Optimization of 3D-Visualization of Micro-Anatomical Structures of the Human Inner Ear in Osmium Tetroxide Contrast Enhanced Micro-CT Scans

Vestibular Implant Surgery: How to Deal With Obstructed Semicircular Canals-A Diagnostic and Surgical Guide

BACKGROUND

A vestibular implant can partially restore vestibular function by providing motion information through implanted electrodes. During vestibular implantation, various obstructions of the semicircular canals, such as protein deposits, fibrosis, and ossification, can be encountered. The objective was to explore the relationship between preoperative imaging and intraoperative findings of semicircular canal obstruction and to develop surgical strategies for dealing with obstructions of the semicircular canal(s) in patients eligible for vestibular implantation.

METHODS

Patients undergoing vestibulocochlear implantation (in an active clinical trial) were included in the current study when preoperative imaging indicated an obstruction in the semicircular canal. Preoperative imaging consisted of CT and MRI scans. During surgery, the bony semicircular canals were skeletonized ("bluelined") to identify the course of the canals and create a fenestration to insert the electrodes. The aim was to place the electrodes in the semicircular canal ampullae. Surgical strategies were developed to deal with the soft tissue obstructions. These procedures were evaluated intraoperatively with microscopic visualization, postoperatively with CT imaging.

RESULTS

The three included patients suffered from bilateral vestibulopathy and hearing loss due to autosomal dominant nonsyndromic sensorineural deafness 9 (DFNA9). A soft tissue obstruction was predicted in one semicircular canal (2 patients) or two semicircular canals (1 patient), based on preoperative imaging. Intraoperatively, bluelining the semicircular canals aided in identifying these locations, by revealing a "whiteline" instead of blueline. Depending on the nature and location of the obstruction, different surgical procedures were employed to facilitate proper electrode insertion. These were as follows: a dummy electrode was used to probe the soft tissue, the obstructive tissue was removed, and/or a bypass fenestration was created. In all patients, the electrodes could be implanted in the semicircular canal ampullae. Based on these first experiences, a diagnostic and surgical guide to deal with obstructions of the semicircular canals during vestibular implantation was developed.

CONCLUSIONS

Preoperative imaging can indicate locations of obstructions in the SCCs. Different surgical procedures can be applied to enable appropriate electrode positioning in the SCC ampulla. This article describes the first experiences with obstructions of the semicircular canals during intralabyrinthine vestibular implantation and presents a diagnostic and surgical guide.

TRIAL REGISTRATION

ABR NL73492.068.20, METC20-087 (Maastricht University Medical Center) and NAC 11-080 (Geneva University Hospitals).

Advances in 3D Inner Ear Reconstruction Software for Cochlear Implants: A Comprehensive Review

Auditory impairment stands as a pervasive global issue, exerting significant effects on individuals' daily functioning and interpersonal engagements. Cochlear implants (CIs) have risen as a cutting-edge solution for severe to profound hearing loss, directly stimulating the auditory nerve with electrical signals. The success of CI procedures hinges on precise pre-operative planning and post-operative evaluation, highlighting the significance of advanced three-dimensional (3D) inner ear reconstruction software. Accurate pre-operative imaging is vital for identifying anatomical landmarks and assessing cochlear deformities. Tools like 3D Slicer, Amira and OTOPLAN provide detailed depictions of cochlear anatomy, aiding surgeons in simulating implantation scenarios and refining surgical approaches. Post-operative scans play a crucial role in detecting complications and ensuring CI longevity. Despite technological advancements, challenges such as standardization and optimization persist. This review explores the role of 3D inner ear reconstruction software in patient selection, surgical planning, and post-operative assessment, tracing its evolution and emphasizing features like image segmentation and virtual simulation. It addresses software limitations and proposes solutions, advocating for their integration into clinical practice. Ultimately, this review underscores the impact of 3D inner ear reconstruction software on cochlear implantation, connecting innovation with precision medicine.

