Mastoid foramen, mastoid emissary vein and clinical implications in neurosurgery

BACKGROUND

Mastoid emissary vein is especially important from the neurosurgical point of view, because it is located in variable number in the area of the occipitomastoid suture and it can become a source of significant bleeding in surgical approaches through the mastoid process, especially in retrosigmoid craniotomy, which is used for approaches to pathologies localized in the cerebellopontine angle. Ideal imaging method for diagnosis of these neglected structures when planning a surgical approach is high-resolution computed tomography. The aim of this work was to provide detailed information about this issue.

METHODS

We studied a group of 295 skulls obtained from collections of five anatomy departments and the National Museum. Both quantitative and qualitative parameters of the mastoid foramen were evaluated depending on side of appearance and gender. Individual distances of the mastoid foramen from clearly defined surface landmarks (asterion, apex of mastoid process, foramen magnum) and other anatomical structures closely related to this issue (width of groove for sigmoid sinus, diameters of internal and external openings of mastoid foramen) were statistically processed.

RESULTS

The most frequently represented type of the mastoid foramen is type II by Louis (41.2%). The differences between right and left sides were not statistically significant. In men there was a higher number of openings on the right side and in qualitative parameters the type III and IV predominated, whereas in women the types I and II were more frequent. In men, greater distances from the mastoid foramen were observed when evaluating qualitative parameters for defined surface landmarks. Mean size of the external opening diameter was 1.3 mm; however, several openings measured up to 7 mm.

CONCLUSIONS

Despite excellent knowledge of anatomy, however, good pre-operative examination using imaging methods and mastering of microsurgical techniques create the base for successful treatment of pathological structures in these anatomically complex areas.

Endoscopic transorbital route to the petrous apex: a feasibility anatomic study

BACKGROUND

While the subtemporal approach represents the surgical module milestone designed to reach the petrous apex, a novel ventral route, which is the superior eyelid endoscopic transorbital approach, has been proposed to access the skull base. Accordingly, we aimed to evaluate the feasibility of this route to the petrous apex, providing a qualitative and quantitative analysis of this relatively novel pathway.

METHODS

Five human cadaveric heads were dissected at the Laboratory of Surgical NeuroAnatomy of the University of Barcelona. After proper dissection planning, anterior petrosectomy via the endoscopic transorbital route was performed. Specific quantitative analysis, as well as dedicated three-dimensional reconstruction, was done.

RESULTS

Using the endoscopic transorbital approach, it was possible to reach the petrous apex with an average volume bone removal of 1.33 ± 0.21 cm³. Three main intradural spaces were exposed: cerebellopontine angle, middle tentorial incisura, and ventral brainstem. The first one was bounded by the origin of the trigeminal nerve medially and the facial and vestibulocochlear nerves laterally, the second extended from the origin of the oculomotor nerve to the entrance of the trochlear nerve into the tentorium free edge while the ventral brainstem area was hardly accessible through the straight, ventral endoscopic transorbital trajectory.

CONCLUSION

This is the first qualitative and quantitative anatomic study concerning details of the lateral aspect of the incisura and ventrolateral posterior fossa reached via the transorbital window. This manuscript is intended as a feasibility anatomic study, and further clinical contributions are mandatory to confirm the effectiveness of this approach, defining its possible role in the neurosurgical armamentarium.

Anatomical variation of superior petrosal vein and its management during surgery for cerebellopontine angle meningiomas

No systematic study is yet available that focuses on the surgical anatomy of the superior petrosal vein and its significance during surgery for cerebellopontine angle meningiomas. The aim of the present study was to examine the variation of the superior petrosal vein via the retrosigmoid suboccipital approach in relation to the tumor attachment of cerebellopontine angle meningiomas as well as postoperative complications related to venous occlusion. Forty-three patients with cerebellopontine angle meningiomas were analyzed retrospectively. Based on the operative findings, the tumors were classified into four subtypes: the petroclival type, tentorial type, anterior petrous type, and posterior petrous type. According to a previous anatomical report, the superior petrosal veins were divided into three groups: Type I which emptied into the superior petrosal sinus above and lateral to the internal acoustic meatus, Type II which emptied between the lateral limit of the trigeminal nerve at Meckel's cave and the medial limit of the facial nerve at the internal acoustic meatus, and Type III which emptied into the superior petrosal sinus above and medial to Meckel's cave. In both the petroclival and anterior petrous types, the most common vein was Type III which is the ideal vein for a retrosigmoid approach. In contrast, the Type II vein which is at high risk of being sacrificed during a suprameatal approach procedure was most frequent in posterior petrous type, in which the superior petrosal vein was not largely an obstacle. Intraoperative sacrificing of veins was associated with a significantly higher rate of venous-related phenomena, while venous complications occurred even in cases where the superior petrosal vein was absent or compressed by the tumor. The variation in the superior petrosal vein appeared to differ among the tumor attachment subtypes, which could permit a satisfactory surgical exposure without dividing the superior petrosal vein. In cases where the superior petrosal vein was previously occluded, other bridging veins could correspond with implications for the crucial venous drainage system, and should thus be identified and protected whenever possible.

Comparison of microscopic and endoscopic approaches to the cerebellopontine angle

OBJECTIVE

To examine the efficacy of the endoscope as an adjunct to the operating microscope in defining the surgical anatomy of the cerebellopontine angle (CPA).

METHODS

The surgical anatomy of the CPA was examined in cadaveric CPAs through a retrosigmoid approach. The upper, middle, and lower neurovascular complexes and the individual segments of the cerebellar arteries in the CPA were examined with the surgical microscope and 0° and 45° rigid endoscopes.

RESULTS

The microscope provided satisfactory views of the posterior surface of the neural and vascular structures in the central part of the CPA cistern. The endoscope provided superior views of the nerves' junction with the brainstem, their dural exit, and their vascular relationships. The endoscope also provided superior views of the individual segments of the cerebellar arteries.

CONCLUSION

The combination of endoscopic and microsurgical techniques aids in achieving optimal exposure in CPA surgery.

Microsurgical endoscopic-assisted retrosigmoid intradural suprameatal approach: anatomical considerations

AIM

The aim of this study was to evaluate the feasibility of microscopic endoscopic assisted suprameatal tubercle drilling with a retrosigmoid approach and it focuses on the anatomic structures identified with the endoscope. The advantages of the 30 degrees optic view are also described.

METHODS

Fifty dry temporal bones were studied in order to estimate the variability of the prominence of the suprameatal tubercle. Eight fresh cadaveric specimens were prepared for a retrosigmoid approach to allow for microscopic endoscopic assisted suprameatal tubercle drilling. The increase in trigeminal exposure and neurovascular structures visualization with the endoscope, using 0 degrees and 30 degrees optics were then evaluated.

RESULTS

Three major types of the suprameatal tubercle were found: 1) a large size tubercle (> 6 mm, 9/50 cases); 2) a medium size tubercle (3-6 mm, 37/50 cases); and 3) an almost absent suprameatal tubercle (< 3 mm, 4/50 cases). Microscopic endoscopic assisted suprameatal tubercle drilling with opening of the Meckel's Cave was found to be technically feasible in all cases. The increase in trigeminal nerve exposition was of 9 mm on average. Endoscopic exploration with 0 degrees and 30 degrees optics made possible the identification of all neurovascular structures in the area.

