AJR 2002; 179:267-272
© American Roentgen Ray Society
The Palatovaginal Canal: Can It Be Identified on Routine CT and MR Imaging?
Zoran Rumboldt1,2,
Mauricio Castillo1 and
Jeffrey K. Smith1
1 Department of Radiology, CB #7510, University of North Carolina at Chapel
Hill, Chapel Hill, NC 27599-7510.
2 Present address: Department of Radiology, University Hospital "Sestre
milosrdnice," Zagreb, Croatia.
Received August 28, 2001;
accepted after revision January 24, 2002.
Address correspondence to M. Castillo.
Abstract
OBJECTIVE. The palatovaginal canal is a short bone tunnel that
extends from the pterygopalatine fossa to the roof of the pharynx. The primary
purpose of our work was to establish whether the palatovaginal canal can be
identified on CT and MR imaging. The secondary goal was to establish the
frequency of visualization and the appearance of this canal.
MATERIALS AND METHODS. We retrospectively analyzed 150 consecutive
direct coronal CT studies obtained for evaluation of the sinonasal cavities.
Frequency, bilaterality, and appearance of the palatovaginal canals were
recorded. The frequency of the vidian canals was recorded for comparison. We
also analyzed 20 MR imaging studies of that area to assess visualization of
the palatovaginal canals and their contents. A dry skull specimen was examined
using CT, and the images were correlated with those obtained in vivo.
RESULTS. The palatovaginal canal could be identified on CT on at
least one side in 88 (58.7%) of 150 patients. Unilateral complete canals were
found in 14 patients (9.3%), and unilateral semicanals were evident in 17
(11.3%). Bilateral complete canals were seen in 24 patients (16%), and
bilateral semicanals were found in 11 (7.3%). In 22 patients (14.7%), one
complete canal and one semicanal were detected. Fifty-five percent of the
visualized canals were completely formed. The palatovaginal canal and its
internal tubular structure, presumably corresponding to the pterygovaginal
artery, were depicted on 40% of the MR imaging studies. The position and
configuration of this canal as seen on CT of the dry skull specimen correlated
well with the imaging findings.
CONCLUSION. The palatovaginal canals are commonly depicted on CT and
MR imaging.
Introduction
The palatovaginal canal is a short bone tunnel formed by the application of
the sphenoid process of the palatine bone to the vaginal process of the
sphenoid bone [1]. The
palatovaginal canal is found inferomedially on the posterior wall of the
pterygopalatine fossa, in the roof of the nasopharynx. The canal transmits the
pterygovaginal artery (pharyngeal artery and descending pharyngeal artery), a
posterior branch of the internal maxillary artery, and the pharyngeal nerve
from the pterygopalatine ganglion to the pharyngeal orifice of the auditory
tube
[1,2,3,4].
Altogether, the pterygopalatine fossa directly communicates with seven
different regions via eight passageways. It is commonly accepted that seven of
these eight openings are routinely visualized on most high-resolution CT
studies [2,
5,6,7].
Several groups of researchers have stated that the remaining exiting foramen,
the palatovaginal canal, either cannot be identified or is very rarely seen or
poorly depicted [2,
3,
8]. The primary purpose of our
study was to establish whether the palatovaginal canal can be identified on
routine CT and MR imaging. The secondary goal was to establish the frequency
of visualization and the appearance of this anatomic structure.
Materials and Methods
A retrospective review of the studies performed over a period of 14 weeks
was completed. All patients who underwent unenhanced CT covering the area from
the posterior wall of the maxillary sinus to the cavernous sinus with
contiguous images of 3-mm-thick slices obtained in the direct coronal plane
were included in our study group. The studies were assessed for the presence
of the palatovaginal and vidian canals. Of 150 patients, there were 68 females
(45.3%) and 82 males (54.7%); patients ranged in age from 6 to 88 years, with
an average age of 40.7 years. Indications for the studies were chronic or
acute sinusitis (n = 88), trauma and a possible fracture (n
= 31), trauma with a foreign body (n = 7), sinonasal or orbital
neoplasm (n = 7), infection focus or abscess (n = 5),
sinonasal polyp (n = 3), nontraumatic epistaxis (n = 2), and
a miscellaneous cause (n = 7).
