Temporal Bone Imaging Technique

Paul A. Caruso, Jennifer L. Smullen, Robert Liu, Mary Beth Cunnane, Hugh D. Curtin

This chapter on the technique for imaging the temporal bone is a practical “how to” written in two parts. In the first part, we explain how to image the temporal bone: how to run the hardware and obtain the images. We present the imaging modalities and technical parameters routinely used for imaging of the temporal bone. In addition, we provide guidelines for the technologist or radiologist as to how to determine which parameters to enter into the computed tomography (CT) or magnetic resonance imaging (MRI) scanner. In the second part, we explain how to read and report the results of temporal bone imaging studies. Here we provide a plan of action for the radiologist who, faced with a request for a temporal bone imaging study on a particular patient, must protocol the case, interpret the images, and report the findings in a way that answers the referring clinician’s questions. The major indications for imaging of the temporal bone are thus reviewed, and protocols, interpretive strategies, and template reports are provided for each of these indications.

Imaging Modalities and Technical Parameters

CT and MRI are primarily used for imaging of the temporal bone. We first present the standard technique and protocols most often used, then review the special considerations for both modalities. A brief overview of the roles of plain radiographs, ultrasound (US), positron emission tomography (PET), and PET/CT is given at the end of this section.

Computed Tomography

Routine Technique

The patient is placed supine in the gantry and positioned to place the lens of the eye as far as possible out of the pathway of the x-ray beam to minimize exposure to the lens. Gantry tilt may need to be avoided to facilitate image reconstruction and reformats. A lateral topogram is then performed. The scan excursion is plotted from the arcuate eminence (the summit of the temporal bone) through the mastoid tip.

The anatomy and pathology of the temporal bone involve small structures; resolution is thus highly important. Collimation is of optimal importance to achieve high resolution.

We routinely use a collimator of 0.6 mm and most commercially available units can be collimated to at least 1 mm. Collimation wider than 1 mm is not usually used, as the resolution is often insufficient.

For 40 to 64 detector scanners, the effective mAs (defined as the mA × the gantry cycle time/helical pitch) is adjusted according to the age and head size. Usually, it is 150 effective mAs (CTDI_vol [volume CT dose index] 34 milligray [mGy]) for neonates, 200 effective mAs (CTDI_vol 45 mGy) for children ages 1 to 10 years, 250 effective mAs (CTDI_vol 57 mGy) for adolescents, and 320 (CTDI_vol 72 mGy) for adults. The gantry cycle time is set at 1 cycle or gantry rotation/second. The kilovolt peak (kVp) is usually 120.

A helical mode is chosen. Although traditionally, some imagers maintain that nonhelical scans provide better resolution, it is our experience that the difference in resolution between nonhelical and helical acquisitions at thin collimation is not appreciable, and helical acquisitions allow for clearer coronal or oblique reformats and decrease susceptibility to motion artifact.

Intravenous (IV) contrast is usually of the low osmolar type; it is administered by power injector at standard doses of 1 mL/lb to a maximum of 80 to 100 mL for adults. IV contrast is used for the evaluation of vascular pathology (e.g., dissection, tumors) and may be considered for some types of infections such as coalescent mastoiditis or for the evaluation of abscesses. However, it is not routinely used to evaluate for otomastoiditis or hearing loss.

The raw data from each ear are separated and reconstructed into 0.6 mm (slice thickness) axial images in bone algorithm at a dual field of view (DFOV) of 100 mm that effectively magnifies the images. Then the 0.6 mm images for each ear are brought up on the CT scanner console, where the raw data are displayed in three orthogonal planes. The technologist scrolls through the sagittal data to find an image where the anterior and posterior limbs of the lateral semicircular canal are displayed in cross section (Fig. 1.1). An axial dataset is then made in a plane parallel to the lateral semicircular canal (LSCC). The technologist “connects the two dots” of the LSCC and makes a 0.6 mm (image thickness) × 0.5 (distance between images) axial dataset in this plane parallel to the LSCC; 0.6 × 0.5 mm coronal images are made in a plane perpendicular to the axial images. The raw data are also reconstructed into 2 mm axial images in soft tissue algorithm to include both ears and the brain at 180–210 mm DFOV. This protocol generates seven sets of images, three for each ear—the source 0.6 mm images (in a variable axial plane), the 0.6 mm reformats in the axial plane parallel to the LSCC, the 0.6 mm reformats in the coronal plane, and a set of 2 mm axial images in soft tissue algorithm of the entire scan volume.

