Mai Elfarnawany

Measuring Cochlear Duct Length – a historical analysis of methods and results

BackgroundCochlear Duct Length (CDL) has been an important measure for the development and advancement of cochlear implants. Emerging literature has shown CDL can be used in preoperative settings to select the proper sized electrode and develop customized frequency maps. In order to improve post-operative outcomes, and develop new electrode technologies, methods of measuring CDL must be validated to allow usage in the clinic.PurposeThe purpose of this review is to assess the various techniques used to calculate CDL and provide the reader with enough information to make an informed decision on how to conduct future studies measuring the CDL.ResultsThe methods to measure CDL, the modality used to capture images, and the location of the measurement have all changed as technology evolved. With recent popularity and advancement in computed tomography (CT) imaging in place of histologic sections, measurements of CDL have been focused at the lateral wall (LW) instead of the organ of Corti (OC), due to the inability of CT to view intracochlear structures. After analyzing results from methods such as directly measuring CDL from histology, indirectly reconstructing the shape of the cochlea, and determining CDL based on spiral coefficients, it was determined the three dimensional (3D) reconstruction method is the most reliable method to measure CDL. 3D reconstruction provides excellent visualization of the cochlea and avoids errors evident in other methods. Due to the number of varying methods with varying accuracies, certain guidelines must be followed in the future to allow direct comparison of CDL values between studies.ConclusionAfter summarizing and analyzing the interesting history of CDL measurements, the use of standardized guidelines and the importance of CDL for future cochlear implant developments is emphasized for future studies.

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.

Evaluation of Cochlear Duct Length Computations Using Synchrotron Radiation Phase-Contrast Imaging

HYPOTHESIS Evaluation of cochlear duct length (CDL) using novel imaging techniques will help improve the accuracy of existing CDL equations. BACKGROUND Various relationships relating A value measured from a patients computed tomography scan and CDL have been proposed to aid in preoperative electrode selection and frequency mapping. METHODS Ten cadaveric temporal bones were scanned using synchrotron radiation phase-contrast imaging. Reference CDL values were calculated by placing points representing the organ of Corti (OC), lateral wall (LW), and electrode location (I) on the synchrotron radiation phase-contrast imaging slices along the length of the cochlea. The CDL estimates from the existing three equations (OC, LW, I) in addition to two newly proposed equations (OC and LW) were compared with reference CDL values at each respective location. RESULTS When compared with reference CDL values, the new OC equation improved the CDL estimates from a 6.2% error to a 5.1% error while the new LW equation improved the CDL estimate error from 3.9 to 3.6%. Bland-Altman plots revealed both new equations increased similarity to reference values and brought more samples to within clinically significant ranges. Validation of the original electrode location equation to the reference values showed a 4.6% difference. CONCLUSION The newly proposed equations for LW and OC provided an improvement over past equations for determining CDL from the A value by showing improved agreement with reference values. Therefore, these equations can provide quick and accurate preoperative estimates of CDL for improving customized frequency mapping.

Improved objective selection of power Doppler wall-filter cut-off velocity for accurate vascular quantification.

The wall-filter selection curve method is proposed to objectively identify a cut-off velocity that minimizes artifacts in power Doppler images. A selection curve, which is constructed by plotting the color pixel density (CPD) as a function of the cut-off velocity, exhibits characteristic intervals hypothesized to include the optimum cut-off velocity. This article presents an improved implementation of the method that automatically detects characteristic intervals in a selection curve and selects an operating point cut-off velocity along a characteristic interval. The method is applied to subregions within the Doppler image to adapt the cut-off velocity to local variations in vascularity. The methods performance is evaluated in 30-MHz power Doppler images of a four-vessel flow phantom. At high (>5 mm/s) flow velocities, qualitative improvements in vessel delineation are achieved and the CPD in the resulting images is accurate to within 3% of the vascular volume fraction of the phantom.

