Tidswell A T, Gibson A, Bayford R H, Holder D S
Department of Clinical Neurophysiology, University College, London, UK.
Physiol Meas. 2001 Feb;22(1):177-85. doi: 10.1088/0967-3334/22/1/321.
Previous work has demonstrated that electrical impedance tomography can be used to image human brain activity during evoked responses, but two-thirds of the reconstructed images fail to localize an impedance change to the expected stimulated cortical area. The localization failure may be caused by modelling the head as a homogenous sphere in the reconstruction algorithm. This assumption may lead to errors when used to reconstruct data obtained from the human head. In this study a 3D reconstruction algorithm, based on a model of the head as a homogenous sphere, was characterized by simulating the algorithm model, the head shape and the presence of the skull in saline-filled tanks. EIT images of a sponge, 14 cm3 volume with a resistivity contrast of 12%, were acquired in three different positions in tanks filled with 0.2% saline. In a hemispherical tank, 19 cm in diameter, the sponge was localized to within 3.4-10.7% of the tank diameter. In a head-shaped tank, the errors were between 3.1 and 13.3% without a skull and between 10.3 and 18.7% when a real human skull was present. A significant increase in localization error therefore occurs if an algorithm based on a homogeneous sphere is used on data acquired from a head-shaped tank. The increased error is due to the presence of the skull, as no significant increase in error occurred if a head-shaped tank was used without the skull present, compared to the localization error within the hemispherical tank. The error due to the skull significantly shifted the impedance change within the skull towards the centre of the image. Although the increased localization error due to the skull is not sufficient to explain the localization errors of up to 50% of the image diameter present in the images of some human subjects, the future use of a realistic head model in the reconstruction algorithm is likely to reduce the localization error in the human images due to the presence of the skull.
先前的研究表明，电阻抗断层成像技术可用于在诱发反应期间对人类大脑活动进行成像，但三分之二的重建图像未能将阻抗变化定位到预期的受刺激皮质区域。定位失败可能是由于在重建算法中将头部建模为均匀球体所致。当用于重建从人类头部获取的数据时，这种假设可能会导致误差。在本研究中，基于将头部建模为均匀球体的三维重建算法，通过模拟算法模型、头部形状以及在充满盐水的水箱中颅骨的存在来进行表征。在充满0.2%盐水的水箱中的三个不同位置获取了体积为14立方厘米、电阻率对比度为12%的海绵的电阻抗断层成像（EIT）图像。在直径为19厘米的半球形水箱中，海绵被定位在水箱直径的3.4%至10.7%范围内。在头部形状的水箱中，没有颅骨时误差在3.1%至13.3%之间，存在真实人类颅骨时误差在10.3%至18.7%之间。因此，如果将基于均匀球体的算法用于从头部形状的水箱获取的数据，定位误差会显著增加。误差增加是由于颅骨的存在，因为与半球形水箱内的定位误差相比，如果在没有颅骨的情况下使用头部形状的水箱，误差没有显著增加。由于颅骨导致的误差使颅骨内的阻抗变化明显向图像中心偏移。尽管由于颅骨导致的定位误差增加不足以解释一些人类受试者图像中出现的高达图像直径50%的定位误差，但在重建算法中未来使用逼真的头部模型可能会减少由于颅骨存在而导致的人类图像中的定位误差。
