- 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.
FIG. 1: Diagram showing the parts of temporal bones (view from outside) (petrous part is hidden from view).
- 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
- 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.
Frontal Radiographs
Transorbital 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.
Towne's view: This is an anteroposterior radiograph of the skull with a 30° fronto-occipital tilt (Fig. 2).
FIG. 2: Towne's view of skull including both mastoid areas: White asterisk marks the region of pneumatized mastoid part of temporal bone while black asterisk indicates the petrous part of temporal bone.
FIGS. 3A AND B: Schuller's view of mastoid: (A) and (B) depict radiographs without and with graphical annotations. Curved vertical line indicates sinus plate while the straight line depicts dural plate. The angle formed between the two is the sinodural angle (shown). Asterisk marks the area of pneumatized mastoid.
This radiographic view enables visualization of the temporal bones, mastoid antrum, superior semicircular canal (SSC), internal auditory meatus, cochlea, petrous apex, and external auditory meatus.
Lateral Radiographs
Schuller's projection (Figs. 3A and B):
- 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.
Law's projection: This employs a 15° caudal tilt instead of 30°.
Oblique (Stenver's) View
- 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
FIGS. 4A AND B: Stenver' view: (A) and (B) depict radiographs without and with graphical annotations. White asterisk marks the petrous apex while black asterisk denotes the pneumatized mastoid. The region of internal auditory canal is also marked (curved line).
COMPUTED TOMOGRAPHY
High-resolution Computed Tomography
- 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:
- Congenital/acquired hearing loss (both in conductive and sensorineural)
- Trauma
- Neoplasms
- Infections
- Preoperative evaluation (for cochlear implant and mastoidectomy)
- Technique:
- Noncontrast thin collimation volumetric CT of both temporal bones is routinely done.
- Patient lies supine on the CT table and no gantry tilt is employed.
- Axial acquisition is performed from top of petrous apex to inferior tip of mastoid with acquisition plane parallel to infraorbitomeatal line (Figs. 5A and B).
- Small field of view (FOV) (15–20 cm) and high matrix resolution (512 × 512).
- Data is reconstructed into 0.6 mm thickness overlapping axial and coronal sections. High spatial resolution bone algorithm is used. Large window width (4,000 HU) and low window level (e.g., 0–200 HU) is used.
- About 2 mm thickness soft tissue window is also reconstructed.
- Axial images are reconstructed from the top of the petrous apex to the inferior tip of mastoid, parallel to the lateral semicircular canal (LSC).
- Coronal reformation is done from the anterior margin of the petrous apex to the posterior margin of mastoid.
FIGS. 5A AND B: CT planning: (A) Scout CT image showing planning of acquisition for HRCT temporal bone; (B) Coronal multiplanar reformats are then generated from the axial images (plane of reformation shown).
FIGS. 6A AND B: Pöschl plane: (A) Axial CT image depicts section orientation in Pöschl plane (perpendicular to petrous); (B) The resultant CT image at the level of superior semicircular canal shows the canal in its entire extent (small black arrows) with air cells cranial to it (white arrow).
- 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):
- Pöschl plane (Figs. 6A and B):
- Sections are oriented parallel to the SSC and perpendicular to long axis of temporal bone.
- This is the best plane to detect dehiscence of the roof of SSC.
- Stenver's plane (Figs. 7A to D):
-
FIGS. 7A TO D: Stenver's plane: (A) Axial CT image depicts orientation of Stenver's plane (parallel to petrous); (B) The resultant plane provides enface depiction of cochlear turns (asterisks); (C) A slight modification of Stenver's plane results in depiction of entire tympanic and proximal mastoid portions of the bony canal of facial nerve (arrowheads); (D) Adjacent section shows distal mastoid portion of the bony canal of facial nerve (arrowheads).
- The result is an oblique coronal short-axis plane of cochlea with enface depiction of all three cochlear turns.
- This is useful for evaluation of temporal bone fractures.
- A slight modification results in depiction of entire tympanic and mastoid segment of facial nerve canal (Figs. 7C and D).
-
- Unconventional planes: Various single and double oblique planes have been described for optimal depiction of ossicles and inner ear structures.1
- Double-oblique coronal plane is best for depiction of malleus and incus. The primary reference plane is axial, where line is drawn at 150° to the left vector of the horizontal axis.
- The secondary reference plane is sagittal where line is drawn at 85° for malleus (Figs. 8A to C) and 60° for incus (Figs. 9A and B).
- There are separate planes for stapes-oval window complex both in long (double oblique axial) and short axes (double oblique sagittal).
FIGS. 8A TO C: Special plane for malleus: (A) Axial and (B) sagittal CT images show direction of reformatting planes in order to obtain double oblique view of malleus; (C) Depicts the resultant image with demonstration of parts of malleus.
FIGS. 9A AND B: Special plane for incus: (A) Sagittal CT image shows direction of reformatting plane in order to obtain double oblique view of incus. Orientation on axial CT image is same as in Figure 7A; (B) Demonstrates the parts of incus on resultant image.
Cone Beam Computed Tomography
- 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.
MAGNETIC RESONANCE IMAGING
- 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).
Basic Sequences
These include:
- 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:
- Steady-state free precession (SSFP) gradient-echo sequences (FIESTA or TrueFISP or balanced SSFP) highlight structures with high T2/T1 ratios [constructive interference in steady state (CISS) on Siemens, FIESTA+C on GE, bSSFP on Philips]; enabling optimal evaluation of cerebellopontine angles and inner ears.
