MR of the normal and abnormal internal auditory canal

MR performed with thin, contiguous sections has replaced CT for the study of the cerebellopontine angle and for diagnosis of acoustic neuromas. In our experience large acoustic neuromas are well seen in all pulse sequences. Tumors with small extracanalicular components are seen in the T1- and spin-density-weighted sequences whereas purely intracanalicular lesions are often visualized only in the T1-weighted images. Small acoustic neuromas producing thickening of the nerve are easily recognizable in narrow internal auditory canals but may be missed in large canals because of partial volume averaging. Since further enhancement of the signal intensity of tumors can be obtained by IV injection of paramagnetic agents, we foresee the use of such agents in the near future in the diagnosis of acoustic neuromas.     

Screening protocol for MR imaging of the internal auditory canal

We have previously shown that 5 mm axial T1-weighted images following Gd-diethylenetriamine pentaacetic acid (DTPA) administration were sufficient as a screening examination of the internal auditory canal (IAC) and cerebellopontine angle cistern in 80% of cases with sensorineural hearing loss. In the remaining 20% of cases 3 mm slices were necessary for confirmation of the diagnosis. To reduce the number of cases requiring the additional examination, we have modified our protocol to include a coronal "scout" T1-weighted sequence followed by 5 mm axial sections angled through the IACs. Routine axial T2-weighted images of the posterior fossa were also obtained. One hundred eleven patients were studied with this protocol. In all but two of these the 5 mm sections unequivocally provided the diagnosis and in those two cases the coronal images confirmed the diagnosis suspected on the axial study. Three millimeter slices were not necessary in any patient. This three sequence, post-Gd-DTPA protocol, which requires only 12 min of scan time, is recommended as a screening protocol for IAC and cerebellopontine angle disease.     

Role of slice thickness in MR imaging of the internal auditory canal

The relative efficacy of post-Gd-DTPA 5 and 3 mm axial T1-weighted images was compared in the detection of lesions in the internal auditory canal and cerebellopontine angle. One hundred twenty consecutive patients were prospectively evaluated with 5 mm axial T1-weighted slices. If these were negative or questionable. 3 mm axial slices were immediately obtained as the next sequence. Eighteen percent of cases were positive and in none of these was the 5 mm study normal. However, in 22 negative cases and two positive cases, a 3 mm study was necessary for confirmation. Five millimeter axial scanning post Gd-DTPA is recommended as the initial study for detection of masses in the internal auditory canal and cerebellopontine angle. Because this study requires fewer acquisitions than 3 mm sections and can be done satisfactorily on low and midfield systems, there are potential time- and cost-saving benefits to this approach.     

"Truncation" artifact in MR images of the internal auditory canal

Dry skulls and a phantom were studied to determine whether an intracanalicular dark band in MR images of some acoustic neuromas could be artifactual. A "truncation" artifact was detected in the internal auditory canals of the dry skulls and in a simulated internal auditory canal of the phantom when the width of the canal approximately equaled 4 X (field of view) /N, where N equals 128 or 256, depending on the number of gradient steps chosen. The "truncation" artifact should not be confused with CSF between normal nerves when a canal contains tumor.     

T2-weighted MR characteristics of internal auditory canal masses.

PURPOSETo determine whether masses of the internal auditory canal are hypointense relative to cerebrospinal fluid, and therefore visible, on fast spin-echo T2-weighted MR images.METHODSForty-six patients had 50 masses of the internal auditory canal, identified initially on contrast-enhanced MR images, that were evaluated retrospectively for signal intensity of the mass with respect to cerebrospinal fluid and for visibility of the neural elements within the internal auditory canal on T2-weighted images.RESULTSForty-seven of 50 masses were clearly identified on T2-weighted images. Three small abnormalities (2 to 4 mm) were not seen with confidence on T2-weighted images. However, on close inspection of these three masses, the small abnormality on contrast-enhanced MR images corresponded to a hypointense focus on T2-weighted images. All 50 masses were hypointense relative to cerebrospinal fluid on T2-weighted images.CONCLUSIONAll masses of the internal auditory canal in this study were hypointense relative to cerebrospinal fluid on T-2 weighted images, and were therefore visible.     

The effect of patient positioning on MR imaging of the internal auditory canal

Summary Identification of individual cranial nerves and complete exclusion of tumor in the internal auditory canal may be difficult with MR, especially in imperfectly positioned patients. MR studies of the temporal bones in patients and in normal volunteers positioned non-rotated or canted were correlated with corresponding cryomicrotomic sections. Especially in axial images, oblique sectioning of cranial nerves VII and VIII may cause difficulty in identifying individual nerves. A combination of axial and coronal short TR and TE images can be used to confidently exclude intracanalicular tumor in most cases.     

