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 algorithms using Fluid Attenuation Inversion Recovery sequences (FLAIR) that will demonstrate minute amounts of contrast agent in the inner ear [56, 58]. The use of MRI in temporal bone imaging is dependent on the area to be visualized, the patient’s age, the pathology involved, and its severity level. MRI is the gold standard in radiologic evaluation of soft tissue changes in the temporal bone and may serve as a complementary method when CT is used to characterize the bony structures.
MRI diagnosis of MD has been challenging until recent years [59]. The first efforts to demonstrate visualization of fluid spaces in the inner ear with gadolinium chelate (GdC) were carried out in animal studies by using animal MRI equipment of 4.7 T scanner [60]. After demonstrating the contrast of perilymph, Zou et al. [61, 62] were the first to demonstrate that endolymphatic hydrops could be visualized accurately in the guinea pig and that the changes were in accordance with the histological verification of the degree of endolymphatic hydrops. These findings were followed by Niyazov et al. [63] who showed similar results using a clinical 1.5 T machine. In humans using 1.5 T MRI, the passage of GdC delivered transtympanically was shown to accumulate in the inner ear after 12 h post injection and fully contrasted the labyrinth after 24 h post injection. However, 1.5T MRI equipment was not sensitive enough to demonstrate the delicate details of the perilymph and endolymph borders [64]. Figure 3 demonstrates the cochlear fluid spaces and endolymphatic hydrops [59].
The recent development of 3T MRI provides a tool for visualizing endolymphatic hydrops with GdC as the contrast agent [65–67] (Fig. 3). MRI, especially in Japan, Germany and more recently in USA has become a clinically useful tool for the diagnosis of atypical and typical cases of MD. Methodological development in imaging techniques and increase of the magnetic field strength have allowed separation of bone from fluid and contrast agent, and have improved spectral resolution,