The Novel High-accuracy Impedance Tomography Enabled By The Time-of-flight EIT Via CHIRP Current Excitation (CHIRP-EIT)

There is currently no technique that can non-invasively image the functional activity in the brain with sufficient spatial and temporal resolution. In addition, there is a need to have a rapid, precise and portable imaging technique for a variety of medical applications spanning from stroke, where rapid imaging on the back of an ambulance can be life-saving, to conditions like acute respiratory distress syndrome (ARDS) along with a multitude of other acute conditions, treatment of which could be greatly improved with having bedside continuous imaging system.

The traditional Electrical Impedance Tomography (EIT) produces images of the internal electrical impedance of a subject using arrays of electrodes (usually 32) placed around the object of interest (e.g. human head). Imperceptible, very low amplitude known current is injected between a pair of electrodes at a time, while electric potentials are measured on the remaining electrodes. By rapid switching of current injections between the possible pairs of electrodes, multiple measurements are made which then can be reconstructed into the image of internal conductivity, the variations of which from the normal values are indicative of various pathologies (e.g. stroke). EIT could potentially be the technique enabling rapid portable and low-cost imaging solutions, but traditionally it results in poor quality blurry images because of the severe theoretical limitations.

Time-of-flight EIT can overcome all limitations and result in great improvement in spatial resolution, theoretically providing MRI-quality images with millisecond temporal resolution. The theory relies on the fact that if the current is injected in form of an ideal step function, within the conductive object the current spreads and different paths would take different times to arrive at an opposite electrode. By measuring the voltages at different times of arrival, it is possible to distinguish between the conductivities of all the above different paths, which theoretically will result in a clear high-resolution image. Although the technique is theoretically possible, in practice it was never performed because it is impossible to produce an ideal pulse delta function of a current, and there are additional distortions associated with wave propagation inside the complex conductive object.

The above challenges could be solved by employing temporally separated CHIRP excitation patterns (linear frequency modulation). This way of injecting the current is possible to produce in practice, and more importantly, would allow separation between the true time of arrival and all internal distortions within the object. Preliminary calculations showed that these CHIRP pulses would allow resulting images to have 1mm spatial resolution and 1 ms temporal resolution.

This will establish a completely new imaging technique with unique capabilities, which has the potential to revolutionise diagnostic medicine and perform life-saving changes in several areas of medical practice. In particular, this will disrupt neurology where there are no other alternative techniques for non-invasive imaging inside the human brain.