Synchrotron-source micro-x-ray computed tomography for examining butterfly eyes

Comparative anatomy is an important tool for investigating evolutionary relationships among species, but the lack of scalable imaging tools and stains for rapidly mapping the microscale anatomies of related species poses a major impediment to using comparative anatomy approaches for identifying evolutionary adaptations. We describe a method using synchrotron source micro-x-ray computed tomography (syn-μXCT) combined with machine learning algorithms for high-throughput imaging of Lepidoptera (i.e., butterfly and moth) eyes. Our pipeline allows for imaging at rates of ~15 min/mm3 at 600 nm3 resolution. Image contrast is generated using standard electron microscopy labeling approaches (e.g., osmium tetroxide) that unbiasedly labels all cellular membranes in a species-independent manner thus removing any barrier to imaging any species of interest. To demonstrate the power of the method, we analyzed the 3D morphologies of butterfly crystalline cones, a part of the visual system associated with acuity and sensitivity and found significant variation within six butterfly individuals. Despite this variation, a classic measure of optimization, the ratio of interommatidial angle to resolving power of ommatidia, largely agrees with early work on eye geometry across species. We show that this method can successfully be used to determine compound eye organization and crystalline cone morphology. Our novel pipeline provides for fast, scalable visualization and analysis of eye anatomies that can be applied to any arthropod species, enabling new questions about evolutionary adaptations of compound eyes and beyond.

Using x-ray micro computed tomography to quantify intracochlear fibrosis after cochlear implantation in a Guinea pig model

Cochlear implants (CIs) allow individuals with profound hearing loss to understand speech and perceive sounds. However, not all patients obtain the full benefits that CIs can provide and the cause of this disparity is not fully understood. One possible factor for the variability in outcomes after cochlear implantation, is the development of fibrotic scar tissue around the implanted electrode. It has been hypothesised that limiting the extent of fibrosis after implantation may improve overall CI function, and longevity of the device. Currently, histology is often used to quantify the extent of intracochlear tissue growth after implantation however this method is labour intensive, time-consuming, often involves significant user bias, and causes physical distortion of the fibrosis. Therefore, this study aimed to evaluate x-ray micro computed tomography (μCT) as a method to measure the amount and distribution of fibrosis in a guinea pig model of cochlear implantation. Adult guinea pigs were implanted with an inactive electrode, and cochleae harvested eight weeks later (n = 7) and analysed using μCT, to quantify the extent of tissue reaction, followed by histological analysis to confirm that the tissue was indeed fibrotic. Cochleae harvested from an additional six animals following implantation were analysed by μCT, before and after contrast staining with osmium tetroxide (OsO4), to enhance the visualisation of soft tissues within the cochlea, including the tissue reaction. Independent analysis by two observers showed that the quantification method was robust and provided additional information on the distribution of the response within the cochlea. Histological analysis revealed that μCT visualised dense collagenous material and new bone formation but did not capture loose, areolar fibrotic tissue. Treatment with OsO4 significantly enhanced the visible tissue reaction detected using μCT. Overall, μCT is an alternative and reliable method that can be used to quantify the extent of the CI-induced intracochlear tissue response and will be a useful tool for the in vivo assessment of novel anti-fibrotic treatments.

The Next Challenges of Vestibular Implantation in Humans

Patients with bilateral vestibulopathy suffer from a variety of complaints, leading to a high individual and social burden. Available treatments aim to alleviate the impact of this loss and improve compensatory strategies. Early experiments with electrical stimulation of the vestibular nerve in combination with knowledge gained by cochlear implant research, have inspired the development of a vestibular neuroprosthesis that can provide the missing vestibular input. The feasibility of this concept was first demonstrated in animals and later in humans. Currently, several research groups around the world are investigating prototype vestibular implants, in the form of vestibular implants as well as combined cochlear and vestibular implants. The aim of this review is to convey the presentations and discussions from the identically named symposium that was held during the 2021 MidWinter Meeting of the Association for Research in Otolaryngology, with researchers involved in the development of vestibular implants targeting the ampullary nerves. Substantial advancements in the development have been made. Yet, research and development processes face several challenges to improve this neuroprosthesis. These include, but are not limited to, optimization of the electrical stimulation profile, refining the surgical implantation procedure, preserving residual labyrinthine functions including hearing, as well as gaining regulatory approval and establishing a clinical care infrastructure similar to what exists for cochlear implants. It is believed by the authors that overcoming these challenges will accelerate the development and increase the impact of a clinically applicable vestibular implant.

