ELECTRICAL IMPEDANCE TOMOGRAPHY OF BRAIN FUNCTION: OVERVIEW AND TECHNICAL CONSIDERATIONS

Louise C Enfield, David S. Holder
Department of Clinical Neurophysiology University College London, LONDON, UK

Abstract: Electrical Impedance Tomography has the potential to provide a valuable new method for brain function, as it is portable, inexpensive and fast.  It could potentially  be used to image impedance changes related to neuronal depolarization over milliseconds, or those related to metabolic recovery processes over tens of seconds. Recent work, using a new system optimised for imaging brain function with scalp electrodes, has demonstrated reproducible impedance changes in humans during evoked activity, but reconstructed images were disappointing. Design of a system for imaging human brain function includes a flexible electrode addressing arrangement to permit 3-D imaging, and provision of a small headbox on a long lead so that recordings can be made continuously in ambulant subject. 

1. Introduction

Biomedical EIT has been actively researched for about 15 years. Until recently, the principal interest in clinical applications lay in using it to image gastric emptying, lung ventilation and perfusion, lung oedema, and cardiac output. Our group has been developing EIT to imaging these impedance changes to look at brain function. It has unique potential in this respect, as it could, in theory, provide a uniquely useful method for providing a portable and inexpensive imaging system able to give images of brain function like Positron Emission Tomography or functional MRI.

The impedance changes that can occur in the brain are due to two mechanisms: 1) During ischaemia or energy supply failure, water moves from the extracellular space into cells; as current at the frequencies used passes almost entirely in the extracellular space. This causes impedance increases of many tens of per cent. 2) Blood flow, volume and temperature changes during activity also cause impedance decreases of a few per cent. However, the main obstacle is the skull, which will tend to divert applied current. Over the past decade, our group commenced by showing that reproducible EIT images could be obtained in physiological experiments, under the best possible conditions, and using the Sheffield Mark 1 system. We then built our own systems, optimized for use in imaging the head in ambulant humans, and have recently produced the first ever images of physiological activity in the brain in humans. Below, we present an overview of our work. 

2. Brain imaging with EIT

The original reason for developing EIT of brain function was that it may be able to image neuronal depolarization directly, with a time resolution of milliseconds [1]. In the late 1980’s, when this interest commenced, the only commercially available system was the Sheffield Mark 1 [2]. This employed 16 electrodes in a single ring, injected current through adjacent electrodes, and its algorithm made the assumption that the imaged volume was 2-D and had an initially homogeneous resistivity. The purpose of our first series of measurements was to determine if EIT images of brain function could be produced at all, under optimal conditions. Recordings were therefore made with a single ring of electrodes placed directly on exposed cortex in anaesthetised rats or rabbits. In this way, we produced the first images of impedance changes related to longer lasting and larger changes, such as during stroke [3], cortical spreading depression [4], physiologically evoked responses [5] [6], and epilepsy [7]. All of these were obtained in the anaesthetised rabbit with a ring of spring-mounted electrodes placed on the exposed superior surface of the cerebral cortex.

Figure 1. Example of EIT images during visual stimulation in human subjects. Six transverse slices are shown. Stimulation was a flashing light at 8 Hz for 2 min. An impedance increase occurs in the area of the visual cortex during the stimulus. Impedance decreases of about 0.3% were found in 51 subjects during visual, somatosensory, and motor tasks [6]

3. Reconstruction algorithms for EIT of the head

The head presents a unique problem to EIT reconstruction. The head is roughly hemispherical and cannot realistically be modeled other than in 3D. In addition, the high resistance of the skull, coupled with the low resistance short-circuit path of the scalp and CSF, act to reduce current density, and therefore sensitivity, in the brain. Its irregular shape also distorts current flow in the brain. To improve the sensitivity within the brain, it is necessary to maximise the proportion of the applied current which enters the brain. An efficient way of doing this is to optimise the way in which current is applied to the head. Bayford et al. [8] used a 2D finite element model to compare the sensitivity of optimised, adjacent and polar current drives when imaging the head. It was concluded that an impedance change of 1 % (such as that predicted during an evoked response) could be imaged using polar or optimised current drive. Recently a 3D algorithm has been developed, based on inversion of a sensitivity matrix. Initially, this employed an analytical model of a sphere [9], but has recently been extended to a multishell model [10] and one using a realistic FEM of the head. Development and validation of these latter two are currently in progress.  Our hope is that the use of these will permit the transition of EIT from experimental development into robust clinical and physiological use.

