computed tomography has been the benchmark for visualizing bony changes of the ear, but has recently been challenged by cone-beam computed tomography. In both methods, all inner ear bony structures can be visualized satisfactorily with 2D or 3D imaging. Both methods produce ionizing radiation and induce adverse health effects, especially among children. In 3T magnetic resonance imaging, the soft tissue can be imaged accurately. Use of gadolinium chelate (GdC) as a contrast agent allows the partition of fluid spaces to be visualized, such as the bulging of basilar and Reissner’s membranes. Both intravenous and intratympanic administration of GdC has been used. The development of positive endolymph imaging method, which visualizes endolymph as a bright signal, and the use of image subtraction seems to allow more easily interpretable images. This long-awaited possibility of diagnosing endolymphatic hydrops in living human subjects has enabled the definition of Hydropic Ear Disease, encompassing typical Meniere’s disease as well as its monosymptomatic variants and secondary conditions of endolymphatic hydrops. The next challenge in imaging of the temporal bone is to perform imaging at the cellular and molecular levels. This chapter provides an overview of current temporal bone imaging methods and a review of emerging concepts in temporal bone imaging technology.
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Introduction
Rapid development of radiological equipment over the last several decades has significantly promoted the role of imaging in otology. Computed tomography (CT) and magnetic resonance imaging (MRI) have become an integral part of the evaluation of children and adults with hearing loss and diseases associated with temporal bone. The currently used multidetector CT (MDCT) techniques allow bony tissue determination with an accuracy of 0.1 mm. Recently, cone-beam CT (CBCT) technology has become particularly attractive for temporal bone imaging as CBCT imaging reduces the exposure to ionizing radiation when compared with traditional MDCT. However, changes in inner ear fluid spaces became possible only with 3T or higher MRI equipment in combination with contrast agents and special imaging techniques.
Abnormalities on CT or MRI are found in 20–50% of children with sensorineural hearing loss and correlate with the degree of hearing loss [1]. Imaging of the temporal bone by using both MRI and MDCT is likely the future gold standard for temporal bone imaging [2]. Some recent and novel imaging methods have been currently used experimentally in temporal bone studies but have not yet been applied clinically and these may provide additional imaging benefits in the future. 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
