•
Neuroradiology
Radiological Visualization of the Vestibular Aqueduct Technique and Morphological Features 1 Mokhtar H. Gado, M.D., and I. Kaufman Arenberg, M.D. Thin-section tomography in the lateral and off-lateral projections permits radiographic visualization of the vestibular aqueduct, which is seen as an inverted J. The proximal curved portion has a constant configuration, while the distal straight portion varies according to the width of the external aperture and may be tubular, funnel-shaped, or filiform. Nonvisualization of the curved portion does not necessarily mean that it is not present. INDEX TERMS:
Aqueduct, vestibular. Skull, anatomy • Temporal Bone, radiography
Radiology 117:621-626, December 1975
• lymphatic sac within it in the pathogenesis of both Meniere's disease and endolymphatic hydrops, we wish to describe the technique required for study of the radiographic morphology of this structure. The vestibular aqueduct (1, 2) is a bony tunnel that extends from the medial wall of the vestibule to the posterior (medial) surface of the temporal bone and can readily be demonstrated radiographically in a slice taken from the temporal bone (Fig. 1). It passes medially and backward from the medial wall of the vestibule through the otic capsule in close proximity to the medial side of
tomography has proved valuable in demonstrating the minute anatomy of the temporal bone. Using this technique, Valvassori et at. (3, 4) were able to visualize the vestibular aqueduct and noted that the best projections were the lateral and 10° off-lateral. Stahle and Wilbrand (6, 7) used a similar technique, but with a slightly different angle (15°). Both groups of authors reported that the aqueduct was invisible more often in patients with Meniere's disease. Since experimental and clinical evidence has recently focused attention on the role of the vestibular aqueduct and the endo-
H
YPOCYCLOIDAL
4---""",
7---
----5
3---
2 1 6
Fig. 1. A. Radiograph of a 5-mm-thick sagittal slice of a temporal bone at the plane of the crus commune. Note the inverted J configuration of the vestibular aqueduct. B. Schematic line drawing showing the straight portion of the vestibular aqueduct (1), the curved portion of the vestibular aqueduct and crus commune (2), the vestibule and cochlea (3), the superior semicircular canal (4), the posterior semicircular canal (5), the flared outer aperture of the vestibular aqueduct on the medial surface of the petrous temporal bone (6), and the crus commune (7). 1 From the Neuroradiology Section (M. H. G., Chief), Mallinckrodt Institute of Radiology, and the Department of Otolaryngology, Washington University School of Medicine (I. K. A.), St. Louis, Mo. Presented at the Sixtieth Scientific Assembly and Annual Meeting of the Radiological Society of North America, Chicago, III., Dec. 1-6, 1974. Supported in part by Deafness Research Foundation grants NS-1-10448-01, NS-1-F11-NS-11041-03, and NS-O-5T1-5190 and by neuroradiology training grant 5-T01-NSO-5522-08 from the National Institutes of Health.
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I.
Fig. 3. Diagram of the vestibular aqueduct, illustrating reference points for the measurements shown in TABLE J. 1 width of the outer end of the straight portion; 2 = width of the middle third of the curved portion; 3 length of length of the curved the straight portion; 4 portion.
=
=
=
2.-
Table [: Measurements of the Vestibular Aqueduct in Ten Specimens Using the Reference Points Shown in Figure 3 No. of SpeciType mens
3. Fig. 2. Schematic representation of the position of the head in relation to the film in three projections for visualization of the vestibular aqueduct: true lateral (1), off-lateral (2), and off-lateral with depression of the vertex (3). In all three projections, the ear was further from the film than the midsagittal plane. A. Frontal view. B. Vertex view.
the crus commune, a useful radiographic landmark. The proximal course of the aqueduct is marked by a pronounced curve leading to a straight course downward and backward, ending at the external aperture on the posterior surface of the petrous bone between the porus acusticus and the sigmoid sulcus. Ogura and Clemis (5) emphasized the J-shaped configuration of the aqueduct and described the isthmus as the narrowest part, separating the proximal arched portion from the distal straight portion. The diagrams presented by these authors described the tendency of the straight portion to flare remarkably from the isthmus (measuring 0.2-0.4 mm in width) to the external aperture (3-9 mm). MATERIAL AND METHODS
Ten specimens from ten cadavers were used in this study. Each consisted of half a head, with the line of division through the midsagittal plane. Six specimens were right-sided and four were left-sided. Cadaver specimens were used rather than the temporal bones so that landmarks such as the orbitomeatal line and the sagittal plane could be employed as in radiography of living individuals. Each specimen was studied by hypocycloidal tomography in three projections and the visibility of the vestibular aqueduct was compared in all films. Radiographs in all projections were taken before and after opacification of the aqueduct by injection of Pantopaque through a 17
I II III
5 1 4
Measurements (mm)* 1
2
3
4
1.5-2.0 0.5 4-5
0.5 0.5 0.5
5.5-9
3-5
7
3
3.5-11
4-5
*1
== width of the outer end of the straight portion. 2 == width of the curved portion at its middle third. 3 == length of the straight portion. 4 == length of the curved portion.
