among isolated human temporal bones; each temporal bone was independently analyzed by two imaging engineers and 3 otologists; each investigator had at least 15 years of experience with temporal bone CT and were blinded to dose-relevant scan parameters during the review of the scan. In 2D imaging of all temporal bones, the land marks of modiolus, osseous spiral lamina, and bony wall of cochlear duct that isolate scala vestibuli from scala tympani were demonstrated at a level such that the anatomic structures could be assessed in all parts and were of acceptable image quality (Table 1). Basilar membrane was not visualized in the cochlea. Changes in the tube current from 8 to 12.5 mA resulted in a minimal change of the temporal bone image quality. The tube voltage of 80 kV provided best images using 900 frames. In 3D imaging, contrast adjustment allowed very high quality imaging (Fig. 2). The low X-ray dose is mostly a result of the small region scanned and the low kV used in CBCT (79 kV). The reason why the artefacts in the work of Zou et al. [6] were relatively minor in comparison to a previous work is that a suitable adjustment on the “γ curve” of the original images that suppresses the metal artefacts on the images was employed, and in addition improved imaging sharpness was achieved by using a pause during each exposure (e.g., start-stop-expose-start-stop-expose) together with high frames of 2D images, which was not previously reported [38].
Fig. 1. A cochlear implant electrode located in the cochlea imaged with CBCT. Osseous spiral lamina is visible in all turns and the locations of electrodes are typically leaning on the lateral wall in the scata tympani. With permission of Acta Otolaryngol (Stokh.) [32].
With CBCT, the full course of the subarcuate canal from the subarcuate fossa to the mastoid antrum can be accurately visualized, and the accompanying air cells were distinguished from the subarcuate canal in a temporal bone with a pneumatized superior semicircular canal wall [46, 47]. The novel high-resolution CBCT system potentially has the power to detect changes associated with SCCD.
A comparison of low-dose MDCT and CBCT image quality after CI revealed that the visibility of cochlear inner and outer walls and overall image quality were positively correlated with radiation dose on MDCT; image quality was better with clinical MDCT than with CBCT protocols [48]. In other comparisons, differences between systems were found, but a distinction between CBCT and MDCT could not be made [48]. The effective radiation dose of the CBCT protocols was 6–16% of the clinical MDCT dose.
Fig. 2. CBCT imaging using 900 frame numbers and 1.72 magnification factor on a temporal bone. A sharp image of the stapes was demonstrated in the temporal bone. AC, anterior crus; FP, footplate; PC, posterior crus; ISJ, incudo-stapedial joint; LPI, lenticular process of incus. The black scale bar in the right lower corner = 2.5 mm. With permission of Annals of Otology, Rhinology & Laryngology [49].
Yamane et al. [49] evaluated the diagnostic properties of 3D CBCT images among 25 patients with Meniere’s disease (MD) and 13 healthy patients. They developed algorithms to determine the optimal 3D-CBCT window settings for the detection of water, muscle, calcium carbonate, and bone [48]. It was suggested that 3D CBCT imaging changes in the membranous labyrinth may be useful for the objective diagnosis of MD that dislodged saccular otoconia and may have an important role in MD, and that CBCT may be useful even in inner ear membrane imaging [48].
In CBCT, the total radiation dose based on the work of Zou et al. [46] was 13 µSv in male phantom head. The most dominant contributor to the effective dose was bone marrow (36–37%) followed by brain (34–36%), remainder tissues (12%), extra-thoracic airways (7%), and oral mucosa (5%) [46]. It is important to note that in this study the dose was measured with an anthropomorphic model with controlled error marginal. These results were in accordance with the results of a previous study that demonstrated effective doses between 35.2 and 137.6 µSv [50]. In some reports, there is an estimation of even lower dose of radiation in CBCT imaging [30] but the estimated values may significantly deviate from the measured values [51]. In a comparable study Erovic et al. [27] reported that the radiation dose of CBCT per scan is ~10 mGy (~0.35 mSv) (measured with head phantom dosimetry at 100 kVp and 170 mA exposure), and was low compared with a typical 2 to 5 mSv diagnostic head CT in that depending on slide thickness of CT as high as 4.8 mSv have been reported [42, 52].
Magnetic Resonance Imaging
High resolution MR imaging has an advantage over all types of CT imaging of the temporal bone as it provides better characterization of soft tissue and fluid-filled partitions. Despite several advantages, the major limitations of this method are the lack of bony details. Even with the recently developed ultra-short echo time pulse sequence, middle ear ossicles are only partly visualized (Naganawa et al. [53], 2016). This is due to the lack of water-containing material in the dense cortical bones. Another major disadvantage of using this method in temporal bone imaging is the high cost of the examination and the sedation needed for younger patients or patients in severe pain. In addition, this method cannot be used for implant imaging or intra-operative imaging where metal objects are involved in the operations as the superconducting magnets attract metals and electronic devices. For patients who are allergic to certain contrast agents, another type of contrast agent that may be more costly is required.
Fig. 3. Endolymph and perilymph in the inner ear. (a) Normal, (b) endolymphatic hydrops. The endolymph (gray) is surrounded by the perilymph (black) except for endolymphatic duct (ED) and endolymphatic sac (ES). U, utricle; S, saccule; St, stapes; R, round window. With permission of Auris Nasus Larynx [70].
MRI is the modality of choice when investigating the inner ear and suspecting soft tissue growth such as vestibular schwannoma, vascular malformations, endolymphatic hydrops or pathology of the cochlear aqueduct [54]. Naganawa et al. [55–57] developed specific