CONCLUSION

Microscopic endoscopic assisted suprameatal tubercle drilling is a feasible procedure that allows the identification of all neurovascular structures in the cerebellopontine angle and petrous apex region. The opening of Meckel's Cave may be particularly useful for lesions located in the cerebellopontine angle having a minor component that extends anteriorly and laterally in the middle cranial fossa.

[Anatomical study of auditory brainstem implantation through retrosigmoid approach]

OBJECTIVE

To provide anatomic data for auditory brainstem implantation (ABI) through the retrosigmoid approach.

METHODS

Simulated operations were performed on 30 web adult head specimens and the structure around the foramen of luschka was observed. Both microscope and endoscope were employed.

RESULTS

(1)The bony window, the most adjacent but not overlapping with sigmoid sinus, was a circle with a radius of 20 mm. Its center was located behind midpoint of the line from parietal notch to mastoid apex with a distance of (26.42 +/- 1.29) mm. The distance between bony window and transverse sinus, mastoid apex, foramen of luschka, jugular foramen, posterior edge of internal auditory meatus and root entry zone of the IX cranial nerve were (22.45 +/- 1.41) mm, (35.51 +/- 1.65) mm, (43.86 +/- 2.20) mm, (16.56 +/- 1.64) mm, (15.01 +/- 0.63) mm and (46.27 +/- 1.70) mm respectively; (2) The foramen of luschka can be spotted by using microscope or endoscope, especially 30 degrees angled endoscope. By this way, we could obtain a more distinct visual field without over-retraction of cerebellum and achieve the goal of minimally invasive surgery. The distance between the foramen of luschka and internal acoustic porus was (15.01 +/- 0.53) mm; (3) The foramen of luschka lies in the triangle formed by flocculus and root entry zone of glossopharyngeal nerve and rostral margin of biventer lobule. Choroids plexus acts as a direct landmark. the length of lateral recess was (17.53 +/- 1.03) mm. The distance between acoustic tubercle and the foramen of luschka and the root of cochlea never was (16.52 +/- 1.67) mm and (13.77 +/- 1.66) mm respectively.

CONCLUSION

Adjustment of the angle of skull bone window and clarification of cerebellopontine angle are the keys to positioning the foramen of luschka. The usage of 30 degrees angled endoscope can identify the angle are the keys to positioning the foramen of luschka. The usage of 30 degrees angled endoscope can identify the foramen of luschka more accurately and expose the operation area more clearly.

[Observation of cranial nerves in the cerebellopontine angle region by retrosigmoid approach]

OBJECTIVE

To investigate the anatomical structures of cranial nerves in the cerebellopontine angle region to offer anatomical data for clinical operation.

METHOD

A total of 52 adult cadaveric heads fixed in 10% formalin were used for this study. After cutting cerebellum and meningeal between transverse and sigmoid sinus, simulate operating method of retrosigmoid approach to observe the cranial nerves.

RESULT

External diameter and length of left V, VII, VIII, IX cranial nerves are (2.54 +/- 0.84) mm and (6.79 +/- 2.51) mm, (1.18 +/- 0.31) mm and (9.89 +/- 2.66) mm, (2.17 +/- 0.52) mm and (9.92 +/- 2.61) mm, (0.77 +/- 0.24) mm and (10.34 +/- 3.12) mm respectively. External diameter and length of right V , VII, VIII, IX cranial nerves are (2.52 +/- 0.86) mm and (6.91 +/- 2.66) mm, (1.14 +/- 0.31) mm and (10 +/- 2.96) mm, (2.13 +/- 0.63) m and (10.09 +/- 2.93) mm, (0.790.29) mm and (10.17 +/- 3.06) mm. intermedius nerve locate between facial nerve and acoustic nerve, external diameter of intermedius nerve is (0.47 +/- 0.91) mm (left) and (0.37 +/- 0.07) mm (right). Length of vagal nerve is (10.44 +/- 2.57) mm (left), (9.91 +/- 2.91) mm (right), rootlets of f vagal nerve is 6.37 +/- 2.26 (left) and 6.33 +/- 2.38 (right). external diameter of accessory nerve is (0.76 +/- 0.16) mm (left) and (0.81 +/- 0.19) mm (right).

CONCLUSION

This study provide anatomical data for retrosigmoid approach in the cerebellopontine angle region.

Hemifacial spasm: neurovascular compressive patterns and surgical significance

BACKGROUND

The aim of this study was to report further investigation of neurovascular compression as a cause of hemifacial spasm (HFS) and to provide useful surgical guidelines by describing the compression patterns.

MATERIAL AND METHODS

From January 2004 to February 2006, 236 consecutive patients with HFS underwent microvascular decompression (MVD) in a single centre. Based on the operation and medical records, the intraoperative findings and post-operative outcomes were obtained and analysed.

RESULTS

We found that 95.3% of lesions had accompanying causative factors that made the neurovascular compression inevitable. Based on the contributing factors, compression patterns were categorised into six different types including: loop (n = 11: 4.6%), arachnoid (n = 66: 27.9%), perforator (n = 58: 24.6%), branch (n = 18: 7.6%), sandwich (n = 28: 11.9%), and tandem (n = 52: 22.0%). The compression patterns were significantly correlated with the compressing vessels involved. Thirty-two (86.5%) of 37 lesions where the vertebral artery was the compressing vessel involved the tandem type. Anterior inferior cerebellar artery was the compressing vessel involved in 49 (84.5%) of 58 perforator type compressions, while posterior inferior cerebellar artery was the compressing vessel involved in 8 (72.7%) of 11 loop type compressions.

CONCLUSIONS

Once the compressing vessel responsible for the neurovascular compression are identified, the probable pattern of compression can be anticipated; this knowledge could facilitate the application of the appropriate operative procedures and minimise post-operative complications.

Anatomic structural study of cerebellopontine angle via endoscope

BACKGROUND

Minimally invasive surgery in skull base relying on searching for possible anatomic basis for endoscopic technology is controversial. The objective of this study was to observe the spatial relationships between main blood vessels and nerves in the cerebellopontine angle area and provide anatomic basis for lateral and posterior skull base minimally invasive surgery via endoscopic retrosigmoid keyhole approach.

METHODS

This study was conducted on thirty dried adult skulls to measure the spatial relationships among the surface bony marks of posterior cranial fossa, and to locate the most appropriate drilling area for retrosigmoid keyhole approach. In addition, we used 10 formaldehyde-fixed adult cadaver specimens for simulating endoscopic retrosigmoid approach to determine the visible scope.

RESULTS

The midpoint between the mastoid tip and the asterion was the best drilling point for retrosigmoid approach. A hole centered on this point with the 2.0 cm in diameter was suitable for exposing the related structures in the cerebellopontine angle. Retrosigmoid keyhole approach can decrease the pressure on the cerebellum and expose the related structures effectively which include facial nerve, vestibulocochlear nerve, trigeminal nerve, glossopharyngeal nerve, vagus nerve, accessory nerve, hypoglossal nerve, anterior inferior cerebellar artery, posterior inferior cerebellar artery and labyrinthine artery, etc.

CONCLUSIONS

Exact location on endoscope retrosigmoid approach can avoid dragging cerebellum during the minimally invasive surgery. The application of retrosigmoid keyhole approach will extend the application of endoscopic technology.