gauge needle inserted into the external aperture (Fig. 2). In the true lateral projection, the midsagittal plane was parallel to the film, with the temporal bone farther from the film than the sagittal plane. In the off-lateral projection, the midsagittal plane formed an angle of 15° open anteriorly. In the off-lateral projection with depression of the vertex, the base end of the specimen was raised so that the midsagittal plane formed a 15° angle (open caudally) with the plane of the film. To further illustrate certain morphological aspects, tomograms of seven injected specimens were obtained in the basal projection, corresponding to the submentovertical projection of the skull. The upper border of the zygomatic process of the temporal bone was parallel to the film. Two of these specimens were also studied in the anteroposterior projection. Hypocycloidal tomography was performed with the Philips Polytome, using a focal spot size of 0.3 mm and exposure factors of 20 mA and 38 kV with Kodak RPM-54 film and Du Pont Hi-Plus screens (no grid). The radiographic appearance of the vestibular aqueduct was analyzed and four measurements of its two components, the straight portion and the curved portion, were made using a X6 comparator (Fig. 3): (a) Width of the straight portion at its outer end (b) Width of the curved portion (c) Length of the straight portion (d) Length of the curved portion, measured as the shortest straight line between its two ends. Although the reticle of the comparator was calibrated
Vol. 117
RADIOLOGICAL VISUALIZATION OF THE VESTIBULAR AQUEDUCT
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4 - - - -......5---~~J
1 - - -........",
2--~~
6--~-
c Fig. 4. Type I. Thin-section tomogram in the lateral projection, showing the tubular configuration of the vestibular aqueduct. A. Before contrast injection. B. After injection of Pantopaque into the vestibular aqueduct. C. Line drawing. 1 = straight portion of the vestibular aqueduct; 2 curved portion of the vestibular aqueduct; 3 = common crus; 4 = superior semicircular canal; 5 = posterior semicircular canal; 6 = vestibule and junction with the ampulla of the inferior semicircular canal (out of focus).
=
Fig. 5. Type II. Thin-section tomogram in the lateral projection, illustrating the filiform configuration. A. Before contrast injection. B. After injection of Pantopaque into the vestibular aqueduct. C. Line drawing. 1 = straight portion of the vestibular aqueduct; 2 curved portion of the vestibular aqueduct; 3 common crus; 4 = superior semicircular canal; 5 = posterior semicircular canal; 6 = vestibule and junction with the arnpusa of the posterior semicircular canal (out of focus).
=
to 0.1 mm, boundary unsharpness limited the accuracy of our measurement to no better than 0.5 mm. All measurements presented in this work were taken directly from the hypocycloidal tomogram with an inherent magnification of 30 % . RESULTS
The results are shown in TABLE I and Figures 4-7; all radiographs, whether of right-sided or left-sided specimens, are presented in the same orientation to facilitate comparison. The vestibular aqueduct was identified on the tomographic cuts at or within 1 mm on either side of the plane of the common crus. Visualization of the Straight Portion: In all ten specimens, the straight portion of the vestibular aqueduct was visualized in all projections before and after injection of contrast material. In six specimens (Figs. 4 and
5), the straight portion was mere or less uniform in caliber, tapering slightly toward its proximal (internal) end where it joined the curved portion. The junction between the straight and curved portions was not visualized before injection of contrast material. The caliber at the distal end was slightly wider, measuring 1.5-2.0 mm in five specimens (Fig. 4); in one specimen it measured 0.5 mm (Fig. 5). The length of the straight portion in these six specimens varied from 5.0 to 7.0 mm. In the four remaining specimens, the straight portion of the aqueduct was funnel-shaped (Fig. 6), measuring 4.0 mm at the distal end in two specimens and 5.0 mm in two. Visualization of the Curved Portion: Unlike the straight portion of the vestibular aqueduct, the curved portion was visualized in the true lateral projection without contrast lnjection in only seven of the ten specimens studied. In the other three, it could be seen in the off-lateral projection without (one specimen) and with
624
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H. GADO AND I. KAUFMAN ARENBERG
4-----. 5 ----I 1 - - -..... 2 -----.-;
-_.l\.
