The role of radiographic phase-contrast imaging in the development of intracochlear electrode arrays

Assessing the Placement of the Cochlear Slim Perimodiolar Electrode Array by Trans Impedance Matrix Analysis: A Temporal Bone Study

New cochlear implant (CI) electrode arrays provide softer insertion dynamics; however, due to their high flexibility, the possibilities of fold-overs or intraoperative displacements must be taken into account. The position of each individual electrode can only be determined by using high-resolution computed tomography or cone-beam CT. The trans-impedance matrix test (TIM) is an electrophysiological method based on electric field imaging that can provide images of electrode position and electrode folding. Objective: In this experimental research, we evaluated the result of TIM as a method of monitoring cochlear insertion for a precurved slim modiolar electrode array in fresh human temporal bones by analyzing the transimpedance matrix patterns and their correlation with electrode position using high-resolution computed tomography. Material and Methods: Sixteen slim modiolar electrode arrays were inserted into eight fresh Human Temporal Bones. Eight electrodes were inserted according to the correct methodology of insertion, and eight were intentionally folded over. After all insertions, a trans-impedance matrix analysis and a Cone Beam CT (CBCT) were performed in each temporal bone. Results: If we correlated the TIM patterns with the radiological electrode position, we observed that better electrode intracochlear positions indicated more “homogeneous” TIM patterns (intracochlear voltage dropped monotonically as the distance between stimulation and recording contact increased, both toward the apex and toward the base). A correlation where fold-over was detected in the TIM results was found in all eight temporal bone radiological findings. Conclusions: Trans-Impedance Matrix patterns were correlated with the radiological CI electrode position. When a tip fold-over appeared, a matrix with a secondary ridge in addition to the primary ridge was observed in all cases. TIM can be an effective method in the control of electrode positioning.

Micro-CT versus synchrotron radiation phase contrast imaging of human cochlea

High-resolution images of the cochlea are used to develop atlases to extract anatomical features from low-resolution clinical computed tomography (CT) images. We compare visualization and contrast of conventional absorption-based micro-CT to synchrotron radiation phase contrast imaging (SR-PCI) images of whole unstained, nondecalcified human cochleae. Three cadaveric cochleae were imaged using SR-PCI and micro-CT. Images were visually compared and contrast-to-noise ratios (CNRs) were computed from n = 27 regions-of-interest (enclosing soft tissue) for quantitative comparisons. Three-dimensional (3D) models of cochlear internal structures were constructed from SR-PCI images using a semiautomatic segmentation method. SR-PCI images provided superior visualization of soft tissue microstructures over conventional micro-CT images. CNR improved from 7.5 ± 2.5 in micro-CT images to 18.0 ± 4.3 in SR-PCI images (p < 0.0001). The semiautomatic segmentations yielded accurate reconstructions of 3D models of the intracochlear anatomy. The improved visualization, contrast and modelling achieved using SR-PCI images are very promising for developing atlas-based segmentation methods for postoperative evaluation of cochlear implant surgery.

Biomedical studies on temporal bones of the first multi-channel cochlear implant patient at the University of Melbourne

OBJECTIVE

To analyse the temporal bones and implant of the first University of Melbourne's (UOM) patient (MC-1) to receive the multi-channel cochlear prosthesis.

METHODS

The left cochlea was implanted with the prototype multi-channel cochlear prosthesis on 1 August 1978, and the Cochlear versions CI-22 and CI-24 on 22 June 1983 and 10 November 1998, respectively. MC-1 died in 2007.

RESULTS

Plain X-rays of the temporal bones showed that after the CI-22 had been explanted seven electrode bands remained in situ. Micro-CT scans also revealed a partially united fracture transecting the left implanted and right control cochleae. Histology indicated a total loss of the organ of Corti on both sides, and a tear of the left basilar membrane. In addition, there was a dense fibrous capsule with heterotopic bone surrounding one proximal band of the CI-22 array that restricted its removal. This pathology was associated with dark particulate material within macrophages, probably due to the release of platinum from the electrode bands. Scanning electron microscopy (SEM) showed possible corrosion of platinum and surface roughening. Three-dimensional reconstruction of the cochlear histology demonstrated the position of the electrode tracts (C1-22 and CI-24) in relation to the spiral ganglion, which showed 85-90% loss of ganglion cells.

DISCUSSION AND CONCLUSIONS

This study confirms our first histopathological findings that our first free-fitting banded electrode array produced moderate trauma to the cochlea when inserted around the scala tympani of the basal turn. The difficulty in extraction was most likely due to one band being surrounded by an unusually large amount of fibrous tissue and bone, with an electrode band caught due to surface irregularities. Some surface corrosion and a small degree of platinum deposition in the tissue may also help explain the outcome for this long-term cochlear implantation.