Accuracy of radiological prediction of electrode position with otological planning software and implications of high-resolution imaging

OBJECTIVES

In cochlear implantation, preoperative prediction of electrode position has recently gained increasing attention. Currently, planning is usually done by multislice CT (MSCT). However, flat-panel volume CT (fpVCT) and its secondary reconstructions (fpVCTSECO) allow for more precise visualization of the cochlea. Combined with a newly developed otological planning software, the position of every single contact can be effectively predicted. In this study it was investigated how accurately radiological prediction forecasts the postoperative electrode localization and whether higher image resolution is advantageous.

METHODS

Utilizing otological planning software (OTOPLAN®) and different clinical imaging modalities (MSCT, fpVCT and fpVCTSECO) the electrode localization [angular insertion depth (AID)] and respective contact frequencies were predicted preoperatively and examined postoperatively. Furthermore, inter-electrode-distance (IED) and inter-electrode-frequency difference (IEFD) were evaluated postoperatively.

RESULTS

Measurements revealed a preoperative overestimation of AID. Corresponding frequencies were also miscalculated. Determination of IED and IEFD revealed discrepancies at the transition from the basal to the middle turn and round window to the basal turn. All predictions and discrepancies were lowest when using fpVCTSECO.

CONCLUSION

The postoperative electrode position can be predicted quite accurately using otological planning software. However, because of several potential misjudgments, high-resolution imaging, such as offered by fpVCTSECO, should be used pre- and postoperatively.

Human cochlear microstructures at risk of electrode insertion trauma, elucidated in 3D with contrast-enhanced microCT

Cochlear implant restores hearing loss through electrical stimulation of the hearing nerve from within the cochlea. Unfortunately, surgical implantation of this neuroprosthesis often traumatizes delicate intracochlear structures, resulting in loss of residual hearing and compromising hearing in noisy environments and appreciation of music. To avoid cochlear trauma, insertion techniques and devices have to be adjusted to the cochlear microanatomy. However, existing techniques were unable to achieve a representative visualization of the human cochlea: classical histology damages the tissues and lacks 3D perspective; standard microCT fails to resolve the cochlear soft tissues; and previously used X-ray contrast-enhancing staining agents are destructive. In this study, we overcame these limitations by performing contrast-enhanced microCT imaging (CECT) with a novel polyoxometalate staining agent Hf-WD POM. With Hf-WD POM-based CECT, we achieved nondestructive, high-resolution, simultaneous, 3D visualization of the mineralized and soft microstructures in fresh-frozen human cochleae. This enabled quantitative analysis of the true intracochlear dimensions and led to anatomical discoveries, concerning surgically-relevant microstructures: the round window membrane, the Rosenthal's canal and the secondary spiral lamina. Furthermore, we demonstrated that Hf-WD POM-based CECT enables quantitative assessment of these structures as well as their trauma.

Implementation of secondary reconstructions of flat-panel volume computed tomography (fpVCT) and otological planning software for anatomically based cochlear implantation

Purpose

For further improvements in cochlear implantation, the measurement of the cochlear duct length (CDL) and the determination of the electrode contact position (ECP) are increasingly in the focus of clinical research. Usually, these items were investigated by multislice computed tomography (MSCT). The determination of ECP was only possible by research programs so far. Flat-panel volume computed tomography (fpVCT) and its secondary reconstructions (fpVCT_SECO) allow for high spatial resolution for the visualization of the temporal bone structures. Using a newly developed surgical planning software that enables the evaluation of CDL and the determination of postoperative ECP, this study aimed to investigate the combination of fpVCT and otological planning software to improve the implementation of an anatomically based cochlear implantation.

Methods

Cochlear measurements were performed utilizing surgical planning software in imaging data (MSCT, fpVCT and fpVCT_SECO) of patients with and without implanted electrodes.

Results

Measurement of the CDL by the use of an otological planning software was highly reliable using fpVCT_SECO with a lower variance between the respective measurements compared to MSCT. The determination of the inter-electrode-distance (IED) between the ECP was improved in fpVCT_SECO compared to MSCT.

