Imaging of electrode position in relation to electrode functioning after cochlear implantation

Five adult patients were included in this study. All of them were subsequently implanted with a Nucleus Contour cochlear implant (Cochlear Corp., Lane Cove, Australia) between October 2005 and November 2005 at the University Medical Center Utrecht. In all patients, the electrodes were inserted as planned, that is according to the insertion depth that had been set out. There were no surgical problems or complications. The patient characteristics are shown in Table 1. The causes of deafness in these patients were meningitis, a congenital and hereditary etiology, and trauma. In one patient, no etiology could be identified.

The device was activated using the Windows-based Diagnostic and Programming System software (Win-DPS, release version 126) provided by the manufacturer (Cochlear Corp., Lane Cove, Australia). The stimuli consisted of biphasic current pulses presented at a level of 100 clinical current units, which is approximately 76 μA, and an impulse duration of 25 μs/phase. In the present study, the EI was measured in common ground (CG) mode intra-operatively. In this mode, the impedance is measured between an intracochlear electrode and all other intracochlear electrodes coupled in parallel. Shortcut or open-circuit electrodes were not considered for data analysis. According to the specifications of the manufacturer, this implied the exclusion of data with impedances below 0.7 and above 20 kOhm, respectively.

The registration procedure of the ECAP responses was identical to the one we used in previous experiments [17]. The initial examination took place intra-operatively immediately after implantation. The ECAP responses were recorded using a computer equipped with NRT software, version 3.0, distributed by the Cochlear Corporation. We used a modified version of the protocol described by Abbas et al. [18]. Our test parameters are presented in Table 2. The stimulation mode was monopolar (MP1 mode), using the extra cochlear ball reference electrode. Masker advance, which is the masker-probe interval, was fixed at 500 μs. As a rule, the sampling delay, i.e., the interval between stimulation and initiation of sampling, was set at 100 μs. In the event of amplifier saturation, the delay was adjusted until a satisfactory response was obtained. The amplifier gain was set at 60 dB but was decreased to 40 dB in cases of amplifier saturation. We set the number of sweeps at 100, whereas Abbas set it at 50. In conformance with the Abbas protocol, we set the pulse duration at 25 μs per phase. The stimulation levels are described in terms of current level (CL), a quantity defined by the Cochlear Company. The CL ranges from 1 to 255 current units (CU), which corresponds to electrical currents from 10 μA to 1.75 mA. Our aim was to test 22 electrodes (electrodes 1–22) intra-operatively. The electrodes selected for recording were two positions above the stimulation electrode. Thus, for each patient, we selected the second electrode, N + 2, from the stimulation electrode, N, in the apical direction. An exception was made for electrodes 21 and 22, for which the recording electrodes were 19 and 20, respectively.

Multislice CT was performed 6 weeks after implantation but before the first fitting of the speech processor, using the Philips Brilliance 64-slice CT (Philips Medical Systems, Cleveland, OH, USA). The patients were scanned with the following parameters, according to the imaging protocol we used in previous experiments [11]: 140 kV; effective tube current – time product of 260 mA/s; rotation time 0.5 s. In order to maximize spatial resolution, the collimation was reduced to 2 × 0.5 mm. To reduce aliasing artifacts a low pitch factor of 0.35 was used. The corresponding measures of local exposure dose (CT dose indices, CTDI) was 136 mGy. An ultrahigh resolution mode was used for image acquisition. A corresponding ultrahigh resolution filter was applied for image reconstruction. In order to further optimize resolution, a field of view of 51 mm was chosen, resulting in a pixel size of 0.1 mm. Datasets were transferred to a workstation (Easyvision; Philips, Best, The Netherlands) where 3-D reconstructions were created using multiplanar reformation (MPR), i.e., calculating slices along arbitrary sections. In our study, an MPR was made parallel to the basal turn of the cochlea and perpendicular to the modiolus and thus in the plane of the electrode array. Window width and window level were adjusted until both the cochlear tissues and the individual electrodes could be visualized.

All analyses were carried out with Wessa statistical software [19]. The Kendall tau rank correlation was used analyzing the correlation between the ECAP, electrode impedance versus electrode distance.