Fig. 1.1 Making the standard axial and coronal computed tomography (CT) dataset. (A) Once the source data are brought up in the three-dimensional (3D) viewer on the scanner console, the sagittal images are scrolled through until a sagittal image through the anterior and posterior limbs (short white arrows) of the lateral semicircular canal (LSCC) are found. (B) A set of axial images is then generated with 0.1 mm overlap parallel to this plane. (C) If the axial reconstructions have been done correctly, the axial dataset should produce an image where the entire lateral semicircular canal is displayed and, as in (D,E), where the cochlear fossette, modiolus, and stapes footplate are clearly delineated. (F) The coronal reconstructions are made in a similar fashion. Starting with the sagittal image in (A), a plane perpendicular to the LSCC is established, and the coronal reformats are made along this plane. (G) This technique should yield a set of coronal images where Prussak’s space (short white arrow) and (H) the facial nerve canal (short white arrow) are clearly delineated.

Multidetector CT provides shorter acquisition times, a decrease in tube current load, and improved spatial resolution.1 Short acquisition is useful in temporal bone imaging to reduce motion artifact, particularly in children who require sedation or are imaged postprandially without pharmacologic sedation. Although the radiation dose with multidetector scanners in high-quality mode remains an issue compared with single detector scanners, the improved spatial resolution allows for high-quality reformats that essentially obviate the need for rescanning the patient in a second coronal plane.¹

Reformats may, moreover, be obtained in sagittal or oblique planes to improve the detection of pathology in specific clinical settings such as superior semicircular canal dehiscence (SSCD) as discussed below in the section Vertigo and Dizziness.

Stenvers Reformat

Similar to the method explained above, for making the standard axial and coronal images, the 0.6 mm raw data are brought up on the console viewer in three orthogonal planes: axial, coronal, and sagittal. As above, the technologist scrolls through the sagittal plane until a view of the LSCC is obtained represented by the two “dots” of the anterior and posterior limbs (Fig. 1.2). The axial plane is then established by connecting the two dots. The technologist scrolls through the axial dataset until an image of the summit of the SSC is viewed. The Stenvers reformats are then made by tracing a line perpendicular to the long axis of the summit of the SSC at 0.6 0.5 mm intervals. This plane is effectively perpendicular to the roof of the SSC and displays the roof of the SSC in cross section.

Pöschl Reformat

Similar to the method explained above, for making the standard axial and coronal images, the 0.6 mm raw data are brought up on the console viewer in three orthogonal planes: axial, coronal, and sagittal. As above, the technologist scrolls through the sagittal plane until a view of the LSCC is obtained represented by the two “dots” of the anterior and posterior limbs (Fig. 1.3). The axial plane is then established by connecting the two dots. The technologist then scrolls through the axial dataset until an image of the summit of the SSC is viewed. The Pöschl reformats are then made by tracing a line parallel to the long axis of the summit of the SSC at 0.6 0.5 mm intervals. The line must be made as parallel as possible to the axis of the summit of the SSC. A slight obliquity may spuriously obscure a dehiscence by volume averaging with the temporal bone on either side of the summit of the SSC.

Computed Tomography Arteriography and Computed Tomography Venography

CT arteriography (CTA) or CT venography (CTV) of the temporal bone may be used to evaluate for tinnitus. At our institution, the standard CT protocol for temporal bone imaging is employed, but the injection rate is increased to 3 to 4 cc per second for CTA. A power injector is employed if a 22-gauge IV or larger is available.

Radiation Dose Reduction Techniques and Considerations for Pediatric Patients

Compared with most radiography procedures, CT exams deliver higher radiation dosages to patients. The quantity CTDI_vol is used to describe the patient dose. CTDI_vol represents the average dose in a given scan volume. When a scan is prescribed, the system displays the CTDI_vol in mGy on the console. However, the dose displayed is not the true dose for the specific patient under examination. Instead, it is the dose value when the patient is replaced with an acrylic phantom while the same imaging parameters are used. The head phantom is a cylinder with a diameter of 16 cm and a height of 15 cm.

