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sphere. The resolution was between 30 and 35 % o f the diameter o f the head, and the peak

^ The contralateral cortex is that on the opposite side to the stimulation: a right motor stimulus would be expected to activate the left motor cortex. In this case, the left cortex is contralateral, the right

impedance change in the image could be localised to within 20 % o f its expected position. The different noise characteristics o f data recorded from humans do not appear to affect the image quality significantly (Figure 4.15). However, there are features o f the human head which were not modelled in the head-shaped tank, such as the cerebro-spinal fluid, the anisotropy o f tissues in the head, and the interface between the electrode and the skin. Despite this, images obtained from the head-shaped tank did not give any reason to expect significant reconstruction errors in human images.

4 .5 .3 .5 L im ited resolution

The reconstruction algorithm developed in Chapter 3 was able to resolve two objects, under perfect conditions, when they were separated by 32 % of the image diameter. The algorithm used for human imaging was identical, except that 62 singular values were taken instead o f 115. The analysis in section 3.6.8 was repeated, and showed that when 62 singular values were taken, two objects could be resolved, under perfect conditions, when they were separated by 44 % o f the image diameter (Figure 4.16).

100 X

« 60

30 40 50

Separation / % diameter

Figure 4.16 : The ratio o f the central impedance change to the peak change plotted against the separation of two simulated objects. The two inset pictures show the equatorial planes o f 3D images with the two objects separated by 25 % and 62.5 % o f the image diameter, respectively.

This would suggest that under perfect noise-free conditions, two impedance changes occurring in the same hemisphere o f the brain could not be resolved, though changes occurring in opposite sides o f the brain could be distinguished. In human data, the resolution would depend on the noise in the data and errors caused by the shape o f the head, but it is likely that the dip between two impedance peaks would not be distinguishable from noise, even if the peaks were widely separated.

4.5.3.6 Baseline activation

A poor experimental paradigm can lead to brain activation in the baseline period, which can manifest itself as an activation decrease during stimulation. For example, daydreaming, falling asleep or a loss o f concentration during the baseline period, all o f which were reported anecdotally by volunteers, would cause a change in blood flow in the frontal cortex, which would manifest itself as an opposite change in blood flow (and therefore impedance) in the stimulus frames. This effect could be reduced with a faster EIT system, as this would allow a faster paradigm. This would also allow the paradigm to be improved, for example looking at the difference between left and right stimulus or an isoluminant grey screen compared to a chequerboard. The aim o f the paradigm used in this work - stimulus compared to rest - was intended to maximise the difference in activation but this might have led to unwanted activation elsewhere.

4.5.3. 7 Common-mode errors

The differential voltage measured in EIT is superimposed on a common-mode signal which depends on instrumentation and measurement errors. See Rigaud and Morucci (1996) and Boone and Holder (1996) for reviews. A common-mode voltage will result if an unbalanced current is injected on the two current electrodes, or if components are mismatched. It is also possible to generate a common-mode voltage at the measurement electrodes if there is a significant impedance between the current source and the body under test, such as a high skin impedance. If this is the case, the entire body is held at a raised potential which depends largely on the skin-electrode impedance.

The common mode voltage could exceed the differential voltage by up to 70 dB (Boone and Holder 1996). This is similar to quoted values for the common-mode rejection ratio o f an EIT system and the measured common-mode signal could therefore be equal to, or even greater than,

differential voltage, which changes with the stimulus, and a common mode voltage, which remains constant. This means that the measured impedance change is effectively attenuated - if the common mode voltage was equal to the differential voltage, the measured impedance change would be halved. If the impedance o f the skin-electrode interface varied from electrode to electrode, then certain impedance measurements would be attenuated more than others. This would distort the measured impedance profile, which would reduce the accuracy o f the reconstructed images.

This was tested indirectly during experiments used to determine the effect o f removing the noisiest electrodes as part o f the pre-processing technique. In these experiments (section 4.3.3), the impedance changes in measurements selected at random from a full image data set acquired in a saline-filled tank, were set to zero. The localisation o f the peak impedance change in images reconstructed from these reduced data sets did not change if up to half o f all measurements were rejected. If removing random measurements completely had no effect on image quality, it is unlikely that common mode voltages, which would effectively cause random attenuation o f the measurements, would significantly distort the images.

4 .5 .3 .8 Slow sam p lin g rate o f EFT system

As discussed above, a slow, but accurate EIT system was used for this work, meaning that a single image consisted o f only 258 measurements and took 25 s to acquire. This limited both the temporal resolution and the amount o f information available to reconstruct each image. It is possible that the cardiac or respiratory cycles occurring repeatedly within each image could cause some image distortion, but this is unlikely, as between 6 and 8 experiments were repeated in each subject, and this would average out physiological fluctuations.

A potentially more serious problem is the limited number o f measurements available for each image. In principle, 419 independent measurements^ are available from 31 electrodes, meaning that, with a faster imaging system, almost twice as much information can be obtained for each image than was available in this study.

4.5.3.9 A lternative mechanisms f o r impedance change

The discussion so far, and most o f the discussion in the literature, has assumed that changes in blood flow are the dominant mechanism for impedance changes during evoked responses. Most o f the alternative mechanisms by which impedance might change were discussed in Chapter 2 in the context o f epilepsy and much o f that discussion applies equally well here.

Cell swelling is likely to be the largest impedance change after blood flow. Whilst it certainly occurs in experimental animals during epilepsy, there is little evidence that the activation which occurs during evoked responses in humans is enough to cause cell swelling.

During epilepsy, local brain temperature can increase due to the increased volume o f warmer blood from the body core. In the intact human brain, however, the brain is maintained at a similar temperature to the rest o f the body core, so increased blood is unlikely to cause a significant change in brain temperature. Indeed, the temperature o f the brain could be slightly warmer than the body core in humans, particularly in brain-injured patients (Henker et al.

1998).

Finally, during epilepsy, the pressure in the cerebro-spinal space increases (Minns and Brown 1978, Gabor et al. 1984). This could cause movement o f cerebro-spinal fluid, which could lead to significant confounding impedance changes. If the brain were to swell locally, due to an increase in blood volume, CSF could be displaced from the region o f the stimulus. A displacement o f CSF would cause an increase in impedance which could be similar in amplitude to the impedance decrease due to the increase in blood volume. The reliability o f single channel impedance measurements suggests that this mechanism is unlikely to be significant.