[A study of applied microanatomy by endoscope-assisted via retrolabyrinthine approach]

OBJECTIVE

To study the microanatomy by endoscope-assisted via retrolabyrinthine approach.

METHOD

Nineteen cadaveric heads fixed with formalin were dissected in our study. The data that endoscope could be extent and the distance between the important point were measured. By simulated the retrolabyrinthine approach, endoscope was placed to observe the nerves and vessels, the distance between nerves and the central point of the anterior edge of sigmoid sinus were measured.

RESULT

The distance from the inferior margin of petrous ridge to the upper bound of endolymphatic sac was (9.93+/-1.52)mm; and from superior margin of petrous ridge to the inferior margin common bony crus was (4.64+/-0.91)mm;and from the intersection of posterior semicircular canal to the anterior wall of sigmoid was (7.85+/-1.47)mm on the left, and (5.69+/-1.68)mm on the right. The distance from inferior margin common bony crus to the anterior wall of sigmoid was (13.9+/-1.71)mm on the left, and (11.31+/-2.03)mm on the right. The trochlear nerve and abducent nerve could be observed under endoscope. The relationship between the trigeminal nerve, acoustic nerve and the vessels could be identified clearly. The distance from the central point of the anterior edge of sigmoid to the roots of the trigeminal nerve, facial nerve, vestibulocochlear nerve and glossopharyngeal nerve were (29.88+/-2.77) mm, (32.04+/-2.04) mm, (29.17+/-1.65) mm, (35.49+/-1.53) mm respectively.

CONCLUSION

The visual field of the cerebellopontine angle appear wider by the endoscope assisted retrolabyrinthine approach. Nerves, vessel, internal acoustic pore, jugular foramen region can be clearly seen. Some minimal invasive surgery can be done by this approach.

[Pathology of the cerebellopontine angle]

The pathology of the cistern of the cerebellopontine angle is primarily that of the nervous and vascular structures that it contains and of the meninges that line it. Knowledge of its anatomy makes it possible to understand and search for a rare pathology, the hemifacial spasm, due to a conflict between the facial nerve and the vertebral artery and the posterior inferior cerbellerar artery. However, the pathology of the cerebellopontine angle remains especially tumoral. Imaging should not only make the diagnosis but also make an exhaustive, pretherapy, and accurate assessment of the three main tumours found in this area: the vestibular schwannoma, the meningioma, and the epidermoid cyst.

The impact of petrosal vein preservation on postoperative auditory function in surgery of petrous apex meningiomas

OBJECTIVE

The importance of preserving the superior petrosal vein has received increasing attention in the surgical treatment of pathologies involving the petrous apex. Recent reports have associated postoperative auditory nerve dysfunction with petrosal vein sacrifice. However, there is no systematic clinical study available thus far focusing on the postoperative auditory function after petrosal vein obliteration.

METHODS

In 55 patients with meningiomas involving the petrous apex, pre- and intraoperative findings including petrosal vein sectioning were analyzed retrospectively concerning their impact on postoperative auditory function.

RESULTS

The petrosal vein was preserved in 26 (47%) cases. In 27 (49%) cases, this vein was not preserved. Hearing loss occurred in 11% of all cases. In the preserved-vein group, postoperative hearing loss occurred in 3 of 26 (11%) cases and in the sacrificed-vein group in 3 of 27 (11%) cases.

CONCLUSION

Sacrifice of the petrosal vein during surgery of petrous apex meningiomas seems not to have an impact on postoperative auditory function.

Retrosigmoid intradural suprameatal approach: advantages and disadvantages from an anatomical perspective

OBJECTIVE

To assess the advantages and disadvantages of the retrosigmoid intradural suprameatal approach by studying the microsurgical anatomy. This study was performed primarily to assess the advantages of the retrosigmoid intradural suprameatal approach by measuring the amount of increased exposure it provides for lesions of the cerebellopontine and petroclival region as well as to identify the disadvantages of the approach.

METHODS

Twenty sides of 10 cadaver heads (embalmed and injected) were dissected under x3 to x40 magnification. A standard retrosigmoid craniotomy was made. The cerebellopontine cistern was entered to expose the neurovascular structures, and the internal auditory canal was opened by drilling the margin of the internal auditory meatus. After this, the suprameatal tubercle was drilled, followed by additional drilling to resect the petrous apex. The trigeminal root was mobilized completely after opening Meckel's cave. During drilling, care was taken to preserve the posterior and superior semicircular canals, petrosal sinus, and the internal carotid artery.

RESULTS

The approach enhanced the exposure of the cerebellopontine cistern and Meckel's cave. There was an additional exposure of 10.7 +/- 1.16 mm length of trigeminal nerve on the right side and an additional 10.7 +/- 1.25 mm on the left. This helped to mobilize and further retract the trigeminal root. Although it facilitated the view of the neurovascular structures medial to the internal acoustic meatus, the depth of exposure did not vary much from a traditional retrosigmoid approach nor did it increase the angle of exposure or the visualization of the clivus and more medially located structures.

CONCLUSION

This approach is suitable for lesions mainly in the posterior fossa with some extension into the middle fossa in the anterolateral direction. The key benefits of this approach are the length of trigeminal nerve exposure and the subsequent mobilization that improves visualization of the structures medial to the internal auditory canal, to the petrous apex, Meckel's cave, and the posterior end of the cavernous sinus.

[Topography and morphometry of the porus trigeminus]

BACKGROUND AND PURPOSE

The trigeminal nerve passes from the posterior to the middle cranial fossa above the superior edge of the petrous part and below attachment of the tentorium cerebelli. Behind this place it is surrounded by the dural trigeminal cave which opens posteriorly as the porus trigeminus. The aim of this study was to describe the porus trigeminus along with its relation to the trigeminal nerve and selected structures around.

MATERIAL AND METHODS

The study was performed on the material of 20 human, formalin fixed specimens of the cranial base. Measurements were taken with a surgical microscope with an ocular ruler, with precision of 0.2 mm.

RESULTS

Mean width of the porus was 7.3 (+/-1.0) mm and height 2.2 (+/-0.4) mm. Dimensions of the trigeminal nerve within the porus were 5.8 (+/-0.8) mm and 2.1 (+/-0.4) mm, respectively. The porus is surrounded by shallow ellipsoid hollow measuring 12.1 (+/-1.8) over 5.1 (+/-1.1) mm. Relation of the porus to the petrous vein was described, as well as to other veins emptying to the dural sinuses in its vicinity. Distances between the porus and venous dural openings were very variable. The following distances were measured from the porus trigeminus to: the internal acustic porus [6.6 (+/-1.7) mm], jugular foramen [16.2 (+/-1.8) mm], jugular tubercle [18.9 (+/-1.8) mm], abducent nerve [5.9 (+/-1.2) mm], trochlear nerve [4.6 (+/-1.4) mm], oculomotor nerve [8.4 (+/-1.9) mm], posterior clinoid process [14.7 (+/-1.8) mm] and median plane [left 13.7 (+/-1.0), and right 13.5 (+/-1.9) mm].

CONCLUSIONS

Obtained results may be helpful during planning surgical approaches to the Meckel's cave and petroclival region.