4. EIT systems developed at University College London.

Figure 2. The UCH Mark 1b EIT system, showing the head box, base box and controlling laptop.

A new system – the UCLH Mark 1b [11] – is capable of imaging in 3 dimensions with scalp electrodes and has 64 electrodes (Fig. 2). It works over a wide range of frequencies, from 225 Hz to 65 kHz, and allows 16 electrodes to be independently addressed. It has a miniaturised headbox on a 10m lead so it can be used on the ward for ambulatory monitoring for imaging with scalp electrodes, and meets relevant safety standards (BS 5724, IEC 601-1). The base unit controls the electrode switching combinations, supplies power to the head box and provides processing for the returning voltage signal. A 3D algorithm is used to reconstruct the images.

5. Future work

The UCH EIT system is optimised for imaging in the human head. We are currently developing the UCLH Mark 2, in collaboration with Prof. B. Brown, Royal Hallamshire Hospital, Sheffield.  This is similar to the Mark 1b, but will permit collection of multiple frequencies at the same time, which enables better tissue characterisation. After testing, it is proposed that work at The National Hospital for Neurology and Neurosurgery in the study of tumours and stroke will begin. Clinical trials in epilepsy in collaboration with Kings College Hospital and neonatal brain injury at University College London Hospital are also currently in progress.

6. Discussion and conclusion

At present, our research has shown that there appear to be underlying impedance changes sufficiently large to provide images. One potential problem is that the effects of skin impedance act to distort the true impedance change, so that reconstructed images are faulty. This could be addressed by improved electronic and electrode design. The other likely source of error is that the algorithm, which is based on a model of a homogeneous sphere, is too simple.  Work to use a realistic model, based on a finite element model, is in progress.

Our group currently has the capability to develop hardware and reconstruction algorithms, and undertake clinical trials. The current clinical studies are critical in achieving the next step and the interplay between development and testing is essential in refining the system. If it is successful in its present mode, this will constitute a significant addition to neuroimaging methods – the technique will image similar changes to fMRI but have practical advantages and be inexpensive. The long-term goal of EIT of the brain is to image action potentials and neuronal depolarisation with millisecond resolution and this work is taking place in parallel with the clinical trials. If this can be made to work, it will constitute a revolutionary advance in neuroscience technology.

7. References

[1]Holder, DS (1987).  The feasibility of developing a method of imaging neuronal activity  in  the human brain : A theoretical review. Med Biol Eng Comput, 25, 211.

[2]Brown, B H and A D Seagar (1987). The Sheffield data collection system. Clin. Phys. Physiol. Meas. 8(A): 91-7.

[3]Holder DS (1992).  Electrical impedance tomography with cortical or scalp electrodes during global cerebral ischaemia in the anaesthetized rat.  Clin Phys Physiol Meas, 13, 87 - 98.

[4]Boone K, Lewis AM and Holder DS (1994) Imaging of cortical spreading depression by EIT: implications for localisation of epileptic foci.  Physiol Meas, 15,  A189-A198

[5] Holder DS, Rao A and Hanquan Y (1996) Imaging of physiologically evoked responses by Electrical Impedance Tomography  with cortical electrodes in the anaesthetised rabbit Physiol Meas 17 A179-186

[6]Tidswell T, Gibson A, Bayford RH and Holder DS (2001).  Three-dimensional Electrical Impedance Tomography of human brain activity.  Neuroimage 13 283 – 294.

[7] Rao A, Gibson A and Holder D (1997) EIT images of electrically induced epileptic activity in anaesthetised rabbits Med Biol Engin Comput 35 Supp 327

[8] Bayford R. H, Boone KG , Hanquan Y and Holder DS(1996). “Improvement of the positional accuracy of EIT images of the head using a Lagrange multiplier reconstruction algorithm with diametric excitation.” Physiol. Meas. 17(A): 91-98.

[9]Tidswell AT, Gibson A, Bayford RH and Holder DS (2001). Validation of a 3D reconstruction algorithm for EIT of human brain function in a realistic head shaped tank. Physiol Meas 22 177 - 186.

[10] Liston A, Bayford RH, Tidswell AT and Holder DS (2002) A multi-shell algorithm to reconstruct EIT images of brain function. Physiol. Meas. 23, 105-120.

[11]Yerworth R, Bayford RH, Cusick G, Conway M, Holder DS (in press) Design and performance of the UCLH Mark 1b 64 channel Electrical Impedance Tomography (EIT) system, optimised for imaging brain function. Physiol. Meas. 23, 1