3---....&J
c Fig. 6.. . Ty~e III. Polytomogram in the lateral projection, demonstrating the funnel-shaped configuration of the vestibular aqueduct. A. Before contrast mjection, S. After injection of Pantopaque into the vestibular aqueduct. C. Line drawing. 1 straight portion of the vestibular aqueduct; 2 curved portion of the vestibular aqueduct· 3 common crus' 4 superior semicircular canal; 5 posterior semicircular canal; 6 vestibule (out of focus). ' ,
=
=
=
=
B
~----r---3
~
its length, taken as the shortest distance between the two ends of the curve (Fig. 3). was 4.0-5.0 mm. DISCUSSION
2
The vestibular aqueduct is most readily and reproducibly visualized in the straight lateral view; off-lateral views with and without depression of the vertex provided a definite improvement in visibility of the aqueduct on two occasions, particularly the proximal curved portion. The rationale for the off-lateral views can be explained as follows. For best visualization of the vestibular aqueduct, an ideal projection would be one in which the ends of the aqueduct are both in a plane as parallel to the film as possible. The distal end of the aqueduct on the medial surface of the petrous bone lies slightly farther from the midsagittal plane of the skull than the proximal end (Fig. 7) and in a more posterior plane (Fig. 1); thus the aqueduct forms an angle of 10-15 0 (open posteriorly) with the sagittal plane (3). This explains the value of turning the head 10 0 toward Stenver's position in the off-lateral projection," which is done by turning the face away from the film (provided the side being examined is up) while depressing the vertex to correct for the lateral sweep of the curved proximal portion of the aqueduct shown in the basal projection (Fig. 7). In spite of the use of both modifications of the lateral projection, in one of our ten specimens the proximal curved portion of the aqueduct was never visualized without contrast injection. Our material consisted of anatomical specimens from which a considerable part of the soft tissues had been removed during previous dissections and the remaining tissues largely dried; in addition. no motion was present. Thus it follows that we were working under more favorable conditions .than would be true of living subjects; there is less likelihood
. . .r -+----- 6
....1------- 5
Fig. 7. A. Polytomogram of the temporal bone in the basal projection. The vestibular aqueduct has been opacified with Pantopaque. Note the lateral sweep of the proximal part of the vestibular aqueduct as it approaches the vestibule. B. Line drawing. 1 internal auditory canal (partly filled with Pantopaque); 2 = vestibule and ampulla of the posterior semicircular canal (out of focus); 3 cochlea (partly filled with Pantopaque); 4 posterior semicircular canal; 5 distal portion of the vestibular aqueduct (filled with Pantopaque); 6 = proximal portion of the vestibular aqueduct (filled with Pantopaque).
=
=
1
....w-----4
= =
=
=
depression of the vertex (two specimens). In one specimen, the curved proximal portion of the aqueduct was never seen on plain polytomography in any of the projections: however, after injection of contrast material, adequate visualization was achieved in nine specimens. In the remaining specimen, filling was incomplete, but the structure was seen faintly in the plain examination (Fig. 5). The junction between the straight and curved portions was never visualized without contrast material; and even when the curved portion could be seen. it was too faint to be measured. After contrast injection it was possible to measure the caliber of the curved portion in nine specimens. and a value of 0.5 mm was obtained;
2 This is just the opposite of the technique of Valvassori and WiIbrand.-Ed.
Vol. 117
RADIOLOGICAL VISUALIZATION OF THE VESTIBULAR AQUEDUCT
of visualizing the curved portion in clinical experience. Definite patency of the curved portion of the aqueduct was demonstrated by contrast injection in the specimen in which this portion was not seen on the plain tomogram. These data clearly demonstrate that nonvisuaJization of the curved portion of the vestibular aqueduct on the polytomogram does not necessarily mean that it is absent, a fact which should be kept in mind when viewing tomograms of living subjects. Morphological Features in the Lateral (and Modified Lateral) Projection: In reviewing our results, two observations emerge: (a) the relative constancy in the appearance of the proximal curved portion of the vestibular aqueduct and (b) the remarkable variation in the appearance of the distal straight portion. It should be emphasized at the outset that the absence of "flaring" of .the straight portion of the aqueduct is an artifactual impression in most cases. Flaring of the vestibular aqueduct to accommodate the endolymphatic sac is a welt-documented anatomical fact. When the straight portion appears funnel-shaped (Fig. 6), it is expanded In the plane of the tomographic cut; on the other hand, when it appears uniform in caliber (Figs. 4 and 5), it is expanded in a different plane. In the latter case, flaring will be realized on the radiographs of the same specimen by visualization of the same portion of the aqueduct over a series of planes, i.e., tomographic cuts. In their anatomical study, Ogura and Clemis (5) described the curved and straight portions