High jugular bulb in the translabyrinthine approach to the cerebellopontine angle: anatomical considerations and surgical management

BACKGROUND

Evidence of a high jugular bulb position (HJBP) during the translabyrinthine approach may compromise the surgical removal of cerebellopontine angle (CPA) tumours. We report a simple surgical procedure to safely manage this frequent normal variation and comment on various alternative options.

METHODS

The translabyrinthine approach included a complete skeletonization of the sigmoid sinus and of the presigmoid dura. A thin eggshell bone was left at the jugular bulb surface. The dome of the jugular bulb was gently dissected from the jugular fossa and gradually retracted downward in a tailored way, allowing the surgeon to drill below the internal auditory meatus. A small piece of bone was wedged over the jugular dome in order to maintain its lowered position.

RESULTS

Among 178 consecutive translabyrinthine approaches performed for the removal of large CPA tumors, the use of this procedure was required in 44 cases of HJBP. Excepting minimal venous bleeding easily controlled in several cases, we never observed any complication from this procedure nor failure to expose the inferior compartment of the CPA.

CONCLUSIONS

The HJBP can be systematically diagnosed with the preoperative CT-scan using bone window imaging. Our results demonstrate that the described procedure is safe and effective to widen the operative corridor that is required for the exposure of the inferior compartment of the CPA in this anatomical situation.

Anatomie des Kleinhirnbrückenwinkels

The cerebellopontine angle (CPA) is an anatomically complex region of the brain. In this article we describe the anatomy of the CPA cisterns, of the internal auditory canal, the topography of the cerebellum and brainstem, and the neurovascular structures of this area.

[Microanatomical study on the nutrient artery of facial nerve in cerebellopontine angle]

OBJECTIVE

To explore the arterial origin of the facial nerve and the site of the arteries joining it in cerebellopontine angle (CPA), in order to provide anatomical data for clinical application.

METHODS

The nutrient arteries were observed on 22 fresh adult head specimens fixed and perfused with formalin and gelatin under operation microscope.

RESULTS

Of all the nutrient arteries of facial nerve motor root,31 were derived from the artery loops in CPA space (50.82%) and 17 from the branch of anterior inferior cerebellar artery (27.88%). Eight of them originated from the labyrinthine artery (13.1%), 3 from posterior inferior cerebellar artery (4.92%) and 2 from basilar artery (3.28%) respectively. Forty-seven nutrient arteries (77.05%) entered the proximal 1/3 segment of facial nerve motor root. Thirty-six nutrient arteries of nervus intermedius raised from the artery loops in CPA space (73.47%), 7 from the branch of anterior inferior cerebellar artery (14.29%) and 6 from labyrinthine artery (12.24%) respectively.

CONCLUSIONS

The observation of the arterial origin of the facial nerve and the site of the arteries joining it in cerebellopontine angle provided an anatomic basis for the etiology of hemifacial spasm and the microsurgical operation in CPA.

Neuroendoscopic anatomy and surgery of the cerebellopontine angle

To probe the feasibility and utility of neuroendoscopic inspection of the anatomy of the cerebellopontine angle (CPA) and of neuroendoscopic assisted microneurosurgery (NEAMN) for CPA lesions via a retrosigmoid approach, we used retrosigmoid NEAMN in 28 patients with CPA lesions. Prior to this, we undertook anatomical observation of bilateral CPA in two adult cadaver heads using the neuroendoscope. NEAMN tumour resection was performed in eight acoustic neuromas, one meningioma and 14 cholesteatomas and NEAMN vascular decompression was performed in five patients with trigeminal neuralgia. Both the neurovascular structures of the CPA and the ventral surface of the pons, as well as the clivus, can be inspected using the neuroendoscope through a retrosigmoid approach with a 2-3 cm diameter bony opening. Complete excision of the tumour with preservation of the facial nerve was achieved in all eight acoustic neuromas. Likewise, total resection of the tumour was possible in the 14 cholesteatomas and one meningioma. Paroxysmal facial pain resolved after NEAMN vascular decompression in the five patients with trigeminal neuralgia. There were no postoperative complications or deaths in this series. The CPA can be divided into three levels - the cranial, medial, and caudal, and each level contains specific neurovascular structures as seen through the neuroendoscope. Knowledge of these divisions is useful to master the common NEAMN procedures of the CPA. NEAMN for CPA lesions via a retrosigmoid approach is a useful adjunct to standard microneurosurgical techniques effect and may decrease the operative risk.

The cisternal segment of the abducens nerve in man: three-dimensional MR imaging

PURPOSE

The goal of this study was to identify the abducens nerve in its cisternal segment by using three-dimensional turbo spin echo T2-weighted image (3DT2-TSE). The abducens nerve may arise from the medullopontine sulcus by one singular or two separated rootlets.

MATERIAL AND METHODS

We studied 285 patients (150 males, 135 females, age range: 9-72 years, mean age: 33.3 +/- 14.4) referred to MR imaging of the inner ear, internal auditory canal and brainstem. All 3D T2-TSE studies were performed with a 1.5 T MR system. Imaging parameters used for 3DT2-TSE sequence were TR:4000, TE:150, and 0.70 mm slice thickness. A field of view of 160 mm and 256 x 256 matrix were used. The double rootlets of the abducens nerve and contralateral abducens nerves and their relationships with anatomical structures were searched in the subarachnoid space.

RESULTS

We identified 540 of 570 abducens nerves (94.7%) in its complete cisternal course with certainty. Seventy-two cases (25.2%) in the present study had double rootlets of the abducens nerve. In 59 of these cases (34 on the right side and 25 on the left) presented with unilateral double rootlets of the abducens. Thirteen cases presented with bilateral double rootlets of the abducens (4.5%).

CONCLUSION

An abducens nerve arising by two separate rootlets is not a rare variation. The detection of this anatomical variation by preoperative MR imaging is important to avoid partial damage of the nerve during surgical procedures. The 3DT2-TSE as a noninvasive technique makes it possible to obtain extremely high-quality images of microstructures as cranial nerves and surrounding vessels in the cerebellopontine cistern. Therefore, preoperative MR imaging should be performed to detect anatomical variations of abducens nerve and to reduce the chance of operative injuries.

The microsurgical anatomy of the glossopharyngeal nerve with respect to the jugular foramen lesions

Removal of lesions involving the jugular foramen region requires detailed knowledge of the anatomy and anatomical landmarks of the related area, especially the lower cranial nerves. The glossopharyngeal nerve courses along the uppermost part of the jugular foramen and is well hidden in the deep layers of the neck, making this nerve is the most difficult one to identify during surgery. It may be involved in various pathological entities along its course. The glossopharyngeal nerve can also be compromised iatrogenically during the surgical treatment of such lesions. The authors define landmarks that can help identify this nerve during surgery and discuss the types of lesions that may involve each portion of the glossopharyngeal nerve.