Improved middle-ear soft-tissue visualization using synchrotron radiation phase-contrast imaging

ABSTRACT High resolution images are used as a basis for finite‐element modeling of the middle‐ear structures to study their biomechanical function. Commonly used imaging techniques such as micro‐computed tomography (CT) and optical microscopy require extensive sample preparation, processing or staining using contrast agents to achieve sufficient soft‐tissue contrast. We compare imaging of middle‐ear structures in unstained, non‐decalcified human temporal bones using conventional absorption‐contrast micro‐CT and using synchrotron radiation phase‐contrast imaging (SR‐PCI). Four cadaveric temporal bones were imaged using SR‐PCI and conventional micro‐CT. Images were qualitatively compared in terms of visualization of structural details and soft‐tissue contrast using intensity profiles and histograms. In order to quantitatively compare SR‐PCI to micro‐CT, three‐dimensional (3D) models of the ossicles were constructed from both modalities using a semi‐automatic segmentation method as these structures are clearly visible in both types of images. Volumes of the segmented ossicles were computed and compared between the two imaging modalities and to estimates from the literature. SR‐PCI images provided superior visualization of soft‐tissue microstructures over conventional micro‐CT images. Intensity profiles emphasized the improved contrast and detectability of soft‐tissue in SR‐PCI in comparison to absorption‐contrast micro‐CT. In addition, the semi‐automatic segmentations of SR‐PCI images yielded accurate 3D reconstructions of the ossicles with mean volumes in accord with volume estimates from micro‐CT images and literature. Sample segmentations of the ossicles and soft tissue structures were provided on an online data repository for benefit of the research community. The improved visualization, modeling accuracy and simple sample preparation make SR‐PCI a promising tool for generating reliable FE models of the middle‐ear structures, including both soft tissues and bone. HIGHLIGHTSSynchrotron‐radiation phase contrast imaging is proposed for middle‐ear imaging.It provides superior visualization of microstructures over conventional micro‐CT.It is an improved tool to visualize middle‐ear bones and soft tissue simultaneously.These improvements are achieved without the need for staining or decalcification.It is a promising tool for generating reliable FE models of the middle‐ear structures.

An automated A-value measurement tool for accurate cochlear duct length estimation

BackgroundThere has been renewed interest in the cochlear duct length (CDL) for preoperative cochlear implant electrode selection and postoperative generation of patient-specific frequency maps. The CDL can be estimated by measuring the A-value, which is defined as the length between the round window and the furthest point on the basal turn. Unfortunately, there is significant intra- and inter-observer variability when these measurements are made clinically. The objective of this study was to develop an automated A-value measurement algorithm to improve accuracy and eliminate observer variability.MethodClinical and micro-CT images of 20 cadaveric cochleae specimens were acquired. The micro-CT of one sample was chosen as the atlas, and A-value fiducials were placed onto that image. Image registration (rigid affine and non-rigid B-spline) was applied between the atlas and the 19 remaining clinical CT images. The registration transform was applied to the A-value fiducials, and the A-value was then automatically calculated for each specimen. High resolution micro-CT images of the same 19 specimens were used to measure the gold standard A-values for comparison against the manual and automated methods.ResultsThe registration algorithm had excellent qualitative overlap between the atlas and target images. The automated method eliminated the observer variability and the systematic underestimation by experts. Manual measurement of the A-value on clinical CT had a mean error of 9.5 ± 4.3% compared to micro-CT, and this improved to an error of 2.7 ± 2.1% using the automated algorithm. Both the automated and manual methods correlated significantly with the gold standard micro-CT A-values (r = 0.70, p < 0.01 and r = 0.69, p < 0.01, respectively).ConclusionAn automated A-value measurement tool using atlas-based registration methods was successfully developed and validated. The automated method eliminated the observer variability and improved accuracy as compared to manual measurements by experts. This open-source tool has the potential to benefit cochlear implant recipients in the future.

Evaluation of non-rigid registration parameters for atlas-based segmentation of CT images of human cochlea