of the vestibular aqueduct. Their mean measurements are shown in TABLE II; our results (TABLE I) are largely in agreement, but there are a few exceptions. In our radiographic study, the proximal curved portion showed a uniform caliber of 0.5 mm and a more or less constant configuration of a gentle curve with a mean length of 4.5 mm, which is significantly more than the 1.4 mm described by Ogura and Clemis. In their anatomical study, the curved portion was measured between the isthmus and the internal aperture; however, on a radiograph it is not possible to identify the isthmus. Therefore our measurements of the curved portion probably inclL!ded part of the aqueduct distal to the isthmus, to which must be added the 30 010 magnification inherent in the technique. In contrast to the almost invariable appearance of the proximal curved portion with a caliber of 0.5 mm, the distal straight portion showed remarkable variation in the lateral projection, depending essentially upon the width of the external aperture. In five of our specimens the external aperture measured 1-2 mmin width (Fig. 4); hence the straight portion appeared as a more or less uniform structure with slight, almost imperceptible tapering toward the 0.5-mm caliber of the proximal portion. In one specimen (Fig. 5), the external aperture measured 0.5 mm and the structure appeared filiform. In the other four (Fig. 6), the external aperture measured 4.0-5.0 mm and the aqueduct appeared funnelshaped. However, it must be pointed out that the shape of the vestibular aqueduct in the lateral projection in a
Table I r:
Neuroradiology
Mean Values of Measurements of the Vestibular Aqueduct*
Width of the external aperture Width of the isthmus Length between the external aperture and the isthmus Length between the isthmus and the inner aperture . ._---
* Taken
625
6.2 mm 0.3 mm 7.3 mm 1.4 mm
from Ogura and Clemis (5)
polytomographic plane reflects the measurements in that plane. Thus the distinction between these different configurations is an arbitrary classification for descriptive purposes and relates only to the particular plane and projection employed. The different appearances of the straight portion of the aqueduct in the lateral and off-lateral projections could be classified arbitrarily as: (a) Type I: The outer aperture measures 1-2 mm. The straight portion is more or less uniform in caliber, although it tapers almost imperceptibly toward its junction with the proximal curved portion. This type appeared in five of the ten specimens in our series and may be called the tubular type (Fig. 4). (b) Type 1/: The outer aperture measures less than 1.0 rnm. The straight portion has a uniform thread-like appearance and has been called the filiform type by Valvassori. It was the least common in our series, seen in only one of our ten specimens (Fig. 5). (c) Type 11/: The outer aperture measures more than 2 mm. The straight portion tapers appreciably toward the narrow proximal curved portion. This may be called the funnel-shaped type and was seen in four of our ten specimens (Fig. 6). CONCLUSIONS
The best technique for radiographic visualization of the vestibular aqueduct is thin-section tomography in the lateral and off-lateral projections, revealing the aqueduct as an inverted J at or within 1 mm of the plane of the common crus. The proximal curved portion has a constant configuration; the straight portion varies in appearance according to the width of the external aperture in this projection and may appear tubular, funnelshaped, or filiform. The straight portion of a patent vestibular aqueduct should be visualized on thin-section tomography. The curved portion can be seen with difficulty; however, nonvlsuallzatlon does not necessarily mean that it is not present.
Neuroradiology Section Mallinckrodt Institute of Radiology Washington University School of Medicine 510 S. Kingshighway St. Louis. Mo. 63110
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REFERENCES 1. Anson BJ: The endolymphatic and perilymphatic aqueducts of the human ear. Developmental and adult anatomy of their parietes and contents in relation to otological surgery. Acta Otolaryngol 59:140-151, Feb-Apr 1965 2. Clem is JD, Valvassori GE: Recent radiographic and clinical observations on the vestibular aqueducts. A preliminary report. Otolaryngol Clin North Am 1:339-346, Oct 1968 3. Valvassori GE: Tomographic findings in Meniere's disease. IX International Congress ORL, Mexico City. Excerpta Medica (Int Congress Series) 189:50 (abst 112), 1969
December 1975
4. Valvassori GE, Clemis JD: Radiologic observations of the cochlear and vestibular apparatus in Meniere's like disorders. Equilibrium Res (in press) 5. Ogura Y, Clemis JD: A study of the gross anatomy of the human vestibular aqueduct. Ann Otol Rhinol Laryngol 80:813-825, Dec 1971 6. Stahle J, Wi/brand H: The vestibular aqueduct in patients with Meniere's disease. A tomographic and clinical investigation. Acta OtolaryngoI78:36-49, 1974 7. Wilbrand HF, Rask-Andersen H, Gilstring D: The vestibular aqueduct and the para-vestibular canal. An anatomic and roentgenologic investigation. Acta Radiol [Diag] 15:337-355, Jul 1974
~~§~
Neuroradiology
