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Three-dimensional Imaging of the Inner Ear by Volume-Rendered Reconstructions of Magnetic Resonance Data
Randolf Klingebiel, MD;
Nadine Thieme, MD;
Dietmar Kivelitz, MD;
Christian Enzweiler, MD;
Mechthild Werbs, MD;
Rüdiger Lehmann, MD
Arch Otolaryngol Head Neck Surg. 2002;128:549-553.
ABSTRACT
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Objective To evaluate 3-dimensional inner ear visualization by volume rendering
of high-resolution magnetic resonance data in patients with clinically suspected
inner ear abnormality.
Design Prospective comparative study of different postprocessing techniques,
based on blinded film readings.
Setting Tertiary referral hospital.
Subjects Fifty patients (17 females and 33 males) aged 1 to 77 years (average
age, 42 years) with sensorineural hearing loss, vertigo, and/or tinnitus.
Intervention Postprocessing of magnetic resonance data to inner ear reconstructions
by the use of volume rendering as well as maximum-intensity projection; caloric
testing by electronystagmography.
Main Outcome Measures Film was read blindly by 4 radiologists using a 5-point parameter scale
for image quality and diagnostic value. The assessibility of inner ear subsegments
was evaluated. The specificity of volume-rendered reconstructions for detecting
semicircular canal obliterations was assessed in a subgroup of 9 patients
by caloric testing. The time required for data postprocessing as well as film
reading was recorded by means of a stopwatch.
Results Volume-rendered inner ear reconstructions were superior in image quality
(P<.001), diagnostic value (P<.001), subsegment inner ear assessment (P<.01
to P<.001), and film reading time (P<.001) compared with maximum-intensity projections. The data postprocessing
time was comparable for both techniques. Caloric weakness was noted in all
patients assessed by electronystagmography.
Conclusion Volume rendering is the postprocessing technique of choice for 3-dimensional
inner ear visualization, performing better than maximum-intensity projections
with respect to various parameters.
INTRODUCTION
INNER EAR (IE) visualization requires high-resolution (HR) cross-sectional
imaging, regardless of whether computed tomography or magnetic resonance (MR)
imaging is performed.1-2 In cases
where an abnormality of the membranous rather than of the bony labyrinth is
clinically suspected, only MR imaging enables comprehensive IE assessment.
Heavily T2-weighted HR sequences have become an indispensable part of IE imaging,3-4 whereas T1-weighted sequences may be
necessary only when the patient's history suggests a traumatic or acute inflammatory
IE condition.
To cope with the huge number of cross-sectional slices provided by HR
imaging, different postprocessing techniques have evolved. The most widely
used technique, the maximum-intensity projection (MIP), yields about 8 to
12 two-dimensional projection images for each side. Data postprocessing to
generate 3-dimensional (3-D) IE views by means of the volume rendering (VR)
technique is a more recent postprocessing technique that has shown promising
results in terms of image quality and reconstruction flexibility.5-7 To our knowledge, this
is the first study assessing the VR technique in a large number of patients
with clinical evidence of a pathologic IE condition.
PATIENTS AND METHODS
An experienced neuroradiologist (R.K.) assessed MR IE studies of 85
consecutive patients, referred by ear, nose, and throat physicians for HR
IE imaging, for positive or questionable signs of IE disease. All patients
(n = 50; 33 males and 17 females, ranging in age from 1-77 years; average,
42 years) in whom IE disease could not be confidently ruled out on the basis
of the primary, cross-sectional image data were included in the study. The
majority of study patients had sensorineural hearing loss (n = 24) followed
by sensorineural hearing loss and vertigo and/or tinnitus (n = 20). Vertigo
and/or tinnitus were noted in 3 patients, and in another 3 patients conclusive
clinical data were not available.
