Download Three-dimensional Models of the Membranous Vestibular Labyrinth

2011 4th International Conference on Biomedical Engineering and Informatics (BMEI)
Three-dimensional Models of the Membranous
Vestibular Labyrinth in the Guinea Pig Inner Ear
Keqiang Wang
Cardiovascular Disease Institute, Zhongshan Hospital
Fudan University
Shanghai, 200032China
Caiqin Wu
Department of Mechanics and Engineering Science
Fudan University
Shanghai, 200433, China
E–mail:[email protected]
Caiqin Wu, Lin Yang, Peidong Dai*
Research Center, Eye & ENT Hospital
Fudan University
Shanghai, 200031, China
*Corresponding author, Email: [email protected]
Abstract—This paper presents and explains the procedure for
creating three-dimensional (3D) finite element (FE) model of
membranous vestibular labyrinth of guinea pig for numerical
analysis. The model of membranous vestibular labyrinth was
built from a series of micro computer tomography (micro-CT)
images using MIMICS software. In order to visualize the
membranous labyrinth in the micro-CT, the specimen was
stained with Osmium tetroxide (OsO4) solution. An accurate 3D
FE model of membranous vestibular labyrinth was built for
computational fluid dynamics (CFD) and FE analysis.
Keywords- three-dimensional model; membranous vestibular
labyrinth; micro-CT
I.
INTRODUCTION
The vestibular labyrinth in the inner ear provides cues
which detect head motion and posture in order to keep balance
during movement[1]. The vestibular system is divided into
bony and membranous labyrinths. The membranous labyrinth
is similar in shape, but much smaller than bony labyrinths. The
membranous labyrinth consists of three roughly orthogonal
semicircular canals, the utricle and saccule in each labyrinth.
The membranous labyrinth is filled with fluid called
endolymph. The primary sense organs of vestibule, crista
ampullaris and the saccular and utricular maculae are in
membranous labyrinth. The process of balance perception in
the inner ear is a complex interaction between fluid and solid
structure in the membranous vestibular labyrinth. The
mechanical and fluid mechanical modeling of these processes
requires an accurate geometrical model of the membranous
labyrinth structures as a basis. A three-dimensional (3D) finite
element (FE) model of membranous structures in the inner ear
can be used for analysis the biomechanical processes of
balance as well as for applied medical tasks.
The method most frequently used to study the membranous
structure of the inner ear is the histological sectioning by
optical microscopy. This approach is capable of visualizing
membranes as well as bones, but tissue shrinkage may be
caused by fixation, staining, and dehydration[2]. A
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reconstruction based on this method usually does not deliver
sufficiently precise models of the structures.
In this study we reconstructed an accurate 3D FE model of
membranous vestibular labyrinth using the micro computer
tomography (micro-CT) images of guinea pig otocyst.
II.
MATERIALS AND METHODS
A. The specimen preparation for micro-CT
The guinea pig weighing 320g was euthanized with
ketamine hydrochloride (65 mg/kg) and xylazine (6.5 mg/kg)
by intramuscular injection, and fixed by intracardiac perfusion
with 3% paraformaldehyde and 0.5% glutaraldehyde. To
prepare the specimens for micro-CT, the otocyst removed from
the temporal bones was stained with 2% Osmium tetroxide
(OsO4) solution for 5 days to visualize the membranous
labyrinth[3].
B. Micro-CT images
The micro-CT scans were acquired using a high-resolution
micro-CT system (eXplore Locus SP, GE Healthcare). MicroCT protocol is developed for the progressing with tube voltage
65KV at a current of 80μA. A series of 8-bit DICOM format
slices were generated by standard reconstruction software from
the volume data of the vestibular labyrinth with a resolution of
16μm. In total, 700 slice images were obtained in the sagittal,
axial and coronal planes, with a slice increment 0.016 mm and
slice thickness 0.016mm. Scanning Time for each specimen is
about 150 minutes.
C.
Image Processing and 3D Reconstruction
All micro-CT scan images were imported into an image
processing software package for 3D design and modeling
called MIMICS12.0. The image processing started with
selecting an initial cross-section within the region of interest
and defining a threshold for a specific tissue type. In the
software environment, turn up contrast as high as possible
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without causing any loss of detail in the images. Bone and
membranous tissues can be isolated with the cavities of inner
ear using the thresholding toolbar which allowed defining an
object by selecting a range of pixel grey values. A maximum
and minimum value is established and individual pixels are
selected if their value falls in between the threshold values.
Using this threshold value, a color mask was grown.
Segmentation of the canal and membrane was performed using
special editing tools available on MIMICS. The contours
around membranous structure were slightly adjusted manually.
The shape created is projected to the next cross-section and
adjusted to fit the new cross-sectional area. The process
continues until all cross-sections with vestibule and
semicircular canals were added using editing tools. A 3D
model was generated from the selected mask. 3D surface
rendering is performed by means of triangulation of a
segmented 3D area. The interpolation algorithm uses the grey
value interpolation within the slices, but in the Z direction a
linear interpolation between the contours is used. The resulting
surface showed distortions due to the remaining noise. A
simple surface smoothing was used to remove these artifacts at
last.
