الفهرس 
ANATOMY OF THE TEMPORAL BONEOnly 14 pages are availabe for public view
 |  | from | 93 |  |  |
| | | from | 93 | | |
Abstract
from the present essay, we could summarize that tinnitus can be subdivided into subjective and objective types. Subjective tinnitus is sound that is audible only to the affected person; objective tinnitus is sound that is produced within the ear or adjacent structures that can be heard by the examining physician.
Tinnitus can often be further classified as either pulsatile or nonpulsatile based on the quality of sound that the patient perceives. Pulsatile sounds are usually caused by vibrations from turbulent blood flow that reach the cochlea. Pulsatile tinnitus can be subclassified as either vascular or nonvascular in origin.
Most causes of Pulsatile tinnitus are vascular. Arterial causes of Pulsatile tinnitus include carotid artery stenosis, carotid occlusion, dissection,carotid artery fibromuscular dysplasia, a persistent stapedial artery, and an aberrant or lateralized ICA course. Venous etiologies include high-riding or dehiscent jugular vein, venous stenosis, venous diverticulum, abnormal condylar and mastoid emissary veins, or turbulent flow through a dominant jugular vein. Pathologic conditions of the middle ear (such as a glomus tympanicum or cholesteatoma) or labyrinth (such as otospongiosis).
In recent years, three-dimensional (3D) multiplanar reformatted images from conventional cross-sectional computed tomographic (CT) data have been increasingly used to better demonstrate the anatomy and pathologic conditions of various organ systems.
Such images can aid in understanding the temporal bone, a region of complex anatomy containing multiple small structures within a relatively compact area, which makes evaluation of this region difficult. These images can be rotated in space and dissected in any plane, allowing assessment of the morphologic features of individual structures, including the small ossicles of the middle ear and the intricate components of the inner ear.
Contrast enhancement is not essential for evaluation of pathology isolated to bone or air spaces. Intravenous contrast material is often used to assess vascular lesions, areas of breakdown in the blood-brain barrier, or soft tissue changes.
CT is less expensive than MRI, although CT is limited by scan artifacts, radiation exposure, and fewer scanning planes (because of the limitations in gantry and in patient positioning).
Although multiplanar CT reformations can display the complex anatomy of the temporal bone in all of the conventional polytomography projections while exposing the patient to radiation only once, slight motion by the patient is unaccepted because the small bony structures of the temporal bone are in such close proximity that motion artifacts may obscure significant detail. In addition, reformation results in slight image degradation and loss of resolution.
C.T. has its greatest advantage in the detection of the early bone erosion of the lateral & anterior bony walls of the jugular fossa in cases of glomus jugular tumors which differentiates glomus tumor from other jugular fosssa lesions.
MRI is then used for detection of such tumor with gadolinium enhancement. The characteristic MR pattern is salt & pepper. That salt is the enhancing tumor stroma and the pepper is signal voids of tumor vessels.
While, high resolution C.T of the temporal bone is the examination of choice of glomus tympanicum, and it is usually possible to appreciate enhancement of a small tumor confined to the middle ear on a C.T study.
Both high resolution CT and MRI provides definitive diagnosis of schwannomas with characteristic dense homogenous enhancement after contrast administration.
Cerebellopontine angle meningioma, as well displays homogenous enhancement after contrast injection with wide dural base on both CT and MRI. CT is superior to MRI in detecting internal calcifications, bone invasion with hyperostosis.
CT scanning is the imaging modality of choice in cholesteatima. CT scanning can detect the bony defects which may include Scutal erosion, Labyrinthine fistula, Defects in the tegmen, and give us details about the ossicular involvement either erosion or discontinuity, while MRI is used when very specific problems are expected to involve the surrounding tissues.
MRI is outstanding for its ability to evaluate blood vessel-related disorders of the temporal bone with many gradient-echo techniques. Gadolinium- diethylenetriamine-penta-acetic acid (Gd-DTPA) contrast-enhanced MRI studies using TI-weighted images are particularly sensitive for evaluating abnormalities that alter the blood-brain barrier or that are vascular, such as inflammatory disease , carcinomatosis, posterior fossa infarcts, tumors, and demelination.
MRI studies, compared with CT, have higher resolution and are more sensitive to alteration in the fluid spaces of the inner ear and the cerebellopontine angle. The findings of contrast enhancement are more obvious on MR images than on CT scans.