Evaluation von anatomischen Strukturen des Os temporale mittels Digitaler Volumentomographie (DVT)

Due to its complex anatomy, the petrous part of the temporal bone sets high requirements on radiologic diagnostics of ear diseases. Within this field conventional computertomography (MDCT = multi-detector-row-CT) represents the gold standard till this day. Another applicable method, the cone beam computed tomography (CBCT, syn.: digital volume tomography [DVT]), found its way into otorhinolaryngology several years ago and is applied in the anterior skull base in many cases. In comparison, concerning the lateral skull base, section imaging is further being discussed at present. In this field of application, mainly in-vitro studies have been conducted thus far, whereras clinical trials are only infrequently available, exact data about advantages and limitations respectively is therefore not delineated yet. The objective of this study was to find out, how well middle- and inner ear structures, such as the filigree ossicular chain, could be evaluated in patient‘s images through use of CBCT. For this purpose 228 CBCT- data sets of patients who underwent imaging of the lateral skull base, were assessed retrospectively regarding the visualization of 15 clinically relevant anatomical structures based on a three- or four-step scale, likewise a structure’s bony margin if in existence. Anatomical variants were additionally collected and intra- and intervariable correlation analyses were conducted. The results were the following: The measured „sum“ score, assessed as a calculational aid to ascertain the degree of visualization of all selected parameters, averaged 21.9 ± 4.1 points, ranging in the upper quadrant of the rating scale (least evaluation: 43 points; best evaluation: 15 points). Larger anatomical parameters assessed in this work could be visualized well in more than 95% of all cases through use of CBCT (facial nerve, long process of the incus, fenestral niches, semicircular canals and the jugular bulb). Contrary to this, the fine components of the ossicular chain were less clearly visible (joint spaces ̴50% and stapes structures only in 17-28% completely [28-41% partly] evaluable). It turned out that assessment of the evaluated structures was not age-dependent. Upon collection of anatomical variants, facial nerve dehiscences occured often (50%), considerably less frequently presented dehiscences within the semicircular canals (5%) and the jugular bulb (7%). Seven percent of the bulbs stood above the bottom of the external auditory canal on average with 2.4 ± 0.7 mm. The diameter of the internal auditory canal at its ampulla averaged 5.1 ± 0.8 mm. While age did not correlate with the width of the auditory canal, there was a significant discrepancy between the sexes: the average difference between female and male internal auditory canals came to 0.4 mm (m>f). Compared to images of petrous bone specimens, the evaluation of images collected from living patients is reduced due to radiological signs and motion artifacts. The results show, that in principle CBCT is well convenient for the evaluation of bony middle- and inner ear structures. However, it became clear there are limitations, especially in the depiction of filigree components of the middle ear such as the stapes structures. This circumstance was also observed in other in- vivo trials. Depending on the application protocol, CBCT means less radiation exposure for the patient, while image quality is at least equal to or partly superior compared to MDCT. Furthermore, as shown in other studies, less metal artifacts occur in visualization of middle- and inner ear implants than by use of MDCT. These observations call for CBCT to receive more attention in the context of imaging of the petrous bone. Further in-vivo investigations with a larger patient population, more experienced observers and coherent device parameters are required in order to define standards within the use of CBCT in the lateral skull base.