Phase retrieval in in-line x-ray phase contrast imaging based on total variation minimization

State-of-the-art techniques for phase retrieval in propagation based X-ray phase-contrast imaging are aiming to solve an underdetermined linear system of equations. They commonly employ Tikhonov regularization - an L2-norm regularized deconvolution scheme - despite some of its limitations. We present a novel approach to phase retrieval based on Total Variation (TV) minimization. We incorporated TV minimization for deconvolution in phase retrieval using a variety of the most common linear phase-contrast models. The results of our TV minimization was compared with Tikhonov regularized deconvolution on simulated as well as experimental data. The presented method was shown to deliver improved accuracy in reconstructions based on a single distance as well as multiple distance phase-contrast images corrupted by noise and hampered by errors due to nonlinear imaging effects.

An improved cochlear implant electrode array for use in experimental studies

Experimental studies play an important role in establishing the safety and efficacy of cochlear implants and they continue to provide insight into a new generation of electrode arrays and stimulation strategies. One drawback has been the limited depth of insertion of an electrode array in experimental animals. We compared the insertion depth and trauma associated with the insertion of Cochlear Ltd's Hybrid-L (HL) array with a standard 8 ring array in cat cochleae. Both arrays were inserted into cadaver cochleae and an X-ray recorded their anatomical location. The implanted cochlea was serially sectioned and photographed at 300 μm intervals for evidence of electrode insertion trauma. Subsequently two cats were chronically implanted with HL arrays and electrically-evoked potentials recorded over a three month period. Mean insertion depth for the HL arrays was 334.8° (SD = 21°; n = 4) versus 175.5° (SD = 6°; n = 2) for the standard array. This relates to ∼10.5 mm and 6 mm respectively. A similar insertion depth was measured in a chronically implanted animal with an HL array. Histology from each cadaver cochleae showed that the electrode array was always located in the scala tympani; there was no evidence of electrode insertion trauma to the basilar membrane, the osseous spiral lamina or the spiral ligament. Finally, evoked potential data from the chronically implanted animals exhibited significantly lower thresholds compared with animals implanted with a standard 8 ring array, with electrical thresholds remaining stable over a three-month observation period. Cochlear Ltd's HL electrode array can be safely inserted ∼50% of the length of the cat scala tympani, placing the tip of the array close to the 4 kHz place. This insertion depth is considerably greater than is routinely achieved using a standard 8-ring electrode array (∼12 kHz place). The HL array evokes low thresholds that remain stable over three months of implantation. This electrode array has potential application in a broad area of cochlear implant related research.

Cochlear implantation update: contemporary preoperative imaging and future prospects - the dual modality approach as a standard of care

The selection of cochlear implant (CI) candidates requires consideration of a variety of clinical and radiographic factors. The present article reviews the current knowledge regarding the preoperative imaging of CI candidates and explores emerging developments in different imaging modalities. Preoperative radiologic assessment should evaluate the status of the middle/inner ear, auditory nerve and central acoustic pathways. Preoperative computed tomography displays anatomic middle ear variations of surgical importance. MRI can demonstrate fluid/obliteration in the inner ear and depict the retrocochlear auditory pathways. Dual modality imaging with high-resolution computed tomography and MRI of the petrous bone and brain can provide the maximum information regarding surgical landmarks and detect deafness-related abnormalities. Cost-effectiveness issues also justify its use. New systems are now becoming available, offering improved soft-tissue delineation, sophisticated segmentation techniques, volumetric measurements, semitransparent views and superior surface resolution, thus significantly advancing our diagnostic acumen and making the preoperative evaluation of CI candidates more accurate and reliable.

Micro-focus fluoroscopy - a great tool for electrode development

The aim of this study was to utilise micro-focus X-ray fluoroscopy for viewing electrode movement in the cochlea. Various prototypes of newly designed cochlear implant electrodes were evaluated during insertion studies on human cadaver temporal bones. The magnified fluoroscopic images were observed in real-time and recorded for retrospective studies. In 30 insertions of hearing preservation (Hybrid-L) arrays, fluoroscopy provided crucial information on the tip design, length of array and stiffening stylet. In 44 insertions of Contour Advance enhanced (CAe) arrays, the length, curvature, depth of insertion and degree of stiffness were assessed. CAe arrays were successfully inserted to the designated depth and positioned close to the modiolus. High quality micro-focus fluoroscopic images of electrode movement in the cochlea greatly assisted in the validation of newly designed intra-cochlear electrode arrays.