All studies were performed using helical CT scanners (Siemens, Erlangen,
Germany) set at 200-240 mAs and 120 kV. The images were reconstructed using a
bone algorithm and were evaluated at wide (bone) window settings. The scans
were independently reviewed by two observers on computer workstations, and
differences in opinion were solved by consensus. We recorded the incidence and
laterality of the palatovaginal canals and of the vidian canals. The diameters
of all the visualized palatovaginal canals were independently measured by the
two observers, and the mean of the two measurements was recorded as the
diameter. A complete canal was defined as a round or ovoid structure with a
sclerotic rim and a radiolucent center seen on at least one coronal section.
An open (incomplete) canal was defined as a radiolucent area surrounded by a
sclerotic rim that extended less than 50% of the presumed circumference.
We found that the easiest way for us to detect the palatovaginal canal on
coronal CT images was to identify the pterygopalatine fossa and then follow
its course posteriorly. In this direction, the shape of the pterygopalatine
fossa changes from triangular to ovoid and oblique: Its superior and lateral
aspects continue posteriorly as the vidian canal, and the medial and inferior
portions become the palatovaginal canal (Figs.
1A,1B,1C
and
2A,2B).
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Fig. 1B. —32-year-old man with sinusitis. Unenhanced coronal CT image
obtained just posterior to A reveals change in shape of pterygopalatine
fossa (arrows) to ovoid and oblique. Round foramen of sphenoid
(arrowheads) can also be seen.
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Fig. 1C. —32-year-old man with sinusitis. Unenhanced coronal CT image
obtained just posterior to B shows that oblique pterygopalatine fossa
has separated into larger laterosuperior vidian canal (white arrows)
and smaller medioinferior palatovaginal canal (black arrows).
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Fig. 2A. —31-year-old woman with sinusitis. Unenhanced coronal CT image
shows ovoid and oblique shape of pterygopalatine fossa (arrows) in
posterior aspect, with more prominent laterosuperior portion and smaller
medioinferior one. Positioning is slightly oblique, with left-sided structures
being almost one slice anterior to contralateral side.
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Fig. 2B. —31-year-old woman with sinusitis. Unenhanced coronal CT image
obtained at adjacent posterior level to A shows separation of
palatovaginal canal (black arrow) and vidian canal (white
arrow) on right. On left, subtle demarcation between vidian canal and
palatovaginal canal (arrowhead) is visible.
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Exclusion criteria were technically suboptimal scans of the region of
interest as a result of motion or metallic artifacts and severe traumatic or
destructive lesions in that region. On the basis of these criteria, four
studies were excluded: two because of artifacts and two because of bone
destruction. A total of 150 scans were included in this study. The chi-square
test and Student's t test were used for statistical evaluation of the
data.
We then retrospectively assessed the presence of the palatovaginal canal on
MR imaging studies obtained for the evaluation of the pituitary gland in 20
patients on a 1.5-T scanner (Vision or Symphony; Siemens). T1-weighted MR
images obtained in the coronal and sagittal planes that were judged to be of
adequate quality were reviewed. Technical parameters for MR imaging studies
were as follows: TR range/TE, 510-810/14; slice thickness, 2-3 mm; matrix, 512
x 256; field of view, 120-180 mm; number of acquisitions, 1-2; and
percentage of oversampling, 100%. The MR images were reviewed together by both
neuroradiologists. The observers attempted to identify the palatovaginal and
vidian canals in these studies. In addition, the presence or absence of a flow
void inside the palatovaginal canal was recorded.
To confirm our findings, we performed CT of a dry skull specimen using the
same parameters as those used for the clinical studies. The skull specimen was
imaged in the axial and coronal planes without and with thin metal wires
placed in the palatovaginal canals. The images from the specimen were then
correlated with those obtained in vivo.
Results
CT
Differences in opinion between the two neuroradiologists were encountered
for 23 palatovaginal canals and seven vidian canals. It was decided by
consensus that of 17 possible palatovaginal canals, 13 were absent and four
were present; of six palatovaginal canals of questionable appearance, three
were judged to be complete and three, incomplete. The vidian canals in
question were decided to be present in four and absent in three cases.
