Correlation of the external occipital protuberance with venous sinuses: a magnetic resonance imaging study.

PURPOSE: To date, no study has explored the external occipital protuberance (EOP) using neuroimaging modalities. This study aims to characterize them using magnetic resonance imaging (MRI). METHODS: A total of 96 patients underwent thin-sliced, post-contrast MRI. The sagittal images were analyzed. RESULTS: In 97%, the EOPs were delineated as a focal external protrusion of the midline region of the occiput with varying morphologies. In 89% of 93 patients with identifiable EOPs, parts of the intracranial dural sinuses were found to lie just below the inion, the most prominent point of the EOP. The most frequently targeted dural sinus was the confluence of sinuses that was found in 57%, followed by the superior sagittal sinus. In 16%, a bony foramen and transmitting vessel were detected in the EOP, connecting between the diploic channels and the subcutaneous veins. Furthermore, in 33%, bony foramina and transmitting venous structures were identified in the region just below the EOPs, connecting between the diploic channels and the subcutaneous veins. CONCLUSIONS: The intracranial dural venous sinus is located just below the EOP with a high probability. Most bony foramina in the EOP and midline suboccipital region may transmit veinous structures connecting to the diploic channel.

Central retinal artery delineation using magnetic resonance imaging.

PURPOSE: Few studies have explored the central retinal artery (CRA) using neuroimaging. Our study aimed to explore this using magnetic resonance imaging (MRI). METHODS: A total of 81 patients with intact orbital structures and visual function underwent thin-slice contrast MRI. RESULTS: The identified CRAs showed highly variable morphologies on both axial and sagittal images. On the axial images, the CRAs were detected in the right orbit in 11.1% and in the left orbit in 19.8%. The distance between the site of CRA branching from the ophthalmic artery to the posterior limit of the bulb was 18.8 ± 3.9 mm (12.8-24.6 mm) on the right and 18.9 ± 3.3 mm (14.6-26.7 mm) on the left. On the sagittal images, CRAs were detected on the right in 76.5% and on the left in 85.2%. The distance between the CRA branching site and the posterior limit of the bulb was 20.4 ± 3.8 mm (14.2-28.2 mm) on the right and 19.2 ± 3.7 mm (11.3-27.1 mm) on the left. CONCLUSIONS: Thin-sliced, contrast sagittal MRI can be used to explore the proximal part of the CRA. In particular, serial sagittal imaging may be useful for detecting the CRAs and their relationship with relevant structures.

Fluid-Filled Dehiscences in the Anterior Cranial Fossa Floor: A Magnetic Resonance Imaging Study.

OBJECTIVE: To date, only limited information regarding the anterior cranial fossa floor (AFF) and the appearance of sites of dehiscence and potential channels has been available. We aimed to evaluate this region with thin section magnetic resonance imaging (MRI). PATIENTS AND METHODS: A total of 65 patients underwent thin-sliced coronal T2-weighted MRI. The AFF was divided into 3 parts for analysis: the anterior, middle, and posterior. RESULTS: Dehiscences were identified in 84.6% of 65 patients with apparently transmitting channels. In 49.2% of the patients, the dehiscences were located in the anterior part, whereas they were located in the middle and posterior parts in 52.3% and 12.3%, respectively. The morphology and number of these dehiscences were highly variable. In 12.3%, channels in the dehiscences were distributed extradural. Statistically, dehiscences were more frequently identified on the left side in any part of the AFF. CONCLUSIONS: With thin-sliced, coronal T2-weighted MRI, dehiscences were frequently identified in the anterior two-thirds of the AFF. Further study is warranted to determine the role of AFF channels and dehiscences, including possibly for cerebrospinal fluid drainage.

Vertebral artery compression of the medulla oblongata: A benign radiological finding?

BACKGROUND: To the best of our knowledge, no study has documented the natural history of rostral medullary compression of the vertebral artery (RMCVA) as radiological finding. The aim of this study was to explore it. METHODS: A total of 57 patients with RMCVA and not presenting symptoms of medullary compression syndrome were enrolled. These participants underwent cerebral magnetic resonance imaging with contrast, and 19 of them who were followed for 5.7 ± 1.9 years (range: 3.0-10.3 years) were analyzed in detail. For comparison, clinical courses of two other patients with vertebrobasilar dolichoectasia (VBDE) were presented. RESULTS: RMCVA was well delineated in all 57 patients. In the 19 patients analyzed, RMCVA was found in 17 sides on the right and 15 on the left. Moreover, the ventrolateral medulla was the most frequent compression site, and it was found in 69% of cases, with 84.2% presenting as mild compression and 15.8% as considerable compression. During the follow-up period, no patients showed neurological deterioration or radiological progression. In contrast, the two VBDE patients demonstrated both neurological and radiological progressions during the follow-up period. CONCLUSION: Unlike VBDE, RMCVA seems to be a benign condition without progression, even when with a considerable compression. Degree of the compression in RMCVA may not be relevant to the patient's neurological status.

