gms | German Medical Science

Electrical Field Imaging (EFI) involves recording the intra-cochlear distribution of the electrical field that is produced in response to cochlear implant stimulation. The circuit required is in principle the same as used for recording a neural response: the EFI response being what would be considered stimulus artefact were a physiological response being recorded. A recording amplifier built into the implant is coupled to the recording electrodes. The measurement is taken at the same time as stimulation is performed, stored in the implant's memory and sent back to the fitting system via a back telemetry link. For each of the 16 contacts a recording is taken while stimulating one contact, respectively. This results in a 16x16 matrix with conventional impedances on the diagonal. As well as providing traditional electrode impedances, EFI gives insight into the current conduction paths inside the cochlea and may provide further anatomical information.  Processes on the surface of the electrode contacts can be separated from processes that influence the intra-cochlear current conduction. EFI was measured in 36 adults implanted with the Advanced Bionics CII cochlear implant system. 14 subjects had a HiFocus I electrode array without positioner, 19 with positioner and three a HiFocus II electrode system. In all subjects a non-uniform current spread was found with at least one area where a considerable amount of current leaves the cochlea. This area was found between contacts two and thirteen, which corresponds to the individual insertion depth between 20 and 580 degrees. Bone thickness, intracochlea tissue growth and proximity of the electrode contact to the medial wall influence the current path. Thus an area of current flow out of the cochlea may indicate the position of anatomical andmarks, e.g. the facial nerve canal.