Noteworthy to mention are optical coherence tomography imaging [3–5], microtomography (µCT) [6] and endoscopes using coherent anti-Stokes Raman spectroscopy (CARS) technique [7] and development of advanced of contrasting agents [8]. This chapter provides an overview of current temporal bone imaging methods and a review of emerging concepts in temporal bone imaging technology.
Computed Tomography
CT is the most common modality for assessing the bony anatomy of the temporal bone. CT can detect signs of perilymphatic fistulae (i.e., pneumolabyrinth) but fails to detect subtle traumatic lesions within the inner ear, such as labyrinthine hemorrhage or axonal damage along central auditory pathways [9]. Many anatomic structures of the middle and inner ears are not optimally depicted using conventional CT with image reconstruction in the standard axial and coronal planes. In the early development of CT, cochlear partitioning and soft tissue membranes were not adequately visualized [10, 11]. Recent advances in MDCT, including the development of scanners with 32 detector rows (64 effective sections) for depiction of normal anatomy and pathologic states in the temporal bone, allow the acquisition of isotropic voxels that can be reconstructed and used in the multiplanar reconstructions of volumetric CT images [12]. This technique gives radiologists the opportunity to visualize the anatomic structures of the middle and inner ears accurately (Table 1) [13]. Recent reconstruction methods in MDCT may also allow visualization of the cochlear partitioning [9]. A recent paper by Maillot et al. [12] indicated that MDCT allows radiologists to examine the complex anatomy of the temporal bone with sub-millimeter resolution and is the first modality of choice. Indeed, it is capable of revealing a broad spectrum of ossicular lesions that may not be apparent on the basis of clinical findings alone.
For MDCT, the slice thickness is a critical point and detailed anatomical evaluation as small as 0.2 mm slice intervals have been used [12]. The MDCT technique may help overcome the limitations imposed by restrictions in gantry angle and patient positioning and improves diagnostic accuracy. The main advantages of MDCT for temporal bone imaging are shorter acquisition times, a decrease in tube current load, and better spatial resolution. The short acquisition time is an advantage, especially when dealing with younger patients, or those with claustrophobia or severe pain that often need sedatives for appropriate image acquisition. The ability of MDCT to obtain images of temporal bones bilaterally in one scan is another reason why MDCT is effective for imaging the temporal bone. Additional techniques such as virtual otoscopy with 3-D reconstructions of MDCT images can provide a different view on ossicular chain anomalies in traumatic conditions [14]. CT has been considered the gold standard method for postoperative imaging of the electrode position after cochlear implantation (CI), although plain X-ray films have been used [15, 16].
In detecting a thin bony coverage of a superior semicircular canal, digital volume tomography scans seem to be superior to MDCT scans [13]. Giesemann and Hofmann [9] indicated that CT is the gold standard in imaging diagnosis of semicircular canal dehiscence syndrome (SCDS); however, it has a high false-positive rate and may be misleading in terms of diagnosis because it overestimates the size of the dehiscence and prevalence [17]. In addition, many patients with imaging findings of superior canal dehiscence do not suffer from a clinical dehiscence syndrome. In those with SCDS, there is no clear linear relationship between the size of the dehiscence and the extent of clinical symptomatology; however, the dehiscence length does correlate positively with the maximal air-bone gap [17]. Nevertheless, a definite diagnosis of SCDS is difficult with any radiologic imaging technique [18]. It has been reported that subarcuate venous malformations cause audio-vestibular symptoms similar to those of SCDS and should be excluded in temporal bone imaging [19].
Table 1. Synopsis of the anatomically important structures and the respective primary criteria for image quality assessment
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