Hyperintense areas in the cisternal segments of the cranial nerves: a magnetic resonance imaging study.

PURPOSE: The study aimed to explore hyperintense areas in the cisternal segments of the cranial nerves using magnetic resonance imaging (MRI). METHODS: Seventy outpatients underwent thin-sliced, coronal constructive interference steady-state (CISS) sequence and sagittal T2-weighted MRI following conventional MRI examination. RESULTS: With the coronal CISS sequence, hyperintense areas were located in the central parts of the olfactory bulbs in 65.7% of patients. For the intracranial optic nerve and optic chiasm, hyperintense areas were detected in 98.6% of the CISS sequences and 100% of the T2-weighted images. In the optic tract, hyperintense areas were detected in 51.4% of cases. In 35% of the patients who underwent the CISS sequence, the intracranial optic nerves were considerably compressed by the internal carotid and anterior cerebral arteries, with hyperintense areas similar to those in patients without vascular compression. Hyperintense areas of the cisternal segments of the oculomotor nerve and trigeminal root were identified in 52.9% and 87.1% of the patients, respectively. CONCLUSIONS: The hyperintense areas found within the cisternal segments of the cranial nerves delineated on the coronal CISS sequence and sagittal T2-weighted imaging may indicate the intracranial part of the glymphatic pathway through the cranial nerves. The cranial nerves may function as part of the glymphatic pathway.

"Hammock" suspending the superior ophthalmic vein: a magnetic resonance imaging study.

PURPOSE: The present study aimed to explore the hammock-like structure suspending the superior ophthalmic vein (SOV) using magnetic resonance imaging (MRI). METHODS: Following conventional MRI examination, 93 outpatients underwent thin-sliced, coronal T2-weighted and contrast imaging of the orbit. RESULTS: SOVs were consistently detected in all 93 patients. In 90.3% of patients, a hammock-like structure suspending the SOV was identified, which was present on both sides in 64.5% of patients. The structure was frequently located in the anterior and middle thirds of the retrobulbar orbit, suspended from the superolateral corner of the orbital walls. The medial edge of the hammocks did not reach the orbital walls; therefore, they partially encased the SOV. The morphology of the hammock was highly variable between patients, although none were tethered to the extraocular muscles. In addition, a septal band connecting the hammock and optic sheath was identified in 36.6% of patients, most frequently located in the posterior third of the retrobulbar orbit. CONCLUSIONS: The hammock suspending the SOV and the septal band connecting the hammock and optic sheath may be structures that loosely anchor the SOV to the orbital fat to maintain a constant SOV flow, in addition to preventing excessive bends and obstructions.

Root exit zone of the trochlear nerve on the dorsal brainstem: an MRI study.

PURPOSE: To date, no study has explored the root exit zone of the trochlear nerve (TroN) on the dorsal brainstem; therefore, we aimed to characterize the location using magnetic resonance imaging (MRI). METHODS: A total of 85 patients underwent thin-slice axial T2-weighted MRI. RESULTS: TroN was identified in 91% of 85 patients, 60 (71%) on the right side, and 67 (79%) on the left. The distances between the apex of the inferior colliculus and the original site of TroN on the dorsal brainstem were variable, with mean values of 2.4 ± 1.8 mm (range 0-8 mm) on the right and 2.2 ± 1.7 mm (range 0-5 mm) on the left. Most of the root exit zones were distributed within 0-5 mm below the apex of the inferior colliculus. In addition, the distances between the midline and the root exit zones of the TroN were variable, with mean values of 4.4 ± 1.4 mm (range 1.9-7.5 mm) on the right and 4.6 ± 1.6 mm (range 1.1-7.8 mm) on the left. Most of the root exit zones were located within 1-7 mm range lateral to the midline. CONCLUSIONS: The root exit zone of the TroN may be mostly located in a small square area measuring 8 mm × 8 mm, lying at and below the apex of the inferior colliculus. The TroN may arise from any site in the square area, and significant attention is necessary when performing surgical maneuvers in and around it.