At least one palatovaginal canal was identified on one or two contiguous
images in 88 patients (58.7%). The palatovaginal canal was visualized
bilaterally in 57 patients (38%) and unilaterally in 31 patients (20.7%).
Unilateral semicanals were found in 17 individuals (11.3%), and unilateral
complete canals in 14 (9.3%). Bilateral complete canals were seen in 24
patients (16%), and bilateral incomplete canals in 11 (7.3%). In 22 patients
(14.7%), one complete canal and one semicanal were detected. Complete
palatovaginal canals were found in 27.3% of the patients with at least one
visualized palatovaginal canal and in 42.1% of those with bilateral canals
seen.
If we assume that the canals should have been present in all individuals,
then of 300 possible palatovaginal canals in 150 patients, 146 canals (48.7%)
were visualized (females, 47%; males, 50%). There were 81 complete
palatovaginal canals (27%), which correspond to 55.5% of all visualized
palatovaginal canals. The palatovaginal canal on the right was observed
slightly more frequently than that on the left (females, 48.5% on the right vs
45.5% on the left; males, 52.8% on the right vs 46.3% on the left), and
visualization of complete canals was also somewhat more common on the right
side (females, 63.6% on the right vs 51.6% on the left; males, 61.4% on the
right vs 44.7% on the left). As expected, none of these differences between
the sides or patient sex were statistically significant (p > 0.1,
2 = 0.52-1.65).
The average diameter of the palatovaginal canals was 0.9 mm (range, 0.5-2.5
mm) (Figs. 3 and
4). The differences in diameter
between the left palatovaginal canal and the right one in either sex or
between sexes were not statistically significant (p > 0.1). Only
three patients had palatovaginal canals that were wider than 1.4 mm, and one
of them presented with recurrent epistaxis. In the other patient with
epistaxis, the palatovaginal canals were not visualized.
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Fig. 3. —14-year-old boy with recurrent epistaxis. Unenhanced coronal
CT image shows prominent palatovaginal canal (solid black arrow) on
right, of approximately same size as ipsilateral vidian canal (white
arrow), which measures 1.6 mm in diameter. These findings influenced
treatment planning, and embolization was considered. Patient was lost to
follow-up before angiogram was obtained. Leftsided palatovaginal canal is seen
as semicanal (open black arrow). This case represents typical oblique
alignment of three posterior pterygopalatine fossa foramina, with round
foramen of sphenoid bone (arrowhead) as most superior and lateral
one.
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Fig. 4. —37-year-old man with orbital foreign bodies. Unenhanced
coronal CT image shows bilateral endosinusal vidian canals (white
arrows) with associated dehiscence of roof. Round foramina of sphenoid
are also protruding into sphenoidal sinus (arrowheads). Both
palatovaginal canals (black arrows) are shown as complete canals.
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Of the 300 vidian canals in our study group, 294 (98%) were visualized, of
which 46 (15.3%) were intrasphenoid (endosinusal). The palatovaginal canal was
seen in only 12 of 46 patients with endosinusal vidian canals, corresponding
to 26.1% (Fig. 4). The
frequency of visualization of palatovaginal canals associated with endosinusal
vidian canals compared with that of palatovaginal canals and vidian canals
that were not endosinusal differed significantly (p < 0.01,
2 = 10.05).
MR Imaging
On the T1-weighted MR images, the maxillary nerves and neurovascular
structures in the vidian canal were usually well visualized on images obtained
in the sagittal and coronal planes. The course of the internal maxillary
artery and its main branches, as well as the sphenopalatine ganglion, could
also be seen on coronal images. The palatovaginal canal was seen medial and
inferior to the vidian canal; internal tubular flow void or
low-signal-intensity structures were considered to be consistent with the
pterygovaginal artery, the vidian artery, or the corresponding nerves (Figs.
5A,5B
and 6), or both. We could not
confidently identify the palatovaginal canal on the sagittal MR images in any
of the patients. The palatovaginal canal with the presumed pterygovaginal
artery was bilaterally visualized in eight of 20 patients, corresponding to
40% of the cases. The depiction of these small anatomic structures depended on
the amount of surrounding fatty tissue present to provide intrinsic
contrast.
