EEG dipole modelling attempts to reconstruct the position o f dipoles within the brain, given voltages recorded from the scalp (section A 1.2.4). This problem is closely related to that o f EIT o f the head where we attempt to locate impedance changes within the brain, given scalp voltages. Both techniques need to compensate for the dismption the skull makes to current flow in the head.
As in EIT, the position o f a current dipole is found from scalp potentials by inversely solving Poisson’s Equation. This is typically done numerically (for a realistically shaped head model), iteratively ( if the head is modelled as a series o f concentric spheres) or analytically (if the head is taken to be homogeneous). Ary et al. (1981) attempted to map the concentric sphere model onto a homogeneous model in order to simplify the calculation. They calculated the scalp potentials for a dipole in the concentric spheres model and for the homogeneous case and asked if, by moving the dipole within the homogeneous model, they could minimise the difference in scalp potential predicted by the two models. They produced a table giving the position o f a dipole in the concentric spheres model and the corresponding position in the homogeneous model which best replicated the scalp potentials. There is almost a linear relation between the two, equivalent to simply increasing the thickness o f the skull from 0.05 to 0.3 times the radius o f the head and treating the resulting model as homogeneous. A second order correction becomes significant at 0.7 - 0.8 times the radius o f the head (roughly corresponding to the cortex). They estimated that the error in localisation could be as low as ±2 % after taking into account variations in scalp and skull thickness, resistivity and geometry and the asphericity o f the head. This is now an accepted technique in dipole modelling (Scherg 1990).
1.7.7
Safety Considerations
The sensitivity o f an EIT system depends on the current which is applied to the object under examination. This is limited by the effect o f electricity on the body, which in turn depends on the frequency o f the applied current (Smallwood et al. 1983). Below 0.1 Hz, the tissue directly underneath the electrode can be electrolysed if the applied current is greater than 100 pA. At higher frequencies, an applied current can directly stimulate neural tissue if a few tens o f mV are developed across the nerve membrane within half a cycle o f the applied current. This effect is greatest at the mains supply frequency o f 50 - 60 Hz where the threshold for perception o f a current applied through the skin is around 1 mA. At this
fibrillation and death. These threshold values remain roughly constant up to 1 kHz, above which the threshold increases by a factor o f 10 for each tenfold rise in frequency. This remains the predominant effect throughout the range o f frequencies used for EIT. Above 100 kHz, heating is the major effect, and is often used intentionally in diathermy and hyperthermia. Localised burning o f the skin can be expected if the applied current density exceeds 5 mA mm'^ for 10 s.
The relevant British Standard is BS BN 60601 (British Standards Institute 1990) which allows a patient auxiliary current (which is the current intentionally applied to the patient) o f 0.1 mA kHz'^ above 1 kHz to a maximum o f 10 mA. These limits are intended to be an order o f magnitude below the minimum current level known to cause damage. The UCH-EIT system (section 1.3.4) was built to this standard and the current amplitude is limited to these levels.
BS EN 60601 treats the heart as a special case to which a more stringent set o f limits apply, and it is worth considering whether it is also appropriate to treat the head separately. The implications o f stimulating the brain are likely to be more severe than, for instance, the arm. However, the stimulation threshold for neurones in the brain is the same as that elsewhere, and any current applied to the scalp is attenuated by the skull. This suggests that the current limits in BS EN 60601 are appropriate for scalp electrodes.
These safety considerations have implications for the choice o f frequency to be used in EIT. If the impedance change is independent o f frequency, a greater current can be applied at higher frequencies, thus increasing sensitivity - although lead capacitance and instrumentation problems also increase with frequency.
There have been claims from epidemiological studies that current amplitudes below those which cause stimulation or heating may have other physiological or psychological effects such as cancer and depression but these are still controversial. These claims have generally been made for frequencies much lower {e.g. mains power transmission) and much higher
{e.g. mobile phones) than those used in EIT. In a comprehensive review o f theoretical mechanisms, and experimental and epidemiological evidence for biological effects o f 50 - 60 Hz electromagnetic fields, Adair (2000) found that field strengths are less than those due to the Brownian motion o f ions in the body and found no experimental or epidemiological evidence o f biological effects o f these fields, with the possible exception o f some weak, non
Hulbert 1998) are less dismissive and are inconclusive as to whether there is an increased risk o f brain cancer or leukaemia in individuals exposed to low frequency fields at home or at work. The study by Linet et al. (1997) was typical in finding an increased risk for some conditions (odds ratio 1.53 for time-weighted magnetic fields in the home > 0.2 pT for leukaemia) but a contradictory result (odds ratio 1.01 for fields > 0 .5 pT) for others. It is important to monitor these studies but even if there is an increased risk, it is unlikely that it will have an effect over the timescale o f an EIT examination.