20.1 Imaging Modalities

20.1.1 Computed Tomography (CT)

• Describe areas of high or low attenuation

• Uses conventional X-rays (ionizing radiation) with computer processing of digital information obtained from layer or spiral tomographs

• For imaging of the temporal bone use high-resolution algorithm, bone windows, thin slices (0.5–1 mm); spiral scanning allows for multiplanar reformatting

• Problems of partial volume averaging can apply to thin bony divisions, especially when slice thickness is higher

20.1.2 Magnetic Resonance Imaging (MRI)

• Describes areas as high or low signal intensity

• Uses the magnetic properties of biological materials and generates images without the use of ionizing radiation

• Tissues contain protons; apply strong magnetic field and these align, then impart energy (radio wave) to excite photons, which resonate; turn offradio wave and protons relax and in doing so give offa radio wave that is detected as the MR signal

• Excellent for contrasting soft tissues:

T1: good for anatomy; fluid (water, CSF, vitreous, mucus) black; fat white; hyperintense structures most likely fat, protein, methemoglobin, or contrast

T2: good for pathology; fluid (water, vitreous, CSF) white; pathological tissue usually bright; fat, muscle intermediate; blood variable; air, bone, rapid vascular flow black

STIR: (short tau inversion recovery) suppresses fat to help show tumour; applied to T2 images; water white, fat black

Can fat suppress either T1 or T2; latter fat-suppressed virtually indistinguishable from T2 STIR sequence

Gadolinium can be used as contrast

Various other sequences can help manipulate image, e.g., FIESTA (fast imaging employing steady-state acquisition), which allows fast acquisition of thin slice information that is good for spatial resolution at expense of contrast resolution

DWI (diffusion-weighted imaging): looks at how water moves through cells; “dead” cells (e.g., post-CVA) trap water (so restrict diffusion) showing up as high intensity, as does keratin, so useful for epidermoid cysts and cholesteatoma; echo-planar (EPI) and non-echo-planar imaging types

20.2 Imaging of Ear Pathology

20.2.1 External Ear

• Congenital atresia of EAC: CT scan can help with surgical planning

• Osteomyelitis (malignant otitis externa):

CT with contrast yields bony detail; coronal CT may show effacement of the tympanic ring

MRI (± gadolinium) can define medial extent of soft tissue disease at the skull base, dural enhancement and cerebral involvement

Technetium-99 m bone scanning and gallium-67 scanning helpful; their sensitivity for the presence of infection is far greater than their specificity for the cause

Tc-99 m gives excellent information about bone function (but poor for bone structure); +ve scan thought to represent osteoblastic activity as little as 10% above normal; +ve in acute and chronic osteomyelitis and in areas of active bone repair without infection (e.g., trauma); increases in Tc-99 m uptake between 4 and 24 h post-injection is the most sensitive indicator of temporal bone osteomyelitis

Ga-67 will highlight an acute infective focus but not the full extent of an osteomyelitic process; as treatment progresses the scan will revert to normal (Tc-99 m will lag behind for months)

Baseline studies of both thus considered, and sequential imaging is used to monitor the response to therapy

Indium-111 labeled leucocyte planar scintigraphy can yield even better results for detection of osteomyelitis and may replace above

• ME effusion will often be an incidental finding on MRIs of brain; especially common in intubated patients; normally hyperintense on T2, hypointense on T1, non-enhancing with contrast

• Routine assessment of COM best with CT, but intracranial complications (dural venous sinus thrombosis, abscesses, meningitis) best evaluated by MRI

• Sigmoid sinus thrombosis: contrast-enhanced CT, acute thrombus hyperdense with contrast enhancing dura around thrombus (but not thrombus)—when chronic it revascularizes so enhances; hypo/isointense with flow voids on T1 and T2 MRI

• Petrositis: CT—opacified cells and lysis of septations; MRI—meningeal enhancement where dura affected with hyperintensity on T2 (not T1)

• Cholesterol granuloma: expansile cyst within pneumatized apex, so smooth expansion of cortical margins on CT; high signal on both T1 and T2 MRI (because high on T1, probably due to methemoglobin, distinguished from simple fluid, apicitis, cholesteatoma); can have hypointensity patches on both T1 and T2

• Cholesteatoma:

CT most helpful to see pattern of bony destruction, but cannot differentiate cholesteatoma from other soft tissue; key features: scutum, lateral Scc, ossicular chain (e.g., long process of incus) erosion

May be helpful in surgical planning, e.g., a small mastoid or low tegmen might mean canal wall up surgery is difficult and atticoantrostomy is better indicated

MRI late T1 Gd get rim enhancement with non-enhancing cholesteatoma (keratin)—inflammatory tissue, granulation enhances; sequence takes 45 min but can show lesions down to 3 mm

T2 moderately hyperintense and can differentiate cholesteatoma from brain herniation if dehiscent tegmen

DWI EPI: detects lesions ≥5 mm, high signal; but temporal lobe artifact

DWI non-EPI: lesions 2 mm+, less artefact, poor bony anatomy; may be suitable to screen before 2nd-look canal wall up surgery

• Neoplasia: CT shows irregular bony erosion for meningioma, glomus, chordoma, chondrosarcoma; smooth erosion for schwannoma, chondroma; metastases vary

20.2.3 Inner Ear

• To assess cochlear anomalies associated with SNHL, both CT and MRI help

• Superior Scc dehiscence: both CT and T2 MRI can be reconstructed in plane and at right angles to plane of superior Scc to demonstrate defect

• Precochlear implantation need to confirm a patent cochlea—both CRT and MRI helpful; MRI also provides information on the presence of VIII n

• Halo sign may be seen around otic capsule in otosclerosis, with CT