Comparison of flow artifacts between 2D-FLAIR and 3D-FLAIR sequences at 3 T

It has been reported that 3D-FLAIR can reduce the flow artifact resulting from cerebrospinal fluid (CSF) at 1.5 T compared to 2D-FLAIR. Flow-related artifacts tend to be worse at 3 T than at 1.5 T. The purpose of this study was to compare the CSF flow artifacts of 2D-FLAIR and 3D-FLAIR sequences at 3 T in eight healthy volunteers. The grade of CSF-related artifacts were scored through observing the perimedullary cistern, cerebellopontine angle cisterns, fourth ventricule, prepontine cistern, suprasellar cistern, ambient cisterns, sylvian fissures, third ventricle and lateral ventricles. Grading was performed on either axial or sagittal images. The CSF in-flow artifact scores were significantly higher on axial 2D-FLAIR than on axial 3D-FLAIR MPR images in all areas except the bilateral sylvian fissures, and higher on sagittal 2D-FLAIR than on sagittal 3D-FLAIR MPR images in perimedullary, bilateral CP angle and suprasellar cisterns. The CSF-related flow artifacts were significantly reduced by 3D-FLAIR, while structures in the cistern were depicted more clearly, even at 3 T. Further study is necessary to compare the clinical efficacy between 2D-FLAIR and 3D-FLAIR in depicting subtle abnormalities.

Relationship between the concentration of supplemental oxygen and signal intensity of CSF depicted by fluid-attenuated inversion recovery imaging

BACKGROUND AND PURPOSE

Prior reports have described increased signal intensity (SI) of CSF on fluid-attenuated inversion recovery (FLAIR) images of anesthetized patients receiving 100% O(2). This appearance can simulate that of diseases. We evaluated the relationship between the concentration of inhaled O(2) and the development of increased SI of CSF on FLAIR images.

METHODS

FLAIR was performed in 25 healthy volunteers breathing room air and 100% O(2) through a face mask for 5, 10, and 15 minutes. MR imaging, including FLAIR imaging, was performed in 52 patients with no potential meningeal abnormalities under general anesthesia: 21 received an equal mixture of N(2)O and O(2), and 31 received 100% O(2). The SI of CSF in volunteers and patients was graded in several locations by using a three-point scale.

RESULTS

SI of CSF significantly increased (P <.05) in various locations, in both volunteers and patients breathing 100% O(2), when compared with SI in the same volunteers breathing room air. Hyperintensity of CSF was not significantly different in volunteers receiving 100% O(2) through a face mask compared with anesthetized patients receiving 100% O(2) through a laryngeal airway or an endotracheal tube. No significant increase in SI occurred in patients receiving 50% O(2), when compared with the SI of volunteers breathing room air.

CONCLUSION

Supplemental oxygen at 100% is a main cause of artifactual CSF hyperintensity on FLAIR images, regardless of the anesthetic drug used. This artifact does not develop when 50% O(2) is administered.

Evaluation of hippocampal infolding using magnetic resonance imaging

To investigate developmental morphological variation of the hippocampal formation, we evaluated the degree of hippocampal infolding in cross-sectional oblique coronal images of the cerebral peduncle and the superior cerebellar peduncle. We defined the hippocampal infolding angle as the angle between the vertical midline and the straight line connecting the medial superior margin of the subiculum with the lateral margin of the cornu ammonis. The angle increased slightly with age, and was larger in the superior cerebellar peduncle than in the cerebral peduncle and larger in the right superior cerebellar peduncle than in the left superior cerebellar peduncle. This suggests that this angle and its variation with age and location merit our attention in morphological evaluation of the hippocampal formation.

[Neurovascular structures of the posterior surface of the petrous pyramid: correlation with the approaches of the cerebellopontine angle]

A topographic study of posterior surface of the petrous pyramid was performed in 20 human cadaveri heads. The distances between the neurovascular structures were measured in the points where they contact the posterior surface of the petrous pyramid. The study also points out the relationship between the bone landmarks and the transverse and the superior petrous sinuses. The result of this study was correlated with the approaches to the cerebellopontine angle.

[Computed tomography and magnetic resonance tomography of the normal temporal bone]

The normal anatomy of the temporal bone and the inner ear will be described in detail on high resolution computed tomography (HRCT) and magnetic resonance images. The imaging technique of computer tomography--either single detector or multi detector CT--is normally obtained in an axial plane without the intravenous application of contrast material. The images are reconstructed in a high resolution bone window level setting. The coronal images are reconstructed either if used single detector or multi detector CT. Only in some cases a scan in the coronal plane is directly obtained using a single detector CT. MR imaging of temporal bone is usually performed in a head coil. Axial high resolution 3D-T2-weighted sequences either in fast spin echo technique or gradient echo technique--for example CISS-sequence--are obtained, then an axial high resolution T1-weighted sequence before and after the application of gadopentate dimiglumine is performed. HRCT excellently demonstrates the osseous structures of the temporal bone as well as of the inner ear, while MRI excellently depicts soft tissue structures especially those of the inner ear. Due to the susceptibility artifacts MRI is not very suitable for imaging the external auditory canal or the middle ear or the pneumatic system. In conclusion HRCT is so far excellent to delineate the osseous structures of the temporal bone and inner ear while MRI excellently depicts the soft tissue structures of the inner ear, the internal auditory canal and the cerebellopontine angle. Reissner's membrane, the cochlear duct, and the organ of Corti cannot be visualized even using high-resolution MRI. HRCT and MRI are therefore used as complementary methods for imaging the temporal bone.

Endoscopic examination of the cerebellar pontine angle

This anatomic study describes how to optimize the use of the endoscope in the cerebellar pontine angle (CPA) through a retrosigmoid approach. Unlike the microscope, that only permits visualization of structures directly ahead, the endoscope can see 'around the corner', also showing the structures down narrow. Nevertheless, to use the endoscope it is necessary to insert it into the CPA that is full of neurovascular structures which limit its movements. Thus, to avoid damages it is important to inspect this region accurately, by means of preferential trajectories to insert the endoscope. A retrosigmoid approach was performed in cadaver heads, and the CPA region was inspected employing 0 degrees, 45 degrees rigid endoscopes (4 mm in diameter). The neurovascular structures of the CPA have been visualized using three trajectories. The limits of view offered by each trajectory as well as the neurovascular structures together with the obstacles encountered on each route have been described. The systematic adoption of three different endoscope trajectories at the CPA level permits to view from different angulation the same structure as well as its relationships with the surrounding nerves and vessels. Considerable experience on cadavers should have already been obtained with the endoscope prior to any application in the operating room.

[Anatomic investigation of the labyrinthine artery]

OBJECTIVE

To observe the origin, the course and the location of the labyrinthine artery[LA], and to investigate the spacial relationship between LA and facial nerve and vestibulocochlear nerve (VCN).

METHODS

The specimens including 49 sides cerebellopontine angle (CPA) and internal acousticmeatus (IAM) infused with red galatin into artery were dissected and observed under operating microscope.

RESULTS

LA arised from anterior inferior cerebellar artery (AICA) (83.6%), or basilar artery (BA) (12.3%) or vertebral artery (VA) (4.1%). AICA formed a loop in cerebellopontine angle (CPA) or internal acoustic meatus (IAM). The loop was located in the extra-meatus of IAM (28.6%), or at the opening of the IAM (18.4%), or in intrameatus of IAM (36.7%). AICA passed over ventral side of VII and VIII cranial nerves (18.4%) or across between VII and VIII cranial nerve root (81.6%). LA was a mono-arterial (51.1%), or bi-arterial (40.8%), or tri-arterial (4.1%) vessel. The calibre of LA was (0.18 +/- 0.05) mm. In CPA and IAM, facial nerve located anteroinferior to VIII cranial nerve, but the location of VCN was posteriolaterally. LA most commonly coursed between VII and VIII. LA divided into anterior vestibular artery, vestibulocochlar artery and cochlear artery. On the base of IAM, the facial nerve situated in anterosuperior, but cochlear nerve in anteroinferior and vestibular nerve in posterosuperior.