The effective dose E is used to assess the radiation detriment from partial-body as opposed to whole-body irradiation (e.g., irradiation of only the head or only the abdomen). The effective dose is a weighted sum of the doses to all exposed tissues. E = Σ (w_t × H_t), where H_t is the equivalent dose to a specific tissue and w_t is the weight factor representing the relative radiosensitivity of that tissue. The unit of effective dose is sievert (Sv). The effective dose for a typical CT exam of the temporal bone is ∼ 1 mSv (i.e., 1/1000 Sv). In comparison, the average effective dose from cosmic rays, radioisotopes in the soil, radon, and so on, is ∼ 3 mSv per year in the United States. The effective dose can be estimated from the dose-length product (DLP = CTDI_vol × scan length), which is also displayed on the CT scanner console. The effective dose for a head study in mSv is 0.0021 DLP (mGy cm).2

Fig. 1.2 How to make a Stenvers reformat. (A) Once the source data are brought up in the three-dimensional viewer on the scanner console, the axial images are scrolled through until an image through the summit of the superior semicircular canal (SSC; short white arrows) is found. (B) The Stenvers reformats are made in a plane perpendicular to the long axis of the SSC, and (C) should yield a cross-sectional view of the SSC (short white arrow).

Radiation Risks The biological effect of radiation is either deterministic or stochastic. The deterministic effect will not occur unless a threshold dose is exceeded. However, the stochastic effects may occur at any dose level, and the probability of occurrence increases with dose linearly according to the linear nonthreshold dose–response model.³

For CT of the temporal bone, the primary concern for deterministic effect is the dose to the lens. The minimum dose required to produce a progressive cataract is 2 Gy in a single exposure.⁴ If the lens is in the direct x-ray beam, the dose to the lens from CT of the temporal bone is in the range of 0.03 to 0.06 Gy, but it could be as high as 0.13 Gy. If the patient is positioned in such a way that the lens is outside the direct x-ray beam, the dose is in the order of 0.003 Gy.⁵ Although the typical dose to the lens from a single CT scan is much lower than the threshold value for a cataract, multiple nonoptimized scans in a short time with the lens in the x-ray beam can result in a lens dose close to the threshold. Every effort should be made to keep the lens outside a direct x-ray beam if it is possible.

Fig. 1.3 How to make a Poschl reformat. (A) Once the source data are brought up in the three-dimensional viewer on the scanner console, the axial images are scrolled through until an image through the summit of the superior semicircular canal (SSC) is found. (B) The Poschl reformats are made in a plane perpendicular to the long axis of the SSC, and (C) should yield a view of the entire excursion of the SSC (short white arrows) from anterior to posterior.

Stochastic effects include carcinogenesis and the induction of genetic mutations. Children are inherently more sensitive to radiation because they have more dividing cells, and radiation acts on dividing cells. Also, children have more time to express a cancer than do adults.6

Factors Influencing the Patient Dose The CT scanning protocols should be optimized such that the quality of images is sufficient for diagnosis and the patient dose is kept as low as reasonably achievable (ALARA). To get the best balance of the image quality and patient dose, it is important to understand the effects of imaging parameters on the dose and imaging quality.

Patient dose depends on three factors: equipment-related factors, patient-related factors, and application-related factors.

The factors in the first group include x-ray beam filtration, x-ray beam collimation, system geometry, and detector efficiency. Although users do not have control of most of these factors, it is important to understand that the z-axis dose efficiency is reduced when the total x-ray beam width becomes very small for multidetector CT due to the need to keep the beam penumbra out of any detector row.

Additional Considerations

Standard MRI safety considerations obtain in the temporal bone, and the reader is referred to publications that list the safety of various prostheses.7 For those institutions with a busy otology service, the issue of MRI compatibility of stapes prostheses, total ossicular replacement prostheses (TORPs), and partial ossicular replacement prostheses (PORPs) may arise. These pros-theses are listed as well in the standard MRI safety references. Most stapes prostheses do not deflect significantly in a 1.5 T unit.

Plain Film Radiography

Plain film radiographs have limited application to imaging of the temporal bone. A plain radiograph in the Stenvers projection, however, may be used for intraoperative or postoperative confirmation of position of a cochlear implant lead. The patient’s head is placed in a 45-degree obliquity contralateral to the implanted ear that places the axis of the implanted temporal bone parallel to the film and then in a 15-degree Townes projection. For example, if the left ear has been implanted, the radiographer would turn the head 45 degrees to the right, with the film behind the head of the patient, and shoot a single radiograph with the beam tilted 15 degrees inferiorly toward the patient (Fig. 1.4). Familiarity with this projection and with the proper position of a cochlear implant is one of the few instances in current imaging where plain film radiography is critical, as the intraoperative assessment is often made while the patient is still anesthetized on the operating room table.