Emissary Foramina of the External Occipital Protuberance: A Magnetic Resonance Imaging Study.

OBJECTIVE: The posterior interparietal region is known to be a distinct area of the skull, predisposing to a variety of bony variants and congenital anomalies. The bony canals in this region have not yet been explored in detail. This study aimed to characterize them. PATIENTS AND METHODS: Overall, 82 patients underwent thin-slice contrast magnetic resonance imaging. Coronal and sagittal images were used for the analysis. RESULTS: At the rostral limit of the posterior interparietal region, the parietal foramen (PF) and transmitting emissary vessels were identified in 84% of cases, mostly delineated as single channels. They were located in the left parasagittal region (PS) in 26%, lateral to the left PS in 33%, right PS in 40%, lateral to the right PS in 12%, and midline in 16% of the cases. In 4% of the cases, the PF presented an obstruction around the outer opening. In the more caudal part of the posterior interparietal region, 51% of the patients presented with fine bony canals and transmitting emissary vessels, lying in the midline and parasagittal levels. CONCLUSIONS: The frequency of PFs may be underestimated for superficially hidden outer openings of the canals. The midline posterior interparietal region may present fine bony canals for transmitting emissary vessels that function complementarily with PFs.

The Arcuate Eminence and Superior Semicircular Canal: Magnetic Resonance Imaging Study.

OBJECTIVE: The present study aimed to explore the relationship between the arcuate eminence (AE) and superior semicircular canal (SSC) using the constructive interference steady-state (CISS) sequence. PATIENTS AND METHODS: After conventional magnetic resonance imaging, a total of 71 patients underwent the CISS sequence in thin-sliced coronal sections. RESULTS: In all patients, the SSC was delineated on both sides. In contrast, the AE was identified only in 29 of 71 patients (40.8%) on both sides, varying in shape and relative location to the SSC in the mediolateral dimension. The shortest distance between the highest point of the SSC and middle fossa floor was 1.3 ± 1.1 mm on the right side and 1.3 ± 0.9 mm on the left with considerable variability. A dehiscent SSC with a distance less than 0.2 mm was found in 11.3% of 142 sides. On 22 sides (15.5%), the site on the middle fossa floor, reaching the SSC with the shortest distance (reference point) corresponded to the apex of the AE, equally on the right and left. On 36 sides (25.4%), the distance between the reference point and the apex of the AE was measured as 3.0 ± 1.1 mm on the 18 right sides and 3.7 ± 1.6 mm on the 18 left sides. CONCLUSIONS: The relationship between AE and SSC is highly variable. Arcuate eminence was not a reliable landmark of the SSC. High-resolution CISS sequence is useful for exploring these structures.

Meckel Cave: An Anatomical Study Using Magnetic Resonance Imaging.

OBJECTIVE: To our knowledge, few studies have investigated anatomy of the Meckel cave with neuroimaging modalities. The present study aimed to characterize it using magnetic resonance imaging (MRI). PATIENTS AND METHODS: Following conventional MRI examination, a total of 101 patients underwent T2-weighted imaging in thin-sliced coronal and sagittal sections, and 11 patients underwent constructive interference steady-state sequences in thin-sliced sagittal sections. Moreover, 3 injected cadaver heads were dissected. RESULTS: In the cadaver specimens, the size and extent of the cerebrospinal fluid-filled space between the Gasserian ganglion and surrounding arachnoids were difficult to define. On the T2-weighted imaging, the Meckel cave was delineated with variable morphologies and left-right asymmetry. On the sagittal images, the shape of the Meckel cave could be classified into 3 different types, bulbous, oval, and flat, with the oval being the most frequent that comprised 60%. Furthermore, on the sagittal constructive interference steady-state images, parts of the trigeminal nerve distributed in the Meckel cave were delineated in all patients. The ophthalmic, maxillary, and mandibular divisions were clearly distinguished on both sides. CONCLUSIONS: The Meckel cave is a structure characterized by diverse morphologies and left-right asymmetry. Thin-sliced T2-weighted imaging is useful for exploring the anatomy of the Meckel cave.

Massa intermedia of the thalamus: an anatomical study using magnetic resonance imaging.