Radiological Visualization of the Vestibular Aqueduct Technique and Morphological Features 1 Mokhtar H. Gado, M.D., and I. Kaufman Arenberg, M.D. Thin-section tomography in the lateral and off-lateral projections permits radiographic visualization of the vestibular aqueduct, which is seen as an inverted J. The proximal curved portion has a constant configuration, while the distal straight portion varies according to the width of the external aperture and may be tubular, funnel-shaped, or filiform. Nonvisualization of the curved portion does not necessarily mean that it is not present. INDEX TERMS:
Aqueduct, vestibular. Skull, anatomy • Temporal Bone, radiography
Radiology 117:621-626, December 1975
• lymphatic sac within it in the pathogenesis of both Meniere's disease and endolymphatic hydrops, we wish to describe the technique required for study of the radiographic morphology of this structure. The vestibular aqueduct (1, 2) is a bony tunnel that extends from the medial wall of the vestibule to the posterior (medial) surface of the temporal bone and can readily be demonstrated radiographically in a slice taken from the temporal bone (Fig. 1). It passes medially and backward from the medial wall of the vestibule through the otic capsule in close proximity to the medial side of
tomography has proved valuable in demonstrating the minute anatomy of the temporal bone. Using this technique, Valvassori et at. (3, 4) were able to visualize the vestibular aqueduct and noted that the best projections were the lateral and 10° off-lateral. Stahle and Wilbrand (6, 7) used a similar technique, but with a slightly different angle (15°). Both groups of authors reported that the aqueduct was invisible more often in patients with Meniere's disease. Since experimental and clinical evidence has recently focused attention on the role of the vestibular aqueduct and the endo-
H
YPOCYCLOIDAL
4---""",
7---
----5
3---
2 1 6
Fig. 1. A. Radiograph of a 5-mm-thick sagittal slice of a temporal bone at the plane of the crus commune. Note the inverted J configuration of the vestibular aqueduct. B. Schematic line drawing showing the straight portion of the vestibular aqueduct (1), the curved portion of the vestibular aqueduct and crus commune (2), the vestibule and cochlea (3), the superior semicircular canal (4), the posterior semicircular canal (5), the flared outer aperture of the vestibular aqueduct on the medial surface of the petrous temporal bone (6), and the crus commune (7). 1 From the Neuroradiology Section (M. H. G., Chief), Mallinckrodt Institute of Radiology, and the Department of Otolaryngology, Washington University School of Medicine (I. K. A.), St. Louis, Mo. Presented at the Sixtieth Scientific Assembly and Annual Meeting of the Radiological Society of North America, Chicago, III., Dec. 1-6, 1974. Supported in part by Deafness Research Foundation grants NS-1-10448-01, NS-1-F11-NS-11041-03, and NS-O-5T1-5190 and by neuroradiology training grant 5-T01-NSO-5522-08 from the National Institutes of Health.
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Fig. 3. Diagram of the vestibular aqueduct, illustrating reference points for the measurements shown in TABLE J. 1 width of the outer end of the straight portion; 2 = width of the middle third of the curved portion; 3 length of length of the curved the straight portion; 4 portion.
=
=
=
2.-
Table [: Measurements of the Vestibular Aqueduct in Ten Specimens Using the Reference Points Shown in Figure 3 No. of SpeciType mens
3. Fig. 2. Schematic representation of the position of the head in relation to the film in three projections for visualization of the vestibular aqueduct: true lateral (1), off-lateral (2), and off-lateral with depression of the vertex (3). In all three projections, the ear was further from the film than the midsagittal plane. A. Frontal view. B. Vertex view.
the crus commune, a useful radiographic landmark. The proximal course of the aqueduct is marked by a pronounced curve leading to a straight course downward and backward, ending at the external aperture on the posterior surface of the petrous bone between the porus acusticus and the sigmoid sulcus. Ogura and Clemis (5) emphasized the J-shaped configuration of the aqueduct and described the isthmus as the narrowest part, separating the proximal arched portion from the distal straight portion. The diagrams presented by these authors described the tendency of the straight portion to flare remarkably from the isthmus (measuring 0.2-0.4 mm in width) to the external aperture (3-9 mm). MATERIAL AND METHODS
Ten specimens from ten cadavers were used in this study. Each consisted of half a head, with the line of division through the midsagittal plane. Six specimens were right-sided and four were left-sided. Cadaver specimens were used rather than the temporal bones so that landmarks such as the orbitomeatal line and the sagittal plane could be employed as in radiography of living individuals. Each specimen was studied by hypocycloidal tomography in three projections and the visibility of the vestibular aqueduct was compared in all films. Radiographs in all projections were taken before and after opacification of the aqueduct by injection of Pantopaque through a 17
I II III
5 1 4
Measurements (mm)* 1
2
3
4
1.5-2.0 0.5 4-5
0.5 0.5 0.5
5.5-9
3-5
7
3
3.5-11
4-5
*1
== width of the outer end of the straight portion. 2 == width of the curved portion at its middle third. 3 == length of the straight portion. 4 == length of the curved portion.