All patients underwent HR MR imaging in a 1.5-T scanner (Magnetom Vision;
Siemens, Erlangen, Germany) with the use of a standard circular polarized
head coil. A 3-D Fourier transformation–constructive interference in
steady state sequence was applied, defined by the following variables: repetition
time, 12.3 milliseconds; echo time, 5.9 milliseconds; slice thickness, 0.5
mm; flip angle, 70°; number of acquisitions, 1; time of acquisition, 13.5
minutes; matrix, 256 x 256; field of view, 130 mm. Subsequently, the
study data were transferred via an internal network to a workstation consisting
of a computer (Ultra 60; Sun Microsystems, Inc, Palo Alto, Calif) and a software
package with a module for volume rendering (EASY VISION 4.1; Philips Medical
Systems, Best, the Netherlands).
The VR postprocessing protocol was defined in a preliminary study using
perspective views, threshold values, depth cuing, and other parameters.6 Figure 1
shows the impact of parameter variation on the image quality of VR views.
In brief, a frontal 3-D shaded-surface view of the cochlea and vestibule was
generated as well as a view of all 3 semicircular canals from a craniolateral
angle (Figure 2A-B). The MIP reconstructions
(Figure 2C), featuring 9 different
projection images, were generated at the console of the MR imager. The postprocessing
time was recorded by the use of a stopwatch. Subsequently, 4 radiologists
(R.K., D.K., C.E., and R.L.) with varying degrees of neuroradiologic training
(2 years, 1 year, 3 months, and 35 years, respectively) assessed the VR as
well as the MIP reconstructions in a blinded manner on a 5-point parameter
scale (1, insufficient; 2, poor; 3, sufficient; 4, good; 5, excellent) for
image quality and diagnostic value. In addition, anatomic subsegments of the
membranous labyrinth, defined as basal cochlea turn, middle and upper cochlea
turn, vestibulum, and lateral, posterior, and superior semicircular canals,
were selectively checked for their assessability.
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Figure 1. Impact of threshold value and
depth cuing on image quality of volume-rendered reconstructions of the inner
ear.
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Figure 2. Different postprocessing techniques
for inner ear visualization, based on high-resolution magnetic resonance data:
volume-rendered views of the cochlea (A) and semicircular canals (B) and maximum-intensity
projection (C).
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Statistical analysis was performed by means of a Wilcoxon rank-sum test
(paired samples) for the 5-point parameter scale, a McNemar test for the subsegmental
labyrinth assessibility, and t test for the results
of stopwatch measurements for reconstruction and film reading time. The charts
of all patients with evidence of lateral semicircular canal abnormality, as
suggested by VR IE reconstructions (n = 16), were reviewed for caloric test
results; if these results were not available, the patients were asked to undergo
caloric testing. In 9 of 16 patients, caloric testing could be performed by
electronystagmography.
RESULTS
The average time required for generating the MIP (3.4 minutes) and VR
(3.1 minutes) images of one IE were comparable without statistically significant
differences. The mean ± SD time required for assessing VR views amounted
to 24.9 ± 17.5 seconds vs 37.8 ± 22.1 seconds for MIP images,
resulting in a reduction of 34.1% for VR (P<.001, t test). The VR displays were assigned a higher score (P<.001, Wilcoxon test) for image quality (mean ±
SD, 4.1 ± 1.1 vs 3.0 ± 1.3) as well as diagnostic value (mean
± SD, 4.4 ± 1.1 vs 3.3 ± 1.3) (Figure 3).
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Figure 3. Results of blinded reading of
hard-copy printouts of volume-rendered (VR) and maximum-intensity projection
(MIP) labyrinth reconstructions by means of a 5-point scale.
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In total, 600 IE subsegments were selectively assessed for each postprocessing
technique (VR and MIP). Figure 4
provides an overview of the imaging findings, based on VR reconstructions.
The overall assessability of IE subsegments was significantly increased by
the use of VR reconstructions as compared with MIP images (P<.001, McNemar test), showing a decrease in the significance level
for judging the vestibulum (P<.01, McNemar test).
Monosegmental or multisegmental obliterations of the labyrinthine fluid signal
considered pathologic were encountered in 38 IEs (38%), showing the following
distribution: cochlea (16 IEs), vestibulum (1 IE), superior semicircular canal
(12 IEs), lateral semicircular canal (16 IEs), and posterior semicircular
canal (8 IEs). In 14 IEs evidence of multisegmental lesions was detected.