PURPOSE: Few studies have explored the morphology of massa intermedia (MI). The aim of the present study was to characterize it using magnetic resonance imaging (MRI). METHODS: A total of 205 patients were enrolled in this study. Following initial examinations with conventional MRI sequences, thin-slice coronal and sagittal T2-weighted imaging was performed. For MI localization, the third ventricle was arbitrarily divided into nine areas on the midsagittal image. RESULTS: MI was identified in 93% of the total patients-89% in male and 91% in female patients. Among them, 68% showed a single, styloid-shaped MI with variable thickness and cross sectional configuration, followed by broad and double MIs that were found in 18% and 10% patients, respectively. In the anteroposterior dimension, 99% of the MIs were identified in the middle third area, followed by the posterior third area. In the supero-inferior dimension, 95% of the MIs were identified in the middle third area, followed by the upper third area. With a significant difference, a broad MI was more frequently found in women than in men. CONCLUSIONS: MIs are commonly located in the middle third of the third ventricle as a single commissure with high morphological variability. Compared to men, women may have a well-developed, broader MI.

Calvarial diploic venous channels: delineation with maximal intensity projection technique.

PURPOSE: To date, very few studies have explored the three-dimensional architecture of calvarial diploic venous channels (CDVCs). This study aimed to characterize the three-dimensional architecture of CDVCs using maximum intensity projection (MIP) images based on contrast-enhanced magnetic resonance imaging (MRI). METHODS: A total of 77 patients with intact calvarial hemispheres and underlying dura mater and dural sinuses underwent contrast-enhanced MRI. Among them, we extracted the data of 49 with at least a part of the major CDVC pathways identified on the MIP images for analysis. RESULTS: On serial contrast-enhanced MRI images, the CDVCs were commonly detected as curvilinear structures with inhomogeneous diameters and tributaries, while the MIP images delineated the three-dimensional architecture of the developed CDVC pathways. More than such CDVC pathway was entirely delineated on the right in 67.3% and on the left in 71.4%, most frequently in the frontal and temporal regions, with their connecting sites to the sphenoparietal and superior sagittal sinuses. The morphology, distribution, and course of the identified CDVCs were highly variable. In 55.1%, the CDVCs formed fenestrations that were variable in size, shape, and number. CONCLUSIONS: The developed CDVC pathways may be characterized by morphological variability and fenestrations. Thin-sliced, contrast-enhanced MRI is useful to depict diploic veins, while MIP images allow for better appreciation of the entire course of the developed CDVC pathways. Traumatic and intraoperative disconnection between the dura mater overlying the dural sinuses and the adjacent inner table of the skull can cause epidural venous bleeding.

Arachnoid granulations bulging into the transverse sinus, sigmoid sinus, straight sinus, and confluens sinuum: a magnetic resonance imaging study.

PURPOSE: Few studies have explored arachnoid granulations (AGs) bulging into the cranial dural sinuses using contrast-enhanced magnetic resonance imaging (MRI). This study aimed to explore such AGs in the transverse (TS), sigmoid (SigS), and straight (StS) sinuses, and confluens sinuum (ConfS) using thin-sliced, contrast MRI. METHODS: A total of 102 patients with intact dural sinuses underwent thin-sliced, contrast MRI in the axial, coronal, and sagittal planes. RESULTS: In 88.2%, more than one AG was identified in the TS and SigS, StS, and ConfS. In the TS, AGs were identified in 40.2% on the right side and 37.3% on the left and were frequently located in the middle and lateral thirds. In the SigS, AGs were identified on the right in 17.6% and on the left in 18.6% in the distal region. In the StS, AGs were identified in 35.3% of cases, most frequently located in the proximal third, followed by the distal third. In the ConfS, AGs were identified in 20.6% of cases. Furthermore, in 23.5%, a collection of multiple AGs of varying sizes was found in the TS. A statistical difference was not shown between the mean age of 90 patients with AGs and that of 12 patients without identifiable AGs. CONCLUSIONS: Bulging AGs may more frequently found in the TS. Thin-sliced, contrast MRI is useful for delineating AGs.

Inferior sagittal sinus: magnetic resonance imaging study.

