Imaging Techniques in Temporal Bone EvaluationCHAPTER 1

Temporal bone constitutes the lateral skull base and contributes to middle and posterior fossa and the lateral calvarium. It is situated lateral to the temporal lobe of brain.

It comprises of five portions: (1) Squamous; (2) Mastoid; (3) Tympanic; and (4) Petrous parts; and (5) Styloid process (Fig. 1).

The small tympanic part houses the tympanic cavity (middle ear), which communicates with the mastoid.

Petrous portion houses the otic capsule (inner ear) at its base.

Lower six cranial nerves and major vessels (internal carotid artery and jugular vein) pass in relation to the temporal bone.

4This book focusses on the imaging of ear and related structures, including the petrous apex and lateral skull base.

The role and techniques of various imaging modalities are discussed in this chapter.

Always compare with the opposite side on cross-sectional imaging modalities and look for symmetry while reporting.

Radiographs give an overview of the entire temporal bone and indicate the status of pneumatization of the mastoids and petrous pyramids.

Opacification/sclerosis of air cells indicates otomastoiditis and large lesions can produce osseous lytic destruction, evident on the radiographs.

Radiographs are also useful to evaluate the position and integrity of cochlear implants, demonstrating the continuity of the wires.

In addition to intraoperative use, pre- and postoperative radiographs are also very useful.

Serial radiographs serve to assess the clinical course and disease progression/response.

Commonly performed projections include frontal view, lateral view, and oblique view.

It is obtained with orbitomeatal line of the patient perpendicular to the detector and radiation beam going anteroposteriorly or posteroanteriorly, directed at the center of the orbit.

It provides a frontal projection of the mastoid and petrous pyramid.

Petrous apex is foreshortened because of its obliquity and internal auditory canal (IAC) is seen as lucent canal extending through it. Vestibule, semicircular canals, and cochlea are also discernible.

This is a lateral view of the mastoid with the sagittal plane of the patient's skull parallel to the table and X-ray beam directed 30° caudally to separate the two mastoids.

It gives accurate depiction of degree, extent, and distribution of aeration of mastoid with superimposition of IAC and external auditory canal (EAC).

The sinus plate is seen as a dense vertical line posterior to the EAC and merges posterosuperiorly with similar dense line (dural plate).

The sharp angle at the junction of these plates is called as the sinodural angle of Citelli, which becomes obtuse in secondary sclerosis [chronic suppurative otitis media (CSOM)].

More anteriorly is the superior petrous ridge, which crosses the radiolucency of the EAC and extends to the mandibular condyle.

In this view, the patient's head faces the film with slight flexion and 45° rotation to the opposite side, so that ipsilateral orbit touches the image receptor. The X-ray beam is directed 12° cranially.

The entire petrous apex is visualized in full length while the porus acusticus is seen enface (Figs. 4A and B). Vestibule, semicircular canals, and mastoid are also seen.6

High-resolution multidetector CT is the most frequently employed imaging modality for evaluation of temporal bone.

Widespread availability, relatively low cost, high spatial resolution, and quick acquisition make it a robust investigation.

Indications of temporal bone CT include:

Technique:

Indications of contrast administration include suspected tumor, abscess formation, vascular involvement, and complications (like sigmoid sinus thrombosis or intracranial spread of disease).

Special planes of reconstruction for each temporal bone (reconstructed in 0.6 mm thickness bone window):

Three-dimensional volume-rendering postprocessing tools enable easy understanding of the normal structures and pathologies (discussed later).9

Cone beam CT (CBCT) employs a cone-shaped X-ray beam, directed toward the temporal bone onto a two-dimensional (2D) detector. Since it uses the entire FOV of the 2D X-ray detector, only a single gantry rotation is required to acquire a 3D-volumetric dataset.

Advantages over multidetector computed tomography (MDCT) include low radiation dose despite high spatial resolution and less metallic and beam hardening artifacts.

However, acquisition time is long and thus it is prone to motion artifacts.

Advantage is that no ionizing radiation is involved. It can be done at 1.5–3T. Sedation is required for children.

Indications: For evaluation of brainstem, cerebellopontine angle, IAC, 7th to 8th nerve complex and signal intensity of cochlea-vestibular apparatus (modality of choice for inner ear evaluation).

Three-dimensional axial isotropic thin-section (0.3–0.6 mm) cisternography-type sequences of bilateral inner ear and IAC regions. Two types of MR sequences may be done:

Thin section (1.25–2 mm) axial and coronal T2W.

Thin section (2–3 mm) axial and coronal T1W.

Thin section axial and coronal (or 3D) postcontrast T1W (fat-suppressed).

A multiphasic scan may also be done to evaluate the contrast kinetics as well as for perfusion studies in case of mass lesions. Detection of delayed enhancement may help in differentiating postoperative granulation tissue from residual cholesteatomas.

