The National Cancer Institute of United States estimates that approximately 5–9 million CT examinations are performed annually on children all over the country [20]. Despite the many benefits of CT, a disadvantage is the inevitable radiation exposure. Although CT scans comprise approximately 12% of diagnostic radiological procedures in large U.S. hospitals, it is estimated that they account for approximately 49% of the U.S. population’s collective radiation dose from all medical X-ray examinations [20]. An association between increased risk of cancer and CT scans has been reported, especially in children [21–23]. The National Cancer Institute of United States estimates that a radiation dose of 50 mSieverts (mSv) increases the risk of cancer by threefold, particularly in children [20, 22, 24]. This dose reflects CT imaging performed in 5–10 head CT scans. The lifetime cancer mortality risk is estimated to be approximately 14% per Sv for a 1-year-old child, 5% per Sv for a middle-aged adult, and based on current models less than 2% per Sv for an individual older than 60 years [23].
To avoid high radiation dose, the novel “low dose protocols” of MDCT have been developed and a compromise between dose and resolution made [25, 26]. The tube current-time product and voltage can be reduced by 50% without increasing the artefacts [26]. Low-dose postoperative MDCT scans are sufficient for localizing the CI electrode [26] and reduce metal artefacts [25]. The application of conventional CT in otolaryngology might be replaced by cone-beam CT (CBCT) in the future [27, 28]. For congenital malformations of the inner ear, MDCT has a long history and is an outstanding tool for visualizing anatomical changes. Ultra-high resolution CT with collimation of 0.25 mm is coming to clinical field soon [29]. Future study is warranted to compare the ability of CBCT and ultra-high resolution-CT, although CBCT might have the advantage in terms of radiation dose reduction.
Cone-Beam CT
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