In 9 of 16 patients in whom VR images suggested lateral semicircular canal
abnormality, caloric test results were obtained. In all of these patients
caloric weakness was encountered, indicating a high specificity of VR reconstructions
for noninvasive detection of lateral semicircular canal abnormality. Examples
of various kinds of IE abnormalities as visualized by VR are presented in Figure 5 and Figure 6. In 8 IEs, image quality was severely compromised in VR
reconstructions because of motion artifacts (7 IEs) and pulsation artifacts
caused by the internal carotid artery (1 IE), making IE assessment impossible.
In 21 IEs, minor image artifacts caused by the pulsating internal carotid
artery and/or basilar artery as well as by fluid-retaining mastoid cells could
be resolved by parameter variations of the postprocessing protocol.
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Figure 4. Imaging findings, based on volume-rendered
3-dimensional reconstructions of the inner ear (IE).
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Figure 5. Volume-rendered image reconstructions
of the cochlea and vestibulum, frontal view, visualizing various abnormalities.
A, Mondini malformation, showing an incomplete cochlea partition (arrow).
B, Pseudo-Mondini malformation, combined with a vestibule–lateral semicircular
canal dysplasia (arrow). C, Labyrinthitis ossificans; a circumscribed basal
turn obliteration is depicted (arrow). D, Appearance after translabyrinthine
surgery; only the cochlea is preserved (circle).
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Figure 6. Volume-rendered image reconstructions
of all semicircular canals, viewed from a craniolateral angle. A, Aplasia
of the posterior semicircular canal. Note the enlarged vestibular aqueduct
(arrow). B, Mondini malformation, showing a vestibule–lateral semicircular
canal dysplasia (arrow). C, Labyrinthitis ossificans; irregular multisegmental
labyrinthine obliterations are demonstrated (arrows). D, Appearance after
tympanoplastic surgery; an almost complete obliteration of the lateral semicircular
canal fluid signal is noted (arrows), subsequent to a labyrinthine fistula.
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COMMENT
Inner ear imaging with the use of unenhanced heavily T2-weighted, HR
MR protocols has become an established visualization technique during the
past decade. Most commonly a gradient-echo1, 3
or a fast spin-echo4-5,8-9
sequence is used. Even though fast spin-echo sequences are generally less
susceptible to image artifacts by tissue interfaces, both sequence types are
widely used, and the 3-D Fourier transformation–constructive interference
in steady state sequence (gradient echo) has been considered to have superior
detail resolution with respect to the identification of cranial nerves in
a recent study.10 The data acquisition protocol
applied, also featuring a 3-D Fourier transformation–constructive interference
in steady state sequence, yielded isotropic image volume elements (voxels)
of 0.53 mm, thus enhancing any kind of 3-D visualization technique.5
Both sequence types (gradient echo and fast spin-echo) yield numerous
cross-sectional images and require data postprocessing to condense and communicate
the important MR study findings effectively. Maximum intensity projection,
surface rendering, and, more recently, VR have been used for visualizing the
complex IE architecture on the basis of HR cross-sectional data.2, 5, 11-13
The MIP, as the most widespread postprocessing technique, takes insufficient
advantage of the high spatial resolution available, reducing the complex 3-D
structure of the labyrinth to various 2-dimensional projection images.5, 7 The surface rendering technique typically
models surfaces from overlapping polygons and is limited with respect to detail
resolution and reconstruction flexibility as compared with VR.7, 14
Volume rendering, as the only technique that incorporates the entire data
set into the 3-D image, has not been available for routine imaging purposes
until recently because of limitations in hardware performances.14
A preceding study by our group showed that threshold-based direct VR, combined
with user-defined postprocessing protocols and standardized IE views, allows
for advanced 3-D IE visualization within about 5 to 6 minutes per ear.15 A further reduction of the postprocessing time for
VR images, down to about 3 minutes per ear in the present study, was most
probably due to increasing experience in handling the software and hardware
tools. Volume rendering provides valuable tools for reducing image artifacts
that severely compromise image quality in MIP reconstructions, such as those
caused by fluid-retaining mastoid cells.5, 15
The statistically significant differences in parameter scores, subsegmental
labyrinthine assessability, and image evaluation time between VR and MIP reconstructions
in our study underline the superior performance of the VR technique.
