。对磁体外频闪灯刺激时眼睛位置的数字化视频记录的分析显示，在扫描的短时间内，与应用频闪灯频率没有相关性。根据GRACE理论，在10名志愿者的13项研究中，只有两项研究显示，在TE = 39 ms的所有回波时间内，视神经图像在与应用刺激一致的位置上存在微弱但明显的一谐波鬼影，检出率仅为15%，因此在这些实验中没有明确的证据可以直接检测。(C) 2008 Elsevier B.V.版权所有
A number of studies have now shown that direct detection of neuronal firing by MRI may be possible. The optic nerve carries all visual information from the eye to the brain and is a particularly promising target for these measurements. However, it has been assumed that the effects of axonal firing may not be detectable, as a single firing event produces a bipolar waveform of around 1 ms duration whose effects should cancel oil MR. A simulation of the magnetic modulation which could feasibly be produced by the optic nerve over an extended period and with different firing rates has been developed. The Hodgkin-Huxley equations were calculated for ail array of model ganglion cell axons which were assumed to act as voltage to pulse frequency converters. Dependence of the modulating waveform on relative action potential firing start time was investigated. Although the simulated waveforms were bipolar at the beginning, during the period of MR acquisition the different frequencies combine to produce a largely positive waveform. The simulation included only contrast luminance changes and not color, spatial correlations or other more sophisticated processing in the retina. A gradient echo sequence was used at 3 T to create images for analysis by the ghost reconstructed alternating current estimation (GRACE) method from phantoms subject to current modulation by the actual modelled axonal waveforms. The optic nerve was also imaged using the same method during visual stimulus by a strobe light in adult human volunteers at 3 T. Analysis of digitized video recordings of eye locations during strobe stimulation outside the magnet showed no correlation with the applied strobe frequency over the short duration of the scans. images of the optic nerves at ail echo time of TE = 39 ms had weak but significant first harmonic ghosts in locations consistent with the applied stimulus as calculated from GRACE theory in just two out of thirteen studies on 10 volunteers and a detection rate of only 15% providing no clear evidence for direct detection in these experiments. (C) 2008 Elsevier B.V. All rights reserved.