Conclusion

The combination of fpVCT_SECO and otological planning software permits a simplified and more reliable analysis of the cochlea in the pre- and postoperative setting. The combination of both systems will enable further progress in the development of an anatomically based cochlear implantation.

IE-Map: a novel in-vivo atlas and template of the human inner ear

Brain atlases and templates are core tools in scientific research with increasing importance also in clinical applications. Advances in neuroimaging now allowed us to expand the atlas domain to the vestibular and auditory organ, the inner ear. In this study, we present IE-Map, an in-vivo template and atlas of the human labyrinth derived from multi-modal high-resolution magnetic resonance imaging (MRI) data, in a fully non-invasive manner without any contrast agent or radiation. We reconstructed a common template from 126 inner ears (63 normal subjects) and annotated it with 94 established landmarks and semi-automatic segmentations of all relevant macroscopic vestibular and auditory substructures. We validated the atlas by comparing MRI templates to a novel CT/micro-CT atlas, which we reconstructed from 21 publicly available post-mortem images of the bony labyrinth. Templates in MRI and micro-CT have a high overlap, and several key anatomical measures of the bony labyrinth in IE-Map are in line with micro-CT literature of the inner ear. A quantitative substructural analysis based on the new template, revealed a correlation of labyrinth parameters with total intracranial volume. No effects of gender or laterality were found. We provide the validated templates, atlas segmentations, surface meshes and landmark annotations as open-access material, to provide neuroscience researchers and clinicians in neurology, neurosurgery, and otorhinolaryngology with a widely applicable tool for computational neuro-otology.

Visualization of different anatomical parts of the enucleated human eye using X-ray micro-CT imaging

Micro-CT visualization allows reconstruction of eye structures with the resolution of light microscopy and estimation of tissue densities. Moreover, this method excludes damaging procedures and allows further histological staining due to the similar steps in the beginning. We have shown the feasibility of the lab-based micro-CT machine usage for visualization of clinically important compartments of human eye such as trabecular outflow pathway, retina, iris and ciliary body after pre-treatment with iodine in ethanol. We also identified the challenges of applying this contrasting technique to lens, cornea, and retina and proposed alternative staining methods for these tissues. Thereby this work provides a starting point for other studies for imaging of human eyes in normal and pathological conditions using lab-based micro-CT systems.

Micro-anatomy of the ear of the southern white rhinoceros (Ceratotherium simum simum)

The white rhinoceros is the largest of the five extant rhinoceros species. The population is declining rapidly because of intense poaching. However, normal anatomical descriptions in this species are lacking. The purpose of this study is to describe the osseous anatomy of the middle and inner ear of the southern white rhinoceros using micro-focus X-ray computed tomography imaging. Four temporal bones obtained from two 1-day old southern white rhinoceros preserved in 10% formalin were scanned. Tri-dimensional reconstructions were obtained and volumes of the middle ear ossicles and inner ear structures were calculated. Excellent high spatial resolution 3D images were obtained for all samples and virtual models of the auditory ossicles and bony labyrinth were generated. Visualization of the tympanic membrane, middle ear and inner ear structures was possible in all samples. Whereas the stapes and incus had a shape similar to their human or equine counterparts, the malleus showed a unique appearance with a long rostral branch projecting latero-distally to the manubrium. The cochlea described 2 turns rostro-laterally around its axis, with a medial direction of rotation. However, identification of the soft tissue structures of the middle ear was sometimes difficult and visualization of the small structures of the membranous labyrinth was not possible using this formalin fixation and alternative techniques should be investigated. Further investigations are needed in order to provide a complete virtual model including both soft and bone tissues of this difficultly accessible region.

Electrical stimulation in the cochlea: Influence of modiolar microstructures on the activation of auditory nerve fibres

Existing computational studies of cochlear implants have demonstrated that the structural detail of threedimensional (3D) cochlear models exerts influence on the current spread within the cochlea. Nevertheless, the significance of including the microstructures inside the modiolar bone in a cochlear model is still unclear in the literature. We employed two different multi-compartment neuron models to simulate auditory nerve fibres, and compared response characteristics of the fibre population between a detailed and a simplified 3D cochlear model. Results showed that although the prediction of firing is dependent on the details of the neuron model, the responses of the fibre population to the electrical stimulus, especially the location of the initiation of action potential, varied between the detailed and the simplified models. Therefore, the inclusion of the modiolar microstructures in a cochlear model may be necessary for fully understanding the firing of auditory nerve fibres.