MR imaging of cerebellopontine angle and internal auditory canal lesions at 1.5 T

The high-field, thin-section (3-5 mm) MR imaging characteristics of 49 cerebellopontine angle and internal auditory canal lesions were reviewed. The diverse abnormalities include 20 acoustic neurinomas, eight neurinomas of other cranial nerves (six involving the fifth cranial nerve and two involving cranial nerves IX-XI), seven meningiomas, five subdural fluid collections, four brainstem tumors with exophytic components, three glomus jugulare tumors, one epidermoid tumor, and one basilar artery aneurysm. T1-, T2-, and proton-density-weighted images were obtained in all cases. T1-weighted images most accurately showed the margins of the seventh and eighth nerves in the internal auditory canal and were most sensitive in detecting small tumors in the cerebellopontine angle. Differentiation of meningioma from acoustic neurinoma by MR was provided most reliably by separation of the meningioma from the porus acusticus and seventh and eighth nerves and not by signal-intensity differences. A hypointense vascular rim was noted on MR in seven of 13 extracanalicular acoustic tumors and in three of seven meningiomas.     

Dumbbell schwannomas of the internal auditory canal

BACKGROUND AND PURPOSE: Benign tumors of the internal auditory canal (IAC) may leave the confines of the IAC fundus and extend into inner ear structures, forming a dumbbell-shaped lesion. It is important to differentiate dumbbell lesions, which include facial and vestibulocochlear schwannomas, from simple intracanalicular schwannomas, as surgical techniques and prognostic implications are affected. In this article, the imaging and clinical features of these dumbbell schwannomas are described. METHODS: A dumbbell lesion of the IAC is defined as a mass with two bulbous segments, one in the IAC fundus and the other in the membranous labyrinth of the inner ear or the geniculate ganglion of the facial nerve canal, spanned by an isthmus. Twenty-four patients with dumbbell lesions of the IAC had their clinical and imaging data retrospectively reviewed. Images were evaluated for contour of the mass and extension into the membranous labyrinth or geniculate ganglion. RESULTS: Ten of 24 lesions were facial nerve dumbbell lesions. Characteristic features included an enhancing "tail" along the labyrinthine segment of the facial nerve and enlargement of the facial nerve canal. Dumbbell schwannomas of the vestibulocochlear nerve (14/24) included transmodiolar (8/14), which extended into the cochlea, transmacular (2/14), which extended into the vestibule, and combined transmodiolar/transmacular (4/14) types. CONCLUSION: Simple intracanalicular schwannomas can be differentiated from transmodiolar, transmacular, and facial nerve schwannomas with postcontrast and high-resolution fast spin-echo T2-weighted MR imaging. Temporal bone CT is reserved for presurgical planning in the dumbbell facial nerve schwannoma group.     

Unilateral dilatation of the internal auditory canal

Summary An unusual and functionally insignificant variant of the internal auditory canal is presented. Marked dilatation of one canal was demonstrated by petrous bone tomography. Pantopaque cisternography revealed a large intracanalicular extension of the subarachnoid space with no evidence of tumor. No patients should undergo exploration of this region without prior Pantopaque studies.

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Einseitige Erweiterung des inneren Gehörgangs

Zusammenfassung Es wird eine ungewöhnliche und funktional unbedeutende Variation des inneren Gehörgangs beschrieben. Dabei wurde die Erweiterung des Kanals mit der Tomographie dargestellt. Die Cisternenfüllung mit Pantopaque zeigte eine große Ausdehnung des Subarachnoidalraumes in den inneren Gehörgang ohne Zeichen eines Tumors. In der Arbeit wird darauf hingewiesen, daß die einseitige Erweiterung des inneren Gehörgangs zur Operationsindikation nicht ausreicht.

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Dilatation unilatérale du conduit aduitif interne

Résumé L'auteur décrit la dilatation unilatérale du conduit auditif interne par extension de l'espace sous-arachnoïdien dans le canal. Ce mécanisme peut être démontré par cisternographie opaque.