Image quality may not be as important in a grossly affected IE with
extensive labyrinthitis ossificans. Yet, for complex abnormalities and/or
syndromal diseases such as various IE dysplasias, the precise definition of
the principal pathologic changes and associated IE lesions may be crucial
for clinical management. The potential benefit and the specific procedure
of a surgical intervention such as cochlear implant, possible intraoperative
complications, and implications for genetic counseling all are influenced
by the type of dysplastic entity encountered.16-20
Moreover, precise IE assessment may have therapeutic implications not only
in dysplastic but also in other pathologic conditions, such as a labyrinthitis.
Although a T1-weighted, contrast-enhanced study is necessary in acute disease,1 a protracted course or chronic disease may lead to
irregular obliterations of the labyrinthine fluid signal in T2-weighted HR
MR studies. In our investigation, dysplastic and postinflammatory obliterations
differed markedly in their appearance. Thus, the imaging protocol presented
here may be especially beneficial in cases where anti-inflammatory treatment
is considered as well as in patients where successful cochlear implant surgery
depends on the timely recognition of labyrinthine obliterations before the
occurrence of cochlear calcifications. Postoperative patients showing smoothly
demarcated signal obliterations in our study could be reliably identified
on the basis of their clinical history. Yet, not only the morphologic characteristics
but the topography of IE lesions may help to differentiate between various
causes of IE pathologic changes. Himi et al21
reported a preference of meningogenic IE lesions for the basal cochlear turn
and semicircular canals as compared with tympanogenic labyrinthitis. Both
patients in our study whose clinical history suggested meningogenic labyrinthitis
in childhood showed markedly affected basal cochlear turns and semicircular
canals (Figure 5C and Figure 6C).
A potential pitfall of 3-D postprocessing techniques is the choice of
inappropriate rendering parameters, causing loss of relevant image information.14 As a reference method for detecting peripheral vestibular
lesions as suggested by VR image reconstructions, caloric testing by using
electronystagmography was chosen. Electronystagmography is widely recognized
as a gold standard in terms of vestibular function tests.22
All patients with evidence of lateral semicircular canal fibrosis by VR reconstructions
who agreed to undergo electronystagmographic testing showed caloric weakness,
indicating a high specificity of the visualization protocol presented for
detecting peripheral vestibular disease. The corresponding sensitivity of
VR IE reconstructions, however, remains unclear, as caloric testing was not
performed on a regular basis in all study patients.
CONCLUSIONS
Our results suggest that VR should be the method of choice for postprocessing
HR IE images. Only direct VR allows for comprehensive IE assessment with a
limited number of 3-D IE reconstructions and meets the need for rapid labyrinthine
visualization in an easily appreciable fashion. The image quality of 3-D VR
views provides detailed information on morphologic features and topography
of labyrinthine lesions, permitting differentiation of various kinds of IE
disease in many cases.
AUTHOR INFORMATION
Accepted for publication October 2, 2001.
Corresponding author: Randolf Klingebiel, MD, Neuroradiology Section,
Department of Radiology, Charité CM, Schumannstr 20/21, 10098 Berlin,
Germany (e-mail: Randolf.Klingebiel{at}charite.de).
From the Neuroradiology Section (Drs Klingebiel, Thieme, and Lehmann),
Department of Radiology (Dr Kivelitz), and Ear, Nose, and Throat Department
(Dr Werbs), Charité Campus Mitte, Humboldt University, Berlin, Germany;
and the Department of Radiology, Massachusetts General Hospital, Harvard Medical
School, Boston (Dr Enzweiler).
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