gauge needle inserted into the external aperture (Fig. 2). In the true lateral projection, the midsagittal plane was parallel to the film, with the temporal bone farther from the film than the sagittal plane. In the off-lateral projection, the midsagittal plane formed an angle of 15° open anteriorly. In the off-lateral projection with depression of the vertex, the base end of the specimen was raised so that the midsagittal plane formed a 15° angle (open caudally) with the plane of the film. To further illustrate certain morphological aspects, tomograms of seven injected specimens were obtained in the basal projection, corresponding to the submentovertical projection of the skull. The upper border of the zygomatic process of the temporal bone was parallel to the film. Two of these specimens were also studied in the anteroposterior projection. Hypocycloidal tomography was performed with the Philips Polytome, using a focal spot size of 0.3 mm and exposure factors of 20 mA and 38 kV with Kodak RPM-54 film and Du Pont Hi-Plus screens (no grid). The radiographic appearance of the vestibular aqueduct was analyzed and four measurements of its two components, the straight portion and the curved portion, were made using a X6 comparator (Fig. 3): (a) Width of the straight portion at its outer end (b) Width of the curved portion (c) Length of the straight portion (d) Length of the curved portion, measured as the shortest straight line between its two ends. Although the reticle of the comparator was calibrated
Vol. 117
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Neuroradiology
4 - - - -......5---~~J
1 - - -........",
2--~~
6--~-
c Fig. 4. Type I. Thin-section tomogram in the lateral projection, showing the tubular configuration of the vestibular aqueduct. A. Before contrast injection. B. After injection of Pantopaque into the vestibular aqueduct. C. Line drawing. 1 = straight portion of the vestibular aqueduct; 2 curved portion of the vestibular aqueduct; 3 = common crus; 4 = superior semicircular canal; 5 = posterior semicircular canal; 6 = vestibule and junction with the ampulla of the inferior semicircular canal (out of focus).
=
Fig. 5. Type II. Thin-section tomogram in the lateral projection, illustrating the filiform configuration. A. Before contrast injection. B. After injection of Pantopaque into the vestibular aqueduct. C. Line drawing. 1 = straight portion of the vestibular aqueduct; 2 curved portion of the vestibular aqueduct; 3 common crus; 4 = superior semicircular canal; 5 = posterior semicircular canal; 6 = vestibule and junction with the arnpusa of the posterior semicircular canal (out of focus).
=
to 0.1 mm, boundary unsharpness limited the accuracy of our measurement to no better than 0.5 mm. All measurements presented in this work were taken directly from the hypocycloidal tomogram with an inherent magnification of 30 % . RESULTS
The results are shown in TABLE I and Figures 4-7; all radiographs, whether of right-sided or left-sided specimens, are presented in the same orientation to facilitate comparison. The vestibular aqueduct was identified on the tomographic cuts at or within 1 mm on either side of the plane of the common crus. Visualization of the Straight Portion: In all ten specimens, the straight portion of the vestibular aqueduct was visualized in all projections before and after injection of contrast material. In six specimens (Figs. 4 and
5), the straight portion was mere or less uniform in caliber, tapering slightly toward its proximal (internal) end where it joined the curved portion. The junction between the straight and curved portions was not visualized before injection of contrast material. The caliber at the distal end was slightly wider, measuring 1.5-2.0 mm in five specimens (Fig. 4); in one specimen it measured 0.5 mm (Fig. 5). The length of the straight portion in these six specimens varied from 5.0 to 7.0 mm. In the four remaining specimens, the straight portion of the aqueduct was funnel-shaped (Fig. 6), measuring 4.0 mm at the distal end in two specimens and 5.0 mm in two. Visualization of the Curved Portion: Unlike the straight portion of the vestibular aqueduct, the curved portion was visualized in the true lateral projection without contrast lnjection in only seven of the ten specimens studied. In the other three, it could be seen in the off-lateral projection without (one specimen) and with
624
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H. GADO AND I. KAUFMAN ARENBERG
4-----. 5 ----I 1 - - -..... 2 -----.-;
-_.l\.