     

Optimizing MR imaging for detecting small tumors in the cerebellopontine angle and internal auditory canal

Relative resolving power was used to determine the optimal MR imaging pulse sequence for detecting small tumors of the internal auditory canal and the cerebellopontine angle. Resolving power takes into consideration these important image characteristics: signal-to-noise ratio, contrast, and spatial resolution. The study was performed on a 1.5-T magnet using a 256 X 256 matrix and a 3-mm slice thickness. The TR ranged from 400-2000 msec; the number of excitations was either two or six; and the pixel size was 0.94, 0.78, or 0.63 mm. Theoretical calculations of relative resolving power were compared with the relative resolving power of 45 control patients and 15 patients with small tumors of the cerebellopontine angle or internal auditory canal. A TR of 800 msec was optimal from theoretical calculations and proved optimal in control and tumor patients. Scans obtained with TR = 2000 msec, TE = 80 msec were inferior to short TR scans; such scans could fail to detect intracanalicular tumors. The relative resolving power in patients exceeded theoretical calculations because of greater than expected image contrast caused by low CSF signal intensity secondary to CSF pulsation.     

3D-FIESTA序列在内耳及内听道磁共振检查上的应用

目的:探讨3I-FIESTA序列高分辨率影像在内听道成像技术上的应用.方法:对86例共172只内耳应用3D-FIESTA序列做颞骨的三维容积扫描,采用最大强度投影(MIP)进行后处理,观察由3D-FIESTA序列获得的高分辨率影像对内耳及内听道解剖结构及病变的显示.结果:3D-FIESTA序列扫描及MIP重建对内耳耳蜗、迷路及内听道内神经及血管显示清晰,能够清晰显示病变与周围结构的关系.结论:3D-FIESTA序列可以提供非常好的分辨率,对内耳及内听道充满水的结构及病变显示清晰,在内耳、内耳道和桥脑小脑角区具有较高的临床应用价值.     

MR cisternography of the cerebello-pontine angle and internal auditory canal in diagnosis of intracanalicular acoustic neuroma

Summary One hundred and fifteen patients, suffering from sensorineural hearing loss were tested with a 1.5 T superconducting magnet. The authors describe utility of both T1-weighted multiple slice and T2-weighted multiple echo images for the evaluation of cerebello-pontine angle, internal auditory canal and their neurovascular content. In seventy-three cases MR cisternography was normal. The remaining forty-two cases were subdivided into twenty extracanalicular masses, eleven small intra-extracanalicular and nine purely intracanalicular lesions. All the lesions were histologically proven acoustic neuromas, except one intracanalicular mass which was a meningioma. Examination was inconclusive only in two cases and decision was then made to follow the clinical course. Advantages of MR cisternography over CT and air CT cisternography, such as absence of ionizing radiation and contrast material, easy multiplanar evaluation of the region of interest and the possibility to delineate both the cisternal and canalar extremities of the tumor mass are pointed out.     

Hybrid phased array for improved internal auditory canal imaging at 3.0-T MR

PURPOSE: To develop and evaluate a hybrid phased array for internal auditory canal (IAC) imaging at 3.0 T. MATERIALS AND METHODS: A hybrid phased array was designed and built as two circular surface receive-only coils combined with a volume transmit-receive birdcage head coil for simultaneous image acquisition. Phantom and volunteer images were obtained to assess the coil performance. RESULTS: The phantom data show that significant signal-to-noise ratio (SNR) improvement was achieved in the region corresponding to the inner ear, i.e., by a factor of 2.5 compared to the standard head coil data. Volunteer IAC image quality was deemed superior as compared to images acquired at 3.0 T using a standard head coil. CONCLUSION: This hybrid array combined with three-dimensional fast spin-echo (FSE) acquisition resulted in improved high spatial resolution IAC imaging.     

MRI of unusual lesions in the internal auditory canal

We report the MRI findings of six unusual lesions of the internal auditory canal: three haemangiomas, one lipoma, one metastasis and one traumatic neuroma. We compare the findings to those of 20 intracanalicular schwannomas. We noted the site and size of the tumour, its signal intensity, borders and the homogeneity of enhancement were studied on T1-weighted images before and after intravenous contrast medium and T2-weighted images. Most schwannomas were homogeneous lesions, isointense on T1- and T2-weighted images, and strongly enhancing. Spontaneous high signal on T1-weighted images, heterogeneous contrast enhancement and extranodular enhancement were helpful for recognising lesions other then schwannomas; site, size and signal on T2-weighted images were not. All the haemangiomas had a specific pattern of contrast enhancement, with an anterior core intensely enhancing portion and a posterior portion which enhanced moderately or not at all. Received: 4 November 1999/Accepted: 19 July 2000