CONCLUSION

LA was a mainly artery supplied to facial nerve, VCN and vestibulocochlear organs. Once LA is damaged, facial weakness (or paralysis), vertigo or hearing loss would be developed.

Selective retrosigmoid vestibular neurectomy without internal auditory canal drill-out: an anatomic study

OBJECTIVE

It is well established that selective vestibular nerve section by means of the retrosigmoid or posterior fossa approach can be accomplished with or without drill-out of the internal auditory canal (IAC) by virtue of the presence or absence of a surgically accessible cleavage plane between the vestibular and cochlear nerves. Some reports have indicated that a majority of patients would be amenable to successful separation of the vestibular nerve from the cochlear nerve medial to the IAC, thus obviating the need for IAC drill-out and associated complications. However, other reports have indicated routine difficulty in finding a satisfactory vestibulocochlear cleavage plane within the cerebellopontine angle. This in situ cadaver study was undertaken to determine whether normal anatomic relationships support the hypothesis that selective vestibular nerve section can be accomplished by means of the posterior fossa approach without the need for concomitant IAC drill-out in a majority of circumstances.

METHODS

A retrosigmoid approach to the posterior fossa was performed bilaterally on 36 intact human cadavers. After displacement of the cerebellum, an operating surgical microscope was used to visualize the cerebellopontine angle in the surgical position. The ability to develop a satisfactory cleavage plane between the vestibular and cochlear nerves without the need for drill-out of the IAC was established in each case.

RESULTS

Seventy-two vestibulocochlear nerve bundles in 36 intact human cadavers were analyzed. A vestibulocochlear nerve cleavage plane within the cerebellopontine angle amenable to neurectomy medial to the porus of the IAC was observed in 81% left and 69% right vestibulocochlear nerve bundles (average, 75%). The facial nerve was found deep or anterior to the vestibulocochlear nerve bilaterally in all cases examined. The anterior inferior cerebellar artery, or a branch of the artery, was found to cross the plane between the facial and vestibulocochlear nerve bundles within the lateral cerebellopontine angle in 47% of the cases on the left and in 50% of cases on the right.

CONCLUSIONS

A vestibulocochlear nerve cleavage plane amenable for selective vestibular nerve transection without drilling the IAC was found in 75% of the 72 cerebellopontine angles studied. The facial nerve consistently lies deep or anterior to the vestibulocochlear nerve within the cerebellopontine angle with the retrosigmoid approach. These findings support the rational and feasibility of avoiding drill-out of the IAC in the majority of circumstances when performing selective vestibular neurectomy by means of the posterior fossa approach for Ménière's syndrome and other vestibular disorders.

Anatomical variations of landmarks for implantation at the cochlear nucleus

A knowledge of the microanatomy of the cochlear nucleus complex and its variations is essential for successful implantation and for the design of stimulation devices. One hundred cerebellopontine angle specimens were dissected under surgical conditions using the Zeiss NC31 surgical microscope. The topographical anatomy of the exit of the vestibulocochlear nerve, the cochlear nucleus and the surface of the medulla and their relation to the surrounding structures was recorded and measured. The mean distances between the exits of the VIIth and VIIIth cranial nerves were 4.7 +/- 0.9 mm, between the VIIth and IXth 6.3 +/- 1.2 mm and between the VIIIth and IXth 5.5 +/- 1.0 mm. The visible area of the cochlear nucleus covered a square of 10.0 +/- 2.9 by 3.3 +/- 1.0 mm. A major AICA-loop had to be re-routed in 17 per cent of specimens. The taenia of the choroid plexus was present in 92 per cent and had to be cut in 51 per cent in order to enter the foramen of Luschka, that had a mean size of 3.5 by 2.0 mm. It was wide open in 24 per cent, open only after incision of the arachnoid in 53 per cent, functionally closed but opened by extensive dissection in 18 per cent and anatomically occluded in five per cent of the specimens. The typical straight vein at the cochlear nucleus leading to the entrance of the foramen of Luschka was found in 76 per cent of specimens. Constant anatomical landmarks are very helpful for finding the cochlear nucleus, but variations may endanger dissection and implantation in a remarkable number of cases.

Usefulness of high-resolution magnetic resonance cisternography in patients with hemifacial spasm

To analyse the usefulness of preoperative magnetic resonance (MR) cisternography in patients with hemifacial spasm (HFS), MR observations were compared with surgical findings. High-definition images were obtained using MR cisternography which employed a long echo train length, fast spin-echo sequence, which revealed both nerves and blood vessels without any contrast media. In 35 HFS patients, certified radiologists examined the presence of vascular compression of the facial nerve and identified the offending vessels. MR cisternography depicted neurovascular compression in 31 patients (sensitivity 88.6%) and correctly determined the offending vessels in 28 patients. In 4 patients, MR cisternography revealed the presence of the vessel near the facial nerve, but the radiologist evaluated the compression as questionable (false-negative rate 11.4%). Even in these patients, MR cisternography revealed the precise anatomy of cerebellopontine (CP) cistern. High-resolution MR cisternography is useful in identifying the vessels and nerve bundles in the CP cistern, and in so doing can provide surgeons with valuable information regarding neurovascular decompression for HFS.

Topographic anatomy of the cochlear nuclear region at the floor of the fourth ventricle in humans

OBJECT

The development of appropriate methods to stimulate the dorsal and ventral cochlear nucleus by means of an auditory brainstem implant in patients with acquired bilateral anacusis requires a detailed topoanatomical knowledge both of the location and extension of the nuclear surface in the fourth ventricle and lateral recess and of its variability. The goal of this study was to provide that information. Anatomically, it is possible to use a midline surgical approach to the fourth ventricle rather than the translabyrinthine and suboccipital routes of access used hitherto. This is especially useful if severe scarring, which occurs as a result of tumor removal in the cerebellopontine angle, make the orientation and placement of an auditory brainstem implant via a lateral surgical approach difficult. There have been only a few studies, involving single cases and small series of patients, in which the focus was the exact extension of the cochlear nuclei, whose microsurgically relevant position in relation to the surface structures is not known in detail.

METHODS

Landmarks that are important for the placement of an auditory brainstem implant through the fourth ventricle were examined and measured in a large series of 28 formalin-fixed human brainstems. In all cases, these examinations were supplemented by addition of a histological section series. For the first time values of unfixed fresh brainstem tissue were determined. Anatomical features are discussed with regard to their possible neurosurgical relevance, taking into account inter- and intraindividual variability.

CONCLUSIONS

The midline approach would provide an opportunity to stimulate the whole area of the dorsal as well as the ventral cochlear nucleus with an auditory brainstem implant.

High-resolution MR cisternography of the cerebellopontine angle: 2D versus 3D fast spin-echo sequences

BACKGROUND AND PURPOSE

The clinical usefulness of MR cisternography of the cerebellopontine angle, applying 2D or 3D fast spin-echo sequences, has been reported recently. Our purpose was to investigate the cause of signal loss in CSF in the prepontine or cerebellopontine angle cistern on 2D FSE MR images and to compare the cisternographic effects of 2D and 3D FSE sequences.

