Inner ear fluid volumes and the resolving power of magnetic resonance imaging: can it differentiate endolymphatic structures?

Magnetic resonance imaging (MRI) can accurately recognize minute volumes as small as 1 mm3. The volumes of the utricle and saccule of the inner ear are within the resolving power of MRI, but these structures cannot be recognized because the endolymph and perilymph signals are identical. To clarify the interpretation and description of inner ear structures on MRI, we measured and calculated the volumes of the perilymphatic and endolymphatic spaces of the human ear. We found the total volume of the bony labyrinth to be approximately 192.5 mm3 (endolymph, 34.0 mm3; perilymph, 158.5 mm3).

Magnetic resonance imaging of the internal auditory canal.

Magnetic resonance imaging (MRI) is presently the study of choice for assessment of the internal auditory canal (IAC). MRI provides excellent assessment of the IAC and the bony changes occurring in the canal walls, and it provides excellent demonstration of the content of the canal. Pathological processes arising within the IAC are well visualized by various MR sequences. The possibility of demonstrating masses as small as 2 mm has propelled MRI into the leading role for diagnosis of vestibular schwannoma. Unfortunately, the high cost of MR has been a limiting factor in its use as a screening test for patients with sensorineural hearing loss (SNHL) of unknown origin. Auditory brain stem response has been widely used as a screening procedure, but this test fails to recognize small lesions and cannot be used whenever hearing loss is severe. In this article, we will discuss our approach to assessment of the IAC in patients with retrocochlear SNHL or vestibular symptoms of central origin, review the pathological processes involving the IAC walls or arising within the canal, emphasizing the appropriate MRI sequences used for diagnosis.

Imaging of orbital lymphoproliferative disorders.

Lymphomas and leukemias account for a large portion of orbital tumors. Orbital lymphoma accounts for 55% of malignant orbital tumors in adults. Idiopathic orbital inflammatory pseudotumors are pathologic entities that often challenge ophthalmologists and radiologists. This article describes the MR and CT features of orbital lymphoma, leukemia, and some other lymphoproliferative disorders.

Predictability of magnetic resonance imaging in differentiation of orbital lymphoma from orbital inflammatory syndrome.

Twenty-two patients with clinical presentations of either orbital lymphoma or orbital inflammatory syndrome (OIS) involving 25 orbits were examined by magnetic resonance imaging (MRI) before biopsy. To determine whether MRI adds specificity to radiographic diagnosis, the MRI signal characteristics of all tumors were examined. Attention was focused on the signal intensity of each tumor on T1- and T2-weighted images. In nine of 12 (75%) orbital lymphomas, the tumors appeared hyperintense to fat on T2-weighted images and became brighter relative to their appearance on T1-weighted images. In 11 of 13 (85%) orbital infiltrates with OIS, the tumors appeared isointense to fat on T2-weighted images and became slightly darker or unchanged relative to their appearance on T1-weighted images. Tumor density and homogeneity were fairly similar in all 25 lesions and were therefore not useful for further differentiation. Similarly, the presence of moderate enhancement with gadopentetate dimeglumine was seen in all but one tumor, and it was therefore not useful for further differentiation.

The internal auditory canal revisited. The high-definition approach.

The internal auditory canal (IAC) is a bony canal that contains nerves and vessels and is lined by meninges. Pathologic processes may arise from each of these structures. Congenital, developmental, and acquired pathologic conditions may involve the bone of the canal and lead to excessive narrowing or expansion. Lesions formed within the canal include hemangiomas, vascular malformations, and inflammatory and neoplastic processes arising from the facial and acoustic nerves and meninges. Finally, other rare lesions, such as choristomas and metastases, may occur within the IAC, which are not related to its normal content. High-definition computed tomography, MR imaging, and MR angiography allow diagnoses of IAC or cerebellopontine-angle cistern lesions, some of which could not be identified by older techniques.

Update of computed tomography and magnetic resonance in otology.

Since my report of 4 years ago at the 8th Shambaugh-Shea Workshop in Otology, equipment and software techniques have continued to evolve with consequent further refinement of imaging studies. Three-dimensional images have greatly improved and have acquired an important role in maxillofacial reconstructive surgery. In otology their value is limited except in congenital atresia. New developments in imaging technique (3-D Fourier transformation) allow sections as thin as 1 mm in any plane. The facial nerve, endolymphatic sac and vestibular aqueduct are well demonstrated but differentiation of the components of the membranous labyrinth is limited. Applications of postcontrast imaging have become more defined for cerebellopontine angle masses, inner ear pathology, and middle ear and facial nerve lesions. Gradient echo techniques and bipolar flow encoding have enabled the development of magnetic resonance angiography. This new, noninvasive procedure is currently useful for the study of aneurysms, vessel stenosis, vascular tumors, and arteriovenous shunts and malformations.

Imaging of otosclerosis.

Imaging studies play an important role in the diagnosis of otosclerosis and in the clinical management of this disease. CT scanning at present is the tool of choice for the assessment of the labyrinthine windows and cochlear capsules. MR imaging thus far has had limited applications in the examination of the cochlear capsules but has been more useful in the assessment of the cochlear lumen prior to cochlear implant in patients with profound bilateral hearing loss.