Light sheet microscopy of the gerbil cochlea

Light sheet fluorescence microscopy (LSFM) provides a rapid and complete three-dimensional image of the cochlea. The method retains anatomical relationships-on a micrometer scale-between internal structures such as hair cells, basilar membrane (BM), and modiolus with external surface structures such as the round and oval windows. Immunolabeled hair cells were used to visualize the spiraling BM in the intact cochlea without time intensive dissections or additional histological processing; yet material prepared for LSFM could be rehydrated, the BM dissected out and reimaged at higher resolution with the confocal microscope. In immersion-fixed material, details of the cochlear vasculature were seen throughout the cochlea. Hair cell counts (both inner and outer) as well as frequency maps of the BM were comparable to those obtained by other methods, but with the added dimension of depth. The material provided measures of angular, linear, and vector distance between characteristic frequency regions along the BM. Thus, LSFM provides a unique ability to rapidly image the entire cochlea in a manner applicable to model and interpret physiological results. Furthermore, the three-dimensional organization of the cochlea can be studied at the organ and cellular level with LSFM, and this same material can be taken to the confocal microscope for detailed analysis at the subcellular level.

Accuracy and Reliability of Computer-aided Anatomical Measurements for Vertebral Body and Disc Based on Computed Tomography Scans

Objective

To assess whether the computed tomography (CT)‐based method of three‐dimensional (3D) analysis (Mimics) was accurate and reliable for spine surgical anatomical measurements.

Methods

A total of 40 lumbar segments and 32 inter‐vertebral discs from eigth adult male cadavers without fractures or deformities fixed with the classical formaldehyde method were included in this research on 5 June 2017. CT scans including seven dimensions: anterior height of the vertebral body (VBHa), middle height of the vertebral body (VBHm), posterior height of the vertebral body (VBHp), width of the upper endplate (EPWu), depth of the upper endplate (EPDu), anterior height of the inter‐vertebral disc in the median sagittal plane (IDHa), and posterior height of the inter‐vertebral disc in the median sagittal plane (IDHp). They were performed based on uniform conditions (slice thickness: 0.625 mm) using a CT scanner on 8 June 2017. Afterwards, the surgical anatomical measurements were conducted with a Vernier caliper on 12 June 2017. The computer‐aided anatomical measurements were conducted by three investigators using Mimics 16.0 to perform 3D reconstructions of CT bone on 16 June 2017. Finally, the length and angle were measured with associated measurement tools, yielding a verified accuracy of 0.01 mm and 0.01°, respectively. Each measurement was repeated three times, and all anatomical data was analyzed using the statistical software and P‐value < 0.05 was considered statistically significant.

Results

The results showed no statistically significant difference was observed between the surgical anatomical and computer‐aided anatomical measurements (P > 0.05) for lumbar vertebra measurements, and the absolute difference between surgical and computer‐aided data were all less than 1.0 mm (for the VBHa, VBHm, VBHp, EPWu, and EPDu were 0.12, 0.03, 0.03, 0.31, and 0.03 mm, respectively). Moreover, although the absolute differences of discs was larger than those of lumbar vertebras, no significant differences were detected between the computer‐aided and surgical anatomical measurements for the IDHa, as well as IDHp in the vast majority of measurements (P = 0.543, 0.079 or 0.052 for IDHa, and P = 0.212, 0.133 or 0.042 for IDHp). In addition, excellent reliability correlation was observed between the measurements of each investigator, and the reliability coefficients in the intra‐groups were all greater than 0.9 except for IDHp (reliability coefficient = 0.892). Additionally, the reliability coefficients were greater than 0.9 for the all between‐group correlations, and a significant correlation was also observed. Furthermore, no statistically significant difference for three anatomical values was found in the computer‐assisted measurements of the lumbar bone structure (P > 0.05). Similarly, we did not observe a statistical difference in the anatomical data of the lumbar discs from the three measures (P > 0.05).