Cochlear implant surgery is a hearing restoration procedure for patients with profound hearing loss. In this surgery, an electrode is inserted into the cochlea to stimulate the auditory nerve and restore the patient’s hearing. Clinical computed tomography (CT) images are used for planning and evaluation of electrode placement, but their low resolution limits the visualization of internal cochlear structures. Therefore, high resolution micro-CT images are used to develop atlas-based segmentation methods to extract these nonvisible anatomical features in clinical CT images. Accurate registration of the high and low resolution CT images is a prerequisite for reliable atlas-based segmentation. In this study, we evaluate and compare different non-rigid B-spline registration parameters using micro-CT and clinical CT images of five cadaveric human cochleae. The varying registration parameters are cost function (normalized correlation (NC), mutual information and mean square error), interpolation method (linear, windowed-sinc and B-spline) and sampling percentage (1%, 10% and 100%). We compare the registration results visually and quantitatively using the Dice similarity coefficient (DSC), Hausdorff distance (HD) and absolute percentage error in cochlear volume. Using MI or MSE cost functions and linear or windowed-sinc interpolation resulted in visually undesirable deformation of internal cochlear structures. Quantitatively, the transforms using 100% sampling percentage yielded the highest DSC and smallest HD (0.828±0.021 and 0.25±0.09mm respectively). Therefore, B-spline registration with cost function: NC, interpolation: B-spline and sampling percentage: moments 100% can be the foundation of developing an optimized atlas-based segmentation algorithm of intracochlear structures in clinical CT images.

Intra- and Interobserver Variability of Cochlear Length Measurements in Clinical CT.

HYPOTHESIS The cochlear A-value measurement exhibits significant inter- and intraobserver variability, and its accuracy is dependent on the visualization method in clinical computed tomography (CT) images of the cochlea. BACKGROUND An accurate estimate of the cochlear duct length (CDL) can be used to determine electrode choice, and frequency map the cochlea based on the Greenwood equation. Studies have described estimating the CDL using a single A-value measurement, however the observer variability has not been assessed. METHODS Clinical and micro-CT images of 20 cadaveric cochleae were acquired. Four specialists measured A-values on clinical CT images using both standard views and multiplanar reconstructed (MPR) views. Measurements were repeated to assess for intraobserver variability. Observer variabilities were evaluated using intra-class correlation and absolute differences. Accuracy was evaluated by comparison to the gold standard micro-CT images of the same specimens. RESULTS Interobserver variability was good (average absolute difference: 0.77 ± 0.42 mm) using standard views and fair (average absolute difference: 0.90 ± 0.31 mm) using MPR views. Intraobserver variability had an average absolute difference of 0.31 ± 0.09 mm for the standard views and 0.38 ± 0.17 mm for the MPR views. MPR view measurements were more accurate than standard views, with average relative errors of 9.5 and 14.5%, respectively. CONCLUSION There was significant observer variability in A-value measurements using both the standard and MPR views. Creating the MPR views increased variability between experts, however MPR views yielded more accurate results. Automated A-value measurement algorithms may help to reduce variability and increase accuracy in the future.

A new three-component signal model to objectively select power Doppler wall filter cut-off velocity for quantitative microvascular imaging

The wall-filter selection curve (WFSC) method was developed to automatically select cut-off velocities for high-frequency power Doppler imaging. Selection curves are constructed by plotting color pixel density (CPD) as a function of wall filter cut-off velocity. A new three-component mathematical model is developed to guide the design of an online implementation of the method for in vivo imaging. The model treats Doppler imaging as a signal detection task in which the scanner must distinguish intravascular pixels from perivascular and extravascular pixels and includes a cost function to identify the optimum cut-off velocity that provides accurate vascular quantification and minimizes the effect of color pixel artifacts on visualization of vascular structures. The goodness of fit of the three-component model to flow-phantom data is significantly improved compared to a previous two-component model (F test, p < 0:005). Simulations using the new model indicate that selection curves should be sampled using at least 100 cut-off velocities to ensure robust performance of the automated WFSC method and determine an upper bound on CPD variability that ensures reliable vascular quantification accuracy, defined as CPD within 5% of the reference vascular volume fraction. Results of the simulations also provide evidence that limiting the selection of the cut-off velocity to a binary choice between the middle and right end of the characteristic interval is sufficient to meet the quantification accuracy goal. The model provides an intuitive, empirical description of the relationship between system settings and blood-flow detection performance in power Doppler imaging.

Implications of tumor oxygenation and blood flow for cancer treatment monitoring using photoacoustic imaging and power Doppler

Photoacoustic (PA) imaging has been proposed for cancer treatment monitoring. Tumor oxygen saturation (sO2) should in principle be related to vascular parameters such as blood flow. In this work, in-vivo PA estimates of sO2 were compared to power Doppler (pD) measures of vascularity hours after the administration of microbubbles (MB), radiation therapy (XRT), individually or combined (MB-XRT).