Ultrasound

Ultrasound may be used for evaluation of periauricular cystic lesions such as first pharyngeal arch anomalies or ultrasound-guided biopsies of periauricular lesions.

Positron Emission Tomography (PET) or PET/CT

PET or PET/CT may be used for the assessment of temporal bone masses or nodal metastases.

Referrals and Imaging Strategies

In this part of the chapter, we address the major clinical indications for which a patient may be referred for temporal bone imaging. The indications are listed in alphabeticalorder, and for each clinical indication, the following points are addressed.

Fig. 1.4 A Stenvers radiograph for cochlear implantation. If the Stenvers radiograph is performed correctly, the cochlear implant lead (short white arrow) should lay coiled in the cochlea, anterior and inferior to the vestibule and lateral and superior semicircular canals (SSCs; summit of SSC marked by long white arrow).

1. 
   

   How to protocol the case. You have the requisition in hand. What type of scan do you ask the technologist to do?

   
   
2. 
   

   How to interpret the study. An interpretive strategy is provided that first considers the clinical background, then highlights the clinical questions most commonly asked by clinicians. Accordingly, a checklist approach to the images is presented.

   
   
3. 
   

   How to report the findings. A report template is provided to help with the dictation.

Acute Otitis Media

Protocol

A routine CT scan of the temporal bone is the standard protocol.

If there is clinical concern for sinus thrombosis or coalescent mastoiditis, the study is performed with IV contrast. MRI is a more sensitive modality for evaluation of some of the complications of acute otitis media (AOM), such as suppurative labyrinthitis, facial nerve inflammation, and meningitis, but CT is overall the preferred initial study.

Interpretation

Clinical Background

Commonly, patients with AOM present with a bulging ear drum and otalgia and appear sick with fever, malaise, and lethargy. Depending on the level of clinical concern for severity and sequelae of the AOM, the clinician will refer the patient for imaging to evaluate for intratemporal and extratemporal complications of AOM. Long-term antibiotic therapy may be indicated, for instance in the setting of coalescent mastoiditis.8

Clinical Questions

• 
  

  Are there intratemporal (local auricular) complications of AOM, such as coalescent mastoiditis, osseous erosion, facial nerve involvement, or suppurative labyrinthitis?

  
  
• 
  

  Are there extratemporal complications of AOM, such as sigmoid sinus thrombosis, epidural abscess, or meningitis?

Approach

Evaluate for evidence of intratemporal complications of AOM.

• 
  

  Inspect the osseous margins of the mastoid for evidence of demineralization (bone algorithm images) and for subperiosteal abscess (soft tissue algorithm images) that may reflect a coalescent mastoiditis. It is our experience that asymmetry of the central trabeculation of the mastoids is not specific for the diagnosis of coalescent mastoiditis and that demineralization along the external or cisternal walls of the sinus is a more reliable sign.

  
  
• 
  

  Inspect the osseous margins of the facial nerve canal for demineralization that would suggest inflammatory dehiscence. This evaluation is problematic on CT, as the wall of the tympanic segment is naturally papyraceous. An MRI would be a more sensitive modality for detection of neuritis.

  
  
• 
  

  Evaluate for erosion of the walls of the membranous labyrinth that would suggest a suppurative labyrinthitis. Again, an MRI would be a more sensitive modality for evaluation for enhancement of the labyrinth.

  
  
• 
  

  Evaluate for a middle ear cavity cholesteatoma.

Evaluate for extratemporal sequelae of AOM.

• 
  

  Inspect the margins of the middle ear cavity (MEC) and mastoid for evidence of epidural or Bezold abscess.

  
  
• 
  

  Evaluate the transverse and sigmoid sinus for evidence of thrombosis.

  
  
• 
  

  Evaluate the meninges for evidence of meningitis. Here again, an MRI would be a more sensitive modality.

Related posts:

The External Auditory Canal and Pinna Temporal Bone Trauma Anatomy and Development of the Facial Nerve Temporal Bone Vascular Anatomy, Anomalies, and Disease, with an Emphasis on Pulsatile Tinnitus The Inner Ear and Otodystrophies The Middle Ear and Mastoid

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