Conclusions

Computer‐aided anatomical measurement for spine based on CT scans presents the high accuracy and reliability for improving spinal surgical procedures.

Application value of magnetic resonance hydrography of the inner ear in cochlear implantation

OBJECTIVE

This study aims to investigate the application value of magnetic resonance (MR) hydrography of the inner ear in cochlear implantation.

METHODS

146 patients were enrolled. MR hydrography and spiral CT examinations for the intracranial auditory canal were performed before surgery, and all imaging results were statistically analyzed in order to explore the application value of MR hydrography of the inner ear in cochlear implantation.

RESULTS

146 patients (292 ears) were examined. Among these patients, 13 were diagnosed with abnormal vestibular aqueducts (20 ears) by MR hydrography, while five were diagnosed with this disease by CT; 15 patients were diagnosed with inner ear malformation (19 ears) by MR hydrography, while 11 were diagnosed by CT (four were misdiagnosed); five patients were diagnosed with internal acoustic canal stenosis (eight ears) by MR hydrography, while two were diagnosed by CT (three were misdiagnosed); and four patients were diagnosed with cochlear fibrosis (five ears) by MR hydrography, while four were diagnosed by CT (four ears). The correct rate of diagnosis was 77.40% (113/146) based on CT, while the rate was 93.84% (137/146) based on MR hydrography.

CONCLUSIONS

MR hydrography imaging technique can be applied to the preoperative evaluation of cochlear implantation, providing accurate and reliable anatomic information on the inner membranous labyrinth and nerves in the internal acoustic canal and an accurate basis for the diagnosis of cochlear fibrosis and nerve development. This has a guiding significance for the selection of treatment schemes.

Electrical Stimulation in the Human Cochlea: A Computational Study Based on High-Resolution Micro-CT Scans

Background: Many detailed features of the cochlear anatomy have not been included in existing 3D cochlear models, including the microstructures inside the modiolar bone, which in turn determines the path of auditory nerve fibers (ANFs).

Method: We captured the intricate modiolar microstructures in a 3D human cochlea model reconstructed from μCT scans. A new algorithm was developed to reconstruct ANFs running through the microstructures within the model. Using the finite element method, we calculated the electrical potential as well as its first and second spatial derivatives along each ANF elicited by the cochlear implant electrodes. Simulation results of electrical potential was validated against intracochlear potential measurements. Comparison was then made with a simplified model without the microstructures within the cochlea.

Results: When the stimulus was delivered from an electrode located deeper in the apex, the extent of the auditory nerve influenced by a higher electric potential grew larger; at the same time, the maximal potential value at the auditory nerve also became larger. The electric potential decayed at a faster rate toward the base of the cochlea than toward the apex. Compared to the cochlear model incorporating the modiolar microstructures, the simplified version resulted in relatively small differences in electric potential. However, in terms of the first and second derivatives of electric potential along the fibers, which are relevant for the initiation of action potentials, the two models exhibited large differences: maxima in both derivatives with the detailed model were larger by a factor of 1.5 (first derivative) and 2 (second derivative) in the exemplary fibers. More importantly, these maxima occurred at different locations, and opposite signs were found for the values of second derivatives between the two models at parts along the fibers. Hence, while one model predicts depolarization and spike initiation at a given location, the other may instead predict a hyperpolarization.

Conclusions: Although a cochlear model with fewer details seems sufficient for analysing the current spread in the cochlear ducts, a detailed-segmented cochlear model is required for the reconstruction of ANF trajectories through the modiolus, as well as the prediction of firing thresholds and spike initiation sites.

Vestibular Implantation and the Feasibility of Fluoroscopy-Guided Electrode Insertion

Recent research has shown promising results for the development of a clinically feasible vestibular implant in the near future. However, correct electrode placement remains a challenge. It was shown that fluoroscopy was able to visualize the semicircular canal ampullae and electrodes, and guide electrode insertion in real time. Ninety-four percent of the 18 electrodes were implanted correctly (<1.5 mm distance to target). The median distances were 0.60 mm, 0.85 mm, and 0.65 mm for the superior, lateral, and posterior semicircular canal, respectively. These findings suggest that fluoroscopy can significantly improve electrode placement during vestibular implantation.