D. A 3D finite element model of membranous vestibular
labyrinth
The 3D reconstruction can be meshed using the remeshing
module in MIMICS. The protocol includes the steps of
smoothing, normal triangle reduction, split-based automatic
remeshing, and quality preserving triangle reduction, et al. This
is needed in order to raise the quality of the triangles so that a
tetrahedron mesh can built from them.
III.
Figure 1. Scans of the labyrinth of guinea pig specimens, the arrows show the
location of utricular macula (UM), superior semicircular canal(SC). (a) shows
the membranous vestibular labyrinth which was stained with OsO4. (b) shows
the vestibular labyrinth which wasn’t stained. The membrane of SC is not
been shown in the image.
RESULTS
The micro-CT images with a resolution of 16μm allowed
visualizing the membranous structures of vestibular labyrinth.
The 3D geometrical model of bony and membranous labyrinth
including the semicircular canals, ampullae, utricular vestibule
and common crus was reconstructed. A 3D FE model of
membranous structures on the basis of the 3D geometrical
model was built to analyze the balance mechanism.
Fig.1 shows the image of the labyrinth of guinea pig
specimens with a resolution of 16μm. Fig. 1a shows an
essential improvement concerning especially the visualization
quality of the membranous vestibular labyrinth that had been
stained with OsO4 compared to membranous vestibular
labyrinth that hadn’t stained with OsO4 in Fig.1b.
Fig.2 shows the membranous structures of semicircular
canal, utricle and saccule.
Fig.3 shows a series of micro-CT scan images every
0.08mm in the axial plane from the posterior to anterior. The
membranous semicircular duct and its course can be seen.
A sample of the segmentation of the membranous
semicircular duct with bony canal is shown in Fig.4. The
membranes have been reconstructed even in those regions
where they are hardly visible.
Figure 2. The membranous structures of vestibular labyrinth. The arrows
show the location of superior semicircular canal (SC), lateral semicircular
canal (LC), crista of posterior semicircular canal (CP),utricle macula (UM)
and saccule (S).
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Figure 3. A series of micro-CT scan images every 0.08mm in the axial plane.
The figures from ① to ⑧ represent the sequence images of the micro-CT
from posterior to anterior. The black arrow shows the location of the lateral
semicircular canal.
Figure 5. A 3D model of the inner ear. Light blue areas indicates bony
labyrinth and the red areas indicates the membranous vestibular labyrinth,
including three semicircular ducts, utricle and saccule. 3D model and the
axial, coronal and sagittal CT images are simultaneously shown in Fig.4b
Figure 6. The surface mesh of the constructed models of membranous vestibular
labyrinth.
Figure 4. Segmentation of endolymphatic and perilymphatic spaces. Red areas
indicate endolymphatic spaces and light blue areas indicate perilymphatic
spaces.
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Fig.5 presents the 3D vestibular labyrinth models
reconstructed using the image processing technique described
above. The bony vestibular labyrinth model created from the
micro-CT images is shown in transparency. Fig.5b shows that
the reconstructed 3D model superimposed on the original CT
data in axial, coronal and sagittal planes. The 3D reconstruction
allows the configuration of the membranous vestibular
labyrinth to be clearly identified in the labyrinth.
Fig. 6 shows the surface mesh of the reconstructed models
at the end of the meshing protocol. The mesh consists of almost
equilateral triangles.
IV.
DISCUSSION
The thickness of membranous labyrinth is about from 15 to
50μm [4]. Standard medical CT procedures do not have high
enough spatial resolution to display the membranes. In this
study, the membranes were visible in microtomography
because sufficient Osmium was deposited on the wall of the
membranous labyrinth after the specimens had been fixed in
2% OsO4. This application has confirmed this method in the
guinea pig animal model [3]. The membranes of membranous
labyrinth were able to be detected in the micro-CT images.
Some morphological changes, especially shrinkage, are still
present due to a fixation process. The shrinkage was minimized
by avoiding decalcification and dehydration steps that would
invariable in a serial section approach.
The reconstructed 3D model can be superimposed on the
original CT data to examine their consistency in the 3D view
which simultaneously shows 3D model and the axial, coronal
and sagittal CT images. We can measure the volumes of
perilymphatic and endolymphatic spaces using the 3D model of
the inner ear. The results of the geometrical modeling of the
vestibular labyrinth not only provide a basis for the
understanding 3D endolymphatic and perilymphatic structures
but also present an important source for studying the
biomechanics of inner ear diseases. Currently, the temporal
response dynamics and directional sensitivity of the human
semicircular canals cannot be investigated completely with
existing measuring techniques[5].
The geometric model could be used to construct the FE
model [6]. Numerical simulations using mathematical
procedures, such as FE method, remain the most promising
approaches to understand physiological processes and their
pathology [7]. With these FE model, fluid-structure interaction
of endolymph and crista in 3D FE model can be simulated to
explore the macro and mircomechanics of vestibular
proceeding in the inner ear.
V.
CONCLUSION
An accurate 3D geometrical model of membranous
vestibular labyrinth was generated from serial micro-CT
images. The process of staining specimen with OsO4, setting
appropriate parameter values for scanning and processing
images are the preconditions for the membranous labyrinth
visualization in the micro-CT. The models are useful for not
only the membranous labyrinth morphology but also
computational fluid dynamics and FE analysis.
ACKNOWLEDGMENT
This work was supported by the National Natural Science
Foundation of China (Grant No. 30971528), and the Graduate
Innovation Fund of Fudan University (EYH2126023).
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