3---....&J
c Fig. 6.. . Ty~e III. Polytomogram in the lateral projection, demonstrating the funnel-shaped configuration of the vestibular aqueduct. A. Before contrast mjection, S. After injection of Pantopaque into the vestibular aqueduct. C. Line drawing. 1 straight portion of the vestibular aqueduct; 2 curved portion of the vestibular aqueduct· 3 common crus' 4 superior semicircular canal; 5 posterior semicircular canal; 6 vestibule (out of focus). ' ,
=
=
=
=
B
~----r---3
~
its length, taken as the shortest distance between the two ends of the curve (Fig. 3). was 4.0-5.0 mm. DISCUSSION
2
The vestibular aqueduct is most readily and reproducibly visualized in the straight lateral view; off-lateral views with and without depression of the vertex provided a definite improvement in visibility of the aqueduct on two occasions, particularly the proximal curved portion. The rationale for the off-lateral views can be explained as follows. For best visualization of the vestibular aqueduct, an ideal projection would be one in which the ends of the aqueduct are both in a plane as parallel to the film as possible. The distal end of the aqueduct on the medial surface of the petrous bone lies slightly farther from the midsagittal plane of the skull than the proximal end (Fig. 7) and in a more posterior plane (Fig. 1); thus the aqueduct forms an angle of 10-15 0 (open posteriorly) with the sagittal plane (3). This explains the value of turning the head 10 0 toward Stenver's position in the off-lateral projection," which is done by turning the face away from the film (provided the side being examined is up) while depressing the vertex to correct for the lateral sweep of the curved proximal portion of the aqueduct shown in the basal projection (Fig. 7). In spite of the use of both modifications of the lateral projection, in one of our ten specimens the proximal curved portion of the aqueduct was never visualized without contrast injection. Our material consisted of anatomical specimens from which a considerable part of the soft tissues had been removed during previous dissections and the remaining tissues largely dried; in addition. no motion was present. Thus it follows that we were working under more favorable conditions .than would be true of living subjects; there is less likelihood
. . .r -+----- 6
....1------- 5
Fig. 7. A. Polytomogram of the temporal bone in the basal projection. The vestibular aqueduct has been opacified with Pantopaque. Note the lateral sweep of the proximal part of the vestibular aqueduct as it approaches the vestibule. B. Line drawing. 1 internal auditory canal (partly filled with Pantopaque); 2 = vestibule and ampulla of the posterior semicircular canal (out of focus); 3 cochlea (partly filled with Pantopaque); 4 posterior semicircular canal; 5 distal portion of the vestibular aqueduct (filled with Pantopaque); 6 = proximal portion of the vestibular aqueduct (filled with Pantopaque).
=
=
1
....w-----4
= =
=
=
depression of the vertex (two specimens). In one specimen, the curved proximal portion of the aqueduct was never seen on plain polytomography in any of the projections: however, after injection of contrast material, adequate visualization was achieved in nine specimens. In the remaining specimen, filling was incomplete, but the structure was seen faintly in the plain examination (Fig. 5). The junction between the straight and curved portions was never visualized without contrast material; and even when the curved portion could be seen. it was too faint to be measured. After contrast injection it was possible to measure the caliber of the curved portion in nine specimens. and a value of 0.5 mm was obtained;
2 This is just the opposite of the technique of Valvassori and WiIbrand.-Ed.
Vol. 117
RADIOLOGICAL VISUALIZATION OF THE VESTIBULAR AQUEDUCT
of visualizing the curved portion in clinical experience. Definite patency of the curved portion of the aqueduct was demonstrated by contrast injection in the specimen in which this portion was not seen on the plain tomogram. These data clearly demonstrate that nonvisuaJization of the curved portion of the vestibular aqueduct on the polytomogram does not necessarily mean that it is absent, a fact which should be kept in mind when viewing tomograms of living subjects. Morphological Features in the Lateral (and Modified Lateral) Projection: In reviewing our results, two observations emerge: (a) the relative constancy in the appearance of the proximal curved portion of the vestibular aqueduct and (b) the remarkable variation in the appearance of the distal straight portion. It should be emphasized at the outset that the absence of "flaring" of .the straight portion of the aqueduct is an artifactual impression in most cases. Flaring of the vestibular aqueduct to accommodate the endolymphatic sac is a welt-documented anatomical fact. When the straight portion appears funnel-shaped (Fig. 6), it is expanded In the plane of the tomographic cut; on the other hand, when it appears uniform in caliber (Figs. 4 and 5), it is expanded in a different plane. In the latter case, flaring will be realized on the radiographs of the same specimen by visualization of the same portion of the aqueduct over a series of planes, i.e., tomographic cuts. In their anatomical