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Fig. 5A. —30-year-old woman evaluated for pituitary microadenoma.
Unenhanced coronal T1-weighted MR image obtained at posterior aspect of
pterygopalatine fossa shows two linear flow-void structures (arrows)
that arose bilaterally from internal maxillary artery and are directed
posteromedially.
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Fig. 5B. —30-year-old woman evaluated for pituitary microadenoma.
Unenhanced coronal T1-weighted MR image obtained posterior to A shows
that flow-void structures revealed in A are now found in endosinusal
vidian canals (white arrows) and palatovaginal canals (black
arrows), corresponding to vidian and pterygovaginal arteries,
respectively.
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Fig. 6. —28-year-old woman with pituitary microadenoma. Unenhanced
coronal T1-weighted MR image shows two low-signal-intensity structures that
are bilaterally extending posteriorly from pterygopalatine fossa. These
structures presumably correspond to pterygovaginal artery and, possibly,
pharyngeal nerve (solid arrows) and vidian artery and nerve (open
arrows), although they are not depicted as flow voids. Maxillary nerves
are bilaterally visualized in round foramina of sphenoid
(arrowheads). Note oblique linear alignment maxillary nerve, vidian
artery and nerve, pterygovaginal artery. Compare this image with Figures
1A,1B,1C
and 3.
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Dry Skull Specimen
The pharyngeal orifices of the palatovaginal canals were bilaterally well
visualized in a dry skull specimen (Fig.
7A,7B,7C,7D).
The location and appearance of the palatovaginal canals on CT images of the
specimen corresponded well with those visualized in vivo, confirming our
findings (Fig.
7A,7B,7C,7D).
Compared with the in vivo results, the diameters of the canals were slightly
smaller: 0.8 mm on the right and 0.7 mm on the left.
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Fig. 7A. —Dry skull specimen from cadaver of adult male. Magnified
photograph of skull base as seen from below shows posterior (pharyngeal)
orifice of palatovaginal canals on both sides (arrows).
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Fig. 7C. —Dry skull specimen from cadaver of adult male. Direct coronal
CT image of specimen with opaque markers (0.018-inch guidewire) placed through
both palatovaginal canals (black arrows). Vidian canals (white
arrows) and round foramina of sphenoid (arrowheads) are well
depicted.
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Fig. 7D. —Dry skull specimen from cadaver of adult male. Direct coronal
CT image of specimen obtained at level slightly more posterior to C
reveals wires in palatovaginal canals (black arrows) descending
toward pharynx. Vidian canals (white arrows) are also well
depicted.
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Discussion
Recognition of skull base anatomic structures is important for
understanding the complex regional neurovascular anatomy and for
distinguishing normal from abnormal findings. The pterygopalatine fossa is of
clinical importance because it represents the central intersection of vascular
and nervous structures in the head. The pterygopalatine fossa is an elongated,
funnel-shaped space located behind the posterior wall of the maxillary sinus
and below the body of the sphenoid bone. This structure is bounded medially by
the perpendicular lamina of the palatine bone and posteriorly by the fused
pterygoid plates. The pterygopalatine fossa contains the terminal portion of
the internal maxillary artery and its branches, the pterygopalatine
(sphenopalatine) ganglion and its connections, and the maxillary nerve
[1,2,3,4,5,6,7].
The contents of the pterygopalatine fossa may be divided into two distinct
layers: an anterior one, containing all the blood vessels, and a posterior
one, where the nervous structures are found
[1,
9,
10]. The pterygopalatine fossa
is laterally continuous with the infratemporal fossa of the masticator space
via the pterygomaxillary fissure, and inferiorly it communicates with the oral
cavity through the greater and lesser palatine foramina. Anterosuperiorly the
pterygopalatine fossa is connected to the orbital apex via the infraorbital
fissure, and medially it communicates with the nasal cavity through the
sphenopalatine foramen
[2,3,4,5,6,7].