METHODS

Preliminary experiments were performed in four volunteers to assess the causes of signal loss. Initially, using a 2D cardiac-gated cine phase-contrast method with a velocity encoding value of 6 cm/s, we measured the velocity and flow pattern of CSF. Comparisons were made to assess the effects of intravoxel dephasing, amplitude of the section-selecting gradient, echo time (TE), and section thickness. Four healthy subjects and 13 patients with ear symptoms were examined, and multisection 3-mm-thick 2D images and 30-mm-slab, 1-mm-section 3D images were compared qualitatively and quantitatively. Then, 3D MR cisternography was performed in 400 patients with ear symptoms, and qualitative evaluation was performed.

RESULTS

In volunteers, the average peak velocity of CSF was 1.2 cm/s. With TE = 250, CSF may move an average of 3 mm, and can be washed out of a 3-mm-thick 2D section volume. The CSF signal relative to that of a water phantom decreased gradually as TE increased on single-section 3-mm-thick 2D images. The CSF signal relative to that of the water phantom increased gradually as section thickness increased. No significant differences were noted in intravoxel dephasing and amplitude of the section-selecting gradient. The contrast-to-noise ratio (CNR) between CSF and the cerebellar peduncle, and the visibility of the cranial nerves and vertebrobasilar artery were significantly improved on 3D images in 17 subjects. In images from 400 patients, no significant signal loss in the cistern was observed using 3D FSE.

CONCLUSION

CSF signal loss in thin-section 2D MR cisternography is mainly attributable to the wash-out phenomenon. 3D acquisition can reduce this phenomenon and provide thinner sections. The scan time for 3D acquisition is not excessive when a long echo train length and half-Fourier imaging are used. MR cisternography should be performed using a 3D acquisition.

Suprameatal extension of the retrosigmoid approach: microsurgical anatomy

OBJECTIVE

This study was conducted to determine whether removing the bony prominence located above the porus of the internal acoustic meatus, called the suprameatal tubercle, and surrounding bone using the retrosigmoid approach would aid in the exposure of tumors that are located predominantly in the cerebellopontine angle but that also extend into the middle cranial fossa in the region of Meckel's cave and thus avoid the need for a supratentorial craniotomy.

METHODS

Thirty cerebellopontine angles from 15 cadaveric heads examined using 3 to 40x magnification provided the material for this study. A retrosigmoid craniotomy was completed and the exposure obtained before and after removing the suprameatal tubercle, and the surrounding bone was examined. In some cases, Meckel's cave and the tentorium lateral to the porus of Meckel's cave was opened to aid in the exposure.

RESULTS

Removing the suprameatal tubercle and surrounding bone increased the exposure an average of 10.3 mm (range, 8.0-13.0 mm) forward of the exposure, which could be obtained without suprameatal drilling. The extent of bone removal was limited on the lateral side by the posterior and superior semicircular canals and their common crus.

CONCLUSION

The suprameatal extension of the retrosigmoid approach will permit removal of some tumors that are located mainly in the posterior fossa but that extend into the middle fossa in the region of Meckel's cave. The exposure can be increased by opening the superior petrosal sinus as it crosses in the upper margin of the porus of Meckel's cave and by opening the tentorium lateral to Meckel's cave.

Microanatomy of the cerebellopontine angle and internal auditory canal: study with new magnetic resonance imaging technique using three-dimensional fast spin echo

OBJECTIVE

We report a new magnetic resonance imaging technique that uses three-dimensional fast spin echo and the minimum intensity projection method. Using this technique, detailed images of the cerebellopontine angle (CPA) and internal auditory canal (IAC) were obtained in normal volunteers and in patients with acoustic neuromas or hemifacial spasm.

METHODS

Ten normal volunteers, 44 patients with acoustic neuromas, and 31 patients with hemifacial spasm were studied using the three-dimensional fast spin echo magnetic resonance imaging protocol. The CPA and IAC were scanned by using a 1-mm slice thickness in the axial and parasagittal planes.

RESULTS

Normal anatomy was as follows. 1) The vestibulocochlear nerve was ovoid near the brain stem and changed to a slightly crescentic configuration (C shape) as it traveled laterally. 2) Separation of the cochlear and vestibular nerves was observed near the central part of the IAC. 3) Discrimination between the superior and inferior vestibular nerves was also possible near the fundus of the IAC. 4) The facial nerve was easily identifiable as a discrete nerve at the anterior aspect of the vestibulocochlear nerve. 5) The meatal loop of the cerebellar artery was located medial to the porus in 44% of 95 CPAs and reached the porus or protruded into the porus in 56%. Acoustic neuromas were as follows. 1) In a patient with a very small intracanalicular tumor, the nerve on which the tumor was located could be identified. 2) In 22 of 44 acoustic neuromas, cerebrospinal fluid was present between the tumor and the fundus of the IAC. Hemifacial spasm was as follows. The relationship between the responsible artery and the facial nerve could be precisely observed.

CONCLUSION

The three-dimensional fast spin echo method offers ultrahigh-resolution images, which are extremely useful in understanding the surgical anatomy of the CPA and IAC.

MR imaging of the inner ear and cerebellopontine angle: comparison of three-dimensional and two-dimensional sequences

OBJECTIVE

The aim of the study was to compare the ability of three-dimensional (3D) T2-weighted turbo spin-echo and gadolinium-enhanced 3D T1-weighted gradient-echo sequences with two-dimensional (2D) T2-weighted turbo spin-echo and gadolinium-enhanced T1-weighted spin-echo sequences to reveal anatomic and pathologic structures of the inner ear and cerebellopontine angle.

SUBJECTS AND METHODS

Thirty-one patients underwent axial 2D T2-weighted turbo spin-echo and 3D T2-weighted turbo spin-echo MR imaging, axial and coronal 2D T1-weighted spin-echo MR imaging before and after i.v. injection of gadopentetate dimeglumine, and gadolinium-enhanced axial 3D T1-weighted gradient-echo MR imaging. The visualization of anatomic and pathologic structures on the different sequences was evaluated. Statistical analysis was performed from the data obtained from the visual evaluation of the anatomic structures on the different sequences. Signal-to-noise and contrast-to-noise ratios were calculated for the gadolinium-enhanced 3D T1-weighted gradient-echo and 2D T1-weighted spin-echo sequences, and statistical evaluation was performed.

RESULTS

The 3D sequences enabled excellent visualization of 94% of all evaluated anatomic structures, and the 2D sequences enabled excellent visualization in only 3% of these structures. Pathologic structures were revealed in all cases by one or both of the 3D sequences. Diagnosis in all patients could be made by using the combination of the 3D T2-weighted turbo spin-echo and the gadolinium-enhanced 3D T1-weighted gradient-echo sequences. However, the 2D sequences failed to show pathologic structures in three patients. We found a significant statistical difference for the visualization of anatomic structures with the 3D and 2D sequences (p < .0001) and no significant statistical difference for the signal-to-noise and contrast-to-noise ratios with the 3D T1-weighted gradient-echo and 2D T1-weighted spin-echo sequences.