study, Ogura and Clemis (5) described the curved and straight portions of the vestibular aqueduct. Their mean measurements are shown in TABLE II; our results (TABLE I) are largely in agreement, but there are a few exceptions. In our radiographic study, the proximal curved portion showed a uniform caliber of 0.5 mm and a more or less constant configuration of a gentle curve with a mean length of 4.5 mm, which is significantly more than the 1.4 mm described by Ogura and Clemis. In their anatomical study, the curved portion was measured between the isthmus and the internal aperture; however, on a radiograph it is not possible to identify the isthmus. Therefore our measurements of the curved portion probably inclL!ded part of the aqueduct distal to the isthmus, to which must be added the 30 010 magnification inherent in the technique. In contrast to the almost invariable appearance of the proximal curved portion with a caliber of 0.5 mm, the distal straight portion showed remarkable variation in the lateral projection, depending essentially upon the width of the external aperture. In five of our specimens the external aperture measured 1-2 mmin width (Fig. 4); hence the straight portion appeared as a more or less uniform structure with slight, almost imperceptible tapering toward the 0.5-mm caliber of the proximal portion. In one specimen (Fig. 5), the external aperture measured 0.5 mm and the structure appeared filiform. In the other four (Fig. 6), the external aperture measured 4.0-5.0 mm and the aqueduct appeared funnelshaped. However, it must be pointed out that the shape of the vestibular aqueduct in the lateral projection in a
Table I r:
Neuroradiology
Mean Values of Measurements of the Vestibular Aqueduct*
Width of the external aperture Width of the isthmus Length between the external aperture and the isthmus Length between the isthmus and the inner aperture . ._---
* Taken
625
6.2 mm 0.3 mm 7.3 mm 1.4 mm
from Ogura and Clemis (5)
polytomographic plane reflects the measurements in that plane. Thus the distinction between these different configurations is an arbitrary classification for descriptive purposes and relates only to the particular plane and projection employed. The different appearances of the straight portion of the aqueduct in the lateral and off-lateral projections could be classified arbitrarily as: (a) Type I: The outer aperture measures 1-2 mm. The straight portion is more or less uniform in caliber, although it tapers almost imperceptibly toward its junction with the proximal curved portion. This type appeared in five of the ten specimens in our series and may be called the tubular type (Fig. 4). (b) Type 1/: The outer aperture measures less than 1.0 rnm. The straight portion has a uniform thread-like appearance and has been called the filiform type by Valvassori. It was the least common in our series, seen in only one of our ten specimens (Fig. 5). (c) Type 11/: The outer aperture measures more than 2 mm. The straight portion tapers appreciably toward the narrow proximal curved portion. This may be called the funnel-shaped type and was seen in four of our ten specimens (Fig. 6). CONCLUSIONS
The best technique for radiographic visualization of the vestibular aqueduct is thin-section tomography in the lateral and off-lateral projections, revealing the aqueduct as an inverted J at or within 1 mm of the plane of the common crus. The proximal curved portion has a constant configuration; the straight portion varies in appearance according to the width of the external aperture in this projection and may appear tubular, funnelshaped, or filiform. The straight portion of a patent vestibular aqueduct should be visualized on thin-section tomography. The curved portion can be seen with difficulty; however, nonvlsuallzatlon does not necessarily mean that it is not present.
Neuroradiology Section Mallinckrodt Institute of Radiology Washington University School of Medicine 510 S. Kingshighway St. Louis. Mo. 63110
626
MOKHTAR
H. GADO AND I. KAUFMAN ARENBERG
REFERENCES 1. Anson BJ: The endolymphatic and perilymphatic aqueducts of the human ear. Developmental and adult anatomy of their parietes and contents in relation to otological surgery. Acta Otolaryngol 59:140-151, Feb-Apr 1965 2. Clem is JD, Valvassori GE: Recent radiographic and clinical observations on the vestibular aqueducts. A preliminary report. Otolaryngol Clin North Am 1:339-346, Oct 1968 3. Valvassori GE: Tomographic findings in Meniere's disease. IX International Congress ORL, Mexico City. Excerpta Medica (Int Congress Series) 189:50 (abst 112), 1969
December 1975
4. Valvassori GE, Clemis JD: Radiologic observations of the cochlear and vestibular apparatus in Meniere's like disorders. Equilibrium Res (in press) 5. Ogura Y, Clemis JD: A study of the gross anatomy of the human vestibular aqueduct. Ann Otol Rhinol Laryngol 80:813-825, Dec 1971 6. Stahle J, Wi/brand H: The vestibular aqueduct in patients with Meniere's disease. A tomographic and clinical investigation. Acta OtolaryngoI78:36-49, 1974 7. Wilbrand HF, Rask-Andersen H, Gilstring D: The vestibular aqueduct and the para-vestibular canal. An anatomic and roentgenologic investigation. Acta Radiol [Diag] 15:337-355, Jul 1974
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