On the posterior wall of the pterygopalatine fossa are thre exiting foramina
with an oblique alignment. The round foramen of the sphenoid bone is lateral
and superior to the pterygoid (vidian) canal, which is lateral and superior to
the palatovaginal canal
[1,2,3,
8,
11]. The round foramen of the
sphenoid bone connects the pterygopalatine fossa with the middle cranial
fossa, and it connects the pterygoid canal with the foramen lacerum.
The palatovaginal canal, one of the foramina exiting the pterygopalatine
fossa, is rarely mentioned in the medical literature and is even omitted in
articles and textbook chapters describing the anatomy of the pterygopalatine
fossa [7,
12]. We found two studies in
the radiology literature that clearly show the anatomy of palatovaginal canal,
both of which used dry skull specimens
[8,
11]. The palatovaginal canal
may be occasionally identified on CT images in figures found in the
literature, but it is not labeled as such. In a review of the anatomy of the
pterygopalatine fossa [7], we
could see the left palatovaginal canal in one of the images, but the canal was
not mentioned in the text or figure legend. On the basis of their CT study of
the pterygopalatine fossa and its communications, a group of researchers
concluded that the palatovaginal canal cannot be visualized because of its
small size [2]; however, we
think that the palatovaginal canal is depicted in a number of the figures in
that article [2]. In another
publication, the palatovaginal canal is described in the text but is marked as
a "probable Vidian canal duplication" in one of the figures
[6]. In their article about the
anatomy of the nasopharynx and the floor of the middle cranial fossa, Teresi
et al. [13] stated that the
nerves and maxillary artery branches inside and adjacent to the
pterygopalatine fossa are inconsistently and rarely seen on MR imaging. The
authors' inability to see these structures probably resulted from the slice
thickness and resolution available 15 years ago.
On CT images, we found at least one palatovaginal canal in more than 50% of
our patients and bilateral complete canals in 16%. We were surprised by such
frequent visualization of this small anatomic structure, especially in light
of the previously mentioned reports
[2,
5,
7,
8,
12]. Visualization of the
vidian canals can be used as an internal control because it can be compared
with the findings from two previous studies. Pandolfo et al.
[14] identified this anatomic
structure in 95% of their patients, whereas Kim et al.
[2] were able to identify the
vidian canal in all their patients. Our results were similar: We identified
the vidian canal in 98% of our patients. The fact that Kim et al. had a 100%
detection rate for the vidian canal (similar to our findings) but do not
mention visualization of the palatovaginal canal leads us to believe that
these researchers probably did not recognize the palatovaginal canal as such.
Depiction of the palatovaginal canal in some of their figures supports this
conclusion. We found an endosinusal course of the vidian canals in 15.3% of
our patients, which is a slightly higher detection rate than those reported in
previous studies. Pandolfo et al.
[14] found it in 13% of their
patients and Kim et al., in 7.5% of their patients. We speculate that the
lower frequency of palatovaginal canal visualization in patients who have an
endosinusal vidian canal could possibly be explained by an associated
endosinusal course of the pterygovaginal artery and pharyngeal nerve in the
same bony ridge as the vidian canal structures.
Our study shows that the palatovaginal canal and the anatomy of the
pterygopalatine fossa are frequently shown and are well depicted on coronal CT
and MR imaging studies. The pterygopalatine fossa is at the crossroads of
nerves and vascular structures and represents a major pathway for spread of
malignant tumors and infections. Nasopharyngeal malignancies grow along the
path of least resistance, and potential pathways of extension include growth
in the mucosa and submucosa and along fascial planes, muscle bundles, and
neurovascular bundles [15].
The most commonly recognized pattern of nasopharyngeal tumor spread is via the
parapharyngeal space through the sinus of Morgagni along the auditory tube.
From here, a tumor can extend into the foramen ovale of the sphenoid and then
into the cavernous sinus. Tumors can also invade the foramen lacerum and
spread through the vidian canal to the pterygopalatine fossa. Another known
route of pterygopalatine fossa involvement is through the sphenopalatine
foramen, following anterior tumor extension into the nasal cavity
[15,16,17,18,19].
Teresi et al. [16] also
mentioned tumor spread from the vidian canal to the foramen lacerum and
cavernous sinus, but these authors did not explain how tumors gain access to
the vidian canal.
