![]() ![]() If a mass is not visible, then a more complex contrast-enhanced CT with CT angiography and CT venography approach is necessary (Appendix A). If a mass is visible, then non–contrast-enhanced CT is done. The primary study for this indication depends on whether there is a mass visible behind the tympanic membrane. ![]() However, such imaging triage can help avoid clinical misadventures––for instance, biopsy of a highly vascular mass such as a paraganglioma or injury to an important normal vessel such as the internal carotid. More often, they are negative and therefore only exclusionary diagnostic efforts. These studies may establish the precise etiology of the mass or identify the cause of objective or subjective pulse synchronous tinnitus. Patients with pulse synchronous tinnitus or a mass of possible vascular origin will often come to diagnostic imaging. The clinical situation may also guide the sequencing of CT and MRI. A combination of morphologic features, associated findings, and clinical settings needs to determine the next best course of action when abnormal enhancement of the membranous labyrinth, nerves, meninges, or a mass is present.įacial nerve dysfunction may require clinical triage to determine whether CT and/or MRI are indicated at all. Contrast enhancement is sometimes nonspecific. Screening for lesions producing such dysfunction should include use of paramagnetic contrast and three-dimensional (3D) steady state images. The most common use of MRI is in patients with sensorineural hearing loss (SNHL) and vestibular signs and symptoms. MRI becomes the more likely primary examination when a complete evaluation of the brain stem, cerebellopontine angle (CPA), and intracanalicular course of cranial nerve (CN) VIII is necessary. MRI may also provide adjunctive information about the membranous labyrinth ( Table 104.1). As a secondary study, MRI may help characterize the nature or extent of a mass––for instance, petrous apex cholesterolosis versus phlegmon. MRI becomes the adjunctive study where bone detail is of primary interest. Given different practice preferences or mitigating clinical circumstances, either CT or MRI can be an appropriate starting place for some of these indications.ĬT should be the primary examination for indications that require bone detail. These general indications and others are outlined in Table 104.1, where they are keyed to the likely preferred primary study. Other common indications for CT and MRI of the temporal bone and related posterior fossa structures include facial nerve dysfunction, vertigo, tinnitus, otitis media, mass lesions in and about the temporal bone, and patients with unexplained otalgia. Patients with sensorineural and conductive hearing loss make up the bulk of temporal bone study referrals. Relative Indications for Computed Tomography and Magnetic Resonance Imaging General applications of radionuclide and related techniques are discussed in Chapter 5. Ultrasound is essentially not used in this anatomic area of interest. More refined protocols are presented when appropriate in the disease- and anatomy-specific discussions. General imaging parameters used in this region of interest are presented in Appendixes A and B. These issues are discussed in detail in conjunction with general CT and MRI parameters in Chapters 1 through 3. In general, computed tomography (CT) and magnetic resonance imaging (MRI) in this anatomic region almost always require the highest possible spatial resolution, sometimes needing to balance that against required low-contrast resolution. TEMPORAL BONE, POSTERIOR SKULL BASE, POSTERIOR FOSSA, AND CRANIAL NERVESĪNTHONY A. ![]()
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