High resolution micro-CT scanning as an innovative tool for evaluation of the surgical positioning of cochlear implant electrodes

X-ray microtomography (micro-CT) is a new technique allowing for visualization of the internal structure of opaque specimens with a quasi-histological quality. Among multiple potential applications, the use of this technique in otology is very promising. Micro-CT appears to be ideally suited for in vitro visualization of the inner ear tissues as well as for evaluation of the electrode damage and/or surgical insertion trauma during implantation of the cochlear implant electrodes. This technique can greatly aid in design and development of new cochlear implant electrodes and is applicable for temporal bone studies. The main advantage of micro-CT is the practically artefact-free preparation of the samples and the possibility of evaluation of the interesting parameters along the whole insertion depth of the electrode. This paper presents the results of the first application of micro-CT for visualization of the inner ear structures in human temporal bones and for evaluation of the surgical positioning of the cochlear implant electrodes relative to the intracochlear soft tissues.

Digital Volume Tomography: Radiologic Examinations of the Temporal Bone

OBJECTIVE

We evaluated the clinical applicability and the value of digital volume tomography for visualization of the lateral skull base using temporal bone specimens.

MATERIALS AND METHODS

Twelve temporal bone specimens were used to evaluate digital volume tomography on the lateral skull base. Aside from the initial examination of the temporal bones, radiologic control examinations were performed after insertion of titanium, gold, and platinum middle-ear implants and a cochlear implant.

RESULTS

With high-resolution and almost artifact-free visualization of alloplastic middle-ear implants of titanium, gold, or platinum, it was possible to define the smallest bone structures or position of the prosthesis with high precision. Furthermore, the examination proved that digital volume tomography is useful in assessing the normal position of a cochlear implant.

CONCLUSION

Digital volume tomography expands the application of diagnostic possibilities in the lateral skull base. Therefore, we believe improved preoperative diagnosis can be achieved along with more accurate planning of the surgical procedure. Digital volume tomography delivers a small radiation dose and a high resolution coupled with a low purchase price for the equipment.

Cochleostomy site: implications for electrode placement and hearing preservation

CONCLUSIONS

With recent increased interest in minimizing intracochlear trauma and preserving residual hearing during cochlear implantation, increased attention must be paid to the cochleostomy site. The results of this paper demonstrate that the cochleostomy must be made inferior, rather than anterior, to the round window to ensure scala tympani insertion and to decrease the likelihood of insertion-induced intracochlear damage during electrode insertion.

OBJECTIVE

To describe the complex anatomy of the hook region of the cochlea, specifically in relation to the optimal placement of the cochleostomy for cochlear implant electrode insertion to potentially achieve hearing preservation. The authors believe that previous industry recommendations and described surgical techniques have resulted in cochleostomies being placed in anatomical positions that possibly result in electrode insertions that damage the basilar membrane and/or other cochlear structures.

MATERIAL AND METHODS

The results of a number of temporal bone studies were reviewed with attention being paid to the anatomical relationship of the basilar membrane and spiral ligament to the round window membrane. For different cochleostomy sites the potential for damage to intracochlear structures, particularly the basilar membrane and organ of Corti, was assessed.

RESULTS

The review of electrode insertion studies into human temporal bones, as well as a post-mortem anatomical study of implanted temporal bones, showed an increased risk of scala vestibuli insertions and insertion-induced damage to intracochlear structures when the cochleostomy was performed more anterior to the round window. These results were endorsed by studies detailing the anatomy of the hook region of the cochlea.

Cochlear implants in children: safety as well as speech and language

The development of cochlear implants for children at the University of Melbourne and the Bionic Ear Institute, has consisted of a routine of biological and engineering safety followed by evaluation of speech processing strategies on adults before they are undertaken on children. The initial safety studies were to ensure that insertion was atraumatic, the electrical stimulus parameters did not lead to loss of ganglion cells and that the electrode could be inserted without the risk of middle ear infection leading to meningitis. The initial second formant extraction scheme was shown to produce significant open-set speech understanding in adults and was approved by the US Food and Drug Administration (FDA) in 1985. Following this, an international study was undertaken for the FDA on children using a strategy that also included the first formant, and was approved in 1990. Additional advances in speech processing have been evaluated on adults. However, before using one with high rates of stimulation, it was tested for safety on experimental animals. Further advances have been anticipated in particular through the development of a peri-modiolar array, the Nucleus Contour. Prior to its use on adults, it was tested in the human temporal bone and found to lead to minimal trauma. It was evaluated in adults and found to lead to better current localization and lower thresholds. A study was undertaken in children using a spectral maxima scheme at high rates (advanced combination encoder (ACE)) and the Contour array as it had given best results in adults. It was approved as safe and effective for use in children in 2000. Studies were also undertaken to look at plasticity and visual dominance particularly through cognitive studies and the use of the McGurk effect. This demonstrated that deaf children with implants rely heavily on visual information and there is a great need to have unambiguous auditory stimuli to get best results.