It is reasonable to assume that all passage-ways can be used for neoplastic
spread and extension along the vidian or maxillary nerves, although cases of
tumor involvement of the foramen lacerum or of bone invasion have not been
reported [18,
19]. In one of the figures in
the report by Chong and Fan
[19], tumor invasion of the
palatovaginal canal can be seen along with tumor spread into the vidian canal.
Therefore, these cases likely represent the direct spread of nasopharyngeal
cancer into the pterygopalatine fossa through the palatovaginal canal and then
to vidian or maxillary nerves. We believe that this possible route of tumor
spread is not commonly considered. Recognition of the early tumor extension
into the palatovaginal canal on imaging studies may prove to influence
management and treatment.
The pterygopalatine fossa is also the branching point for the third
(pterygopalatine) segment of the internal maxillary artery. Intractable
epistaxis predominantly arises from the sphenopalatine artery; however, other
branches of the pterygopalatine internal maxillary artery may also be the
origin of posterior nasal bleeding. The management of this condition includes
posterior packing and arterial ligation as well as endovascular embolization,
which is probably the treatment of choice in recurrent cases. The failure rate
of internal maxillary artery ligation may be as high as 15%
[8]. Failure occurs as a result
of incomplete ligation of the collateral vessels or the presence of misplaced
arterial clips [9,
10]. Anatomic variations of
the pterygopalatine branches—including the course of the pterygovaginal
artery, which runs through the palatovaginal canal—have been noted as
possible explanations of ligature failure
[9,
10]. The pterygovaginal artery
was found to have an anomalous origin, arising early from the internal
maxillary artery lateral to the pterygomaxillary fissure, in five of 20
specimens in a cadaveric study
[9]. The pterygovaginal artery
then coursed medially across the posterior wall of the pterygopalatine fossa
and posterior to the plane of the nerves. It is accepted that all blood
vessels should be located in an anterior plane, separate from the nervous
structures [1,
9,
10]. Thus, this variant course
of the pterygovaginal artery is hidden from view, and the pterygovaginal
artery may also be of considerable size, equal to that of the sphenopalatine
artery [9]. The pterygovaginal
artery anastomoses with the ascending pharyngeal and ascending palatine
arteries may carry blood in a retrograde fashion, making it a probable cause
of occasional surgical failures. A prominent pterygovaginal artery may be
angiographically visualized with selective injections into the internal
maxillary artery to show a descending posterior course from the
pterygopalatine fossa on lateral projections
[8,
11]. Successful embolization
of the pterygovaginal artery in a case of traumatic epistaxis has been
reported [8]. A large
palatovaginal canal on CT may indicate a prominent pterygovaginal artery. We
found bilaterally wide palatovaginal canals in one of our patients with
recurrent epistaxis (Fig. 3)
and in only two other patients with unrelated symptoms. Thus, a wide
palatovaginal canal seen on CT in patients with recurrent epistaxis could
raise suspicion of this anatomic variant, which may serve to guide
embolization.
The contents of the pterygopalatine fossa are embedded in fibrofatty
tissue, which provides high intrinsic contrast on T1-weighted MR images. Our
study shows that the course of the third segment of the internal maxillary
artery and its branches can, at least in some patients, be assessed on MR
imaging. The palatovaginal canals with an internal tubular flow void, probably
consistent with the pterygovaginal artery, were bilaterally detected in 40% of
our patients. The depiction of these small anatomic structures primarily
depended on the amount of surrounding fatty tissue present to provide good
intrinsic contrast. We believe that visualization of a prominent presumed
pterygovaginal artery as an unusually large flow void is possible on MR
imaging in some cases of spontaneous or traumatic epistaxis, which arises from
this vessel. Extension of malignant tumors through the palatovaginal canal
could also conceivably be detected during the early phase on T1-weighted MR
images. The neoplasm is expected to be visualized as low-signal-intensity
tissue replacing high-signal-intensity fat on unenhanced images and as an
abnormal enhancing area after administration of gadolinium.
In conclusion, the palatovaginal canal can be visualized on routine coronal
CT images and MR images. Identification of this anatomic structure may play a
role in detecting the spread of malignant nasopharyngeal tumors and in
evaluating patients with intractable epistaxis.
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Abstract
Introduction
