High-resolution in situ imaging of cochlear implant electrode arrays in cat temporal bones using Tuned Aperture Computed Tomography (TACT).

OBJECTIVE: To determine the suitability of Tuned Aperture Computed Tomography (TACT) to generate high-resolution images of intracochlear electrode arrays, in situ, with sufficient anatomic and electrode detail to relate the location of individual electrode contacts to important anatomic landmarks in cat cadaveric temporal bones. The ultimate objective is to develop an imaging technology whereby variations in electrode location, relative to the target neural tissues, can be accurately determined and related to variations in performance with the cochlear implant. DESIGN: Cat temporal bones were implanted with an experimental scala tympani electrode array and an external fiducial landmark. A series of conventional 2D digital radiographs were collected from a variety of x-ray source projection angles and served as for generation of 3D volume renderings using the TACT software toolbox. The 3D renderings were then reoriented and resliced interactively to view the cochlear and electrode features of interest. RESULTS: Significant electrode and anatomical details could be visualized including the course of the electrode wires (<40 microm diameter), the location of all electrode contacts and the outline of the scala tympani. CONCLUSIONS: TACT generates high-resolution 3D images from 2D conventional radiographs. With TACT, the 3D renderings can be interactively reoriented and resectioned to permit visualization of any cochlear or electrode feature. In the present study, this aspect of TACT affords the opportunity to view of the location of each electrode contact relative to the adjacent cochlear features, such as the scalar walls. Because TACT uses conventional radiographic images to generate the volume renderings, the quality and resolution of the resulting 2D images do not suffer from artifacts characteristic of CT. These findings suggest that TACT may be a powerful tool for understanding the contribution of electrode placement to perceptual performance with the cochlear implant.

Optimization of TACT imaging protocols for in situ visualization of cochlear electrode arrays in cat temporal bones.

OBJECTIVE: To explore the effect of the number of two-dimensional (2D) images and x-ray projection angles on the resolution of reconstructed three-dimensional (3D) volumes of intracochlear electrode arrays in cadaveric cat temporal bones using Tuned Aperture Computed Tomography (TACT). DESIGN: Multiple 2D radiographs (basis images (BI)) of implanted cadaveric cat temporal bones were acquired using a range of projection angles, and imported into the TACT workbench. 3D volumes were reconstructed using varying numbers of BIs. Contrast resolution in the image was determined by comparing the contrast ratio (using maximum and minimum grayscale values) in specified anatomic areas of interest. RESULTS: Systematically increasing the number of BIs used in the reconstruction process resulted in a systematic increase in contrast resolution. Likewise, increasing the range of effective projection angles, as also the number of such angles used in the TACT computation also increased the contrast resolution of the resulting images. CONCLUSIONS: Precise determination of the location of cochlear implant electrodes in situ is critical to understanding the factors influencing efficacy of electrical stimulation of the deaf ear. Renderings generated with the TACT algorithm produce 3D images permitting visualization of implant electrode features and anatomic details with resolution sufficient to accurately localize electrode contacts within scala tympani. The quality of resulting images, evaluated as a function of image contrast, improved with a larger number of BIs in the reconstruction. Wider projection angles also improved image detail in addition to generating thinner slices. Any loss in contrast was compensated for by the number of BIs. TACT can thus be optimized to provide useful data to help characterize the location of intracochlear electrode arrays.

Quantitative anatomy of the guinea pig endolymphatic sac.

The endolymphatic sac is believed to represent one of the primary loci for endolymph volume regulation in the inner ear. Quantitative analysis of physiologic measurements from the endolymphatic sac requires knowledge of the anatomy of the structure, specifically the luminal volume and the variation of cross-sectional area with distance along the sac. Recently techniques have become available to make these measurements. In the present study, fixed, isolated specimens of the guinea pig endolymphatic sac were imaged by high-resolution magnetic resonance microscopy (MRM) or by histological serial sections. Structures were reconstructed and quantified using image analysis software. In specimens imaged by MRM the endolymphatic sac volume, including tissue and lumen, was 359 nl for the intraosseous region and 106 nl for the extraosseous region, totaling 465 nl for the entire structure. The luminal volumes were 131 nl for the intraosseous region and 13 nl for the extraosseous region, totaling 144 nl. In histological specimens the volume, including tissue and lumen, was 414 nl for the intraosseous region and 121 nl for the extraosseous region, totaling 535 nl for the entire structure. The luminal volumes were 152 nl for the intraosseous region and 26 nl for the extraosseous region, totaling 179 nl. Differences in volume estimates obtained by the two methods were not statistically significant and variation was dominated by inter-specimen variation. Pooling the data, the total volume of the endolymphatic sac in the guinea pig including tissue and lumen was 506 nl (S.D. 100, n=17) and the volume of the lumen was 169 nl (S.D. 48, n=14).