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In CBCT, a pulsed cone-shaped X-ray beam performs a single rotation around a patient simultaneously acquiring all of the necessary volumetric data for the reconstruction of separate images in the sagittal, coronal, and axial planes. In the temporal bone, any interface between 2 materials (as from fluid to the bone), is a step in tissue anatomy, but like a ramp in the CBCT image. The width of the ramp depends on factors such as detector pixel size, image acquisition geometry, and radiation source focal spot size. This process differs from traditional MDCT that reconstructs images using a series of axial slices. CBCT creates less streak artefacts and offers higher spatial resolution than MDCT (cubic voxel size of approximately 0.07 vs. 0.1–0.6 mm, respectively). Erovic et al. [27] promoted the use of CBCT in intraoperative monitoring of the temporal bone. CBCT has a smaller footprint compared with MDCT, which makes it more feasible for use in outpatient departments or operating theatres. CBCT is reportedly capable of demonstrating fine structures of the middle and inner ears as well as pathological diseases within temporal bone, such as otosclerosis [30]. In the assessment of SCDS, CBCT is suggested to be more reliable than MDCT [31].

CBCT appears to provide sufficient spatial accuracy for intraoperative usage during temporal bone surgery [27, 32]. Identifying the location of implants is becoming increasingly important in modern otology, which involves applications such as the Vibrant Soundbridge implant and CI [6, 33, 34]. With regard to monitoring the electrode array location during CI surgery, there is an emerging need to improve the imaging quality to better assess electrode trauma to the fine structures of the cochlea, such techniques include low-dose iterative reconstruction, scatter correction, lag correction, and metal artifact reduction [35–38]. Gupta et al. [39] refined the technology for temporal bone imaging using a smaller detector element and acquired a total of 900 cone-beam projections under a field of view of 15.5 cm; however, the image quality was not perfect. Furthermore, metal electrodes tend to cause artifacts that may blur the images [40]. In assessing the location of the cochlear implant with CBCT, Pearl et al. [41] could not clearly detect the basilar membrane to designate exact scalar location, which is consistent with previous reports. Zou et al. [6] used the osseous spiral lamina of cochlea to assess the location of cochlear implant and to delineate between the scalae (Fig. 1). The scalar position has also been evaluated by Ruivo et al. [42] and Verbist et al. [43], and in their study the osseous spiral lamina was visible in all sections.

Ruivo et al. [42] concluded that CBCT provides high-resolution and almost artefact-free multiplanar reconstruction images allowing assessment of the precise intracochlear position of the electrode and visualization of each of the individual contacts; the CBCT visualization consisted of 23 middle and inner ear structures using a 5-point scale. They showed that insufficient image quality scores were more frequent in low-dose scans compared to high-dose scans; however, the difference was only statistically significant for otologists but not for neuroradiologists. Image quality was critical for small structures (such as the stapes or lamella at the internal auditory canal fundus). The CBCT images can discern individual electrode contacts, often not possible on MDCT [44, 45]. In addition, Pearl et al. [41] demonstrated that CBCT improved imaging of the cochlea and facial nerve canal, enabling identification of electrode contacts in close proximity to the fallopian canal, and thus being most prone to induce inadvertent stimulation of the facial nerve.

Zou et al. [28, 42] evaluated the effects of filters, voltage, and frame numbers on the visualization of the inner ear using CBCT 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].