Natural history of acoustic neuromas.

The growth rate of acoustic neuromas was studied in a series of 50 patients who underwent follow-up imaging studies, since surgery was not performed after the initial diagnostic study or only performed after the follow-up studies. In 50% of the cases the tumor did not grow during the length of the follow-up. In the other patients the growth was less than 50% of the original tumor size in all but four. No correlation was observed between tumor growth, length of the follow-up, and age of the patient. Our results suggest a change in the management of this benign tumor.

Further studies on the effects of magnetic resonance imaging fields on middle ear implants.

We investigated the effects of magnetic resonance imaging (MRI) fields on 21 stapedectomy prostheses and other middle ear implants and two different receiver-stimulator modules from 22-channel cochlear implants. None of the middle ear implants was displaced by the magnetic field, except for one platinum-stainless steel stapedectomy piston. Magnetism was not induced in any of the middle ear implants subjected to prolonged exposure in the MRI scanner. We conclude that MRI could pose a hazard to patients who have had stapedectomy using certain platinum-stainless steel piston prostheses and to patients with cochlear implants. Magnetic resonance imaging should pose no hazard to patients who have had the other middle ear implants reported on in this and our previous investigation.

Imaging studies in patients with central vestibular disorders.

MR is the radiologic examination of choice when a patient's symptoms indicate the presence of some retrolabyrinthine pathology. CT dynamic studies now permit both heart and brain assessment for poor blood circulation problems of the hindbrain.

CT and MR imaging of intralabyrinthine schwannoma: report of two cases and review of the literature.

Two cases of intralabyrinthine schwannoma were studied with computed tomography (CT) and magnetic resonance (MR) imaging. On CT scans, a soft-tissue mass was identified in the round window niche in both cases. Widening of the basilar turn of the cochlea and characteristic erosion of the promontory were noticed in one case. Preoperative MR imaging (performed in only one case) revealed a soft-tissue mass in the labyrnth, extending into the round window niche. Findings at CT and MR imaging are discussed, and a differential diagnosis is given. The literature is reviewed. The authors' findings suggest that CT and MR imaging may prove very valuable in the previously difficult preoperative diagnosis of these tumors. In the setting of progressive sensorineural hearing loss, atypical Meniere disease, or recurrent vertigo, the presence of a mass in the labyrinth or labyrinthine windows- delineated on CT or MR images, despite a normal internal auditory canal, cerebellopontine angle, or brain stem - is highly suggestive of intralabyrinthine schwannoma.

Growth rate of acoustic neuromas.

During the past 25 years the authors have diagnosed over 1,600 acoustic neuromas. In almost all of the cases surgery for the removal of the tumor was performed shortly after the diagnostic tests. However in 35 cases one or more follow-up imaging studies was performed since the patients refused surgery, or surgery was not performed because of the poor general condition of the patients or the small size of the lesion, particularly in individuals of advanced age. In the latter group there was a clear understanding that surgery would be performed if the follow-up study showed progression of the lesion. In this paper the authors review the evolution of the tumor in the 35 cases, try to establish the rate of growth in cases where the tumor became larger, and correlate tumor evolution to patient's age and clinical findings.

Magnetic resonance imaging of the posterior cranial fossa.

Magnetic resonance imaging has acquired a leading role in the assessment of the posterior cranial fossa, base of the skull, and neck. Several technical improvements, such as further refinement of surface coils, fast scanning, and paramagnetic agents, have enhanced the diagnostic capability of this imaging technique. It is extremely useful in the differentiation of pathologic processes arising from or involving the petrous pyramids, such as congenital cholesteatomas, cholesterol granulomas, and glomus tumors. It is the study of choice for both extra- and intra-axial lesions in the posterior cranial fossa. The magnetic characteristics of the most common lesions are reviewed in this paper.

Otogenic intracranial inflammations: role of CT.

This article reviews the authors' results using CT in the diagnosis of 18 selected patients who were clinically suspected of having otogenic intracranial complications, demonstrating with illustrative examples the important role of CT in diagnosing various stages of acute coalescent mastoiditis and its associated complications.

Cerebellopontine angle tumors.

Acoustic neuromas account for approximately 80 to 90 per cent of cerebellopontine angle tumors. Useful imaging studies include (1) high-resolution CT of the internal auditory canals using extended scale and "bone algorithm" techniques, (2) infusion study of the cerebellopontine cisterns, (3) gaseous or opaque CT cisternography, and (4) MRI.

Tumors and tumor-like conditions of the middle ear and mastoid: role of CT and MRI. An analysis of 100 cases.

CT and MRI scanning provide valuable information in determining location and extension of middle ear and mastoid tumors and tumor-like conditions. The bone anatomy depicted by CT better delineates the involvement of the middle ear diseases. In glomus tumors, facial nerve tumors, and other cases including temporal bone metastases, the combination of CT and MR imaging is exceedingly useful in the diagnosis and management of patients.

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.