PURPOSE: To date, no study has explored the inferior sagittal sinus (ISS) using neuroimaging modalities. This investigation aimed to characterize it using magnetic resonance imaging (MRI). METHODS: A total of 77 patients with intact cerebral hemispheres and covering meninges underwent thin-sliced, contrast-enhanced MRI. RESULTS: The ISS was well delineated as a linear structure with a constant diameter in 97% of the patients. The maximum intensity projection (MIP) images well delineated the three-dimensional architecture of the ISS and relevant veins. The identified ISSs could be classified into three different types, with the underdeveloped type being the most frequent at 47%. In addition, the ISSs showed considerable variability both in the original site and course along the lower margin of the falx cerebri. Furthermore, in 22% of the cases, fenestrations were identified in the falx cerebri adjacent to or near the ISS. More than 70% of them were located in the middle third of the falx, followed by the anterior and middle thirds of the falx. CONCLUSIONS: The ISS is a constant venous structure characterized by morphological variability and may function as an adjunctive or assistive venous drainage route. Thin-sliced, post-contrast-enhanced sagittal MRI combined with MIP imaging is useful for exploring the ISS.

Bridging veins of the cerebellum: a magnetic resonance imaging study.

PURPOSE: To date, no study has yet explored the bridging veins (BVs) of the cerebellum using neuroimaging modalities. Therefore, this study aimed to characterize them using magnetic resonance imaging (MRI). METHODS: A total of 90 patients with intact cerebellar hemispheres and intracranial dural sinuses underwent thin-sliced, contrast-enhanced MRI. RESULTS: The BVs were classified into six routes based on the draining pattern into the dural sinuses. The superior vermian vein emptying into the straight sinus was delineated in 100% of the patients. The inferior vermian vein emptying into the confluence of the sinuses was identified in 66.7% of the patients. The inferior hemispheric and cerebellar cortical veins emptying into the transverse sinus were identified in 54.4% and 26.7% of the patients, respectively. The inferior vermian and cerebellar cortical veins emptying into the straight sinus were identified in 77.8% and 12.2% of the patients, respectively. The cerebellar cortical vein emptying into the tentorial sinus was identified in 83.3% of the patients; it was delineated on 54 sides with an average number per right hemisphere of 1.9 and 63 sides with an average number per left hemisphere of 2. The pontine-trigeminal and anterior hemispheric veins emptying into the superior petrosal sinus were identified in 42.2% of the patients. CONCLUSIONS: The BVs of the cerebellum can be classified into six distinct routes. Radiological classification may be useful for understanding the drainage pattern of the cerebellum.

Trochlear cistern of the cavernous sinus: an anatomical study using magnetic resonance imaging.

PURPOSE: The present study aimed to explore the trochlear cistern (TC) of the cavernous sinus using magnetic resonance imaging (MRI). METHODS: Following conventional MRI examination, a total of 73 patients underwent the constructive interference steady-state (CISS) sequence in thin-sliced coronal sections. Moreover, three injected cadaver heads were dissected. RESULTS: In the cadaver specimens, the extent of the TC was difficult to identify on any dissected side. On the CISS images, the TC was identified in 98.6% on the right side and 94.5% on the left, while transmitting the trochlear nerve (TN) was identified in 83.6% on the right and 79.5% on the left. Most TNs were delineated as a single trunk, while duplication of the nerve was found in 3% of cases. The TC, commonly located inferior or inferolateral aspect of the oculomotor trigone. The size and extent of TC were highly variable. The TN location in the TC was also variable and was identified throughout the upper, middle, and lower parts of the TC. Moreover, relationships between the TC and Meckel's cave were highly variable. CONCLUSIONS: TC shows morphological variability. The coronal CISS sequence is useful for exploring TC and TN in clinical practices.

Hyperintense areas in the intraorbital optic nerve evaluated by T2-weighted magnetic resonance imaging: a glymphatic pathway?

PURPOSE: The present study aimed to explore the glymphatic pathway in the intraorbital optic nerve (ON) using magnetic resonance imaging (MRI). METHODS: Following conventional MRI examination, a total of 89 outpatients underwent T2-weighted imaging in thin-sliced coronal and sagittal sections. Moreover, three injected cadaver heads were dissected. RESULTS: In the cadaver specimens, differences in appearance between the central and peripheral parts of the ON were not observed. On the axial T2-weighted MRI performed in the initial examination, the central part of the intraorbital ONs was delineated as a well-demarcated, linear hyperintense area in 19% of patients. On the thin-sliced serial coronal images, the hyperintense areas were identified on both sides in 91% of patients. They were delineated as continuous hyperintense areas in the ONs with an inconsistent appearance even in the same nerve. In 12.4% of patients, the areas were divided into the upper and lower parts by a horizontal septum, while others showed variable morphologies, lacking a septum. On thin-sliced sagittal images, hyperintense areas were identified in 46% of patients. CONCLUSION: Hyperintense areas in the intraorbital ON detected on T2-weighted sequences may involve a glymphatic pathway with perivascular spaces of the ON and central retinal artery. These may be collapsed and difficult to identify on surgical and cadaver specimens.