Screening fluid-attenuated inversion recovery (FLAIR) (axial or 3D) and diffusion-weighted imaging (DWI) of entire brain with postcontrast scans.11

These include 3D surface reconstructions (Figs. 11A to C) [surface-shaded display (SSD)] and volume rendering technique (VRT) to have an overview of fluid-filled cochlea-vestibular apparatus [in magnetic resonance imaging (MRI)] and/or associated auricle anomalies (in CT).

Image segmentation can be done and various structures (like ossicles) can be seen in isolation in three dimensions after clipping out surrounding regions.

Virtual endoscopy of the middle ear structures is also possible because of high contrast between the bony structures and air spaces in CT. These structures can be rotated in space and dissected in different planes, enabling understanding of the complex anatomy of ossicles and inner ear structures.

Other postprocessing tools include maximal intensity projection (MIP) and minimal intensity projections (MinIP).

These include SPACE FLAIR on Siemens, cube FLAIR on GE, and FLAIR VISTA on Philips (please see Chapter 20).²

Fluid signal is suppressed and brain signal is accentuated.

This is useful in evaluation of Ménière's disease because it permits distinct visualization of endolymphatic and perilymphatic components.³

It is also useful to detect labyrinthine hemorrhage and to monitor the degree of functional impairment in case of vestibular schwannoma.12

This sequence has also been found to be helpful after contrast administration, for the evaluation of Ménière's disease.

Endolymphatic hydrops may be detected on 3D heavily T2W sequences where saccular dilatation may be seen as enlarged hypointense structure.

However, postcontrast 3D FLAIR imaging is better validated for this purpose. Contrast may be administered intratympanically or more commonly intravenously followed by delayed phase imaging.

Usually, double dose of contrast is administered intravenously and acquisition is done at 4 hours after contrast injection. 3T acquisition is better to accentuate the contrast opacification of perilymph and better assess the nonenhancing cochlear duct. Also, the perilymph enhancement has been found to be more dramatic in the ears affected by Ménière's disease.

Recently, constant high flip-angle technique has been sound to have higher intensity ratio compared to heavily-T2 variable flip-angle method.⁴

Cisternal and intracanalicular parts of facial nerve are well seen on 3D cisternography sequences mentioned above. However, it is difficult to visualize the labyrinthine, tympanic, and mastoid segments. Parotid part can be seen by 3D double-echo steady state (DESS) sequence with water excitation (Figs. 12A and B).13

Postcontrast T1W 2D sequence: Facial nerve faintly enhances in the region of geniculate ganglion, tympanic and mastoid segments after contrast administration. However, rest of the segments does not enhance and thus any enhancement in these regions may suggest inflammation/neoplastic pathology.

Postcontrast T1W 3D: Spoiled gradient acquisition in the steady state [spoiled gradient recalled (SPGR) in GE, VIBE in Siemens and FFE in Philips].

Three-dimensional inversion recovery fast spoiled gradient recalled (IR-FSPGR) sequence: Compared to SPGR, facial nerve shows significantly higher signal on this sequence in majority of the segments on both unenhanced and contrast-enhanced images.

Pointwise encoding time reduction with radial acquisition (PETRA) ultrashort echo time sequence: Usually done at 3T, this MR sequence enables visualization of facial nerve from its origin, through the temporal bone, to the stylomastoid foramen.⁵

Magnetic resonance arteriography and venography can be employed to assess the vascular structures in the jugular foramen and carotid canal, especially in case of large mass lesions.

Time-of-flight (ToF) sequence enables this evaluation without the need of contrast, but the method of choice is contrast-enhanced angiography.

Three-dimensional MR angiography provides excellent demonstration of both intracranial and extracranial vessels and arteries and veins can be separately imaged (Figs. 13A to D).14

It detects the Brownian motion of water molecules, which is facilitated or restricted based on tissue characteristics (Figs. 14A to C).

Single-shot echo planar imaging (EPI) based sequence is prone to susceptibility artifacts and geometric distortion.

Thus, multishot or segmented EPI sequences are used.15

Non-EPI sequences:

b-values of 0 and 800–100 s/mm² are often used.

Restricted diffusion is the hallmark of congenital cholesteatomas with smooth bony remodeling.

Residual/recurrent cholesteatomas can also be detected with high accuracy along with delayed postcontrast imaging.

Tumors and abscesses also show restricted diffusion.

Magnetic resonance spectroscopy (MRS) and perfusion MRI may be employed for characterization of mass lesions, especially in retrocochlear region. Elevated choline on MRS indicates increased cell turnover, suggesting a neoplastic lesion (Figs. 15A and B).

Diffusion tensor imaging may be helpful in detection of disruptions of the brainstem auditory pathway (Fig. 16).

Digital subtraction angiography (DSA) is primarily reserved for preoperative embolizations and demonstration of cross circulation.