CONCLUSION

The 3D sequences revealed anatomic structures significantly better than did the 2D sequences and showed pathologic structures considerably more often than did the 2D sequences in all patients. MR imaging of the inner ear and cerebellopontine angle performed with 3D T2-weighted turbo spin-echo and gadolinium-enhanced 3D T1-weighted gradient-echo sequences provided the most accurate imaging leading to diagnosis in cases of abnormality.

[MRI of the regions of the inner ear and cerebellopontine angle using a 3D T2-weighted turbo spin-echo sequence. Comparison with conventional 2D T2-weighted turbo spin-echo sequences and T1-weighted spin-echo sequences]

PURPOSE

To assess the value of a three-dimensional (3D) T2-weighted turbo spin-echo sequence (3D T2-TSE) in comparison to conventional two-dimensional (2D) T2-weighted TSE and unenhanced and enhanced T1-weighted spin-echo sequences (SE) in imaging anatomic structures and pathologic changes of the inner ear and cerebellopontine angle.

PATIENTS AND METHODS

The inner ear and cerebellopontine angle were investigated by MRI in three healthy volunteers and 18 patients performing a 2D T2-weighted turbo spin-echo sequence and a 3D T2-TSE in the axial plane. In the patient study, 2D T1-weighted SE sequences both before and after the i.v. injection of gadopentetate dimeglumine in both the axial and coronal plane were performed in addition.

RESULTS

Only the 3D T2-TSE enabled an accurate imaging of the anatomic structures. In cases of pathology, the 3D T2-TSE provided additional information to the performed 2D sequences. The combination of the 3D T2-TSE with unenhanced and enhanced 2D T1-weighted SE enabled the most accurate diagnosis in cases of pathology.

CONCLUSIONS

Accurate depiction of anatomic structures of the inner ear and cerebellopontine angle could be obtained by 3D T2-TSE only. The most accurate diagnosis in cases of pathology was provided by the combination of the 3D T2-TSE with unenhanced and enhanced 2D T1-weighted spin-echo sequences.

MR imaging of acoustic neuroma with high resolution fast spin echo T2-weighted sequence

OBJECTIVE

To assess the value of high resolution 2D fast spin echo T2-weighted sequence (HR 2D-FSE T2w) for evaluating the internal auditory meatus (IAM) in patients with asymetric or unilateral sensorineural hearing loss, vs. gadolinium-enhanced T1-weighted (T1w) sequence; to suggest a screening protocol to exclude the diagnosis of acoustic neuroma in a patient with isolated unilateral sensorineural hearing loss.

MATERIALS AND METHODS

One-hundred ten patients with suspected acoustic neuroma were evaluated with 1.5 T MRI system. The protocol included axial images focused on the IAM: HR 2D-FSE T2w images (4000/63, ETL = 16, 3-mm sections with 1.5 mm overlap, 18 FOV, 512 x 384 matrix) and gadolinium-enhanced T1w images (600/23, 3-mm sections, 18 FOV, 256 x 192 matrix). Two criteria for normality of the HR 2D-FSE T2w examination are defined: high homogeneous signal of the cerebellospinal fluid (CSF) and linear low signal of the nerves visible throughout the IAM.

RESULTS

Overall results show no false-negative and six false-positive with HR 2D-FSE T2w sequences vs. gadolinium-enhanced T1w sequences. The sensitivity of HR 2D-FSE T2w sequences is 100%, specificity 93%, and negative preditive value 100%: normal images using HR 2D-FSE T2w sequence can rule out the diagnosis of acoustic neuroma.

CONCLUSION

Using this protocol we can exclude the diagnosis of acoustic neuroma in case of normal HR 2D-FSE images and no additional gadolinium-enhanced T1w sequence is necessary. This protocol might reduce examination time, must promote recourse to MRI in the event of clinical suspicion of acoustic neuroma, and also enables savings by proposing MRI examination as a first-line exam.

Salient anatomic features of the cortico-ponto-cerebellar pathway

Recent studies of the primate corticopontine projection show that the neocerebellum--in addition to connections from motor and sensory areas--receives connections from various association areas of the cerebral cortex, some of which are thought to be primarily engaged in cognitive tasks. The quantities of such connections in relation to those from more clearly motor-related parts of the cortex need to be more precisely determined, however. Furthermore, the anatomic data on origin of corticopontine fibers needs to be supplemented with physiological experiments to clarify their functional properties at the single-cell level. For example, nothing is known of the functional role of the large input from the cingulate gyrus, nor is the input from the posterior parietal cortex physiologically characterized. Finally, the scarcity of corticopontine connections from the prefrontal cortex in the monkey (and probably also in man) may not seem readily compatible with a prominent role of the neocerebellum in certain cognitive tasks. We discuss data--in particular from three-dimensional reconstructions--indicating that both corticopontine projects and pontocerebellar neurons are arranged in a lamellar pattern. Corticopontine and pontocerebellar lamellae have similar shapes and orientations but appear to differ in other respects. Corticopontine terminal fields are sharply delimited, apparently without gradual overlap between projections from different sites in the cortex, whereas pontocerebellar lamellae are more fuzzy and exhibit gradual overlap of neuronal populations projecting to different targets. In spite of the sharpness of the corticopontine projection, there may be many opportunities for convergence of inputs from different parts of the cortex. Thus, the wide divergence of corticopontine projections produces many sites of overlap, and extensive interfaces between different terminal fields enabling convergence of inputs onto each neuron. We suggest that the lamellar arrangement of corticopontine terminal fields and of pontocerebellar neurons serve to create diversity of pontocerebellar neuronal properties. Thus, each small part of the cerebellar cortex would receive a specific combination of messages from many different sites in the cerebral cortex. The spatial arrangement of cerebrocerebellar connections have to be understood both in terms of fairly simple large-scale, gradual topographic relationships and an apparently highly complex pattern of divergence and convergence. Developmental studies of corticopontine and of pontocerebellar projections together with three-dimensional reconstructions in adults suggest that the highly complex adult connectional pattern may be created by simple rules operating during development.

Problem in diagnostic imaging: have you got the nerve?

This article poses a diagnostic problem commonly encountered in neuroradiology and otolaryngology. The solution and ensuing discussion focus on the anatomy of the posterior cranial fossa (with emphasis on the cerebellopontine angle) and the relevant pathology. Current methods of imaging the posterior cranial fossa are explained and their relative merits discussed.

Visualization of inner ear structures by three-dimensional high-resolution magnetic resonance imaging

High-resolution computed tomography (CT) has long been the method of choice in the visualization of the petrous bone, the internal auditory canal, and the cerebellopontine angle. The introduction of magnetic resonance imaging (MRI), especially of the three-dimensional Fourier transformation constructive interference in steady state (3DFT-CISS), has proved to be superior in the detection of soft-tissue lesions in the inner ear. The aim of this study was to visualize small anatomic structures of the inner ear and cerebellopontine angle. The examinations were performed with a standard head coil on a 1.5-T Magnetom ("Vision"; Siemens, Erlangen, Germany). The three-dimensional reconstruction of the cochlea, semicircular canals, and vestibulum allowed detailed visualization, as well as the imaging of cranial nerves VII and VIII. Our results indicate that 3DFT-CISS MRI is a valuable diagnostic tool in the evaluation of inner ear anatomy and pathology; in most cases, however, it must be supplemented by HR-CT.