- An alternative is heavily T2-weighted (T2W) fast/turbo spin-echo sequence (RESTORE on Siemens, FRFSE on GE, DRIVE on Philips) (Figs. 10A to C). Susceptibility artifacts are lesser compared to balanced SSFP sequences.
- Sagittal oblique reformatted images are then generated perpendicular to the 7th to 8th nerve complex in the IAC and cerebellopontine angle. The fluid-filled spaces of membranous labyrinth and semicircular canals are well seen.
- 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.
FIGS. 10A TO C: MRI for depiction of nerves in internal auditory canal (IAC): (A) Axial T-2 weighted (T2W) MR image at the level of 7th to 8th nerve complexes shows both IACs (white arrows) with cochlea and vestibule; (B) Coronal T2W MR image, at the level of IAC, showing 7th to 8th nerves (arrows) with bilateral semicircular canals; (C) Sagittal oblique image of constructive interference steady state (CISS) sequence shows anterosuperiorly facial nerve (superior long arrow), anteroinferiorly cochlear nerve (inferior long arrow), superior and inferior vestibular nerves posteriorly (short arrows).
ADVANCED IMAGING TECHNIQUES
Postprocessing Tools
- 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).
Three-dimensional Variable Flip-angle FLAIR MRI Sequence
- 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
FIGS. 11A TO C: Three dimensional reconstructions: (A) Three-dimensional surface-shaded display (SSD) generated from CT data shows the normal semicircular canals (arrows) and cochlea (asterisk); (B and C) Volume rendered images generated from heavily T2-weighted thin section MRI dataset. (B) SSD and (C) using colonography preset, good depiction of cochlear turns (asterisk), semicircular canals (arrows), and vestibule is possible.
- 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
Facial Nerve Visualization
- 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
FIGS. 12A AND B: Facial nerve: (A) Coronal double-echo steady state image shows the facial nerve in mastoid and parotid segments (arrow); (B) Corresponding coronal T2-weighted magnetic resonance image depicts pleomorphic adenoma in left parotid gland (asterisk).
- 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
Contrast Magnetic Resonance Angiography
- 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
FIGS. 13A TO D: MR angiogram: MR angiogram arterial phase postcontrast MIP image provides excellent depiction of arteries of head and neck. (A) No tumor feeder arteries seen; (B) MR angiogram venous phase postcontrast MIP image (seen from anterior) depicts nonvisualization of left-sided sigmoid sinus (compressed due to mass effect). Arrow shows abrupt termination of left transverse sinus; (C) Axial 3D FLAIR image of the same patient demonstrates heterogeneously hyperintense mass lesion in left retrocochlear region; (D) Axial T1W image shows peripheral hyperintensity, suggesting hemorrhage within. Histopathology revealed endolymphatic sac tumor.
FIGS. 14A TO C: Diffusion-weighted MR imaging in tumors: (Same patient as in Figure 13). (A) Axial diffusion-weighted image at low b-value (0 s/mm2); (B) High b-value (800 s/mm2), and (C) Apparent diffusion coefficient) map shows that there is no restriction of diffusion within the mass lesion, suggested by low signal intensity on (B) and high signal on (C) (asterisks).
Diffusion-weighted MRI
- 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.
-
- These include MUSE in GE, RESOLVE in Siemens, and multishot EPI in Philips.
- Reduced FOV technique can be employed to generate high resolution images in EPI-based diffusion (FOCUS in GE, ZOOMit in Siemens, and ZOOM Diffusion in Philips).
- Non-EPI sequences:
- These include fast spin-echo sequences like PROPELLER in GE, BLADE AND HASTE in Siemens, and MULTIVANE in Philips.
- There is higher spatial resolution, thinner sections can be acquired (for smaller lesions) and less susceptibility artefacts using this technique.
- 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.
FIGS. 15A AND B: MR spectroscopy in tumors (Same patient as in Figures 11 and 12): (A) Demonstrates placement of region of interest (ROI) for MR spectroscopy on the enhancing solid component of the mass lesion in all the three planes. ROI must be confirmed on all three planes; (B) Resultant spectrum shows elevated choline, suggesting a neoplastic lesion.
FIG. 16: Diffusion-tensor imaging. Axial image generated from three-dimensional tractography with color-coded fractional anisotropy map depicts mass effect on the 7th to 8th nerves in the cisternal and canalicular portion by cystic schwannoma.Courtesy: Professor Ajay Garg.
CONCLUSION
Knowledge of basic imaging techniques and indications is important to optimally utilize different modalities. Imaging advancements have enabled increased role of imaging in taking patient management decisions.
REFERENCES
- Lane JI, Lindell EP, Witte RJ, DeLone DR, Driscoll CLW. Middle and Inner Ear: Improved Depiction with Multiplanar Reconstruction of Volumetric CT Data. Radiographics. 2006;26(1):115–24.
- Jambawalikar S, Liu MZ, Moonis G. Advanced MR Imaging of the Temporal Bone. Neuroimaging Clin N Am. 2019;29(1):197–202.
- Bernaerts A. MRI in Menière's Disease. J Belgian Soc Radiol. 2018;102(S1):13.
- Nahmani S, Vaussy A, Hautefort C, Guichard JP, Guillonet A, Houdart E, et al. Comparison of Enhancement of the Vestibular Perilymph between Variable and Constant Flip Angle–Delayed 3D-FLAIR Sequences in Menière Disease. Am J Neuroradiol. 2020;41(4):706–11.
- Guenette JP, Seethamraju RT, Jayender J, Corrales CE, Lee TC. MR Imaging of the Facial Nerve through the Temporal Bone at 3T with a Noncontrast Ultrashort Echo Time Sequence. Am J Neuroradiol. 2018;39(10):1903–6.