Subependymal hyperintense layer on CISS sequence: An MRI study.

PURPOSE: The present study aimed to explore the subependymal layers overlying the cerebral ventricles using magnetic resonance imaging. METHODS: A total of 69 outpatients underwent constructive interference in steady-state (CISS) sequence in thin-sliced, coronal, and sagittal sections. RESULTS: The subependymal layers were delineated as linear hyperintensities, coursing along the outer margins of the ventricular walls. On coronal images, the hyperintensities surrounding the anterior horn of the lateral ventricle were identified in 97% of patients, while those of the third ventricle were identified in 96% of patients. In the trigone and posterior horn of the lateral ventricle, the hyperintensities were delineated in all patients. On sagittal images, subependymal hyperintensities were identified in all. At the level of the anterior horn and third ventricle, the subependymal hyperintensities were found to communicate with the Virchow-Robin spaces (VRSs) in 68% and 65% of patients, respectively. At the level of the trigone and posterior horn of the lateral ventricle, the VRSs communicated with the subependymal hyperintensities in 83% of patients. CONCLUSIONS: Subependymal hyperintensity may represent an inflow passage of the VRSs that jointly contribute to efficient transependymal migration of the interstitial fluid into the ventricular cerebrospinal fluid.

Cortical and bridging veins of the upper cerebral convexity: a magnetic resonance imaging study.

PURPOSE: There is no study exploring the cortical veins (CVs) and connecting bridging veins (BVs) with neuroimaging modalities. The present study aimed to characterize these veins of the upper cerebral convexity. METHODS: A total of 89 patients with intact cerebral hemispheres and covering meninges underwent thin-sliced, contrast magnetic resonance imaging (MRI). In addition, three injected specimens were dissected in this study. RESULTS: In cadaver dissection, the BVs were observed to course in the arachnoid sheaths, suspended from the dura mater. The medial parts of the BVs, located near the superior sagittal sinus (SSS)-BV junction site, were occasionally exposed subdurally. The CVs were formed by venous channels arising from the cerebral gyri and those emerging from the sulci. On MRI, the CVs and connecting BVs were identified in the medial and latera convexity areas and medial surface of the cerebrum. These veins were highly variable in number, thickness, length, course, and distribution. In the medial convexity area, the CVs arising from the gyri were identified in 58% of patients, while they were found only in 11% of patients in the lateral convexity area. CONCLUSION: In the medial convexity area, involving the parasagittal region, the CVs connect more densely with the BVs that may predispose to injury during neurosurgical procedures. Mechanical impact exerted the area, diameter of the veins in the craniocaudal direction, and number of venous afferences may affect the SSS-BV junctional region in an indirect manner and lead to the development of acute subdural hematoma.

Positional relationship between the external acoustic meatus and sigmoid sinus: an MRI study.

PURPOSE: There is no study exploring the positional relationship between the external acoustic meatus (EAM) and the sigmoid sinus (SS) in detail. The present study aimed to characterize the relationship using contrast magnetic resonance imaging (MRI). METHODS: In total, 85 patients with an intact EAM, SS, and surrounding structures underwent thin-sliced, contrast MRI. Imaging data were transferred to a workstation for analysis. RESULTS: In all patients, the EAM and SS were well depicted on both the sagittal and axial images. The relationships and distances between the EAM and SS, in addition to the shape of cross sections of the EAM, were highly variable with left-right asymmetry. The positional relationships between the EAM and the anterior edge of the SS were classified into superior, intervening, and inferior types. The intervening type was the most predominant, accounting for 85%. The shortest distance between the posterior wall of the EAM and the anterior margin of the SS was 12.3 ± 3.9 mm on the right side and 13.0 ± 2.9 mm on the left. In three women, the distance was less than 5 mm on the right side. CONCLUSIONS: The positional relationship between the EAM and SS is highly variable and inconsistent. These structures may be adjacent, especially on the right side, and presurgical contrast MRI should be included when planning surgeries around the EAM.