2 research outputs found
Functional imaging of the human brain using electrical impedance tomography
Electrical Impedance Tomography (EIT) is a technique for imaging the
spatial distribution of conductivity inside a body using the boundary voltages, in
response to applied current patterns, to reconstruct an image. Even though EIT has
been proved useful in several medical applications such as mechanical respiration
and ventilation monitoring of the lungs, its reported success in localising cerebral
conductivity changes due to brain stimulation is very scant.
In the case of the human head, the amplitude of the brain response to
stimulation is usually very small and gets contaminated with physiological noise
initiated from inside the cranium or the scalp. Three types of evoked responses were
experimentally investigated: auditory startle response (ASR), CO2 reactivity
response, and transient hyperaemic response (THR). ASR is expected to be a result
of the brain’s functioning processes. However, the responses to CO2 and THR are
expected to be due to cerebral blood volume or flow, due to physiological
intervention in blood supply. According to the results, even when the amplitude of
EIT measurements shows profound variation as in the case of CO2 reactivation, those
could not be physiologically linked to the targeted responses and have been shown to
be initiated from the scalp. The consistency of the measurements in the case of CO2
reactivation response was poor (37.50-50%). Meanwhile in the case of THR,
although the magnitude of conductivity changes was overall 50% smaller than the
previous cases, the subject movement was not necessary. This could be a reason that
the consistency of THR case was very good (87%), and this can emphasize the
necessity to maintain the changes in the scalp at minimum levels. In the case of ASR
the response magnitude was very small (six times smaller than the CO2 reactivity
case), and the evoked response can be detected with only 50% consistency.
To measure very small EIT signals (such as those expected due to brain
function) effectively, one must improve the sensitivity of the measurements to
conductivity changes by increasing the excitation current. The functional EIT for
Evoked Response (fEITER) system used in our investigations was modified from its
initial configuration to increase its excitation current from 1 mApk-pk to 2 mApk-pk or 1
mArms. The bit-truncation in the process of Phase-Sensitive Detection (PSD) has also
been improved, to modify the original 16-bit data readout to be 24-bit data readout.
These improvements have doubled the instrument’s sensitivity, and have
substantially reduced the truncation error to about 183 times. The quality of the
physiological waveform was also significantly improved. Therefore, one could study
more effectively very fast brain response using the modified system. For example,
the latency of responses can be more precisely extracted, or the monitoring of the
conductivity change in a period of only a few tens of milliseconds is then possible.
The reconstruction of brain images corresponding to these physiologically
evoked responses has been the ultimate goal of this thesis. To ensure obtaining the
correct images, some crucial issues regarding EIT reconstruction were firstly
investigated. One of these issues concerns the modelling error of the numerical head
models. The reconstruction requires an accurate model capturing the geometry of the
subject’s head with electrodes attached and accurate in-vivo tissue conductivities.
However, since it is usually impractical to have a personalised model for each
subject, many different head models (including a subject model) were constructed
and investigated, to evaluate the possibility of using a generic model for all subjects.
The electrode geometry was also carefully included into the models to minimise
error. Another issue concerns the appropriate reconstruction algorithm. A novel nonlinear
reconstruction method, based on the difference imaging approach and
Generalized Minimal Residual method (GMRes) algorithm, with optimal parameters
and prior information, was proposed to deal with significant modelling errors. With
this algorithm, the experimental results showed that it is possible to use a generic
model for reconstructing an impedance change, but the magnitude of the change
should be rather small. The last issue tackled was regarding the a priori choice of
model parameters, and in particular the tissue conductivities. The tissue
conductivities of the scalp and the skull were also estimated by a proposed
methodology based on the Gauss-Newton method. The estimation showed that,
compared to previous reported values, the conductivity of the scalp was higher, at
0.58 S/m, and that of the skull lower, at 0.008 S/m. Eventually, by exploiting the
hardware and firmware advances in the measuring instrument in conjunction with the
proposed modelling and reconstruction algorithm, processing our experimental EIT
data captured on human heads and a head-like tank confirm that the localisation and
imaging of conductivity changes occurring within the head is indeed possible.
From the low quality measurements in the case of the CO2 reactivity
response, the reconstructed images of this response do not reflect the true
conductivity change. The consistency of the images to localise the sources of the
changes was very poor (0-50%), i.e. the conductivity changing locations in the
images were likely to be random. Our analysis suggests that the changes inside the
cranium are likely to be due to the large change in the scalp. In the case of THR, the
reconstructed images were able to localise the response in a similar manner to what
had been found on the measurements, and the consistency was quite high (76%).
Meanwhile, in the case of ASR, surprisingly the consistency of the images was 82%,
much higher than the consistency of the measurements, which was only 50%. This
was because the changing amplitude of the measurements was too small to be
noticed by visualisation, and it was practically cumbersome to investigate all
measurements. This statistic confirms that image reconstruction can reveal
information that is not directly apparent by observing the measurements.
In summary, EIT can be used in brain (function) imaging applications to
some extent. The targeted response, which typically originates from inside the
cranium is always infused with neurophysiological noise or physical noise at the
scalp, and the amplitude of noise determines the possibility to localise the changes. It
is also necessary for the desired response to have sufficiently large amplitude. These
results show that EIT has been successful in THR and ASR, but for CO2 reactivity
response, EIT lacks the necessary sensitivity